Fatigue Assessment of Container Ships – a Contribution to Direct Calculation Procedures

Doctoral thesis, 2013

Within the field of marine structural engineering, the introduction of new materials and material classes, the development of production techniques and new design, and new requirements imposed by authorities are examples which justify a need for continuous revisit and update of fatigue assessment methodologies of marine structures. This thesis contributes to the development and improvement of fatigue assessment methodology for ship structures. Although the work is demonstrated using container vessels here, the methodology provides good insight into the direct calculation procedure of fatigue in general. A practical time-domain procedure is proposed that utilises more sophisticated numerical tools to take account for realistic sea conditions and actual operational profiles in the direct calculation of fatigue damage in ship structures. The feasibility and sensitivity of this time-domain procedure were investigated through case studies. In-depth studies are presented for nonlinear wave loads and their influence on fatigue damage estimation in the deck, bilge, and side-shell structures of container ships. The proposed time-domain procedure provides reasonable and conservative fatigue life estimations in comparison with the fatigue methodologies in widespread use. A method is proposed to derive local stress for fatigue calculation from the ship’s global nominal stress. The idea behind this method is that a stress concentration factor should be based on realistic loadings. This method utilises stress ranges extracted from the stress history, which is particularly suitable for time-domain fatigue analyses. Nonlinear finite element (FE) analyses are carried out to investigate the cyclic material response during severe wave loading conditions. The results indicate that the amount of accumulated plastic strain is often low and that the material exhibits an elastic shakedown response after repeated cyclic loadings. It was therefore concluded that elastic FE analyses followed by stress-based fatigue assessments can be successfully employed, at least for the studied cases. Furthermore, important influencing factors causing uncertainties in fatigue analyses are identified and quantified by comparing fatigue damages estimated using various numerical methods. Uncertainties connected with the wave environment are also studied through the comparison of fatigue assessment using different wave descriptions for a specific sea condition and different distributions of the long-term wave environment. In addition, concerning that the girder stresses occur as a combination of torsion, horizontal and vertical bending moment loadings, an analytical derivation for the separation of wave-induced girder stresses is presented. As a result, it was possible to identify the contribution to fatigue damage, in various locations of containership structures, from stress components that are caused by these different loading modes. The practical usefulness of the results is demonstrated in an example of ship fatigue routing between Europe and North America.