Abstract

Three inch bolts have been fatigue tested to quantify the fatigue strength in air environment and seawater with cathodic protection. The motivation behind the laboratory test project was to justify the use of cathodic protection against corrosion for bolts used in subsea systems. In fatigue design standards such as DNV-RP-C203, the use of cathodic protection is not recommended for bolts because high strength steel is prone to hydrogen embrittlement when subjected to high loads. However, based on published fatigue test data of bolts the last 2–3 years the revised DNV-RP-C203 that is currently undergoing industry hearing has proposed the following update: “The recommended practice is valid for bolts in air environment for grades up to 10.9, ASTM A490 or equivalent. In seawater with cathodic protection the recommended practice is valid for grades up to 8.8 or equivalent.

Present industry subsea practice is to use preloaded bolts exposed to cathodic protection. The objective of this paper it to assess the fatigue strength of bolts with cut threads in seawater with cathodic protection. It will further be considered whether standard design principles can be assumed, more specifically if the difference in fatigue life between air and seawater environment can be assumed equal to a factor of 2.5. The long-term aim is to gather sufficient relevant laboratory test data such that a revision of the design recommendations for bolt operating in marine environment can be justified. This is considered important as the industry seem to prefer cathodic protection as primary means of protecting bolts against corrosion.

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