Review and Assessment of Fatigue Data for Offshore Structural Components Containing Through-Thickness Cracks

In recent years, structural integrity management schemes for offshore installations have placed increased reliance on the use of flooded member detection (FMD) as the principal inspection method. This method can be routinely employed in a remotely operated vehicle, which enables a large number of members to be inspected fairly quickly at a much reduced cost compared to using diver operated techniques. However, reliance on FMD for safety assurance requires that welded joints retain sufficient fatigue life and static strength after through-thickness cracking. A comprehensive examination of published work containing data on fatigue lives beyond through-thickness cracking in offshore structures was carried out, resulting in the development of a database of 281 relevant tests. The database was used to perform a statistical assessment of the effects of different testing conditions and geometrical parameters on the remaining fatigue life beyond the occurrence of through-thickness cracking, N₃, which was represented by a parameter Re. Whilst the data showed a large amount of scatter, it was found that Re depends strongly on chord thickness, loading mode, type of joint and testing environment. In some cases, a significant amount of remaining life existed. This was often associated with T-type tubular joints with thin chord thickness under out-of-plane loading and a seawater (with CP) environment. The influence of the relevant parameters on Re is discussed and attributed to their effect on crack shape, stress distribution, cracking location and crack propagation path.