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The FSRU topside facilities comprise of process modules for cargosendout, regasification, metering and boil-off gas handling system.

LNG is exported from the cargo tanks 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 ofcompact 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.

Figure 2 Schematic flow diagram of IFV

Boil-off gas consists of the gas which is produced by the vaporizationof 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 aship 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 issufficient to handle vapor from operational upset conditions in the

storage and vapor handling areas. Simulations have been developedusing HYSYS v7.1. This suite of simulations allows 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 LNG-FSRU. We quantify the amount of BOG produced in the worst case

scenarios while a ship is unloaded and in abnormal and emergency

operations

Table 1 Design Basis Specification of LNG-FSRU

Description Specification

Sendout

LNG loading

Cargo storage tank

Mooring system

BOG handling system

LNG Vaporizer

Max. 600 mmscfd

80 barg, 5C

TDR = 1 : 10Peak rate 12,000 m3/h

Side-by-side loading

Availability up to 220K LNGCCapacity 270K

45,000 m3x 6

Design pressure 25 kPaBoil-off rate 0.15 %/day

Internal turret mooring systemRecondenser

IFV circulation system

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 m3of LNG stored in

three or four separate compartments 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 loadingarm 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 ships 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 ships 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 leavingthe tanks varies in temperature from - 160to - 90.

Sendout from the FSRU is accomplished by pumping the liquid from

the storage tanks and passing it through vaporizers. During periods ofno 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 isprovided on the sendout stream. The topside process block diagram of

FSRU is shown in Figure 3.

LNG TankHP

PumpRe-

condenserMeteringSystem

Vaporizer

BOGCompressor

SuctionKO

Drum

Heater

BOG BOG

BOG

LNGLNG

LNG NG NG

IF IF

To GasNetwork

BOG

BOG

Heating

MediumSupply

HeatingMediumReturn

To PowerGenerators

Figure 3 Topside process block diagram of FSRU

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 canbe 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 ofthe 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.

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.

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 isoperated 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 liquidphase 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 Heat leak into the systeml

Energy input due to the ships 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)

l Displacement due to the incoming liquid (A4)l Vapor generated due to the pressure difference between the ship

and the FSRU storage tanks (A5)

l Displacement due to the sendout liquid (A6)

Thus the net vapor from storage tanks with ship unloading is A1 + A2

+ A3 + A4 A5 A6.

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, thecirculating 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 theflash vapors generated in cargo storage. Vapor generation during the no

unloading and maximum sendout case occurs as a result of the

following factors.

l System heat leak (A1)l Energy input to the recirculation stream returning the storage

tank (A2)

l Displacement gas make-up for the liquid removed (A3)

Thus the net vapor from storage tanks with no ship unloading is A1 +

A2 A3.

Process Simulation

Simulations have been developed using HYSYS v7.1. This suite ofsimulations 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,

and no ship unloading mode at maximum sendout, to allow the sizing

of the LNG pumps. Four cases have been considered:

l Case 1A : Ship unloading with maximum sendoutl Case 1B : Ship unloading with minimum sendoutl Case 2A : No ship unloading with maximum sendoutl Case 2B : No ship unloading with minimum sendout

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 tankparameters 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 PFDs. Unit operations are used to

allow input of heat leak and pressure drop data, as well as routing ofappropriate flows.

Table 2 Principle Parameters of Simulation Basis

Description Parameter

LNG composition

Loading rateShip pressureShip heat leak basis

Storage tank pressure

Storage tank vapor temperature

Storage tank heat leak basis

Recondenser pressure

Sendout rate

Lean/Rich LNG

12,000 m

3

/h130 mbarg0.15% per day

190 mbarg

-157C

0.15% per day

6 barg600 mmscfd

These scenarios are used to calculate the maximum amount of BOG

produces and hence to size the BOG compressors and their associatedequipment. BOG generation rate for the operation modes is shown in

Table 3.

Table 3 BOG Rate of Operation Modes

Case Mode BOG rate (t/h) Sendout rate (t/h)1A1B

2A2B

Ship/Max. sendoutShip/Min. sendout

No Ship/Max. sendoutNo Ship/Min. sendout

16.719.2

5.27.5

59452

594

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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 noship 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 continuousservice.

For sendout rates below approximately 180 t/h then the sendout gas

compressor with a discharge pressure of approximately 80 barg will berequired. The decision to install a spare is dependent on the frequency

of the low sendout rate scenarios and whether flaring is an acceptablealternative 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 environmentallyunacceptable, it is recommended that spare machine is installed.

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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.

Vent and relief valve discharges are routed to a single elevated flare

stack. During normal operation, the FSRU does not produce any excessvapors 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 bereduced in rate or shutdown to match the vapor which can be returned

to the ship in order to minimize flaring.

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-operationare relieved to atmosphere. The flare system is sized for the disposal of

vapor produced during abnormal and emergency operation.

An assessment of relief loads for various vapor generating scenarios is

based on the methods in EN 1473 and NFPA 59A. These flows arebased on worst case scenarios with allowance for barometric pressure

change. Flows comprising the flare relief system are given below.

A. Vapor displacement during tank fillingB. Flash vaporization of hot ship unloadingC. LP pump at recirculation, flash vaporizationD. Storage tank heat leakE. LNG tank roll-overF. Piping heat leakG. Drop in barometric pressureH. Make-up control valve failure

Pressure relief scenarios and design cases are as follow.

a) Power failure : C + D + F + G

b) Zero sendout : A + C + D + F + Gc) Hot ship unloading : A + B + C +D + F + Gd) LNG tank Roll-over : C + D + E + F + Ge) Make-up contol valve failure : A + C + D + F + G + H

The maximum continuous venting rate to the flare occurs during make-up 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 continuousavailability 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 fromthe 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

B. BOG compressors runningC. Barometric pressure increase

The total flow required for the make up gas control valve is A + B + C.

CONCLUSIONS

As part of the topside design of the LNG-FSRU, the method of

handling the vapor generated within the FSRU were studied. This vaporarises from the general boil-off from the storage tanks. A number ofoperating 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 maximumamount 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 arecondenser to condense boil-off vapor which is routed to the sendout

stream.

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.

REFERENCES

Daejun Chang, et al. (2007). "Safety and Environmental challenges inDeveloping a Closed-circuit LNG Regasification System for Offshore

Natural Gas Export" 26th Int Conf on Offshore Mech and Art

Eng,San Diego, USA.

Y. S. Han, et al. (2002). "Design Development of FSRU from LNGCarrier and FPSO Construction Experience" 2002 Offshore

Tech Conf, Houston, USAKOGAS 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

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