Numerical Simulation of the Deployment of a Hybrid ROV Optical Fiber Tether

This paper describes a numerical code for simulating the dynamics of an unmanned, underwater vehicle system that is self-propelled and tethered to a surface ship through an optical fiber tether. The vehicle, called a hybrid ROV, uses a buffered, single-mode optical fiber with a maximum working load of 1.7 N for communication and data transmission. The vehicle is designed to go to the deepest parts of the ocean and for exploring beneath the Arctic ice cap. The optical fiber tether is stored in a pair of canisters, one mounted on the vehicle and one mounted on a garage that is lowered from the ship. The canisters each hold 20 km of fiber, which is pulled out during operations when the tension at the canister reaches a threshold value, which is set to the maximum working load of the fiber. The numerical simulation is based on the two-dimensional version of WHOI Cable, a finite-difference solver of the cable equations that includes bending stiffness to model low-tension effects. A velocity/tension based payout algorithm was incorporated into the code to model the behavior of the canisters. In the payout model, the payout velocity is set equal to zero below the threshold tension and varies linearly with tension above the threshold value up to a maximum pay out velocity. Hydrodynamic drag models for axial and normal fluid loading, whose values are a function of Reynolds number, are used to calculate local drag coefficients of the optical fiber. Examples of the vehicle being lowered to the sea bottom in uniform and shear currents are used to demonstrate the capabilities of the code and the performance of the tether and payout system.