Modelling and Control of a Two-Body Offshore Wave Energy Converter - Evaluation of Passive Control Strategies
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- Institutt for marin teknikk 
The main motivation behind this thesis is using the renewable resource of wave energy to solve two of the most important challenges for sustainability; energy and freshwater. The DynOcean is a two-body offshore wave energy converter concept with a relative motion power take-off system, intended for desalination of seawater. This design was the inspiring concept for the Ocean Oasis wave power device developed during the 2014 DNV GL Summer Project, for which this thesis is an extension off. In order to improve the design further, it is of interest to evaluate the possibility of using control action to increase mean power extraction. In this context, a mathematical model was developed which allowed for the implementation of passive control strategies known as latching control, declutching control and latched-operating-declutched control. In this thesis, it was shown that it is possible to model the dynamics of the DynOcean buoy by using a nonlinear state space. The model developed was able to capture the main dynamics of the converter motion and provided an intuitive structure for easy implementation in a numerical solver, giving time-efficient simulations in the time-domain. The model included nonlinear viscous drag forces and estimated radiation forces with fluid memory effects given by a linear time-invariant state space. The power take-off system was modelled as a linear relative motion damper. By a comparison study, it was shown that the model gave satisfactory results when compared to similar analyses for the same design, both in the frequency- and time-domain. The mathematical model allowed for the implementation of control action and control design. Three different control schemes was designed and implemented using an optimal control formulation and a non-causal iterative solution algorithm. A simulation-based evaluation study was performed for the control designs, and neither of the passive strategies was able to generate any significant mean power amplification for the DynOcean design. Simulations indicated adequate behavior for latching and declutching control designs while latched-operating-declutched control showed signs of unfeasible behavior. It was also shown that the effect of the control action was improved if a linear spring model was added to the control design models. According to the findings in this thesis, it was concluded that passive control of the relative motion power take-off system by latching or declutching control was ineffective for the DynOcean design.