A Jacket Sizing Tool for Offshore Wind Turbines Within the Systems Engineering Initiative

With a few exceptions, all of the offshore wind turbines in existence have been installed on monopile foundations in shallow waters (depths < 50 m). However, monopiles become progressively uneconomical as turbines become larger (> 5 MW) and as water depths increase beyond 25–30 m, primarily because of modal requirements that force structural dimensions to grow onerously, even beyond current fabrication and installation capabilities. Space-frame structures (e.g., jackets), derived from the oil and gas industry, offer a lighter and yet stiff alternative to monopiles. Modeling of these structures within the turbine system dynamics, however, is resource intensive. Thus, although jacket foundations could potentially contribute to the offshore wind industry's quest for lower levelized cost of energy (LCOE), research is needed to support their basic design and analysis. Moreover, the choice of one support strategy in place of another has ramifications on the whole turbine system mechanics and costs. To address these challenges, the National Renewable Energy Laboratory (NREL) has developed the wind energy systems engineering initiative (SEI). The initiative aims to develop a toolset that integrates a variety of models for the entire wind energy system, including: turbine and plant equipment, operation and maintenance (O&M), and cost modeling. The toolset allows for trade-off studies and guides the design of components as well as overall systems towards a configuration that can achieve minimum overall LCOE through multidisciplinary analysis and optimization. This paper discusses one of the software modules in development—the jacket sizing tool (JST). This tool integrates with the remainder of the system's technical modules (e.g., the rotor, drivetrain, and turbine-to-turbine interaction module) and allows for preliminary design of a space-frame substructure, given turbine mass and load input at the tower top and environmental and soil conditions for the foundation. The JST can perform modal and static analyses while running design and code checks on frame members and joints per API RP 2A (WSD), AISC, GL, and IEC standards. Two case studies discuss the preliminary structural design of supports for a baseline 10-MW offshore wind turbine and the NREL 5-MW offshore reference turbine. Parametric trade-off studies conducted for these cases are discussed, including secondary considerations that are important for transportation and installation. Configurations optimized for minimum mass are presented in terms of main geometric parameters (such as batter, brace geometry, and tower layout) together with comparisons to results from a commercial finite element code. The examples intend to capture overall trends more than definitive figures on best designs, and in particular they shed light on the relative importance of tower-top mass and on the role played by the tower stiffness on the design of the entire support systems. Reducing tower-top mass by 15% may lead to a savings of about 10% of the overall mass for 10-MW rated systems. It is also shown that, for large turbines in deep waters, the tower and substructures should be designed simultaneously to achieve optimized configurations that lower the expected costs of materials. While land-based towers could be utilized offshore (in truncated, marinized versions) in cases of moderate hub heights, in cases where the hub heights are raised higher than on land, the use of land-based towers becomes either questionable or outright economically unfeasible. As a result, the JST can help the designer with important choices on main parameters and to assess the relative significance of various components, especially when combined with other SEI modules, that can track the effects on subsystems, the balance of plant, and O&M.