Numerical and Experimental Study of Shared Mooring Systems for Prototype Floating Wind Farms

Abstract

Conventional mooring systems represent a substantial portion of the cost of a floating offshore wind farm (FOWF). The concept of shared mooring was initially proposed to reduce the overall usage of mooring lines and the number of anchors required by connecting adjacent floating offshore wind turbines (FOWTs) in an FOWF. Although such a concept offers cost-saving potential, extra complexity has been introduced to the floating system as the platform motions of FOWTs are coupled. To demonstrate the feasibility of shared mooring systems, it is of great interest and importance to understand the fundamental influence of shared mooring systems on the dynamic characteristics of FOWFs. Limited research has been conducted concerning detailed dynamic analyses of shared mooring systems and experimental investigations of FOWFs. This thesis addresses this knowledge gap by conducting numerical and experimental studies on prototype dual-spar FOWFs with shared mooring systems. Extensive investigations are carried out considering various shared mooring configurations, different turbine spacings and water depths, and varying mooring properties, configurations, and load scenarios. In the numerical study, a modeling method is developed for shared mooring systems utilizing Irvine’s elastic catenary theory for hanging cables, which serves as the basis for the mooring stiffness linearization and eigenvalue analysis of FOWFs with shared mooring systems. For a dual-spar FOWF, a shared line connecting two FOWTs is the basic shared mooring configuration. The influence of the shared line on the system’s natural periods and eigenmodes are investigated through a comparison to a single spar FOWT. The investigation reveals the significant influence of the shared line on the natural periods of surge and sway degrees of freedom (DOFs) of the spar platforms due to the dominance of mooring stiffness in the relevant terms of restoring stiffness. Moreover, the natural periods of these DOFs are found to be sensitive to variations in the mooring properties of both the single lines connecting the FOWTs to the seabed and the shared line connecting adjacent FOWTs. The basic shared mooring configuration is then modeled with a numerical simulation tool to perform fully coupled time-domain simulations under varying environmental conditions (ECs). A comparison with a single spar FOWT demonstrates larger motion ranges and dynamic motions of FOWTs in the FOWF, along with higher tension levels in the single lines. The shared line experiences high dynamic tension and snap load events. The studied mooring layout displays sensitivity to loading directions due to its inherent asymmetry. An alternative shared mooring system is considered by connecting neighboring FOWTs to a shared tethered buoy and by replacing the single lines of the FOWTs with hanging lines connected to individual tethered buoys. Through dynamic analyses under various load scenarios and comparison against the basic shared mooring configuration, it is concluded that the proposed tethered-buoy shared mooring system can achieve a significant reduction in tension levels across all mooring lines and alleviate potential threats associated with snap loads, with a trade-off for increased mean offset and dynamic motions of the FOWTs. Therefore, the proposed tethered-buoy shared mooring system is a preferred solution for future applications involving inter-array cables, where the excursion restrictions of FOWTs become less critical. A hydrodynamic model test campaign is conducted for dual-spar FOWFs with different shared mooring systems. Decay tests are carried out, and regular and irregular wave tests are performed under various ECs. Detailed physical modeling of the FOWTs and shared mooring systems are documented. Through a comparison to the results of a previously tested single spar FOWT, the findings from the numerical study are verified. A clump weight is introduced at the midpoint of the shared line in the basic shared mooring configuration, and its effects are investigated through a comparative analysis where the natural periods, motion response, and mooring tension before and after the inclusion of the clump weight are analyzed. The added clump weight leads to a decrease in the natural periods of horizontal DOFs and reduced dynamic motions of the FOWTs. Despite an increase in the pretension of all mooring lines, dynamic tension in these lines is reduced, and a reduced number of snap load events in the shared line is observed.

In summary, this thesis investigates shared mooring systems for prototype dualspar FOWFs by numerical and experimental approaches. A quasi-static modeling method of shared mooring systems is developed and applied. The dynamic characteristics of FOWFs with five shared mooring configurations are revealed, and the station-keeping performance of the FOWF systems are highlighted in the comparison. These research outcomes augment the current understanding of and expand the existing knowledge base related to shared mooring systems. The analysis methods developed and employed in this study can be further utilized in the design and optimization of FOWFs.