Risk Management of Through-Thickness Cracks In Offshore Fixed Structures Available to Purchase

Despite the increasing number of ageing structures in the North Sea, the overall trend is for the level of inspection to decrease. The increased use of flooded member detection and visual techniques in preference to other techniques for in-service inspection of fixed platforms leads to situations where through-thickness cracks (TTCs) may be present in welded components, prior to their detection. TTCs have been observed offshore and in some cases growth of these cracks has led to member separation. It is important to consider the capacity of welded components containing TTCs in the light of the original design requirements and the redundancy in the overall structure. Equations on the performance of welded joints and members with TTCs have been derived from limited test data. In practice, such cracks can grow substantially beyond the through-thickness stage, depending on the frequency of inspection. In some joint types (e.g. ring stiffened) it is also known that when TTCs develop the remaining life is less than in conventional joints and the loss in load carrying area is also increased. Cracks in circumferential welds in tubulars, particularly when single-sided, are often only found at the through-thickness stage; data on the performance of this type of cracked joint are very limited. The implications on structural integrity of these factors are assessed. There is limited information on the effect of TTCs on the structural integrity of welded joints in frame structures. Additionally, it is important to note that the repair of components with TTCs requires a considerable amount of preparation time. Consequently, the integrity of the platform may be at risk for the duration of the repair. Overall, the extensive use of flooded member detection requires careful consideration of the additional risk factors arising from the presence of TTCs in components.