Designing Oceanographic Surface Moorings to Withstand Fatigue

Abstract

A method is presented for predicting the fatigue resistance of a single-point oceanographic mooring. The method determines the fraction of life that the components of a mooring lose during a deployment by combining results of dynamic analysis, data from laboratory fatigue tests of mooring components, and observations of sea-state occurrence. Calculations based on the 1989 and 1991 moorings used in the Marine Light-Mixed Layers experiment confirm the failure of a pear-shaped sling link on the 1989 mooring and the success of the redesigned 1991 mooring. Dynamic analysis shows that the large oscillatory tensions that cause the fatigue in oceanographic moorings are due primarily to the mass and damping of the instruments that are attached to the upper, wire-rope portion of the mooring line. Developing new instruments that are smaller and lighter would greatly increase fatigue resistance of oceanographic moorings and allow for longer deployments.