Abstract
This report presents a collection of research on mooring solutions for large U.S. West Coast floating offshore wind farms that are standardized to enable serial production and leverage the domestic supply chain. It is the final report of the project Standardized Scalable Mooring Solutions Optimized for the U.S. Supply Chain, which was funded by the National Offshore Wind Research and Development Consortium, led by the National Laboratory of the Rockies, and supported by Delmar Systems. The large range of water depths in existing U.S. West Coast offshore wind lease areas may lead to a large variety of mooring designs and component sizes unless standardization methods are developed. The capacity of the domestic supply chain for mooring components is a key constraint on floating wind energy project deployment rates, an effect exacerbated by variation in component types and sizes. This project explores how standardization and careful selection of component types and sizes can lead to more scalable and domestically manufacturable mooring systems under realistic West Coast site conditions with large depth variations. It involves mooring system design at the array level accounting for site bathymetry and integration of a supply chain model within the design loop to enable optimization for supply chain lead time. The findings include estimation of the current and possible future supply chain capacities, engineering of preliminary mooring system designs for California lease areas, and comparison of costs and supply chain impacts across mooring design alternatives.
The first task of the project develops two baseline designs, one in a Humboldt lease area and one in a Morro Bay lease area, that do not use standardization but rather have each mooring line and anchor individually designed for its specific water depth and location in the wind farm. Each farm consists of 40 floating wind turbines arrayed in a rectangular grid layout. The floating wind turbine design is the International Energy Agency Wind Technology Collaboration Programme (IEA Wind) 15-MW reference turbine on the VolturnUS-S reference semisubmersible platform. To suit the deep water, the mooring designs use taut mooring lines of polyester rope with short chain sections at the seabed attaching to suction pile anchors as well as short chain sections near the waterline attaching to the floating platform. In certain seabed areas with rock (rather than mud), drilled-and-grouted pile anchors are used instead. The Humboldt baseline design uses three mooring lines and anchors per turbine. The Morro Bay baseline design uses six mooring lines and anchors per turbine, providing a more redundant mooring option with smaller individual component sizes. An efficient design methodology allows each mooring system to be individually sized to suit the water depth at its anchor’s position and meet ultimate and fatigue load design requirements across all 40 turbines in each array without having to run the full loads analysis for every turbine. This design process resulted in baseline mooring system designs with 35 unique chain diameters and 112 unique anchor diameters for the Humboldt site and 29 unique chain diameters and 55 unique anchor diameters for the Morro Bay site. In contrast, only two rope diameters were needed for each site. Each mooring line section had its own length to cater to its local depth and extreme and fatigue loads. This wide variety in component sizes (with the exception of rope diameter) demonstrates the complexity that arises if mooring systems are designed individually, which would require the supply chain to deliver many different component sizes, each in relatively small quantities.
The second task characterizes the domestic supply chain of these mooring system components, using the mooring component volumes from the baseline designs as reference points. We identified existing domestic suppliers of chain, rope, and anchors that could potentially support the required level of manufacturing and transportation to port for the baseline designs. The domestic supply chain for chain has sufficient workforce but currently can only manufacture chain with nominal diameters up to 127 mm (5 inches), where the nominal diameter is the diameter of the steel rod used to form the chain links. These chains would also require upgrades to meet the necessary offshore grade and certification for permanent floating wind moorings. The domestic supply chain for rope can support the expected mooring rope sizes but production rate is limited by the workforce and processes available for splicing the ends of each rope. The domestic supply chain for anchors has ample suppliers on the Gulf Coast but would require long trips to ship by barge from the Gulf Coast through the Panama Canal to the West Coast. The supply chain and the suppliers’ capacities (e.g., throughput, transportation speed) were implemented into the existing Offshore Renewables Balance-of-System and Installation Tool (ORBIT), allowing for full arrays of mooring system designs to be simulated through a supply chain model and enabling calculations of lead time. The total existing supply chain can support the needs of single floating wind projects at the scale studied in this project (600 MW), but some supply chain capacities would be saturated for more than a year. Larger deployment scenarios at the scale of 1 GW per year would therefore not be possible without domestic supply chain expansion. Expansion scenarios were discussed with suppliers of each component type to identify likely expansion steps to support larger amounts of floating wind components. In general, there is an appetite to expand, but demand certainty would be required before investing in expansion.
The third task explores standardization techniques for mooring system design. Different techniques are identified, including homogenizing component types and sizes (like diameters, lengths), sharing components between turbines (like lines and anchors), or other strategies to vary the design or layout to simplify materials or logistics. The techniques are initially studied in isolation using quasi-static mooring design tools and the supply chain model in ORBIT to assess their potential to influence the total mooring system material cost or supply chain lead time. The cost of manufactured and delivered components is calculated only as a function of component mass, in which cost effects of economies of scale, manufacturing setup, or shipping logistics are not considered. These initial studies show that shared anchors provide the largest reduction in supply chain lead time due to a reduction in the total anchor count in the array while marginally increasing anchor sizes to account for multi-line loads. Standardizing mooring line and anchor diameters, by binning diameters to specific standardized sizes, shows a trade-off between increasing component material costs (due to binning and increasing component sizes) and decreasing supply chain lead time (due to decreasing the number of unique sizes to manufacture). Other standardization techniques can provide additional benefits to a project related to staging and installation logistics, but these aspects are not yet quantified.
The fourth task develops and describes new standardized designs for the two sites, improving on the baseline designs using the most effective standardization strategies in combination. These designs use multiple standardized chain diameters and anchor diameters within new shared anchor layouts. The anchors were resized to account for the shared anchor loads before the component size standardization. The choices of standardized sizes are selected following a multi objective study using the new ORBIT supply chain model within the design loop that aimed to minimize both cost and lead time. The standardized designs increased component material costs relative to the baseline designs by 9% for Humboldt and 20% for Morro Bay but provided reductions in supply chain lead time of 4.5 months and 8 months, respectively. The supply chain modeling assumptions factor into the lead time results considerably. For example, the anchor supply chain modeling assumes two parallel assembly lines, but additional lines could be used.
The fifth task performs a levelized cost of energy (LCOE) analysis between the baseline and standardized designs for the two sites, using the most updated LCOE modeling tools available with many assumptions such as turbine costs, platform costs, and port fees. The results found an increase in LCOE of 1.0% between the standardized and baseline designs for the Humboldt site and a decrease in LCOE of 1.0% for the Morro Bay site. Even though the mooring system material cost is larger in the standardized designs for each site, the changes in annual energy production due to the new shared anchor layouts and the inclusion of higher failure rates of the shared anchors have a larger effect on LCOE.
The overall finding from the project is that standardization of mooring system components for deep-water floating wind farms reduces logistical complexity, improves supply chain efficiencies, and has mixed effects on costs. Standardization reduces supply chain lead times and increases total mooring system material weights, which can result in increased component costs unless countered by savings from economies of scale. The existing domestic supply chain has the capacity to supply a single project, but supplying multiple large-scale project deployments would require significant expansion. Standardization of chain and anchor diameters and the use of shared anchors have the greatest potential for supply chain lead time reductions and can either increase or decrease LCOE on the order of 1% depending on the specific site and design characteristics. Additional aspects of standardization, such as mooring line interchangeability or port storage and handling, can also provide benefits but have yet to be quantified.
General conclusions from the design process are that (1) rope size standardization can be achieved with little change to the design characteristics, (2) standardization of chain and anchor diameters involves a balance between material cost and manufacturing considerations, and (3) shared anchors help reduce both cost and lead time. For continued investigation, development of higher-fidelity supply chain models could provide more comprehensive results and help identify new supply chain strategies to support varying levels of deployment. Specific improvements include accounting for port handling and installation processes and the manufacturing and transportation cost impacts of different component quantities and sizes. Overall, this work set up a new supply chain model, identified different mooring standardization strategies, and applied them to two floating wind farms with individually sized mooring system components, finding that including the supply chain (and other total project processes) in the design process yields different results than conventional design studies.