Floating wind energy has attracted substantial interest since it enables the deployment of renewable wind energy in deeper waters. Compared to the bottom-fixed turbines, floating wind turbines are subjected to more disturbances, predominantly from waves acting on the platform. Wave disturbances cause undesired oscillations in rotor speed and increase structural loading. This paper focuses on investigating the potential of using wave preview measurement in the control system labeled as wave feedforward to mitigate the effects of the wave disturbances. Two wave feedforward controllers were designed: one to reduce generator power oscillations and the other one to minimize the platform pitch motion. In this study, a software-in-the-loop wave tank experiment is presented for the purpose of investigating the potential of these wave feedforward controllers. In the experiment, a 1:40 scaled model of the DTU 10 MW reference wind turbine is used on top of a spar platform, with the baseline feedback control functionalities. Different environmental conditions, including wind speed, significant wave height, turbulence intensity, and wave spreading, were applied during the experiments to test the feedforward control performance and their effect on the turbine dynamics in general. It was found that the feedforward controller for the generator power reduces the power fluctuations properly with a fair control effort, while the one for platform pitch motion requires almost double the actuation duty for the same percentage reduction. Furthermore, the feedforward controller was able to counteract the wave disturbance at different wave heights and directions. However, it could not do much with increasing turbulence intensity as wind turbulence was found to have more dominance on the global dynamic response than waves.@en

Floating wind energy has attracted substantial interest since it enables the deployment of renewable wind energy in deeper waters. However, floating wind turbines are subjected to disturbances, predominantly from turbulence in the wind and waves hitting the platform. Wave disturbances cause undesired oscillations in speed and increase structural loading. This paper focuses on mitigating these disturbance effects with feedforward control using knowledge of the incoming wavefield. The control problem is formulated in an H∞ optimization framework designing two wave feedforward controllers: one to reduce rotor speed oscillations, and the other one to minimize the platform pitch motion. Mid-fidelity time-domain simulations demonstrate the improved performance of the proposed control algorithm regarding wave disturbance mitigation at the cost of higher actuator duty.

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The control of Floating Wind Turbines (FWTs) is challenging, as they possess much lower natural frequencies related to the structure's rigid body motion, which creates an undesirable coupling between tower motion and the blade pitch control. As a result, the tower motion is negatively damped triggering instability. This is because of the presence of Right Half Plane Zeros (RHPZs) imposing fundamental limitation on the control bandwidth. To address this problem, different solutions were proposed with varying control structures ranging from Single-Input, Single-Output (SISO) controllers to Multiple-input, Multiple-output (MIMO) ones. In this paper, a new control structure, of Single-Input, Multiple-Output (SIMO) is proposed that is able to lift the bandwidth limitation, while using simple elements that match the industry demands.

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The workability of various types of operations offshore are largely affected by waves and wave induced motions. Examples are crew transfer from crew transfer vessels or service operation vessels to offshore wind turbines for maintenance, landing of helicopters in (navy) vessels and various crane operations. Over the recent years quite some effort has been put in technology aiming to provide a real time on-board prediction of approaching waves and wave induced vessel motions some minutes in advance. Enabling crew to anticipate, thus enhancing the safety and operability of these operations. This paper addresses the performance during a field test of the system as being under development by Next Ocean enabling such predictions, based on using an off-the-shelve (noncoherent) navigation radar system as a remote wave observer. Briefly summarizing (earlier publications on) the technical approach, focus will be on results obtained from a field test where the system was validated. Good agreements between ship motions as measured by an on-board motion reference unit and predictions obtained by the wave and motion prediction system during a field test on the North Sea near the Dutch coast on a 42 m patrol vessel will be shown in the results section, from which the usefulness of the system for operational decision support can be concluded.

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With many operations at sea carried out by ships or or other floating vessels, risks are involved because of the waves and resulting motions of the ships. Examples are the landing of helicopters on ships, transferring crew from a ship to a wind turbine, or working on the deck of an anchor handling tug. It is common practice to assess the workability of a certain operation by statistical criteria. The rationale behind such criteria is in general that the probability of some phenomenon (e.g. the vertical motion of a helicopter landing deck) exceeding a certain threshold value has to be less than a chosen acceptable level. Operability analysis usually qualify a given wave condition as workable or not workable based on such statistical criteria, assuming that no information is available about when critical wave induced events occur. In this thesis, the feasibility is investigated to obtain a short term, ’deterministic’, i.e. time-specific prediction of the critical response: making available a short term prediction of approaching waves and vessel response real time, on-board, would give crew the opportunity to anticipate and chose the optimal moment to perform a critical operation. The research is motivated by two possible advantages of such a deterministic prediction: 1. It further enhances safety in conditions that were considered as workable from a statistical point of view.
2. It possibly increases workability by pointing out windows of opportunity in conditions that were considered as unworkable from a statistical point of view.
The chosen approach to obtain the mentioned deterministic prediction of waves and induced motion response, is to use the ship’s navigation radar as a remote wave sensor. The spatial domain that can be covered by a navigation radar to observe the sea surface is of course limited: both its minimum and maximum range is limited. Besides it is obvious that the wave observation will only be available in the past, and by no means in the future. Therefor, the first chapter answers the theoretical question where in the spatio-temporal domain waves can be accurately predicted, given a perfect spatio temporal observation of the waves. An indicator is proposed that specifies predictability in space and time based on the spatio-temporal observation and based on a given wave condition. It is confirmed that the group velocity of the waves is governing concerning this question. In the remaining chapters basically 2 different approaches are proposed and investigated to solve a linear wave representation based on input from synthesized radar images of sea waves. Finally the methods are applied to real radar data acquired during a sea trial. Based on the solution of the linear wave field, ship motions were predicted using pre-computed linear motion tranfer functions. Correlation coefficients up to 0.86 were obtained for the heave motion predicted 60 sec in advance. @en

For numerous offshore operations, wave induced vessel motions form a limitation for operability: Installation of wind turbines, removal and placements of top sites on/from jackets, landing of helicopters etc. can only be done safely in relatively benign wave conditions. In many cases the actually critical phase takes no more than some tens of seconds. An on-board prediction of vessel motions would enable crew to anticipate on these near future vessel motions and avoid dangerous situations resulting from large ship motions. This paper presents results from a field campaign in which non-coherent raw X-band navigation radar data was used as input for a procedure that inverts the radar data into a phase resolved estimation of the wave elevation. In combination with a wave propagation and vessel response model, this procedure can compute a prediction of phase resolved vessel motions, some tens of seconds up to minutes into the future, depending on radar range and sea state. We compare predictions obtained this way with actual measurements of a well intervention vessel that were obtained during a sea trial performed at the North Sea. It was concluded that the method results in very accurate predictions: correlations between 0.8–0.9 were obtained for predicted ship motions of/around the COG and the vertical motions of the helicopter deck.@en