LNG is now considered a mature alternative fuel option. However, there are many technology choices that need to be made depending on specific vessel design and operational requirements. Making the right choices is decisive for having a competitive design.
One of the biggest challenges for LNG fuelled vessels is finding the most efficient use of a vessel’s available space for the fuel tank and the associated systems. LNG storage on board requires more space than conventional fuel oil storage. This is primarily because LNG has a lower energy density than fuel oil and therefore requires a larger tank to provide the same operational range. In addition, due to the low temperature of LNG, the tank insulation and required gas handling systems additional space is needed
The IMO has defined three basic, independent LNG tank types: Type A, Type B and Type C. In addition, there are membrane tanks which are fully integrated into the ship structure. The main differences between the tank systems are:
- design pressure,
- design of the secondary barrier,
- shape of the tank,
- tank size
An important requirement of the IGF Code is for the system to avoid venting natural gas to atmosphere for a period of 15 days. Various methods for tank pressure control are available:
- Energy consumption by the ship (engines, gas turbines, boilers etc.)
- Thermal oxidation of vapours (gas combustion unit)
- Pressure accumulation
Engine selection is also a key consideration. Currently there are both high-pressure (diesel cycle) and low-pressure (Otto cycle) 2-stroke engines available. High-pressure engines offer lower fuel consumption and practically eliminate methane emissions, while low-pressure engines offer simpler designs at a somewhat lower investment cost. Smaller 4-stroke engines are also available, both of dual-fuel and spark-ignition (gas only) type. LNG engine design has been steadily improving as the technology becomes more widely adopted, with increases in efficiency and reductions in methane slip emissions.