Wave load on the elements of offshore structures in the shallow water zone. Part 1

We analyze the applicability of the theory of long nonlinear waves to the problem of determining wave loads on elements of offshore structures in the coastal zone of the sea. It is shown that for calculating wave loads in the shallow sea zone, applying the theory of cnoidal waves is necessary, which gives the best agreement with experimental data compared to linear theory and other approaches. Basing on a three-dimensional model of wave diffraction on an obstacle, which has the shape of a body of rotation around a vertical axis, it is shown that the principle of harmonic superposition can be applied in determining wave loads. The nonlinear interaction between waves impinging on an obstacle and reflected from it is estimated. Its nonlinearity is shown negligible at distances of the order of the characteristic size of the obstacle. Therefore, the problem of diffraction of the cnoidal waves on bodies of rotation can be reduced to a sequence of the problems of diffraction of harmonic waves obtained by proper decomposing the cnoidal wave impinging on an obstacle. Within the proposed approach, wave loads on a vertical circular cone obstacle were assessed for various values of the parameter characterizing the wave steepness. The effect of the steepness of the oncoming waves on the behavior of wave loads and overturning moments is analyzed. For fairly short waves and the cone with a 45°-inclined generatrix, the change of the sign of the overturning moment is observed. This occurs in the cases when the maximum total overturning moment is directed towards the oncoming wave. The obtained results may be useful for the design and calculation of the buffer marine oil storage with the submersible tanks installed on coastal shelves and exposed to the active influence of wind waves.