The dynamic response of large-diameter offshore structures to wave loading is studied. Emphasis is placed on spar platforms which have recently emerged as promising concepts for the deep-water recovery of hydrocarbons. Rigid body response analyses (surge, pitch, and heave) are performed in the frequency domain, investigating the effect of various nonlinearities. Using second-order potential theory, first-order forces and radiation coefficients, and different types of second-order forces are computed and presented as functions of the incident wave frequency and body motions. These hydrodynamic force quantities are then used in the dynamic equations of motion. The relative importance of these forces in the accurate prediction of responses is assessed.
Three primary damping mechanisms are also explored: radiation damping, drag damping, and wave drift damping. Adequate consideration of damping is necessary for resonant-dominated structures such as spar platforms. Response predictions are compared to experimental data obtained in the Offshore Technology Research Center’s (OTRC) wave basin on two 1:55 scale model spars. A large taut-moored spar and a small taut-moored spar with a vertical tether were subjected to unidirectional monochromatic, bichromatic, and irregular wave loading in a prototype water depth of 3000 ft.
Response results are presented by comparing experimental and predicted time series, power spectra, and marginal statistics. The ensemble statistics are predicted for the response of the large spar to the survival Gulf of Mexico storm seas. A similar prediction is made for the small spar to operational seas.