The high frequency vibration of the tendons of tension leg platforms (TLPs) with slender columns and pontoons is investigated in this paper. The equations for the two-dimensional rigid body motion of a TLP in heave, surge and pitch are solved in the time domain using a Runge-Kutta time-stepping procedure. The wave loads are computed using the relative velocity formulation of the Morison equation. High frequency tendon loading occurs at wave periods that are integer multiples of the platform heave and pitch periods. This is a result of different types of nonlinearities in the fluid loading. Here specifically, the contribution of two nonlinear effects are considered: the effect of integrating the kinematics up to the free water surface and the effect of the nonlinear drag term of the Morison equation.
The tension leg platform is one of the leading concepts for oil production in deep waters. The vertical tethers or tendons which connect the platform to the seabed are kept in tension by the excess buoyancy of the platform. The large axial stiffness of the tendons results in natural frequencies for the heave, pitch, and roll motions well above the exciting wave frequencies. However, recent studies have shown that the high frequency modes of motion can be excited, leading to high frequency vibrations of the tendons or tendon ringing. While the variance of the high frequency component of the tendon force is often much smaller than that of the wave frequency component, a proper understanding of this phenomenon is required for an accurate estimation of the probability distribution of the extreme tendon tensions, and tendon fatigue life predictions. Most previous investigations of tendon ringing phenomenon have assumed it to be due to second order wave loading effects (e.g. De Boom et al. (1984), and Petrauskas (1987)).