Hamworthy holds a worldwide license for the sale and fabrication of the Moss RS LNG Reliquefaction system, which helps owners to realise savings by adopting diesel or diesel electric propulsion. Unlike all other cargo ships, liquefied natural gas (LNG) carriers have continued to use steam turbine propulsion plant despite more efficient diesel engines being available. This is because the gas that naturally evaporates from the cargo (called boil-off) is used as fuel for the steam turbines, and until recently there was no other use for it. The ability to reliquefy the gas given off by the cargo now makes it possible to increase the amount of LNG delivered to the discharge port, which is more profitable than using it as ships’ bunkers.
The patented process offered by Hamworthy reliquefies boil-off gas and returns LNG back into the cargo tanks. This, in combination with an efficient propulsion system, offers great technical and economical advantages over the steam turbine based propulsion system. Reliquefaction paves the way for the installation of more efficient propulsion systems on LNG carriers. The efficiency of diesel engines is up to 50 per cent, compared with approximately 30 per cent for a steam turbine plant. The economic advantage of diesel engine propulsion translates to minimum savings of 2 to 5 million USD per year for a LNG carrier depending on size of vessel and LNG price.
The liquefied natural gas is kept in a liquid state at -163°C in the tanks. Due to warming up during transportation, gas naturally evaporates from the cargo and the boil-off gas can be used in the boiler for production of some of the steam required for a steam-turbine propulsion plant. In principle there are two ways of handling the boil-off gas. It can either be burnt in a boiler, gas turbine or dual-fuel diesel engine to provide power for the propulsion of the vessel, or the boil-off gas can be reliquefied and returned to the cargo tanks, resulting in increased cargo quantity delivered. Reliquefaction is based on a closed nitrogen cycle extracting heat from the boil-off gas. Several novel features such as separation and removal of incondensable components have resulted in a compact system with low power consumption. A boil-off gas reliquefaction system in combination with slow speed diesel-mechanical or diesel-electric propulsion has both economic and technical advantages, according to Hamworthy.