Direct Calculation of Wave-Induced Loads and Fatigue Damage of Container Vessels

Licentiatavhandling, 2011

Container ships and their rules for fatigue design are in several ways different compared with other types of commercial ships such as tankers and bulk carriers. For example, most modern container ships have a pronounced bow flare and an overhang stern. This unique hull form, in combination with high service speed, gives rise to large ship motions that require nonlinearity in wave loads must be taken into account. Another important characteristic of container ships is that the U-shaped cross sections due to the large deck openings make the ship structure sensitive to wave-induced torsion, especially in high waves. A consequence of the open cross section is low torsion rigidity of the ship hull, which, together with the high service speed and large ship motions, demands new stricter requirements for fatigue design of the future container ships. The objective of the current thesis was to review the design methodology in current ship design regarding wave-induced structural loads and fatigue strength assessment. The outcome and contribution to both industrial and scientific relevance of the research work is a novel and comprehensive calculation procedure on the direct calculation of wave-induced loads and fatigue damage assessment with the target application of container ships. It comprises hydrodynamic analysis, finite element (FE) analysis followed by fatigue assessments. A 4400TEU Panamax container ship is used for case study in the thesis. The wave loads and ship structural responses are based on the nonlinear time-domain hydrodynamic analysis, with particular attention to wave-induced torsion. Together with full-scale measurement data, the nonlinear vertical bending moments from hydrodynamic simulations are employed for the extreme hogging and sagging prediction. Global and local FE models of the ship are designed and used in the structural analysis. A procedure for calculation of the stress concentration factor (SCF) for local details is proposed which compares the ranges of the hot spot stress and the nominal stress. The results from the FE analysis are used in a fatigue assessment procedure. Fatigue damages in two structure details are calculated using the rainflow counting approach. Additionally, a designed wave scatter diagram for the North Atlantic was introduced for the computation of a long-term fatigue damage accumulation. The approach and models presented in the thesis have been validated against full-scale measurements of ship motions and stress responses. In addition, a numerical code for fatigue route planning and monitoring is presented, which will be further developed in future work. Finally, it is believed that the numerical procedure proposed contributes to enhanced accuracy in the estimation of fatigue damage of container ships.