Zhihuang Ran, Daejun Kim, Serdar Celebi (PhD students)
Nonlinear Coupled Motion Analysis of Spar Platforms
For the reliable motion prediction of most compliant platforms, conventional linear wave-body interaction theory is not adequate and nonlinear analyses have to be used. Various nonlinear phenomena, such as nonlinear interaction with multidirectional waves and currents, loading by large amplitude waves, half-mooring interaction and slow drift damping, are not very well understood yet.
The spar is a new cost-effective platform concept for deepwater development. Particularly, this platform is the 'theme structure' of the coordinated OTRC research. The capability of the performance evaluation of this kind of compliant platforms for realistic sea conditions using nonlinear analysis computer programs is one of the main subjects in fluid/structure thrust area.
Recently, a JIP experimental program for a spar in the OTRC wave tank was completed. A series of experiments were conducted for monochromatic, bichromatic, and random waves with and without currents, and the experimental results are now available. We developed a nonlinear motion analysis computer program which can numerically simulate these kinds of experiments. The higher-order boundary element method was used for the computation of nonlinear wave loading. The comparison between theory and experiment looks promising. To further improve our numerical capability, more rigorous approaches pertaining to wave-current-body interaction, wave drift damping, and mooring line damping are developed. In addition, a coupled tether-hull-mooring dynamics program is developed to assess the coupling effects. A fully nonlinear 3D BEM program is being developed in the time domain by MEL method and can be used as a numerical wave tank.
Computer programs for wave drift damping and nonlinear wave loading are also being developed in MIT, Oxford University, and University of British Columbia.
The initial phase of our comparative study indicates that our numerical simulation agrees fairly well with the JIP experimental results. We believe that the developed computer program can be used for an extensive parametric study to find the optimum design of a spar. A similar program can also be used for the hydrodynamic optimization of other compliant platforms, such as TLPs.
September 1993 to September 1997
New second-order slender-body formulas were developed and have been published. They produce a means to produce simpler and faster reliable results for relatively slender structures, such as SPARS and ALPs. A time-domain higher-order boundary element approach is being developed to compute nonlinear wave loading, wave-current-body interaction, and wave drift damping on compliant offshore platforms. A study comparing numerical predictions and experimental results for the theme SPAR platform is summarized in ISOPE '95 and '96 papers. A fully coupled tether-hull-mooring dynamics program is developed and the program will be licensed through OTRC in the future.
Through a related JIP for the PC-based computation of the second-order wave loads on multi-column structures, the WINTCOL program was developed and is being used worldwide. Recently journal and conference papers describing the nonlinear motion analysis were published and more are planned.