Two-Point Mooring System for Spar Buoy

A 181-ft long spar buoy was anchored in 1,800 ft of water off the coast of Oregon with a two-point mooring by the Department of Oceanography at Oregon State University. A single mooring line was not used because it can create large downward loads on the buoy due to horizontal drag forces from wind and current. Such a condition is undesirable because of the inherently small reserve buoyancy relative to the total displacement. The two-point mooring consists of two lines, pretensioned with submerged spring buoys, which are attached to two widely spaced anchors. Two chain catenaries lead from the spring buoys to the spar buoy. The design of the mooring was based on a hydraulic model study and a structural analogue and it was to withstand a 10-yr storm. It was shown that the type of mooring had no undesirable influence on the stable dynamic action of the buoy in wind and waves. Experimental information and data are presented which support the prototype design.