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Document Title:
RECENT DEVELOPMENTS WITHIN LPG CARGO HANDLING SYSTEMS
Wärtsilä Oil & Gas Systems AS
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Table of Contents
1 2 3 4 5 6 7 8 9 10
INTRODUCTION ............................................................................................................................. 3
WÄRTSILÄ OIL & GAS SYSTEMS..................................................................................................... 4
THE NEW DEVELOPED LPG RELIQUEFACTION SYSTEM................................................................. 6
HANDLING OF SECOND GRADE CARGO VAPOR............................................................................ 8
CARGO LOADING........................................................................................................................... 9
COEFFICIENT OF PERFORMANCE ................................................................................................ 10
VARIABLE FREQUENCY DRIVE AND FUEL ECONOMY .................................................................. 12
REDUNDANCY CONSIDERATIONS AND CAPACITY DURING MAINTENANCE............................... 16
RELIQUEFACTION IN COMBINATION WITH VENT GAS COOLER / CONDENSER UNIT................. 17
VESSEL ARRANGEMENT .............................................................................................................. 18
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1
Introduction
Recent developments within Wärtsilä‐Hamworthy for the design of cargo handling systems for fully refrigerated VLGC’s have resulted in a number of improvements and advantages that offers reductions in fuel consumption, reduction in loading time, simpler operations and cost reductions, all compared with traditional installations. Traditionally, VLGC’s have been equipped with four identical reliquefaction units. The capacity has been sufficient to handle the pressure build up in the cargo containment system caused by heat input from the surroundings, to lower the cargo pressure towards atmospheric before un‐loading and to ensure sufficient reliquefaction capacity to manage acceptable loading rates without vapor return to shore side. Reliquefaction capacity used in this paper is to be understood as net reliquefaction capacity. Plant design is normally based on requirements related to operation. The plant should then fulfill all rules set forth by the IGC Codei and Classes related to capacity and redundancy. Using the recent experience from operating LNG vessels with reliquefaction systems where two large reliquefaction systems have been installed in order to provide the required cargo pressure / temperature control, it would be natural to consider that two large reliquefaction units would be sufficient for a LPG VLGC. Such a configuration satisfies the IGC redundancy requirements. However, opposed to the LNG trade, LPG carriers may carry two grades of cargo at the same time. A third measure for cargo pressure / temperature control is recommended to handle the second and less volatile cargo if one of the installed reliquefaction units is not available. In the last few years there has also been a tendency that the cargo list has been shortened, particularly for the very large gas carriers (VLGCs). Elimination of some grades from the cargo list simplifies somewhat the design of the cargo handling system. During the economic recession in 2009 – 2010, extensive development work was made within Hamworthy in order to meet the market when it recovered with a new and more cost and energy efficient system. The development work has been successful and thanks to both Hyundai Heavy Industries and Solvang ASA who both believed in the new developed system, an order of two ship sets have been received.
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Wärtsilä Oil & Gas Systems
Wärtsilä Oil & Gas Systems AS, with its origin from the Kværner group has been part of the gas shipping industry since early 1960. The company formerly known as Kværner Ships Equipment was acquired by Hamworthy December 1998 and has recently become a member of the Wärtsilä group. The LNG and Gas Recovery business segments are offspring from the LPG segment and have during the few years shown to be highly successful with innovative solutions for the industry. The below figure shows the gas business development within Wärtsilä Oil & Gas Systems.
1999 - 2012 7 Ship based VOC recovery systems, 4 shore terminal VOC recovery systems
Gas Recovery Business Stream
1999
2015 1999 LPG reliq system with vent gas condenser
1965 Havgas LPG Carrier Delivered
1986 Berge Troll LPG FPSO
2004 Sanha LPG FPSO
1996 Escravos LPG FPSO
1963 - 1999 125 LPG reliquefaction systems
2011 New LPG Reliq award Two vessels 2009 Aquires Aibel T&P
1999 - 2014 90 LPG cargo handling systems
1963
LPG Business Stream
2015 2008 Aquires Baltic Design 2004 Berge Sisar Centre 2012 LPG FPSO Member of the Wärtsilä Group 2010
1998 Member of the Hamworthy Group
2003 Small scale LNG plant delivered
2007 Small scale LNG plant delivered
Small scale LNG plant delivered 2012 Biogas liquefaction plant contract award
2004 - 2010 34 LNG Carriers w/ Reliq
1996
LNG Business Stream
2008 - 2013 9 Floating Regasification Systems
2008
2013
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Wärtsilä Oil & Gas Systems delivers design, systems, solutions, services and training for any type of gas carriers and we have an extensive reference list for:
Fully pressurized (FP) vessels Semi refrigerated / pressurized (SP / SR) vessels o LPG / Chemical o LEG
Fully refrigerated
LNG carriers
Last 12 years the company has 90 references for the supply to the LPG/C segment only, typical systems delivered are: Ballast control system Motor control centre Cargo control system Reliquefaction plant and auxiliary systems Cargo heater and booster pumps Deepwell cargo pumps Nitrogen generator Ship design with class drawings, hull model testing, cargo tank design and cargo tank delivery Cargo handling system, process and arrangement design including construction supervision and commissioning o Cargo handling system equipment delivery; valves, instruments, gas detection, level gauges and alarms, cargo piping,… o o o o o o o o o
Together with ship owners we conduct extensive data logging of the performance of the existing fleet enabling the owners to take corrective actions if a vessels energy consumption deviates from the expected or if the general health condition indicates that preventive maintenance is due. As part of the Wärtsilä organization, we can now offer a global service network with more than 70 locations throughout the world. Crew training can to a much larger extent be offered closer or close to the recruitment offices. An increased R&D pool will ensure that ongoing and future development of cargo handling systems will be accelerated and with a scope increase to cover total environmental solutions from both propulsion and auxiliary engines, fuel supply and power management ensuring an optimum fit between power producers and consumers reducing the environmental emissions of the future vessels or retrofitted vessels.
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3
The New Developed LPG Reliquefaction System
It is a tendency that larger LPG carrier new‐buildings are designed for dedicated LPG transport only Vessel specifications (cargo handling section) and requirements are reflecting this. The traditional solution for a VLGC is to have four identical reliquefaction units installed. During loading all units are normally running, and when the vessel is loading two grades of cargo, at least one unit is dedicated to the less volatile cargo, normally Butane. Butane is iso‐Butane, normal‐Butane or any mixture thereof.
Figure 1: Traditional reliquefaction system
The development team was given the mandate to develop a system that offers sufficient operational benefits and cost improvements in order to become a valid candidate for installations on next generation LPG vessels. Early in the development phase the concept was presented to Class. Essential for any development are Rule compliance. An approval in principle from a major classification society was received confirming that the concept complied with applicable acts and regulations. The ambitions for the development were to develop a system with higher efficiency than the traditional system, and that offers a better utilization of the installed generator facilities, i.e. resulting in reduced fuel consumption and thus reduced carbon foot print. Secondly, the new system should offer increased loading capacity and reduced cool down time. Thirdly, the system should offer improvements and simplifications in plant operation. Fourthly, the system should offer cost reductions, both CAPEX and OPEX, benefitting both yard and ship owner. Doc Id.:
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The operational benefits were met. This paper discusses some of these results. The new development is based on two large and identical reliquefaction units with a combined reliquefaction capacity in excess of the conventional four units. Additionally, a separate condenser unit for the less volatile cargo is installed. This condenser unit is using condensate from the more volatile cargo boil off vapor, normally Propane, as refrigerant.
Figure 2: New Reliquefaction System
The flow schematics for the two reliquefaction units are in principal identical to the conventional system. However, individual improvements are made on process equipments in order to further improve the overall performance. The heart of any reliquefaction unit is the compressor. Traditionally four compressors of type Burckhardt 3K140‐3A running at fixed speed are installed on a VLGC. The new system uses two compressors type Burckhardt 4K165‐3P running at variable speed; offering improved COP, improved power management control and improved fuel economy. Each of the new reliquefaction units has a reliquefaction capacity significantly exceeding the IGC requirements. The most efficient means of operation during laden voyage in case of two grades is to run both reliquefaction units and leave the condenser unit as redundancy. However, even though the condenser unit is not a thermodynamically optimal means of condensing the second grade cargo vapor, it may offer overall operational benefits. This will be discussed later. The new design is however able to handle all traditional types of cargo and cargo combinations including Propane with high Ethane content (>8 %). This is based on 36°C seawater temperature and 1 bara suction pressure.
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Handling Of Second Grade Cargo Vapor
Liquefaction of second grade cargo vapor can either be done by one of the reliquefaction units or by the dedicated Butane condenser unit. The Butane condenser unit offers benefits during both loading and when loaded. During loading of two grades of cargo, currently for traditional systems at least one reliquefaction unit needs to be dedicated for the less volatile cargo. The available reliquefaction capacity is normally in excess of what is needed for loading and cool down of this cargo and means of transferring the excess reliquefaction capacity to the more volatile cargo would improve the loading rate. This is because loading takes place without vapor return and loading rate is thus controlled by the installed reliquefaction capacity. By introducing a dedicated condenser taking a side stream of the condensate from the more volatile cargo, the installed reliquefaction capacity is better utilized for both cargoes during loading. Vapor can either free flow to the condenser or by the aid of a dedicated blower, preferably free flow shall be used. The condensed vapor is returned back to the cargo tanks using a dedicated return pump, this is regardless of free flow or blower operation. Particularly for blower operations it is important to minimize the compression heat input and thus a return pump is beneficial. Normally, blower operation is not required. Free flow of cargo vapor is driven by the temperature differences between the first grade condensate and second grade vapor and from experience with free flow on LNG vessels, the required driving temperature difference will be significantly less than what is available for LPG in order to develop and sustain a stable vapor flow through the condenser.
Figure 3: Butane condenser loop
The blower is designed to handle all vapors when loading two cargo tanks with iso‐Butane. Operations of the blower will be further studied and optimized when the two vessels are put in operations. Doc Id.:
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5
Cargo Loading
Since vapor return from the LPG carriers are rarely an option, the loading rate is governed by the cargo handling system’s ability to handle the vapor generation from the cargo tanks being loaded. The new system offers higher loading rate than the traditional system by primarily higher installed reliquefaction capacity (both increased speed capabilities and more than double volumetric capacity at same speed) and more available reliquefaction capacity for the most volatile cargo when loading two grades. The latter is by using the dedicated condenser unit for the high temperature cargo. SYSTEM Traditional New
PROPANE ONLY 136 hours 115 hours
3xPROPANE, 1x‐iBUTANE 133 hours 93 hours
The table is valid for Commercial Propane with 3.5% Ethane. Cargo loading temperatures are ‐39°C and ‐4.5°C, Propane and iso‐Butane respectively. The table should be regarded for information only.
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Coefficient of Performance
The Coefficient of Performance (COP) is the ratio between reliquefaction capacity and power consumption and describes the energy efficiency of the reliquefaction system and is dependent on several factors, both external to the system and internal. External factors are cargo vapor composition, inlet conditions and ambient temperatures, internal factors are process flow path, equipment selection, specification and design. For a given reliquefaction system the COP reduces (worsens) with increasing cargo volatility, increasing ambient temperatures and increasing inlet temperatures. Of many internal design factors that have a positive influence on the COP the following can be mentioned: minimum temperature approaches in heat exchangers, minimum pressure loss design, compressor piston configuration, valve design and compressor speed. The piston diameters (configuration) can to some extent be varied within a fixed compressor frame size to optimize on COP or reliquefaction capacity. This new process has also been developed for smaller vessels using a special designed 3K160‐3S Burckhardt Compression compressor. The process has been tuned to yield acceptable liquefaction capacity at favorable energy consumption. Variations in system performance based on piston diameters are given below:
3K160‐3S1 Base Case Configuration case 1 Configuration case 2 Configuration case 3
Reliquefaction Capacity kW 32°C SW 36°C SW 456 445 446 434 468 457 441 430
Shaft Power kW 32°C SW 36°C SW 384 396 373 384 407 421 365 377
COP 32°C SW 1.188 1.196 1.150 1.208
36°C SW 1.124 1.130 1.086 1.141
The new reliquefaction system based on 4K165‐3P is optimized on both capacity and COP and has for the same conditions as the 3K160‐3S a COP of 1.116 and a reliquefaction capacity of 702 kW. This new system does also use variable speed drive and maximizes the benefits from this drive system. The reliquefaction capacity of 702 kW is for a speed of 710 rpm, during loading the speed can be increased to 750 rpm giving an additional 36 kW of reliquefaction capacity. Valve losses reduce with reduced speed and during voyage the compressor is best operated at lower speeds where the efficiency is at a more optimum value. As can be seen from the graph below, the new system is superior in COP when matching capacities with traditional system. As mentioned above, the advantage of variable speed drive gives another 5% capacity during loading which can be important for this operation and slight worsening of COP in this period should be accepted. The decline in COP from about 450 rpm is due to a reduction in labyrinth sealing efficiency. However, since the labyrinth sealing efficiency is dependent on both speed and differential pressure, the rate of decline at the lower speeds depends on both ambient conditions and cargo composition. It has further been investigated if the COP declines for loading cases with moderate Ethane content. The results show that this is not the case.
1
Calculations are based on commercial Propane with 5% Ethane. IGC conditions are 32°C seawater temperature. Doc Id.:
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The right hand side graph is for commercial Propane
with 3.5% Ethane and shows that there is still an efficiency gain at the lowest speeds for most cargoes at more
moderate seawater temperatures.
Running configuration do not differ much from the traditional system apart from that condensation of second
grade vapor can either be handled by one of the two reliquefaction units or by the dedicated condenser skid.
But the way of operating the cargo handling system when transporting two grades of cargo is simplified. and
the system offers increased fuel efficiency which is further described in the next section.
The following charts matches the reliquefaction capacity with the traditional system, and as can be seen there
is a minimum of 6.5% efficiency gain with the new system at these conditions.
Using variable speed drives enables the use of electric drive motors with more optimum pole configuration, i.e.
less poles and hence better efficiency. The overall power factor improves with variable speed drive and as a
result the generator can be loaded higher.
‐
338 rpm
‐
710 rpm
710 rpm
‐
‐
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Variable Frequency Drive and Fuel Economy
Chasing for high COP alone will not necessarily benefit the operational economy if the vessels overall power balance is neglected. Variable speed drive enables a more efficient operation of the auxiliary engines. A VLGC is typically equipped with 3x1250kW generator capacity. When running one generator approximately 360kW is available for cargo operations before the Power Management System will request start of the second generator. With the new system, 360 kW is sufficient for most voyage operations which is not the case for the traditional system. It is not required to start the second generator in order to start the variable speed driven motor, but for the traditional system with star‐delta starter it is. A number of cases have been investigated comparing the fuel economy for the traditional system against the new system for laden voyages. The investigation has assumed commercial Propane with 5% Ethane and iso‐ Butane as cargoes. In case 1 the vessel is loaded with commercial Propane in all cargo tanks.
Due to the higher COP at reduced speed a moderate fuel saving was expected, however the better utilization of the generators further improves fuel savings significantly. At moderate seawater temperatures compared with IGC conditions there are fair savings with the new system. In case 2 the vessel is loaded with two grades of cargo, three cargo tanks with Commercial Propane and the fourth cargo tank with iso‐Butane. For seawater temperatures above 16°C, the traditional system cannot liquefy both Propane and Butane without starting the second generator set whilst the new system requires the second generator running from above 32°C. Doc Id.:
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The new system offers a better fuel economy than the traditional system at all seawater temperatures, however for IGC conditions the gain is fair, primarily due to more optimized generator utilization.
In case 3 we have investigated how the new system performs with the Butane condenser system in operation to reliquefy Butane boil off. Calculations shows that the traditional system has marginally better fuel economy than the new system looking at reliquefaction plants only. The new system has in overall better generator utilization and thus a marginally better overall fuel economy than the traditional system.
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To conclude, running the Butane condenser yields practically the same fuel economy as the traditional system. The significant gain is that one compressor can handle two grades of cargo.. Finally, operation of the Butane condenser has been compared with base case. Base case is defined as both reliquefaction units in operation, one for Propane and one for Butane. Interestingly, the Butane condenser operation gives marginally better fuel economy than base case for low to moderate seawater temperatures.
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Rather than only focusing on system COP, though highly important, one should focus on the total chain from auxiliary engine, generator, starter and electrical equipments all the way to the cargo handling system. Only by matching these systems fuel savings can be guaranteed. All calculations accounts for electrical losses, heat dissipation in equipment room and increased HVAC load due to VFD starters. Generator data and consumption figures are as per yard supply.
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Redundancy Considerations and Capacity during Maintenance
Redundancy requirements are set forth by the IGC code and two reliquefaction units are comparable with the philosophy made for LNG carriers; apart from that for LPG each unit has a significantly higher reliquefaction capacity than what the total heat leakage into the cargo containment system is. The vessel is fully capable of handling any loading configuration using only one reliquefaction unit in combination with the dedicated condenser unit. Loading rates will naturally be influenced. Loading rates will only be influenced when the temperature of the incoming liquid is of a magnitude higher temperature than the cargo temperature in the tanks. However, the loss of one unit before loading commences have been investigated. The loading time calculations are based on no vapor return and 2°C higher incoming liquid temperature than the cargo temperature.
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Reliquefaction in Combination with Vent Gas Cooler / Condenser Unit
Rarely there have been requests for vent gas coolers (also known as purge condensers) in order to either be able to recover an increased amount of cargo vapor during purging or to be able to carry a cargo with increased amounts of lighter components as e.g. Ethane in Propane. However, since the new system is capable of handling ethane content to above 8 mole%, this additional equipment is normally not required. Wärtsilä Oil & Gas Systems have references dating back to 1999 in the supply of such systems integrated into the standard reliquefaction units. Due to the higher capacity of the new reliquefaction unit, it is probably more suited than the traditional systems for integrating a vent gas condenser system.
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10 Vessel Arrangement Up until now, the cargo machinery house has been segregated into one electrical equipment room and one compressor room. The first is safe area and the latter gas dangerous area. Due to recent rule changes (vessels constructed after 01.01.2007) this segregation is no longer required provided that all electrical equipment has the required degree of protection as outlined in current acts and regulations. It was decided that this new development should utilize the benefits from this rule change and the two hulls under construction at HHI are the first LPG carriers with a cargo machinery house that are constructed according to the new rules. Benefits are several, and the following is avoided: The previous electrical motor room required a separate HVAC system. Previously an air lock with separate ventilation was required at inlet to the electrical motor room Bulkhead wall between compressor room and electrical room Gas tight compressor shaft penetration Flexible shaft connection Cofferdam underneath the cargo machinery house Arrangement inside the house simplifies with the new rules and allows for common lay down areas, hatches and so forth.
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