A joint environmental model for a long term response calculations is proposed. The model is based on experience gained from measurements and hindcast data from the Norwegian Continental Shelf. The model is an extension of an existing joint environmental model for Haltenbanken (off central Norway). The present model includes the following environmental parameters: 1-hour mean wind speed, current speed, significant wave height, spectral peak period, main wave direction (wind and current are assumed to be collinear with the main wave direction), and water level. Additionally, a simultaneous description of wind sea and swell is also introduced. The model is expected to be a useful tool for long term reliability calculations of offshore structures, especially as dynamics and/or other frequency dependent loading mechanisms become important. The model accounts for the lack of full correlation among the various environmental processes and it can be used for assessing the conservatism of the traditional design approach. Herein, as an illustration, the model is applied in connection with a reliability analysis of an idealized structural system, where the degree of dynamics is varied artificially. A first order reliability method (FORM) is used for this purpose.
A consistent prediction of extreme response and/or fatigue damage requires a joint probabilistic description of the main environmental characteristics. Traditionally, structural design relies on an assumption that the most important load processes, wind, waves and current, are fully (or nearly fully) correlated. Thus, the design load effects are calculated by combining worst cases of wind, waves and current and multiply the results with a proper load coefficient. The assumption of full correlation is most probably rather conservative but the degree of conservatism is generally unknown.