Thermal cyclic resulting in alternating thermal stress in pipe has been identified as the primary cause of high cycle thermal fatigue failure. From several literatures, most of which involve nuclear power plant industry, the failure is caused by ageing. The monitoring and assessment methodology has been focused and developed continuously. Conversely, in upstream oil and gas industry, the failure due to thermal cyclic stress in piping system has not much been reported in open literatures.

In recent year, at the upstream production platform of PTTEP's Greater Bongkot South Field, there was a gas seeping incident at a regeneration gas piping. The overstress due to thermal fatigue at the weldment was suspected to be one of the root causes of the incident apart from the quality of welding profile. Pipe stress analysis for thermal cyclic condition is conducted to check capability of the piping system to withstand a thermal cycling effect due to overstress in the current operating condition. The thermal cyclic in a regeneration pre-treatment unit is brought into the justified case as it has operated in the large thermal range and possibly failed due to the accumulative repeat of the stresses.

The pipe stress analysis result reveals that the existing piping routing has overstress in some locations. The overstress is caused by insufficient flexibility in the piping system. The feasible solutions i.e. modifying the pipe route and modifying the pipe supports are modelized and tested in the study. The final solution is to modify the pipe support as it induces less local stress amplitude in the system and it could be implement with less modification work at site. The solution also provides a margin in the number of operating cycles. Thus, the system is able to operate over 60,000 cycles (as the base case is at 20 years design life) thru approximately 120,000 cycles (40 years) with the current operating condition.

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