Session: 09-02-03 Wind Energy: Moorings I

Submission Number: 156633

Conceptual Design Exploration of Gigawatt-Scale Floating Wind Farms With Shared Mooring Systems 

This research explores innovative mooring solutions for gigawatt (GW)-scale floating offshore wind farms, addressing the high costs that limit their competitiveness in the energy market. Mooring systems, which ensure platform stability and stationkeeping, significantly contribute to the overall levelized cost of energy. Shared mooring systems offer a cost-effective approach by allowing neighboring platforms to share various mooring components such as anchors, mooring lines, and midpoint connections (e.g., buoys). For instance, multiple platforms can be connected to a single anchor, two platforms can be tethered directly without anchors, or two or more platforms can share a single midpoint with or without a tendon connecting it to an anchor. These solutions can potentially reduce the total length of the required mooring lines, anchor count, and subsequent cost of the mooring system. The most effective shared mooring system designs for large floating wind farms are not yet established, and existing methods are not well-suited for evaluating the breadth of design options at this scales. 

We present and demonstrate a new approach for efficiently exploring conceptual shared mooring system designs for GW-scale farms. The approach uses a unit cell representation for identically repeated patterns within the array. A linearized 2D force-displacement model facilitates rapid evaluation of numerous unit cell configurations. The unit cell is then extrapolated to build the full array model, interconnecting the cells with shared anchors, shared mooring lines, multi-turbine connectors, and vertical tendon lines. This extrapolation is constrained by either a periodicity vector (number of repeated unit cells in each direction) or by the designated wind energy area. Partial unit cells are supported at farm boundaries to maximize capacity. Array-level optimization that explores design variables related to platform and anchor spacing, farm orientation, multi-line connectors’ depths and locations and other geometric arrangements is carried out aiming to minimize material mass of the required mooring systems and the associated cost and to improve array efficiency by analyzing annual energy production and losses due to intra-array wake interactions based on the site-specific wind rose. 

Detailed mooring component specifications are refined within a quasi-static 3D model that accounts for material properties, line profiles, anchor type, and multi-line connectivity.  This process incorporates geometric constraints, such as preventing rope contact with seabed and allowing a minimum subsea clearance for vessel access, in addition to tension and stiffness constraints ensuring compliance with safety factors and platform offset limits derived from the linear model. The iterative coupled evaluation of the linear and quasi-static models ensures convergence on optimal designs. These designs are benchmarked against baseline configurations to evaluate cost reductions and feasibility. 

This work contributes to the development of preliminary tools for evaluating shared mooring solutions for large-scale floating wind farms, paving the way for the evaluation and assessment of most promising layouts at a higher fidelity level to study their behavior under realistic environmental conditions. Future efforts will also expand performance metrics to cover environmental impacts, advanced loading and cost analysis, risk and failure assessment, and design and installation feasibility.  

Presenting Author: Rudy Alkarem National Renewable Energy Laboratory

Presenting Author Biography: Rudy has just got his PhD from the University of Maine last month. During his PhD, he worked with the researchers across the spectrum from the Advanced Structures and Composites Center staff at UMaine to professors at the Norwegian University of Science and Technology (NTNU) to researchers at the National Renewable Energy Laboratory where he did a graduate research internship. His research focuses on low- to medium-fidelity design aspects of stationkeeping systems for floating offshore wind farms at a large scale and how to bring down the cost and footprint of such systems on the surrounding environment.