Model-free Positioning Control of a Payload by use of Wires

Abstract

With the highly increased interest in offshore wind turbines and their technologies, the sector has witnessed rapid development in the past decade. For installed offshore wind turbines, there has been a lot of research conducted in the field of aero-, hydro- and structural dynamics, wind turbine-control, operation & maintenance, foundations and moorings. The research field of offshore wind turbine installation is on the other hand relatively new, and the studies regarding this topic are limited. The lifting of heavy objects is one of the most commonly performed offshore installation operations and has become more challenging due to the trend of increasingly larger and heavier payloads. Especially for substantial waves, the pendular motions of the payload may cause operations to be halted.

This thesis performs a study on a positioning control strategy for a complex lifting control scenario, i.e., position-keeping of a complex-shaped 6-DOF payload using a floating vessel equipped with multiple tugger winches. As the system is highly complex and contains non-linear and time-varying dynamic phenomena, it is an impracticable task to formulate a model that meticulously describes the actual system. For this reason, a fully integrated simulation model in Orcaflex has been used to capture the non-linear dynamic behaviour of the system.

The preassembly operation of a Jacket Lifting Tool on a monohull vessel is adopted as a case study to verify the proposed control strategy. Two scenarios are considered -installation and decommissioning- for which an outrigger configuration is used to position the tugger winches. Due to the difference in setpoint (i.e. the desired position) in the two scenarios, the proposed controller is solely implemented in the decommissioning scenario. Damping tuggers, the current state-of-the-art when it comes to motion mitigation, is considered suitable in the installation scenario.

The proposed controller does not consider the state–space equations of the system and only relies on real-time motion and tension measurements of the vessel and suspended payload. In addition, the controller considers the system's velocity tension and power limitations. The controller's impact is evaluated based on the positional error and verified by the peak reduction in the power spectral density spectra of the simulations. Despite its simple form, results show a significant reduction in the positional error, and therefore the possibility to extend the working conditions of the installation vessel. To improve the controller's performance it is recommended to involve derivative control, consider payload motion prediction and to optimise the tugger winch configuration. For further studies, experimental testing is needed to verify the effectiveness of the control scheme as it could appear that the controller does not exhibit similar performance in the real system. However, it is deemed unlikely that the latter would occur as a sensitivity study regarding measurement error indicates a stable response of the controller.