The linear diffraction theory used for calculating pressures, forces and moments on large structures in. regular long crested waves is extended to short crested multi-directional random waves and applied to the case of a large surface piercing cylinder. Computer calculated theoretical results are compared with the results of measurements on a cylinder in laboratory generated short crested seas. Comparisons between theory and experiment were made in terms of transfer functions, calculated in the latter case from the measured spectra of the waves, pressures, forces and moments. The linearity of the experimental results and their agreement with theory even in severe sea states show that the theory can be used for design purposes.


Large water depths and severe environmental conditions have led offshore engineers to consider the use of monolithic concrete structures with members large enough to modify the wave field. This means that the wave-structure interactions are generally in the diffraction regime(1).

Linear diffraction theory(2) is usually used for design in this situation. This theory, which neglects viscous effects, has been used to calculate the pressures, forces and moments on large bodies in long crested (uni-directional) sinusoidal waves which were also used in the laboratory to test the theory.

However real seas are both random and short crested (i.e, multi-directional with wave energy propagating simultaneously over a wide spread of directions). In this paper the theory is extended to short crested random seas and tested against experiments on a surface piercing cylinder in the Complex Sea Basin at the Hydraulics Research Station where seas with an angular spread of energy can be generated.

The theoretical and experimental comparisons are made in terms of the transfer functions which relate the incident wave spectra to the spectra of the responses of interest, namely pressures, forces and moments. These transfer functions are of great importance in design because they describe the response of the structure and enable its response spectrum corresponding to any wave spectrum to be calculated. From the moments of the response spectrum the extreme values required for design can be estimated(3).

Multi-directional random waves

Multi-directional (short crested) random seas can be considered as linear sums of trains of long crested random waves from all directions; the wave trains from different directions being uncorrelated. The pressures, forces and moments on the cylinder in short crested waves can be derived from the sum of all the contributions from long crested waves from all angles. Thus the instantaneous total pressure at a point on the cylinder is the linear sum of the instantaneous pressures from the long crested waves incident from every angle in the directional spectrum. The instantaneous total forces and moments result from integrating this total pressure and consequently they act in any direction.

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