A number of studies have been reported recently where the dynamic response of a TLP IS obtained in the time domain, through step by step integration of the equations of motion, using Monson's equation to predict the hydrodynamic forces on the members. One difficulty with this approach, as reported by a number of authors, is the computation of the vertical hydrodynamic forces acting at the base of the main vertical cylinders as well as the fluid reactions to the heave motion of these members to help in the selection of these forces a number of parametric studies have been conducted on a single vertical truncated cylinder using linear potential theory. Periods from 4 to 18 seconds (wave period or period of cylinder oscillation corresponding to diffraction or radiation, respectively) and cylinder aspect ratios (D/h. diameter/draft) varying from 0.05 to 1.6 were used m the analyses with water depths exceeding 3000 ft. The results have been arranged in dimensionless form and are shown in graphs. Simplified formulae have then been fitted to these results in order to obtain approximate expressions winch can be used in practice.
Recently there has been an increased interest in time domain solutions of TLP response to investigate transient motions (i.e. ringing) and the effects of various nonlinearities. If nonlinearities can be assumed to be small, frequency domain solutions are convenient to determine the frequency-dependent radiation and diffraction forces for large diameter bodies Second order diffraction formulations allow nonlinear effects associated with sum frequency and difference frequency terms to be included. There are, however, other nonlinearities which cannot be reproduced at present m frequency domain solutions. If these other nonlinearities are considered significant or must be investigated a time domain solution is desirable. Such nonlinearities are, for instance, the drag forces associated with vortex shedding or the nonlinearity associated with computing the forces in the deformed position of the structure.
