BOG combustion systems are used only onboard LNG carriers. Excess BOG is sent to the engine room via gas heaters by low capacity compressor and is burned by the main boilers as fuel. The main boilers are capable of operating under different fuel combustion modes such as exclusively BOG mode, combined BOG and fuel oil mode, and exclusively fuel oil mode. Although steam turbine systems have been the main form of propulsion used onboard LNG carriers, diesel engines capable of using BOG as fuel have become perfect solution due to their higher operating efficiencies.