An Empirical Study on Reliability of Strength for Independent Prismatic Tank Type B

Abstract

It has been reported that a contract was signed for building an LNG carrier of independent (self supporting) prismatic tank Type B system. This news must be received with a great interest as this is the first case of adopting the independent prismatic tank Type B system for a large LNG carrier.The idea of the independent prismatic tank Type B system was crystallized in 1970's and a group of ship builders started the development of its design standards in which Nippon Kaiji Kyokai (a Classification Society-NK) played an active role. With the design standards thus developed, number of LPG carriers and Ethylene carriers of this system were built with the NK class in the beginning of 1980's, and nothing has been reported of serious deficiencies and troubles.Nevertheless, on the occasion of adopting this system for the large LNG carrier, the authors attempted to review the design standards to verify their reliability. For this purpose, the authors reviewed the tank strength analysis, tank test procedure, design and inspection records of ships in service, and made an empirical research on probability of failure of the tanks.The study concluded that the prevailing design standards for the system were considered quite reasonable and this independent prismatic tank Type B system could be one of the LNG tank systems of high reliability.