ABSTRACT When predicting the resonant surge and sway motions of moored structures, it has until recently been customary to neglect the contribution from mooring line drag upon the system damping. In 1986 there was the first publication (OTC paper 5204) presenting a theoretical discussion of the effect, together with experimental results. In many cases the effect turns out to be very significant in reducing the resonant surge amplitudes of moored offshore structures. The present paper presents a simple calculation procedure for practical estimation of damping due to mooring lines. It is based on systematic calculations, with the results shown in suitable diagrams. By means of these diagrams it takes only a few minutes to calculate the surge damping of a given mooring system by hand calculation. The paper also presents experimental verification of the calculation procedure. It discusses in detail how superposed first order wave-induced vessel motions will lead to a further increase of the low frequency damping. INTRODUCTION 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 mainly by second order wave forces. In surge direction, for instance, the first order motions occurring at wave frequencies are normally 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 theoretical 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 accurate prediction of system damping becomes very important in the prediction of motions, maximum offsets and peak mooring loads. Until very recently it has been customary to neglect the drag forces on the mooring lines when estimating the surge and sway damping of moored offshore platforms and ships. The argument for this has been that the drag area represented by the mooring lines is very small compared to the drag area of the vessel itself. However, as explained in Ref. 1 and illustrated in Fig. 5, the motion amplitude, ? of the line can be many times larger than the corresponding surge amplitude, so' of the vessel. Furthermore, the energy dissipation due to drag force is proportional to the third power of the amplitude. Thus, in spite of their small drag area, the mooring lines may still represent a dominant contribution to the total surge damping. SCOPE OF INVESTIGATION The objectives of the present investigation are:–To develop a simple calculation procedure for estimating the surge damping contribution from the mooring system.–To verify the theory.–To study the effect of high frequency (first order) motions upon the low frequency (resonant) surge damping. (The term "surge damping" is used throughout this paper, but the theory and considerations will of course be identical for sway motions.)