Wave motion compensation in dynamic positioning of small autonomous vessels
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ORIGINAL ARTICLE
Wave motion compensation in dynamic positioning of small autonomous vessels Håkon S. Halvorsen1 · Henning Øveraas1 · Olav Landstad1 · Vidar Smines2 · Thor I. Fossen1 · Tor A. Johansen1 Received: 17 June 2020 / Accepted: 18 August 2020 © The Author(s) 2020
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. Keywords Autonomous vessels · Dynamic positioning · Wave compensation · Acceleration feedback · Roll damping · Wave prediction
1 Introduction 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 also that many thrusters do not have a sufficiently fast dynamical response. It would also increase fuel consumption, and fast power load variations cause excessive wear of the machinery system and the thrusters themselves. Moreover, for diesel-electric power systems that do not utilize batteries for peak-shaving, highly dynamic loads may cause variations in electric frequency * Tor A. Johansen [email protected] 1
Center for Autonomous Marine Operations and Systems (AMOS), Department of Engineering Cybernetics, Norwegian University of Science and Technology, Trondheim, Norway
Kongsberg Maritime, Ålesund, Norway
2
and voltage that may cause electric power blackout unless mitigating control such as power limitation, reduction, biasing or modulation is implemented in the DP, thruster control, or power management systems [8, 12, 13, 18]. Our research is motivated by new unmanned surface vessels (USV) concepts that are emerging for offshore operations such as inspection, maintenance, and repair (IMR). Some new vessel concepts are designed to be autonomous and much smaller in size than conventional IMR vessels, see Fig. 1 for an example. For smaller and unmanned vessels, the first-order wave-driven oscillatory motion may
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