1 13th International Conference on Fracture June 16-21, 2013, Beijing, China Fatigue Failure of Outlet Pipe Work of Flare System at Natural Gas Processing Plant Abdel-Monem El-Batahgy 1,*, Martin Wheeler2 1 Manufacturing Technology Department, Central Metallurgical R&D Institute, Cairo, Egypt 2 Process and Operational Safety Department, Rashid Petroleum Company, Cairo, Egypt * Corresponding author: elbatahgy@yahoo.com Abstract The natural gas produced from subsea wells is processed through different stages before being supplied through pipelines for local market or export. Processing of the produced natural gas is started with separation of gas and liquid using a slug catcher that is connected to a flare system. The flare system consists of piping made of 16" diameter branches connected to 20" diameter header using circumferential weld with a total of seven joints. The pipe work of the flare system is made of 316L stainless steel and it is operated in an interrupted mode. Design and operation pressures are 142 bar g and 88 bar g while design and operation temperatures are -40oC and 20oC, respectively. After five years of normal service, one of the seven joints of 16" branch/20" header has experienced failure. Based on different non-destructive and destructive tests, it is concluded that this premature failure is attributed mainly to fatigue damage. It is believed that acute or sharp angle of 16" branch/20" header connection that acts as a local stress raiser played a remarkable role in initiation of fatigue damage on outer surface, just beside circumferential weld. In addition, low stress high frequency vibration of the subject pipe work, due to mainly several emergency shut down operations, has accelerated initiation of fatigue damage. In order to minimize the possibility of such failure in future, design of 16" branch to 20" header connection was modified where a compensation plate fitted was used to minimize stress concentration at this connection zone. Keywords: Natural gas processing, Flare system, 316L stainless steel, Stress concentration, Vibration, Fatigue 1. Background A natural gas production plant has been set into operation about twelve years ago. The natural gas produced from subsea wells is processed through different stages before being supplied through pipelines for local market or export. Processing of produced natural gas is started with separation of gas and liquid using a slug catcher that is connected to flare system to be used in case of emergency plant shutdown. The flare system consists of piping is made of 16inch diameter pipes connected to 20inch diameter header using circumferential weld with a total of seven joints. Nominal thickness of 16" pipes is 4mm while that of 20" header is 6mm. The pipe work of the flare system is made of 316L stainless steel and it is operated in an interrupted or irregular mode. Design and operation pressures are 142 bar g and 88 bar g, respectively. Design and operation temperatures are -40oC and 20oC, respectively. After five years of normal service, one of the seven joints of 16" pipe/20" header has experienced failure where
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