Of the three options for meeting the 2020 sulphur rules, LNG would seem to be ideal as it contains no sulphur and thus engines running on it cannot produce SOx. Proponents of LNG have been forecasting its role as the fuel of the future for most of the 21st century but the lack of international standards and rules has been an impediment to a greater take-up
although that is now changing.
LNG-burning engines have been used for onshore power generation for many years but their use for marine purposes is a more recent phenomenon. Initially, they were marketed almost solely as an alternative to the steam turbines in LNG carriers then later as a solution to meeting increasingly stringent exhaust emission requirements.
In spite of its attractions on environmental grounds, LNG has had a slower take up than proponents expected. There are many reasons for this including lack of bunkering infrastructure, higher capital outlays, LNG’s lower energy density compared to oil fuels and a lack of international regulation as to the use of gas as a fuel.
Those disadvantages are gradually being addressed and while the second two will remain an issue for shipowners to decide on merits, the first is underway and the fourth has been resolved by the IMO which in 2015 adopted the International Code of Safety for Ships using Gases or other Low-flashpoint Fuels (IGF Code), along with amendments to make the code mandatory under SOLAS with effect from 1 January 2017.
The impending introduction of the 2020 global sulphur cap has seen a growing acceptance of LNG as a marine fuel and the number and type of ships employing dual-fuel engines has advanced. Today there are dual-fuelled vessels in the largest container carriers, bulkers and tankers and cruise ships as well as the smaller vessel types that were early adopters.
Because gas and other low-flashpoint fuels pose their own set of safety challenges and prior to the IGF Code their regulation was only possible by individual flag states, there had been no universal standard. The IGF Code addresses this and has le
ad to more gas and dual-fuel ships being built.
The amendments to SOLAS chapter II-1 as a result of the IGF Code include changes to Part F ‘Alternative design and arrangements’. These provide a methodology for alternative design and arrangements for machinery, electrical installations and low-flashpoint fuel storage and distribution systems while a new Part G Ships using low-flashpoint fuels, to add new regulations to require ships constructed after 1 January 2017 to comply with the requirements of the code, together with related amendments to chapter II-2 and Appendix (Certificates). The code contains mandatory provisions for the arrangement, installation, control and monitoring of machinery, equipment and systems using low-flashpoint fuels, focusing initially on LNG with the intention to expand the provisions as new alternative fuels gain acceptance.
The code addresses all areas that need special consideration for the usage of low-flashpoint fuels, taking a goal-based approach, with goals and functional requirements specified for each section forming the basis for the design, construction and operation of ships using this type of fuel. The MSC also adopted related amendments to the STCW Code[EU1] , to include new mandatory minimum requirements for the training and qualifications of masters, officers, ratings and other personnel on ships subject to the IGF Code. These amendments also entered into force on 1 January 2017, in line with the SOLAS amendments related to the IGF Code.
Growing market attracts makers
Wärtsilä had been developing dual-fuel engines for shore-based use since the late 1980s and was the first maker to transition the idea to marine applications. In 2001, Wärtsilä was contracted to supply the FPSO Petrojarl I with a pair of its 18V32DF dual-fuel engines. This was followed by contracts for a series of LNG carriers built in France and two offshore ships.
For many years, Wärtsilä was the main proponent of dual-fuel engines although Rolls-Royce was also promoting a spark-ignited gas versions of its Bergen Diesel engines. Regardless of maker all gas-fuelled engines were medium-speed variants. That has changed and now there are dual-fuel low-speed two stroke engines produced by MAN Energy Solutions and by Wartsila’s successor in the two-stroke sector, Winterthur Gas & Diesel better known as WinGD.
In the four-stroke sector, the number of makers producing dual-fuel engines is higher. Wärtsilä, MAN, MaK, EMD, ABC, Himsen, and Niigata all have dual-fuel engines in their ranges and more makers are soon to join the list. Rolls-Royce is following a different path with its Bergen engines offering them only as oil burning or pure gas engines. Dual-fuel engines ordinarily make use of a pilot ignition system using diesel fuel, but the Rolls Royce engines are spark-ignited.
The four-stroke engines are being installed in many vessel types. Many of the engines are being installed in vessels that are ‘dual-fuel ready’ meaning they have the engines but not necessarily an LNG fuel system which will be added later if the operating profile permits.
Dual-fuel engine types and development
Wärtsilä’s range of dual fuel engines currently comprises five basic models
the longer established 20DF, 34DF, 46DF and 50DF and the most recent the 31DF launched in 2015. All are four-stroke engine that run on oil fuels (LFO and HFO) and can switch over from gas to oil and vice versa smoothly during engine operation. The Wärtsilä dual-fuel engines are available in power range from 0.9 –18.3 MW having speed range from 500 –1200 rpm.
MAN Energy Solutions was a later entrant to the dual-fuel market. Rather than concentrate on four-strokes, it has played to its strength and is the undisputed leader in dual-fuel two-strokes although it does have four-stroke dual-fuel offerings and has sold several for propulsion engines in LNG carriers and for gensets in vessels with two-stroke dual-fuel propulsion engines.
The two-stroke engines in MAN Energy Solutions’ portfolio are identified by four different suffixes to the engine designation. GI engines are intended for gas fuels particularly methane, GIE engines for ethane, LGI engines are designed for liquid gas fuels with LGIM indicating methanol and LGIP indicating LPG such as propane or butane.
MAN Energy Solutions dual-fuel two-strokes operate according to the high-pressure Diesel principle while the WinGD engines employ the low-pressure Otto cycle. The higher temperatures of the Diesel cycle means more NOx formation but the lower pressure of the Otto cycle can lead to methane slip where unburned fuel passes out in the exhaust. Methane has a higher greenhouse gas potential than CO2 and so is considered undesirable. The Diesel cycle is considered more energy efficient but the higher pressures employed mean more costly and complex fuel systems.
MaK is also a strong player in the dual-fuel sector and has contracted for at least 15 engines of different variants of its M46 range. MaK has traditionally enjoyed good support from owners in the cruise market. The cruise market has been targeted by MAN Diesel & Turbo for its new 45/60CR
A volatile future
In any typical fuel system for oil-fuelled engines, the fuel is stored in bunker tanks on board the ship. The same is true for LNG fuel supplies except on LNG carriers where the fuel comes from the boil off from cargo tanks. Ethane carriers with a dual-fuel engine adapted to run on ethane are similarly equipped.
In 2018, a project involving WinGD, Wärtsilä Gas Systems and shuttle tanker operator AET developed a system that makes use of a new source of fuel available to tankers. Most oil cargoes emit volatile organic compounds (VOCs) during a voyage and for safety purposes these must either be vented to the atmosphere or recirculated into the cargo.
In the project – which involves a WinGD X-DF engine – instead of returning or venting the VOCs from crude oil they were diverted to a holding tank and then injected into the natural gas supply to the engine. The engine was able to run normally with up to 20% VOCs in the fuel mix. This reduces LNG fuel consumption by a comparable amount.
The engine used in the tests was also running on fuel oil and the transitions between running on gas, gas/VOC and oil were all achieved easily and without problems. No changes were made to the engine’s normal operating parameters and there was no significant increase in NOx emissions. As a consequence of the tests, AET has ordered two vessels to make use of the new concept.