the growing number of ECAs around the world. They can switch to low sulphur fuel, install scrubbers to remove the sulphur from the exhaust gas or switch to LNG as fuel. At DNV, we believe that implementing
LNG as bunker fuel represents an attractive and cost-effective solution for reducing air pollution and securing ECA compliance for new ships. An LNG fuelled ship reduces the emissions of NOx by 85-90% (depending on engine type) and SOx and particulates by close to 100% compared to the use of conventional fuel. In addition, LNG fuelled ships come with a 15-25% reduction in net greenhouse gas emissions. However, for existing vessels, this solution is less attractive due to both the complexity and costs associated with a conversion to run on LNG. Marine Gas Oil and Marine Diesel Oil
can be supplied with sulphur content below 0.1% and switching to such fuel only requires minor modifications to the fuel system on board ships. However, the availability of low sulphur fuel is already limited and rising demand is expected to increase its price uncertainty. A scrubber can be installed to remove
sulphur from the engine exhaust gas using chemicals or seawater. Scrubbers require significant alterations on board including additional tanks, pipes, pumps and a water treatment system. Te sulphur rich sludge produced is categorised as special waste to be disposed of at dedicated facilities. But scrubbers also increase
power consumption, thereby increasing CO2
emissions. Other hydrocarbon gases could be
considered as bunker fuel but for example LPG (liquefied petroleum gas) is more expensive than LNG and supply is limited. It also carries safety concerns as it has different flammability and dispersion properties to LNG. Another alternative might be CNG (compressed natural gas). Te existence of distribution grids is an advantage for CNG but twice the volume of CNG is required to achieve the same sailing distance as LNG. Biofuel blends and alternative energy
sources such as wind power, fuel cells and nuclear power still face significant technical and economic challenges and require implementation timetables longer than that of LNG. Biofuel, for example still has unresolved problems relating to sustainability, fuel instability, corrosion, susceptibility to microbial growth and poor cold flow properties. Te main barriers to the adoption of
nuclear energy for commercial shipping relate to societies’ reservations, to the uncontrolled proliferation of nuclear material, to decommissioning and to the storage of radioactive waste. Tese challenges present significant investment costs and would need to gain greater acceptance from governments and the general public before nuclear energy would become viable. In summing up the alternatives,
LNG today offers the best economics to shipowners and the best environmental
effect for society when considering ship newbuildings. Natural gas is the cleanest form of fossil fuels available, and when fuelling a ship with LNG, no additional abatement measures are required in order
to meet ECA requirements.
LNG is considerably cheaper on a per-energy-content basis than oil-based fuels and, taking a 20-year perspective, a conservative lifetime for a ship, it is estimated that
the LNG alternative
has the lowest net present value cost compared with the other alternatives outlined above. The technical performance of LNG
as a fuel for shipping is proven daily by more than 20 ships in operation in Norway. The first ship with LNG propulsion, the passenger ferry Glutra, was launched in 2001. Since then, more ferries and offshore supply vessels have been built, along with three patrol vessels for the Norwegian Coast Guard. Together, these ships demonstrate the application of a wide range of engine set-ups and manufacturers. Moving from the ships already sailing
on LNG to the current orderbook of around 20 ships, we see a greater variation again in ship types. The list contains ro-ro vessels, passenger ships, liquid bulk tankers, special cargo ships and high speed ferries. As these ships are being delivered over the next couple of years they will add to the experience base for LNG fuelled ships and lead to further design optimisation. Gas fuelled engines are now
available from several suppliers, e.g. 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 dual fuel engine, which runs on both LNG and conventional fuel, is a flexible solution when the availability of LNG fuel is uncertain. In LNG mode, these engines only consume a minor fraction of conventional fuel. The lean burn mono fuel engine gives a simpler installation onboard when LNG availability is secure.
Cutaway showing the tank configuration for a diesel fuel bulker.
The Naval Architect January 2012 23
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