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

Container ships and their rules for fatigue design are in several ways differentcompared with other types of commercial ships such as tankers and bulk carriers. Forexample, most modern container ships have a pronounced bow flare and an overhangstern. This unique hull form, in combination with high service speed, gives rise to largeship motions that require nonlinearity in wave loads must be taken into account.Another important characteristic of container ships is that the U-shaped cross sectionsdue to the large deck openings make the ship structure sensitive to wave-inducedtorsion, especially in high waves. A consequence of the open cross section is lowtorsion rigidity of the ship hull, which, together with the high service speed and largeship motions, demands new stricter requirements for fatigue design of the futurecontainer ships.The objective of the current thesis was to review the design methodology in currentship design regarding wave-induced structural loads and fatigue strength assessment.The outcome and contribution to both industrial and scientific relevance of theresearch work is a novel and comprehensive calculation procedure on the directcalculation of wave-induced loads and fatigue damage assessment with the targetapplication 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 waveloads and ship structural responses are based on the nonlinear time-domainhydrodynamic analysis, with particular attention to wave-induced torsion. Togetherwith full-scale measurement data, the nonlinear vertical bending moments fromhydrodynamic simulations are employed for the extreme hogging and saggingprediction. Global and local FE models of the ship are designed and used in thestructural 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 thenominal stress. The results from the FE analysis are used in a fatigue assessmentprocedure. Fatigue damages in two structure details are calculated using the rainflowcounting approach. Additionally, a designed wave scatter diagram for the NorthAtlantic was introduced for the computation of a long-term fatigue damageaccumulation.The approach and models presented in the thesis have been validated against full-scalemeasurements of ship motions and stress responses. In addition, a numerical code forfatigue route planning and monitoring is presented, which will be further developed infuture work. Finally, it is believed that the numerical procedure proposed contributesto enhanced accuracy in the estimation of fatigue damage of container ships