Sensitivity Study of Calculated Jacket Fatigue Damage due to Long Term Distribution of Wave Heights

Marine structures like jacket structures are often highly utilized structures operating in an environment dominated by dynamic loading. The fatigue limit state is of main concern and is to a large extent governing the structural dimensions and the amount of resources utilized in inspection and maintenance of members and joints. There is a considerable degree of uncertainty related to the parameters determining the fatigue damage. The models applied, both for describing the fatigue driving mechanisms e.g. the wave-description and load modeling and the deterioration mechanism, are always compromises between the ability to accurately describe the nature and computationally efficiency. The main focus in this paper is to show how sensitive the calculated fatigue damage of a jacket is to different models for the short term variability of wave heights. To obtain consistent basis for comparison a deterministic fatigue analysis is considered and a potential structural dynamic amplification is not included in the comparison study. Sensitivity to selection of wave spectra will not be addressed. In a deterministic approach the long term distribution of individual wave heights is used to calculate the stress ranges occurring in the joints and butt welds. Typically, the long term variability of sea state conditions is given by a scatter diagram of significant wave height (Hs) and the peak period (Tp). When converting the scatter diagram of sea states to the long term distribution of wave heights, it is common to assume that the individual waves in the sea states are Rayleigh distributed. Later developments indicate that a Forristall distribution may be a more accurate assumption. The following cases have been considered: 1. Assuming that the individual waves in each sea state are Rayleigh distributed. 2. Assuming that the individual waves in each sea state follows a Forristall distribution. 3. Calculating the long term wave height distribution from time domain simulations. In the third method, second order wave theory was used to simulate all sea states in the Hs/Tp scatter diagram. I.e. extensive time domain simulations were carried out to cover the complete scatter diagram of possible sea states. The study is performed for an 8-legged jacket. The analyses are performed for a typical North Sea wave environment for water depth about 110 m. The objective of this study is to investigate the robustness in the current design practice for jacket structures where the individual waves in the sea states are Rayleigh distributed. The paper documents the calculated fatigue lives for main joints along the height of the jacket for the three wave height distributions. Further, the paper gives advice on application of wave distribution models for design of new structures and reassessment of existing structures.