The purpose of the initial cooling down is to place the cargo tanks in a suitable cold condition for loading LNG. The initial cooling down must be gradually undertaken to reduce the cargo tanks’ temperature down to cryogenic temperature to:

• Avoid thermal shock on heavy structure such as pump tower and equipment.
• Limit the amount of vapour generation during loading.
• Limit the nitrogen flow rate in the Interbarrier Spaces (IBS) during loading.

The initial cooling down normally follows immediately on from the gassing-up operation and is achieved using LNG supplied from the terminal. This is delivered to the spray line and is sprayed into each cargo tank, which will cool down the cargo tank and surrounding insulation. A large amount of vapour will be generated and this is returned to the terminal by using the High Duty (HD) compressors.

The rate of cooling down the cargo tanks is pre-calculated and this, along with the terminal’s ability to receive the returning vapours, are the time limiting factors. The cooling down rate with the amount of LNG required is presented to the vessel in a tabulated format.

To control the spray rate according to the cargo tank pressure, the spray lines are adjusted by the operator to achieve the cooling down schedule. The spray line pressure supplied from terminal is to the vessel’s request.

Initial starting up

On initial start-up, the requested pressure should be low in order to permit gradual cooling down of the cargo tank and structures. Once the spray line and rail and line are cooled, shown by frosting on the spray line into each cargo tank, the pressure can be increased to full pressure.

During cooling down, nitrogen flow to the IBS will significantly increase. It is essential that the rate of cooling down is controlled so that it remains within the limits of the nitrogen system to maintain the IBS pressure.

Cooling down of the cargo tanks is considered complete when the temperature sensors in each tank indicate the target average temperature. When this average temperature has been reached, and the Custody Transfer System (CTS) registers the presence of liquid, bulk loading can begin.

Vapour generated during the cooling down of the cargo tanks is returned to the terminal via the HD compressors and the vapour manifold.

Once the cooling down is complete and loading has commenced, the cargo tank membrane will be at or near to the cargo temperature, but 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 during the first 3-4 days after loading to fully cool all of the cargo tank insulation to the loaded voyage operating temperature.

During the cooling down of the cargo tanks, cooling down the cargo lines can be undertaken. The purpose of cooling down lines is to slowly reduce the line temperature to a cryogenic level to avoid thermal shock, prior to loading LNG.

Once the cooling down of the cargo tanks is nearing completion, cooling down the lines can start, this includes the liquid manifold crossovers, cargo liquid line and filling lines to be used for loading.

It is achieved by the terminal supply of LNG being partially diverted from the spray line into the manifold crossover valves to the cool down valves on each manifold and through the double-shut valves, crossover lines and along the liquid lines to the cargo tank filling lines and valves.

As LNG slowly fills the lines the temperature probes at the manifold crossover and forward and aft sections of the liquid line should be carefully monitored for temperature drop and subsequent frosting.

Filling valves to the cargo tanks should be adjusted accordingly to ensure that there is a minimum flow through the valves to permit excess vapor out and allow the liquid line to fill and cool.

Exercise Description

This exercise focuses on the operation of cooling down cargo tanks onboard an LNG carrier. The exercise begins with all tanks in a “gassed-up” condition, where the atmosphere inside the tanks is fully filled with LNG vapor.

Exercise Overview:

1. Initial Setup:

Liquid LNG is introduced from the shore and directed toward the cargo tanks via spray lines, which contain spray nozzles inside the tanks. The LNG is sprayed into the tanks, gradually lowering the temperature. The goal is to reach a loading temperature of -130°C inside all cargo tanks. The vapor pressure inside the tanks is released to the shore using the high-duty compressor (HD compressor) onboard.

2. System Preparation:

Prepare the high-duty compressor and generators for operation. Connect the manifold arms and hoses (referred to as “spool pieces”). Ensure the correct valves are lined up to direct the LNG into the tanks via the spray lines.

3. Assessment Process:

The assessment begins once the lineup is complete and all equipment is ready for operation. The LNG flow is started from the shore, and the user must achieve a temperature of -130°C inside the tanks. Throughout the assessment, the user must maintain the correct LNG flow in order to achieve the temperature drop in the requisite time period and respond to any rising tank pressure by operating the HD compressor appropriately.

4. Completion Criteria:

The exercise is considered complete when the target temperature of -130°C is reached inside all cargo tanks within the stipulated time. The user is assessed based on their response to various remarks, cautions, and alarms presented during the exercise.

Learning Objectives for the Exercise

Participants will be able to effectively demonstrate the process of cooling down cargo tanks on LNG ships, understanding the necessary safety protocols, operational procedures, and the importance of achieving the required temperatures for safe LNG loading.

Key Components:
Understanding Cooling Down:

• Explain the purpose of cooling down cargo tanks to prepare them for LNG loading.
• Identify the significance of achieving appropriate temperatures to prevent thermal shock and ensure safe operations.

Operational Procedures:

• Detail the step-by-step process for cooling down cargo tanks, including:
• Preparation of cooling equipment and systems.
• Controlled introduction of LNG or cold nitrogen into the tanks.
• Monitoring the temperature drop and managing flow rates to prevent rapid cooling.
• Explain the importance of maintaining proper tank pressure and venting as necessary.

Monitoring and Verification:

• Demonstrate the use of temperature gauges and pressure monitoring systems during the cooling down process.
• Describe how to ensure that temperatures are within safe operational limits before loading LNG.

Practical Demonstration:

• Simulate the cooling down process in a controlled environment or through hands-on training exercises.
• Role-play scenarios to address potential challenges or deviations from the procedure, emphasizing critical thinking and teamwork.

Assessment Criteria:

• Ability to articulate the significance of cooling down cargo tanks.
• Accurate execution of the cooling down procedure and effective monitoring.

By achieving this objective, participants will gain the knowledge and skills necessary for safely and efficiently cooling down cargo tanks on LNG ships, ensuring operational readiness for LNG loading.

Extended Cooldown from Shore-HD Preparation

Extended Cooldown from Shore-Line up and Manifold connection

Cooldown – Operational flow for temperature management

Extended Cooldown from shore-Operational Assessment

Milestones

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

1. Prepare HD Compressors And Generators For Use.
2. Connect Manifold Arm L3 and V.
3. Open Required Valves for Line Up.

• Assessment Phase – Operational flow and Assessment:

4. Start One HD Compressor.
5. Achieved Required Temperatures In All Tanks.

• Achieved -30°C Temperature In All Tanks.
• Achieved -100°C Temperature In All Tanks.
• Achieved -130°C Temperature In All Tanks.

Exercise Actions and Warnings

1. Close One Nozzle in Cargo Tanks with Temperature Below 0°C:
   • When a tank’s temperature reaches 0°C, the user must close one of the spray nozzles in that tank.
   • If not resolved within 5 minutes, a low-priority alarm will be issued. If unresolved for another 5 minutes, a high-priority alarm will trigger. Failure to correct this within 1 minute of the high-priority alarm will result in termination of the exercise.
2. Starting HD Compressors as Per Requirement:
   • When the tank’s pressure reaches 14 kPa, an alarm will be issued to start one of the HD compressors. Failure to do so within 1 minute will terminate the exercise.
3. Increasing HD Compressor Veins:
   • When the tank’s pressure reaches 14.5 kPa, an alarm will instruct the user to increase the veins of the running HD compressor. Failure to do so within 1 minute will terminate the exercise.
4. Starting Second HD Compressor:
   • If the pressure reaches 15 kPa, the user will receive an alarm to start the second HD compressor. If the veins of the first HD compressor have not been reduced to 0%, the second compressor cannot be started. Failure to start the second compressor within 1 minute will terminate the exercise.
5. Increasing Both HD Compressors Veins:
   • When the tank’s pressure reaches 15.5 kPa, an alarm will instruct the user to increase the veins of both HD compressors. Failure to do so within 1 minute will terminate the exercise.

6. Deviation from Exercise

   • Important: After starting or resuming the cooldown process, do not open non-required valves or close any required valves.
     • A caution message will appear if this happens, instructing the user to undo the action.
     • If the user fails to correct it, the exercise will be terminated.

Alarms & Remarks

Step By Step Procedure

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

Prepare Both HD Compressors and Required Generators for Use

• Generators: The HD Compressor requires at least two generators to be operational.
  1. Navigate to the PMS page either through the side navigation or by clicking the “number of generators running” indicator on the HD Compressor page.
  2. Once on the PMS page, turn on one of the three generators that are not running (Note: One generator will already be running by default).
• After the required number of generators is running, the user must:
  1. Visit each of the two HD screens.
  2. Start the Aux LO Pump and switch ON the Heater for the Oil Reservoir.
  3. Open the inlet and outlet valves for the HD compressors.

Connect Manifold Arm Spool Piece

• Locate the highlighted manifold arm positioned above the liquid manifold.
• Click on the highlighted arm to initiate the connection process.
• Once all required arms are connected, the milestone will be marked as completed.

Complete the Line-Up by Following the Required Steps

1. Cargo Tank Vapour Valves:

2. Vapour Line to No.1 Vent Mast and Regulating Valve:

3. Spray Line Valves:

4. Cargo Tank Spray Line Valves:

5. HD Compressor Valves:

6. Vapour Line Valves:

7. Manifold Valve:

Note: The user must ensure that the above valves are set correctly across the various pages for proper line-up.

Note: Valve CG702 is only available in the Cargo System page.

• Assessment Phase – Operational flow and Assessment:

Starting the Cooldown Process

• Set the flow rate from shore and begin the cooldown process.
• If the drop rate exceeds the maximum allowed for the given flow rate and spray nozzle valve combination, alarms will be triggered.
• If the drop rate is set too low, it will prevent achieving -140°C within the given duration.

HD Compressor Requirements

• Pressure Reaching 14 kPa:
  • An alarm will instruct the user to start one of the HD compressors.
  • The compressor must be properly prepared before starting.
• Pressure Reaching 14.5 kPa:
  • An alarm will instruct the user to increase the veins of the running HD compressor.
• Pressure Reaching 15 kPa:
  • An alarm will instruct the user to start the second HD compressor.
  • The veins of the first HD compressor must be reduced to 0% before starting the second compressor.
• Pressure Reaching 15.5 kPa:
  • An alarm will instruct the user to increase the veins of both HD compressors.

Achieve Required Temperatures Inside the Tanks

• Achieve -30°C in all four tanks.
• During Stage I cooldown:
  • If the temperature drop exceeds 17°C/h, a caution message will be shown, instructing the user to reduce the drop rate.
  • If the temperature drop exceeds 20°C/h, a low-priority warning will be issued.
  • If the temperature drop exceeds 25°C/h, a high-priority alarm will be triggered. This alarm will remain active for 5 minutes. If the user continues the cooldown with a caution alarm, the respective tank valves will lock. The valves need to be reset to continue.
  • Each cargo tank valve can lock twice during the exercise. If the valve locks for a third time, the exercise will be terminated.
• Achieve -100°C and -130°C in all four tanks.
• During Stage II & III cooldown:
  • If the temperature drop exceeds 11°C/h, a caution message will be shown.
  • If the temperature drop exceeds 13°C/h, a low-priority warning will be issued.
  • If the temperature drop exceeds 15°C/h, a high-priority alarm will be triggered. Same procedure as above for valve locking.

Completion of Exercise

• Once the user achieves -130°C in all four tanks, the exercise is completed successfully.

Proper line up for cargo tank cooldown from shore: