Evaluation of Extreme Design Wave Loads for Hull Girder Strength Assessment
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- Institutt for marin teknikk 
Ships may operate in severe sea states, which might induce large vertical bending moments. The classification rules have developed empirical curves for the extreme wave-induced vertical bending moments. It is therefore of interest to see how a vessel satisfying the requirements of the classification rules cope with extreme wave loads and freak wave events. A cross section amidships of an LNG carrier is designed according to DNV GL rules. The design loads from the classification rules are checked against the capacity of the cargo hold by using Smith s method. Smith s method gives ultimate capacities of 4.86 GNm and -5.24 GNm in hogging and sagging, respectively. The iterative procedure of Smith s method shows that the ultimate capacity is larger than the environmental loads found according to the classification rules. Partial safety factors are included according to DNV GL s requirements to account for un- certainties. The ultimate capacity of a one cargo hold model is then found in terms of NLFEA (Nonlinear Finite Element Analysis) in Ansys, in order to obtain more accurate results. For this purpose, FE (Finite Element) models in sagging and hogging conditions were created separately, with a finer mesh in the regions where compressive forces were evident. A mesh refinement study has been carried out for both models. Results from the NLFEA show that the ultimate capacities are found as 5.04 GNm and -5.4 GNm in hogging and sagging, respectively, for the one cargo hold model. The capacities are larger than the environmental design loads according to the classification rules. NLFEA in Ansys has been performed for a three cargo hold model as well. Only a half model is used for the analyses, such that computational time could be saved. Symmetry boundary conditions have been applied along the centerline. The capacities in pure vertical bending moment conditions have been assessed, where the capacities were found to be 5.2 GNm in hogging and -5.6 GNm in sagging. Long-term analyses using All short-term sea states and Long-term extreme value methods have been performed. The results show that the 25-year extreme wave-induced vertical bending moment amidships is equal to ±1.84 GNm for the Statfjord field with head-on sea conditions. On contrary, the envelope curve gives 1.5 GNm and -1.75 GNm in hogging and sagging, respectively. Comparing the results to the freak wave event New Year wave, the 100-year extreme is closest to the vertical bending moment induced by the freak wave event. The RAO (Response Amplitude Operator) for the vertical bending moment has been calculated in HydroD. Three fields have been looked into in this thesis, namely: Heidrun, Statfjord and Southern North Sea (Ekofisk). It is seen that the Statfjord field gives the largest vertical bending moments from the long-term analysis. Even though the wave-induced extreme vertical bending moments are greater than the values from the envelope curve, the capacity of the vessel satisfies the limit state safety format for ultimate strength with the extreme loads from the statistical analyses. Long-term analyses for all direction have been performed, assuming that the wave heading angle follows a uniform distribution. This approach is in agreement with the approach of the classification rules. However, the assumption of uniform distribution for the heading angle is questioned in this thesis. Subsequently, a long-term analysis in a range of 160◦ - 200◦ has been performed, assuming a uniform distribution of the wave heading angle in this range. Finally, a more reasonable distribution of the heading angle has been proposed for further work.