High frequency effects of modifications to Morison`s equation on the response of a TLP

The work described in this paper is part of a research project conducted at the University of Texas at Austin in conjunction with the Offshore Technology Research Center. The research studies the contribution of different nonlinearities to the global response of tension leg platforms. A rigid body model with three degrees of freedom is selected to represent a TLP hull which consists of four columns and four pontoons. The tendons are modelled as elastic springs allowing for the variation of the axial forces and the effect of setdown. The forces are computed using Morison`s equation with and without the nonlinear modifications formulated by Rainey. Forces on the bottom of a truncated cylinder are employed to account for the diffraction and radiation effects. The effects of these modifications on the high frequency response and on the tendon forces are discussed for different aspects ratios of column diameter to wavelength. Among other conclusions, the tendons forces are dominated by the heave response at the wave frequencies and by the pitch response at the pitch natural frequency.