On the Design of Single-Point Cable-Linked Moorings for Ocean Observatories

The goal of this paper is to provide design criteria for moorings proposed for long-term ocean observatories that use electro-optical-mechanical (EOM) cables and to identify the parameters that affect their dynamics. The EOM cables are much stiffer than traditional mooring lines, and therefore, the moorings may experience higher loads under most environmental conditions if not designed properly. Two designs that provide geometric compliance yet keep all the cable in the water column are considered. One of the designs uses a single subsurface float (SSF) with an "S-tether" positioned above it. The other design uses distributed buoyancy to create an S-tether in the lower half of the mooring line. Both designs use an elastomeric snubber hose attached inline just below the buoy. The maximum tensions in the hose and at the anchor and Raoof's contact stress-slip parameter at critical locations are used to evaluate the designs. The present results show that the most important design parameter in terms of the maximum tensions is the length of the mooring, the most important parameters in terms of the bending fatigue damage is the length of the buoyant section and the distribution of cable floats, and the most important design parameter in terms of the dynamic motion of the cable is the length of the snubber hose. In the design with an SSF, it is shown that there exists an optimum S-tether length that minimizes the maximum tensions for a given SSF depth. The general rule of thumb that the S-tether be about twice the depth of the SSF breaks down when the SSF is placed near the surface. In general, the design with an SSF should be used if a small watch circle and small tension in the snubber are required. The design without an SSF is preferred if small overall tension along the length of the mooring is required, if the bending fatigue failure is of concern, and if a longer smooth section of the cable below the surface buoy for use with a profiler is required. In both designs, the snubber length should be at least 30 m so that the dynamic motion is kept reasonably small