Motion and Control of Tethered System with Fluid Drag

Tethered systems, which offer the advantages of low weight and efficient tether storage, are used as important components of marine, terrestrial and space installations. However, in deep sea operations, long tethers become heavy and their tension often becomes compromised by the buoyancy of the sea. Consequently, the inertia forces become significant and large displacements, rotations and elastic deformations of the tether occur. For such nonlinear systems, motion and control of the system with fluid drag and time-varying length are discussed. Applying Absolute Nodal Coordinate Formulation (ANCF) procedure can be applied. The ANCF is particularly useful when the effect of the fluid drag is to be accounted for. In this approach, the action of the fluid drag can be accommodated by using the space slope in combination with the nodal coordinates of the ANCF. In this paper, the motion of the tethered system that consists of a mother ship and tether in an approximate current and the position control of the tether lower end using the combination of length control of tether and the movement of the mother ship are discussed.