The deployment of an oceanographic deep sea Tri-Mooring

Abstract

A three-legged deep-sea mooring was successfully deployed and recovered by the Woods Hole Oceanographic Institution in the fall of 1973. The Tri-mooring supported a rigid three-dimensional array of current and temperature sensors for measuring internal waves in the main thermocline (600-1000 meters depth). The mooring was deployed in 5455 meters of water 700 kilometers southwest of Bermuda for a period of six weeks. Deploying the mluti-legged mooring was made complex by (1) the configuration of the mooring which put the apex 300 meters below the surface when any two anchors were in place, and (2) the use of 1/4" wire with an R.B.S. of 6600# as the legs of the mooring. The problem of attaching all three mooring legs to the apex float before it submerged was overcome by the use of an auxiliary mooring which supported one leg on the surface throughout the deployment sequence. The light weight nature of the mooring made it susceptible to failure due to dynamic and high static loading. Nylon working lines and good sea condition prevented dynamic loading. R/V Knorr, equipped with twin eycloidal propulsion, provided the position keeping capability and fine control maneuverability to avoid stressing the system. An acoustic navigation system was used to continually monitor ship position and to track the anchors as they were lowered to the bottom. Three-dimensional ship and anchor positions relative to a network of three acoustic transponders were accurate to ten meters. The maneuverability of the R/V Knorr combined with the reliability and precision of relative navigation enabled the anchors to be placed within twenty meters of the ideal target positions. The apex of the mooring stabilized within two meters of the design depth.