The gas tank is one of the single most expensive elements in an installation. In addition, the
full redundancy required
under some class rules can dictate the added complication of power take-in arrangements on each main shaft; and even, in some applications, a second auxiliary set running on MDO – thus negating the main advantage of a single-fuel gas installation.
A pure gas-fuelled vessel could also require 48 per cent more gas storage capacity than a DF vessel to secure an identical operational endurance (and allow for higher power demands in bad weather). Furthermore, the total cost of a DF installation will be significantly less than that of a gas-only system. Another cited merit is that a DF plant allows a vessel to be transferred more easily from one operational area to another, without concern over a limited LNG supply infrastructure. Future developments in medium speed DF engine technology, Wärtsilä reports, will focus on reducing methane slip, and work will also be undertaken on further engine optimisation, which can be achieved through software tuning in the control system and hardware optimisation. Excellent experience is reported from Viking
Energy, the world’s first LNG-fuelled supply vessel, which was delivered by Kleven Maritime in April 2003 for a 10-year charter to Statoil in North Sea service. The 94.9m-long, 20.4m-wide VS 4403 PSV-class vessel was designed by Vik-Sandvik and specified with an electric power station based on four gensets, each driven by a 6-cylinder Wärtsilä L32DF engine developing 2,010kW at 720 rpm. The engines are arranged to run on LNG and/or MDO in any proportion, with automatic switching without power interruption. After nine years’ operation, owner Eidesvik Shipping has reported 100 per cent reliability from the machinery, which has run on gas for around 97 per cent of the power generating time; and not a single hour of technical off-hire due to the gas system had been logged. Diesel fuel mode is only engaged during LNG refuelling and certain minor preventative maintenance procedures or when triggered by alarms. Viking Energy was followed into service by other LNG-powered PSVs commissioned by Eidesvik, Simon Møkster, DOF, Solstad Rederi, Olympic Shipping, Island Offshore and REM Offshore. All are classed by DNV. Eidesvik’s Viking Prince, a Wärtsilä VS 489 Gas PSV- class vessel completed in 2012 by Kleven Maritime, features an electric power station based on a pair of W34DF-driven and a pair of W20DF-driven gensets. Twin 2,450kW azimuth thrusters provide the 89.6m-long design with a speed of 16.4 knots. Harvey Gulf will deploy the first US-flagged PSVs with gas propulsion
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and become the world’s second largest operator of such tonnage when its five STX Marine SV310DF-class newbuildings are delivered by Trinity Offshore from 2013. Each ABS-classed vessel will be powered by 6-cylinder W34DF- driven gensets. The number of LNG-fuelled PSVs in service or on order was extended to 19 in December 2012 with the fifth Harvey Gulf contract. The previous month saw Wärtsilä secure a contract to supply power and propulsion systems for a newbuilding at Hellesøy Verft; this Wärtsilä ship design VS 4411 LNG-class vessel will be owned and operated by Siem Offshore (its first LNG-fuelled commitment) after delivery in 2014 for charter to Total and service in the Martin Linge oilfield, north west of Stavanger. Of those 19 gas-powered PSVs, Wärtsilä has designed nine and supplied or will supply dual-fuel machinery to 17; seven of the vessels are also specified with Wärtsilä integrated gas supply systems.
Another well-known enginebuilder, MAN Diesel & Turbo can also now target OSV propulsion opportunities with the 35/44DF series, a 350mm-bore dual-fuel design derived from the German group’s established 320mm-bore 32/44 medium speed diesel engine. The newcomer complements MAN’s 510mm-bore 51/60DF engine which has references in LNG carrier propulsion and power generation on floating production, storage and offloading (FPSO) vessels. Testing of the 35/44DF prototype started in 2012 and full production is planned in 2014. A rating of 530kW per cylinder will enable in-line 6- to 10-cylinder models to cover an output range from 3,200kW to 5,300kW at 750 rpm. V-cylinder (12 to 20) versions will eventually extend the upper limit to 10.6MW. As Mr Shock explained, Caterpillar Marine
Power Systems is initially targeting the gas market with a dual-fuel derivative of its MaK M43 medium speed diesel engine. The new 460mm-bore M46DF series shares the same
Terje Nordtun: dual-fuel technology is Tier III compliant and suitable for ECAs
footprint as the successful 430mm-bore design, fostering simple retrofit projects for existing installations. In-line 6, 7, 8 and 9-cylinder models with a rating of 900kW per cylinder at 500/514 rpm are due for commercial availability from the first-quarter of 2014, with V-cylinder versions later doubling the power threshold of the range.
Caterpillar is simultaneously developing
an LNG-fuelled high speed engine for outputs of under 2,000kW, designed primarily for electric propulsion and offshore and ferry applications. The US group also plans platforms in the 2,000-3,000kW power range and a 320mm-bore dual-fuel contender (M 3X DF). Both MAN and MaK DF challengers will meet IMO Tier II NOx emissions limits in diesel mode (burning heavy fuel, marine diesel or marine gas oil) while operation in gas mode will satisfy Tier III limits. Other engine builders such as Rolls-
Royce are also working on pure gas engine installations, and the company was recently boosted by US EPA approval for the group’s Bergen C-series medium speed designs to be sold in the country. The latest 260mm-bore C26:33 gas engine is offered in 6, 8 and 9-cylinder configurations to cover a maximum power
band from 1,460kW to
2,430kW.
Outputs up to 7,000kW can be delivered by the 350mm-bore Bergen B35:40 gas engine in in-line and V-cylinder versions. The first offshore vessel to feature lean-burn gas main engines entered service in 2012, Island Crusader being the first of a pair of UT 776 CDG (Clean Design Gas)-class PSVs from STX Offshore Norway for
Island Offshore.
John Shock: dual-fuel and pure gas have differing applications
A quadruple-genset installation for electrical propulsion and power generation incorporates two 9-cylinder C26:33 gas engines and two 6-cylinder C25:33 diesel engines, the latter pair being used when the vessel is deployed away from gas fuel supplies. OSJ
Annual Offshore Support Journal Conference and Awards 2013 I 19
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