Synthetic Electro-Optical-Mechanical Cables for Ocean Observatory Moorings

A major capital component of future ocean research will be single-point surface moorings, which will be "hard-wired" to the sea bottom with electro-optical-mechanical (EOM) cables. We consider an alternative to traditional high-modulus EOM cables for single-point surface moorings that is based on using low-modulus Nylon fibers as the strength member. High-modulus EOM cables, such as those that use Vectran fibers as the strength member, are usually constructed with the conductors and optical fibers in the core and the strength member on the outside. The key aspect of the design that uses Nylon fibers is that the strength member is placed in the center of the cable and the conductors and fibers are wrapped around the outside with a high helix angle. When the cable stretches, the conductors and fibers can alter their geometry without straining themselves. In a comparison of the static and dynamic responses of moorings constructed with Nylon and Vectran EOM cables (the Vectran EOM cable uses a rubberized "snubber hose" at the top to offset the effects of the high elastic stiffness of the Vectran cable), our results show that the maximum total tensions were lowest for the mooring constructed with a Nylon EOM cable. In comparison, the maximum total tensions for a mooring constructed with a Vectran EOM cable and snubber hose were 79% higher at the anchor and 21% higher at the surface buoy. The substantially lower anchor tensions for the Nylon mooring are due to the hydrodynamic damping that occurs over the length of the Nylon EOM mooring cable. Most of the motion of the Vectran/snubber mooring is confined to the short snubber hose. Since there is little hydrodynamic damping over the length of the Vectran EOM cable, the tensions pass from the surface buoy to the anchor without attenuation.