Nonlinear forces and response of floating platforms in regular and random waves

Chitrapu, Srinivasamurthy
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Floating platforms in the ocean are subjected to mean and low-frequency drift forces, and high-frequency springing forces in addition to the first-order forces in the wave-frequency range. These forces, which are caused by both potential and viscous effects, can induce large amplitude, resonant response of platforms due to the near absence of damping at such frequencies. In this study, a frequency-domain method based on the relative velocity formulation of Morison's equation is presented to compute viscous drift forces and moments in regular and random waves. The method, which is applied to a semisubmersible and tension-leg platforms, indicates significant mean forces and moments in surge, pitch and yaw modes of the platform motion. The combined effect of waves and current, variable submergence of platform members and computation of forces in the displaced location of the platform appear to have a pronounced effect on the computed drift forces and moments. It is shown that the viscous drift forces are important in the long-period range and hence must be considered under design wave conditions. A time-domain model which uses Morison's equation method for force computations is developed to simulate platform motions. This model can include most of the nonlinearities such as the nonlinear drag force, effect of finite wave elevation, nonlinear restoration of the positioning system and nonlinearities in the equations of motion of the platform. The viscous drift forces and response obtained from the frequency-domain method are compared with results from time-domain simulations. Good agreement has been found for the forces and response, both in regular and random waves. This frequency-domain method can be used to predict viscous drift forces and response in the preliminary design stage and for parametric studies due to its superior computational efficiency as compared to the time-domain simulations. The effect of the nonlinear drag force, in inducing higher-harmonic forces and tether-tension response, has been studied using the time-domain simulation results together with power spectral methods. For the wave and current conditions used in this study, second- and higher-harmonic drag force and tether-tension response are observed in regular, bi-chromatic and random waves and current. Inclusion of current is shown to affect the nonlinear response of the platform. Another theoretical model, based on the application of linear potential theory in the time domain, to simulate large amplitude nonlinear motions of platforms is also presented. The theory is based on the combination of potential and viscous flow effects in the time domain to determine forces acting on the platform. Hydrodynamic coefficients and wave excitation forces, obtained a priori from the linear, three-dimensional potential theory, are included in the nonlinear, large amplitude simulation model for platform motions. First-order memory effects are included through velocity based convolution integrals. The results obtained from this simulation model are compared with those obtained using Morison's equation model and the agreement is found to be good. It is believed that this method, due to its ability to model both potential and viscous-flow effects accurately in a large amplitude motion simulation model, will give better predictions to the various nonlinear effects mentioned above.
Thesis (Ph. D.)--University of Hawaii at Manoa, 1992.
Includes bibliographical references (leaves 206-218).
xix, 218 leaves, bound ill. 29 cm
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Theses for the degree of Doctor of Philosophy (University of Hawaii at Manoa). Ocean Engineering; no. 2806
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