Jack-up platforms generally behave like highly dynamic system because of high structural flexibility. This paper discusses how this dynamic behaviour influences the selection of sea states for design of such platforms. A jack-up platform in seven different operational conditions giving basic natural period of the platform in the range 4 to 10 sec is studied. Results from stochastic long term analysis show that the contribution to extreme long term response from different sea states in the wave scatter diagram is highly influenced by the dynamic characteristics of the structure. For the example platform, operational conditions giving natural periods in the range 6 to 8 seconds give much lower sea states as the most important sea state for the design value than conditions with natural period around 4 sec. The study also includes the effect of wave spreading as well as the effect of using double peaked wave spectra. Finally it is shown how the results from this work can form the basis for development of future design methodologies for dynamic sensitive structures.
The use of Jack-up platforms for drilling and as production facility in relatively deep water has increased in the past few years. The Jack-up platforms are highly flexible and hence they exhibit significant dynamic behaviour. These platforms are also very sensitive to nonlinear drag loading from waves and current. Due to the highly nonlinear nature of these forces the response can not be treated as a Gaussian process (Leira and Karunakaran, 1990 and Gudmestad et al., 1990). They must therefore generally be analysed by nonlinear dynamic response methods. Furthermore, due to their dynamic behaviour, such Jack-up structures may have a lower sea state as the design sea state than the extreme sea state, depending on the dynamic properties of the structure.