LNG Operational Practices

Contents Preface

iii

Introduction

vii

Section 1: Post-Refit Operations

1

1 Primary and Secondary Insulation Space 1.1 1.2

1.3 1.4 :

1.5

3

Inerting/Drying Membrane Ships Drying of Cargo Tank Hold Spaces (Moss-Rosenberg) 1.2.1 Drying and Inerting of Tank Insulation and Annular Space Drying of Cargo Tanks Inerting of Cargo Tanks 1.4.1 Overview 1.4.2 Flammability of Methane, Oxygen and Nitrogen Mixtures 1.4.3 Plant Comparison IG/N 2 1.4.4 Inerting Procedure 1.4.5 Pressurisation Post-Refit Ballast Passage

2 Gassing-up Cargo Tanks 2.1 2.2 2.3

Operational Overview Operational Description Other Useful Points

13 14 15

17

Operational Overview Basic Cooldown Procedure (Membrane Vessel) Basic Cooldown Procedure (Moss-Rosenberg Vessel)

17 17 18

Section 2: In-Service Operations

21

4 Loading Operation 4.1 4.2 4.3 4.4

Pre-Loading Procedure Loading Procedure Topping-off Post Loading Procedure

5 Loaded Passage 5.1 5.2 5.3

Overview Normal Boil-off Gas Burning 5.2.1 Operation Forced Boil-off Gas Burning 5.3.1 Operation

7 7 8 8 9 10 11 12 12

13

3 Initial Cooldown of Cargo Tanks 3.1 3.2 3.3

3 6

23 '

23 24 28 29

30 30 30 30 32 32

LNG Operational Practice

5.4 5.5 5.6 5.7

Loaded Passage Essential Safety Equipment Loaded Passage Administration/Records Inner-Huil Inspection (IHI) - Cold Spotting Loaded Voyage Discharge Preparations 5.7.1 Two days prior to arrival at the Disport 5.7.2 Day of arrival at the Discharge Terminal 5.7.3 Day prior to arrival at the Disport

33 33 34 35 35 36 .36

6 Discharge Operation 6.1 6.2 6.3

6.4 6.5 6.6

38

Overview. Pre-DischaYge Procedure Discharge Procedure 6.3.1 Older Class of Vessel (GT or TGZ) - Using a Main Cargo Pump 6.3.2 Newer Vessels (GTT Membrane or Moss-Rosenberg) - Using a Stripping/Spray Pump Additional Notes Regarding Cryogenic Cargo Pump Operation Completion of Discharge Procedure Post Discharge Procedure

38 38 40 40 42 44 46 46

7 Ballast Passage - In Service 7.1 7.2

7.3

47

Overview Ballast Voyage Loading Preparations 7.2.1 Two Days before Arrival at the Load Port 7.2.2 Day before Arrival at the Load Port 7.2.3 Day of Arrival at the Load Port Cold State of the Vessel on Arrival at the Load Port

47 48 48 48 49 50

Section 3: Pre-Refit Operations

51

8 Cargo Tank Warm-up 8.1 8.2 8.3

Tank Stripping / Line Drainage Warm-up Operation Useful Points

9 Inerting of Cargo Tanks 9.1 9.2 9.3

Inerting Overview Inerting Operation Useful Points

10 Aeration 10.1 Aerating Overview 10.2 Aerating Operation 10.3 Useful Points

53 :

*

53 &4 55

56 56 56 57

57 57 58 58

Glossary

59

Index

65

VI

Introduction

Introduction On occasions, the text will refer to various Officers by rank. To clarify how cargorelated responsibilities are usually managed between the Officers, here is'.a typical manning structure for the Cargo Control Room (CCR): Chief Officer: Overall responsibility for the cargo, reporting directly to the Master and Chief Engineer as appropriate. Cargo Engineer: Responsibility assigned to the Cargo Engineer is dependant on the operating company. Typically the following would be the case, split-operational responsibility for the cargo with the Chief Officer, but with a direct reporting line to the Master and the Chief Engineer with regard to aspects of maintenance. OiC (Officer In Charge): Chief Officer and/or the Cargo Engineer. OOW (Officer of the Watch): In charge of watchkeeping responsibilities, that is, moorings, security, safety and providing assistance with cargo/ballast duties as directed. If the time frame for cargo operations and/or compliance with Hours of Rest regulations . requires handover of operational responsibility between C/O and Cargo/Eng, then that process should be formal and recorded appropriately in the Deck Operations Log. In order to conduct cargo operations safely and efficiently, synergetic teamwork from Jhe CCR is an essentia? requirement. Operator's own Safety Stem (SMS) must define the fSponsibility clearly and without ambiguity.

Emergency Procedures and Communications This section provides guidance in the event of an abnormal condition during cargo loading or discharge operations. An abnormal condition is anything that could compromise the vessel's ability to carry out a smooth, incident-free cargo operation. An abnormal condition need not be cargo-related. For example, it might be in the Engine Room (E/R) or involve deck machinery, such as a mooring winch failure.

Emergency Procedures Many emergency procedures are covered regularly by the shipboard Safety Management Drills which are applicable to in-port operations. Some examples are: • • • • •

Ship/Shore Fire Blackout Internal Loss of Cargo Compressor House Gas Detection Pollution.

All such procedures should be documented in the relevant Information Books and referred to regularly to ensure all shipboard personnel are fully aware of the appropriate action to be taken. In compliance with the ISM Code clauses 7 and 8 - 8.3, these drills must be regularly rehearsed by the ship's personnel according to an approved calendar to ensure that everybody is familiar with all essential response procedures. If considered necessary, suspend operations to stabilise an abnormal situation. If there is any doubt, a/ways take the safest option. From experience, the incidents listed here have the potential to cause serious disruption to the vessel's continued trading pattern:


Cargo Pump Failure: If you suspect that the cargo pump load current (amps) is abnormal or the discharge pressure is fluctuating excessively and the condition cannot be stabilised by conventional means, stop the pump immediately. The OIC (officer in charge) will inform all appropriate interested parties - the Master, Chief Engineer, Buyer's Representative, Terminal Control and (in most cases) the Superintendent in HO.* After such an action, do not restart the pump without permission from the appropriate party. Primary Barrier (Membrane) Failure: If you suspect that the primary membrane integrity has been compromised, inform all interested parties - the Master, Chief Engineer, Buyer's Representative, Terminal Control and (in most cases) the Superintendent in HO. If liquid cargo is suspected to have entered the primary insulation space, stop all cargo discharge from the tank. After such an action, do not restart the pump without permission from the appropriate party.

Communications In the event of a serious abnormal condition during cargo operations, the OIC should follow the guidelines laid down by the vessel's operator. Typically, these are: On the vessel (Ship's Staff) - Inform the Master, Chief Engineer, Chief Officer and Cargo Engineer. On the vessel (Non-Ship's Staff) - Inform the Buyer's Representative and/or the Loading Master Off the vessel - Inform Terminal Control and the vessel's Superintendent in HO. Brief all interested parties correctly and promptly about the nature of the abnormal condition. Keep your explanations concise and include any possible effects for the

VIII

current cargo operation. Cover the safety implications where appropriate.

Physical Properties, Composition and Characteristics of LNG Natural gas is a mixture of hydrocarbons that, when liquefied, forms a clear colourless and odourless liquid. This LNG is usually transported and stored at a temperature very close to its boiling point at atmospheric pressure (approximately -160°C). The actual LNG composition of each loading terminal will vary depending on its source and on the liquefaction process, but the main constituent will always be methane. Other constituents will be small percentages of heavier hydrocarbons, e.g. ethane, propane, butane, pentane, and possibly a small percentage of nitrogen. For most engineering calculations (e.g. piping pressure losses) it can be assumed that the physical properties of pure methane represent those of LNG. However, for custody transfer purposes, when accurate calculation of the gross heating value and density is required, the specific properties based on actual component analysis must be used. During a normal sea voyage, heat is transferred to the LNG cargo through the cargo tank insulation, causing vapourisation of part of the cargo, i.e. boil-off. The composition of the LNG is changed by this boil-off because the lighter components, having lower boiling points at atmospheric pressure, vapourise first. Therefore, the discharged LNG has a lower nitrogen content than the LNG as loaded, and a slightly higher percentage of ethane, propane and butane, due to methane and nitrogen boiling-off in preference to the heavier gases.

Introduction

The flammability range of methane in air (21% oxygen) is approximately 5.3 to 14% (by volume). To reduce this range, the air is diluted with nitrogen or inert gas until the oxygen content is reduced to 2% prior to loading after dry-dock. In theory, a.n explosion cannot occur if the O2 content of the mixture is below 13%, regardless of the percentage of methane, but for practical safety reasons, purging is continued until the 02 content is below 2%. This is because the flow path of gases through the tanks can make mixing less effective. At 2%, we can be sure that all areas are well away from the flammable range. The boil-off vapour from LNG is lighter than air at vapour temperatures above -110°C or higher depending on LNG composition. Therefore when vapour is vented to the

atmosphere, the vapour will tend to rise above the vent outlet and will be rapidly dispersed. When cold vapour is mixed with ambient air, the vapour-air mixture will appear as a readily visible white cloud due to the condensation of the moisture in the air It is normally safe to assume that the flammable range of vapour-air mixture does not extend significantly beyond the perimeter of the white cloud. The auto ignition temperature of methane, i.e. the lowest temperature to which the gas needs to be heated to cause self-sustained combustion without ignition by a spark or flame, is 595°C. Volumetric expansion at the liquid/vapour phase change is 600-620 times the liquid volume, hence the dangers of trapping residual liquid between valves.


For clarity, the following style has been used to distinguish between the various types of text in this book: A text block like this will be used for general comment or a description of a step. An indented text block like this will be an instruction. • A bulleted point like this will either be a check-list item or part of some other sort of list. Items in this italic type are extremely important and should have special note taken of them.

Items in this italic type relate to safety and must always be closely reviewed and be given full consideration.

Normal Trading Cycle for a Modern LNG Carrier (membrane type used as an example)

Normal Cargo Operation

Post-Refit Operations

Section


1 Primary and Secondary Insulation Space

1.1 Inerting/Drying Membrane Ships

ships operating manual should be followed as there may be slight differences in the operating procedures required.

Before putting a cargo tank into its initial service, or after dry-docking, it is necessary to replace the ambient humid air in the insulation space with dry nitrogen (N2).

The following outlines the more complex procedure involved where the use of vacuum pumps is required:

To do this, use the ship's vacuum pumps to evacuate the insulation spaces and refill them with N2. Repeat this procedure until the oxygen content is reduced to less than 2% and the humidity to less than -25°C.

To avoid major damage to the secondary barrier, never evacuate a primary insulation space while the associated secondary space is under pressure and never fill a secondary space whilst the primary space is under a vacuum.

Vessels fitted with the GTT Mklll type of containment system, or similar, are not equipped with vacuum pumps. Instead the by-pass valves on the exhaust from the interbarrier and insulation spaces are opened and a continuous supply of Nitrogen is provided so that the spaces are 'swept' with a free flow of Nitrogen. Once the exhaust from the spaces meets the above criteria, the exhaust by-pass valves are closed and the spaces pressurised. In all cases the detailed instructions in the

Measurement devices which may otherwise be damaged, should be isolated prior to the commencement of the test. Install temporary manometers of known accuracy, each bearing an in-date test certificate, to monitor the pressure in the space concerned. At all times, the barrier spaces must be protected against over-pressure, which might cause membrane failure.


Diagram 1.1 Vacuum Pumps

Typically, evacuation of the insulation spaces will take approximately 8 hours. Three cycles are usually necessary to reduce the oxygen to less than 2% by volume. Before being refilled with N2, the insulation spaces are evacuated to 200 mbarA. It is important to have a clear understanding of the pressures involved. For example, a primary space evacuated to 200mbarA exerts the same force on the primary

A

membrane as a positive pressure from within the tank of 800mbarG - that is, 3.5 x the tank relief valve lifting pressure. This procedure for evacuating the insulation spaces is also used to check the integrity of the barriers during the periodic global test and the same stringent precautions to avoid major damage must be in place. To avoid possible damage to the secondary membrane, evacuate the secondary insulation spaces before you evacuate the


primary insulation spaces. In normal design, the pipe work at the vacuum pump's suction ensures that either: (a) the evacuation of the primary spaces cannot take place without having first evacuated the secondary spaces, or (b) that both primary and secondary insulation spaces are evacuated simultaneously. Two electrically-driven vacuum pumps, installed in the cargo compressor room and usually cooled by fresh water, draw from the appropriate headers and discharge to the vent riser. Changes in temperature or barometric pressure can produce differentials far in excess of 30 mbar in insulation spaces that are shut in. With the cargo system out of service and during inerting, always maintain the secondary insulation space pressure to a level at or below the primary insulation space pressure. Severe damage to the membranes will result if the differentials exceed 30 mbar. After evacuation, the next step (or cycle) is to fill the insulation spaces with N2. Repeat the cycle until the oxygen content in the spaces is less than 2%. Adjust the opening of the primary space supply valves to balance the pressure rise in all the spaces. (This procedure is a valvespecific matter.) During filling, maintain the pressure in the primary space at 100 mbar above the secondary space. When the pressure in the primary spaces reaches 300 mbar A (100 mbar above the pressure in the secondary spaces), crack open the secondary space supply valves on each tank. Again, adjust the opening of these valves to balance the pressure rise in all the spaces.

For this operation, liquid N2 is supplied from shore to the liquid manifold. Where it passes to the stripping/spray header through the appropriate manifold shore-connection liquid valve. Then, it is fed to the LNG vapouriser. The N2 gas produced is passed at a temperature of +20°C to each insulation space. The initial filling process is supplied from a bulk liquid N2 source where the ship's N2 generating plant has insufficient capacity for this purpose. The final filling of the insulation spaces up to 2 mbar is carried out at a reduced rate of flow. Three cycles are usually necessary. After the final filling, check the 02 content in all the spaces. If it is higher than 2%, repeat the inerting operation. Also check the 02 content at the vacuum pump discharge at regular intervals. Although the N2 produced by vapourisation from a liquid source is very dry, the final humidity of the insulation spaces is an important consideration. As with the 0 2 , on completion, confirm it as acceptable i.e. better than-25°C. Do not shut down the LNG vapouriser until it has warmed back to the ambient temperature. The primary and secondary insulation spaces are filled with dry N2 gas. This is automatically maintained by alternate relief and make-up as the atmospheric pressure or the temperature rises and falls. It is necessary to maintain pressures of between 2 mbar and 4 mbar above atmospheric. Because of this, reinstate the vessel's normal supply from the A/2 generating plant and the associated pressure control systems as soon as possible after the procedures listed above have been completed.


Ensure that all relief valves have been reinstated and any blanks are removed. To prevent membrane damage, vigilance, careful planning and tight operational control are essential at all times. Note these key points that cover the importance of maintaining insulation spaces in the inerted condition: The N2 provides a'dry and inert medium for the following purposes: • To prevent the formation of a flammable mixture in the event of an LNG leak. • To permit easy detection of an LNG leak through a barrier. *© To prevent corrosion within the insulated spaces. • To deter the migration of leaking LNG vapour from primary to secondary spaces. In normal service, the last point which is more associated with the older class of membrane vessels, is achieved by controlling the secondary space pressure at approximately 1-2 mbar higher than the primary space. Now, Gas Transport Technology (GTT) recommends equal pressures of between 2-4mbar in both spaces. N2 produced by the two N2 generators is stored in a pressurised buffer tank, commonly 22 m3 in size.'This supplies the primary and secondary supply or pressurisation headers through make-up regulating valves located in the cargo compressor room. From the headers, branches are led to the primary and secondary insulation spaces of each tank. Excess N2 from the insulation spaces is vented to the appropriate mast riser through the relief regulating valves.

band of 10%. (Within this control deadband, neither supply nor relief is taking place.) Now, GTT recommends a very slight control range overlap, to ensure a continuous minimal N2 flow through the space, thus maintaining dryness and corrosion protection.

1.2 Drying of Cargo Tank Hold Spaces (MossRosenberg) During dry-docking or inspection, hold spaces contain a certain amount of moist air. These must be dried before any continuation of service, mainly to avoid the formation of corrosive agents within the hold spaces. This formation can occur if the moist air is allowed to combine with sulphur and nitrogen oxides, which may be contained in the inert gas. Dry air is introduced into the hold space through the inert gas filling pipeline. The displaced air is expelled from the top of each hold to the atmosphere. Dry air is supplied by the refit yard or .{if available) by the vessel's own Inert Gas (IG) plant operating in the Dry Air mode. If the vessel has its own IG plant, it can take some 20 hours to reduce the Dew Point to less than -25°C. But, if the dry air is supplied by the refit yard, the result can be better than -45°C. From an operational perspective, the inlet valves to the hold spaces are usually manually operated. The vent valves are situated on each tank top. •

Ensure the IG plant is operating in the Dry Air mode and ticking over while discharging to the atmosphere.

@ Open the vent valves to the holds. Note that on the older class of membrane vessels, the regulating N2 supply and relief valves for insulation space pressure control, can be set-up with an operational Dead-

R

@ Open the inlet valves to the holds. • Activate the Consumer Select facility on the IG control panel, or its equivalent.


Once the O2 level is satisfactory (approximately 21%) and the Dew Point is at (or lower than) -45°C, the IG plant discharge valve to the aeration header will automatically open and the discharge valve to atmosphere will close. The Dew Point for each hold is monitored at the respective outlet pipe. When a Dew Point of -25°C or lower is attained, the appropriate filling valve is closed and the corresponding vent valve also closes.

id Inerting of sulation and pace The insulation around the cargo tanks on spherical vessels is part of the leak protection system. The insulation is fitted in such a way that there is a space between the tank material and insulation. This space is known as the Annular Space'. In the event of a leakage of vapour from the tank, the vapour will be transported around the annular space to the gas detector by a constant flow of Nitrogen, enabling the leak to be detected quickly. In the event of a liquid leak the insulation acts as a splash back, directing the liquid into the drip tray located beneath the tank. Prior to introducing cargo vapour or liquid into the cargo system, the insulation and annular spaces around the tanks are purged with Nitrogen. This process removes any moisture that may be present within the insulation, which if left would cause damage through the formation of ice. The Oxygen in the atmosphere is also removed, resulting in an inert atmosphere around the tank. This procedure is undertaken by opening the exhausts valves from the Annular Spaces to atmosphere, and then opening the by-pass valves on the Nitrogen supply to the spaces on each tank. Nitrogen is supplied from the N2 generators as

required. The process is complete when the Oxygen content is below 2% and dewpoint is below -20°C. Once complete, the Nitrogen inlet by-pass valves are closed and Nitrogen supply to the Annular Spaces is regulated by the appropriate control valves.

1.3 Drying of Cargo During a dry-docking or inspection, cargo tanks that have been opened and contain humid air must be dried. Mainly, this is to avoid the formation of ice when they are cooled, but is also to avoid the formation of corrosive agents if the humidity combines with the sulphur and nitrogen oxides (which might be contained in excess in the inert gas). Normal humid air is displaced by dry air. Then, as the next part of the post-refit procedure, the dry air is displaced by inert gas. Dry air is introduced at the bottom of the cargo tanks through the filling pipes. The air is displaced from the top of each tank through the dome arrangement, into the vapour header and up the appropriate vent mast, usually No.1 (for'd). The operation (performed either from shore or at sea), will take approximately 20 hours to reduce the dew point to less than -25°C. During the time that the inert gas plant is in operation for the drying and subsequent inerting of the cargo tanks, the inert gas is also used to dry and inert all other LNG and vapour pipework to below -45°C. Before the introduction of LNG or the associated vapour, any pipework not purged with inert gas must be purged with N2. A modern shipboard IG plant can produce dry air with a dew point of -45°C and a flow rate of 14,000m3/h.


Area EDFE flammable ! Caution This diagram assumes complete mi which, in practice, may not occur.

Mixtures of air and methane cannot be produced above line BEFC

40

50 Methane

60 %

Area ABEDH not capable of forming flammable mixture

with air

Diagram 1.2 The relationship between gas/air composition and flammability for all possible mixtures of methane, air and nitrogen

Wet air, which may be contained in the discharge lines from the cargo pumps, float level piping and any associated pipe work in the cargo compressor room, must also be purged with dry air. In order to avoid the formation of corrosive agents, it is necessary to use dry air to lower the cargo tank's dew point to at least -25°C, before purging with inert gas.

1.4.1 Over Commence this procedure immediately after the drying process is complete. Inerting is the process that reduces the oxygen level in the atmosphere of the cargo tanks, pipelines and associated equipment to a level where combustion cannot take place. Removing the flammability hazard


means that, when gassing-up, the atmosphere in these spaces will never pass through the flammable zone.

Flammability of Methane, Oxygen and Nitrogen Mixtures Using the diagram 1.2 page 8: At all times, the ship must b'e operated to avoid a flammable mixture of methane and air. The relationship between gas/air composition and flammability for all possible mixtures of methane (CH4), air and nitrogen (N2) is shown on the diagram. The vertical axis A-B represents O2 - N2 mixtures with no methane present, ranging from 0% 02 (100% N2) at point A, to 2 1 % 02 (79% N2) at point B. The latter point represents the composition of atmospheric air. The horizontal axis A-C represents CH4_N2 mixtures with no 02 present, ranging from 0% CH4 (100% N2) at point A, to 100% CH4 (0% N2) at point C. Any single point on the diagram within the triangle ABC represents a mixture of all three components, CH4) 02 and N2, each present in specific proportion of the total volume. The proportions of the three components represented by a single point can be read off the diagram. For example, at point D: CH4: 6.0% (read on axis A-C) 0 2 : 12.2% (read on axis A-B) N2: 81.8% (remainder) The diagram highlights three major sectors: 1. The Flammable Zone Area EDR Any mixture whose composition is represented by a point that lies within this area is flammable. 2. Area HDFC. Any mixture whose composition is represented by a point that lies within this area is capable of

forming a flammable mixture when mixed with air, but contains too much CH4 to ignite. 3. Area ABEDH. Any mixture whose composition is represented by a point that lies within this area is not capable of forming a flammable mixture when mixed with air. Assume that point Y on the 02 - N2 axis is joined by a straight line to point Z on the CH4- N2 axis. If an oxygen-nitrogen mixture of composition Y is mixed with a CH4- N2 mixture of composition Z, the composition of the resulting mixture will, at all times, be represented by point X, which will move from Y to Z as increasing quantities of mixture Z are added. In this example point X, representing changing composition, passes through the flammable zone EDR that is, when the CH 4 content of the mixture is between 5.5% at point M, and 9.0% at point N. Applying this chemistry to the process of inerting a cargo tank prior to cooldown, first assume that the tank is initially full of air at point B. N2 is added until the 02 content is reduced to 13% at point G. The addition of CH4 will cause the mixture composition to change along the line GDC. It will be noted that this does not pass through the flammable zone, but rather is tangential to it at point D. If the 02 content is reduced further, before the addition of CH4, to any point between 0% and 13%, (between points A and G), the change in composition with the addition of CH 4 will not pass through the flammable zone. Theoretically, it would only be necessary to add N2 to air when inerting until the 02 content is reduced to 13%. However, the 02 content is typically reduced to 2% during inerting because, in practice, complete mixing of air and N2 may not occur within the tank, and we are always looking to provide a healthy safety margin.


When a tank full of CH4 gas is to be inerted with N2 prior to aeration, a similar procedure is followed. Assume that N2 is added to the tank containing CH 4 at point C until the CH4 content is reduced to about 14% at point H. As air is added, the mixture composition will change along line HDB; which, as before, is tangential at D to the flammable zone, but does not pass through it. For the same reasons as when inerting from a tank containing air, whe.n inerting a tank full of CH4 it is necessary to go well below the theoretical figure to a CH 4 content of 5% because complete mixing of CH4 and N2 may not, in practice, occur. To summarise procedures for avoiding flammable mixtures in cargo tanks and piping:

Do not confuse the IG/Dry Air generating plant with the N2 generating plant. Both have very different functions. (a) the IG/Dry Air plant has a high capacity, low pressure throughput, from a combustion-based source that is used for inerting large volume spaces with high quality IG. (b) the N2 generating plant is a low capacity, reasonably high-pressure process that is used to supply relatively pure N2 for pressure maintenance purposes, that is, a low flow, low 02 content and without any undesirable by products associated with the combustible source. They provide these onboard services: IG/Dry Air Plant - used to:

;' Before admitting CH4, tanks and piping containing air are to be inerted with N2 until all sampling points indicate 2%Vol or less oxygen content.

# Inert cargo tanks, cargo pipelines and associated equipment.

Before admitting air, tanks and piping containing CH 4 are to be inerted with N2 until all sampling points indicate 5%Vol CH4 or lower.

€> Aerate cargo tanks, hold spaces and cargo pipelines.

Note that some portable instruments for measuring CH 4 content are based on oxidising the sample over a heated platinum wire and measuring the increased temperature from this combustion (catalytic process). This type of analyser will not work with CH4-nitrogen mixtures that do not contain oxygen (0 2 >13% Vol). For this reason, special portable instruments such as the infra-red type have been developed and these are capable of detecting and measuring the hydrocarbon content in inert atmospheres (a purely non-combustible process).

4.3 Plant Comparison IG/N 2 Unlike oil tankers, gas carriers do not use the ship's boilers to create inert gas (IG). Instead, it is produced by a purpose-built IG/Dry Air Generating Plant.

10

% Dry cargo tanks, hold spaces, cargo pipelines and associated equipment.

N2 Generating Plant - used to: if Inert/purge the primary/secondary insulation/annular space (membrane).

Procedure

prevents accumulated debris from entering the cargo system. Start the IG generator and let it run, but do not connect it to the system until the oxygen content and dewpoint are acceptable (02<2% and the dewpoint at or lower than -45°C).

Inert gas and pure N2 can kill you. In order to avoid asphyxiation from O2 depletion, take great care to guarantee the safety of all personnel involved with any operation that uses IG. Make sure there has been a Risk Assessment and all personnel involved in the inerting procedure are familiar with the associated hazards.

The inerting procedure follows the completion of the drying process. Line set-up and preparation are very much the same. With most pipeline configurations, preparations for the drying/inerting procedures involve the fitting of cross-over bends into the cargo piping system. Three dedicated bends are normally required. They are: i IG/Dry Air Plant to Liquid Header (already fitted before Drying) IG/Dry Air Plant to Compressor House (already fitted before Drying)

Monitor tank pressures and adjust the opening of the fill valves to maintain a uniform pressure in all of the tanks. On membrane vessels, ensure that the tank pressures are always higher than the insulation space pressures by at least 10 mbar. Do not allow the tank pressures to exceed 180 mbar above atmospheric pressure. During the inerting procedure, the pressure in the tanks must be kept as low as possible to maximise on the 'piston effect'. Approximately once an hour, take samples of the discharge from the vapour dome at the top of each tank and test them for 02 content. Also test at a purge valve at the filling line of one of the tanks being inerted to verify that the oxygen content of the inert gas remains below 2%.

IG/Dry Air Plant to Vapour Header Note: depending on the vessel's cargo piping configuration and whether or not an onboard IG/Dry Air Plant is fitted, this may vary. To be sure, follow the directions contained in the relevant Cargo Operations Manual. As with the drying process, the flow loop directs IG from the plant to the liquid header and into the cargo tanks via the tank branch valves and the loading drop lines. Purged air from the tanks is exhausted to the vapour header via the tank dome vapour valves and released to the atmosphere through the vent mast riser. Before the IG Plant discharge is allowed to connect to the cargo system, use the IG Blower to blow it through with air. This

Purge all the unused sections of pipelines, blind ends, machinery, associated equipment and instrumentation lines for at last 5 minutes. Inerting is a multi-stage process and involves sufficient stages to purge: Cargo Tanks. Gas Heaters. i- Forcing Vapouriser and LNG Vapouriser. LD and HD Compressors. Spray Header /'Spray Pumps / Spray Rails. •

Main Cargo Pumps and Discharge Lines.

s Manifolds.


Gas Main. Emergency Cargo Pump Columns (Membrane only). Whessoe Columns / Radar Still Pipes. ' Instrumentation (Deck Boxes, impulse lines, safety relief lines). > Fuel Gas Supply Piping to the E/R. When the 02 content sampled from a tank outlet reaches 2% (dew point <-25°C), isolate and shut down the tank. On completion of all stages and final pressurisation, stop the inert gas supply and shut down the IG/Dry Air Plant.

Pressurisation Inerting should happen between leaving the dry-dock and achieving full plant availability. It may be many days before the vessel arrives at the first-load port for gassing-up, cooldown and loading. An essential principle for all membrane vessels dictates that tank pressures must be maintained within designed limits at all times while the vessel is in service. It is the internal tank pressure that keeps the membrane pressed against the underlying load bearing insulation. If this pressure is allowed to fall outside of the maker's recommended parameters, then membrane fatigue stresses, induced by the vessel's motion and vibration, increase dramatically. Prolonged exposure to fatigue stresses can contribute to an overall reduction in the life expectancy of the membrane or cause serious membrane-damaging conditions. In the final stages of inerting, a new membrane vessel can pressurise to 1160mbarA. If the cargo system is tight, the IG plant will not need to be run up for topup purposes quite so often.

important that these vessels depart refit fully pressurised with IG and with a cargo system that is completely free from leakage. In a standard post-refit set-up, a cargo system has each cargo tank isolated from the connecting headers, except for No.1 (usually the smallest tank), where the appropriate isolating valves are opened fully. This provides the headers with volume and ensures that small pipeline leakages do not induce air into the piping spaces as they breathe under the influence of varying temperatures. Moss-Rosenberg vessels can pressurise to 15kPa or 2kPa above atmospheric. Due to their robust construction, these rigid cargo tanks are not internal-pressure dependent to the same extent as are the membrane tanks.

1 R Poct-Pafit Rallact Although the Ballast Passage is covered under In-Service Operations (Section 2-4) the ballast passage immediately after refit requires a separate mention. It happens every 2J£ years (approximately) and is probably the only time in that period that the vessel is in service in the fully inert, condition. An experienced team will be aware of the extensive invasive maintenance that happens during refit. From a cryogenic perspective, no matter how much the associated equipment was tested during the refit after maintenance, the test will have been unavoidably warm and quite unlike the rigors of the cold operating conditions that may be experienced. These are the specific implications:

Some of the older class of membrane vessels (TGZ and GT) do not have an onboard IG plant, which means that they cannot top-up on voyage. Therefore, it is

12

(a) the Work Planning Committee will consider what maintenance can be done, while the ship is in the inerted condition, that would otherwise be


impossible due to the Gas Dangerous Zones. For example, cosmetic maintenance such as manifolds, cargo tank domes, inside the compressor house, vent risers (flame trap arrangements) and similar equipment. A complete absence of the. hydrocarbon gas hazard makes Risk Assessment for this work far more favourable at this time. . (b) The team should consider where extra vigilance will be required when cryogenic equipment, which has been exposed to invasive refit maintenance, contracts during first time operations under cold conditions. Consider tankloading valves, manifold valves, tank gauging systems and gas compressors. Safety awareness under these conditions is invaluable. (c) although a load port is next, it is normal practice to spin-test the cargo pumps at some point in the loading procedure. Questions to be answered are: i During the procedure, when is the most opportune time? Are the pumps turning in the right direction? i; Is the starting current acceptable? Do all measured parameters settle back to normal after starting? S Has the motor winding resistance to earth and between phases been monitored throughout the ballast passage and are all readings acceptable? Has the cargo pump motor heating facility been disconnected correctly? (d) On completion of loading, it is a usual Class/CCT requirement for the cargo tank High/High Levels to be tested by actual cargo transfer within the vessel. Consider these issues: Is the team familiar with this procedure? Has a Risk Assessment been carried out?

$> Have allowable limits for the test been clearly established? ® If these limits are exceeded, what procedure should be followed? J As each independent High/High Level sensor operates, have the implications of the resulting ESD Trip and required response been fully considered? ® Has the terminal been forewarned, guaranteeing official permission and co-operation? # Has a plan been drawn up and agreed? It is clear that this ballast passage is not only busy from the post-refit maintenance perspective, but it also demands a great deal of preparation, forward planning and elevation of awareness levels, especially among key members of staff.

2 Gassing-up Cargo Tanks (Sometimes Referred to as Purge Drying)

After lay-up or dry-dock, the cargo tanks are filled with inert gas or N2. If the purging has been done with inert gas, the cargo tanks will have to be purged with LNG vapour and cooled when the vessel arrives at the loading terminal. Unlike N2, inert gas contains approximately 15% carbon dioxide (C0 2 ), which will freeze at around -56.6°C and produces a white powder which can block valves, filters and nozzles. During purging/the inert gas in the cargo tanks is replaced with warm LNG vapour. This action removes any freezable gases, such as carbon dioxide, and completes the drying of the tanks. This is known as Purge Drying.


Diagram 1.3 An LNG Vapouriser

1.2 Oimrmromu LNG liquid is supplied from the terminal to the liquid manifold where it passes to the stripping/spray header through the appropriate shore connection liquid valve or, in some cases, a dedicated cooldown bobbin. Then it is fed to the LNG vapouriser and the LNG vapour produced is passed at approximately +20°C to the vapour header and into each tank through the vapour domes. (Note: 20°C is a typical process value.) At the start of the operation, the piping system and LNG vapouriser are vapour locked i.e. the reduced liquid supply for this purpose may be insufficient to prime the

1/1

pipework by displacing the vapour, requiring an alternative venting arrangement. The stripping/spray header can be purged into the cargo tanks through the vapour dome through the arrangement of spray valves and associated control valve until liquid reaches the LNG vapouriser. As the LNG vapour is lighter than the inert gas its introduction at the top of the tank creates a clearly defined interface that forces the IG in the cargo tanks to be exhausted by displacement up the tank filling line into the liquid header. The inert gas then vents to the atmosphere, usually through No.1 vent mast. Because the flow of liquid from the terminal is relatively small for this operation, problems experienced may be due to vapour locking at the loading arms. Where a cooldown bobbin is used, there is provision


for intermittent venting of the loading arm through the appropriate vent mast. When 5% Vol CH 4 (the percentage figure will be specified by the particular port authority) is detected at the No.1 vent mast riser, the exhausting gas is directed ashore via the HD compressor's bypass line, or to the boilers through the gas burning line. If there is enough backpressure, this operation can be done without the compressors. But if there is not enough backpressure, one or both of the HD compressors in service can be used. However, as the compressors create unnecessary turbulence inside the tanks, it is better not to use them.

carried out with the exhausting gases being returned to the shore facility.

2,3 Other Useful Points Depending on the type of vessel, the pipeline configuration as described in previous procedures usually requires purpose-fabricated bends to be fitted. Two are required: Liquid Header to Vapour Header and Liquid Header to HD Compressor. Good practice requires this equipment to be purged with cargo vapour: f; Main cargo pump discharge pipes. LD/HD compressors.

The operation is considered complete whe the CH4 content exceeds 88% by volume a measured at the top of the cargo filling pipe. The target values for the N2 gas and the inert gas CO2 are equal or less than 1%. These values should be matched with the requirements of the LNG terminal. This normally entails approximately 1.8-2.0 changes of the volume of the atmosphere the cargo tank. Due to local regulations on venting CH4 ga to the atmosphere, some port authorities may require the entire operation to be

Gas Heaters. When a vapouriser is in use, it is essential to monitor the operating parameters of associated equipment with care. Before introducing LNG, carefully warm the unit. Test all associated trips, alarms and controls beforehand and prove them operable, (refer to the post-refit ballast passage routines). The NG outlet temperature (+207+25°C) and the temperature of the heating steam condensate returns are both critical. Take great care when shutting-down the unit, for example, maintain the heating steam until all residual LNG has been dissipated from the unit.

Required Heat Energy at Initial Purging Tank

TK volume (m3)

Required NG (m3)

Required LNG (kg)

Required LNG (kg)

Heat Energy (MMBTU)

No.1

21,943

39497.04

32071.6

. 68.4

1648.5

No.2

40,432

72777.06

59095.0

126.0

3037.5

No.3

40,443

72796.9

59111.1

126.0

3038.3

No.4

37,831

68095.62

55293.6

117.9

2842.1

Total

140,648

253,167.00

205,571.0

438.0

10,566.0

Note: 1. LNG Density: 469.0 kg/m3 2. LNG Heating Value: 51.400 MMBTU / ton


Diagram 1.4 Cargo tank cooldown

Required heat energy at initial purging for a typical 140,000m3 membrane vessel: The inert gas is purged and replaced by Natural Gas (NG) produced by the LNG vapouriser (+20°C outlet temperature) and fed with LNG supplied by the loading terminal. It requires approximately 1.8 complete volume changes to displace the inert gas and attain a CO2 content of < 1 % by volume. © This operation takes approximately 20 hours. Natural Gas Total Required 1.8 x 140,648 m3 (100% of Total Volume) 253,167 m3 OF NG

16

Required LNG LNG Total Requirement = 1.8. V ( 1 0 0 % ) . P N G = 1.8 x 140,648x0.8120 = 205,571 kg = 430.2 m3 of LNG (Density p NG = 0.8120 kg/m3 AT +20°C and 1,060 mbarA) On completion of the gassing-up procedure, the cooldown of the cargo tanks and associated system normally follows immediately after.


3 Initial Cooldown of Cargo Tanks Cooldown is the process that brings the containment system to a temperature that will not cause excessive boil-off during loading or unacceptable stresses in the support structures. It follows a procedure that prevents a thermal shock to the primary containment system. Cooldown is achieved by pumping LNG through the spray header and cooldown grids at the top of each tank. This allows the LNG to vapourise at the sprays and allows cold gas to enter the tank. A controlled and design-compliant rate is essential to avoid damage to the primary containment and overcome potential problems: \ To avoid excessive stresses being induced in the pump tower or trellis. To prevent vapour generation exceeding the capabilities of the HD Gas Compressors at any time during the process. To avoid structural deformation to the insulation arrangement being caused by non-uniform contraction. <$> To maintain the pressures in the insulation spaces to the requirements of the N2 generating plant and associated system. In the initial stages of cooldown, pressures in these spaces can collapse and slow the rate at which we can complete the procedure. This is particularly the case on the older class of membrane vessel.

Procedure (Membrane Vessel) Unlike rigid cargo tank designs, vertical thermal gradients in the tank walls are not a significant limitation on the rate of cooldown. LNG is supplied from the terminal to the spray header, which is open to the cargo tank spray rails. Once the cargo tank cooldown is almost complete, the liquid manifold cross-overs, liquid header and loading lines are also cooled. To avoid splashing the cargo tank bottoms with LNG liquid, control the stripping/spray header pressure, especially in the initial stages. Cooldown of the cargo tanks is considered to be complete when the mean temperatures (except for the 2 top temperature sensors in each tank) indicate 130°C or lower. When these temperatures have been reached and the Cargo Transfer System (CTS) registers the presence of liquid, bulk loading can begin. (GTT has defined that LNG loading is possible when the mean target temperature is lower than -80°C). In practice however, GTT recommends that the cooldown operation is continued until -130°C has been attained, in-line with most LNG terminal requirements. Vapour generated during the cooldown of the cargo tanks is returned to the terminal through the HD compressors and discharged to the vapour manifold, as in the normal manner for loading. During cooldown, N2 flow to the primary and secondary spaces will increase significantly. It is essential that the rate of cooldown is controlled so that it remains within the limits of the N2 system to maintain the primary and secondary insulation space pressures between 2 mbar and 4 mbar.


Once cooldown is completed and the buildup to bulk loading has commenced, the tank membrane will be at, or near to, liquid cargo temperature. But it will take some hours to establish fully cooled temperature gradients through the insulation. Consequently, boil-off from the cargo will be higher than normal at-this stage. On a typical new 140,000m3 vessel, cooling the cargo tanks from +40°C to -130°C, over a period of 10 hours, will require a total of about 800 m3 of LNG to be vapourised. Therefore, the cooldown rate in the cargo tank and insulation spaces is dependent on the degree of LNG spraying. Preparation for Tank Cooldown: Prepare the cofferdam steam or gycol heating system as appropriate. Prepare the records for the cargo tank, secondary barrier and inner-hull steel temperature readings. Check that the insulation space A/2 supply system is in automatic operation and has the capability of supplying the additional N2 necessary to compensate for the initial pressure drop experienced as the primary space atmospheres collapse. If appropriate to the system, before cooling, raise the N2 pressure inside the primary insulation spaces to 8 mbar as extra compensation for the inevitable initial pressure drop. Ensure the buffer tank is maintained at maximum operating pressure throughout the procedure. Check that the gas detection system is in normal operation. Prepare the N2 generating system for maximum output. Prepare both HD compressors for use.

18

Cargo Tank Cooldown: After cooling the lines, a liquid line pressure of 2 bar is required on the vessel's spray rail. This is controlled by a shore request from the vessel and adjusted to meet required parameters onboard. Adjust the spray rail pressure to obtain an average temperature drop of 20°C per hour in the first five hours - and 10/15°C per hour thereafter. Start one HD compressor (or both as necessary) to maintain the tank pressures at about 100 mbarG. Check the N2 pressure inside the insulation spaces. If there is a continuing downward trend that is outstripping the N2 supply system, reduce the rate of cooldown accordingly. When the average temperature shown by the CTS sensors is -130°C, inform the terminal that cooldown is complete and prepare for a gradual ramp-up to bulk loading status.

3.3 Basic Cooldown Procedure (MossRosenberg ;;> ôe:; As with the membrane vessels, the cargo tanks are gradually cooled by spraying LNG received from the loading terminal through the spray nozzles located round the centre column of the tank. This operation, which produces cold vapour that has to be returned ashore, must continue until the equatorial region of the tank is at least 115°C. Typically, the maximum allowable rate of cooldown is 9°C per hour and must never be exceeded. To avoid thermal stresses on the tank shell and tank support structure, the cooldown procedure must be smooth and uniform.


In normal service, the ship will arrive with the equatorial region of the tanks at about (but not warmer than) -119°C. Do not commence full rate loading until this figure is attained. LNG enters the cargo tanks through the spray nozzles and the vapour is returned to shore using the HD compressors as necessary to maintain tank pressures within acceptable limits. On a typical 135,000m3 vessel, the operation will take approximately 24 hours. On most vessels, when the HD gas compressors are running, service from the LD unit is not required. The E/R fuel gas burning requirement can be supplied by a regulated bleed-off from the discharge of the HD compressors. Preparation for Tank Cooldown Use the established Time/Temperature graph forms to prepare the records for the cargo tank equatorial temperature monitoring. Before commencement, set the HD and LD gas compressors for free-flow operation with the appropriate low demand gas heater warming through. Request shore control to supply LNG at the agreed reduced rate. The spray lines are cooled first. Check that the gas detection system is in the normal operating mode. Cargo Tank Cooldown: On completion of line cooldown, LNG is introduced through the liquid crossover, the spray main and branch lines to the spray nozzles.

Expect a spray rate of 1,000kg/hr per tank. Monitor the pressure in the cargo tanks throughout the cooldown procedure, particularly in the initial stages when it will rise rapidly. Start the first HD compressor when the pressure reaches 150mbar. If the tank pressure is allowed to fall to 40mbar below the void space pressure at any time, the HD compressors will automatically shut-down and at the same time the shut-off valves at the domes will close. If not already in use, commence fuel gas supply to the E/R. After approximately 2 hours, you may increase the spray rate, either by requesting more flow from shore control or by opening up more spray valves, or by doing both. The time value is usually clearly defined in the cargo manual. Cargo line cooldown is usually carried out at some point during the tank cooldown procedure. When: (a) the temperature in the liquid header at all the cargo tanks has been reduced below -120°C, (b) the equatorial temperature in all cargo tanks has been reduced to below -115°C (c) tank pressures are fully under control then you may fully open the gate crossover valves in readiness for loading ramp-up.

Jperational Practice

In-Service Operations

Section


4 Loading Operation Procedure Upon completion of mooring, engage the main engine turning gear and close the master steam stop valve as the gangway is brought onboard and the loading arms are connected. Note: On steam propulsion plants, old and modern, while the vessel is alongside the loading/discharge berth: Maintain a slow rotation of the prop shaft at all times while the turbines are hot. You must also raise the engine vacuum and use warming-through steam to maintain a state of readiness. On older Stal-Laval turbine vessels, the maximum period allowed for a stationary prop shaft under these conditions is only 4 minutes, and on the newer Kawasaki turbine ships, it is only 3 minutes. If these periods are exceeded, unequal expansion may cause turbine shaft damage, which leads to rotor deformation. Under FWE (Finished With Engines) conditions, that is, between arrival and departure standby, you can arrange slow rotation of the main engine turbines by engaging the electrically-driven Turning Gear. While on standby, lengthy periods of prop shaft rest are automatically broken by a programme in the Main Engine Bridge Control System called Auto Blasting, which uses steam from the manoeuvring valves to roll the turbines intermittently. If a system failure occurs during autoblasting, the prop shaft rpm can increase and cause the vessel to move, relative to the jetty, which is why the

gangway and loading arms are not connected until the Main Engine Turning Gear has been engaged and the turbines appropriately secured. The normal arrangement at most terminals is two and four loading arms and one vapour return arm. Connect the CommunicationiESD cable, usually a male/female plug arrangement, fibre-optic or electrical, and provides the various channels for ship/shore communication and ESD interconnection. Connect the ESD umbilical cable, usually a pneumatic connection ISGOTT no longer recommends the use of a ship shore bonding cable as the insulating flange at the cargo connection is effective in removing the current and any sparks. However, some national/local regulations still insist on the connection of a ship/shore bonding cable. If so it should be checked to make sure it is mechanically sound and be connected well clear of the manifold area. When the loading arms have been connected, they are purged and pressurised with N2 supplied by the terminal to a pressure of 200kPa in each arm. The responsible Ship's Officer checks all associated joints for leakage. (Soapy water is still the preferred means of leakdetection.) Once the integrity of the loading arms has been proved, they are depressurised in a controlled manner. While depressurising, use a portable O2 meter to check the gas. The usual requirement is below 1%. Dryness of the gas is an important issue and can be part of the standard checks at some terminals. The usual requirement is lower than -50°C. With the vessel upright and even keel, it is usual for the Chief Officer and Cargo


Surveyor (or appointed person) to run the official Initial Cargo Custody Transfer data, commonly referred to as the Initial Gauging.

the loading procedure. There are two recognised ways of doing this: 1.

the Low Demand (LD) compressor is shut-down and the Engine Room (E/R) receives the required fuel gas by a controlled bleed system. This is sensitive to combustion control requirements and tapped from the gasto-shore line.

2.

requires continued use of the LD compressor during loading, with the ship's boilers maintaining duel firing throughout. With this system, the LD compressor is removed from the ESD trip logic circuits. (This prevents the LD compressor from being affected by cargo operations.)

In the Cargo Control Room (CCR), the ship/shore communication system is powered up. The Hot-Line and Plant phone are tested and proved satisfactory. Bring the manifold spray-water curtain into service. . Back in the CCR or Ship's Office, complete the ship/shore safety checklist and ISPS security check-list. When the vessel and terminal are ready to carry out ESD tests, confirm whether it is the vessel or terminal that will activate the shut-down. In normal operations, the Officer on Watch (OOW) attends at the manifold to assist with and report on the progress of the tests. With terminal permission, open the manifold/ESD valves. When the valves are confirmed fully open, reset the ship's ESDS ensuring appropriate indicators all show Healthy. Once the terminal has similarly re-set, switch any associated override facility on the vessel's system to Override Off. Conduct the Hot ESD test as agreed. It is normal at this time for the terminal to require Manifold/ESD valve closure to be timed. The IGC Code requires a closure time of less than 30 seconds. Once ship and shore have confirmed correct ESD operation, both can. reopen the appropriate valves (Manifold/ESD) and reset the ESDS. At this stage, the ship may request permission from the terminal to send gas ashore. Associated lines and valves are set accordingly. Normally, the terminal allows the vessel to burn boil-off gas throughout

24

Enable and ensure all cargo tank level alarms are operative (recommended as Low Level @ 0.5Mtr, Hi Level @ 95% capacity, Tank Fill Level @ 98.5% capacity and HiHi Level @ 99% capacity). On passage, you may use the speciallyinstalled blocking circuits to inhibit certain lalarm systems. The vessel is now ready to begin loading.

42 Lo&dhig Prae&dyre In accordance with the IGC Code, the maximum filling capacity for any cargo tank is 98% by volume. However, in compliance with contractual requirements, backed by Flag State approval and allowing for an average daily boil-off rate of 0.15%, slightly higher loading capacities are in force for LNG vessels. On average, a membrane vessel will load to 98.5% volume and a Moss-Rosen berg ship will take up to 98.8%. Closure of the tankfilling valve at these levels is usually automatic, but is closely monitored from the CCR and at the tank dome in case manual


Diagram 2.1 Ship-Shore Optic Fibre Transmission

intervention is needed. The loading valve must close as the required level is reached.

correctly lined-up and set for line cooldown.

Critical to this procedure is the level at which the valve starts to close.and the time it takes to close. Therefore, partial-loading valve closure at the topping-off stage is not part of the normal procedure.

To make it easier to empty the ballast tanks, the ship may be trimmed within the terminal's maximum draught requirements during loading.

Assuming that the vessel has arrived in the Cold Condition, this is the recommended sequence of events: Ensure manifold/ESD valves, stripping/spray main crossover connections, cargo loading valves and the cargo tank spray rails are

The structural loading and stability as determined by the loading computer, must always remain within recommended safety limits. HD compressors must be ready for service. The secondary float level gauge system must be ready for operation.


Temperature recording systems, alarms for the cargo tank barriers and double hull structure must be in continuous operation. The gas detection system and alarms must be in continuous operation.

When cooldown on the vessel is complete, inform the terminal and request control to standby pending the start of loading. Tank loading valves are set in the prescribed manner, according to the approved ship specific procedure.

Request the terminal to "start cooldown. Terminal control will gradually increase the cooldown rate, simultaneously cooling lines ashore and on the vessel. On deck, the OOW and assistants check the cargo system and report cooldown progress. Frosting provides a very clear indication in this respect, especially at the loading arms, which should be carefully monitored during the procedure. Any sign of liquid/vapour leakage must be reported back to the CCR immediately. As a guide, the progress of cooldown is indicated by a sudden reduction in liquid header/crossover temperature, frosting along the liquid header and frosting along the spray header. In the CCR, the OIC is responsible for monitoring tank pressures, vapour header pressure, liquid header/crossover pressure and temperature. At the same time, the N2 supply to the holds/interbarriers is closely monitored to ensure the correct pressures are maintained. Usually, the terminal is the first to report that cooldown is complete. The vessel will then request the terminal to standby pending cooldown completion on the vessel. Line cooldown, as monitored in the CCR, is considered complete when the liquid header temperature is approximately 157°C. Other indicators include (usually at No.1 Tank), frosting along the loading line, a reduction in noise level at the loading valve, a noticeable drop in the interbarrier space pressure and a significant drop in the tank bottom temperature.

26

Request terminal control to start loading at the pre-arranged reduced rate. On deck, the OOW and assistants continue to monitor the integrity of loading arms, cargo lines, associated flanges and instrument connections/impulse lines. When tank pressures reach the appropriate level, (design dependent) start the first HD compressor, returning gas ashore. But before starting, inform terminal control and the E/R. On a modern Moss-Rossenberg vessel, this pressure is around 12kPaG. While on the older TGZ or GT vessels, approximately 100mbG would be more appropriate. Providing that boil-off gas generation and N2 supply to the holds are both under control, request the terminal to increase the loading rate up to maximum over the pre-arranged time scale. Start the second HD compressor as boil-off generation begins to exceed the capacity of the in service unit. Again, inform terminal control and the E/R before starting. With both HD compressors running steadily in parallel, tank pressures are lowered to that required for the duration of the bulkloading period, recommended as 12mbG (12kPaG). The OIC should remember that in the interest of cargo conditioning considerations while on-passage, achieving


Diagram 2.2 HD Compressor

maximum gross heating value out-turn for the receiver, while loading at the lowest pressure appropriate for the containment system, is an important safety factor.

loading rate and manifold pressures. The terminal will normally require QOB and % cargo loaded as recorded by the vessel every hour.

Once the full loading rate is achieved, loading valves can be adjusted to prevent flow preference (unequal filling, which may occur in all designs).

The Cargo Control Room (CCR) officers wil also maintain a close scrutiny on the weather data (wind and sea state) and mooring tensions, provided a mooring tension monitor is part of the ship/shore installation.

Commence de-ballasting operations as per the Cargo/Ballast Plan requirements. The procedure usually runs in parallel with loading, achieves good drainage of the required ballast tanks and keeps the vessel upright. In the CCR, an hourly record is maintained of tank levels, Quantity On-board (QOB),

Loading operations also provide the opportunity to log-HD compressor parameters. These'are normally recorded every hour.


As bulk loading progresses, boil-off tends to reduce and it may prove necessary to stop one of the two HD compressors. Adjust loading valve openings to provide the desired stagger between the tanks. At some point towards the end of the procedure, lower the back-up manual float gauges (typically Whessoe). Record the readings at final gauging and compare them to the remote system (typically Fox IV Trans-sonics or Radar).

T Topping -cm All Responsible Officers - for example, the Chief Officer, Cargo Engineer and the OOW- must report to the CCR in good time. Give one hour's notice of the first loading rate reduction to terminal control, that is, when the first pump is to stop. Plan to complete de-ballasting approximately one hour before completion of cargo and before topping-off the first tank. Ensure the vessel is upright and on an even keel. When the 95% Hi-Level alarm operates, the OOW on duty at the tank dome will test the loading valve (and/or branch valve) operation locally. When this task is complete, the Officer should report positively back to the CCR that the valve concerned operates correctly, automatic' mode has been re-engaged and that the valve is full open. On the tank designated for receiving the line drains, automatic closure is disabled in case the line drains exceed the appropriate level, even by a minimal amount. The spray valves on each tank are also closed at this time, except on the tank nominated for line drainage where the valve is left fully open.

28

Give 15 minutes notice to terminal control before the first reduction in loading rate is anticipated. Then give terminal control a 5 minute warning. As the first tank shut-down level is approached, request loading rate reduction. The loading valve must begin to close at the known and pre-determined level. As each tank successively approaches topping-off, the vessel should provide terminal control with 5 minutes notice for the next loading rate reduction. As each tank loading valve commences auto closure, the OOW on duty at the tank dome will tell CCR when valve closure has commenced and when valve closure has been completed. Best practice dictates that on completion of closure of each valve, the OOW should disengage the remote activation and, where possible, confirm full closure using the manual gear. In common with sound tanker practice, the CCR will make sure that the level in any closed tank does not continue to creep upwards. Throughout the topping-off procedure, a Nominated Officer in the CCR should record all critical cargo tank levels in the Deck Operations Log (DOL), which will include the point at which the 95% Hi-alarm activated and when the level auto-closure of the loading valve commenced and completed. Stop the final HD compressor, inform terminal control and the E/R accordingly. If appropriate, commence preparation of the LD compressor for the reinstatement of fuel gas burning. When the last tank approaches the finish level minus the line drain


quantity, ask terminal control to stop loading. Once terminal control has confirmed all cargo loading stopped, commence line drainage. With the terminal's permission, you can close the gas-to-shore connection reinstate gas fuel burning with the LD compressor. On completion of draining, and with the terminal's permission, close the liquid manifold/ESD valves. When closure is confirmed, final gauging is conducted in the presence of the Chief Officer, Loading Master and/or Surveyor.

fk n U ^ ^ i f ^ T I ! ••-.:-. •'-=•• ••'.-.•v:.

#*•%. *"» f \ » **% f*a " , , - . J c i L - . ; : ; iu

On completion of final gauging, the Loading Master will request the terminal to pressurise the liquid lines and vapour return line to the agreed pressure with N2 (typically 2bar). This procedure pressurises/ depressurises the loading arms to purge them of hydrocarbons. The actual method will vary according to the terminal and vessel type, but whichever way purging is arranged, the principle remains the same. Purging will continue in the approved manner until the CH4 content is reduced to less than 1% by volume (1.2% at some installations). As the QRCs (Quick Release Connections) are released, you can start the disconnection after the final depressurisation. Follow the standard procedures and safety precautions. Return back-up float level gauges to their stowed position. Make sure that the in-house reading is correct in each case.

Once the loading and vapour arms have been secured and re-stowed ashore, the vessel should secure and blank the manifold connections for sea conditions. With the terminal's permission, the fibre-optic/electrical communication IESD link is disconnected and re-stowed ashore. Shut-down the manifold overside water sprays. When all shore personnel have cleared the vessel, remove the gangway. In the CCR cargo tank level, reengage the alarm blocking circuits. The Master/Chief Officer will now complete and issue all paperwork as appropriate. Experience shows that giving the vessel a slight list towards the berth so that she assumes (as near as possible), an upright position after letting-go counters the tension effect of taut mooring wires. Other vessel-departure procedures are run concurrently with Post-Loading procedures: Conduct a stowaway search in compliance with ISPS requirements. Once clear of the jetty, adjust the ballast to bring the vessel upright and trimmed appropriately for the sea passage. After leaving the-berth, set cargo lines to keep one tank open to the liquid and spray header. This action prevents the build-up of internal pressure through vapourisation of residue liquid. Associated valves are usually manually cracked-back for the first day.


5 Loaded Passage 5.1 Overview During the laden voyage, the movement of the vessel and the ingress of external heat through the tank insulation generates convection currents in the bulk cargo mass. This causes the relatively warm LNG to rise to the surface, producing boil-off gas that becomes available'as a supplementary fuel source for the vessel's power plant. This process can account for 0.15% of the liquid cargo per day. It is the responsibility of the vessel's operators to achieve these goals: > Safe carriage of the cargo in compliance with the operator's SMS and associated international regulations. Achievement of maximum out-turn across the manifold for the customer. !

Delivery of the cargo within the customer's quality parameters and/or as per the contractual requirements of an established Sale & Purchase agreement. These may include a maximum delivery temperature (>-158.8DegC) and a window of allowable vapour pressure. If required, SG and N2 content may be determined at source by sampling, as arranged between supplier and receiver. Cargo venting avoidance in-line with the operator's EP Policy. Full use of boil-off as a supplementary fuel source for the vessel's power plant. Maintenance of cargo containment integrity, particularly the inner-hull steel work temperatures.

;

I nrnviuA Son^yfi Gas Burn-in®

During a sea passage where the cargo tanks contain LNG, the naturally-generated boil-off from the tanks is burned in the

^n

ship's boilers. The operation is started on deck and controlled by the ship's engineers in the CCR and ECR. If the boil-off cannot be used for gas burning purposes, or if the volume is too great for the boilers to handle, then excess vapour as a last resort may be vented into the air through the No.1 vent mast as a last resort.

5.2.1 Op The cargo tank boil-off gas enters the common vapour header through the cargo tank vapour domes. It is then directed to the in-service LD compressor, which delivers the fuel gas to the E/R through a gas heater. The heated gas is delivered to the vessel's power plant at approximately +30°C by a fuel gas control valve. Compressor throughput is controlled by speed of the prime mover and/or inlet guide vanes at the compressor suction. On a modern carrier, the various control requirements are consolidated by DCS (Distributed Control System), which includes the ACC (Automatic Combustion Control). Associated logics use a pressure input from the cargo tank vapour system to make sure the gas delivery to the E/R does not suppress the cargo tank vapour pressure below a pre-determined allowable minimum, or allow it to rise above a predetermined maximum. The system is designed to burn all boil-off gas normally produced by a full cargo maintaining the cargo tank pressure and temperature at the required level. On a standard steam propulsion plant, if fuel consumption is not sufficient to burn the generated amount of boil-off, the tank pressure will increase. This pressure increase can be controlled in one of two ways either provision of a steam dump system, or alternatively, by increasing the speed of the vessel. In most cases, the main steam dump system is designed to dump sufficient steam to let the boilers burn the boil-off gas, even when the ship has stopped. A combination of both methods may be used.


Although venting is also a means of automatic vapour pressure control, it is not an option under normal operating conditions and, in the interest of Environmental Protection (EP) compliance and operating efficiency, it should be avoided. This is the standard logic arrangement for boil-off gas pressure control by a Distributed Control System (DCS): To control the flow of gas through the LD compressors, adjust the inlet guide vane position. When gas-burning is initiated, this is directed by the DCS. Select the normal boil-off in the boiler combustion control. Select the maximum/minimum allowed tank pressures. Select the tank pressure at which the main steam dump operates. For normal operation, the normal boil-off value is selected at approx 60%, that is, boil-off provides 60% of the fuel required to produce 90% of full steaming capacity. The minimum/maximum tank pressures are selected at (for example), 1050 and 1090 mbarA (for a standard membrane-type carrier). If the normal boil-off control value has been correctly adjusted, the tank pressures will remain within the selected values. Should the selected normal boil off value be too large, tank pressure will slowly reduce until it reaches the minimum value selected. If the tank pressure value continues to fall below the minimum value selected, the DCS will reduce the normal boil-off value until the tank pressure has increased again above the selected value. If the selected normal boil-off rate is too small, the tank pressure will slowly increase until it reaches the maximum setting selected. If the tank pressure value increases above the maximum selected

setting, the normal boil-off rate will be increased until the tank pressure falls to a level below the selected setting. If the tank pressure continues to increase because the steam consumption is not sufficient to burn all the required boil-off, the steam dump will open. The steam dump is designed to open when the normal boil-off rate is 5% above the original selected value and/or when the tank pressure has reached the pre-selected dump operating pressure. At this setting, an increase of 5% of the normal boil-off corresponds to an increase in tank pressure of approximately 40 mbar above the maximum tank pressure selected. In each case, the Automatic Combustion Control (ACC) compensates for the boiler fuel requirements by varying the amount of HFO delivered -which is variable between 0% HFO (100% gas) and 100% HFO (0% Gas). If anything should stop the gas from being burned in the ship's boilers, the cargo vapour and gas burning piping system is arranged so that excess boil-off can be vented automatically. An automatic control valve, usually sited at the No.1 vent mast, is normally set at approximately 25-30mbar below the tank relief valve (typical "luceat") upper operating level. If the gas-burning system shuts down for any reason, an integral part of the shutdown sequence automatically initiates a thorough purging of all associated lines with N2. Typically the gas burning security valve G will operate (shut) under these circumstances: P Gas to E/R temperature Low/Low. ' Gas detected in boiler combined gas hood. Gas detected in the vent duct. .•'••v Gas duct exhaust system failure. } Gas pressure High/High. Gas pressure Low/Low.

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Manual operation (E/R, CCR, Bridge). ;

: Loss of authorisation from CCR or Bridge. E/R exhaust fan failure. i E/R C0 2 fire-fighting release. • Blackout.

If, during a loaded passage, additional fuel gas from the cargo tanks is required to be burned in the ship's boilers over and above current natural generation, it can be made available by forced vapourisation, using a dedicated Forced Vapouriser. This operation, called Forced Boil-Off, can be used to complement gas burning for up to 100% of the boiler's fuel requirement.

o,;;; Pureed BoH-Df? Gas Burning Before undertaking forced boil-off, consider the economics of gas versus fuel oil burning and the charter agreement, if applicable.

5.3.1 Operatior The normal gas burning arrangement is maintained and the forcing vapouriser is brought into operation. This uses a single stripping/spray pump in conjunction with the LNG forcing vapouriser. The excess flow from the pump is returned to the same tank

Diagram 2.3 Forcing Vapouriser

3?


through the stripping header pressure control valves. The generated vapour then combines with the natural boil-off gas from the vapour header before being drawn into the suction of the LD compressor, and reducing the risk of droplet carry-over. The process is controlled by the DCS. In normal forced vapouriser operation, the controlled return from the pump is always directed back to the same tank where the liquid is being drawn from as an insurance against cargo transfer between tanks.

•'••;>i Loaded Passage Essential Safety

• Catalytic Gas Detection System -Atmospheric sampling of strategic spaces round the accommodation, E/R and compressor house. •i Insulation and Inner-Hull Temperature Sensors # - One of the main line defences against primary containment leakage and inner-hull steel work cold-failure protection. ® Automatic Sequence Controlled N2 Purge System -When a gas burning shut-down is initiated, it renders an inert atmosphere within all gas burning pipe work. A Fully Integrated Alarm System - Sounds in the CCR, E/R and Bridge, as appropriate.

This is a list of essential safety equipment: Air Swept Duct - Conveys fuel gas line, purge lines and N2 lines through the E/R space to the boiler furnace fronts/top. Double-Walled N2-Jacketed Gas Line - Used where the fuel gas lines exits the air-swept duct and connects to the furnace front/top burner gas valves.

Records Maintain these records at all times: ;i Daily Cargo Log. © Daily LD Compressor Log.

Air-Swept Duct Exhaust Fans - Maintains air flow through the air-swept duct. Stand-by fans automatically cut-in on failure of in-use units. Initiated by ampere load-sensing relays.

•s Monitor inner-hull and insulation space temperatures, as appropriate.

Gas Burning Security Valve G - Independent and operated by any of the above listed elements.

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Dedicated Fuel Gas Detection System - Samples furnace front/top fuel gas line canopy/hood arrangements and the swept-duct exhaust air for gas leakage. Positive detection incorporated in the gas burning security chain. Main Permanent Gas Detection System - Infra-red, sampling from the N2 rich interbarrier spaces, insulation spaces and hold spaces.

;i Inner Hull Inspection (IHI) Record.

-' Daily trend monitoring of Average Liquid Temperature (for delivery). Daily Fuel Oil Equivalent (usually calculated as part of the Voyage Abstract).

•1 Alarm Test Register (on-going from an established register or PM). tl Weather Report (sea state, barometric pressure, etc, all have an effect regarding cargo conditioning considerations).


•^6 . h ^ r - r h ^ l ^^pecllon

These are the inspection points: • The position and temperature of cold spots, or the absence of cold spots where previously reported.

At low temperatures, structural steels can suffer brittle fracture. Such failures can be catastrophic because once brittle steel starts to fracture, only a small amount of energy is required to make it spread.

§; On Moss-Rosenberg vessels, condition of the spray-shield and taped joints.

However, in a tough material, considerably more energy is required to turn a small crack into a larger fracture.

© Condition of inner-hull paint work or appropriate tank coating.

Plain carbon steels have a brittle to ductile behaviour transition that generally occurs in the range of -50°C to +30°C. Because of this, the composition of structural steels used in the containment system needs to be carefully chosen and protected from the cold cargo temperatures (-160°C) throughout the service life of the vessel. IHI (or Cold-Spotting) is an important procedure, more so on membrane vessels than the Moss-Rosenberg designs. It is a Classification requirement for the granting of a valid Certificate of Fitness for ships carrying liquefied gasses in bulk that routine cold spot inspections are carried out and that the results are recorded in the LNG survey record book. As a Class requirement, all spaces around the cargo tanks must be inspected once in every six months. To meet this requirement, it is good practice to divide your spaces into zones and inspect a number of zones on each occasion, meaning that all spaces will be inspected during the designated period as defined by Class. These spaces include ballast tanks, cofferdams, whaleback spaces, MossRosenberg hold spaces and duct keels. On completion of loading, and to standardise the entries in associated records, conduct your IHI after the same time has elapsed.

$ Condition/wastage of ballast tank sacrificial anodes.

• The extent of corrosion on both inner and outer hulls, particularly under ballast tank suction strums in-way-of striking plates and behind heating coils. @ Position and amount of sediment in the ballast tanks. 9 Any damage or fractures, with particular attention paid to the external portion of the inner-hull and at the associated turn of bilge areas, especially within the midships section of the vessel. :::: Evidence of hydraulic oil or heating coil leakage (steam or glycol). .;- Condition of scupper pipes. # Condition of fittings for any ballast level detecting systems. Ballast line condition, especially at bends and expansion pieces. # Ballast line/valve integrity checked if appropriate by applying a static head pressure. When the inspection is complete, the report is signed by the Master, Chief Engineer and Cargo Engineer. These are the IHI hazards and safety precautions: 9 Risk Assessment completed and tabled with all those involved present, and then posted at the tank entry location. & Full enclosed space entry procedures implemented in accordance with the operator's SMS and permit to work

O/l

n-Service Operations

system. Permit posted at the tank entry location. A worksite Toolbox Talk (TBT) given by the Supervisor to all those involved. ;

> Operation discussed at the previous Daily Work Plan Meeting and'the associated requirements for all departments clearly identified in the Daily Work Plan, which is then posted and circulated accordingly. In accordance with standard enclosed space entry procedures, initiate ventilation in good time and maintain it throughout the inspection. Multi-point/level testing for hydrocarbon gas and the presence of enough O2. The contingency safety trolley is fully equipped and standing-by. Inspection route for each team clearly established. All personnel wearing adequate Personal Protective Equipment (PPE). All equipment must be approved, fitness certificates in date as appropriate and electrical equipment Intrinsically Safe (IS) by design. Personal O2 meters worn by all personnel and their correct operation confirmed before entry.

;;

Each person equipped with a torch or helmet mounted headlamp, plus an emergency chemical light-stick.

# The dangers posed by the presence of N2 pockets highlighted and understood by all concerned. ® All participants invited to express concerns before entry, usually conducted at the TBT In the event of a cold-spot forming, the response is controlled and graded according to severity. The following procedures would be normal: i> Note size, location and minimum surface temperature of the affected steel. # Assess the level of stress and loading in the steel structure affected. For this, Head Office should provide back-up. # Take no further action, but continue to monitor until a temperature approaching the brittle to ductile transition range for the steel is reached. $•• If the situation continues to deteriorate, consider the stresses on the vessel and arrange to flood the space concerned. Even on the older TGZ and GT membrane vessels (now in excess of 28 Years old), it would be a very rare and isolated case.

Personal pocket-sized motion sensors with activated locating beacon worn by each team member. One person in each team equipped with a radio, tested and tuned to the correct channel. Temporary in-tank aerial rigged and tested. Communication with the Bridge confirmed and the reporting system fully established. Standby man present at the main deck level to liaise between the in-tank team and the Bridge.

D? s e n a r y Preparation 5,7.1 Two day' at the Disport This is a check-list of tests to be carried out and items to be'verified: ESD system -Tested and proved fully operational.


Cargo valves - Remote/manual operation tested. - Speed of remote operation checked and confirmed to be within surge avoidance parameters which should be 25-30sec. (Critical with regard to the loading arms and damage-avoidance in the event of an ESD.) Ballast valves - Remote/manual operation tested. - Use line-surge avoidance to check the speed of remote operation. (Important where the ballast lines are made of GRR which is the preferred type of construction on many Moss-Rosenberg vessels.)

Cargo savealls - Where appropriate, fill with fresh water. In the designated areas, place stainless steel buckets filled with fresh water filled and provide and adequate supply of fresh clean rags and PPE. (Used as a temporary way to stem a minor leak). •& Prepare the Oil Pollution Trolley and its associated equipment. Prepare the deck fire-fighting equipment, rig/arm dry powder monitors and open dry-powder house doors. ® Rig the standard international ship/shore hose connection. CCTV - Check and ensure it is operable on all points.

Manifolds - Check for moisture ingress. Manifold bobbins, filters, gaskets and presentation flanges - Check for mechanical damage. Gas Detection - Check systems (normally infra-red and catalytic) and prove operational on all points. Pressure-test the liquid and vapour manifolds. If using N2, the test pressures should be 5Bar at the liquid manifold and 1 Bar at the vapour manifold. Use soapy water to detect for leaks.

ay of Arrival at the Discharge Terminal Ballast - Adjust for appropriate arrival draught and trim. Deck scuppers - Make sure all closed. Mooring winch savealls -Clean and mop out. - Replace drain plugs. Bunker/hydraulic savealls -Clean and mop out. - Replace drain plugs.

3fi

5,7,3 Day prior tc the Disport This is a check-list of tests to be carried out and items to be verified: @ Cargo system - Inspect and ensure the intended offshore manifold is secured and pressurised with N2. ;

Cargo/spray pump motors and associated transmission cables - Check the insulation resistance to earth. Record the results as they may warn of potential electrical failures and a possible break-down of electrical insulation. - Depending on the system, prove the integrity of the electrical safety chain for each pump at the same time As far as is practical, set the cargo lines for the intended discharge procedure. - The Chief Officer and/or the Cargo Engineer are responsible for this. - On the older membrane class, this procedure opens the spray valve on the tank to be used for cooldown, ensures that all other tank spray valves are closed, opens (fully) the loading valve on the same tank, opens the crossover


valve between liquid and spray headers and cracks open all discharge valves slightly to remove any possibility of jamming during cooldown. Moorings and associated equipment. - Prepare/check all. -The high freeboard of an LNG carrier and the fine tolerance regarding the ships position relative to the loading arms often places a high demand on this equipment. Cargo boil-off control systems. - Check and prove all systems operative. - Prove the emergency closing facility for the For'd Vent Riser (activated) from the Bridge. Valve opening is usually achieved by a manual adjustment of the controller's Set Value (SV) from the CCR and emergency closure is observed as the Bridge activates the appropriate control. Manifold overside water sprays. - Test the operation to ensure hull protection against LNG leakage. Fire pump, IMO pump, Emergency fire pump and emergency generator - Ensure/confirm that all are available for the loading procedure. Cargo/Ballast Plan - Chief Officer completes the approved plan and obtains the authorising signatures as required. Pilot hoists and/or accommodation ladders - Test as appropriate. Chief Officer to prepare arrival/sailing stability conditions. Chief Officer to prepare arrival/departure paperwork. In accordance with the Company's SMS, the Master chairs the pre-port meeting that provides the interface between the various departments of the SMT At this meeting, terminal requirements, safety, ISPS security/compliance, known system/vessel defects, shore-leave control, intended

reliefs, storing/bunkering requirements and the arrival/departure programme are discussed. Note On the newer Moss-Rosenberg and GTT Membrane vessels, the cargo lines can be cooled before arrival at the disport terminal. This can be done the day before or on the same day, depending on the Expected Time of Arrival (ETA). A similar procedure will usually apply at the load port. Cargo lines and cargo plant are cooled to the lowest possible temperature before arrival at the terminal so that cargo operations can begin as soon as the vessel is moored and all procedures have been completed. As the cargo lines are being cooled, use the actual cooldown liquid to pressure-test the system, that is, as distinct from the N2 pressure test procedure mentioned above. To do this, wait until the manifold spoolpieces, reducers and discharge filters have been fitted - and then increase the system pressure to approximately 5Bar. (This test is not a requirement but it ensures that you have a tight system.) To run a line cooldown procedure, use one spray pump (from No.3 Tank, for example) to pump LNG through the spray header to the liquid manifold pipe work. Vapour displaced from the crossover pipe work passes through the liquid header, the spray bypass, the return valves of No.1, 2 & 4 cargo tanks and then back to No.3 tank, through the filling line. Vapour generated by this process is burnt in the boilers, using the LD compressor and gas heater system. If there is more vapour than is required by the propulsion plant at this stage in the voyage, the excess steam dump is used. When line cooldown is complete, stop the spray pump. If the time between line cooldown completion and berthing is


extensive, restart the spray pump as required. Return of cooldown liquid to the bottom of the cooldown source tank through the loading line (in this case No.3), can cause a local temperature increase at the appropriate tank bottom sensor. Allow sufficient time for this to stabilise before the post discharge gauging.

6 Discharge Operation

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Most discharge terminals require the manifold valves to be fully shut and put into manual operating mode before the discharge arms are connected or disconnected. Before the operation begins, this condition should be checked and confirmed by a Responsible Officer (usually the Cargo Engineer) to the Terminal Representative on duty at the manifold.

The main engine turning gear is engaged and the master steam stop valve closed prior to the gangway being brought onboard and the loading arm connections made. Most terminals use between two and four discharge arms and one vapour return arm. At most LNG terminals, you are not permitted to immobilise the main engine for maintenance or any other purpose and the main engine must remain in a state of readiness throughout the process. Connect the Communication I BSD cable. This is usually a male/female plug, (fibre-optic or electrical) and provides the required channels for ship/shore communication and ESD interconnection. Connect the ESD umbilical cable, usually a pneumatic connection Connect the shore ground cable at the approved location on the vessel.

To replace the LNG being discharged from the vessel's cargo tanks, return vapour is normally supplied from the terminal or, if the terminal supply is unavailable, by LNG vapourisation on board. Both methods are supplemented by normal boil-off from the liquid in the cargo tanks. These procedures are as detailed as it is possible to make a non-specific class description. Common underlying principles should be evident, especially when compared to the previously-described loading procedures.

When the discharge arms have been connected, they are purged and pressurised with N2 supplied by the terminal. A pressure of 200kPa may be raised in each arm. The Responsible Ship's Officer checks all associated joints for leakage. Use soapy water to check for leaks. As appropriate, use NEW gaskets on all connections.

As with the previously described Loading Procedures, record all key events in the Deck Operations Log (DOL)

QP.

Once the discharge arms have been proved leak-free, they are depressurised in a controlled manner. While depressurising,


use the portable O2 meter to check the gas. Below 1% is the usual requirement. Dryness of the gas is important and can be part of the standard checks at some terminals. The usual requirement is lower than -50°C. Most LNG terminals have a representative in attendance at the manifold while the discharge arms are being connected. At the same time a Responsible Ship's Officer will witness the connection process and the leakage tests, O^moisture checks and (if required), report the progress of any storing operations to be completed before cargo operations can commence. Back in the CCR or Ship's Office, a predischarge meeting is held to confirm discharge procedures and to exchange relevant information with respect to the vessel's time alongside. At this time, the ship/shore safety check-list and ISPS security check-list are completed. With the vessel upright and even keel, it is usual for the Chief Officer and Cargo Surveyor (or appointed person) to run the official Initial Cargo Custody Transfer data. All parties should confirm their agreement with the calculated quantities. At this point in the procedure, many terminals require EIR fuel gas burning to be terminated and secured. At such terminals, EIR fuel gas burning may not restart until discharge and final gauging have been completed. In the CCR, the ship/shore communication system will be powered up. Test the HotLine and Plant phone and prove that it works. When it has been confirmed that no oil leakage from any of the deck hydraulics has taken place, no matter how slight, bring the manifold spraywater curtain into service. When the vessel and terminal are ready to carry out ESD tests, confirm

whether the ship or the terminal will activate the shut-down as they run through. The OIC normally attends at the manifold to assist and report on the progress of the tests. With permission from the terminal, open the manifold/ESD valves, which are now in the Automatic mode. When the valves are confirmed fully open, reset the ship's ESDS and make sure that the appropriate indicators all show Healthy Once the terminal has completed a similar reset, switch all the ship's associated override facilities to Override Off. Conduct the "Hot ESD" test as agreed with the terminal representative. At this time, it is quite normal for the terminal to ask for the closure of the Manifold/ESD valve to be timed. Now the ship can request permission from the terminal to open the return gas from the shore vapour valve. Set associated lines and valves accordingly. On the older class of vessels, the liquid manifold valves remain closed until the line/discharge arm cooldown procedures are complete and the terminal gives permission. The ESDS, however, is reset and seen to be healthy both terminal and shipside by utilising the appropriate "Manifold Valve not fully open" over-ride facility. On the newer Moss-Rosenberg and GTT vessels, manifold/ESD valves are opened fully at this stage. Enable and ensure all cargo tank level alarms are. operative (Low Level @ 0.5Mtr, Hi Level @ 95% capacity, Tank Fill Level @ 98.5% capacity and HiHi Level @ 99% capacity).


On passage, it is normal practice to use the dedicated blocking circuits to inhibit certain level alarm systems. The vessel may now begin the discharge procedures, starting with the cooldown of the discharge arms.

are in close radio communication if one is absent

6.3.1 Olc Main Carg Make sure that vapour return from the shore is available.

6.3 Discharge Procedure When the vessel is informed by the shore representative that the terminal is ready for cooldown, depending on the age of the vessel, one of these two procedures will applyThis assumes that both Chief Officer and Cargo Engineer are present in the CCR or,

Fully open the loading and spray valves on the nominated cooldown tank (normally No.3 or 4 on a 5-tank vessel). With the terminal's permission, open the liquid manifold bypass valves by the approved amount (normally 1 A-1 turn).

Diagram 2.4 Custody Transfer System

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Inform the EIR that you are about to start the first cargo pump. Make sure that a Responsible Officer (usually the OOW) is standing-by at the tank and in close radio communication with the CCR. With the discharge valve fully closed, start the nominated main cargo pump. When the ampere loading and discharge pressure stabilises, crack the discharge valve (typically 1 turn) to elevate the amps. Remember that pump protection usually includes a time-delayed trip facility on Low Amps and Low Discharge Pressure. When the discharge valve body is completely frosted, gradually increase opening until the ampere loading is steady at the approved first level (this is nominally 40Amps below normal full load). Close the loading valve until the liquid header pressure is between 1.8 to 2.0BarG. As cooldown progresses, a further adjustment may be necessary to maintain this pressure. Use the midships crossover temperatures and the frosting along the liquid manifold bypass lines and chiksan arms to monitor the progress of cooldown. Regulate the manifold bypass valves to maintain an even rate of cooldown on all discharge arms. When the terminal informs the ship that cooldown is complete, fully open the tank loading valve and allow the liquid header pressure to fall below LObarG. If necessary, throttle in the discharge valve on the pump being used for cooldown to make sure that the chiksan arms are not subjected to

hydraulic shock as the liquid manifold valves are opened. In some discharge ports, a manifold/ESD valve Cold Function test is carried out on completion of cooldown to verify the correct operation of the valves when they are in the cold state. With the terminal's permission, open the liquid manifold valves. When manifold valves are confirmed fully open, switch the ESDS 'Manifold Valve Not Fully Open' override facility to the Off position. This means that the ESDS is now fully enabled. Close the liquid manifold bypass valves. Close the loading valve on the cooldown tank. Adjust the pump ampere value back to the first level by adjustment of the discharge valve opening. Close the cross-over between liquid and spray headers. After informing the EIR, start the remaining pumps with the approved time interval between each start and as agreed at the pre-discharge meeting. This is normally ten minutes for the second pump and five minutes thereafter. Each pump is started in the approved manner. Remain aware of the mechanical damage that flow disruption or repeated starts can cause. When all the pumps are running, inform the EIR and adjust the discharge valves to give either the maximum flow rate permitted by the terminal, the maximum allowable continuous running amps for the pumps or a stipulation regarding EIR


generating capacity, whichever is the prevailing parameter. This procedure is called Ramp-Up. At all times during Ramp-Up, patrolling watchkeepers will keep a watch on deck lines, fittings and the manifold (including the off-shore manifold). They will remaining in close radio contact with the CCR throughout the process. When the pressure in the tanks has fallen to approximately 65mbarG, ask the terminal to start the Return Gas Blower (RGB). If an RGB is not available at some terminals, then return gas may be supplied by FreeFlow only. If tank pressure continues to fall, you must either reduce the discharge rate or use the ship's dedicated vapouriser to generate vapour internally. Depending on the design, do not allow tank pressures fall below a minimum level, which must be clearly established in the operating procedures (on the older membrane class, normally 40mbar). Ballast operations are carried out concurrently with cargo discharge and in accordance with the Chief Officer's Cargo/Ballast Plan. During discharge, hourly rates and individual tank finishing times are calculated and relayed ashore as required by the terminal. Cargo valves are adjusted to maintain the maximum permitted rate and for all tanks to finish closely together. Under normal circumstances, a certain amount of cargo (heel) is retained onboard in each cargo tank after discharge. This figure is calculated according to an agreed formula that considers the Charterer's requirements, the length of the ballast passage and fuel gas requirements.

Reference to previous records will give you the optimum distribution of this quantity between the tanks because, in practice, the cargo heel does not vapourise equally from each tank.

6.3.2

Me IT

To cool the manifold and discharge arms, use a liquid supply from the cooldown source tank (typically No.3 on a 4 tank vessel) with the stripping/spray pump discharging into the spray header. The spray header is cross-connected to the liquid manifold through cooldown bypass valves which go round the manuallyoperated liquid manifold discharge block valves. For this procedure, open the liquid discharge manifold/ESD valves. Line-up the spray manifold for return to the source tank through the spray return line. Cool the spray header first. Once you have permission from the terminal, start the spray pump with the discharge valve approximately 10% open. Once the spray header has cooled, obtain permission from the terminal to open the spray pump discharge valve a little more, to ease the spray return into the source tank. Liquid is then forced into the discharge manifold and the discharge arms at a controlled rate. Once the manifold and discharge arms have been cooled (-130°C) and the terminal has given their permission, stop the spray pump and close the manifold cooldown bypass valves.


Diagram 2.5 Stripping/Spray Pumps

The spray line is drained back to the source tank through the tank's spray master and spray nozzle valves, relieving the expanding residual liquid in the spray header.

When the cooidown procedure is complete, some designs of vessel use the spray pump to prime the discharge columns on the first start main cargo pump(s). This avoids a pressure surge in the lines. To


.— Diagram 2.6 Stripping/Spray Pumps Start Control

accommodate this, the columns can be vented through appropriate vent valves. The vessel is now ready to start discharge and the principles involved are similar to the procedure already described.

Cargo Pump Operation Operator vigilance and the correct operation of the cargo pumps is of prime importance. Good practice recommends that you maintain an hourly log of associated critical parameters, for example: Ampere Loading / Discharge Pressure / Flow Rate. Investigate abnormal events immediately.

44


Diagram 2.7 Cargo Pump Start Control Diagram

For operator guidance, there should be a clear CCR response procedure, just in case a cargo pump parameter(s) becomes unstable and cannot be steadied out by normal means. If there are fluctuations on the motor ammeter or pump discharge pressure gauge, reduce the flow rate until the readings stabilise down to the NonPumpable Suction Height (NPSH). On a new 135,000 M3 Moss-Rosenberg vessel, when

the flow rate is throttled down to approximately 230 M3/Hr, the required NPSH will be about 10cm. This represents the minimum level attained by pumping. Do not restart a cargo pump with a low suction head of Jiquid remaining in the tank. As a general rule, when the liquid reaches 1 mtr or less, try not to stop the pump until the cargo has been fully discharged. If the shore facility is unable to accept the cargo for intermittent periods, keep the pump


going by re-circulation back to the tanks until discharge can be resumed and completed.

6.5 Completion of Discharge Procedure Most terminals require one hour's notice before the first pump.can be stopped. One hour before, inform the EIR that your cargo discharge is about to finish and that the first cargo pump will stop. Normally, the vessel should be upright and on an even keel for the completion of the cargo. The OOW should standby each tank, as the discharge valves are closed, to monitor the operation. The OOW will confirm this to the CCR. As the suction head reduces on the final pump in each tank, adjust the discharge valve to maintain back pressure and ampere loading above the associated tripping levels. Before stopping the pump, throttle-in the main cargo pump discharge valve to 40%. If two main cargo pumps are in use in a tank, when the level reaches an approved predetermined level (typically 0.8Mtr), throttle-in the discharge valve on one pump to 40%, then stop that pump. This action reduces turbulence around the pump suctions as the suction head is being reduced. As the predetermined heel quantity is approached, stop the main cargo pumps in each tank and close the discharge valve. This makes sure that the CCR indicators display healthy operation and full closure.

4fi

When all of the pumps have been stopped and the discharge has been confirmed as complete, the terminal will request closure of the liquid manifold/ESD valves. Before controlled closure of the manifold/ESD valves can be accomplished, the ESDS 'Manifold Valve Not Fully Open' override function must be switched to override. This avoids initiating an ESD trip. When manifold/ESD valves are closed, most terminals request the mode of operation to be switched to manual for eventual disconnection of the discharge arms.

6,6 Post Ofeeh&rge Procedure On completion of cargo discharge, and after all cargo pumps have been stopped, open the discharge valve on the tank nominated to receive line drainings. The terminal will then request the officer at the manifold (usually the Cargo Engineer) to open the manifold/ESD bypass valves to drain the discharge arms. On completion of draining, and as requested by the terminal, the bypass valves are closed and the liquid discharge arms are pressurised for purging with N2 supplied from the terminal. In a normal operation, the N2 purge pressure is steadily increased to 2.5barG. Then the supply is stopped and the manifold/ESD bypass valve re-opened to depressurise the discharge arm. This procedure may be repeated as required to purge each liquid arm in turn, i.e. one at a time On the final purge depressurisation, and with a pressure of 1barG still remaining in the chiksan arm, the purge gas is sampled for hydrocarbon content. For most terminals, purging is complete when this is below 1% by volume.


After complete depressurisation, disconnect the appropriate discharge arm from the ship's manifold. Normally, the vapour manifold remains open until just before final gauging. Then with permission from the terminal, close the valve. It is good practice to change the operating mode to manual and make sure that there has been a full closure. Purge the vapour arm (as the discharge arms were purged) and disconnect. Once all discharge arms have been disconnected, stop and secure the manifold overside sprays. On the receiving tank for line drains, leave the loading valve partially open so that the final liquid residues can all drain back. If discharge strainers have been used, remove them for inspection. Check for debris. The Cargo Engineer should now prepare to reinstate the LD gas compressor and the supply of fuel gas to the EIR. With the vessel upright and on an even keel, perform the final gauging with all interested parties present, who must all agree with the quantities shown on the final OCT document. Reinstate the fuel gas supply to the EIR. Once the loading and vapour arms have been secured and re-stowed ashore, start securing/blanking the manifold connections for sea conditions. With the permission of the terminal, disconnect the fibre-optic/electrical communication/ESD link and re-stow it ashore.

When all shore personnel have cleared the vessel, remove the gangway. The Master/Chief Officer must be sure that all paperwork has been completed and issued as appropriate. The vessel is then given a slight list towards the berth so that she assumes, as near as possible, an upright position after letting-go. Other vessel departure procedures will have been running concurrently. A stowaway search should be conducted in compliance with ISPS requirements. Once clear of the jetty, adjust ballast to bring the vessel upright and trimmed appropriately for the sea passage. This completes the discharge procedure.

7 Ballast Passage In Service 7.1 Overview There are certain fundamental objectives that the SMT must consider on every ballast voyage. Before itemising these objectives, there is a basic underlying principle to be made clear. Most experienced operators believe that when a gas carrier is'in a fully laden condition, you should avoid serious maintenance (especially of an invasive nature) that could cause a serious disruption. For cargo/deck or related work, the status of the hazard or risk will only increase when a vessel is fully laden.


Commitment to the customer/receiver means that you should avoid non-essential work in the E/R that has the potential to cause a delay to the schedule.

7.2.

BsMm

Vc-V':;Vz:

Loading Preparations 7.2.1 Two Days bef< at the Load Port

Although following this guideline may put ballast passage under pressure, from a safety and commercial point of view, this is a better time to manage such an issue.

This checklist details the tests to perform and the items to verify:

On the ballast passage, these are the main objectives for SMT consideration:

® ESD system: -Tested and proved fully operational.

fr Cold maintenance of the cargo tanks, resulting in the best possible Cold State on arrival at the load port.

• Cargo valves: -Test remote/manual operation. - Check and confirm speed of remote operation to be within surge avoidance parameters, as recommended 25-30sec. (In the event of an ESD, this is important with regard to the loading arms and damage avoidance.)

Maximisation of the use of the LNG boil-off from the retained heel as a supplementary fuel source will help to conserve bunkers. Maintain cargo tank pressures within maker's recommendations - that is, cargo tank pressure on a membrane vessel > 70mbarG or 1083mbarA at all times. This avoids exposing the membrane to unacceptable levels of fatigue stress. @ Completion of any potentially disruptive unscheduled maintenance. Completion of any potentially disruptive planned maintenance (PM). With operator approval, completion of any cosmetic maintenance that is in or adjacent to a gas-dangerous zone. Completion of E/R maintenance on the vessel's power plant that has the potential for blackout, loss-of-way through the water or a delay to the schedule. f • Completion of any alarm or trip tests that could disrupt the schedule.

Ballast valves: -Test remote/manual operation. - Use line surge avoidance to check the speed of remote operation. 1 Manifolds: - Check for moisture ingress. 1' Manifold bobbins, filters, gaskets and presentation flanges: - Check for mechanical damage. s Gas Detection: - Check and prove system operational on all points.

722 Da Load Port This check-list details the tests to perform and the items to verify: # Cargo system: - Inspect the intended offshore manifold. Ensure it is secure and pressurised with N2. # Set Cargo Lines for the intended load procedure. This is the responsibility of

48


the Chief Officer and/or the Cargo Engineer.

programme are all potential issues for the agenda for this meeting.

Moorings - Prepare and check all associated equipment. Note: that the high freeboard of an LNG carrier, and the fine tolerance regarding the ship's position relative to the loading arms, places a high demand.on this equipment.

7.2.3 Day of Arrival at the Load Port

# Cargo boil-off control systems. - Check and prove all these systems operative. These include the HD compressors and associated security/safety circuits.

e Ballast: - Adjust for appropriate arrival draught and trim. <§> Deck scuppers. - Make sure all deck scuppers are closed. ® Mooring winch savealls: -Clean and mop out. - Replace drain plugs.

Manifold overside water sprays. - Test the operation to ensure hull protection against LNG leakage

# Bunker/hydraulic savealls: -Clean and mop out - Replace drain plugs

Fire pump, IMO pump, Emergency fire pump and emergency generator. - Ensure/confirm that all are available for the loading procedure

# Cargo savealls: -Where appropriate, fill with fresh water.

Cargo/Ballast Plan - Chief Officer completes the approved plan and obtains the required authorisation signatures. Pilot hoists and/or accommodation ladders - Test as appropriate. Chief Officer to prepare arrival/sailing stability conditions. Chief Officer to prepare arrival/ departure paperwork. In accordance with the Company's SMS, the Master will chair a pre-port meeting. This is an important interface between the various departments within the SMT Terminal requirements, safety, ISPS security/ compliance, known system/vessel defects, shore-leave control, intended reliefs, storing/ bunkering requirements, arrival/departure

<§ Designated areas: - Place the fresh water filled stainless steel buckets with enough clean rags and PPE. (These are for temporary stemming of minor leakages.) ® Oil Pollution Trolley: - Make ready the trolley and its associated equipment. ? Prepare the deck fire fighting equipment: - Rig/arm dry powder monitors and open dry powder house doors. # Rig the standard international ship/shore hose connection. c CCTV - Check and ensure it is operable on all points. ft Elevate interbarrier/insulation space pressures (design dependent). - Recommended elevation = 2mb each to provide a buffer against the


inevitable collapse in the pressure that always occurs in the initial stages of the loading procedure.

7.3 Cold State of the Vessel on Arrival at the Load Port On the ballast voyage, you must maintain the cold state of the vessel, depending on the Charterers requirements. Maintaining the cold state allows the vessel to start loading at the terminal on arrival and so reduce port time and turn-round to a minimum. It will also determine the load procedure required at the terminal. Remember: if the ballast voyage is too long, the lighter fractions of the liquid can evaporate. Eventually, the CH4 content will be minimal, with only the heavy fractions (for example, propane, butane, ethane) remaining. Consequently, this liquid mass will have a relatively high saturation temperature and density, which may prevent pumping because of the high ampere loads induced in the pump motor. As a spray medium, it also becomes ineffective, carrying a risk of droplet entrainment into the vessel's vapour system. On extended ballast voyages, Heel Ageing is a phenomenon for the operator to consider. These are the four main methods of maintaining the vessel in a cold state during the ballast voyage: Heel out at the end of discharge and subsequently cooldown the tanks with LNG, supplied by the terminal before loading.

50

; Cooldown the tanks just before arrival at the loading terminal. At the end of the previous cargo discharge, the LNG heel is retained in one of the tanks. As well as the quantity to be sprayed, calculate the amount of LNG heel to be retained by assuming a boil-off equal to 45% of the boil-off under laden conditions. To keep the cargo tanks cold during a ballast voyage, periodically spray the LNG so that the average temperature inside the tanks does not exceed the designed maximum. This would be the Moss-Rosenberg equatorial temp of -113DegC or -130DegC for GTT vessels. As above, the LNG heel is consolidated in one of the tanks. # For short voyages, for example, 6-8 days, each tank retains enough LNG heel from the discharge for the tank bottom to remain covered until it reaches the load port. This removes the need for spraying during the ballast voyage.

For reasons of safety, spraying is normally carried out during the hours of daylight. In many cases, control of the operation is automatic and DCSprogrammable. On Moss-Rosenberg vessels, all equatorial temperatures can be maintained within 2DegC. Other operational considerations include gas burning requirements and the elevation of cargo tank pressures.


Pre-Refit Operations

Section


8 Cargo Tank Warm-up Formal written confirmation by management sets out the requirements and a timetable of operations. This is normally done before refit. During the last loaded voyage before refit, conduct a full inner-hull inspection of all ballast tanks, cofferdams, void spaces and duct keels and send the appropriate report to management. Usually, this is a Class requirement that confirms the absence or presence of cold spots. A Class surveyor may attend the last discharge before refit to inspect selected ballast tanks and cofferdam spaces. The ship will carry out a maximum discharge. Tank levels should be reduced to the point where the main cargo pumps trip on low current. Using the stripping/spray pumps, remove the last of the cargo until they also trip on low current (procedure referred to as Tank Stripping I Line Drainage). Note: the quantity remaining onboard must still be gaugable for commercial reasons regarding measurement of cargo ou-turn. The ship will then proceed to sea and begin the warm-up, inerting and aerating procedures before its arrival at the refit yard. Underlying Principle Vapourisation of residual cargo and warmup of the primary containment are not the only objectives. By creating a 'Heat Sink' effect within the tanks, you can extract as much of the cold from the insulation as well. Then when the procedure is terminated, less cold will ingress back into the primary containment from the insulation, and

keeping the temperature of the cargo vapour elevated. At the next stage, when IG is introduced into the tank bottoms, the temperature differential between the inflowing IG and the cargo vapour within the tank will be as low as possible. This will produce a maximum density differential between the two mediums. From this, the drive moves towards optimum efficiency for the process of inerting by displacement. An effective warm-up will produce a successful inerting process.

8.1 Tank Stripping / Line Drainage With permission from the terminal, the vessel is trimmed by the stern (normally 2.5Mtrs) and upright to maximise discharge at the discharge port. At most terminals, the liquid remaining in the tanks when the stripping process is complete must still be gaugable by the level-measuring system. This is a Customer I CCT requirement. For an up-todate trans-sonic level measuring system, the minimum gaugable level is approximately 50mm. One main cargo pump in each tank is used until it trips on Low Ampere Loading or Low Discharge Pressure. Previous statements regarding damage to cargo pumps caused by running for extended periods with a partially disrupted flow still apply. It a condition that should be avoided whenever possible. Start the spray pumps soon enough to avoid any possible starting problems that could be caused by very low tank levels. As with the main cargo pump, when the stripping/spray pump loses suction, stop it and change to the next tank.


When all pumpable cargo has been stripped ashore, drain the system back to the nominated drain tank. All remaining LNG in the downward leg of the discharge arms and manifold connections is usually drained back to the tanks through the spray Ifhe. This is helped by pressure from the terminal supplied purge N2.

8,2 V^J arm-up Operation Tank warm-up is part of the gas freeing operations performed before dry-docking or when preparing tanks for inspection purposes. A successful warm-up operation is essential preparation for the inerting process. As well as warming up the membrane, efficient warm-up of the insulation is also essential. By continuing the warm-up process as long as possible, even beyond the target figures stipulated, cold ingress from the insulation on completion of the warm-up will be minimised (heat sink effect). This ensures that the temperature of the cargo vapour remains elevated and so has as low a density as possible. It also ensures maximum difference against the density of the incoming IG, which in turn enhances the results of inerting by displacement. Tanks are warmed-up by re-circulating heated LNG vapour. The vapour is recirculated by the two HD compressors and heated by the cargo heaters to a preset value. (1st stage = 0°C, 2nd stage = 75°C). On the older class of vessel, single stage heating may be the only option, and on these vessels, the final discharge temperature from the compressors is restricted to 50°C. This avoids the risk of damage to gas compressor seals or the deformation of primary containment/ insulation.

54

In a first step, hot vapour is introduced through the filling lines to the bottom of the tanks. This causes any liquid remaining in the tanks to evaporate. In a second step, when the temperatures have a tendency to stabilise, hot vapour is introduced through the vapour piping at the top of the tanks (new GTT membrane vessels only). Excess vapour generated during the warmup operation is vented to atmosphere when at sea or, if in port, returned to shore. The following points apply to the standard situation of venting to the atmosphere at sea: On new GTT membrane vessels, the warmup operation continues until the temperature at the coldest point of the secondary barrier of each tank reaches +5°C. On new MossRosenberg vessels, warm-up is considered complete when the temperature of tank equator is above -20°C (tank wall temperature approximately +5°C). On the older membrane vessels (GT and TGZ), this parameter is determined by achieving +15°C at the tank bottom sensor. The period of time the warm-up operation requires depends on the amount of liquid, the composition of liquid remaining and the initial temperature of the tanks and insulation spaces. In normal operation, the warm-up will take about 48 hours for membrane vessels and 60 hours for MossRosenberg spheres. Initially, the tank temperatures will rise slowly as evaporation of the LNG proceeds. This process is accompanied by high vapour generation and venting. Expect an approximate venting rate of 8,000 m3/h at 60°C. On completion of evaporation, tank temperatures will rise rapidly and the venting rate will fall to between 1,000 and 2,000 m3/h at steadily increasing temperatures. Temperatures within the tank and insulation are displayed in the OCR on the appropriate system (DCS or similar).


. Diagram 3.1. Gas Heater

Rolling and pitching of the vessel will assist evaporation. On membrane vessels, temperature sensors at the aft end of the tank give a good indication of the progress of warm-up. A slight listing of the vessel will help to correct uneven warm-up in any one tank. Continue gas burning for as long as possible. Normally, that means until all the liquid has evaporated, venting has ceased and tank pressures have started to fall, i.e. before the process goes into the closedcycle. Keep the Interbarrier/lnsulation space pressures under close observation as the spaces warm-up and the N2 atmospheres

expand. This guarantees that the automatic relief systems can handle the increased demand.

In cases where you are not required to warm-up all the tanks, follow the same procedure as for all tanks, even if you are preparing for a single tank entry. In such a situation, the tank(s)'to be inspected must be completely separated from the other tanks and associated cargo systems.


With the vessel underway and venting, make sure the prevailing wind direction at all times allows the vapour to disperse away from the accommodation block and machinery intakes. The highest level of the No Smoking Policy should be clearly posted and in force during the operation.

Before you admit the cold CH4 vapour, preheat the vapour heater(s) thoroughly with steam. This prevents the formation of ice which could damage the equipment. When returning heated vapour to the cargo tanks, do not exceed the maximum stipulated heater outer temperature for the system design. This is to avoid possible damage to the cargo piping insulation, cargo valve seals, gas compressor seals, primary membranes, membrane load bearing insulation or sphere equatorial structure.

9 Inerting of Cargo Tanks After the tanks have been warmed up, and before the final Aerating stage can begin, the LNG vapour is displaced with inert gas. Inert gas from the inert gas plant passes through the LNG loading lines to enter at the bottom of the tanks. Displaced gas from the cargo tanks is vented from the top of the tank through the vapour header to vent-mast No.1 (most forward riser), or to shore if the vessel is in port.

56

These points apply to the standard situation of venting to atmosphere while the vessel is at sea: Inerting is necessary to prevent the risk of having an air/LNG vapour mixture in the flammable range. The operation continues until the hydrocarbon content is reduced to less than 1.5%Vol. A typical operation will take about twenty hours to complete. In addition to the cargo tanks, all cargo pipe work, associated fittings, impulse lines, compressor house machinery and fuel gas supply lines must also be freed of gas. Fittings and lines are usually filled with inert gas or N2 while the plant supplying gas to free the tanks. It is a multi-stage process that requires continuous planning and monitoring.

8,2 Porting CUU:W:M^}. Make sure that all warm-up parameters have been completed (as described in the previous section). Prepare to use the inert gas plant in the inert gas mode. Set the lines as defined in the procedure from the 'Cargo Operations Manual'. Before connecting inert gas generator to the cargo system, start and run it until the oxygen content and dew point have reached the recommended levels (02 <2% and dew point <-45°C). Monitor tank pressures and adjust the opening of the fill valves to maintain a uniform pressure in all the tanks.


On membrane vessels, make sure that tank pressures are always higher than insulation space pressures by at least 10 mbar, and that the tank pressures do not exceed 180 mbar above atmospheric pressure. During gas freeing, keep the tank pressure low to maximise the piston effect. At about one-hourly intervals, take samples of the discharge from the vapour dome at the top of each tank and test for hydrocarbon content. On one of the tanks being inerted, test the purge valve on one of the filling lines to verify that the oxygen content of the inert gas remains below 2%. All unused sections of pipelines, blind ends, machines, equipment and instrumentation lines must be purged for five minutes. When the hydrocarbon content sampled from a tank outlet falls below 1.5%, isolate and shut in the tank. On completion of tank and pipeline inerting, stop the inert gas supply and shut down the inert gas plant. Reset the system for the aerating process.

With the vessel underway and venting, make sure the prevailing wind direction at all times allows the vapour to disperse away from the accommodation block and machinery intakes. The highest level of the No Smoking Policy should be clearly posted and in force during the operation. Inert gas and pure N2 can kill you. In order to avoid asphyxiation from 02 depletion, take great care to guarantee the safety of all personnel involved with any operation that uses IG. Make sure there has been a Risk Assessment and all personnel involved in the inerting procedure are familiar with the associated hazards.

Before the inert gas line is connected to the cargo system, blow it through with air. Use the inert gas blower to avoid any debris entering the cargo system.

10 Aeration 10., 1 Aerfdii:;g Overvêvy Before anyone can enter the cargo tanks, replace the inert gas from the previous procedure with dry breathable air. Include the pipelines (liquid and vapour), cargo machinery, associated heat exchangers and fittings in this process. With the Inert Gas and Dry-Air Plant in DryAir production mode, purge the cargo tanks with dry air until you reach a reading of 2 1 % oxygen by volume.


Note: the difference in density between IG and dry air is only slight, even when optimum temperatures are achieved. On the older class of membrane vessels (GT and TGZ), the line configuration for Aerating is the same as for the Inerting operation, i.e. the cool dry air enters the tank bottoms through the loading lines and the displaced IG is expelled through the vapour header system. On the newer class of vessels (both GTT and Moss-Rosenberg), this trend has been reversed, and dry air is usually introduced at the top of the tanks through the vapour header system. Most Cargo Operation Manuals on these vessels include a procedure for both methods.

10.2 Aerating Operation The Inert Gas and Dry-Air Plant produces dry air with a dew point of -45°C. The dry-air enters the cargo tanks through the vapour header to the individual vapour domes. The inert gas/dry-air mixture is exhausted from the bottom of the tanks to atmosphere at No. 1 vent mast through the tank filling pipes, the liquid header, and appropriate system spool piece. During aerating, keep the pressure in the tanks relatively low to maximise the piston effect. The operation is complete when all the tanks have a 20-21% oxygen value, a CH4 content of less than 0.2% by volume (or a locally-required equivalent) and a dew point less than -40°C.

Before anyone can enter a tank, test for traces of noxious gases (C0 2 less than 0.5% by volume, and CO less than 50ppm) which may have been constituents of the inert gas. In addition, take appropriate precautions as given in /SG07T*and other relevant publications. Adjust the pressure in the tanks to approximately 120 mbar. When aeration is performed at sea (as a continuation of pre-refit procedures) it will take approximately twenty hours on a 135,000m3 vessel.

•uKM2£iîiy y 111*9 N2 gas in confined spaces is hazardous to personnel and all possible precautions are required to avoid a concentration. Before entering such an area, test for sufficient oxygen > 20% and for traces of noxious gases: C02 < 0.5% and CO < 50 ppm.

Cryogenic cargo tanks are not opened when this stage is completed. Strict procedural guidelines are subject to rigorous and specific safe-working procedures. During pre-refit preparations, you can only implement cargo tank entry procedures when the shore chemist has tested and passed all associated spaces. On membrane vessels, differential pressure across the barriers, both primary and secondary, must be maintained within acceptable limits.

International Safety Guide for Oil Tankers and Terminals, Fifth Edition - published by Witherbys Publishing.

5fi

Glossary and Index

Glossary

Glossary CCR: Cargo Control Room, focal point for the control of cargo operations. LNG: Liquefied Natural Gas. . SMS: Safety Management System, rules and regulations in a manual format, (electronic or hard-copy) as prescribed and kept updated by the operators, for the safe manning and operation of their vessels. IGG Plant: Inert Gas Generating Plant. Inerting: Exchanging a gaseous atmosphere in an enclosed space or tank, using Nitrogen or Inert Gas (IG) from an IGG, either by forced displacement or mixing. Inerted: The condition that exists in a space on completion of an atmosphere exchange, using Nitrogen or Inert Gas (IG) from an IGG. The atmosphere is then unable to support life or sustain a fire/explosion. mbarA (Absolute): Pressure expressed in terms of mbar as measured from absolute zero pressure. mbarG (Gauge): Pressure expressed in terms of mbar as measured above atmospheric pressure, i.e. as displayed on a standard pressure gauge. mbar. A unit of pressure equal to onethousandth of a bar. A bar is a unit of pressure equal to 105 Newton/m3, or approximately equal to one atmosphere (1,000mbar±). Relief: In the context of this publication, a term used to describe a device or system for controlled pressure release from an enclosed space or vessel.

Makeup: In the context of this publication, a term used to describe a device or system for supplementing or maintaining the pressure in a vessel or enclosed space. For'd: An abbreviation for Forward, as in the leading section of a ship or structure, sometimes expressed as Fwd. ESD: Emergency Shut Down.(system) IG: Inert Gas, as produced by an IGG. HD: High Demand, as used to describe the HD Compressors, i.e. compressors designed with the capability of a large volumetric throughput. LD: Low Demand, as used to describe the LD Compressors, i.e. compressors designed for a lower volumetric throughput, but usually capable of higher discharge pressures. GTT: Gas Transport / Technigaz, modern day builders of membrane type gas carriers. EIR: Engine Room. DCS: Distributed Control System. A computer based alarm, monitoring, data acquisition and control system. SMT: Ship's Management Team. A committee usually chaired by the ship's Master, comprising of department heads and associated supervisors. PPE: Personal Protective Equipment. QRC: Quick Release Coupling, as used to connect the loading arms to the vessel's manifold, affording an instantaneous dry break. ACC: Automatic Combustion Control system, sometimes referred to as Automatic Boiler Control.


N2 Gen: An abbreviation used for the Nitrogen Generating Plant and not to be confused with the Inert Gas Generating Plant. CCT: Cargo Custody Transfer system, is an approved/certificated mea'ns by which the quantity of cargo onboard -can be determined at any point in time, primarily to determine the gross heating value delivered to the customer. System components comprise of, cargo tank level gauging system (Radar or Trans-sonics), trim/list indicator, vapour pressure measurement system, cargo temperature sensors and an Input/Output box for the communication interfaces. The system is sometimes referred to as the CTS (Cargo Transfer System). Hot ESD: A term used for an ESD test, which is carried out when the associated equipment is still at ambient temperature, i.e. not at -160°C(±) cargo temperature. SG: Specific Gravity (density). EP Policy: Environmental Protection Policy. IHI: Inner Hull Inspection. Disport: Discharge Port. GRP: Glass Reinforced Plastic. IMO Pump: A term used to identify a dedicated high pressure fire pump, the primary purpose of which is to supply the various spray-water fire curtains round the vessel, i.e. tank domes, accommodation, compressor house. RGB: Return Gas Blower. Usually part of the shore installation used for returning pressurised gas to the vessel, to replace the liquid volume discharged. GT: Gas Transport, builders of membrane type gas carriers.

62

TGZ: Technigaz, builders of membrane type gas carriers, now part of GTT ISM Code: The purpose of this code is to provide an international standard for the safe management and operation of ships and for pollution prevention, the objectives of which ensure safety at sea, prevention of human injury or loss of life, and avoidance of damage to the environment and to property. ISM Drills: Drills regularly rehearsed onboard in compliance with the ISM Code, in particular clauses 7 and 8. Information Books: Manuals compiled onboard to provide a readily available source of ship specific information for all personnel. The manuals provide ease of reference on all critical onboard operations and for new or recently transferred personnel, a means of enhancing familiarisation with their duties. The manuals are normally categorised into Bridge (BIB), Deck (DIB) and Engine Room (EIB). Deck Operations Log (DOL):A hand-written continuous record of events in real time, correlating all deck, cargo and bridge operations associated with a particular voyage. Deadband: A control engineering term used to describe that portion of a split-range control band in which both of the controlled functions are normally closed and inoperative. Overlap: A control engineering term used to describe that portion of a split-range control band in which one of the controlled functions commences response before the other has completed its full responsive action.

Glossary

Purge: In this context, a term used to describe any operation that involves replacing one medium with another in a controlled manner and to prescribed limits, i.e. hydrocarbon gas with IG, or IG with air, etc. Spin-Test: In this context, a term used to describe the testing of a cargo pump by initiating the start arrangement and then observing associated parameters. The test is usually brief, i.e. just long enough to prove the pump's operation without disrupting other conditions prevailing at the time. Class: A general term for, Classification Society, for example, Lloyds or DNV. Saturation Temperature: The temperature at which, a liquid mass starts to change state into vapour.

Heel Ageing: In this context, a term used to describe the process whereby the light factions contained in a liquid mass evaporate, leaving behind the heavier ends. Impulse Lines: Small bore piping connecting a transducer/sensor to a measuring device, i.e. a pressure gauge. The medium in the impulse line is usually static, i.e. a non-flow process. Out-Turn: In this context, a term used to describe the quantity of cargo delivered by the vessel, i.e. the actual amount received by the buyer.

Index ACC (Autoc Combustion Control), 30-31 accommodation ladders, 37,49 Aeration, 57 Air Swept Duct, 33 Alarm Test Register, 33 ampere loading, 4 1 , 44, 46, 53 arrival/departure paperwork, 29, 37, 47, 49 atmospheric pressure, 5,-11, 57 Auto Blasting, 23 Average Liquid Temperature, 33 ballast tanks, 25, 27, 34, 53 Ballast valves, 36, 48 ballast voyage, 47, 48, 50 barometric pressure, 5, 33 blackout, 32, 48 blocking circuits, 24, 29, 40 boil-off, 17, 18, 24, 26, 28, 30-33, 37, 38, 48,50 brittle fracture, 34 buffer tank, 6, 18 butane, 50 bypass valves, 40-46 calculated quantities, 39 carbon dioxide (C0 2 ), 13, 15, 16, 32, 58 cargo conditioning, 26, 33 Cargo Operations Manual, 11, 56 cargo out-turn, 53 cargo pump, 11, 13, 15, 40, 43, 44, 53 Cargo Surveyor, 24, 39 Cargo Tank Hold Spaces, 6 cargo tank wedge, 10 Cargo Tanks, 7-11, 13, 16, 17, 30, 50, 53 Cargo/Ballast Plan, 37, 49 catalytic process, 10

CCTV, 37, 49 Certificate of Fitness, 34, 35 chemical light-stick, 35 Class requirement, 34, 53 closed-cycle, 55 CO, 58 cofferdam, 18, 34, 53 cold conditions, 12 Cold Function, 41 Cold maintenance, 48 Cold Spotting, 34, 53 compressor seals, 56

conserve bunkers, 48 Consumer Select, 6 control range overlap, 6 Cool down, 16-19 cooldown bobbin, 14 cooldown tank, 40 corrosive agents, 6 cosmetic maintenance, 13 creep, 28 cross-over bends, 11 Daily Cargo Log, 33 Daily Work Plan, 55 DCS (Distributed Control System), 30 Deck Operations Log, 38 Deck scuppers, 37, 49 Dew Point, 6, 7, 12, 56, 58 discharge arms, 38 discharge columns, 43 Discharge Preparations, 38 Discharge Procedure, 40 discharge strainers, 47 discharge valve, 41 displacement, 14, 53 dome arrangement, 6 double-wall piping, 10 Double-Walled N2-Jacketed Gas, 33 droplet carry-over, 33 droplet entrainment, 50 Dry Air mode, 6 dry powder monitors, 36, 49 Dry-Air Plant, 57 dry-docking, 6, 54 duct keels, 34, 53 E/R maintenance, 53 emergency generator, 37, 49 EP Policy, 30 equatorial region, 19, 20, 50, 56 ESD, 13,23-29, 35,38-46,48 ethane, 50 fatigue stress, 12, 48 fibre-optic, 23, 29, 38, 47 filling pipes, 6, 58 filters, 36, 48 Fire pump, 37, 49 Flag State, 24 flammable mixture, 69 flammable range, 57 flammable zone, 9, 10


flow loop, 11 flow preference, 27 Forced Boil-Off, 32 freeboard, 37, 49 Free-Flow, 42 freezable gases, 13 Frosting, 26, 41 fuel gas control valve, 30 Fuel Gas Supply Piping, 12 Fuel Oil Equivalent, 33 . FWE, 23 Gas Dangerous Zones, 13 Gas Detection, 18, 26, 33, 36, 48 gas heater, 11, 15, 30, 37 gas hood, 31 gaskets, 36, 48 gassing-up, 13, 16 gauging systems, 13 gross heating value, 27 guide vane, 31 gycol heating system, 17 heat energy, 15 heating coils, 34 Heel Ageing, 50 HFO, 31 High/High Levels, 13 Hot ESD test, 24, 39 hot vapour, 54 Hot-Line, 24, 39 humidity, 6 hydraulic shock, 41 hydrocarbon gas hazard, 13 IG plant, 6, 11, 12 IGC Code, 24 IMO pump, 37, 49 impulse lines, 26, 28 inert gas, 6, 10, 11, 13-15, 56-58 inerted condition, 12 Inerting/Drying 1, 5, 7-12, 53-57 Initial Cargo Custody Transfer, 24, 39 Initial Gauging, 24, 39 inner-hull, 18,30,33, 34, 53 insulation, 36, 53 insulation spaces, 3-6, 17, 33, 54 international ship/shore hose connection 37,49 intrinsically safe (IS), 35 invasive maintenance, 12

ISPS security checklist, 24 Kawasaki turbine ships, 23 lay-up, 13 LD and HD Compressors, 11,12-18, 24-30, 46, 49, 54, 56 life expectancy, 12 line drainings, 46 liquid header, 41 LNG spraying, 18 load bearing insulation, 12, 56 Load Port, 37, 47, 48 loading arms, 15, 23, 26, 29, 37, 38, 48 Loading Master, 29 Low Discharge Pressure, 53 low suction head, 45 Main Engine Bridge Ctl Sys, 23 main engine turning gear, 23, 38 make-up regulating valves, 6 Manifold bobbins, 36, 48 manifold bypass valves, 41 Manifold Valve override facility, 39 manifold valves, 12, 38, 39, 41 Manifolds, 36, 48 manoeuvring valves, 23 master steam stop valve, 23, 38 methane (CH4), 9, 15, 29, 50, 56 midships crossover, 41 moisture ingress, 36, 48 Moorings, 37, 49 Moss-Rosenberg hold spaces, 6 motion sensors, 35 motor ammeter, 45 motor winding resistance, 13 N2 generators, 5, 6 N2 pockets, 35 nitrogen (N2), 3-9, 10, 13, 18, 19, 23, 29, 30 31,35,46, 55-58 nitrogen oxides, 6 Non-Pumpable Suction Height, 45 non-uniform contraction, 17 normal boil-off value, 31 noxious gases, 58 offshore manifold, 36, 48 Oil Pollution Trolley, 37, 49 operational Dead-band, 6 out-turn, 30, 53

Glossary and Index

override facility, 24, 39 overside water sprays, 29, 37, 49 permit to work, 34 Personal O2 meters, 35 Pilot hoists, 37, 49 piping insulation, 56 piston effect, 11, 57 Plain carbon steels, 34 planned maintenance (PM), 48 Plant phone, 24, 39 portable instruments, 9 PPE, 35, 36 pre-port meeting, 37, 49 presentation flanges, 36, 48 Pressurisation, 12 primary insulation space, 3, 18 primary space supply valves, 5 prop shaft, 23 propane, 50 pump protection, 41 pump tower, 17 pumpable cargo, 54 Purge Drying, 13 purge pressure, 46 purging, 8, 10, 13, 14, 29, 30,46

spray header, 11, 14, 17,26,29, 37,41,42 Spray Rails, 11, 17, 25 spray nozzles, 18 spray-shield, 34 stability conditions, 37, 49 Stal-Laval turbine vessels, 23 starting current, 13 steam dump, 30, 37 striking plates, 34 stripping/spray pump, 32, 42, 53 suction strums, 34 sulphur, 6 surge, 35, 43, 48 tank shell, 18 tank-loading valves, 12 taped joints, 34 temperature sensors, 17, 33, 54 thermal gradients, 17 thermal shock, 17 Time/Temperature graph, 18 Toolbox Talk (TBT), 35 transition range, 35 transmission cables, 36 turbulence, 15, 46 unscheduled maintenance, 48

QRCs, 29 quantity on board, 27 radio contact, 42 ramp-up, 18, 19,42 re-circulation, 46 return gas blower (RGB), 42 Risk Assessment, 13, 34, 58 sacrificial anodes, 34 safety margin, 9 safety trolley, 35 samples, 11, 33, 57 savealls, 37, 49 scupper pipes, 34 secondary barrier, 3, 17, 54 secondary insulation spaces, 4, 6 secondary membrane, 4 secondary space supply valves, 5 security valve G, 31 ship/shore safety checklist, 24 SMS, 30, 34, 37, 49 SMT, 30, 47, 49 spool piece, 57

valve seals, 56 vapour generation, 54 vapour header, 6, 11, 14, 26, 30, 33, 56, 57 vapour locked, 14 vapour return, 40 vent duct, 31 vent mast, 6, 15,30, 31, 56 void space, 19, 53 Voyage Abstract, 33 whaleback spaces, 34, 53 Whessoe Column, 12, 28 Work Planning Committee, 12

LNG Operational Practices

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