Theoretical development and validation of a fatigue model for ship routing

Artikel i vetenskaplig tidskrift, 2012

Fatigue cracks are observed much earlier than expected in ships and marine structures due to uncertainties in the fatigue design process, such as encountered sea environments and variability in S-N curves, etc. The current study presents the theoretical development and validation of a fatigue model useful for ship routing which may contribute to better utilisation of the materials and structures by more wise operation of them. During the theoretical development of the ship routing fatigue model, various models to estimate fatigue damage intensity, including both cycle counting calculations and spectral approximations, are reviewed. The proposed ship routing fatigue model is a function of operation profiles, i.e., heading angle and ship speed, as well as encountered wave environments (significant wave height), which are easily available in today’s commercial routing tool systems. Concerning the characteristics of ship response, the so-called narrow-band approximation is adopted and further simplified in order to estimate the fatigue damage in ships under an arbitrary stationary sea state. Long-term fatigue damage is estimated by a summation of fatigue damages in all encountered sea states during, for example, one voyage or a period of several years. Further, fatigue damage due to wave-induced vibrations (whipping and springing) and fatigue damage caused by various stress components are also studied and discussed. A validation using response from full-scalemeasurements and numerical analysis is presented. It shows that the proposed model works very well in comparison with the rainflow counting method. Finally, the proposed ship routing fatigue model is applied on real case scenarios, where its applicability in shipping industry is demonstrated.