The results of experimental measurements of the surge drift motion of a soft-moored barge in random waves are compared to a numerical simulation employing a complete quadratic frequency response function for surge motion. Estimates of typical statistical parameters derived from a large ensemble of model tests and time domain simulations are found to be in good agreement, and illustrate the weakly non-Gaussian nature of the surge response to Gaussian wave excitation.
The response of moored offshore structures to random seas is often dominated by large amplitude longitudinal and lateral motions with frequencies significantly lower than the frequency range of the individual waves. Preliminary sizing and performance analyses of such soft-moored systems require reliable estimates of the composite effects of the low and wave frequency response for determination of mean, maximum and significant excursions and mooring line loads, as well as, induced motions and loads on ancillary components such as drill strings or product risers. Whereas drift motion is generally characterized as a response to the combined effects of waves, gusting winds and fluctuating currents, the present investigation is confined to the simulation and measurement of wave induced drift motion and damping only. Kim and Breslin (1976) developed a time domain simulation for surge drift motion of a moored ship in random seas in which the equation of motion included theoretically estimated drift force and an assumed damping coefficient. The aforementioned approach was improved by Wichers (1987) through the introduction of experimentally determined viscous damping and statistical mean wave drift damping terms into the equation of drift motion. These mean wave drift damping coefficients are obtained from a series of free oscillation tests in monochromatic waves, and are subsequently used to determine the statistical mean wave drift damping for a given random sea condition.