Fuel Efficient Steam Turbine LNG Carrier
Mitsubishi Heavy Industries (MHI) signed on the dotted line a few days ago to build a 138,000 gt LNG carrier in their ‘Sayaendo’ series for Mitsui O.S.K. Lines . MHI describe this 288m (945ft) long vessel as a ‘New-generation’ LNG carrier, with a multitude of new design features which add up, MHI claims, to deliver a 25% reduction in fuel consumption compared to that of conventional LNG carriers of the same size; but essentially the design is marked by two outstanding features. The first a dramatic change in the shape of the ship compared with the distinctive dromedary-like silhouette of other LNG carriers. Second a new turbine plant that achieves higher thermal energy by steam reheat.
MHI 'Sayaendo' LNG Carrier: Image courtesy of MHI
LNG Tanks – 'Peas in a Pod' Style
The name itself explains the concept: ‘Sayaendo’ in Japanese means ‘peas in a pod’. The ‘peas’ are the spherical LNG tanks, the ‘pod’ is a continuous cover over the four spherical tanks, integrated structurally with the hull, thus adding to overall structural strength. At the same time weight is reduced by eliminating complex support structures for the maze of pipes, electric cable runs and passages that are a feature of the conventional LNG carrier above deck level.
Wind tunnel tests showed that the Sayaendo cover improves aerodynamics by reducing longitudinal wind force drag on the ship’s propulsion, making not only a contribution to reduced fuel consumption in this way, but also to cost-saving in port by simplifying maintenance.
Ultra Steam Turbine (UST) Plant
Higher turbine efficiency is achieved by application of a reheat cycle adopting main steam with higher pressure and temperature. A picture (schematic) is better than a thousand words as will be seen below.
MHI Ultra Steam Turbine Diagram: Image courtesy of MHI
In essence, the intermediate pressure turbine is integrated with the high pressure turbine for the steam reheat plant, and the associated higher temperatures (560C) in the turbine rotor and single cylindrical turbine casing are taken care of by using materials already proven in the land-based use of steam reheat system turbines.
Taken in combination with other main engine improvements the manufacturers claim the UST can improve performance over conventional steam turbines by between 22 – 23%.
The UST also brings flexibility of fuel use. It can run entirely on cargo boil-off LNG, or bunker oil fuel, or any combination of the two. Interestingly, the LNG fuel supply system, is capable of gas-only use in all conditions of operation, including when the ship is in harbour.