EngineeringMarine EngineeringOcean EngineeringOffshore StructuresStructural DynamicsStructural EngineeringShip Motions In WavesSurface Wave DynamicsWave DiffractionWave LoadingWave Propagation In StructuresFree Surface ModelingNumerical ModellingOscillatory IntegralsWave Load AnalysisWave Load AssessmentWave Load EstimationWave Load PredictionWave Loading On Structures
The second-order wave loads which act on an offshore structure include mean, difference-frequency, sum-frequency, and second harmonic components. These are of special importance to vessels with low- or high-frequency resonant features outside the spectral range of wave encounter. Special numerical techniques are required to evaluate the second-order loads. The most difficult component, associated with the boundary condition on the free surface, requires the evaluation of slowly-convergent oscillatory integrals over this surface. Numerical methods are described for evaluating the second-order loads, based on extensions of the first-order panel method and applicable to three-dimensional structures of arbitrary geometric form. Illustrative results are presented for the second-harmonic and sum-frequency diffraction loads on the ISSC TLP, in monochromatic and bi-chromatic waves. Emphasis is given to the solution of the second-order free-surface condition, which is the dominant source of sum-frequency vertical loads.