When predicting the surge and sway motions of moored marine structures, it has until now been customary to assume that the mooring system influences rig motions only through its stiffness, which determines the resonant periods. Possible damping effects from the mooring system have been neglected. This has been a usual assumption in theoretical calculations and also a basis for model testing techniques. The present paper shows that drag forces on the mooring lines, and to some extent line friction on the seabed, can give rise to a substantial energy dissipation. This in turn can represent a very significant surge damping of the rig. Theoretical calculations and experimental verifications presented in this paper show that this can 1ead to a reduction in surge amplitudes of the order of 20 percent or more. Such reduction will have a very significant effect on for instance riser system performance and peak tensions in the mooring lines. It is concluded that the damping effect of the mooring system should be taken into account when predicting the surge and sway motions of moored marine structures.
A characteristic feature of moored offshore structures is their slow oscillatory motions at resonant frequencies. They take place well outside the frequency range of the wave spectrum, and are excited by second order wave forces. In surge direction, for instance, the first order motions occurring at wave frequencies are normally much smaller than the resonant surge. Since the resonant motions are often dominating, they are correspondingly important to the peak offsets of the platform, to the peak loads in the mooring lines, riser design requirements, etc.
First order motions of platforms and ships can today be calculated quite accurately by theoretica1 methods. Second order motions are more uncertain. Present-day theories for calculating the excitation forces have limitations, and particularly the system damping cannot be calculated on a purely theoretical basis. Since we are dealing with low-damped systems at resonance, the motion amplitude is to a first approximation inversely proportional to the damping. The accurate prediction of damping therefore becomes very important in the prediction of motions, maximum offsets and mooring line peak loads.
The main contributions to the surge damping of moored platforms and ships arise from:
radiation of waves due to motions
wave drift damping
drag and friction damping of main structure including effects of appendages, roughness, marine growth etc.
drag forces on mooring lines
friction between mooring lines and seabed
Items a. above can be treated by linear potential theory. There are also methods at hand to deal with item b, see for instance Ref. 1. Item c. is for most offshore structures considered to be the most important contribution. For practical applications one has to rely on new model tests in each case or on empirical data from previous tests. Item e. can also be a significant contribution as shown in Ref. 2.