Economic Design Through the Reliability Analysis of Instrumentation and Control System for the LNG Shipment

In order to improve the system reliability it may necessary to change the system by doing reliability analysis. This requires the identification of those components or subsystems which contribute most to system failure. In my research, explosion is studied as the main hazard associated with the control of the LNG shipment as result of accident history survey. Therefore, it has been concluded that the undesirable accident is fire and explosion on the ship, following release of carrier during loading, transportation and discharging, in the relation to the control system. Since the 'Top event' is known, Fault Tree Analysis(FTA) is a suitable means of identifying how the control system can contribute to or protect against the hazard. It is required to find cutsets of fault trees for its system analysis. From the FTA, the frequency of fire and explosion by the gas release is resulted in the 89.04289 f/106hr and the probability of occurrence is 89.02146×10-7 in the original system. By using Fussell-Vesely importance measures, relative importances of basic events are calculated. The failure of level transmitter has most dramatic effect on the gas release, which value was over 0.7. This item should be improved unless this action is relatively costly. The redundancy of level transmitter will be prepared for the controlling of fire and explosion in LNG carrier from this importance measure. The frequency of fire and explosion by the gas release is improved to the about 50 times reliable, 1.585709 f/106hr and the probability of occurrence is about 10 times reliable, 7.930963×10-7 by adopting only transmitter redundancy. I found that the most economical and reliable design can be determined by considering this importance measures.