LNG
Inert Gas from Engine Room
LD Compressor No. 1 (Inboard) HD Compressor No. 1
(Inboard)
HD Compressor No. 2 (Outboard)
LD Compressor No. 2 (Outboard)
4.2.2 Gas Freeing With Other Tanks In Service Cargo Tank Stripping With Other Tanks In Service (see illustration 4.2.2a)
It may become necessary to displace LNG vapour with inert gas in a cargo tank with other tanks still in service in order to prepare a tank for inspection. There are two possibilities when this can be carried out i.e
During the first laden voyage.
During the ballast voyage.
These two possibilities use the same procedures. Tank No.1 will be demonstrated for this example. This vessel is assumed to in gas burning mode. (Only the valves concerned with the gas burning operation are open, all other valves are closed.)
1 Strip all possible LNG from tank No.1. During the laden or ballast voyage, remove the maximum LNG with the stripping pump and transfer to the other tanks via the stripping/spray main, liquid header and the filling pipes.
2 Open valves 170, 090 and 190.
3 Open the filling valves on the other tanks, 020, 030 and 040.
4 Ensure the filling valve on tank No.1 is shut.
5 Start the stripping pump in tank No.1 and open the discharge valve 110.
! Caution
Changes in temperature or barometric pressure can produce differentials far in excess of 30mbar in the insulation spaces which are shut in. With the cargo system out of service and during inerting, always maintain the primary insulation space pressure at or below tank pressure and always maintain the secondary insulation space pressure at or below the primary insulation space pressure. Severe damage to the membranes will result if the differentials exceed 30mbar. In case of emergency, put in communication the primary and secondary membranes.
4.2 Cargo Tank Stripping With Other Tanks In Service Issue: 1
Cargo Systems and Operating Manual LNG LERICI
4.2.3a Cargo Tank Warm Up With Other Tanks In Service Issue: 1
Key
Degassing line into Main Cargo Pump Cable Penetration
LNG Warm Vapour
LNG Vapour
Inert Gas from Engine Room
LD Compressor No. 1 (Inboard) HD Compressor No. 1
(Inboard)
HD Compressor No. 2 (Outboard)
LD Compressor No. 2 (Outboard)
Gas Freeing With Other Tanks In Service
Cargo Tank Warming Up With Other Tanks In Service (see illustration 4.2.3a)
Warming up tank No.1
During the laden or ballast voyage, tank No.1 is warmed by re-circulating heated LNG vapour. This warm vapour is re-circulated by one L.D. compressor and heated via the cargo heaters to 50°C. The hot vapour is introduced through the filling pipe at the bottom of the tank to evaporate any remaining LNG liquid that was unable to be removed via the stripping out process.
Excess vapour generated during the warming-up operation of the tank is burned in the main boilers or vented to atmosphere.
The warm-up procedure is as follows;
1 Stop the L.D. compressor in use for gas burning and close gas heater valve outlet 453.
2 Install spool piece CL.045FO and open valve 063 to discharge heated vapour to the LNG header.
3 Prepare gas heaters YA/5141B for use. The temperature set point is adjusted to 50°C (corresponding to gas burning case).
4 Start the L.D. compressor YA/5122B.
5 At the vent mast No.2 open valves 481, 483, 484 and 486. The set point of regulating valve 487 is adjusted to 1080mbar a. Vent heater YA/5142 is now prepared for use.
6 Open the compressor suction valve 404 from the vapour header.
7 Open the inlet and outlet valves from the compressor 411 and 431.
8 Open the heater inlet and outlet valves 451 and 453.
9 Open the vapour valves on each tank 491, 492, 493 and 494.
10 Open the filling valve 010 on tank No.1.
11 Monitor the pressure in tank No.1 and adjust the compressor flow in order to maintain the pressure in the tank at 1060 mbar a.
12 At the end of the operation, when the coldest temperature of the secondary barrier is at least +5°C stop and shut down the gas burning system, stop the L.D. compressor and shut the tank filling valve 010.
13 Close the tank vapour.valve 491.
! Caution
Changes in temperature or barometric pressure can produce differentials far in excess of 30mbar in the insulation spaces which are shut in. With the cargo system out of service and during inerting, always maintain the primary insulation space pressure at or below tank pressure and always maintain the secondary insulation space pressure at or below the primary insulation space pressure. Severe damage to the membranes will result if the differentials exceed 30mbar. In case of emergency, put in communication the primary and secondary membranes.
4.2 Cargo Tank Warm Up With Other Tanks In Service Issue: 1
Cargo Systems and Operating Manual LNG LERICI
4.2.4a Cargo Tank Gas Freeing With Other Tanks In Service Issue: 1
Key
Degassing line into Main Cargo Pump Cable Penetration
Inert Gas from Engine Room
LD Compressor No. 1 (Inboard) HD Compressor No. 1
(Inboard)
HD Compressor No. 2 (Outboard)
LD Compressor No. 2 (Outboard)
Gas Freeing With Other Tanks In Service Cargo Tank Gas Freeing (Version 1) (see illustration 4.2.4a)
This following procedure is not undertaken whist the vessel is in gas burning mode on either laden or ballast voyage.
After the cargo tank has been warmed up (see Cargo Tank Warming Up With Other Tanks In Service), the LNG vapour is displaced with inert gas. The inert gas from the inert gas generating plant is introduced into the bottom of the cargo tank via the LNG filling pipe. The gas from the tank is vented from the top of the tank through the gas dome safety valves to the vent mast.
The gas freeing procedure is as follows;
1 Prepare the inert gas generating plant for use in inert gas mode.
2 Open the vapour valves on the tanks that are not being gas freed 492, 493, 494.
Ensure the vapour valve on tank No.1 remains shut.
3 At vent mast No.2 open valves 481, 483, 484 and 486.
Adjust the set point of regulating valve 487 to 150mbar gauge.
4 Prepare vent heater YA/5142 for use.
5 Install spool piece CL.045FO and open valves 460 and 063 to supply the inert gas to the LNG header.
6 Open the filling valve 010 on tank No.1.
7 Remove the plug on the gas dome safety valves on tank No.1. This is in order to evacuate the inert gas in tank No.1 to the vent mast.
8 Start the inert gas plant delivering to No.1 tank.
Monitor the methane content inside the tank until it has reached the acceptable level.
9 Open valve XH/5321G upstream of the two non return valves on the dry air /inert gas discharge line.
10 Monitor tank No.1 pressure and check that tank No.1 pressure is always higher than the insulation space pressure, taking into account the pipe losses between the gas dome safety valves and vent mast.
11 When the hydrocarbon content from tank No.1 has fallen below 2.5%, isolate and shut off the tank. Stop the inert gas supply and shut down the plant.
! Caution
Changes in temperature or barometric pressure can produce differentials far in excess of 30mbar in the insulation spaces which are shut in. With the cargo system out of service and during inerting, always maintain the primary insulation space pressure at or below tank pressure and always maintain the secondary insulation space pressure at or below the primary insulation space pressure. Severe damage to the membranes will result if the differentials exceed 30mbar. In case of emergency, put in communication the primary and secondary membranes.
4.2 Gas Freeing With Other Tanks In Service - Version 1 Issue: 1
Cargo Systems and Operating Manual LNG LERICI
4.2.4b Cargo Tank Gas Freeing With Other Tanks In Service Issue: 1
Key
Degassing line into Main Cargo Pump Cable Penetration
Inert Gas from Engine Room Flexible Hose
LD Compressor No. 1 (Inboard) HD Compressor No. 1
(Inboard)
HD Compressor No. 2 (Outboard)
LD Compressor No. 2 (Outboard)
Gas Freeing With Other Tanks In Service Cargo Tank Gas Freeing (Version 2) (see illustration 4.2.4b)
It is not possible to use gas burning and the gas freeing procedure at the same time. Due to the fact that the gas freeing operation of a cargo tank will take approximately 20 hours, a significant amount of LNG vapour from boil-off, will have to be vented to atmosphere. In order to avoid this the following procedure can be adopted.
The following procedure calls for the separation of the fuel gas line from the inert gas line by installing additional pipe elements.
The gas freeing procedure is as follows;
The vessel is in gas burning mode.
1 Prepare the inert gas generating plant for use in inert gas mode.
2 Open the vapour valves on the tanks that are not being gas freed 492, 493, 494.
Ensure the vapour valve on tank No.1 remains shut.
3 At vent mast No.2 open valves 481, 483, 484 and 486.
Adjust the set point of regulating valve 487 to 150mbar gauge.
4 Prepare vent heater YA/5142 for use.
5 Connect the flexible hose between the inert gas line and liquid line.
6 Ensure the jettison discharge valve 000 is shut.
7 Open the inert gas supply valve 061 to the LNG header.
8 Open the filling valve 010 on tank No.1.
8 Remove the plug on the gas dome safety valves on tank No.1. This is in order to evacuate the inert gas in tank No.1 to the vent mast.
8 Start the inert gas plant delivering to No.1 tank.
Monitor the methane content inside the tank until it has reached the acceptable level.
9 Open valve XH/5321G upstream of the two non return valves on the dry air /inert gas discharge line.
10 Monitor tank No.1 pressure and check that tank No.1 pressure is always higher than the insulation space pressure, taking into account the pipe losses between the gas dome safety valves and vent mast.
11 When the hydrocarbon content from tank No.1 has fallen below 2.5%, isolate and shut off the tank. Stop the inert gas supply and shut down the plant.
! Caution
Changes in temperature or barometric pressure can produce differentials far in excess of 30mbar in the insulation spaces which are shut in. With the cargo system out of service and during inerting, always maintain the primary insulation space pressure at or below tank pressure and always maintain the secondary insulation space pressure at or below the primary insulation space pressure. Severe damage to the membranes will result if the differentials exceed 30mbar. In case of emergency, put in communication the primary and secondary membranes.
4.2 Gas Freeing With Other tanks In Service - Version 2 - page 2 Issue: 1
Cargo Systems and Operating Manual LNG LERICI
4.2.4c Initial Insulation Space Inerting Issue: 1
Tank 1
Tank 2
Tank 3
Tank 4
From No 3 Tank Secondary Space
Vacuum Pump (Outboard) Vacuum Pump (Inboard)
Supply from Nitrogen Storage Tank
Main Vaporiser No 5 Cofferdam
From No 4 Tank Secondary Space
No 4 Cofferdam
From No 2 Tank Secondary Space No 3 Cofferdam
Primary Space
Secondary Space
From No 1 Tank Secondary Space No 2 Cofferdam
No 1 Cofferdam Insulation
Spaces Exhaust Control
542 541
532
531
521 522
512 511
572
571
480
AW/826VX AW/827VX
568 569