What are ship turbines and how do they work?
Two varieties of turbines are found on ships, steam turbines which are external combustion systems and gas turbines which are internal combustion engines developed from aero engines.
Steam still has a place
Steam turbines have a long history in ship propulsion but are now limited to just LNG carriers as regards newbuildings. They produce rotary motion that is used to generate electricity by the simple expedient of directing a jet of stream at the turbine rotor. The rotor blades can be arranged in stages so as to take advantage of steam at different pressures which increases their efficiency.
A variety of fuels can be used to heat the water including waste heat from engines recovered through heat exchangers. On LNG carriers, the fuel for a turbine is the same as that used in gas carriers fitted with dual fuel engines and is the boil off gas from the cargo. On the remaining merchant vessels still operating with a steam turbine the fuel is usually oil and in a very few cases coal. In addition, there are a number of Russian-owned nuclear-powered icebreakers in which the heat comes from a nuclear reactor.
Until 2004 virtually all LNG carriers except some very early examples were powered by steam turbines but since then diesel and dual-fuel engines have become the favoured choice. Most of the steam turbines were of Japanese design and this holds true for the small number of ships still being built with them.
Most active is Mitsubishi Heavy Industries which has evolved its product into the Ultra Steam Turbine Plant (UST), a new turbine that provides higher thermal efficiency through effective use of thermal energy by reheating steam. The turbine’s intermediate pressure turbine is integrated with the high-pressure turbine for the steam reheat plant, and the associated higher temperatures (560ºC) 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.
Another Japanese builder that has had recent success is Kawasaki HI. Its URA-450 reheat steam turbine propulsion plant can provide fuel savings of around 15% relative to conventional steam turbine installations. Reheating is not a new technology in fact Kawasaki fitted marine reheat turbine technology to eight oil tankers in the VLCC and ULCC categories back in the 1970s.
To raise the thermal efficiency of the plant relative to conventional marine steam turbine steam generated by the main boiler is first led into the high-pressure turbine of the main turbine and returned to the boiler for reheating. The reheated steam is then introduced into the intermediate-pressure turbine and then the low-pressure turbine.
From air to sea
Gas turbines used for power in a variety of situations including power generation, aircraft, some military land vehicles and for marine purposes. In the transport sphere their main use is for aircraft engines and it is in this sector where most research and development has been carried out.
The gas in the term gas turbine does not relate to the fuel choice which can be anything from heavy fuel oil through to LNG, but to the method of operation. Incidentally most marine gas turbines have been run on MDO or MGO with the 2012-built fast ferry Lopez Mena being the first LNG–fuelled gas turbine powered vessel.
A gas turbine might be described as a cross between a cylinder engine and a turbocharger and consists of four parts. First is the inlet where air is drawn or forced into the second part – the compressor. Here the air is compressed into the combustion chamber where fuel is also injected. The burning of the fuel and air mixture causes it to expand and in doing so to turn the turbine which is shaft connected to the compressor as in a turbocharger. The spent air is then exhausted but in some cases may pass through a waste heat recovery system to extract further energy.
In a marine application, the shaft connecting the turbine and compressor is extended to also power a generator for production of electricity to feed the propulsion motors. Most of the energy produced by burning the fuel is needed by the turbine to drive the compressor so in effect a gas turbine is only about 40% efficient – much less than a modern diesel.
Although gas turbines can run on fuel oil most of those in operation run on distillates (MDO and MGO) to avoid problems with fouling and soot that occurs when using heavier fuels.
In terms of numbers installed, gas turbines are very much the last choice for merchant vessels and have had a very chequered history since the first trials in the late 1940s through to the early 1960s. There are two good reasons why this has been so; a poor efficiency level compared with diesels and the issues of maintenance and repair which is almost impossible
to carry out in the same way as with diesels.
A small field
There are several manufacturers of gas turbines around the globe but in the marine sphere only two can be considered as contenders in the merchant sector and a third - Textron Lycoming active in the yacht sector. The two makers that appear to have commercial ship potential are Cincinnati-based GE Marine Engines and Rolls-Royce. Both companies are highly active in the naval sectors where some characteristics of gas turbines are considered to have special merit. Gas turbines, particularly in the lower range of outputs up to 5MW, are significantly less efficient than diesels and also less flexible as a diesel has a wider load range before efficiency falls to sub-optimal levels. With fuel consumption running around 20% higher than a comparable diesel and capital costs also around 20% higher, there would seem to be little to recommend a gas turbine. However, in military use economics is not necessarily a prime consideration and, unlike most diesel engines, gas turbines can be brought into action quickly and have high acceleration levels.
In July 2015, GE and Hyundai Heavy Industries announced receipt of Approval in Principle (AiP) for a gas turbine powered LNG carrier. If built the ships would mark the first time that a new version of GE’s improved gas turbine COGES system (Combined Gas turbine, Electric & Steam) has been applied to an LNG carrier.
The two companies say that gas turbine powered LNG carriers will require no additional emission reduction equipment to meet IMO Tier III requirements and that weight requirement with a gas turbine system is about 60% lower compared to diesel engines and costs including lube oil, and pilot burning fuel will be eliminated.