Sample calculations illustrate uncertainties in predicting second-order wave forces and low-frequency motions of a large moored structure. The results, for a "deep-draught floater" in regular and irregular waves, demonstrate sensitivity to numerical modelling procedures and to assumptions made about the damping. When there was no current, the damping was affected by first-order velocities, and the damping coefficient by boundary layer forces and by the partial development of flow separation and vortex shedding at low Keulegan-Carpenter numbers. The introduction of a current substantially increased the damping, but had generally a simplifying influence. The calculations also illustrate different ways of estimating the extreme second-order response, and combining this with the first-order response.


In 1989 BMT took part in a comparative evaluation of computer programs, organised by the Royal Norwegian Council for Scientific and Industrial Research (NTNF). The results of this investigation were discussed at a Workshop in Bergen, and will no doubt be published in due course. The project highlighted several sources of uncertainty in predicting low-frequency responses of floating and moored structures in waves, in particular those of estimating the damping and extreme responses. The present paper describes results from a follow-up study at BMT, which demonstrated how recent research data may be used in calculations of this type, and how changes in the assumptions and procedures affect the results. The problem posed by NTNF was to calculate wave forces on, and motions of, two vessels in regular and irregular waves; to estimate maximum responses in a storm of six hours duration for first and second-order motions separately, and then for the combined process. The two vessels were a turret-moored production ship of 230m length, and a four-column deep-draught floater (DDF), the latter being a very large semi-submersible type structure.

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