Tighter limits on air emissions are driving a move towards powering ships with gas instead of oil. The first container ship trading worldwide using LNG as fuel will be the 5,000TEU 'Maersk Drury'. Two of her four main MAN Diesel generators are currently being converted for dual-fuel operation and will be used when the ship is in port or sailing through emission control areas (ECAs).
An LNG-fuelled ship reduces nitrogen oxide (NOx) emissions by 85-90 percent and sulphur oxide (SOx) and particle emissions by close to 100 percent compared with the use of conventional fuel. This means they comply with the Tier III emissions reductions being imposed by the International Maritime Organisation (IMO), effective after 2016, which specify an 80 percent reduction of NOx in ECAs. In addition, LNG-fuelled ships can reduce net greenhouse gas emissions by around 20 percent.
Despite the environmental credentials of LNG, drawbacks currently exist for its widespread adoption. Classification society Germanischer Lloyd suggests that LNG power adds around 15 percent to the cost of a newbuilding. The elimination of equipment like heavy fuel oil service tanks, pre-cleaning and pre-heating equipment, clean oil tanks, fuel oil drain tanks and oil-water separators reduces costs but this is offset by the need for more sophisticated fuel tanks and insulation.
"Gas power for ships will reduce emissions dramatically," says Andrea Cogliolo, head of the machinery sector of Italian classification society RINA. "But we must ensure that lower emissions don't mean less safety. Gas carriers have a good safety record, but they also have crews with special training, a culture of safety, and experience with gas on ships, which is not found on most other ship types."
Industry group BIMCO warns that the green credentials of LNG may seem impressive but may not be the whole story when the true carbon cost of extracting the LNG, liquefying, storing and transporting it are taken into account. Its availability will be a serious problem for some time, though it might be expected that distribution networks, tankage and floating bunker vessels capable of supplying it will be provided to answer any demand, reports the organisation.
Gas-fuelled engines are now available from several manufacturers including Wärtsilä, Rolls-Royce, MAN Diesel and Mitsubishi Heavy Industries. There are two main engine concepts: dual fuel engines and LNG lean burn mono fuel engines.
The first LNG-fuelled passenger ferry 'Glutra', operated by Møre & Romsdal Fylkesbåtar of Norway, was launched in 2001 with four Mitsubishi GS12R-PTK ultra lean-burn natural gas engines. Now there are over twenty gas-fuelled ships in operation in Norway and the orderbook globally is around twenty vessels, including ro-ro vessels, passenger ships, liquid bulk tankers, special cargo ships and high-speed ferries.
Currently under construction is a dual-fuel high-speed passenger ro-ro being built by Incat Tasmania of Australia for South American owner Buquebus. In each catamaran hull a GE Energy LM2500 gas turbine will drive a Wärtsilä LJX 1720 waterjet, a departure from the usual use of two engines and two waterjets per hull used in diesel-powered Incat vessels. The new vessel will be the first to be built under the High Speed Code that is powered by gas turbines using LNG as the primary fuel and marine distillate for standby and ancillary use.
Two new platform supply vessels are being built by STX Offshore Norway for Norwegian owner Island Offshore. Rolls-Royce has developed a gas-electric diesel-electric propulsion system featuring two dedicated gas generators and two liquid fuelled generators. Rolls-Royce claims the LNG storage tanks have not impacted the vessels' dead weight or cargo volume.
Large cargo vessels are expected to adopt LNG propulsion in the future and design work is already underway. For example, Oshima Shipbuilding and DNV are developing ECO-Ship 2020, a concept design for an open-hatch bulk carrier featuring lean-burn four-stroke medium-speed gas engines. The vessel has been specifically designed to be fully compliant with future ECA and Tier III emission requirements.
Wendy Laursen