Elastic Characteristics of Moorings

The weight, strength, and elastic properties of steel-wire and fiber mooring ropes are examined for purposes of predicting their elastic behavior under dynamic conditions. Most of the essential properties can be generalized by empirical formulas that apply to all ropes when the appropriate values of certain constants are known. Thus, tension is found to be a simple nonlinear function of elastic strain. Elastic modulus in consequence is a nonlinear function of stress. These moduli are given for steel-wire, nylon, and coir mooring ropes. The effect of the elastic properties of mooring ropes in generating restraint-motion functions, governing the behavior of a moored object, are discussed with reference to anchor-line mooring and to normal harbor mooring. For anchor lines, the restraint is a nonlinear, odd, nonsymmetric function of the motion; for the harbor moorings it tends to be a nonlinear, odd, symmetric function. The fatigue properties of mooring ropes, inferred by analogy from the known fatigue properties of steels, are also examined.