When using LNG as a fuel, the design of the ship has to be adjusted in such a way so as to adapt to the properties of LNG. The shipping industry has witnessed a remarkable change of opinion regarding the use of liquefied natural gas (LNG) as a bunker fuel. In the last couple of years, the debate around the viability of LNG as a bunker fuel has moved from polite curiosity to seriously considering it. In this week's industry spotlight we return to the drawing board to sketch out how changing to use LNG can affect the design of ships.
There are currently about 40 LNG fuelled ships (excluding LNG carriers) in operation worldwide, and another 40 newbuildings are now confirmed. While these numbers are still relatively small and no deep-sea LNG-fuelled ships are in service as of yet, the demand for LNG powered ships - including deep-sea - is expected to grow rapidly in the next couple of years. A recent study conducted by Lloyd's Register on bunkering infrastructure predicts that by 2025, there could be 653 deep-sea LNG-fuelled ships in service, consuming 24 million tonnes of LNG annually. This is only under the base case scenario with current Emission Control Areas (ECAs) and a 0.5 percent global sulphur limit in bunker fuel implemented from 2020. When the study modelled relatively cheap LNG – for example, 25 percent lower than current market prices – the projected number of LNG-fuelled ships rose to approximately 1,960 units in 2025. If ship owners are to turn their heads and if this turning of heads gains velocity, a number of ship design considerations need to be taken into account.
Why Does LNG Have an Impact on Design?
When using LNG as a fuel, the design of the ship has to be adjusted in such a way so as to adapt to the properties of LNG. LNG has half the density of diesel fuel which means that larger storage tanks are needed for the same range. Also, it is liquid only at very low – cryogenic – temperatures (-163°C) so it requires special storage tanks, pipe systems and handling to avoid contact with personnel and with the ship's structure. it is advised to provide the ship with a backup diesel fuel option to ensure fuel availability Only special materials unaffected by cryogenic temperatures, such as stainless steel, aluminium, and Invar, can be exposed to the liquid. Given that LNG bunkering facilities are still not widely available, it is advised to provide the ship with a backup diesel fuel option to ensure fuel availability. When in gas state, LNG can be highly volatile, especially when stored in an enclosed space at the right mixture with air, therefore a ventilation system is needed for safety. It should also be considered that when stored, LNG will normally slowly evaporate so a means to deal with boil off gas is required – venting to air is not allowed.
Adequate LNG storage is the major design concern when using LNG as fuel. This is because when liquefied, the storage space required for natural gas is about four times higher than for conventional fuels. In addition, well-insulated tanks with a safe area in case of accidental spillage are needed. Consequently, the required storage space on a vessel will be greater than that needed for conventional fuel oils which may impact on the available cargo volume for the ship. Several LNG tank options are available, however some are not feasible for ships using LNG as fuel. Most of the membrane tank systems that are used on the very large LNG carriers, for example, are sensitive to sloshing, and therefore cannot carry partial loads making them unsuitable for ships running on LNG. IMO type A (self-supporting tanks designed like ship structures) and type B (self-supporting prismatic or spherical) tanks would generally be feasible, but they require a secondary barrier and cannot contain pressurised gas and thus continuously have to consume boil off gas. These problems have not yet been solved in a technically and commercially sound way. Nonetheless, these tank types could potentially be a future solution, especially for ships carrying large amounts of LNG as fuel. The first LNG carriers started operating in the 1960s and since then have used LNG as fuel For today, IMO type C tanks are mainly used because they allow a pressure build-up of 7 - 10 bar, which is sufficient to contain boil off gas for 10 to 20 days, meaning that there is little wasted fuel. Furthermore, these tanks are very safe and reliable and are easy to fabricate and install. Their big drawback, however, is their high space requirements. Besides already requiring more space than conventional fuel oil tanks, type C tanks also take up more space because of their cylindrical shape. What makes matters even worse is the fact that fuel oil tank space for the ship's full range may still need to be provided if there is no certainty of LNG availability. Of course, the large volume of LNG tanks takes away the space available for cargo, another issue that must be taken into consideration. The burning of LNG in internal combustion engines is not a new concept. The first LNG carriers started operating in the 1960s and since then have used LNG as fuel. So LNG as fuel is now a proven and available solution, with gas engines being produced covering a broad range of power outputs. This truly is a time of change, with exciting prospects for a new era of shipping fuels and design LNG-fuelled engines include gas-only engines, dual-fuel engines and steam turbine systems. In commercial vessels, the use of a turbine plant is unlikely. When fitted with a gas-only engine system, the IMO International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk Code (IGF Code) applies. It requires the installation of a backup LNG tank and propulsion system, neither of which is required with a dual-fuel engine system. A lot of projects in the market are based on dual-fuel engines that are able to run on conventional fuels (HFO, MDO, MGO) as well as on gas. These engines can switch over to conventional fuels without interruption. This gives the operator flexibility to choose the fuel that is more easily available or cheaper on short notice and provides a backup in case the gas system was to fail. As a whole, dual-fuel engines in operation have totalled well over a million hours. This experience shows that when compared to conventional engines, time periods between overhauls are extended and component lifetimes are longer. Also, the combustion space in the engines remains much cleaner.
Are Certain Ship Types and Designs More Suitable?
Against this backdrop and considering the current LNG infrastructure, using LNG as a fuel is more suitable for some ship types than others. It is well suited for ships and boats with a set route and a short range (4,000 nautical miles or less) as the storage tank capacity is not overly large and fixed bunkering ports can be established. It is also good for ships that operate mostly within ECAs and for ships that trade where a source of LNG is available. In contrast, tramp ships with long voyages and uncertain routes are not considered suitable in the near future. Neither are high-powered ships as they may not have the available space for the required large bunker storage tanks. This truly is a time of change, with exciting prospects for a new era of shipping fuels and design. The seriousness of the industry for the uptake of LNG as a viable fuel can be seen by the amount of time and money being spent by research institutes, class societies, ship owners, manufacturers, ports and governments alike.
Source: ship and bunker