Diamond LNG Shipping, a joint venture between Mitsubishi Corp and NYK Line, has taken delivery of Diamond Gas Orchid, the world’s first LNG carrier designed to the Sayaringo steam turbine and gas engines (STaGE) concept.
The 165,000 m3 twin-shaft Moss spherical tank ship has been built by Japan’s Mitsubishi Heavy Industries (MHI) and is classed with ABS. The vessel will be chartered by Mitsui & Co for lifting cargoes from the Cameron LNG terminal in the US state of Louisiana. Long transpacific voyages to Asia will feature prominently in Diamond Gas Orchid’s operating life.
MHI led the development of both the Sayaringo version of the Moss containment system and the vessel’s STaGE propulsion system. In combining a more efficient hull structure with an innovative hybrid propulsion system, the aim was to provide a next-generation LNGC with improved cargo-carrying capacity and fuel performance.
The Sayaringo containment system features cargo tanks, somewhat modified from the true spherical shape, enclosed in a continuous steel cover. The cover enhances the ship’s structural strength and cuts down on wind resistance, while the shape of the tanks allows more cargo to be carried for a given set of hull dimensions.
Sayaringo is an evolutionary step on from MHI’s earlier Sayaendo (“peas in a pod”) Moss tank concept. The Japanese word “ringo” means “apple”, and the new containment system is so-named because the upper hemisphere of the cargo tanks is larger than the lower hemisphere. Sayaringo tanks are not true spheres, due to the incorporation of top and bottom toroidal sections and a larger cylindrical mid-section than that used on Sayaendo tanks.
The STaGE propulsion system encompasses an ultra steam turbine (UST) on the port side and three combined sets of dual-fuel diesel engines (DFEs) and propulsion electric motors (PEMs) on the starboard side. MHI also utilised UST plants on the Sayaendo LNGCs it has built to date.
With the STaGE system the exhaust gas and jacket waste heat from the DFEs is recovered to heat the feedwater utilised in the UST. The arrangement results in improved propulsion system performance and enables high-efficiency navigation over a full range of ship speeds.
The waste heat from the DFEs is also recycled to generate auxiliary steam as well as the drive steam for the main turbine, adding to the overall efficiency rating. Instead of the turbine generator used in conventional steam turbine plants, the power generator of the DFE plant supplies power to the ship, resulting in a simpler plant configuration and higher efficiency.
MHI points out that from a maintenance point of view, the STaGE propulsion system has only half the number of cylinders compared to an LNGC powered by a full dual-fuel diesel-electric system.