Estimation of Impulsive Wave-induced Loads on a FPSO

The effect of impulsive loads like slamming and green water on deck on the wave-induced bending moment is estimated by a semi-analytical approach. The impulse loads leading to transient vibrations are described in terms of magnitude, phase lag relative to the wave-induced peak and decay rate. These loads can be due to bow flare slamming, bottom slamming or green water loads as they all can be characterized by a short duration relative to the wave cycle. The magnitude of the bow flare slamming loads is estimated using accurate results from wedge-shaped sec-tions, Zhao and Faltinsen (1993) and for green water loads from the results of Buchner (1995) and Wang et al. (1998) are applied. The phase lag relative to the wave-induced peak and the decay rate are derived mainly from published experimental results, Sikora, (1998). The results are given in closed-form expressions and the required input information for the procedure is re-stricted to the main ship dimensions: Length, breadth, draught, block coefficient and bow flare coefficient. The formulas make it simple to obtain quick estimates in the conceptual design phase and to perform a sensi-tivity study of the variation of the ship’s main dimensions and operational profile. The procedure is applied to an FPSO and used to investigate the hull girder reliability taking into account structural degradations due to corrosions. The strength of the hull is modeled as a time function of corrosion rate. The hull girder reliability was thus determined as a time dependant function of corrosion. Both fatigue and extreme hull girder failure are considered.