Investigations Into Uncertainties in Fatigue Damage During Transport Phase for Topside Structures

Up to now, the uncertainties in the fatigue utilization during operation (long-term, typical 20 to 30 years) phase have been widely investigated for various oil and gas installations while limited attentions have been paid to the fatigue damage during the transport phase. In normal, the fatigue damage during the transport phase is assumed to contribute limited proportions (for example, less than 5%) of the total fatigue damage for the whole life since the transport duration is at most several months. However, as the size of oil and gas installations increases, the fatigue damage during the transport phase may increase noticeably considering the inertia forces in moderate or severe sea states. During the transport, the weather encountered may deviate significantly from the long-term statistical values. It becomes crucial to determine the uncertainties in the calculated fatigue damage during the transportation phase. Nowadays, the uncertainties are mainly accounted for using Design Fatigue Factor (DFF) while the value of DFF may be different in different standards. In this paper, the fatigue damage for a topside module during the transport phase is studied. Three different vessels are to be used for comparison purpose. The uncertainties due to the sea states encountered are focused. Simplified approach is adopted to investigate the factors influencing fatigue damage. In addition, the calculated fatigue damage is also compared with the fatigue damage based on simplified fatigue analysis.