JAPANESE engine maker, Mitsubishi Heavy Industries, has redesigned its steam turbines to provide more efficiency as it fights against diesel electric in the LNG sector, writes David Tinsley.
Reliability, easy maintenance and use, along with the possibility of a steam turbine installation to easily use cargo boil-off, ensured that LNG carrier propulsion remained almost entirely a steam province for 40 years.
However, with technical innovation and LNG cargo prices, a seachange has now taken place. Low-speed diesel propulsion machinery in combination with shipboard reliquefaction systems has become the technology of choice for the emergent generation of ships over 200,000 cu m, and the dual-fuel diesel-electric solution has wrested huge newbuilding market share from conventional steam turbine power in the under 200,000 cu m range.
Japanese engineer Mitsubishi has now revitalised the steam offering, with a higher-performance steam turbine claimed to confer efficiency improvements of upto 15%.
The safety record of steam-powered LNG vessels has been one of the best throughout the shipping industry.
However, a need by gas tanker operators for more flexible powering arrangements, coupled with higher power densities to suit a new generation of larger LNG carriers, stimulated the uptake of alternatives, and ended the dominance of the steam turbine in the newbuilding market.
Wärtsilä was the prime mover in introducing LNGC electric propulsion based on dual-fuel medium-speed engines of the 50DF type, and dual-fuel diesel-electric has become the system of choice for many operators.
BP Shipping’s 155,000 cu m British Emerald put down a new milestone for the industry last year, as the first LNG carrier to incorporate dual-fuel diesel-electric propulsion.
The GTT MkIII membrane-type tanker has four Wärtsilä DF50 engine-based generators, feeding electrical energy to a pair of 14.8 MW synchronous propulsion motors. MAN secured an opening contract for its 51/60DF dual-fuel engine targeted at the LNG carrier market. Five engines of the 51/60DF type have been selected to power a 173,600 cu m newbuilding booked by a Spanish owner from STX Shipbuilding.
Just as Japan’s position as the world’s leading maker of steam turbines for commercial marine applications underpins Japanese shipbuilders’ still widespread use of such plant, South Korean yards’ adoption of alternative systems can be expected to accelerate with the scheduled launch later this year of a new factory in Korea to produce the Wärtsilä DF50 engine.
The joint venture Wärtsilä Hyundai Engine Company will serve clients throughout Korea, Japan, China, and Taiwan, effectively bringing the entire field of large LNG tanker production within the ambit of the joint undertaking.
Since achieving its breakthrough in the LNG carrier market in 2004, MAN’s electronically controlled, two-stroke ME engine family has been specified for every project involving ships in excess of 200,000 cu m capacity. Recent months have seen the commissioning of the initial vessels in the total of 45 Q-Flex and Q-Max newbuildings entrusted to Hyundai, Samsung and Daewoo, and destined for the Qatar export trade.
Each of the 210,000 cu m Q-Flex and 266,000 cu m Q-Max ships has been specified with two MAN low-speed engines and onboard reliquefaction plant.
As well as maximising the value of the delivered cargo, onboard reliquefaction paves the way to the use of diesel propulsion and diesel engines of up to 50% thermal efficiency, compared with approximately 30% for a steam turbine plant.
The higher efficiency results in lower energy consumption and reduced operating costs. Determining the relative worth of boil-off can be difficult under conditions of varying gas and fuel oil prices. Some operators, owners, charterers and supply companies may opt to burn the evaporated gas in the engine.
MAN is therefore also promoting its dual-fuel, electronically-controlled ME-GI engine for LNG ships, which is able to run on either natural gas or heavy fuel oil.