Gas fuelled ships into the future

The ‘Sapphire Blue’ LNG-fuelled cruise ship concept design The ‘Sapphire Blue’ LNG-fuelled cruise ship concept design
Industry Database

The Motorship discusses the way forward for LNG at sea with Oskar Levander, vice-president of innovation, Rolls-Royce.

Ship technology and design moves forward thanks to two main development drivers. First, there is the technology ‘push’: new technologies and materials; new design procedures, which includes such technologies as CFD and simulations; and the ability to handle, analyse and communicate large amounts of data. But this has little purpose without the market ’pull’: in the case of shipping this includes factors such as: regulations and environmental consideratoins; cots of fuel and the possibility of alternatives; cost of manning, and competence levels of the crew; the need to earn revenue; and safety and reliability.

One of the most important ‘pulls’ focusing the attention towards development of new designs and technologies is the introduction of emission control areas, where sulphur, and in the near future NOx, emissions will be very tightly controlled. With the choice of heavy and distillate fuels, with scrubbers and SCR installations cutting emissions when using the cheaper and more polluting fuels, and the possibility for gas fuels, in both dual fuel and pure gas engines, the palette is becoming more diverse. There is no clear answer as to which option to choose, the optimum choice will depend on a host of economic and operational variables, all of which are well documented.

Mr Levander is convinced that LNG as fuel will play a significant role in the future, particularly as ship and engine design evolve to take full advantage of its benefits. The number of LNG-fuelled ships in operation and on order is growing all the time; earlier this year the total passed the 100 mark, covering many different sectors from tugs up to large container ships and tankers.

Rolls-Royce is one of the main players in the LNG-fuelled propulsion arena, and its products – Bergen engines, now part of the Rolls-Royce Power Systems business, along with gas storage and handling systems - fall firmly within the pure gas camp, which the company feels offers significant benefits in certain markets over the dual-fuel option. Wider adoption of gas as fuel will depend on several factors, not least the supply and cost of the fuel, over which the industry has little control. But another important consideration is to have ship designs ready that exploit the advantages of LNG fuel, and this is where the industry can prepare itself.

Mr Levander explained to us a number of new LNG concepts from Rolls-Royce. First is a LNG-fuelled cruise ship, known as ‘Sapphire Blue’. The concept maximises passenger comfort and convenience, with all cabins extending across the whole width of the superstructure, and outside corridors, allowing the best possible sea views and maximum light. Generous space is devoted to public walkways, decks and facilities such as pools and lounges. A modular construction principle is proposed for the passenger accommodation.

Sapphire Blue would have four C-type pressurised LNG tanks in the lower hull, offering full redundancy as well as the ability to cruise for three weeks or undertake a transatlantic passage without refuelling. A typical machinery installation would include five Bergen B:35:40 engines, giving 21.6MW electrical power for both propulsion and hotel loads from 48 cylinders. A range of propulsion options are proposed, using thrusters or pods, with or without a central propeller, including Promas electric drives. These options cater for different priorities in efficiency, manoeuvrability, capital and operating cost, noise and reliability. Given the popularity of cruising in North America, Sapphire Blue would most likely be able to take advantage of the competitive LNG pricing in the US, and Mr Levandar estimates a saving in annual fuel cost of over US$6 million compared with MGO, and even a saving of around US$2 million compared with HFO.

LNG, particularly in conjunction with energy storage such as batteries, should prove equally attractive for North American tow vessels – see Design for Performance in this issue (pages 44-45)- and Rolls-Royce has a concept design ready for such applications.

The cleanliness and low noise and vibration of gas fuelled ferries is now a fact of life, and Rolls-Royce’s two ferry concept designs aim to build on this experience. ’Clear Blue’ and ‘Dynamic Blue’ offer different approaches to future ferries, the former encompassing low investment cost and high efficiency in a minimalistic design, and the latter featuring high-end technology and low operating cost, leading to highest efficiency and optimum revenue-earning potential.

Dynamic Blue has a hybrid mechanical/electrical drive, based on Bergen pure gas engines, efficiency aided by a waste heat recovery system. The mechanical element is a shaft-driven central Promas combined propeller and rudder, supplemented by two Azipull electric wind thrusters. Two C-type gas tanks are mounted low in the hull. The ship is designed with asymmetric superstructure, and drive-through loading on two levels.

The simpler Clear Blue concept also employs Bergen pure gas engines (only three in this instance, and with no steam system or economisers), and a hybrid electrical/mechanical system. This is based on a shaft-driven CP propeller and a single Azipull unit mounted directly aft of the propeller to provide a CRP effect. A rudder provides safe return to port. The basic design is far simpler, with stern-only loading using elevated external ramps, and no moving ramps on the cargo deck. There is a large void space below the main deck and public spaces are concentrated forward for fire safety reasons, while the cabins are in self-supported modules. Cooled containersare used for storage rather than onboard freezers, and the gas fuel is carried in trailers situated astern, with no built-in LNG tanks.

Operational and investment costs for both concepts have been tightly controlled to provide overall costings close to equivalent MGO or HFO fuelled ferries.

Taking a more general look into the future of LNG as ship fuel, Mr Levandar believes that adopting the pure gas propulsion option, where this is feasible, will lead to ships becoming simpler and more efficient. He believes that it should prove possible to remove diesel systems and other non-essential systems, while bunkering LNG will become simpler as alternative concepts emerge. He believes it is wrong to copy fuel oil bunkering solutions – more efficient gas transfer methods will be developed.

The location of the onboard LNG storage tank will depend on economic and operational considerations. Inside-hull tanks will make use of low-value space and aid ship stability. On the other hand, they will need dedicated access and dedicated ventilation systems, while the question of low flashpoint fuel being stored below accommodation has to be resolved. Tanks on deck overcome the access and ventilation problems, but this depends on suitable deck area being available and stability might be a concern.

Another critical question is bunkering. Higher loading rates and larger amounts of LNG will be needed for future gas-fuelled ships, and from the time factor it would seem essential that bunkering can be carried out during cargo operations. Zero gas release must be allowed, and a gas return system will be needed for most tank types. There is, at present, a lack of clear international standards for equipment – work is going on to set standards, but these must be agreed and adopted universally.

Current bunkering options include truck-to-ship, shore-to-ship and ship-to-ship, either from a bunker barge or from an LNG tanker ship. Ship-to-ship would seem to be the best option from the point of view of providing large quantities and offering flexibility in port arrangements. Barges or small LNG tankers should be able to bunker from the sea side while cargo is being handled from the port side. Bunker suppliers and port authorities will need to establish suitable vessels for bunkering LNG – either self-propelled bunker barges, tug-operated barges, or small LNG carrier ships. Rolls-Royce already has designs available for various types and sizes of small LNG carriers.

There are various considerations to be borne in mind with using ships for bunkering. An appropriate level of ‘freshness’ of the LNG must be assured, and the delivery system should be able to cope with different tank types – e.g. atmospheric or pressurised – as well as different pump rates and heads, and vapour return. There has to be a way of dealing with boil-off gases. All of these will incur costs.

Mr Levander said that a full study had been carried out on a 7,500m3 LNG carrier suitable for bunkering, looking at six different tank technologies – A tanks; B tanks; cylindrical, bilobe or spherical C-type tanks; and membrane tanks. It is necessary to optimise both tank and vessel design, as well as look at capital and operating costs in order to fully understand the conditions where each option is applicable.

Other bunkering methods are still likely to be used where suitable, and the use of LNG trucks or containers for onboard fuel storage has attractions for some applications. This offers quick bunkering – changing over containers or trailers is faster than refilling a tank, and no dedicated bunker station is needed. Investment cost is likely to be lower, trailers will not be part of any shipbuilding contract and there could be attractive finance options. On the other hand, the fuel storage will occupy potential cargo deck space.

What is certain is that marine use of LNG will open up new opportunities. With its low emissions and the potential for attractive pricing and more efficient and simpler ships it is, says Mr Levandar, the next major marine fuel. It will certainly be part of the future ship, which is likely to encompass many other innovations for high efficiency, use of renewable energies, energy optimisation and automation. Rolls-Royce says it is determined to continue to introduce novel and more efficient ship concepts based on LNG fuel.


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