Low Cycle Corrosion Fatigue in the North Sea Environment

Abstract

Low cycle corrosion fatigue is an interesting issue in civil Engineering because it may effect the lifetime of structures in negative and unexpected ways. However, analysis of design codes has revealed that low cycle corrosion fatigue is not a major issue in the design of offshore structures because these structures are designed with a global elastic philosophy. This means that the number of cycles that a structure can withstand usually stays well within the high cycle range. The joints ( welded or otherwise ) revealed to be the less resistant elements to low cycle corrosion fatigue. Furthermore, it was revealed that in the ultra low cycle region the effect of the environment is heavily reduced. It was also revealed that the effect of cathodic protection is strongest in the upper spectrum of the high cycle range and deteriorates in the lower spectrum of the high cycle range. It was also revealed that a structure in air and a structure in seawater with cathodic protection have essentially the same fatigue resistance when in the ultra low cycle range. In the ultra low cycle range, the problem appears to turns into mostly a low cycle fatigue problem. In the ultra high cycle region the problem appears to turns into mostly a corrosion problem. S-N curves that can predict the low cycle fatigue life of structures in a deleterious environment are very rare. The only S-N curve available that can be used for structural steel subjected to low cycle corrosion fatigue is one for tubular sections. However, Mathematical modeling based on regression and a principle of constant slopes in a fatigue cycle region ( low cycle, high cycle, ultra high cycle ) has made it possible to not only calculate the low cycle corrosion fatigue life of tubular joints but the low cycle corrosion fatigue life of other types of joints as well.