Design Development of BOG Handling System in LNG-FSRU Youngsoon Sohn, Donghyuk Kim, Sunghee Choi, Youngmyung Yang R & D Division, Korea Gas Corporation Incheon, Korea
INTRODUCTION ABSTRACT The concept of an offshore floating, storage and regasification unit has been proposed as alternatives to conventional onshore LNG import terminals. The advantage is that risk to the public is negligible and it could be remobilized at any time. But it is more vulnerable to storms than the land based terminal for LNG trans-shipment.
The concept of an offshore floating, storage and regasification unit(FSRU) has been proposed as alternatives to conventional onshore LNG import terminals. The advantage is that risk to the public is negligible and it could be remobilized at any time. Boil-off gas(BOG) consists of the gas which is produced by the vaporization of a small portion of LNG storage tanks due to ambient heat leak into the storage tanks and their associated pipe works. The other major source of BOG is the gas which is displaced by incoming LNG when a ship is unloaded into the LNG storage tank. In abnormal and emergency operation, the flare system is provided for the safe disposal of hydrocarbon vapors from the LNG-FSRU. The capacity of the flare will be sufficient to handle vapor from operational upset conditions in the storage and vapor handling areas. Simulations have been developed using HYSYS v7.1. This suite of simulations will allow equipment to be sized for the possible BOG handling methods. This study is to determine the optimum method of handling the boil-off gas produced from LNG storage tanks during the operation of LNGFSRU. We quantify the amount of BOG produced in the worst case scenarios while a ship is unloaded and in abnormal and emergency operations.
The capacity of the hull of the FSRU provides a nominal 270,000 m3 LNG storage capacity. The deck space available for the topside equipment is 300 m by 50 m. The FSRU is anchored to the sea bed by an internal turret mooring system which allows it to weather vane. Transfer of LNG from the tanker to the FSRU is in a side by side arrangement in mild environmental sea conditions while in hostile conditions a tandem arrangement. Process and utility facilities are located above the storage tanks. Generation of electric power and some utilities are necessary. LNG vaporization equipment needs to operate with FSRU moving with winds and waves. The shell and tube vaporizers which are not motion sensitive are used. Gas export to shore occurs through the turret mooring system. The system includes internal that swivel which allow the high pressure gas to pass from the process area to the export pipeline on the sea bed while allowing the FSRU to weather vane. Onshore equipment spacing standards can not be applied due to lack of space and risk analysis is used to determine of the design is acceptable. Some progress has been made in the development of guidelines for the design of offshore LNG facilities by organization such as Lloyds Register and Den Norske Veritas.
KEY WORDS: LNG; FSRU; BOG; Regasification NOMENCLATURE BOG FSRU HP IFV K.O. LNG LP PCV PFD TDR
boil-off gas floating, storage and regasification unit high pressure intermediate fluid vaporizer knock out liquefied natural gas low pressure pressure control valve process flow diagram turn down ratio
Figure 1 Conceptual Layout Layout of LNG-FSRU
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The FSRU topside facilities comprise of process modules for cargo sendout, regasification, metering and boil-off gas handling system. LNG is exported from the cargo t anks by submerged LP pumps and HP pumps elevate the pressure to the required level before regasification. Vaporized gas is subsequently metered and exported via internal turret. There are several methods to vaporize the LNG for export. The use of compact heat exchangers in LNG/Seawater contacting is not practical due to the freeze up & fouling risk. Compared to the direct seawater vaporizers, the use of an intermediate fluid circulation system is often considered for the actual vaporization in LNG-FSRU. The heating medium is typically seawater or a water glycol mixture with a freezing point below -30℃.
BOG HANDLING SYSTEM DESIGN Description of Process Flow LNG is delivered to the FSRU in the storage tanks. The LNG ship normally contain approximately 75,000 to 220,000 m 3 of LNG stored in three or four separate c ompartments within the ship. Once the tanker is moored and the loading arms have been connected, the LNG is pumped using the ships in-tank pumps. During the transfer of the LNG into the FSRU storage tanks, vapor is displaced from these tanks. Some of this vapor is returned to the ship via a separate loading arm to make up the volume of LNG removed from ship. Additional vapor is generate in the storage area due to various energy inputs into the system such as heat leak and pumping energy and flashing due to a higher operating pressure in the ship’s tanks relative to the FSRU storage tanks. The vapor leaving the storage area passes to the BOG compressors which discharge the vapor at a suitable pressure for distribution within the FSRU. After supplying the ship’s vapor make up requirements, the balance of the compressed vapor is distributed between the fuel system and the high pressure compressor. Any excess vapor is routed to flare. Depending on the level of LNG in the storage tanks, the vapor leaving the tanks varies in temperature from - 160℃ to - 90℃.
Figure 2 Schematic flow diagram of IFV Boil-off gas consists of the gas which is produced by the vaporization of a small portion of LNG storage tanks due to ambient heat leak into the storage tanks and their associated pipe works. The other major source of BOG is the gas which is displaced by incoming LNG when a ship is unloaded into the storage tank. In abnormal and emergency operation, the flare system is provided for the safe disposal of hydrocarbon vapors from the FSRU. The capacity of the flare is sufficient to handle vapor from operational upset conditions in the storage and vapor handling areas. Simulations have been developed using HYSYS v7.1. This suite of simulations allows equipment to be sized for the possible BOG handling methods.
Sendout from the FSRU is accomplished by pumping the liquid from the storage tanks and passing it through vaporizers. During periods of no ship loading liquid discharging from the LP pumps is circulated through the loading system to keep it cool. Vaporized natural gas is exported via an internal turret to the gas network. A metering system is provided on the sendout stream. The topside process block diagram of FSRU is shown in Figure 3. Suction KO Drum
BOG
BOG
BOG Compressor
To Power Generators
BOG
BOG
This study is to determine the optimum method of handling the boil-off gas produced from LNG storage tanks during the operation of LNGFSRU. We quantify the amount of BOG produced in the worst case scenarios while a ship is unloaded and in abnormal and emergency operations
BOG Recondenser
LNG Tank LNG
LNG
HP Pump
LNG
Vaporizer
IF
NG
Metering System
IF
NG
To Gas Network
Heating Medium Supply
Heater
Table 1 Design Basis Specification of LNG-FSRU Heating Medium Return
Description Sendout
Specification Max. 600 mmscfd 80 barg, 5 C TDR = 1 : 10 Peak rate 12,000 m3/h Side-by-side loading Availability up to 220K LNGC Capacity 270K 45,000 m3 x 6 Design pressure 25 kPa Boil-off rate 0.15 %/day Internal turret mooring system Recondenser IFV circulation system
Figure 3 Topside process block diagram of FSRU
°
LNG loading
Cargo storage tank
Mooring system BOG handling system LNG Vaporizer
BOG Handling System One or more boil-off gas compressors are required to handle boil-off gas generated during normal operation. The balance of the installed boil-off compressors are required to the additional boil-off gas generated during ship unloading. A desuperheater is provided in the compressor inlet line so that a sufficiently low suction temperature can be achieved by the injection of LNG into the inlet stream. After compression, vapor is condensed into the cold LNG sendout by contacting in the recondenser vessel. Some of the vapor is routed to the fuel gas system for consumption by the power generation units and utilities.
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During ship unloading some vapor is returned to the ship tanks in order to maintain the ship pressure. This vapor transfer occurs by virtue of the difference in operating pressure between the FSRU storage tanks and the ship tanks. The balance of the vapor is preferentially sent to the recondenser. At low sendout rates, when the recondenser is fully loaded, vapor is sent to fuel gas or to the flare for final disposal.
and no ship unloading mode at maximum sendout, to allow the sizing of the LNG pumps. Four cases have been considered: Case 1A : Ship unloading with maximum sendout Case 1B : Ship unloading with minimum sendout Case 2A : No ship unloading with maximum sendout Case 2B : No ship unloading with minimum sendout
l l l l
The recondenser also acts as a liquid buffer volume for the sendout pumps. The recondenser and associated lines are designed to achieve a hydraulic capacity equivalent to maximum sendout rates.
In the cases of ship unloading(operation cases 1A and 1B), the simulation models the ships discharge pumps and associated heat leaks from the ship unloading lines, loading arms and lines from the loading arms to the storage tank. The boil-off gas rate generated from each tank is modeled as a sub-flowsheet. The sub-flowsheet is used to define tank parameters such as discharge rate, tank volume, boil-off percentage, tank pressure and temperature. The aim of the main flowsheet is to model each mode of operation and develop stream data for those streams marked on the relevant PFD’s. Unit operations are used to allow input of heat leak and pressure drop data, as well as routing of appropriate flows.
BOG Generation The maximum vapor generation from the cargo storage tanks occurs when LNG ship is unloaded at maximum rates and when there is no sendout from the FSRU. Normally, a FSRU as a receiving terminal is operated such that prior to ship unloading the desired operating pressure is raised so that the heat leak into the system goes into the LNG liquid in storage. Heat will continue to accumulate in the liquid phase until the liquid has reached a temperature equivalent to its bubble point at the operating pressure. Vapor generation in the storage tank during ship unloading mode is composed of the following elements. l
l l l
Table 2 Principle Parameters of Simulation Basis Description LNG composition Loading rate Ship pressure Ship heat leak basis Storage tank pressure Storage tank vapor temperature Storage tank heat leak basis Recondenser pressure Sendout rate
Heat leak into the system l Energy input due to the ship’s pump (A1) l Heat leak in the piping in FSRU (A2) l Heat leak into the cargo storage tanks through the walls of the tank (A3) Displacement due to the incoming liquid (A4) Vapor generated due to the pr essure difference between the ship and the FSRU storage tanks (A5) Displacement due to the sendout liquid (A6)
Thus the net vapor from storage tanks with ship unloading is A1 + A2 + A3 + A4 – A5 – A6.
l l
°
These scenarios are used to calculate the maximum amount of BOG produces and hence to size the BOG compressors and their associated equipment. BOG generation rate for the operation modes is shown in Table 3.
Ship unloading is a relatively infrequent operation occurring once or twice a week depending on the sendout from the FSRU. Thus for a large proportion of the time the FSRU operates in a no loading mode. In this mode the basic requirement of maintaining the liquid filled piping cool is still valid. If there is no sendout from the FSRU, the circulating liquid stream is returned to the tanks where it flashes giving up the heat leak that it has absorbed and generates flash vapors. When there is a sendout requirement from the FSRU, part of the circulating liquid stream is routed to the vaporizers to satisfy the demand and as a result some of the absorbed heat leak is exported thereby reducing the flash vapors generated in cargo storage. Vapor generation during the no unloading and maximum sendout case occurs as a result of the following factors. l
Parameter Lean/Rich LNG 12,000 m3/h 130 mbarg 0.15% per day 190 mbarg -157 C 0.15% per day 6 barg 600 mmscfd
Table 3 BOG Rate of Operation Modes Case 1A 1B 2A 2B
Mode Ship/Max. sendout Ship/Min. sendout No Ship/Max. sendout No Ship/Min. sendout
BOG rate (t/h) 16.7 19.2 5.2 7.5
Sendout rate (t/h) 594 52 594 52
From the process simulation the boil-off rates from the FSRU storage tanks were found to range from 5.2 - 19.2 t/h. Boil-off when there is no ship unloading is in the range 5.2 - 7.5 t/h. Boil-off when ship unloading is in the range from 16.7 - 19.2 t/h. The BOG flows are handled by centrifugal compressors driven by electric motor. From considerations of maintenance and operability it is proposed that three 50% centrifugal compressors are installed since they are in continuous service.
System heat leak (A1) Energy input to the recirculation stream returning the storage tank (A2) Displacement gas make-up for the liquid removed (A3)
Thus the net vapor from storage tanks with no ship unloading is A1 + A2 – A3.
For sendout rates below approximately 180 t/h then the sendout gas compressor with a discharge pressure of approximately 80 barg will be required. The decision to install a spare is dependent on the frequency of the low sendout rate scenarios and whether flaring is an acceptable alternative to vapor recovery of the compressor is unavailable. Assuming that low sendout rates may be frequent in the years of operation, and that flaring product is financially and environmentally unacceptable, it is recommended that spare machine is installed.
Process Simulation Simulations have been developed using HYSYS v7.1. This suite of simulations allows equipment to be sized for BOG handling system. In essence there are two operating cases to be modeled i.e. ship unloading at minimum sendout, for the sizing of the BOG handling equipment,
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B. BOG compressors running C. Barometric pressure increase
FLARE SYSYTEM DESIGN The flare system is designed for the safe disposal of hydrocarbon vapors and liquids in the FSRU. This system is designed according to API RP521 and EN 1473.
The total flow required for the make up gas control valve is A + B + C.
Vent and relief valve discharges are routed to a single elevated flare stack. During normal operation, the FSRU does not produce any excess vapors for discharge to the flare system. In the event of total power failure, the FSRU is shut down and any loading operation is stopped and boil off gas from the storage tanks is routed to the flare header system. In the case of partial power failure, it will affect some or all the boil off gas compressors. An unloading operation may have to be reduced in rate or shutdown to match the vapor which can be returned to the ship in order t o minimize flaring.
As part of the topside design of the LNG-FSRU, the method of handling the vapor generated within the FSRU wer e studied. This vapor arises from the general boil-off from the storage tanks. A number of operating scenarios need to be looked at ship unloading, peak and minimum sendout as well as the possible range of imported LNG compositions. These scenarios are used to calculate the maximum amount of BOG produces and to size the BOG compressors and their associated equipment. From previous experience of the onshore LNG terminal it has shown that it is the most cost effective to utilize a recondenser to condense boil-off vapor which is routed to the sendout stream.
CONCLUSIONS
The capacity of the flare will be sufficient to handle vapor resulting from operational upset conditions in the storage and vapor handling areas. Excessive vapor loads due to equipment failure or mal-operation are relieved to atmosphere. The flare system is sized for the disposal of vapor produced during abnormal and emergency operation.
The flare system is sized for the disposal of hydrocarbon vapors and liquid resulting from abnormal operating conditions and emergencies. The FSRU is designed in accordance with a philosophy of minimum flaring. The capacity of the flare is sufficient to handle vapor resulting from operational upset conditions in the storage and vapor handling areas.
An assessment of relief loads for various vapor generating scenarios is based on the methods in EN 1473 and NFPA 59A. These flows are based on worst case scenarios with allowance f or barometric pressure change. Flows comprising the flare relief system are given below. A. B. C. D. E. F. G. H.
REFERENCES Daejun Chang, et al. (2007). "Safety and Environmental challenges in Developing a Closed-circuit LNG Regasification System for Offshore Natural Gas Export" 26 th Int Conf on Offshore Mech and Art Eng San Diego, USA. Y. S. Han, et al. (2002). "Design Development of FSRU from LNG Carrier and FPSO Construction Experience" 2002 Offshore Tech Conf , Houston, USA KOGAS R & D CENTER (2009) "Samchuck LNG Receiving Terminal Basic Engineering Data Book" Youngsoon Sohn, et al (2005), “Expansion of Pyeongtaek LNG Receiving Terminal of Korea Gas Corporation” Proc 15th Int Ocean and Polar Eng Conf, Seoul, ISOPE, Vol 1, www.isope.org
Vapor displacement during tank filling Flash vaporization of hot ship unloading LP pump at recirculation, flash vaporization Storage tank heat leak LNG tank roll-over Piping heat leak Drop in barometric pressure Make-up control valve failure
,
Pressure relief scenarios and design cases are as follow. a) b) c) d) e)
Power failure : C + D + F + G Zero sendout : A + C + D + F + G Hot ship unloading : A + B + C +D + F + G LNG tank Roll-over : C + D + E + F + G Make-up contol valve failure : A + C + D + F + G + H
The maximum continuous venting rate to the flare occurs during makeup control valve failure and rollover case. This gas is relieved through the tank atmospheric relief valves. The design flow for the flare system is selected for the zero sendout case. The flare stack is designed for safe and efficient combustion of natural gas at flow rates up to the design capacity. The stack incorporates pilots to ensure the continuous availability of the flare, as well as flame-out alarms and igniters. The storage tanks are protected from under pressure by the letdown of the send-out gas via a pressure control valve. This PCV is sized for all of BOG compressors running at the rated capacity, LP pump out from the storage tanks plus barometric pressure change. In the event of this make up gas failing the ultimate under protection is provided by atmospheric vacuum breakers at each tank. An assessment of the loads for the vacuum relief system is based on the methods in EN 1473 and NFPA 59A. These flows make allowance for barometric pressure change. Flows comprising the vacuum relief system are given below. A. Maximum liquid withdrawal
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