Badak LNG operates two loading docks for loading LNG to transporting vessels. Each loading dock is served by two main piping systems. One piping system used to transfer the LNG from the storage tanks to the loading arms and another piping system to transfer the boil off gas (BOG) from the vessels back to the plant, to be utilized as fuel. Both piping systems are equipped with expansion bellows installed to compensate piping contraction and expansion cycles, caused by temperature cycle during operation. Several gas leaking events have occurred on expansion bellows of BOG piping, resulting in dock unavailability and may result in interruption of LNG shipping schedule. Preliminary specification review suggested that the subjected bellows operational cycles were far below the designed number of cycle. This fact, along with the evidence of bulging sections found at five to seven o'clock position, indicated possible overstress phenomenon. This study verifies the overstress phenomenon and recommends the counter measures to prevent similar problem by implementing three approaches i.e.: reviewing the bellows design and specification, developing piping stress model to obtain general overview of stress experienced by piping system in comparison to the result against the construction code, and developing finite element model to obtain more detailed stress profile within the bellows convolutions. The study results showed that the main cause of the overstress condition is a steep temperature gradient between the bottom and the top side of the bellows during transient period of cooling process of BOG piping, which happened in every loading schedule. To prevent similar failure, a change in bellows specification, modification of piping supports and installation of temperature transmitter to enable monitoring and control of BOG cooling process are recommended to be implemented.

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