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Ocean Systems Engineering
  Volume 4, Number 4, December 2014, pages 327-342

Tethers tension force effect in the response of a squared tension leg platform subjected to ocean waves
Amr R. El-gamal, Ashraf Essa and Ayman Ismail

    The tension leg platform (TLP) is one of the compliant structures which are generally used for deep water oil exploration. With respect to the horizontal degrees of freedom, it behaves like a floating structure moored by vertical tethers which are pretension due to the excess buoyancy of the platform, whereas with respect to the vertical degrees of freedom, it is stiff and resembles a fixed structure and is not allowed to float freely. In the current study, a numerical study for square TLP using modified Morison equation was carried out in the time domain with water particle kinematics using Airy\'s linear wave theory to investigate the effect of changing the tether tension force on the stiffness matrix of TLP\'s, the dynamic behavior of TLP\'s; and on the fatigue stresses in the cables. The effect was investigated for different parameters of the hydrodynamic forces such as wave periods, and wave heights. The numerical study takes into consideration the effect of coupling between various degrees of freedom. The stiffness of the TLP was derived from a combination of hydrostatic restoring forces and restoring forces due to cables. Nonlinear equation was solved using Newmar\'s beta integration method. Only uni-directional waves in the surge direction was considered in the analysis. It was found that for short wave periods (i.e., 10 sec.), the surge response consisted of small amplitude oscillations about a displaced position that is significantly dependent on tether tension force, wave height; whereas for longer wave periods, the surge response showed high amplitude oscillations that is significantly dependent on wave height, and that special attention should be given to tethers fatigue because of their high tensile static and dynamic stress.
Key Words
    tethers tension; tension leg platforms; hydrodynamic wave forces; wave characteristic
Amr R. El-gamal: Department of Civil Engineering, Faculty of Engineering at Benha, Benha University, Assistant Lecturer, Egypt
Ashraf Essa: Department of Civil Engineering, National Building Research Center, Egypt
Ayman Ismail: Department of Steel and Structure Engineering, National Building Research Center, Egypt

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