Statoil has over the last 25 years increasingly used duplex stainless steels for their offshore and onshore installations. The main applications include process piping systems, pumps, valves, vessels, manifolds, umbilicals and flowlines. The duplex materials are selected mainly due to their good corrosion resistance, in combination with excellent mechanical properties (high yield strength) and the resulting weight saving potential. As the knowledge of the material properties has improved, the duplex stainless steels have been increasingly used for progressively tougher process conditions. Correct use of these materials requires good knowledge of the metallurgy and microstructural development during manufacturing and welding to achieve a sound austenite - ferrite balance, as well as avoiding detrimental intermetallic precipitates. The service experience has in general been very good, but operational failures happen from time to time. A number of failures are every year analyzed by the Statoil material laboratory in order to establish the cause of failure, but only a few of these cases involve duplex stainless steel materials. Internal statistics from the last ten years show that the predominant failure mode for the duplex family is fatigue, mainly due to improper design and unexpected cyclic loads. Approximately 50% of the examined failures are caused by fatigue, while about 20% are related to different corrosion mechanisms. The remaining failure modes observed are brittle overload, sand erosion, cavitation and casting/forging defects. This paper summarizes some failure cases related to the use of duplex and super duplex stainless steels at Statoil operated offshore installations in the North Sea, and the following cases and failure modes are presented in further detail: 1. Fabrication defects/forging laps in duplex hydraulic tubing. 2. Casting defects of a superduplex impeller from produced water pump. 3. Brittle overload fracture of valve stem. 4. Fatigue of a duplex stainless steel thermowell. 5. Corrosion of weld between pipe stub and weldolet
Duplex and super duplex stainless steels in the Norwegian Petroleum Industry - some failure case histories during the last ten years
Nichtrostende Duplex- und Superduplex-Stähle in der norwegischen Erdöl-Industrie: einige Ursachen der Schadensfälle während der letzten zehn Jahren
2007
12 Seiten, 20 Bilder, 7 Quellen
Aufsatz (Konferenz)
Englisch
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