LNG Owners Choose Low-pressure, Two-stroke Engines


The present LNGC orderbook features 16 vessels which will be powered by these units, which WinGD terms as its Generation X dual-fuel (X-DF) engines.

Of the 13 conventional LNG carriers contracted in 2017, four were fitted with low-pressure, dual-fuel, two-stroke engines developed by Winterthur Gas & Diesel (WinGD).

These newbuilds will join the two LNG carriers (in service) fitted with a low-pressure, two-stroke propulsion systems. They 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, 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 at its heart, from around 2002 until December 2012. However, during the same month, 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 over the DFDE technology and steam turbines, which are 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 – 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 more recently, as highlighted by the current 16-ship orderbook. Both propulsion systems are now also being specified as the means of powering a growing array of larger LNG-powered vessels that are not LNG carriers, including container ships and tankers.

Low-pressure advantages

WinGD’s new Generation X engines operates on lower-rated speeds to reduce both fuel consumption and wear, while maintaining power outputs at par with its predecessors. The X-DF dual-fuel engine version uses LNG fuel in the form of low-pressure gas.

The X-DF technology uses lean-burn Otto cycle, in which a compressed lean air-gas mixture, ignited through the injection of a small amount of liquid pilot fuel. The pilot fuel accounts for only 1% of the overall volume of fuel used.

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. It has the ability to burn gas from any source, irrespective of the methane number and to provide high levels of efficiency at partial loads.

X-DF vs ME-GI engines

However, WinGD points out that some of the X-DF technology’s shortcoming were overcome by ME-GI, the overall propulsion system performance is the prime concern.

For example, ME-GI engines require the use of a sophisticated FGSS to inject gas into the cylinders at 300 bar. The piston compressor needs a high-pressure FGSS which can result in a compressor that weighs six times that of the unit utilised in the WinGD’s 16-bar system.

Sophistication in design

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

WinGD estimates that 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.

It is because of the need for exhaust gas treatment facilities, a more elaborate, energy-intensive FGSS and more robust engine room 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.

Win GD’s X-DF engine 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 sistership 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 from Hyundai Heavy industries (HHI). In contrast to SK Audace and SK Resolute, each of which is equipped with a pair of 6X62DF engines, the HHI duo will each be fitted with two five-cylinder, 72cm-bore (5X72DF) units.

Besides SK Shipping, companies such as GasLog, Mitsui OSK Lines and TMS Cardiff Gas also opted for WinGD engines in their recent LNGC newbuilding orders.

These companies believe that their engine choice will not only ensure compliance with all present and future emission regulations, but also bring long-term savings through reduced fuel and maintenance costs.

Win GD’s charters

The Hyundai newbuildings will be chartered to SK E&S and used to lift cargoes from the Freeport LNG terminal in Texas. The facility’s location is within the North American ECA and the ability of the WinGD engines, when running on gas, to meet the Tier III NOx emission rules without the need for exhaust gas treatment equipment was a key factor in the choice of propulsion system for the vessels.

The French liner service operator CMA CGM achieved a major breakthrough in November 2017 as it used WinGD dual-fuel engines for their nine new 22,000 TEU container ships. Each ship in the series is propelled by a pair of 12X92DF units with a power of 63,840 kW at 80 rpm.

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.

X-DF engines were 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 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 are fitted with the similar propulsion systems.

WinGD orders

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, the number of engine orders will grow and also increase the X-DF share of the gas-powered vessel market.

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Source: LNG World Shipping


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