Comparative Risk Assessment for Different LNG-Storage Tank Concepts

Abstract

LNG can be stored either in flat bottom storage tanks or pressurized storage tanks such as bullets or spheres. Safety levels of atmospheric storage tanks are classified by codes as single containment, and containment. For spherical and bullet tanks an analogical definition by codes is missing and containment philosophies for atmospheric storage cannot be applied to pressurized storage. Therefore a new definition is proposed to describe the safety levels of pressurized tank types and flat-bottom tank types consistently.It is supposed to use the definitions single, double and full integrity instead of single, double and full containment in order not to mix up definitions. The integrity level a storage tank has been assigned to gives a direct link to the consequences that would have to be taken into account in case the primary container fails totally. The advantage of these new containment definitions is having available a normalized basis for the comparison of different tanks fulfilling similar requirements on safety design.It is expected, that the integrity level of the LNG storage vessel has influence on the risk to external population. To evaluate this influence, the risk to external population caused by the different LNG storage alternatives has been calculated by means of a quantitative risk assessment. These calculations show, that in general the risk to external population is the higher the lower the integrity level of the LNG storage. Therefore it can be concluded that high integrity solutions should be chosen where exposure of third party population is high. This should be considered when selecting the tank type (i.e. integrity level) with regard to the distance to populated areas as well as the population density.