Moored vessels at sea may experience low-frequency surge, sway and yaw motions, resulting in severe loads in the moorings and operational problems. The causes of these motions are complex; they are correspondingly difficult to predict, and model test results need careful interpretation. This paper is partly a review of recent theoretical and experimental work on low-frequency wave drift forces, damping and response, and partly a description of work now in progress at British Maritime Technology. Numerical calculations, using the NMIWAVE wave diffraction computer program, are compared with model measurements of drift forces and response. Calculations based on the quadratic transfer function technique are compared with the simpler and more approximate Newman method. The paper examines a simple method for approximating the additional damping due to the presence of waves, based on the drift forces in regular waves at zero forward speed. It also compares two methods for deriving the damping from measured responses in irregular waves. A new and computationally efficient method for simulating low-frequency drift forces is proposed, based on generation of a pseudo-random white noise process. Results show that the spectrum of low-frequency response may be unexpectedly sensitive to the method chosen to simulate the random wave sequence. The reasons for this sensitivity are unclear, but suggest that some techniques presently used in both numerical simulation and model testing may give misleading results.
Mooring system are often designed on the basis of static wind, wave and current loads with a large safety factor to take care of dynamic effects. The uncertainties is this approach, coupled with possible unnecessary costs, are becoming increasingly important factors in the design process as the industry seeks to reduce costs and to exploit more marginally economic and deeper water fields. Overprediction of loads might exclude certain mooring options that would otherwise be available to the designer. Despite the attendant difficulties, therefore, there are considerable advantages in taking account of the vessel's dynamic response.
Ships and floating structure in exposed offshore locations for example; floating production systems or offloading systems have to be moored or dynamically positioned in order to keep them on station in wind, waves and current. The mooring system leaves the vessel free to respond to forces acting at wave frequencies, but limits drifting and low-frequency motions. The vessel has natural frequencies in surge, sway and yaw within a range that may be excited dynamically by low-frequency wave drift forces or wind gusting. Typical natural frequencies vary from, say, 0.01-0.02 Hz for a tension leg platform or centenary-moored vessel, to perhaps 0.001 Hz for the fishtailing motions of a ship at a single-point mooring. These low-frequency responses, combined with the other motions of the vessel, may be sufficient to cause: large mooring forces and excessive excursions of the vessel, may be sufficient to cause: large mooring forces and excessive excursions of the vessel, difficulties with the riser angle of production equipment, or (for a tension leg platform) increased draw-down and associated tether loads. The vessel's dynamic response is determined by the amount of damping present.