An efficient nonlinear dynamic approach for calculating wave induced fatigue damage of offshore structures and its industrial applications for lifetime extension

Because traditional frequency domain analysis can hardly properly handle nonlinear effects induced from many sources, based on a nonlinear time domain dynamic analysis approach, the current paper presents a general calculation procedure and comparison for wave induced fatigue damage of a typical jacket structure under two hydrodynamic coefficient specification systems. Compared with the previous version of Norsok Standard N-003 [Norsok Standard. Actions and action effects, N-003. Oslo: Norwegian Technology Standards Institution; 1999], the inertia coefficients adopted by the latest Norsok Standard [Norsok Standard. Actions and action effects, N-003. Oslo: Norwegian Technology Standards Institution; 2007] released in 2007 are significantly decreased while the drag coefficients are slightly increased. The current paper shows that, for fixed offshore structures such as jackets, this change may result in a significantly increased calculated fatigue life. This may indicate that offshore jacket structures currently in service may be allowed a significant life time extension with regard to fatigue. In addition, the influence of the bottom support conditions of the jacket structure on the fatigue life is also investigated. The investigation indicates that, in cases where the pile-soil and conductor soil stiffness data are not available and the structure is high enough, for a rough estimation of the fatigue life on the upper part of a jacket, the jacket’s bottom support condition may be simply modelled as fully fixed. By comparing the fatigue life from dynamic analysis with that from static analysis without taking the inertia effects of the structure, equipment mass and other non-structural installations into account, it also shows the significance of the contribution from the structure’s dynamic response. The nonlinear dynamic analysis method presented in the current paper has been successfully applied in several industry projects for the life time extension of offshore installations subject to wave and wind loads.