The overall objective of the design of a marine structure is to ensure that the structure can resist all foreseen loads with an adequate degree of safety against failure. In this connection the most important load processes, wind, waves and current, are often assumed to be fully (or nearly fully) correlated. The assumption of full correlation is most probably rather conservative and may result in an overestimation of the design loads. This study investigates the effects of utilizing a joint environmental model of wind, waves and current when predicting the extreme response of a deep water jacket and compares the results with the results obtained using design practice. The joint environmental model is limited to the following environmental parameters: 1-hour mean wind speed. current speed, significant wave height (sea and swell), spectral peak period (sea and swell). main wave direction (wind and current are assumed to be collinear with the main wave direction), and is fitted to instrumental data (1980–1985) and hindcast data (1955–1985) from Haltenbanken (65°0"N, 7°36"E). The extreme values of the axial stress in a selected structural member are considered. The effects of using different models for the wave spectrum are indicated and compared to the effects of utilizing a simultaneous description of the environmental parameters.
Standard design procedure for offshore structures is to assume the extremes of wind, waves and current to be fully or nearly fully correlated. A characteristic maximum hydrodynamic load can be evaluated as the expected largest load during the 100 year sea state with a 10-year current field added to the wave kinematics. A characteristic maximum load is then obtained by superimposing the load caused by the 100-year one-minute wind speed on top of the hydrodynamic load. Finally this value is multiplied by a load factor of 1.3 (Norwegian Regulations) in order to obtain a design load.
