LNG fuel gas systems – clean ship propulsion

Today there is no doubt that improvement of ship’s emissions is urgently required, even if shipping is supposed to be the most eco-friendly means of transportation. There are different ways to cut NOx, SOx, particulate matters and CO₂ emissions, but only one solution provides ‘all in one’ reduction of all mentioned emissions: LNG as a ship’s fuel will reduce NOx to clearly below Tier 3 level (for four-stroke engines), SOx to zero, particulate matters to about zero and CO₂ by about 20% without any after treatment of combustion gases or exhaust gas recirculation.

Using LNG as a fuel has been common technology for decades on LNG carriers. There is an excellent safety record for loading/unloading of those vessels as well as for operation of propulsion systems based on burning boil-off gas (BOG). Further there is about 10 years of experience, mainly in Norway, on small ships with LNG propulsion, e. g. ferries and offshore supply vessels.

The current world market situation with rather low LNG prices provides an additional driver for the development of LNG as ship’s fuel. Commercial calculations from different sources result in pay-back times of 3 to 10 years for the additional capital expenditure on LNG equipment for ships mainly operating in SECA/ECA zones. The main uncertainty of all those calculations is the price development of LNG at the bunker flange compared with the traditional and low-sulphur fuel oils.

LNG containment systems

One basic disadvantage of LNG is its low density: for the same energy content LNG takes roughly twice the volume of liquid fuels. There are several types of containment systems for LNG available, but some are not feasible for the given conditions on ships using LNG as fuel following current designs. For example, most of the membrane tank systems as used on the very large LNG carriers are sensitive to sloshing and could therefore not carry partial loads – thus their use for fuel tanks is not possible. IMO type A (self-supporting tanks designed like ship structures) and type B (self supporting prismatic or spherical) tanks are generally feasible for fuel gas tanks, but their requirement for pressure maintenance and secondary barrier rise difficult problems that are not yet solved in a technically and commercially sound way. This will be a future solution for ships carrying large amounts of LNG as fuel.

So IMO type C tanks (pressure vessels based on crack propagation design) turn out to be the preferred solution for today. Those tanks are very safe and reliable, their high design pressures allow for high loading rates and pressure increase due to boil-off; finally they are easy to fabricate and install. The major disadvantage is the space consumption of this tank type that is restricted to cylindrical, conical and bilobe shape. In addition to the unfavorable volume to energy efficiency these tank shapes lead to a factor of three to four times the oil bunker tank volume to carry the same energy in LNG.

Tank insulation is required in order to reduce heat ingress and to protect the ship structures against the cryogenic temperatures of LNG. This may be done by vacuum or foam insulation depending on the operational and tank shape requirements. Vacuum tanks have an excellent insulation performance; however, they are restricted to cylindrical shape, limited in size and usually do not have a manhole for inspection or mounting of in-tank equipment. Foam insulated single shell IMO type C tanks are feasible in conical or bilobe shape in order to better fit to the available space. Those tanks have been investigated in sizes of up to 10,000 m³ for cylindrical and even 20,000 m³ for bilobe tanks. Foam panels are glued to the tank and protected by vapour barrier and steel sheets or foam is directly sprayed to the tank surface and covered by a polymeric layer. Both have been used for small LNG carriers with type C tanks. Even with special high-capacity panels the heat ingress is clearly higher than for vacuum insulated tanks.

LNG process systems

Basically the process system is intended to bring the LNG to the pressure and temperature level as required by the engines. Pressurising may be either done by small vaporisers keeping the entire tank on high operation pressure, by pumps serving the vaporisers or by compressors. All versions are feasible, the plant capacities and operational requirements will dictate the right solution tailor-made for each situation. In the basic system for a four-stroke engine an in-tank pump inside the type C tank is employed to feed the LNG fuel gas vaporiser; the gas is provided to the engines via a fuel gas heater.

Also two-stroke engines will be available as dual-fuel engines quite soon. They require a different process system due to high injection pressure of 300 bar. On LNG carriers this can be done by BOG compressors, but for other ships this will only be a viable solution in some particular cases due to high capital expenditure, power requirements, size and weight of this equipment. High pressure pumps and high pressure vaporiser and heater are the preferred alternative to achieve the required pressure level. Tanks will usually be equipped with in-tank pumps to feed the high pressure system as well as the low pressure fuel gas supply to auxiliary engines.

LNG on ships other than gas carriers – safety systems

With the use of gas on ships a number of hazards have to be addressed (e. g. fire, explosion, cold brittleness). IMO interim guideline MSC 285(86) as a preliminary version of IGF-Code and the Rules for LNG fuelled ships that have been published by all major classification societies are based on several decades of experience with LNG operations.

Double barriers for gas equipment, gas detection, ESD systems and appropriately classed equipment are mandatory. Spill detection and stainless steel drip trays are located wherever LNG might escape and harm the ship structures by cold brittleness. Piping sections not in use are inerted with Nitrogen, e. g. bunkering line after bunkering is finished. Last but not least the control system and crew training will have significant influence on the safe operation of LNG installations.

LNG bunkering

LNG bunkering will have to be as close as possible to the traditional bunkering, if it is to prove acceptable to the mainstream shipping industry; that is more or less agreed. However, there are few details in place about how to bunker large amounts of LNG to a ship while it is alongside at the terminal for cargo operations. Bunker volumes and required bunkering rates will exceed by far the current Norwegian practice, and bunker vessels or barges will be required to cover these needs. The safe handling of heavy equipment like dry break emergency release couplings and double wall hoses or pipes of relevant diameters will ask for mechanically or hydraulically supported installations. Technical solutions do not seem to be the major issue, as ship-to-ship transfer of large amounts is current practice in LNG business. The main challenges are the procedures and the global and local regulations that still need to be developed.

The bunker interface would require better standardisation than for current fuel oil bunkering, as the use of a number of reducers cannot be accepted for LNG. Each flange connection is a hazard for spillage and leakage, therefore a reduction to the minimum is mandatory. Further data connection between the ships including ESD (emergency shutdown) function has to be in place for safe bunkering. Vapour return connection is not a must; however, it will clearly ease bunkering at high loading rates and avoid accidental gas emission via safety relief valves during bunkering.

Technical solutions for safe operation of LNG as fuel are available. Emission control and low LNG prices should be the main drivers to develop LNG as bunker fuel. Currently the lack of related bunker infrastructure as well as the regulatory framework for such operations is the main challenge that needs to be addressed soon in order to make LNG a reliable option for the Owners in their decision about future ships. With the relevant dates for ECA&SECA legislation coming closer the pressure will rise for everybody to find a solution to cover the challenges resulting from it.