Scope of Work:
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 in 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 forces in the deformed position of the structure.
A problem that has been reported by a number of authors conducting time domain analyses is the proper representation of the vertical hydrodynamic forces at the base of the main TLP cylinder legs. Nwogu and Irani (1990) and Johnson, Mekha, and Roesset (1993) have used different approaches and expressed the need to adequately reproduce the vertical hydrodynamic forces in their computer models. Nwogu and Irani (1990) approximated these forces using the Froude-Krylov force, while Johnson et al (1993) used the variable hydrostatic forces to be on the conservative side.
This project presents equations for heave diffraction force(modulus and phase) and heave added mass and damping coefficients for vertical truncated cylinders as a function of cylinder aspect ratio and normalized frequency . The equations were fitted to the results of a parametric study using linear potential theory. Similar equations were then obtained for the horizontal case (surge motion) and rocking (pitch motion). The results of the approximate formulae are then compared to those of a direct solution to predict the response of a freely-floating truncated cylinder to waves (response amplitude operator).
Weggel, D.C. and Roesset, J.M., “Vertical Hydrodynamic Forces on Truncated Cylinders,” Fourth International Offshore and Polar Engineering Conference, Osaka, 1994.