The first step in the response analysis procedure is the mathematical modelling of the sea environment and the loading imposed on the structure. The sea environment may be characterized mainly by overwater wind, surface waves and currents. Overwater wind during severe storm conditions could significantly influence the design of offshore structures because of the large forces it can induce on the exposed parts of the structure. The wind loading, however, has not been considered in the present work. For the fatigue analysis of offshore structures hydrodynamic loadings caused by the surface waves are of major importance. As the fatigue is known to be low stress high cycle phenomenon, even the low to moderate sea could significantly add to cumulative fatigue damage. Finally, currents may also appreciably contribute to the total forces exerted on the structure. The following sections describe the formulation of the mathematical model of the sea environment and the hydrodynamic loading induced on the structure by waves and currents.
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