Design, Numerical Modelling, and Dynamic Analysis of Combined Wind and Wave Energy Systems for Intermediate Water Depths

Abstract

A promising strategy to fully harness the potential of offshore wind and wave energy lies in their combined exploitation. While the Development of integrated wind and wave energy systems is still in the early stages, recent trends indicate that integrating wave energy converters (WECs) alongside the more established offshore wind technology is encouraging. This approach leverages the advantages of shared ocean space, facilities, and operation and maintenance (O&M) costs, potentially reducing the barriers to commercialization. Despite numerous early-stage investigatory attempts on various combined wind and wave energy systems, neither academia nor industry has yet reached a consensus on the most effective method for the combined extraction of these resources. With wind as the primary power source, the addition of WECs should not compromise wind Power absorption. Therefore, a thorough understanding of the combined system’s dynamic response characteristics is essential. From a numerical modelling perspective, the interactions between the floating offshore wind turbines (FOWTs) and WECs can result in complex numerical models which often leads to extended simulation time. Mooring design is another critical factor influencing the feasibility of combined wind and wave energy systems. While BOWTs dominate shallow water regions (10-40 m) and FOWTs are the preferred solutions for deep water regions over 100 m, the intermediate water regions (40-100 m) present a competitive landscape for both technologies, With mooring costs playing a decisive role in system selection. Conventional catenary mooring systems face challenges at intermediate water depths due to the insufficient mooring line weight, which is needed to restrain the floater’s horizontal offset while maintaining acceptable mooring line tension levels. Alternatively, using synthetic fibre ropes in taut configurations can create more compliant mooring systems with generally lower tension compared to chain-catenary systems. However, the resulting mooring restoring stiffness is inversely proportional to the line length, imposing limitations on the acceptable anchor radius. In this thesis, the dynamic responses of two combined wind and wave energy systems—the Semi-submersible Torus Flap Combination (STFC) and the Two-rotor Semi-submersible Torus Combination (2WTSTC)— are investigated. The interactions between the FOWT supporting platforms and the WECs are modelled with two primary objectives: (1) to estimate the power absorption potential based on the WECs’ motions, and (2) to accurately incorporate the damping effect provided by the WECs into the overall system. These objectives serve as critical indicators for evaluating the synergy between the different Components of the integrated wind and wave energy systems. During operation, the torus WEC’s PTO system contributes additional heave damping to the combined system, leading to a notable reduction in heave motion compared to a system without the torus WEC. The integration of flap-type WECs in the STFC further enhances platform stability by increasing damping in the pitch degree of freedom (DOF). The added damping from the WEC PTO systems reduces platform motions and stabilizes wind turbine power output. For station-keeping in intermediate water depths, a novel mooring system, known as the “soft-chain”, is proposed. This system strategically uses polyester rope to handle only the horizontal component of the mooring restoring force, creating a more compliant system under extreme environmental conditions while maintaining adequate restoring capacity during normal operating conditions, assisted by the heavy chain segments. Additionally, the response characteristics of two 5-MW-CSC FOWTs in a shared mooring configuration are examined in detail, demonstrating the feasibility and cost-saving potential of such mooring systems for deep-water applications.