This paper presents a procedure of modelling uncertainties in the spectral fatigue analysis of offshore structures. Uncertainties in the fatigue damage are generally two types as being natural (inherent) and model (parametric), which are embedded in response characteristics of the stress process and the damage-model used. Herein, most of uncertainties are associated with the modelling of structures and random wave environment. There are also uncertainties arising from joint flexibilities, wave-current and water-structure interactions. Uncertainties in joint flexibilities arise from degradation of member connectivity during a response process. Uncertainties introduced by wave-current interaction are related to the modelling of a random seastate, applied wave loads and effects of water-structure interaction in general. The water-structure interaction, which is an important phenomenon, introduces some added hydrodynamic damping. The associated uncertainties are reflected in the response analysis via the damping term. In a quasi-static response analysis, damping related uncertainties disappear. In addition to uncertainties in the stress process, which is assumed non-narrow banded, the spectral fatigue damage contains also some other uncertainties associated with the damage-model used. These uncertainties are related to experimentally determined fatigue data and joint configurations at which damages are likely to occur.


In the long-term performance of offshore structures, fatigue is the most important issue, which needs a proper treatment at the design stage. Because of stochastic loading-condition and uncertain environment, such a treatment is plausible only at the mean value level. In practice, it is of great interest to know the deviation of fatigue damage from the mean value and its sensitivity if a condition, e.g. a parameter in the analysis, is slightly changed. Therefore, a reliability analysis is usually switched on to assess a safety factor or the likelihood of a fatigue failure, which needs a proper modelling of uncertainties inherited in such a modern analysis.

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