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More LNG owners choose low-pressure, two-stroke engines

Fri 09 Mar 2018 by Mike Corkhill

More LNG owners choose low-pressure, two-stroke engines
Terntank’s 15,000 dwt chemical/product tanker Ternsund is the first LNG-powered vessel with a low-pressure, two-stroke engine

Of the 13 conventional LNG carriers contracted in 2017, four were specified with low-pressure, dual-fuel, two-stroke engines developed by Winterthur Gas & Diesel (WinGD). The current LNGC orderbook now features 16 vessels which will be powered by these units, which WinGD terms its Generation X dual-fuel (X-DF) engines.

The newbuildings will join the two LNG carriers with low-pressure, two-stroke propulsion systems currently in service. These are the 14,000 m³ Hua Xiang 8, which is powered by a Wärtsilä 5RT-flex50DF unit, a precursor of the WinGD dual-fuel X-DF technology, and the 180,000 m3 SK Audace which is propelled by a pair of WinGD 6X62DF engines. Both ships were completed in 2017.

Two-stroke breakthrough

The propulsion system of choice for the majority of LNGC newbuildings was the dual-fuel diesel-electric (DFDE) option, with a set of four-stroke, medium-speed diesel generators, from around 2002 until December 2012. The order that month, however, for the first vessel with MAN’s high-pressure diesel engines marked the start of the two-stroke, dual-fuel power train era.

Two-stroke, low-speed engines of the MAN high-pressure and WinGD low-pressure types offer major propulsive efficiency advantages over both the DFDE technology and steam turbines, the most popular propulsion system choice during the early days of LNG transport.

Initial LNG shipowner interest in two-stroke, dual-fuel propulsion was focused primarily on the MAN high-pressure unit, otherwise known as its mechanically operated, electronically controlled, gas-injection (ME-GI) diesel engine. Daewoo Shipbuilding & Marine Engineering (DSME) was particularly successful in gaining newbuilding orders by marketing an ME-GI propulsion package which also included its inhouse designs for a high-pressure fuel gas supply system (FGSS) and a partial reliquefaction plant.

While MAN’s ME-GI option remains a popular LNGC propulsion system choice, the WinGD X-DF technology has been gaining ground recently, as highlighted by the current 16-ship orderbook. Both propulsion systems are now also being specified to power larger LNG-powered vessels that are not LNG carriers, including container ships and tankers.

Low-pressure advantages

WinGD was established as a 70/30 joint venture company established by China State Shipbuilding Corp (CSSC) and Wärtsilä in January 2015 to take over Wärtsilä’s low-speed, two-stroke engine business, and is now fully owned by CSSC. The origins of the two-stroke technology can be traced back to Sulzer, the Switzerland-based engine manufacturer acquired by Wärtsilä in 1997.

WinGD’s new Generation X engines employ lower rated speeds to reduce both fuel consumption and wear while maintaining power outputs comparable to their predecessors. The X-DF dual-fuel version uses LNG delivered to the engine as low-pressure gas.

The X-DF technology is based on the lean-burn Otto cycle, in which a compressed lean air-gas mixture is ignited through the injection of a small amount of liquid pilot fuel. Under the micro-pilot ignition concept, which is the global standard for the four-stroke engines that drive DFDE propulsion systems, the pilot fuel accounts for only 1% of the overall volume of fuel used.

WinGD states that the concept results in significant reductions in nitrogen oxide (NOx) emissions compared with alternative engine types and enables compliance with IMO Tier III NOx limits in emission control areas (ECAs) without the need for the vessel to be fitted with exhaust after-treatment equipment. The first demonstration run of a large-bore X-DF engine, in April 2015 in co-operation with Diesel United of Japan, verified the performance capabilities.

ME-GI engines, which run on the diesel cycle, offer important advantages such as the abilities to deliver the same output as conventional diesel engines; to burn gas from any source, irrespective of the methane number; and to provide high levels of efficiency at partial loads.

However, as WinGD points out, some of the X-DF technology’s shortcomings in those areas where ME-GI offers advantages need to be offset by considering the overall propulsion system performance rather than just that of the main engine as a stand-alone unit. 
For example, ME-GI engines require a sophisticated FGSS to inject gas into the cylinders at 300 bar. The piston compressor set needed for a high-pressure FGSS can result in a compressor skid that weighs six times that of the unit utilised in the WinGD’s 16-bar system.

The piston pumps that feature in the high-pressure FGSS are also more sophisticated and require more maintenance than the simple centrifugal LNG pumps used in the WinGD FGSS.

WinGD estimates the capital cost of a propulsion system for an ME-GI LNG carrier could be up to 40% greater than that for a similar-sized vessel with a low-pressure, two-stroke power train, due to the need for exhaust gas treatment facilities, a more elaborate, energy-intensive FGSS and more robust engineroom feed gas pipework. For LNG-fuelled merchant ships the price disparity would fall to 15% but the advantage still lies with the X-DF option.
X-DF rollout

Eight of the 16 LNGC newbuildings specified with WinGD engines will be completed in 2018. One of the first to be delivered will be SK Resolute, a sister ship of SK Audace. Both were built by Samsung Heavy Industries for a SK Shipping/Marubeni joint venture and chartered to Total.

In August 2016 South Korea’s SK Shipping ordered a second pair of 180,000 m3 LNGCs with WinGD propulsion systems, this time at Hyundai Heavy industries (HHI). In contrast to SK Audace and SK Resolute, each equipped with a pair of 6X62DF engines, the HHI duo will each be fitted with two five-cylinder, 72cm-bore (5X72DF) units.

Other shipowners besides SK Shipping that have opted for WinGD engines for several of their recent LNGC newbuilding orders are GasLog, Mitsui OSK Lines and TMS Cardiff Gas. These companies believe that their engine choice will not only ensure compliance with all existing and likely future emissions regulations but also bring long-term savings through reduced fuel and maintenance costs. 

The French liner service operator CMA CGM achieved a major breakthrough in the use of LNG as marine fuel in November 2017 when it specified WinGD dual-fuel engines for nine new 22,000 TEU container ships. They are not only the largest vessels of this type ever ordered but also the largest ships that are not LNG carriers to be powered by LNG. Each ship in the series will be propelled by a 12X92DF unit. Their rating – 63,840kW at 80 rpm – makes them the largest gas-burning engines ever contracted.

Terntank placed the first ever order for a low-pressure, two-stroke, gas-burning engine to propel a ship in December 2013. The tanker operator specified an RT-flex50DF engine for each of a pair of 15,000 dwt coastal product/chemical tankers it had contracted at the Avic Dingheng yard in China. The deal was subsequently boosted to four ships, all of which are destined for operations in North and Baltic Sea ECAs.

X-DF engines were also chosen for the first-ever gas-fuelled Aframax crude oil tankers to be contracted. Each of the four 114,000 dwt ice-class 1A tankers that Sovcomflot odered at HHI in March 2017 will be powered by 7X62DF engines. Since the Sovcomflot order, nine more Aframax tankers and two twin-screw Suezmax shuttle tankers have been specified with similar propulsion systems.

WinGD reports that 83 X-DF engines have been ordered to date, eight of which are in operation. The orderbook is just about evenly split between the LNG carrier and LNG-powered ship sectors.

As more owners become aware of the benefits of the low-pressure, two-stroke technology, not only will the number of engine orders grow but also the X-DF share of the gas-powered vessel market.

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