Expansion of Offshore Wind Depends on Development of Floating Wind Turbines
Offshore wind is a rapidly maturing sector, increasingly seen as a major contributor to electricity supply in states with coastal demand centers and good wind resources. While an almost 3-decade history exists in European experience, the US only recently is beginning to move forward with grid-scale projects on national and state levels. As floating wind is scaled up, to minimize technical risks experienced in the past, formal processes will help to identify the novel features, novel applications, and highest-risk components.
Technical and Commercial Context: Demonstration Projects
Large offshore wind farms have been built by all countries with coastlines on the southern North Sea, the area with the most favorable conditions: strong, consistent winds; water depths of less than 40 m; sand or clay deeper than 70 m; and close proximity to onshore electrical distribution networks and centers of high demand. Rapid reductions have been realized in the cost of electricity, calculated over the full project lifetime, from well over 200 Euros/MWhr for the first large-scale wind farms to 50/MWhr. Installed electricity generation from renewable sources now outstrips conventional fossil fuels in many areas including the European Union.
Early floating-wind concepts have tended to be conservative but have proven key aspects, including high capacity factors; consistent production; resilience to offshore conditions; and suitable methods of fabrication, installation, redeployment, and decommissioning. Current high costs are expected to fall as the first floating wind farms are constructed with several tens of units. Cost reductions will likely be the result of the following factors:
Integrated design using improved analytical tools
Fabrication through standardized, high-precision methods in centralized locations
Improvements in component reliability and predictive maintenance
Establishment of suitable elements of the local supply chain and infrastructure
Deployed platforms demonstrate many different concepts, from fully buoyant barges, semisubmersibles with catenary moorings, tension-leg platforms, and hybrids. In the pipeline are more-exotic ideas involving integrated redesign of the wind turbine or control system. Deep waters are required for a tension-leg spar, not only at the final site of the wind farm but also for installation of the turbine and tower on the floating platform.
To estimate the future cost of energy from floating wind farms, several studies of current costs and trends have been conducted. Direct comparison between models is not straightforward because cost component definitions vary, not all appear in each model, and the assumptions are not always published. Wind resources are more favorable further away from coasts. Consequently, wind turbines can be operated for a considerable proportion of time at rated capacity (typically greater than 11 m/s). As expected, life-cycle costs are affected strongly by increasing capacity factor. Given that the floating platform is between one-fourth and one-third as a proportion of the initial capital cost, and that 25 years is a typical assumption for the project life of a wind farm at the design stage, designing platforms for extended operational lives may be an option to reduce total life-cycle costs.
The complete paper describes a model-test methodology to explore the aerodynamic challenges posed by the wind-turbine concept. Widespread adoption of offshore wind power will require not only integration of turbine and platform dynamics through improved numerical-analysis software, but also integrated philosophies toward the design of offshore power plants. Platforms have been designed to replicate the behavior of bottom-fixed platforms to provide similar loads and useful lifetimes for the drive train and primary structures. Examples of integrated concepts, planned for deployment within forthcoming years, include X1 Wind, comprising a semisubmersible A-frame, single-point, swiveling turret mooring and downwind rotor. Enerocean and Hexicon both propose multiturbine platforms, which have been evaluated through integrated software and small-scale testing (Fig. 1).
Fig. 1—Multiturbine concepts.
Other Technical Challenges
Mitigating the inadequacies of aerodynamic models for integrated design is just one of the challenges facing floating wind turbines. Costs of floating wind farms are estimated to be high (up to 250% project life-cycle costs as compared with bottom-fixed wind farms) and very difficult to predict for future utility scale. Reducing costs is predicated on addressing all the topics described in this section of the complete paper. Additionally, it must be anticipated that there will be unexpected challenges, as experienced when large numbers of wind turbines were first installed offshore 20 years ago, despite many years of successful operation of onshore wind farms.
Drive-Train Loads and Testing. Floating wind-turbine motions will give rise to displacements and accelerations that normally would exceed those on fixed-foundation turbines. Dealing first with accelerations, the normal maximum axial acceleration on the tower top is approximately 0.2g–0.3g. If the motions of the nacelle caused by the motion of the floating platform reduce or interrupt the lubrication of the gears or bearings, then this would have a negative effect on the wear of bearings and gears. However, if the amount of motion the nacelle will experience is known, then this can be accounted for in the design. This is a common issue on ships and is accounted for during the design of machinery.
Scaling Up From One-Off to Volume Production. Floating wind has been demonstrated at small scale. It is expected that significant reductions in cost can be expected from serial production at high volume even though unusual in the marine sector. Some proposals regarding production of multiple, identical floating platforms on the quayside have been proposed. Suitable ports with adequate water depths, storage space, and high-capacity quays first need to be identified. Eventually, it will become clear whether the more-effective approach will be either to fabricate centrally, using highly skilled personnel and a well-developed supply chain despite long transport distances, or to manufacture in many locations close to wind-farm sites.
Operations and Maintenance. It is still common practice at bottom-fixed offshore wind farms to implement maintenance intervals specified by original equipment manufacturers in service manuals provided with the wind turbines. Often, respective tasks may be performed too frequently, resulting in components being replaced before the end of their safe operational life. Conversely, other components have been found to fail more frequently than anticipated, resulting in unplanned maintenance, multiple turbine visits, excessive down time, lost production, and bottlenecks in the supply of parts or provision of suitably trained technicians. Failure-mode effect and criticality analysis is key to identifying candidates for optimizing task scheduling and prioriting further steps, such as inspections, campaigns of measurments, new condition-monitoring systems, or replacement of major components.
These problems may be magnified for floating wind farms because systems are likely to be more complex, loading more onerous, and distances to maintenance centers and ports further. Costs of operating wind farms offshore may be reduced through improvements in remote monitoring, fault diagnosis, intelligent and adaptable interpretation of limited signals available from turbines, and development of more-accurate measures of current health and remaining life.
Remote Anchor Installation. Locations for floating wind farms may not offer seabed conditions suitable for drag anchors, piles, or suction caissons. Drilling or drill-driving of piles in sedimentary or igneous rock at depth will be required at a suitable level of cost and reliability to enable investment in floating wind farms at acceptable levels of risk. Methods and technologies used currently for floating offshore installations at fixed locations will be further developed, improved, and simplified.
Floating Offshore Electrical Substations. Utility-scale floating wind farms will need offshore electrical substations and converter platforms like those commonly used for bottom-fixed wind farms. So far, none have been constructed. As for other aspects described in the complete paper, costs will need to be lowered and reliability improved. In some locations, substations may serve separate wind farms within a geographical region, as in the German sector. Studies currently under way are attempting to model the hydrostructural dynamics of high-voltage cables needed to analyze these designs.
It is possible that the first utility-scale floating offshore wind farms will not require floating substations if either located within a much larger bottom-fixed wind farm or close to a suitable fixed structure, such as a decommissioned production platform. Bottom-mounted, submerged electrical substations have also been proposed.
The technical feasibility of installing conventional wind turbines on a variety of floating platforms has been proven theoretically, in wave tanks, at reduced scale, and for current utility-scale machines, but only in small clusters; the economics remain unfavorable for utility-scale power plants. Despite having strong wind resources and promising record-breaking capacity factors, most sites globally are unsuitable for bottom-fixed units. Floating wind will provide part of the solution for the majority of countries seeking to decarbonize their electricity through a combination of renewable conversion technologies including offshore wind power. Three topics have been discussed as candidates for significant cost reduction: high-volume production using combinations of local content and centralized facilities, integrated design philosophies and analysis, and accurate estimates of component life and predictive maintenance.
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