The effect of gravity on the dynamic characteristics and fatigue life assessment of offshore structures

This paper intends to draw attention to the influence of gravity on the dynamic response and fatigue damage assessment of offshore structures. In traditional fatigue life calculation, the gravity loads of structures are assumed to only contribute to the mean stress of the structures. This paper examines the significance of P-Delta (P-Δ) effects and the stress “stiffening/softening” induced by gravity. This paper first explains the two aforesaid gravity-induced effects, with studies on their influence on both eigenperiod and fatigue life assessment. A modal analysis of a typical offshore structure with large degrees of freedom is followed, to identify the dynamic characteristics influenced by the gravity effects. It is discovered that gravity load can induce a tendency to cause additional compressive and tensile forces to coexist in various structural components, causing the eigenperiods to increase, decrease or even cross each other. Compared to the stress “stiffening/softening” effects, the P-Delta effects on tuning the structure's stiffness and eigenperiods are insignificant in mild sea states. Furthermore, based on the nonlinear dynamic response calculation and a type of efficient wave energy inputs, a procedure for calculating fatigue damage is adopted and a systematic investigation of fatigue calculation of the structure is performed. It is discovered that ignorance of the gravity loads can underestimate the fatigue damage by up to 24%. Finally, through a series of investigations, it is discovered that gravity can have significant effects on both the response statistics and frequency content of the structural responses.