Rapid advances in the management of boil-off gas are changing fuel supply systems in LNG vessels
A marvel of low-emission technology, the first of MTU’s LNG-fuelled, high-speed eight-cylinder engines will be installed in late 2019 on a new ferry plying Lake Constance on the Rhine, following thousands of punishing hours on the test stand.
One of the few inland passenger vessels in Europe to be powered by a pure gas engine, the ferry was configured for a propulsion system suitable for a pristine environment. And by any measure, MTU has delivered, with one of the cleanest-burning engines on the market.
With a power rating of between 750-1,000 kw, the system more than meets the IMO 3 emission guidelines. According to MTU, particulate mass is below the verification limit and nitrogen oxide emissions are very low. A smaller version of a 16-cylinder LNG engine unveiled in late 2016, the engine is still being put through its paces on a test stand at MTU’s Friedrichshafen plant prior to delivery to the ferry’s owners, Constance Municipal Works.
When MTU’s latest product goes into action in earnest in early 2020, it will showcase the rapid strides being made by engine manufacturers in devising solutions that satisfy sometimes conflicting criteria in gas-powered propulsion. Not only must engines meet tougher emissions regulations, they must do so with constantly improving economies of scale, reliability and compactness, regardless of the type of vessel.
For instance, for its new small-scale, 30,000m3 LNG carrier under construction at Hyundai Heavy’s Mipo yard, Norway’s Knutsen Shipping has opted for a two-stroke, dual-fuel engine from Winterthur Gas & Diesel (WinGD) that will run on marine diesel and natural gas. Importantly, it will also have a re-liquefaction unit for processing boil-off gas (BOG). Burckhardt Compression will provide Laby compressors, an important element of the fuel-gas supply system built by Wärtsilä.
Knutsen opted for Wärtsilä’s all-in-one fuel supply system. Not only does the system deliver the fuel, it comes with cargo-handling and control, and the re-liquefaction module. This package is built around three Wärtsilä 20DF auxiliary engines that drive the thrusters, cargo control system and re-liquefaction.
Coupled with WinGD’s five-cylinder X52DF main engine, the overall system becomes an integral element of the BOG management system. “It monitors and controls the cryogenic cargo that, combined with a Wärtsilä -developed mixed refrigerant re-liquefaction unit, ensures real-time control over the cargo tank pressure and temperature,” Wärtsilä explained in a release.
When the ship is delivered in 2021, it will also be one of the first to carry LNG in the space-saving C-type bilobe cargo tanks.
The propulsion system reflects its future duties – the LNG carrier will be mainly put to work in the Mediterranean for the next 12 years, delivering LNG to Edison. “The engine is high-efficiency, guaranteeing a higher level of operational flexibility and an improvement in terms of containment and environmental impact,” Knutsen said in a statement that summarises the complexity of the briefs that engine manufacturers are increasingly required to meet.
Because the type of propulsion system adopted closely affects charter rates, it is a crucial decision for shipowners. In the last two years spot rates for conventional steam carriers have been at their lowest, while those for main engine gas injection (ME-GI) and X-DF the highest because of their size and fuel efficiency, according to the China Classification Society in a definitive 2019 study of the developing technology of the LNG fleet.
As of early 2018, the order books revealed a marked shift in preference from dual-fuel diesel (DFDE) to low-speed, dual-fuel (LSDF), with the latter accounting for a significant 57% of orders compared with DFDE’s 28%. Simultaneously, the study’s authors conclude, MAN’s ME-GI high-pressure engines have evolved into a popular choice while WinGD’s low-pressure technology has also been gaining ground. Surprisingly, steam propulsion that started it all off in the 1960s could be making a comeback.
As China Classification Society’s study shows, new types of gas-powered systems are developing to suit shipowners’ widely varying requirements, particularly in the fast-growing and increasingly diverse LNG fleet. Entitled Options and evaluations of propulsion systems for LNG carriers, the paper explains: “[These vessels] are undergoing a rapid and profound change, with much larger ships and novel propulsion systems emerging [that meet] the market trends of LNG shipping.” The study cites the competing influences of cost, emissions regulations and safety of navigation on the “technical feasibility and economic viability of LNG carriers”.
In a summary of the relatively short history of propulsion systems for the LNG fleet, the authors recall how steam turbines were the power plants of choice for several decades when LNG carriers were in their infancy. But the last 15 years has seen an explosion of options, ranging from DFDE-based systems to the increasingly common two-stroke diesel engines, boosted by re-liquefaction plants.
In the process, the dominance of steam turbines had steadily been eroded, initially by four-stroke dual-fuel engines and since 2010 by two-stroke versions. The result, say the authors, is that “so far there is no standard propulsion system applicable to all types of LNG carriers.”
BOG as fuel
Three main reasons account for this turnaround. First, changing patterns of trade, mainly triggered by the breakdown of long-term fixed delivery contracts, have forced shipowners to look for more flexible and efficient propulsion systems to accommodate various operation profiles.
Second and most obviously, IMO’s imminent emissions regulations have triggered the need for cleaner and more efficient fuel supply systems.
And finally, constant improvements in the insulation of LNG tanks have led to much-reduced BOG rates, in fact down to a remarkable 0.1% in the latest versions. In turn, this means there is less BOG to be used as fuel, forcing engine-manufacturers to come up with more fuel-efficient power plants that do not rely on waste gas.
Thus by default, the tanks themselves have turned into a crucial element within the fuel supply system. According to the study, over 70% of the fleet are now equipped with a membrane-type tank, with the rest divided between the self-supporting B-type tanks and the C-type tanks typically installed on small-scale LNG ships.
Overall, there are now six options for propulsion systems in LNG carriers, the study explains. At current count they comprise steam turbine propulsion, DFDE, SSDF, the more expensive but highly efficient gas turbine developed by Rolls-Royce, SSDE coupled with a reliquefaction plant, and a hybrid system based on steam turbine and gas engine.
Put another way, the wide variety of options for propulsion and fuel supply systems shows how engine manufacturers and other suppliers are rising to the occasion.
Off the boil
Some of the biggest progress in fuel-supply systems has been made in the technology of boil-off gas (BOG). Typically, LNG is lost from tanks during storage, loading and discharging, and during the voyage. Despite tanks being insulated against outside heat, even a small amount of heat will lead to the slight evaporation of the cargo.
According to a China Classification Society study, entitled Options and evaluations of propulsion systems for LNG carriers, losses generally run at a rate of around 0.15% a day, the percentage being expressed in terms of total liquid volume per unit time. Lately though, tanks have come on the market that claim a boil-off rate of close to 0.1% a day.
While this means more of the original cargo is delivered, it has implications for the fuel supply system. Because BOG must be removed in order to maintain the correct pressure in the tank, the vessel must have a processing system. “BOG can be re-liquefied, used as fuel or burned in a combustion unit,” explains the report, adding, “Excess gas can also be led to the engines which have a capability of burning gas fuel … Another alternative is to burn the unwanted gas in a combustion unit, but this results in a wastage of materials and valuable energy.”
The solution varies according to the vessel’s propulsion system. In a steam turbine-based system, for example, BOG is burned in boilers but, as the study notes, “the steam turbine has the lowest overall efficiency of the propulsion systems available, approximately 35% at full load.”
In the case of dual-fuel diesel-electric (DFDE) engines, one of the most popular propulsion systems in the LNG fleet, they can run on BOG if necessary. When in gas-burning mode, the BOG is injected into the air intake of each individual cylinder where it is mixed with charged air. And because the BOG is used at a relatively low pressure, the complexity of the fuel gas supply system is reduced.
The paper concludes: “With a multi-engine configuration, the DFDE propulsion system provides a superior performance in terms of redundancy and safety.” However, it is also one of the most complex and expensive systems. Importantly, if there is more BOG available than needed for the engines, it is simply despatched to the gas combustion unit (GCU).
That leaves two-stroke slow-speed (SSDR) engines, the dominant choice in merchant shipping because of their efficiency, low-maintenance and ability to burn cheaper fuels. In SSDR systems the BOG is liquefied and restored to the cargo tanks. Recently though, as LNG carriers have got bigger, up to 265,000m3 in Q-max vessels, the volume of BOG has increased significantly to the point where it is within the capacity range of re-liquefaction plants. Thus, none of the cargo is wasted.
According to the authors, this makes SSDE systems coupled with re-liquefaction capability “a feasible and attractive option for ship owners.”
There is however a sting in the tail – re-liquefaction plants are hungry for power. The plant on a 149,000m3 LNG vessel, for example, consumes 3.5-7 MW depending on the amount of BOG generated in the cargo tanks.
As such, the vessel’s net auxiliary power increases to the order of 14-16 MW. “Considering the overall performance of the system, the tremendous power consumption of the re-liquefaction plant substantially diminishes the efficiency advantages provided by the SSDE system,” the paper concludes.
In the business of fuel-supply systems, nothing is easy.