The Society of International Gas Tanker and Terminal Operators (SIGTTO) have taken an initiative to inform and advise the owners and operators of LNG carriers about the issues surrounding rollover.
The guidelines suggested by Society of International Gas Tanker and Terminal Operators (SIGTTO) deal with the circumstances leading to rollover in LNG ships and its migration strategies. Although it is quite unusual to happen, yet if rollover has occurred, these guidelines would help to take the issues surrounding rollover.
Rollover refers to the process of speedy release of LNG vapour that can happen because of the spontaneous mixing of layers of different densities of LNG in a storage or cargo tank. It is applicable to various LNG ships including ships acting as floating storage vessels, LNG Regasification Vessels (LNGRV) and Floating Storage and Regasification Units (FSRU). Traditionally, bulk LNG is stored in heavily insulated tanks which may be vertically cylindrical or in-ground tanks, having a capacity of up to 250,000 m3 and a working pressure of up to 250 mbar.
Rollover mostly happens when gas cargo is carried in large tanks. Spherical or prismatic cargo tanks are used on LNG carriers with individual tank capacities of up to 50,000 m3 and a similar working pressure. Smaller quantities of LNG are usually put away in vacuum insulated tanks (VITs) with pressures of up to 5 bar, even with an extended capacities of up to 10,000 m3. The heat can leak into the tank through the insulation, heating the cargo and creating surface layer to evaporate resulting in “boil-off”. It can cause an increase in the boil off rate up to 10 times than the normal condition, over pressurization of the tank and lifting of relief valve of the tank.
However, stratification and thereby rollover can be stopped by mixing LNG of different densities using top and bottom fill procedures and recirculation of the tank to mix the contents and release superheat through jet nozzles or other mixing devices. The other measures to prevent rollover includes avoidance of prolonged stoppage while loading the ship’s tank and continuously monitoring the boil off rate.