Practical Calculation of Global Design Loads and Load Effects for Large High Speed Catamarans

Methods for calculating design loads and load effects for high speed vessels are investigated. The influence of operational restrictions on design loads is emphasised. Relevant operational criteria for high speed displacement vessels are reviewed. Operational limits and design loads for a 60 m catamaran are calculated based on linear strip theory. Non-linear effects on design loads are assessed from calculations in regular waves. It is found that simplified formulas commonly used by classification societies for predicting operational limits significantly overpredict the reduction of motions and wave loads at reduced speed. For vertical bending, the use of active foils is found to increase the linear loads. Due to reduced motions, the foils reduce the non-linear loads and hence the total loads. When informal operational limits are used, the design loads for vertical and transverse bending found by direct calculations are approximately at the same level as design loads given by classification societies. For torsion, the design values found by direct calculations are substantially lower than design loads given by the rules. Whereas stresses and deformations due to vertical bending and torsion can be found by simple beam theories, transverse strength analysis has to be performed by a finite element analysis. It is found that the flexibility of the partial bulkheads typical for a twin-hull Ro/Ro-vessel is so large that the interaction with the surrounding structure has to be accounted for. A web frame model that does not include any bulkheads, gives large errors and is not recommended.