Coupled Dynamic Analysis Of A Truss Spar With Nonlinear Mooring Force

Truss Spar has been widely recognized as a viable option for offshore deep and ultra-deep water oil/gas exploration due to its excellent motion performance. This thesis focuses on the analysis of nonlinear mooring force in cables and the coupled effects of Truss Spar and deep water mooring system. The analysis can provide references for design and fabrication of Spar platform.The load-displacement behavior of the mooring line is almost linear when Truss Spar has a small displacement. However, the nonlinearity increases drastically when larger excursion happens. It is necessary to study the influence of nonlinear mooring force on the motion of Truss Spar. Considering the coupled effect of horizontal and vertical motion of the top mooring line end, the multi component catenary equation is derived according to the catenary line theory. A numerical simulation program is developed by software MATLAB in order to calculate the nonlinear mooring force caused by excursion of Truss Spar. Parameters analysis is conducted to study the influences on nonlinear mooring force. These influential parameters include initial horizontal force, top slop, water depth and unit weight of clump weight.The natural frequency ratio of heave and pitch may equal to 2:1, this is resulted from the configuration characteristic of Spar. The internal resonance of Spar may occur when the ratio is close to 2:1. It is necessary to consider the coupled relation of heave, roll and pitch. Internal resonance of Truss Spar is investigated in this thesis by numerical methods with considering the nonlinear mooring force. Also the results are compared between conditions with and without mooring force. The influence of wave elevation on the motion is studied and different mooring types are also investigated. As the numerical results show, the mooring force can delay the jumping phenomenon while incapable of preventing internal resonance. However, the jumping phenomenon occurs easily with the wave elevation. The relative heave motion will increase greatly when the wave elevation is large with a phase difference of 90°. The increase in relative heave motion will result in the freeboard dramatic change and thus greatly influence the safety of Truss Spar. For different mooring types, when internal resonance occurs, compared with the grouped mooring type, the equally distributed mooring type has lower surge and pitch amplitude but higher heave amplitude.