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Validation of Wave Response Analysis for Jack-Up Rigs

For dynamically sensitive systems such as jack-up rigs, a dynamic analysis is performed to assess their structural response. In order of their increasing complexity, the available analysis methods may be broadly classified as: (1) Single degree of freedom; (2) Frequency domain; and (3) Time domain approach. Some of these analysis techniques are outlined in SNAME T&R 5-5A.

The random time domain approach is considered most accurate although most complex, whereas the SDOF method is the simplest but approximate. Frequency domain methods are generally applied to simple linearized models. A frequency decomposition of the wave forces on a structure indicates that even for a regular wave, higher harmonic components are present due to non-linearities. Therefore, SDOF method severely overestimates the inertia forces near the natural period, while possibly underestimating the dynamic response further away.

This study recommends a practical frequency-domain dynamic analysis method referred here as “Wave Response Analysis” (WAVRES), which leads to comparable results on extreme values as the random time domain analysis. For regular wave steady state response, the method is based on mode superposition without the assumption of linearization and considers the effect of higher harmonics, which reduces the overall resonance effects predicted by the SDOF method. This method is further extended to random seas. It is computationally very efficient, applicable to reasonably large size models and amenable to simple interpretation.

Therefore, this method is seen as a close alternative to the random time domain analysis, and offers significant time saving to the practicing engineer without sacrificing the accuracy and being unduly conservative. Nonlinear effects such as P-delta effect, foundation stiffness etc. can be included in the method through suitable linearization.

The purpose of the study is to obtain the results from the proposed method and to compare them to more accurate time domain methods. The study outlines the proposed technique, and discusses the various frequency domain methods, such as drag-inertia parameter and Rayleigh distribution. In order to benchmark this method, the study analyzes a few practical examples of jack-up rig models and compares results from the above analyses with the time domain results for both regular and irregular waves. It has been demonstrated that “Wave Response Analysis” (WAVRES), leads to comparable results of extreme values of dynamic response as the random time domain analysis.

The method appears to give good results irrespective of the jack-up model, water depth, wave heights and periods, and foundation fixity conditions. For high current, static response using drag-inertia parameter appears to be over-predicted, but prediction of inertia forces still remains accurate, which is of prime importance for design assessment.

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