NO.3 CARGO TANK
Part 3 Cargo Operations 3.1 Insulation Space Tests
2. Stage Two
(See Illustration 3.2.4b)
Inert gas condition CO2 : equal to or less than 1%.
Nitrogen gas condition N2 : equal to or less than 1%.
When 5% CH content (or the quantity of port authority is allowed) is detected at No.1 vent mast and each vapor dome, request permission from the terminal personnel to direct exhaust gas to the terminal facilities as flare stack.
Normally HD compressor shall not be used to avoid creating turbulence inside the tanks. Cargo tank pressure shall not be maintained in steady condition due to back pressure from shore side, then HD compressor shall be used for the operation.
a) Prepare both HD compressors for use.
b) Install the spool piece connecting the liquid line to the suction for the HD compressors.
c) Adjust the set point of both HD compressor’s flow control.
d) On the HD compressors open the following valves:
CG509 inlet to No.1 HD compressor CG511 outlet from No.1 HD compressor CG513 inlet to No.2 HD compressor CG515 outlet from No.2 HD compressor e) Open the following valves:
CL602, liquid header vapour supply to the HD compressors CG002, compressor supply to the manifold
f) Open the vapour manifold valve CG801 (port side). This will enable a free flow of gas to the terminal and is a check that the pipeline layout on board has been arranged correctly.
g) Once the flow to the terminal has been established, close valve CL107 at No.1 vent mast. Using the IAS, adjust the set point of No.1 mast riser control valve CG106 to the required value (for example 230 mbar, so that this valve will remain closed during normal running of the compressors, but would act in a safety capacity if necessary), and open CG107.
h) If the tank pressure increases too much, using the IAS start one or both of the compressors as necessary.
i) Using the IAS, monitor the pressure inside the tanks.
If the pressure increases, request the terminal to reduce the supply of LNG, or increase the flow through the HD compressor by adjusting the set point on both HD compressor flow rate to be controlled by IGV adjusting.
If the pressure decreases, reduce the flow through the HD compressors by adjusting the set point of both HD compressor’s flow rate. Alternatively, shut down one of the compressors as necessary, or request the terminal to increase the LNG liquid supply to the LNG vaporizer. But normally HD compressors may not be used.
When the cargo tank CH content reaches 98%, throttle in the individual tank loading valve until it is only just cracked open.
During the change of atmosphere, purge the following sections for about 5 minutes each:
a) All sections of the stripping/spray header and tank connections, via the valves at each vapour dome:
No.1 Tank CS105, 106, 107 No.2 Tank CS205, 206, 207 No.3 Tank CS305, 306, 307 No.4 Tank CS405, 406, 407
b) Purge manual and ESD valves. The manifold bypass valves are not in use.
The operation is considered complete when all four cargo tanks have at least an 98% CH content and the acceptable CO2 content and/or N2 content as requested by the terminal.
c) Purge the following lines and equipment for five minutes each:
i) No.1 and 2 boil-off/warm-up heater, forcing vaporizer, venting via the sampling cocks.
ii) HD and LD compressors with the compressor inlet and outlet valves. Make sure to thoroughly purge each compressor in turn.
iii) Vapour crossover and manifolds CG702 and CG802, venting through the manifold flanges CG701 and CG801.
iv) Cargo pump lines, stripping/spray pump lines and emergency cargo pump well via the appropriate line valve and purge sample point.
v) Extremities of vapour header via sample points.
d) Request the terminal to stop the supply of LNG liquid.
e) Stop both HD compressors, if operated.
f) Close CS808, the isolating line to the stripping/spray lines.
g) Do not shut down the LNG vaporizer until it has been warmed through to the ambient temperature.
h) Remove and spool pieces after purging with nitrogen and testing the gas content.
i) Prepare the cargo system for cool down.
Illustration 3.2.5a Cooling Down Cargo Tanks
IG021 IG020
CG405 TO E/R FROM E/R
IG022 CG527
CG525
CG524 CG523
CG522
CG516
CG512
CG508 CG507
CG503 CG504
CG515
CG511 CG514
CG513 CG002
CG509
CG505
CG501
CN683
CG528
CS004
CS003
CS002 CS001
CG510
CG506
CG502 No.2 HIGH DUTY COMP.
No.1 HIGH DUTY COMP.
No.2 LOW DUTY COMP.
No.1 LOW DUTY COMP.
TO INS.PRESS.
DEMISTER CG519
CG518 CG517
CG532
CG530 No.2 B.O/W.UP
HEATER
CG521
CS506
CS505 CS504
CS501
CF101
CF102 CF201
CF202 CF302
CF301
CF402
No.4 CARGO TANK No.3 CARGO TANK No.2 CARGO TANK No.1 CARGO TANK
CF401 CS502
LNG VAPORIZER FORCING VAP.
CS503 CG526
CG520
GAS MAIN
LNG LIQUID LINE
LNG VAPOUR LINE
VAPOUR MAIN STRIPPING/SPRAY MAIN LIQUID MAIN No.1 B.O/W.UP
HEATER
3.2.5 Cooling Down Cargo Tanks
Introduction
Arriving at the loading terminal to load the first cargo after refit, or when repairs require the vessel to be gas free, the cargo tanks will be inert and at ambient temperature. After the cargo system has been purge-dried and gassed up, the headers and tanks must be cooled down before loading can commence. The cool down operation follows immediately after the completion of gas filling, using LNG supplied from the terminal.
The rate of cool down is limited for the following reasons:
To avoid excessive pump tower stress.
Vapour generation must remain within the capabilities of the HD compressors to maintain the cargo tanks at a pressure of 70 mbar (about 1083 mbarA).
To remain within the capacity of the nitrogen system to maintain the primary and secondary insulation spaces at the required pressures.
Unlike rigid cargo tank designs, vertical thermal gradients in the tank walls are not a significant limitation on the rate of cool down.
LNG is supplied from the terminal to the manifold cool down line and from there directly to the spray header which is open to the cargo tanks. Once the cargo tank cool down is nearing completion, the liquid manifold cross-overs, liquid header and loading lines are cooled down.
Cool down of the cargo tanks is considered complete when the mean temperature except two(2) top sensors in each tank indicate temperatures of -130°C or lower.
When these temperatures have been reached, and the CTS registers the presence of liquid, bulk loading can begin. (GTT defined that target LNG loading is possible when mean temperature of cargo tank is lower than –80°C.) But GTT recommended to carry out cooldown operation of cargo tank to -130°C as per LNG terminal requirement.
Vapour generated during the cool down of the tanks is returned to the terminal via the HD compressors (or free flow) and the vapour manifold, as in the normal manner for loading.
During cool down, nitrogen flow to the primary and secondary spaces will significantly increase. It is essential that the rate of cool down is controlled so that it remains within the limits of the nitrogen system to maintain the primary and secondary insulation space pressures between 2 mbar and 4 mbar.
Once cool down is completed and the build up to bulk loading has commenced, the tank membrane will be at, or near to, liquid cargo temperature and it will take some hours to establish fully cooled down temperature gradients through the insulation. Consequently boil-off from the cargo will be higher than normal.
Cooling down 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 vaporized. Cooldown rate in the cargo tank and insulation spaces is depended on amount of spraying LNG.
As typical data taken at gas trial for the vessel refer to attached chart.
Preparation for Tank Cool Down
Place in service the heating system for the cofferdams.
a) Prepare the records for the tank, secondary barrier and hull temperatures.
b) Check that the nitrogen pressurisation system for the insulation spaces is in automatic operation and lined up to supply the additional nitrogen necessary to compensate for the contraction from cooling of the tanks. Prior to the cooling down, the nitrogen pressure inside the primary insulation spaces will be raised to 6 mbar. Pressurise the buffer tank at maximum pressure.
c) Check that the gas detection system is in normal operation.
d) Prepare the nitrogen generators for use.
e) Prepare both HD compressors for use.
Operating Procedure - Gas Return Through Vapour Header (See Illustration 3.2.5a)
Assume that the ship is ready to prepare for cool down after the completion of gas filling.
As reported by several ship operators, it seems accepted that the vapour return through the liquid header instead of the vapour header, makes the cool down operation more efficient and prevents liquid droplets in the vapour stream.
Alternatively, the procedure for cooling down cargo tanks with gas return via the vapour header is as follows:
a) Arrange the nitrogen piping to preferentially feed the primary insulation spaces.
b) Adjust the set point of the nitrogen supply regulating valves CN576, and CN579 at 2 mbar.
c) Adjust the set point of the nitrogen exhaust regulating valves CN277 and CN284 at 4 mbar.
d) Open valve CS003 connecting the stripping/spray header with the forward manifold and CS004, 002 and 001 on the stripping/spray header.
e) Open CS808 to supply LNG from the liquid manifold.
f) At each vapour dome open the spray valves CS105, 106, 107, 205, 206, 207, 305, 306, 307, 405, 406, 407.
g) Open vapour valves CG101, 102, 201, 202, 301, 302, 401, 402 on each tank.
h) At No.1 vent mast, open CG106. Set pressure control valve CG106 at 200 mbar to avoid venting except for safety.
i) Open the HD compressor’s suction discharge valves CG509, 511, 513, 515.
j) Open the HD compressor’s suction from the vapour header CG603 and discharge valve CG002 to the vapour manifold.
k) Open vapour manifold valve CG801.
l) When shore is ready to supply LNG, open ESDS valve CL803.
m) After cooling down the lines, request the terminal to supply a pressure of 2 bar at the ship’s rail. Monitor the tank’s pressure and the cooling down rate.
n) Adjust the flow to the spray bars in order to obtain an average temperature fall of 20°C per hour in the first five hours and then 10/15°C per hour thereafter.
o) Start one HD compressor (or both as necessary) in order to maintain the tank pressure at about 100 mbar.
p) Check the nitrogen pressure inside the insulation spaces. If it has a tendency to fall, reduce the cooling down rate.
In cases where other consumers reduce the availability of nitrogen for the insulated spaces, the pressure may temporarily fall below atmospheric pressure.
This condition is not critical insofar as the differential (Ps-Pp) between the secondary space pressure (Ps) and the primary space pressure (Pp) does not exceed 30 mbar.
(Ps - Pp ≤ 30 mbar)
q) When the average of the temperatures shown by the sensors installed on the pump towers is -130°C, request the terminal to stop LNG supply, and close CL803. The other valves should remain open until the lines have warmed up.
r) Stop the compressor(s) if loading does not take place after cool down.
Illustration 3.3.1a Cooling Down Tanks Prior to Arrival
IG021 IG020
CG405 TO E/R
FROM E/R
IG022 CG527
CG525
CG524 CG523
CG522
CG516
CG512
CG508 CG507
CG503 CG504
CG515
CG511 CG514
CG513 CG002
CG509
CG505
CG501
CN683
CG528
CS004
CS003
CS002 CS001
CG510
CG506
CG502 No.2 HIGH DUTY COMP.
No.1 HIGH DUTY COMP.
No.2 LOW DUTY COMP.
No.1 LOW DUTY COMP.
TO INS.PRESS.
DEMISTER CG519
CG518 CG517
CG532
CG530 No.2 B.O/W.UP
HEATER
CG521
CS506
CS505 CS504
CS501
CF101
CF102 CF201
CF202 CF302
CF301
CF402
No.4 CARGO TANK No.3 CARGO TANK No.2 CARGO TANK No.1 CARGO TANK
CF401 CS502
LNG VAPORIZER FORCING VAP.
CS503 CG526
CG520
GAS MAIN
LNG LIQUID LINE
VAPOUR MAIN STRIPPING/SPRAY MAIN LIQUID MAIN No.1 B.O/W.UP
HEATER
3.3 Ballast Passage
A characteristic of the cargo tanks of the Gas Transport membrane type is that as long as some quantity of LNG remains at the bottom of the tanks, the temperature at the top will remain below -50°C.
However, if the ballast voyage is too long, the lighter fractions of the liquid will evaporate. Eventually most of the methane disappears and the liquid remaining in the tanks at the end of the voyage is almost all LPG with a high temperature and a very high specific gravity, which precludes pumping. Thus operator should consider heel ageing for coolant when ballast voyage is too long.
Due to the properties of the materials and to the design of the membrane cargo containment, cooling down prior to loading is, theoretically, not required for the tanks. However, to reduce the generation of vapour and to prevent any thermal shock on the heavy structures, e.g. the pump tower, loading takes place when the tanks are in a ‘cold state’.
Cold Maintenance During Ballast Voyage
Different methods are used to maintain the cargo tanks cold during ballast voyages:
1. For short voyages a sufficient amount of LNG is retained in each tank at the end of discharge. The level must never be above 10% of the length of the tank and the quantities can be calculated by considering a boil-off of approximately 45% of the boil-off rate under laden voyage condition and the need to arrive at the loading port with a minimum layer of 10 cm of liquid spread over the whole surface of the tank bottom (with the ship even keel).
LNG terminal requirement
ATR (Arrival Temperature Requirement)
T C T T
ATR T ≤ − °
= + + + 130
4
) 6 5 4 3 (
Additional cool down should be carried out at the LNG terminal, when the cargo tank temperature is higher than ATR.
2. Three methods of cooling down are possible, and the one selected will depend on the operating conditions of the ship.
a) Cool down the tanks with LNG supplied from shore.
b) Cool down the tanks just before arrival at the loading terminal.
At the previous cargo discharge, a LNG heel is retained in one of the tanks, provided that the heel does not exceed 10% of the tank length (see sloshing). On top of the quantity to be sprayed, the amount of the LNG heel to be retained will be calculated by assuming a boil-off equivalent of 45% of the boil-off under laden conditions.
c) Maintain the cargo tanks at cold during the ballast voyage by periodically spraying the LNG so that the average temperature inside the tanks does not exceed -130°C. As before, a LNG heel is kept in one of the tanks, provided that the level does not exceed 10% of the tank length (see sloshing). On top of the quantity to be sprayed, the amount of the LNG heel that needs to be retained will be calculated by assuming a boil-off equivalent of 45% of the boil-off under laden conditions and heel ageing in long ballast voyage case.
Whichever method is used, cooling down is carried out by spraying LNG inside the tanks. Each tank is provided with two spray rings, each capable of a same flow rate.
Note!
It is obvious that this system will generate more boil-off than the first proposed system. The quantity of LNG to be retained on board will have to be calculated with enough margin to avoid the situation at mid-voyage where the residual is too heavy for the pump to operate.
Conservation of bunkers is important; consequently, the cooperation of all members of the management team is essential to ensure as much boil-off gas as possible is used to supply boiler fuel demand, thus keeping fuel oil consumption to a minimum.
The LD gas compressor is used for gas burning on the ballast voyage in the same way as on a loaded voyage, with control of the compressor from vapour header pressures (See section 3.5 gas burning operation).
Gas burning during ballast passage can be done with gas to boiler on free flow.
The LD compressor need not to be operated.
If a long delay at the loading port is experienced, the remaining heel will slowly boil-off and the gas available for burning will reduce. Therefore, care must be taken to stop gas burning as the tank system pressures continue to drop as the temperature rises. The degree of natural warm-up will depend on the time factor, voyage and weather conditions.
After refit, the first ballast voyage will have to be made using fuel oil only.
Due to the different calorific values of fuel oil and gas, engine power will require controlling to prevent overloading the boilers.
Note !
The pressure in the insulation spaces shall be maintained between 2 mbar and 4 mbar as per GTT recommendation.
y N2 Supply regulating valve:
Open the valve at 2 mbar Close the valve at 3 mbar y N2 Exhaust regulating valve:
Open the valve at 4 mbar Close the valve at 3 mbar
During cooling down and loading operation, the set point of N2 exhaust regulating valve may be adjusted to 6 mbar in view of safety margin of operating range at lack of N2 supply.
Illustration 3.3.1b Cooling Down
TYPICAL : CARGO TANK TEMP,VARIATION