The role of corrosion fatigue crack growth mechanisms in predicting the fatigue life of offshore tubular joints

The present work examines several aspects of the corrosion fatigue behaviour of tubular welded joints of the type used to fabricate fixed Offshore drilling platforms. It is possible for the structural integrity of such structures to become compromised by the development of fatigue cracks within the tubular joints. Fatigue design of tubular joints has traditionally been based on statistical evaluation of fatigue endurance results from constant amplitude laboratory fatigue tests conducted in air. In contrast, the North Sea environment encompasses highly variable random amplitude loading and corrosion. Safe design and maintenance of fixed jacket Offshore structures relies on accurate prediction of the rate of growth of fatigue cracks in tubular joints during the service life. Fracture mechanics analysis may be used to predict the growth rate under random amplitude loading, by using an appropriate corrosion fatigue growth rate relationship. The accuracy of fatigue crack growth predictions is dependent upon the suitability of the chosen corrosion fatigue crack growth rate relationship, and the techniques used to model crack growth under variable amplitude loading. A series of tubular joint corrosion fatigue tests have been conducted using simulated North Sea wave loading (WASH) under cathodic protection. These tests produced fatigue lives which were, on average, four times less than the mean life in air. There was no trend in the data to suggest that cathodic protection restores the in-air life. The tests also showed that the use of realistic loading produces crack growth behaviour consisting of slow shallow crack growth, followed by very rapid growth to chord wall penetration. The rapid growth rates were observed to be independent of the applied loading. Tests on large compact tension specimens have shown that corrosion fatigue crack growth rates in large specimens are limited by diffusion of dissociated hydrogen from external surfaces to the crack tip region. The compact tension test data have been used to quantify this time-dependent corrosion fatigue behaviour, and to develop a new corrosion fatigue fracture mechanics crack growth methodology which is capable of accurately modelling the crack growth in the tubular joint tests. The new prediction methodology has also been used to make fatigue life predictions at representative service lives