LNG fuel systems
Updated 11 Oct 2019
Fuel systems on gas-only or dual-fuel vessels that are not LNG carriers are something of a novelty and outside the experience of most operators. This is likely to be the case for many years to come as most industry observers now believe that it will be 2025 or even later before the number of LNG powered vessels other than LNG carriers reaches 1,000. That may sound a large number, but it is less than 2% of all ships.
In adopting the IGF Code, the IMO has requested the ISO to develop a standard for quick-disconnect bunkering connections and work on this is progressing. A standard LNG bunkering checklist was not included as part of the request but the MSC has since been requested for this to be included as part of the task. The biggest problem of LNG is that the fuel must be stored at extremely low temperatures and under pressure.
In addition, it has a lower flash point than oil fuels and in the event of a leak in the storage or fuel delivery system, any escape of fuel would be far more difficult to contain and to recover than is the case with oil. However, the system between tank and engine is less complex and there are virtually no waste products to be stored and disposed of ashore.
Until the adoption of the IMO’s IGF Code, approval of LNG fuel storage and delivery systems was done by flag states on a more-or-less case-by-case basis. There are a small number of systems so far developed for LNG although the number must be expected to increase. Several shipbuilders either have or are developing systems and while they will have much in common there will be variations.
Few seagoing engineers apart from those who have been employed on gas carriers or the small number of other ship types with dual-fuel or pure gas engines will have much knowledge of the fuel systems for gas fuelled ships.
According to the IGF rules, the LNG fuel tanks have to be selected from among the “Independent Types A, B, or C”. ‘Independent’ means that the tanks are not formed as part of the ship’s structure. In the first two types the gas is stored at atmospheric pressure but in a type C tank, the gas is contained under high pressure.
Type C tanks require less space inside the ship than A or B types and for this reason have generally been favoured for use as fuel tanks on LNG-fuelled vessels. However, for larger cargo ships running on gas, a type B tank may be considered because there may be more space available – especially if the intended cargo is a high density one such as ores – and because of the cheaper construction costs involved.
Whatever tank type is chosen the need to retain any leakage of gas within a second barrier before it can contact the ship’s hull is an imperative. This is because the cryogenic temperatures needed to keep LNG in its liquid form will mean spills that come into contact with the hull structures will inevitably cause cracking of the steel structure of the vessel.