Active Control of the Wave-Induced Motions of a Float: Real-Time Simulations with a Digital Twin and Experimental Validation
Dahl, J. M.
Grilli, S. T.
Hashemi, R. M.
Alkarem, Y. R.
Kimball, R. W.
Publication Name: Proceedings of ISOPE 2023
Publication URL: https://personal.egr.uri.edu/grilli/Steeleetal_ISOPE23.pdf
Floating offshore structures, which are subject to wave-induced motions, are used in a wide range of ocean engineering applications, from vessels of various sizes, to oil and gas platforms (TLPs, simply tethered, . . . ), and more recently, floating offshore wind turbine (FOWT) design concepts. To optimize a variety of design and operational factors, it is important to minimize these wave-induced motions, which in some cases may require using semi- or fully-active methods that may use phase-resolved infor- mation about incoming waves. In the latter case, a mass or water ballast can be actuated or moved by a pump to counteract effects of the wave- induced forces and moments on the float, on the basis of some control law. Here we present the development and experimental validation of a control system based on real-time simulations of the wave-induced mo- tions of a floating object, with its active control method, using a complete physics-based model, referred to as a digital twin (DT). For demonstra- tion and validation, the system is applied to a barge-like float, for which a 1:19.22 scale model is tested in a laboratory wave tank (30 m long, 3.6 m wide and 1.8 m deep) in our facilities, while being placed transverse to the tank axis, in regular or irregular waves. The float is equipped with a nonlinear model predictive controller based on a mass moving horizon- tally on a slider in the bargeÕs beam direction to control its rolling motion, actuated by a linear motor. The controller is actuated based on the DT modeling of float heave and roll including the effect of the moving mass, informed by the assimilation of past float motions and, optionally, us- ing phase-resolved wave data at the float (either measured or predicted based on a separate model) incorporating disturbance previewing in the controller. In the experiments, the complete system is implemented and run in realtime on a desktop computer. Results will be presented and discussed, showing a significant reduction in float motions when the ac- tive control method is used. Considerations for using a similar concept to control FOWT motions will be presented as well as the potential for using open source software (OpenFAST) as a real time DT.