Snap Loads in Mooring Lines During Large Three-Dimensional Motions of a Cylinder

The nonlinear dynamic response of a moored, rigid, horizontalcylinder is considered. Use of the structure as a breakwater or a waveenergy device is envisaged. Inertial and nonlinear damping effects ofthe external fluid are not included, i.e., a ‘dry’ analysis is carriedout. The four mooring lines are modeled as massless springs which onlyresist tension. The six coupled equations of motion of the cylinder arederived and solved numerically. Linear free vibrations are treatedfirst, followed by the nonlinear three-dimensional response to harmonicforces simulating the action of waves. Normal, longitudinal, and obliqueforces are considered, and time histories of the displacements andmooring line tensions are computed. Snap loads tend to occur in thelines when they are alternately slack and taut. The effects of theforcing direction, amplitude, and frequency are investigated. Chaoticmotions are observed in some cases, and the structure collapses forsufficiently large forcing amplitudes.