BOG ventilation and combustion in re-liquefaction plants of LNG carriers

Abstract

A liquefied natural gas (LNG) carrier is designed to transport LNG over a long distance. During transportation, the leakage of boil-off gas (BOG) along with poor ventilation can pose an explosion risk. The traditional method for dealing with the BOG is to burn the gas to power the ship. As an alternative method, re-liquefaction plants on board re-liquefy the vapours back to the cargo tanks. This project will characterize the technical issues associated with fire protection in a confined space where LNG boil-off re-liquefaction takes place and will demonstrate what the appropriate ventilation rate in an emergency is. In addition, the CFD software (ANSYS CFX) is applied to simulate the ventilation and burning of boil-off gas in the on board re-liquefaction plant to ensure the safety of the cargo tanks for the worst case scenario. The traditional method for dealing with the boil-off gas (BOG) on board an LNG carrier is to burn the gas to power the ship. As an alternative method, re-liquefaction plants on board re-liquefy the vapour back to the cargo tanks [1]. In the last few years, the introduction of LNG re-liquefaction systems has given ship owners a choice of fuels and provides engine redundancy. Nevertheless, flammable natural gas is processed in both methods. Due to the inherent properties of a ship's structure, the re-liquefaction system, gas combustion unit, pipelines, etc. are located in the space enclosed under the deck. Under certain circumstances the leakage of natural gas, along with poor ventilation, can pose an explosion risk. Unfortunately, limited industry codes have been developed for fire protection in the cabin where the innovative LNG re-liquefaction system is located. The purpose of this project is to demonstrate the leaking and burning of boil-off gas in the on board re-liquefaction plant to ensure the safety of the cargo tanks for the worst case scenario by using CFD simulation software.