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.

Unlike common marine vessels, wave-propelled vehicles cannot directly control their speed, but rely instead on the forces exerted by the environment to navigate as intended. The unique navigation capabilities of such vehicles motivate the study of control solutions that adapt the vehicle heading to the prevailing environmental conditions and ensure robust course-keeping performances across different sea states, winds, and currents over extended periods of time. This article presents the design and experimental validation of a path following and course control system for an underactuated, wave-propelled unmanned surface vehicle (USV). The major focus and novelty of this work stands in the analysis of the model nonlinearities that appear when the vehicle propulsion force does not prevail on the wind and sea current forces generated by the environment. In these situations, low maneuverability is experienced depending on the magnitude of counteracting forces and, in some cases, loss of controllability is a risk. Initial investigation of the vehicle’s nonlinear dynamics is followed by derivation of a simplified quasilinear mathematical model that isolates the major source of nonlinearity. This provides a basis for the control design, the theory of which is supported and validated by extensive field experiments. In particular, when the USV’s ground speed is close to zero, the theory shows singularities in the model that lead to instabilities and loss of controllability of the course over ground that is experienced in the field. Our test results verify that an effective solution is to switch to heading control when the ground speed is small.

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Remotely operated vehicle (ROV) operations are today typically supported by large designated vessels. New emerging concepts aims to streamline ROV operations by utilising unmanned surface vessels of a smaller size. Reduction in size may result in first-order wave induced motion being more significant. This motivates the use of dynamic positioning control using thrusters to actively compensate for first-order wave-driven horizontal-plane motion in order to maximise operability. This paper proposes a controller for dynamic positioning based on model predictive control and short-term wave motion prediction intended to actively compensate for first-order waves. By considering the full dynamic sea environment, the controller is able to dampen out some of the oscillatory motion caused by first-order waves. The controller is able reduce the average deviation from the set-point with up to 65% for a variety of sea conditions. The maximum distance error to the reference point is reduced by up to 65% depending on sea state. The dynamics of the thrusters is a limiting factor when counteracting first-order waves and fast thrusters are therefore crucial in achieving best possible positioning. The cost of the wave-compensated positioning is a more dynamic consumption of power.