The stripping of cargo tanks is carried out when the vessel needs to discharge the entire cargo parcel from its cargo tanks. To achieve this, the vessel uses its spray pumps and runs them until the desired level is reached, ensuring the maximum liquid volume is discharged from the tank.

The use of spray pumps is preferred because they are lower-capacity pumps, suitable for handling lower liquid levels inside the cargo tanks. The stripping operation is also performed in preparation for an upcoming dry dock.

The purpose of stripping cargo tanks is the removal of the dead heel, leaving the cargo tanks as empty of LNG as possible. This can be achieved by using the stripping/spray pumps to strip out all the remaining LNG after the cargo pumps have been used to empty the cargo tanks as far as possible.
The stripping/spray pumps would generally be started before the cargo pumps are stopped and at a cargo tank level above the minimum stripping/spray pump start level. The stripping/spray pumps discharge to the spray line, to the stripping crossover, and then to the manifold to the terminal.
On some occasions, it may prove more effective to set the stripping/spray pumps to discharge into one cargo tank to consolidate the remaining LNG. The remaining cargo tank would continue using the cargo pump to discharge all the LNG to the terminal before this cargo tank stripping/spray pump continues to empty the final cargo tank.
The operation can be conducted either with or without vapour return and would normally be conducted in a similar manner to the main unloading operation with regard to vapour handling.

Exercise Overview:

1. Initial Setup:

• The exercise begins after the correct lineup has been achieved.
• The user is required to start the spray pumps and monitor them for any instability.

2. Assessment Process:

• The user must manage any fluctuations in the spray pumps and stabilize them as needed.
• The exercise is designed so that the user must open and close the required valves to complete the stripping operation and establish the correct line-up.
• The user is expected to maintain proper operation of the spray pumps and monitor tank levels, ensuring the desired level is reached.

3. Completion Criteria:

• The exercise is complete when the spray pumps are successfully stabilized, and the desired tank levels are achieved.
• The user must then stop all spray pumps and carry out the closing of the custody transfer system printout.

4. Evaluation:

• The final evaluation is based on how the user responded to various alarms, remarks, and cautions during the exercise, as well as how effectively they managed the stabilization of the spray pumps and carried out the required steps to complete the stripping operation.

Learning Objectives for the Exercise

When planning an LNG (Liquefied Natural Gas) discharging operation some potential learning objectives for a training program focused on the discharge of LNG (liquefied natural gas):
1. Understanding LNG Properties: Explain the physical and chemical properties of LNG, including its boiling point, density, and flammability.
2. Safety Procedures: Identify and apply safety protocols and emergency response procedures specific to LNG operations.
3. Discharge Process: Describe the step-by-step process of discharging LNG from a tanker to a storage facility.
4. Equipment Familiarization: Demonstrate knowledge of the equipment used during the LNG discharge process, including pumps, hoses, and vapor return systems.
5. Regulatory Compliance: Understand the relevant regulations and industry standards governing LNG discharge operations.
6. Environmental Considerations: Assess the environmental impact of LNG discharge and identify measures to minimize potential risks.
7. Monitoring and Control: Explain the monitoring systems in place to ensure safe and efficient LNG discharge, including temperature and pressure controls.
8. Incident Management: Recognize potential hazards during the LNG discharge process and develop strategies for risk mitigation and incident management.
9. Communication Skills: Demonstrate effective communication and coordination with team members and other stakeholders during the discharge operation.
10. Practical Application: Engage in hands-on training to apply theoretical knowledge in real-world LNG discharge scenarios.

Stripping of Cargo Tanks-Intro and Line up

Stripping of Cargo Tanks-Operation and Assessment

Milestones

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

1. Open and Close Required Valves for Stripping

• Assessment Phase – Operational flow and Assessment:

2. Stabilize All Spray Pumps
3. Achieve All Tank Levels
   • A: Achieve required level in Tank 1 (35 m³)
   • B: Achieve required level in Tank 2 (43 m³)
   • C: Achieve required level in Tank 3 (43 m³)
   • D: Achieve required level in Tank 4 (43 m³)
4. Stop All Spray Pumps
5. Close Custody Transfer Print-Out for Official Tank Gauging

Actions During the Exercise

Deviation from Exercise

• Opening or closing incorrect valves:
  If the user opens any non-required valves or closes required ones, a caution message will appear instructing them to undo the action.
  • If the issue is not corrected, the exercise will be terminated.

High Crossover Pressure

• Thresholds:
  • 4 bar: A warning will prompt the user to decrease the pump flow rate.
  • 5 bar: If the crossover pressure reaches 5 bar, the user must reduce it within 1 minute, or the exercise will be terminated.

Pump Trips

• Maximum limit: If more than two pumps trip due to liquid levels or safety concerns, the exercise will be terminated.

Alarms & Remarks

Step By Step Procedure

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

1. Open And Close Required Valves for Stripping.

• Assessment Phase – Operational flow and Assessment:

2. Stabilize Cargo Pump

1. When the tank level drops to 130 mm while the spray pump is running:
   • Pumps may become unstable.
   • A caution message will instruct the user to throttle the discharge valve of the affected pump.
2. Adjust the discharge valve to achieve a 31A output for that pump.

Pump Trips

1. 1. When the tank level drops to 122 mm while the spray pump is running, the pump will automatically trip unless the level override is enabled.
   2. If the level override is enabled, the pump will trip at 110 mm.

3. Achieve All Tank Levels

1. Target levels for tanks:
   • Tank 1: 35 m³
   • Tank 2: 43 m³
   • Tank 3: 43 m³
   • Tank 4: 43 m³
2. Ensure all tanks meet the required levels.

4. Stop All Spray Pumps

1. Once the target levels are achieved:
   • Stop the spray pumps.
2. If more than two pumps trip, the exercise will be terminated.

5. Closing Custody Transfer Print-Out

1. After stopping all pumps:
   • Navigate to the CTS Main Page via the menu.
   • Stop the CTS Report and Setup.
2. Upon closing the CTS Report and Setup, the exercise is complete.
   

Proper line up for stripping of cargo tanks: