Time Domain Simulation of Jack-Up in Second Order Irregular Seas
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- Institutt for marin teknikk 
For dynamically sensitive structures or marine structure sujected to large displacements (such as jack-up paltform) the extreme response is often determined on the basis of short term time domain simulation of extreme sea states using environmental contour line method. A challenge with time-domain simulation is the representation of the sea spectrum. For first order wave (Gaussian seas), there is no big obstacle to performs a complete 3-hour simulation. However, the computational requirements becomes prohibitive for second order irregular waves. The purpose of this report is to observe the effect of second order irregular waves on jack-up platform and also contribute to developement and verification of strategies on decreasing the computational time for time domain analysis. This report mainly consists of seven parts. First part consists of the review about wave theory, the probabilistic model of ocean waves, methods of establishing kinematics, methods for simulating a sea spectrum and method for calculating response of structure. In addition, some strategies for reducing computational time also presented. The second part consists of explanation about the numerical. In the third part, a verification study is performed. The second order model is compared with theoretical wave distribution and 5th Stokes wave. At the end of third part, the static analysis of second order irregular seas on single vertical cylinder with small diameter is performed. The four part deals with strategies to reduce time for calculating time for second order wave while the fourth part contains the metocean analysis. The last part presents the effect of second order irregular wave on jack-up platform. From the study, it is found that for second order wave, Wheeler method gives underestimation for wave horizontal velocity below the sea surface. Compared to 5th Stokes, linear extrapolation produces greater surface horizontal velocity though it produces smaller horizontal velocity below mean sea surface. For jack-up case and Ultimate Limit State purposes, the largest static baseshear could come from wave which has crest smaller than 100-year wave crest but has comparable wave length to distance between jack-up leg. However, the largest static overturning moment tend to occurs from wave with crest close to the 100-year wave crest. In addition, dynamic analysis of the observed jack-up produces 20% larger baseshear and 90% larger oveturning moment than static anaysis.