Diesel-electric power is almost considered the norm for many types of offshore support vessels (OSVs), with most ships having four or six diesel gensets and a pair of main propulsion motors, sometimes driving through a gearbox but often directly linked to the main propulsors, especially where the thrust is provided by azimuth thrusters rather than conventional propellers.
Multiple gensets allow total power output to be matched to demand with the engines running at optimum speed. When less power is needed, there is no need for all the engines to be running, thus allowing for fuel savings. The number of engines also confers a high degree of redundancy on a diesel-electric ship where even one engine will permit the vessel to maintain some controlled motion in all but the heaviest of seas.
The flexibility that diesel-electric propulsion can bring does come at a price, with open seas propulsion efficiency being adversely affected by energy losses in the additional equipment that is necessary. However, for an offshore vessel, where open seas propulsion accounts for perhaps just one-third of the use of the engines, this is much less of a problem because of the optimum running of engines that can be managed at other times. A further benefit comes in the flexibility of layout, as there need not be a clear direct line between engine and propellers. Thus, engines can be located in line of one and another or offset in a layout that can take account of other needs of the vessel.
As diesel-electric has become more popular, so system makers have striven to improve the efficiency and reduce costs simultaneously. The result is a proliferation of concepts from enginemakers and others. Among these are Wärtsilä’s Low Loss Concept (LLC), Siemen’s BlueDrive PlusC, MAN’s EPROX (electric propulsion excellence) and ABB’s Onboard DC Grid.
Wärtsilä LLC dates back to 2004 when a system was ordered for the VS4420 platform supply vessel (PSV) Normand Skipper and has been in continuous development since. Before the advent of the system, the usual way to reduce the harmonic distortion in the distribution network caused by the frequency drives of the propulsion motors was to place a 12-pulse transformer between each frequency drive and the switchboard. Wärtsilä devised a way to overcome this problem by splitting the distribution bus into two sections and placing a single transformer between the two buses.
With Siemen’s BlueDrive PlusC system developed in 2011, individual speed control of each engine over the whole engine speed region is possible. BlueDrive PlusC is based on fully integrated power distribution, with the main switchboard and all drives collected into one compact unit. The main switchboard has in/out AC voltage and supplies clean power to other switchboards. Engine speed is controlled to optimise fuel consumption and reduce load-deviation issues.
Over the past few years, DC distribution systems have been promoted as an alternative that has benefits in simplifying the distribution and reducing components and an added advantage if batteries are to be incorporated into the system. As Siemens and others have noted, battery technology is seen by many as a very promising energy efficiency measure for OSVs and many other vessel types. While they can be integrated into a DC distribution system more easily, their use is not confined to DC grid systems alone. Siemens’ revolutionary propulsion system uses variable rotational speed with optimal operation of the diesel generator in combination with batteries to significantly reduce fuel consumption and the emissions of NOx and greenhouse gases. Siemens claims that the variable speed propulsion concept reduces total energy consumption by 15 per cent compared with earlier diesel-electric systems and by 23 per cent compared to gas/dual-fuel engines. Total NOx and greenhouse gas emissions are lower compared with vessels using conventional diesel-electric or gas-powered solutions. The company says a comparison with the results obtained with dual-fuel offshore vessels with the same operating profile showed that greenhouse gas emissions were 27 per cent lower for the variable speed diesel engine alternative.
MAN has taken a rather different approach with its EPROX system. In a recent presentation, Bernd Friedrich, head of power trains and auxiliary equipment at MAN Diesel & Turbo SE, highlighted the highly flexible and redundant features of a conventional multi-engine diesel-electric arrangement but drew attention to some of the issues with such an arrangement, notably the question of losses between the fuel and combustion to the shaft power achieved.
He described MAN’s new system in which, as he put it, “good old DC is coming back” and existing components are arranged in a new way. As he explained, a DC system removes the switchboard usually found in a conventional AC arrangement, and the alternators are connected via rectifiers. The propulsion motors are connected and their speed controlled via inverters. The diesel engines can operate independently, and no synchronisation of the alternators is required. In the EPROX, it is also possible to integrate energy storage devices such as batteries, with the batteries connected to the DC grid via DC/DC converters. These energy storage sources can be used to reduce transient loads on the engines and help provide a faster, more dynamic system response. Load peaks are ‘shaved’ and buffered by the batteries. Mr Friedrich concluded that this new energy-saving solution would facilitate significant reductions in fuel consumption by operating the engines at variable speed. Energy storage would provide an additional degree of freedom in diesel-electric plant design, and he described additional measures that could be used to improve a diesel engine’s dynamic performance, such as jet assist and boost injection.
The first use of a battery onboard an OSV was on Eidesvik’s Viking Lady two years ago. Eidesvik has been an enthusiastic supporter of innovation, and the ship itself has been used as a test bed for liquefied natural gas, fuel cell and battery technology. The battery system onboard Viking Lady was supplied by Canadian battery specialist Corvus and comprises 68 packs each rated at 6.5 kWh for a total capacity of 442 kWh. It is used to aid the DP system of the ship.
In May this year, Eidesvik announced a new battery retrofit project, this time on the dual-fuelled Viking Queen – a 2008- built 6,200 dwt PSV powered by four Wärtsilä 6L32DF engines. The project will be supported by the vessel’s charterer, Lundin Norway, and Eidesvik along with the system supplier, Norway-based Zem Energy, a newcomer to the marine scene as far as battery systems are concerned that has been selected to supply the battery system.
The energy storage system (ESS) will have a capacity of 650 kWh and can supply up to 1,600kW, which it is hoped will give a fuel saving of approximately 18 per cent for the vessel, with associated reductions in emissions.
To Zem Energy, the contract for delivery of a high efficiency battery system for Viking Queen represents a breakthrough, as CTO Egil Mollestad noted. “Zem has, through several years of research, development and advisory work, built comprehensive competence on battery systems. Therefore, we are now able to deliver competitive solutions to the maritime industry. We are proud that such a leading offshore company as Eidesvik has chosen us,” he said.
Salman Farmanfarmaian, Zem Energy’s co-founder, said the company is working with Nidec ASI, which is supplying the power electronics, and with Wärtsilä to integrate into the ship’s power management system. The battery system will be housed in two containers and will weigh around 32 tonnes. Mr Farmanfarmaian also said that the battery pack will have active cooling and that, for several years, Zem Energy has been continuously testing different battery cells to understand, quantify and model their behaviour – notably the heat generated by different load cycles and their effect on the battery’s degradation. “For this application, we believe it is important to actively cool each and every battery cell individually,” he said, adding that the cold northern waters certainly help to achieve an effective cooling solution.
Eidesvik is not the only offshore operator showing an interest in batteries. In April this year, operator Østensjø Rederi announced that, along with Corvus Energy and Siemens, it would extend a partnership with a second offshore vessel project. The new 150m MPSV, to be named Edda Freya, will incorporate a 1050 VDC, 546 kWh Corvus ESS consisting of 84 Corvus Energy AT6500 advanced lithium polymer battery modules.
This new project follows on the heels of the Edda Ferd hybrid PSV project completed by the same partners in 2014. The Corvus ESS will again be integrated with the Siemens BlueDrive PlusC electric propulsion solution that will provide efficient hybrid propulsion to the new MPSV and also provide backup power in the case of blackout. The two vessels are part of Østensjø Rederi’s environmental concept Mindset. The vessels optimise the use of diesel generators and the ESS to significantly reduce fuel consumption and emissions, allowing a 20–25 per cent energy saving over comparable vessels.
As also highlighted elsewhere in this supplement, the most recent member of the ‘battery pack’ is Icelandic operator Fafnir Offshore, which was another OSJ award winner, having picked up the OSJ Environmental Award for their battery-powered PSV, a Havyard 833 WE ICE design that has its battery supplied by Norwegian Electric Systems (NES).
Article published in the Guide to OSV Propulsion, a supplement to Offshore Support Journal