A scaling procedure for mooring experiments with lines containing buoys attached to them is outlined. It is shown that dynamic similitude is ensured only if the elasticity of each segment of the line is properly modelled through the introduction of elastic springs. The validity of the proposed procedure is shown by comparing full-scale and scaled-down numerical results concerning dynamic amplification for a number of configurations. Preliminary experimental results are also discussed.
The importance of taking into account dynamic phenomena in deep water mooring applications has recently led to a revision of the required design procedure specifications (I.S_S.C_, 1988). The well-known quasi-static approach, which has been proven to be a proper design tool for mooring systems in shallow water (AP.I., 1987), has to be modified to account for dynamic tension in deeper waters. In such cases, the well-documented dynamic tension amplification (Triantafyllou et al, 1985) attains its maximum value within the wave frequency range. This is entirely due to the action of the nonlinear drag force, Which, by preventing the mooring line from lateral motion, forces it to respond by stretching along its tangential direction. It is therefore evident that, for sufficiently high frequencies, the nonlinear dynamic response of an elastic cable in deep water is dominated by its elastic stiffness, the reduction of which would improve the dynamic performance of the line_ This can be achieved, as it has been recently shown both numerically and experimentally (Mavrakos et al, 1989a, b), by inserting properly designed submerged buoys attached along the mooring line, so that advantage can be taken from the dynamic modification caused to the system. Number, location and size of buoys affecting the amount of the reduction of the dynamic tension amplification obtained can then be incorporated within a consistent optimization procedure (Mavrakos et al, 1989c).