One of the primary considerations in the design of an LNG system is safety. The technology for safe and reliable handling of LNG is well established through 40 years of dependable service provided by the LNG industry throughout the world. Special safety procedures and techniques are incorporated in all aspects of the LNG system. However, as in the development of any new system involving a hazardous commodity LNG, careful assessment to detail along each step of the design process is extremely important. Therefore, this paper will address practical considerations and risk analysis for the LNG import system, mainly the LNG terminal and the LNG carrier serving it.
Safety features of LNG safety are described, such as the phenomenon known as "roll-over", ship collision, and leak-before-failure concept. Rollover may occur if stratification of the liquid is allowed to develop when introducing a significant quantity of LNG into a tank containing LNG of a different density or composition. Ship collision is a key factor that must be considered in the risk analysis of LNG systems. Leak-before-failure concept is used to prevent a catastrophic rupture of LNG tanks. Emergency equipment such as quick disconnect hardware will be discussed in the accident case analysis. Quick disconnect hardware can eliminate the need for time consuming operations, but must be used in the safe manner.
Since Japan has been the LNG market leader, Japanese experience in LNG handling will be reviewed and risk analysis of the Himeji LNG terminals will be used as an example. Specifically, two methods commonly used in Japan for assessment of the risk will be contemplated. One is "Fault Tree Analysis" (FTA), which is used to investigate the events through which the assumed accidents are created in a similar manner to the relevant tree method. Another approach is "Event Tree Analysis" (ETA). The ETA investigates the accident through which chain accidents occur one after another, in accordance with a scenario relating to the accident. Methodology and safety measures during transportation, loading and offloading of LNG will be described in this paper.
Finally, a method called "Probabilistic Risk Assessment" (PRA) will be briefly discussed. PRA provides a formal process of determining the full range of possible adverse occurrences, probabilities, and expected costs for any undesirable event. PRA can identify those areas that offer the greatest risk-reducing potential. Once the components with the greatest risk-reducing potential are identified, appropriate technology and management schemes can properly influence risk reduction.