Cyclic fatigue testing of surface mooring hardware for the Arabian Sea mixed layer dynamics experiment

Abstract

The Arabian Sea is strongly forced by monsoon winds. Surface moorings deployed in the Arabian Sea are exposed to high winds and large waves. The waves, generated by strong wind events, impose a dynamic load on all mooring components. The dynamic cycling of mooring components can be so severe that ultimate strength considerations are superseded by the fatigue properties of the standard hardware components. Concerns about all in-line mooring components and their fatigue endurance dictated the need for an independent series of cyclic fatigue tests. The components tested included shackles of various sizes and configurations, wire rope, instrument cages, chain, and a variety of interconnecting links such as weldless sling links and end links. The information gained from these tests was used in the design of the surface moorings deployed in the Arabian Sea by the Upper Ocean Processes group of the Woods Hole Oceanographic Institution. The results of the cyclic fatigue tests conducted in support of the Arabian Sea surface mooring design effort are presented in this report. Recommendations are made with regard to all in-line components for surface moorings where dynamic conditions might be encountered for extended periods. The fatigue test results from shackles, and sling links were compiled to generate an SIN diagram where the cyclic stress amplitude is plotted versus the number of cycles to failure. In addition, the wire rope test results were compiled with historical wire rope data from US steel to generate a SIN diagram for torque balanced 3x19 wire rope. These results can be used in conjunction with future design efforts.