Study On The Calculation Method Of Non-linear Wave-induced Load In Ship Longitudinal Strength Analysis

Wave-induced load is critical in ship longitudinal strength verification. For standard ships, the design value of wave-induced load can be obtained from empirical formulae in classification rules. For non-standard ships, the wave-induced load should be calculated through hydrodynamic methods. Nowadays, the linear method based on frequency-domain linear algorithm and spectrum analysis for long-term predication is the most commonly used method in ship industry, which is also recommended by the classification societies. But due to the linear hypothesis in this method, large error will occur when it is applied to ships with small block coefficient. Especially in the prediction of the design value of wave-induced wave, as the design value only happens in extreme sea state where the non-linear effect of the wave-induced load is not negligible. In this condition, non-linear algorithms and statistical method should be applied. In this paper, the non-linear method to calculate the design value of wave-induced load in ship longitudinal strength verification is studied. The main work and conclusions are listed below:(1) Compared the rule formulae, trochoidal wave method, the frequency-domain algorithm and time-domain non-linear algorithm used in calculation of wave-induced load. It is found that for ships whose block coefficient is small with large bow flare and non-vertical side, only the non-linear algorithm should be applied to obtain correct result.(2) A ship whose block coefficient Cb = 0.52 is studied in the paper. Thewave-induced vertical bending moment is calculated by the frequency domain algorithm and time-domain method in stochastic waves, the long-term extreme value is obtained from statistical method.(3) A simplified time-domain method called non-linear design wave method is presented. The parameter of the design wave is chosen in the same way as the linear method, but time-domain non-linear calculation is then applied to the design wave to calculate the ship’s motion and wave-induced load.(4) The procedure to apply the “non-linear design wave” on whole ship direct calculation is suggested, including the FEM modeling requirement, boundary conditions and loads. The result of structure calculation of the studied ship is presented.