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It is common practice to employ direct calculation procedures for spectral based fatigue assessment. Numerical codes are used to compute direct hydrodynamic loads. There are several complexities and nuances associated with application of loads on finite element (FE) model. It is a computationally expensive task especially when a large number of cases need to be analyzed (for e.g. spectral fatigue analysis). The present paper outlines an approach to evaluate the stress transfer function based on the direct application of moments (vertical bending, horizontal bending and torsional moments) computed using a frequency-domain based sea-keeping code. Multi-point constraint (MPC) method is utilized for application of bending moment. The structural responses computed using direct application of the bending moments (Method 1) and the panel pressures (Method 2) are compared. The evaluated stress transfer function is used for spectral fatigue analysis. Overall, the present study provides a methodology for spectral fatigue assessment using direct application of bending moments.

This paper presents a detail comparative study on wave induced vertical bending moment (VBM) between linear and approximate nonlinear calculations using a 3D numerical wave tank (NWT) method. The developed numerical approach is in time domain where the ambient incident waves can be defined by any suitable wave theory. Certain justifying approximations employed in the solution of the interaction hydrodynamics (diffraction and radiation) enabling the NWT to generate stable long duration time histories of all parameters of interest. This is an extension of our earlier works towards the development of a practical NWT based solution for wave-structure interactions [1]. After a brief outline of the implemented numerical details, a comprehensive validation and verification of vertical shear force (VSF) and bending moment RAOs computed using the linearized version of the NWT against the usual linear results of strip theory and 3D panel codes are presented. Next we undertake the comparative study between the fully linear and approximate nonlinear versions of the present code for different incident wave steepness. In the approximate nonlinear formulation, the ambient incident wave is defined by the full nonlinear numerical wave model based on Fourier approximation method which can generate very steep steady periodic nonlinear waves up to the near wave breaking limit. The nonlinearities associated with the incident Froude Krylov and hydrostatic restoring forces/moments are exact up to the instantaneous wetted surface at the displaced location, but the hydrodynamic diffraction and radiation effects are linearized around the mean wetted surface. The standard S175 container hull is considered for the comparative studies because of its geometric nonlinearities. Numerical simulations are performed for four different wave lengths with increasing wave steepness. It is observed that the computed wave induced VBM amidships from the approximate nonlinear results can be almost 30% higher compared to the results from a purely linear solution, which can be a critical issue from the safety point. Significant higher harmonics are also observed in the approximate nonlinear results which at some times may be responsible for exciting the undesirable whipping/springing responses.

The Spectral Fatigue Analysis (SFA) is a comprehensive fatigue life assessment method. The SFA is performed by following a systematic process at the onset of hydrodynamic analysis, structural analysis, and spectral analysis. Hydrodynamic analysis and finite element based structural analysis are numerically intense stages, and require a substantial amount of computational time and resources. In the present paper, some simplifications are imposed on individual stages to perform the SFA analysis in a practical time scale but not compromising on the underlying theoretical assumptions. Three distinct methods (semi-analytical formulation, 2D strip method, and 3D panel method) have been used to compute the wave-induced loads while the structural responses are obtained using the beam theory based formulations (in case of semi-analytical and 2D strip method) and finite element analysis (in case of 3D panel method). Fatigue damages are calculated using these methods at the selected locations of a bulk carrier and results are compared with each other. It has been shown that the first two methods (semi-analytical and 2D strip based methods) are quick and efficient and can be used in initial design assessment or identifying the fatigue prone locations. The third method is realistic and accurate and can be used in case of a comprehensive assessment of the design.