The purpose of Inerting cargo tanks is to remove the oxygen content to avoid the cargo tanks entering the flammable range during gas filling. As part of the process of preparing the cargo tanks ready to carry the next LNG cargo, they must be Inerted with inert gas, thus removing ignition risk when gas filling.
The Inert gas, provided by the vessel’s inert gas generator, should be introduced at the bottom of each cargo tank simultaneously, via the liquid line. The cargo tank atmosphere will be displaced from the top of each cargo tank into the vapor line and discharged to the atmosphere through the No.1 vent mast.
- Function Replacement of dry-air in cargo tanks with inert gas
- Performance criteria Cargo tank dew point < -40°C
- Cargo tank oxygen content < 2% in volume
- Auxiliaries involved Inert gas generator (inert gas mode). Flow rate 16,500m³/h, dew point -45°C, oxygen about 0.5%.
- Operation duration 24 hours, including line up and piping purge.
- Necessary device Portable dew point meter
- Portable oxygen meter
This exercise focuses on preparing the systems and equipment to successfully make the tank atmosphere completely inert. The objective is to ensure that the dry air present inside the tanks is replaced with inert gas, which is generated by the inert gas generator onboard the LNG carrier.
Exercise Overview:
- System Setup:
The exercise begins with initial milestones that describe how to set up various equipment, including:
- Starting the portable multi-gas detector and dew point meters.
- Installing the necessary connections to the inert gas line.
- Opening and closing various valves to set up the system correctly.
- Preparing and starting the Ballast pump required for the inert gas generator.
- Configuring and starting the inert gas generator itself.
- These steps are prerequisites before the assessment part of the exercise begins.
- Assessment Process:
- The assessment begins when inert gas is introduced into the tanks from the bottom.
- The user must correctly manage the flow of inert gas entering the tanks and the dry air exiting the tanks, venting to the main vent system.
- Throughout the assessment, the user is required to continuously monitor the flow and check the atmosphere inside the tanks via various sampling points.
- Key Parameters to Monitor:
- Oxygen content in the tanks must be reduced to less than 2% by volume.
- The dew point must be below -40°C.
- The tank pressure should be maintained between 5 kPa and 7 kPa.
- Completion Criteria:
- The exercise is considered complete once all cargo tanks have met the required oxygen and dew point specifications.
- The user’s performance is evaluated based on how well they manage the process, respond to various remarks, and address any alarms or questions that arise during the exercise.
