Numerical Modeling of Nonlinear Coupling between Lines/Beams with Multiple Floating Bodies
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Nonlinear coupling problems between the multiple bodies or between the mooring/riser and the offshore platform are incorporated in the CHARM3D-MultiBody, a fully coupled time domain analysis program for multiple bodies with moorings and risers. The nonlinear spring connection module and the three dimensional beam module are added to appropriately solve the structural connection problem. The nonlinear spring connection module includes the hydro-pneumatic tensioner module with the friction & stick/slip implementation, the tendon/mooring disconnection (breakage/unlatch) module with the tendon down-stroke check, and the contact spring with the initial gap with the friction force implemented. The nonlinear coupling may happen in many places for the offshore floating structures, such as hydro-pneumatic tensioner, tendon of TLP down stroke at the bottom joint, stick-slip phenomena at the tie down of the derrick and most of the fender-to-steel or steel-to-steel contact problem with initial gap during the installation. The mooring/tendon broken and unlatch can be a nonlinear connection problem once the transient mode is taken into account. Nonlinearity of the stiffness and friction characteristics of the tensioner combined with stick-slip behavior of riser keel joint is investigated. The relationship between tensions and strokes for hydro-pneumatic tensioner is based on the ideal gas equation where the isotropic gas constant can be varied to achieve an optimum stroke design based on tensioner stiffness. A transient effect of tendon down-stroke and disconnection on global performance of ETLP for harsh environmental condition is also investigated by incorporating the nonlinear boundary condition of the FE tendon model in CHARM3D. The program is made to be capable of modeling the tendon disconnection both at the top and the bottom connection as well as the down stroke behavior for the pinned bottom joint. The performance of the tie-down clamp of derrick is also investigated by using six degrees of freedom spring model and the three(3) dimensional FE beam model. The coupling of the TLP motion with the reaction force at the tie-down clamp is considered by using exact nonlinear dynamic equations of the motion with the reaction forces modeled with the spring or FE beam model. The method reduces too much conservatism when we design the tie-down system by the conventional method, in which all the environmental forces are combined without the phase lag effect between them. The FE beam model is also applied to the connectors between the semisubmersible and the truss for the pre-service and in-place conditions to be verified with the model test results, which shows good agreements.
Yang, Chan K. (2009). Numerical Modeling of Nonlinear Coupling between Lines/Beams with Multiple Floating Bodies. Doctoral dissertation, Texas A&M University. Available electronically from