With the exception of some military vessels, where nuclear reactors can provide large quantities of steam, the use of steam turbines for ship propulsion is mostly now considered to be superseded technology. There are commercial vessels still in operation but, for some operators, it has been viable to convert to diesels engines for improved economy in operation.
The most popular market for commercial steam turbines in recent decades has been for the propulsion of LNG carriers, where boil-off gas could easily be used to raise steam. This market reduced, however, as a result of the successful development of large dual-fuel engines, capable of delivering high operational efficiencies running on either LNG or liquid fuels. Sources suggest that, by the end of 2013, less than 10 per cent of LNG carriers listed as on order were specified with steam turbine propulsion.
Despite this trend, Mitsubishi Heavy Industries (MHI) of Japan continues to offer attractive propulsion steam turbine solutions for the LNG carrier market. In 2014, the company announced that it had completed its "Sayaringo STaGE" as successor to the "Sayaendo1", which has enabled carrying capacity to be raised by over 15 per cent within the same ship width.
The STaGE acronym is derived from the Steam Turbine and Gas Engine equipment that is used in this hybrid propulsion system that combines a steam turbine and engines that can be fired by gas. The steam turbine is from MHI’s Ultra Steam Turbine plant (UST) range, which is based on a high-efficiency reheating steam cycle developed independently by the company. Operating in combination with this plant is a dual-fuel diesel engine, which can use either gas or oil as fuel, while the ship propeller is driven by an electric motor. The efficiency of the UST plant has also been further improved through use of waste heat from the diesel engine.
The overall result is a propulsion system that can operate efficiently across the full range of vessel speeds. MHI states that the new hybrid propulsion system has increased fuel efficiency by more than 20 per cent compared to the original Sayaendo design and by more than 40 per cent compared with earlier carrier designs.
MHI is already seeing orders for the new carrier, including one for delivery to joint ventures established separately between Chubu Electric Power with MOL and NYK Line. Both vessels are scheduled to be delivered in 2018 and will go into service transporting North American shale gas, mainly produced from the Freeport LNG Project1, Chubu Electric Power is involved in. The vessels are being built at MHI's Nagasaki Shipyard & Machinery Works and these latest two follow on from two earlier Sayaringo STaGE orders from NYK Line. These carriers will also transport shale gas, in this case taking on their loads from the Cameron LNG Project Louisiana terminal, in North America.