The Line Cool-Down at Sea operation is carried out while the vessel is en route to the discharge port. The goal is to keep the liquid lines at the required low temperature and ensure they are in a ready-to-discharge condition upon arrival. To maintain the liquid lines in a cold state, the vessel uses the spray pumps inside the cargo tanks to guide the LNG from the spray line into the liquid line, thereby cooling it down. This process ensures the lines remain cold and are ready for the custody transfer system upon arrival.

The purpose of cooling down the LNG lines is to avoid thermal shock on the piping and associated valves which would occur if gradual cooling was not employed.

Therefore, before unloading LNG from the cargo tanks, the cargo liquid lines, crossovers, and cargo filling lines must be cooled down to cryogenic temperatures and this can be undertaken with LNG supplied from the vessel’s cargo tanks.

The cargo tanks will normally be at approximately -160ºC during a loaded voyage with fully loaded cargo tanks and with the supply of fuel gas for the Main Engines (ME) and the Generator Engines (GE).

A stripping/spray pump is used to supply an initial slow flow of LNG to the spray line and crossover into the cargo lines and to the cargo tank filling lines. The operation would normally be undertaken prior to arrival at the port.

Exercise Overview:

1. Initial Setup:

The exercise begins by starting the required generators for use. The system is correctly lined up by opening the necessary valves. The spray pump is started, and parameters are verified to ensure the system is functioning properly.

2. Assessment Process:

• The assessment begins once the spray pump is activated.
• The user must maintain the flow and continuously monitor the drop in temperature and rise in pressure along the liquid line.
• The goal is to lower the liquid header temperature to -100°C.

3. Completion Criteria:

• The exercise is considered complete when the liquid header temperature reaches -100°C.
• The final evaluation is based on the user’s ability to manage the operation effectively, ensuring smooth execution, and responding to various remarks, cautions, and alarms that may arise during the course of the exercise.

Learning Objectives for the Exercise

Participants will be able to effectively demonstrate the process of cooling down the liquid header on LNG ships prior to discharging, understanding the necessary safety protocols, operational procedures, and the importance of achieving appropriate temperatures for safe LNG transfer.

Key Components:
1. Understanding Liquid Header Cool Down:
o Explain the purpose of cooling down the liquid header to prepare for safe discharging of LNG.
o Identify the risks associated with temperature differentials and the significance of thermal management.
2. Operational Procedures:
o Detail the step-by-step process for cooling down the liquid header, including:
 Pre-cool down checks and preparation of systems.
 Controlled introduction of cold LNG or nitrogen into the header.
 Monitoring temperature and pressure throughout the cooling process.
o Explain how to manage flow rates to prevent rapid temperature changes and ensure system integrity.
3. Monitoring and Verification:
o Demonstrate the use of temperature gauges and pressure monitoring systems during the cool down process.
o Describe how to confirm that the liquid header is within safe operational temperature limits before discharging.
4. Practical Demonstration:
o Simulate the cool down process in a controlled environment or through hands-on training exercises.
o Role-play scenarios to address potential challenges or deviations from the procedure.
Assessment Criteria:
• Ability to articulate the significance of cooling down the liquid header prior to discharging.
• Accurate execution of the cool down procedure and effective monitoring.

By achieving this objective, participants will gain the knowledge and skills necessary for safely and efficiently cooling down the liquid header on LNG ships, ensuring safe operations during discharging.

Line cooldown at sea prep and line up

Line Cooldown at sea Operational Assessment

Milestones:

• Learning and Training Phase – Pre-Requisites for the start of operation:

1. Open Required Valves for Line Up
2. Prepare Required Generators for Use

• Assessment Phase – Operational flow and Assessment:
  

3. Start Spray Pump in Cargo Tank 4
4. Stop Spray Pump in Cargo Tank 4
5. Achieve Required Temperatures on Liquid Header

Actions During Exercise:

Deviation from Exercise:

• After starting or resuming the line cooldown, do not open any non-required valves or close any required valves.
• A caution message will prompt the user to undo the last action.
• Failure to correct this will terminate the exercise.

High Crossover Pressure:

• During line cooldown, if crossover pressure reaches 4 bar, the user will be prompted to open valve CL400 to reduce the pressure.
• If the pressure reaches 5 bar and CL400 has not been opened, the exercise will terminate.

Liquid Entering the Tank:

• When CL400 is opened (max 10%) to reduce crossover pressure, liquid will start entering the tank.
• The user will receive a warning to close CL400. If this is not done within 1 minute, the exercise will terminate.

Close Tank Filling Valves CL200, CL300, CL400:

• At 3 minutes into the exercise, the user must close CL200, CL300, and CL400. Failure to do so will terminate the exercise.

Keep Tank Filling Valves CL100 and CL400 Open:

• At the 12-minute mark, the user will be warned to keep CL100 and CL400 open. Failure to keep these valves open will cause the exercise to terminate.

Alarms & Remarks

Step By Step Procedure

• Learning and Training Phase – Pre-Requisites for the start of operation:

Open All Required Valves for Line-Up:

1. CG100 – Open
2. CG101 – Open
3. CG200 – Open
4. CG201 – Open
5. CG300 – Open
6. CG301 – Open
7. CG400 – Open
8. CG401 – Open
9. CG075 – Open
10. CG701 – Open
11. CG702 – Close & Auto
12. CS704 – Open
13. CS705 – Open
14. CS706 – Open
15. CS707 – Open
16. CS071 – Open
17. CS072 – Open
18. CS700 – Open
19. CS701 – Open
20. CL107 – 100%
21. CL207 – 100%
22. CL307 – 100%
23. CL407 – 100%
24. CL100 – 100%
25. CL200 – 100%
26. CL300 – 100%
27. CL400 – 100%
28. CS401 – Between 25% and 30%
29. CS403 – 100%

Prepare Required Generators for Use:

• Navigate to the PMS page through the side navigation or click on the “number of generators running” indicator on the Cargo Tank 4 page.
• Turn on one of the three generators not currently running. (One generator is already running by default.)

• Assessment Phase – Operational flow and Assessment:

Start the Spray Pump in Cargo Tank 4:

1. Open Valve CS401 to 25-30%.
2. Open Valve CS400 to 95% or greater.
3. Ensure two or more generators are running (one is default).
4. Close CS403 (Spray Pump Master Valve) to 5% or less.
5. Once these conditions are met, start the spray pump.

Starting the Line Cooldown:

1. After lineup and starting the spray pump, open Master Valve CS403 to 100%.
2. Close Return Valve CS400 to 0%.
3. Once these actions are completed, the cooldown process begins, and a remark will be issued.

Actions at the 3-Minute Mark:

• At the 3-minute mark, the user will be warned to close CL200, CL300, and CL400.
• Once closed, the tank pressure will decrease by 0.3 kPa per minute, while crossover pressure will increase from 2 to 5 bar.

Managing Crossover Pressure at 4 Bar:

• When crossover pressure reaches 4 bar, the user must open CL400 for 20 seconds to reduce the pressure.
• Failure to open CL400 will result in exercise termination once pressure reaches 5 bar.

Pressure Management:

• After closing CL400, the crossover pressure will rise again. If it reaches 4 bar, repeat the process of opening CL400 for 20 seconds.

Actions at the 12-Minute Mark:

• At the 12-minute mark, keep CL100 and CL400 open.
• Crossover pressure will be maintained at 2 bar while tank pressure increases by 0.3 kPa every minute.

At the 14-Minute Mark:

• The user will be warned to stop the spray pump in Cargo Tank 4.
• Navigate to the Cargo Tank 4 page to stop the pump.
• After stopping the pump, the user will be prompted to close the filling valves in all tanks.

Completion of the Exercise:

• After closing the filling valves, open CS400 and CS404.
• The exercise is complete once the liquid header forward temperature reaches -120°C, the spray pump is stopped, and the required valves are opened.

Proper line up for line cooldown at sea: