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October 2011 www.sname.org/sname/mt Since the launching of Viking Energy , Eidesvik Oshore has continued to expand its LNG-fueled eet of PSVs, and in each case has returned to Wärtsilä Ship Design for the vessel concept and to Wärtsilä dual-fuel engines for the machinery. is would indicate that the original decision to develop an environmentally sound PSV was right, and that the vessels operational record has been successful. is is endorsed by the fact that the original ve-year charter to Statoil has since been renewed and extended. Today, there are six LNG-fueled PSVs in global operation with a further six currently under con- struction. Of these 12 ships, 7 have been designed by Wärtsilä Ship Design, and ten of them are, or will be, powered by Wärtsilä dual-fueled engines. The underlying theme of this development is that, by elim- inating or reducing emissions at source, compliance with International Maritime Organization (IMO) or local regulations becomes that much easier. Development of gas as a marine fuel In general, the regulation of airborne emissions from marine applications is tightening. e IMO is intro- ducing an Energy Eciency Design Index (EEDI), for which LNG oers obvious advantages. However, until now there has been no real enforcement of a transi- tion to more environmentally sustainable maritime solutions and the current eet of LNG-fueled vessels is ahead of the curve as regards global requirements. So too are the stipulations set by pioneering charter- ers such as Statoil. Nevertheless, the trend is already apparent. The use of LNG as a marine fuel is proved to be via- ble and, as the IMO regulations tighten, dual-fuel engines are increasingly a feature of new vessel design concepts. While the background to this development is undoubtedly compliance with envi- ronmental legislation, technical breakthroughs have clearly been the enabler. Although engines capable of burning both a liquid and a gaseous fuel were invented before the Second World War, their practical use is far more recent. The first generation of modern gas/diesel engines was developed in the late 1980s. These ran on a die- sel cycle, were tolerant of fuel quality, and used high-pressure gas. The second generation was intro- duced in the early 1990s as spark-ignited gas engines. These employed the Otto cycle, were mono-fuel, i.e. gas only, with the engines running on low-pressure gas. These were aimed primarily at meeting land- based powerplant needs, where fuel flexibility is not a major requirement. The real breakthrough came with the third generation of dual-fuel engines, intro- duced by Wärtsilä in 1995. These employ both the Otto and diesel cycles, use low-pressure gas, and combine fuel flexibility with environmental perfor- mance and fuel efficiency. By applying modern electronic combustion con- trol, this technology has paved the way to improved performance and greater reliability for gas engines. Viking Energy boasts four 6-cylinder Wärtsilä 32DF dual-fuel engines and is the worlds ?rst PSV to be powered by LNG. is dual-fuel capability means that when running in gas mode, emissions of nitrogen oxides (NO X) are reduced by 85% compared to diesel operation.