Introduction

Function: Replacement of inert gas in cargo tanks with LNG vapour

The purpose of gassing-up cargo tanks is to remove the inert gas contained within the cargo tanks because inert gas contains approximately 15% carbon dioxide which will freeze at approximately -60°C and produce a white powder that can block valves, filters, and nozzles.

LNG liquid is supplied from the terminal to the liquid manifold where it passes to the spray line via an ESD liquid manifold valve to the vessel’s LNG vaporizer. It is then delivered as LNG vapor into the top of the cargo tanks via the vapor line and exits from the cargo tank via the filling lines. From here it can be vented to the atmosphere via the No.1 vent mast for step one of the process. Step two then follows with the LNG vapor being returned to the terminal using a high-duty (HD) compressor.

In some instances venting to the atmosphere is not permitted so options available include sending the vapor to the terminal, using the vapor as a gas fuel supply for the generator engines, or sending it for gas burning in the Gas Combustion Unit (GCU); or a combination of these choices.

It should be noted that the level of methane vented into the atmosphere is dependent upon the terminal regulations, which should be discussed and agreed upon before operations commence.

Performance criteria

• 1st step methane 5% No.1 vent mast
• 2nd step cargo tanks bottom methane content >99%
• 2nd step cargo tanks carbon dioxide content <1% in volume

Auxiliaries involved

• LNG vaporizer
• High duty compressor

Operation duration

• Steps 1 and 2: 20 hours, volume change ratio 1.7 times

Necessary device

• Portable methane analyzer
• Portable carbon dioxide meter

Checkpoints

• Sampling points and sampling lines in the cargo tanks
• No.1 vent mast

This exercise is carried out onboard an LNG carrier, where liquid LNG is provided by the shore and converted into vapor using onboard LNG vaporizer equipment. The LNG vapor is then introduced into the cargo tanks from the top, replacing the heavier inert gas present inside the tanks. The inert gas is extracted from the bottom of the tank and released into the atmosphere.

The exercise is divided into two main steps:

1. Step 1: Release of inert gas to the atmosphere.
2. Step 2: Once the LNG vapor is sufficiently present, it cannot be released into the atmosphere. Instead, it is sent to shore via the HD compressor onboard the LNG carrier.

Exercise Overview:

1. Prerequisites and Setup:

Setting up the correct configuration of emergency shutdown systems. Starting the manifold water curtain. Preparing gas measurement instruments for use. Preparing the LNG vaporizer. Opening the required valves and connecting the necessary hoses (or “spool pieces”).

2. Assessment Process:

The assessment begins when the actual flow of LNG starts from the shore. The user must ensure the following during the assessment:

• The vapor flow is maintained between 11,500 to 12,500 m³ per hour.
• The LNG vapor outlet temperature reaches 40°C.
• All gas sampling points are monitored to assess the current atmosphere status.

3. Part 1 Completion: The first part of the exercise is completed when the LNG vapor properties are visible on the gas detectors, confirming the proper replacement of Inert gas by LNG vapor inside the tank.
4. Part 2: HD Compressor Operation: The second part of the exercise involves preparing the HD compressor and generators for use. The user must ensure the correct lineup by opening the required valves and connecting the necessary hoses. Once the HD compressor is started, the warming-up process begins as part of the assessment.

• During this phase, the user must continuously monitor the tank pressure, ensuring it remains between 7 kPa and 18 kPa throughout the process.

5. Completion Criteria:

• The exercise is considered complete once the tank pressure is correctly maintained within the specified range.
• The user is assessed based on how well they respond to various remarks, questions, and alarms that arise during the exercise.

Line up for the Gassing up Step 1: