On completion of a dry dock period or delivery of a new Vessel, the Interbarrier Spaces (IBS) and Insulation Spaces (IS) may require re-inerting to remove any traces of oxygen, or to reduce the moisture content of the atmosphere inside the insulation spaces. This can be carried out by using the vessel’s nitrogen generation system.

Prior to any inerting of these spaces, great care must taken to ensure all sampling and control systems are fully operational, pressure relief valves are fitted and in full working order and personnel are fully conversant with the operation. If possible, the cargo tanks should be maintained at a pressure of at least 3kPa(g) during the inerting and drying of the insulation spaces to reduce the risk of over-pressurising the spaces.

In the extreme case where the oxygen content in the IS is above 10%, shore supplied vacuum pumps would be required, as the vessel is not fitted with these, to pull a 20kPa absolute vacuum in both the IBS and IS spaces simultaneously and back-fill with nitrogen.

The nitrogen provides a dry and inert medium for the following purposes:

• To prevent the formation of a flammable mixture in the event of a LNG leak.
• To permit easy detection of a leak through a barrier.
• To prevent corrosion.

The nitrogen is produced by two nitrogen generators located in the engine room 2nd deck and is stored in a pressurised buffer tank, ready to be supplied to the nitrogen headers through make-up regulating valves. From the headers, branches are led to the IBS and IS spaces of each tank. Any excess pressure is vented through exhaust valves to the nitrogen vent mast from the IBS and to the deck from the IS.

The system is automatically maintained by alternate exhaust and make-up as the atmospheric pressure and the temperature rises and falls between a pressure of between 0.5kPa and 1.0kPa above atmospheric. Both the IBS and IS spaces on each tank are provided with pressure relief valves which open at 3kPa(g) for the IBS and at 3.5kPa(g) for the IS. A manual bypass with a valve is provided for local venting and sweeping of a space if required.

The nitrogen generators are maintained in automatic mode, during normal operations, one generator should be able to maintain the pressure in the buffer tank. When a high nitrogen demand is detected, the second generator will start automatically.

Insulation Space Pressure Control
Nitrogen is supplied from the buffer tank and pressurisation headers through a pressure reduction unit and supply regulating valve which reduces the header pressure to 50kPa(g). Branch lines lead from the pressurisation header to automatic supply valves for the Interbarrier Space (IBS) and Insulation Spaces (IS) at each tank liquid dome. Nitrogen is released from the IBS and IS by automatic exhaust regulating control valves at each cargo tank gas dome to the respective IBS and IS vent mast.

The IBS and IS on each tank are provided with two pressure relief valves, one on the liquid dome and one on the gas dome, they will open when a pressure of 3kPa for the IBS and 3.5kPa for the IS above atmospheric. A manual bypass is provided for local venting and purging of the space if required. In addition, a manual flow gauge is fitted to a bypass line on the inlet side for the IBS and IS so that manual flow control can be performed in the event of failure of a automatic make-up valve.

Interbarrier and Insulation Spaces
The supply and exhaust control valves for both spaces on each cargo tank are operated under split range control by the output of the reverse-acting pressure controller for that space. Therefore, when the pressure in the space falls below the set value, the supply valve opens and the exhaust valve remains shut. When the pressure in the space rises above the set value, the exhaust valve opens and the supply valve remains shut.

The nitrogen header control valve reacts to the demand from the system to maintain the IBS/IS header at the pressure set by the operator. High, low and differential pressure alarms are fitted to the pressure control systems for each interbarrier and insulation space.

Pressure Control Logic for Interbarrier and Insulation Spaces Set Pressures

• IBS supply valve open setting = 0.5kPa (opens when the IBS pressure decreases below 0.5kPa
• IS supply valve open setting = IBS + 0.2kPa (opens when the IS is less than 0.2kPa above the IBS)
• IBS exhaust valve open setting = 1.0kPa (opens when the IBS pressure increases above 1.0kPa)
• IS exhaust valve open setting = IBS + 0.7kPa (opens when the IS is more than 0.7kPa above IBS)

Nitrogen Purging of Insulation and Inter-Barrier Spaces Exercise

In this exercise, the user is tasked with filling and topping up the insulation and inter-barrier spaces with nitrogen using nitrogen generators on both LNG ships.

Exercise Overview:

1. System Preparation:

The exercise begins by outlining the procedures for starting the nitrogen generators, including the air compressor and feed air compressor. Instructions for starting the portable multi-gas detector are also covered. The procedure for operating the valves for insulation spaces and inter-barrier spaces in manual mode is explained in detail.

2. Objective:

• The main goal of the exercise is to reduce the oxygen content inside the insulation and inter-barrier spaces, replacing the entire atmosphere with nitrogen.

3. Assessment Process:

• Once all system preparations are completed, as outlined in the LNG LCHS exercise instruction manual, the assessment begins with the introduction of nitrogen into the spaces.
• The user must continuously monitor the oxygen levels within the spaces using sampling points.

4. Completion:

• The exercise is considered complete when the oxygen content in the spaces is reduced to less than 2%. After all parameters are checked and verified, the user receives a “completion” notification, indicating the successful completion of the exercise.

IBS Conditioning Intro Operations and Assessment

Milestones

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

1. Set N2 Feed Air Compressor Standby, Start, and Stop Pressure.
2. Start N2 Feed Air Compressor and Set to Auto.
3. Reset N2 Feed Air Compressor on Alarm.
4. Start Portable Multi Gas Detector.
5. Operate IS IBS Valves in Manual Mode and Reduce Oxygen Content by Venting.
6. Check All Vent Mast Sampling Points.
7. Oxygen Content < 2%.
8. Set IS IBS Valves to Auto.
9. Set IBS and IS PID Set Points and Switch to Auto.

10. Set Valve CN511 and CN511 PID to Auto.

• Assessment Phase – Operational flow and Assessment:

11. Completed IS IBS Conditioning of Tanks.

Alarms & Remarks

Step By Step Procedure:

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

Set N2 Feed Air Compressor Standby, Start, and Stop Pressure

1. 1. Navigate to the Compressor Settings Page.
   2. Set the following pressures:
      • Standby Pressure: 6.5 bar
      • Start Pressure: 7.5 bar
      • Stop Pressure: 9.5 bar

Start the N2 Feed Air Compressor and Set to Auto

1. 1. Click on the Compressor Icon to open the control pop-up.
   2. In the pop-up, click Start to initiate the compressor.
   3. Once started, set the mode to Auto to enable automatic pressure regulation.

Reset N2 Feed Air Compressor on Alarm

1. 1. If an alarm is triggered, click on the Compressor Icon to open the control pop-up.
   2. In the pop-up, click Reset to clear the alarm and restore normal operation.

Start Portable Multi Gas Detector

1. 1. Navigate to the MISCELLANEOUS EQUIPMENT page using the side menu.
   2. Press and hold the Power Button on the Portable Multi Gas Detector for 2 seconds until it powers on.

Operate IS IBS Valves in Manual Mode and Reduce Oxygen Content by Venting

1. 1. Set the IS IBS Valves to Manual Mode.
   2. Adjust the valve openings as follows:
      • Open these valves to 100%:
        CN107, CN108, CN207, CN208, CN307, CN308, CN407, CN408
      • Open these valves to 70% or above:
        CN105, CN106, CN205, CN206, CN305, CN306, CN405, CN406
   3. Monitor the venting process to ensure oxygen content decreases.

Check All Vent Mast Sampling Points

1. 1. Inspect all Vent Mast Sampling Points to confirm that oxygen levels are decreasing as expected.
   2. Record readings to ensure the oxygen level is reducing from 20.9% to less than 2%.

Oxygen Content < 2%

1. 1. Wait until the oxygen content drops to less than 2%.
   2. Once this milestone is achieved, proceed to the next step.

Set the IS IBS Valves to Auto

1. 1. Change the mode of the following valves to Auto:
      CN105, CN106, CN107, CN108, CN205, CN206, CN207, CN208, CN305, CN306, CN307, CN308, CN405, CN406, CN407, CN408
   2. Verify that all valves operate automatically.

Set IBS and IS PID Set Points and Switch to Auto

1. 1. Set the following PID Set Points:
      • IBS Exhaust SP: 0.80
      • IBS Supply SP: 0.70
      • IS Exhaust SP: 0.50
      • IS Supply SP: 0.30
   2. Once set, switch all PIDs to Auto Mode for automatic control.

Set Valve CN511 and CN511 PID to Auto

1. 1. Set CN511 PID to Auto.
   2. Set CN511 PID SP to 50.
   3. Change Valve CN511 to Auto Mode for automatic operation.

• Assessment Phase – Operational flow and Assessment:

Completed IS IBS Conditioning of Tanks

1. 1. Allow the IS IBS spaces to fill with nitrogen.
   2. Once nitrogen fully fills the spaces, the IS IBS Conditioning of Tanks process is complete.