Offshore wind offers a major potential source of renewable energy for the nation. Roughly 60% of the U.S. offshore wind energy is in water too deep for fixed platforms; at these depths wind towers supported on floating structures are the primary alternative. The cost of support structures, including the mooring system, for offshore wind turbines can be as much as 15% of the total capital cost, which significantly affects the cost of offshore wind. Noting that capital cost reductions on the order of 70% are needed to make floating offshore wind economically competitive, major reductions in anchor and mooring system costs can make a significant contribution toward achieving this target. The Multiline Ring Anchor (MRA) was conceived in a previous NSF project as a means of reducing the cost of anchors for floating offshore wind turbines (FOWTs.) Basic features of the anchor include: (1) it?s deep embedment permits a relatively compact anchor to have a high capacity for resisting mooring line loads, (2) very quiet suction installation in clays and vibratory installation is possible in sands, (3) precise positioning is possible during installation, leading to a high degree of reliability, (4) it can effectively resist loads ranging from purely horizontal to purely vertical, so it works in any mooring system, (5) it?s compact size leads to reductions in fabrication, transport and handling costs, and (6) it?s circular symmetry permits multiple line attachment to a single anchor, permitting major reductions (about 50%) in the required number of anchors in a wind farm array. An NSF project, validating the technical performance of the MRA, is currently in progress. This I-Corps project was undertaken to explore the commercial feasibility of the anchor.

The I-Corps Team conducted 150 interviews with industry experts and stakeholders in 14 countries, including the U.S., Canada, Brazil, South Korea, Taiwan, Australia, Norway, Spain, France, Germany, Netherlands, Belgium, Denmark, and the United Kingdom. The interviews were targeted toward all playing a role in the decision-making process for anchor and mooring system design for floating offshore wind turbines, including recommenders, decision makers, end users and buyers. Also interviewed were entities that were not direct potential users of the MRA, but who could affect usage and marketing. These included certifying entities such as Det Norsk Veritas (DNV) and the American Bureau of Shipping (ABS). Also contacted were environmental specialists and personnel knowledgeable on ocean use, since these types of issues can place constraints on the purchase and usage of the anchor.

The interviews revealed that stakeholders were receptive to the MRA concept, but some key issues were identified for the path to commercialization to proceed. Project managers pointed out that other considerations apply beyond the direct costs attributed to foundations. For example, the compact MRA requires fewer vessel trips, which can lead to reduced construction periods that reduce overall project costs that are not directly considered as foundation costs. A second major revelation from the interviews is that environmental issues can dominate the selection of the anchor and mooring system, which led the team to reconsider the original anchor concept and expand its applicability to tension-leg mooring system designs. A third significant issue uncovered during the interviews was that there needs to be a realization that the shared anchor concept cannot work everywhere, since geometric constraints can render the system impractical in very shallow and very deep waters. A fourth issue raised during the discussions was that shallow rock is a commonly occurring issue in floating offshore wind farm development; an anchor system that could deal with this problem could be highly marketable. A fifth issue is that, although impacts on marine life are recognized as an issue, guidance is rather nebulous on allowable noise during installation and potential marine mammal entanglement in the moorings. In regard to the path to commercialization of the MRA, the next steps include: continued numerical analyses and laboratory model tests to investigate technical performance issues, validating field tests, and resolving issues associated with actual deployment and installation.

As a final major conclusion, the ring anchor concept has great promise, but its range of application should be extended to include single and multiline mooring systems. While the shared anchor concept is not applicable to tension leg mooring systems (attractive environmentally due to their small footprint) and to some taut mooring systems due to geometric constraints, the efficiencies of a deeply embedded anchor are still sufficient to justify the use of this anchor concept in a single-line system. For this reason, the project team renamed the anchor as a Deeply Embedded Ring Anchor (DERA) to emphasize that it can provide cost-effective anchorage solutions for both single-line and multiline mooring systems.

Last Modified: 05/21/2023
Modified by: Charles P Aubeny