Abstract
Conventional dynamic positioning (DP) systems on larger ships compensate primarily for slowly time-varying environmental forces. In doing so, they use wave filtering to prevent the DP from compensating for the first-order wave motions. This reduces wear and tear of the thruster and machinery systems. In the case of smaller autonomous vessels, the oscillatory motion of the vessel in waves may be more significant, and the thrusters can be more dynamic. This motivates the use of DP to compensate for horizontal wave motions in certain operations. We study the design of DP control and filtering algorithms that employ acceleration feedback, roll damping, wave motion prediction, and optimal tuning. Six control strategies are compared in the case study, which is a small autonomous surface vessel where the critical mode of operation is launch and recovery of an ROV through the wave zone.
Highlights
Dynamic positioning (DP) systems achieve station keeping of vessels only using thrusters and a control system
The DP control algorithms that gave the best results according to the maximum horizontal position error peak were wave prediction and roll damping
Roll damping gave the best results for Hs = 0.5 m and Hs = 5.5 m, and wave prediction gave the best result for the intermediate wave heights
Summary
Dynamic positioning (DP) systems achieve station keeping of vessels only using thrusters and a control system. DP systems on larger ships compensate primarily for the slowly time-varying wind, ocean current, and second-order wave drift forces. They employ wave frequency filtering of the position and velocity measurements, so that the DP feedback control does not compensate for first-order wave motions, [6, 16]. One reason for this is that it may not be necessary for many operations, and that many thrusters do not have a sufficiently fast dynamical response. For diesel-electric power systems that do not utilize batteries for peak-shaving, highly dynamic loads may cause variations in electric frequency
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