Nonlinear interaction of DC cables and magnetically controlled ship-steering autopilots

This paper presents new results on the interaction of DC cables and magnetically controlled ship-steering autopilots. Simulations with a nonlinear model show that a ship using a steering autopilot with a magnetic compass can be captured by a DC cable. That is, the ship tracks the cable, perhaps in an oscillatory fashion. A full-scale experimental study has been performed at the Kontiskan high voltage direct current link to verify calculations and simulation results. Good agreement was obtained between measurements and computer simulations. This paper presents the simulation study, the dynamic model used, and the test results. Simulation and field test show that if the ship is captured by the cable, it can, under certain conditions, track the cable with limit-cycle oscillation. Steady-state stability analysis explains why the ship does not find a stable capture course. Describing function analysis is used to find the limit-cycle amplitude and frequency.