Dual-fuel engines bring multiple benefits to LNGCs

Years of hard work and dedication to a concept are paying off – the order book for Wärtsilä 50DF dual-fuel engines for liquefied natural gas carriers is growing fast. Originally viewed as a stationary power unit, development was carried out at flank speed once its maritime potential was spotted.

– Previously, liquefied natural gas carriers (LNGCs) have only been powered by steam turbines as they can use liquid natural gas (LNG) to fuel their boilers, says Göran Hellén, Head of Exhaust Emission Control at Engine Performance Technologies R&D, who took part in the dual-fuel (DF) development project.
When the Wärtsilä 50DF was launched in 2001, market reaction was cautious. – Steam turbines have a good safety and maintenance record, says Markus Hjerppe, Director, Business Support, Wärtsilä Ship Power Solutions. – So we had to demonstrate that our 50DF engines would at least be the equal of steam turbines in some respects while surpassing them in others. We can now do that.
The “innovation triggers” that Hjerppe outlines to potential customers are simple and few. First is the relative economy of both types of engine. Here the 50DF leads easily: for steam turbines, propulsion and electrical power efficiency are 29% and 25% respectively, the corresponding figures for DF are 43% and 46%.

Much lower emissions
Second, the 50DF’s emission figures, regardless of whether it is using only LNG-BOG or operating in tandem mode with MDO/HFO, is much more environmentally sound. NOx, SOx and CO2 emissions are fractions of those produced by a steam turbine using any of the three fuel options. The value attached to these figures will grow as MARPOL regulations become tighter.
– The 50DF represents a real low-emission solution. Switching to gas fuel is a major step towards environmental soundness, says Hellén.
– Using gas, CO2 emissions are 20% lower than using HFO/MDO. The specific CO2 reduction when the 50DF is running on gas is 30-40% less than produced by a steam turbine. – Reduced levels of CO2 result from both the high hydrogen/carbon ratio in gas and the high shaft efficiency achieved in Wärtsilä DF engines. The lean-burn pre-mixed combustion process results in very low NOx, while SOx and particulate emissions are practically eliminated due to the very low pilot injection fraction (1%), which is the source of sulphur and ash.

Manning levels reduced
Another plus, surprisingly, puts the focus on personnel required in the engine-room. Steam turbines can fairly be described as a sunset technology, with only LNGCs and some naval forces now using them. Hjerppe is happy to point out that normal engine-room personnel can supervise the 50DF, though a one-week training course to familiarize personnel with the engine control system is recommended.

Precise control and flexibility
The heart of the DF concept is its control system, and this is what provides most of its advantages – and flexibility. The system ensures, for example, that the quantity of gas flowing into each cylinder during the air-intake stroke is exactly the amount required before it is ignited by the injection of a minute quantity of diesel fuel.
The system also handles smooth fuel changeovers from gas to liquid or vice-versa. – This is the really beautiful feature of this engine. Automatically or manually, it can switch from one fuel mode to the other without affecting either output or speed, says Hjerppe.
The preferred four main engine configuration also provides operational flexibility. If one engine has to be stopped because of component failure, the other three engines can generate enough power to allow the vessel to proceed at its normal speed.

Shorter maintenance intervals not a problem
One of the most commonly perceived advantages that steam turbines hold over DF solutions is that of maintenance. – We took a close look at the figures to overcome this hurdle, since maintenance intervals for DF engines are shorter, says Hjerppe.
Steam turbines require downtime for boiler and steam-pipe servicing at 30-month intervals and have to undergo a complete inspection in drydock every five years. DF engines require a predetermined stoppage every 2000 hours, but the ‘one-engine-out’ feature means that this can be scheduled in port or at sea. – We’ve demonstrated that a ship can sail while one engine is being serviced, so the timing of this can be planned in advance in the most optimal way, says Hjerppe.

Substantial savings
The figures relating to operating costs also require close examination. Although the DF’s basic spare part cost of USD 4.0/MWh looks incontestable when compared to the figure of USD 0.50 for steam turbines, Hjerppe presents the overall cost profile. – Since the fuel savings achieved with DF mean that total operating costs are so much smaller, ship operators can still make substantial savings even when maintenance costs are higher.”
On a 2-3 week voyage from the Middle East to Mexico or Asia, an outward bound gas-burning ship will use only 2-3% of its load as fuel (at just USD 2.50 for N-BOG and USD 7.60 per mm BTU when on F-BOG). The difference in operating costs means that compared to a vessel powered by steam turbines, a 155,000 m3 LNGC using DF engines can achieve 50% savings on fuel costs alone. With all engines running on gas and liquid fuel, savings can reach 60%. As LNGCs increase in size, the cumulative savings rise accordingly.
By-products of reduced fuel consumption mean fewer bunkers and reduced space and weight, which means more room for extra cargo. It’s bottom lines like these that are swiftly persuading LNG shippers that the future is not as steamy as it once was.

Wärtsilä 50DF at sea
The 50DF is based on the Wärtsilä 46 diesel engine with 80% of its components either identical or closely related, and the remainder from development of the smaller Wärtsilä 32DF.
– We know this engine’s operational history and that knowledge means we can sell them with confidence, says Markus Hjerppe. – And it also explains the low number of teething problems during development.”
The first Wärtsilä 50DF installations were in three LNGCs built in France: Gaz de France Energy with a four 6-cylinder configuration, then Provalys and Gaselys using a combination of three 12- and one 6-cylinder engines. After handover in 2006-7, the engines in these three ships have accumulated more than 50,000 trouble-free running hours.
Gaz de France Energy is a 74,000m3 vessel on the short crossing between France and Algeria, while Provalys and her sister ship are the next size up at 155,000m3. Provalys trades between Egypt and France or Spain while Gaselys sails between Egypt and either Mexico or France.
– We’ve run all three at over 95% gas mode both laden and in ballast, says Technical Superintendent Pierre Baelden of Gazocean, Gaz de France’s shipping subsidiary. – Gazocean is the first company in the world to run Wärtsilä 50DFs onboard, so it’s important to collect information and carry out evaluations.
– We can certainly say the engines have been reliable – yes! And we have a Wärtsilä engineer onboard so any problems can be dealt with very quickly. But comparisons are simply not possible until we run in diesel mode and include the maintenance costs.
So far a total of 212 Wärtsilä DF engines have been ordered for 56 vessels. Confidence in the new technology is so high that shipyards have even placed orders for engines without having a ship to install them in.

The first Wärtsilä 50DF installations were in three LNGCs built in France. Here are pictures of Provalys (photo:marine-marchande.net) and Gaz de France Energy (photo: meretmarine.com).