Research On Moment Similarity Of Truncated Mooring System And Truncation Of Asymmetric Mooring System

New concepts of floating platforms will be used to fulfill thedevelopment of deepwater oil and gas exploration. Wave basin model testis one of the effective ways to verify the platforms and the mooringsystems. Hybrid model method can be used to solve the problem that fullwater depth model test can not be carried out in current wave basins.Therefore it is crucial to perfect the design of truncated mooring systems.Roll and pitch motions are important to a semi-submersible platform,and have a direct effect on its air gap response. Based on asemi-submersible platform operated in1500m water depth, five truncatedmooring systems are designed for the model test. They have similarhorizontal restoring force characteristics, vertical force-horizontaldisplacement characteristics and top tension-horizontal displacement ofsingle line as the full depth system, but different restoring momentcharacteristics. Survival condition in180oand225odirections andoperational condition in180odirection are considered in comparing the rolland pitch motions using these different truncated mooring systems.It is concluded that moment characteristics similarity of the truncatedmooring system has a significant impact on roll and pitch motions for asemi-submersible platform. And the impact is greater as the horizontaloffset of the platform becomes larger. This is due to the fact that thetruncated mooring system has greater restoring moment when thehorizontal offset is bigger and the moment can not be ignored compared tothe restoring moment of the platform. The low-frequency dynamicresponse of the roll and pitch motions are to be influenced and the wave-frequency dynamic response are barely affected with truncatedmooring systems of different restoring moment characteristics.Therefore, keeping the similar moment characteristics with the fulldepth system is essential while designing truncated mooring system forsemi-submersible platforms, in order to make sure the roll and pitchmotion responses are closer to the ones in full water depth condition,especially when considering survival condition with greater motionresponses.In addition, to design truncated mooring systems for asymmetricmooring systems has always been difficult and preliminary research hasbeen conducted in this thesis. The specific difficulties are introduced that itis hard to design truncated mooring systems with multi-objectiveoptimization method to fulfill static characteristics similarity in severaldifferent directions, due to the effect of truncation factors. Based onanother semi-submersible platform, the process to design truncatedmooring systems for survival condition is introduced. Consideringhorizontal truncation factor, the effect of mooring line parameters on thestatic characters of the system is researched. With truncated mooringparameters modified repeatedly, the final design is proposed. It is foundthat the final designed truncated mooring system meet the static characterssimilarity and the comparison curves are provided.