Damping Of Moored Floating Structures

Abstract

1.ABSTRACT The resonant response of moored floating structures due to low-frequency excitation is controlled primarily by the effective damping coefficient of the structure. A Joint Industry Project addressed the feasibility of predicting the various components of the damping coefficient, with particular emphasis on the wave drift damping, and the damping caused by the mooring lines, about which considerable uncertainty exists. Laboratory and full scale data were used, and new predictive tools were developed to obtain benchmark estimates. 2.INTRODUCTION Studies on the dynamic behavior of moored structures have demonstrated the importance of dynamic amplification phenomenal resulting in changes in the API recommended practice (Triantafyllou et al. 1985, Tein et al. 1987, Kwan 19901). One of the principal outstanding issues is the accurate evaluation of the damping coefficient controlling the slowly varying motions of moored systems. The large mass of moored structures and the relatively small effective stiffness constant of the mooring system provide low natural periods, of the order of 100$, while the total effective damping is often well below the critical damping, As a result, low frequency excitation, such as the slowly varying wave drift forces and the unsteady wind forces, can excite resonant oscillations of considerable amplitude. This is of great import ante to mooring system design, since at resonance the maximum amplitude of motion, and hence the peak slowly varying forces in the mooring lines, are controlled primarily by damping. Damping consists of several components, caused by: unsteady drag and lift forces acting on the structure (including current-induced damping); wave drift damping; wind damping; and mooring damping. Considerable uncertainty exists about the magnitude of two of the most important components of damping: the wave drift damping and the mooring damping. The uncertainty is caused by the nature of wave drift damping, which is a small quantity to measure; and because it is caused by a complex nonlinear mechanism, numerical calculations are difficult to perform. In the case of mooring line damping, the uncertainty lies in the drag coefficient, which can be amplified by various mechanisms from a nominal value of 1.2 to a value in excess of 3.0. A one-year Joint Industry Project (JIP), entitled "Mooring Line Damping and Current Loads", was conducted by Noble, Denton & Associates, Inc. and the MIT Testing Tank Facility to address the principal issues in predicting the damping coefficient of floating structures and obtain benchmark estimates. The project was supported by Amoco Production Co., Arco Oil & Gas Co., BP Exploration Inc., El Dorado Engineers Inc., Exxon Production Research Co., NCEL, and Reading & Bates Drilling Co. 3.DAMPING OF MOORED STRUCTURES The following sources of damping were initially considered: Wave drift damping The discovery of wave-drift damping was made with systematic and accurate experimental measurements starting in the early 1970's (Remery & Hermans 1971). The identification of the wave-drift damping was made by Withers & van Sluijs (1979) who showed that this damping force is (i) proportional to the square of the wave height; and @) a function of ambient wave frequency.