Rational design of floating offshore wind turbines requires a trade-off between very stiff structures that support the nacelle in a near-vertical orientation versus less expensive floaters that allow larger angular displacements; the stiffer structures generally cost more but enable greater energy harvest. A computationally friendly method based on power transfer functions is presented in which the total power output from specific designs is computed by convoluting power output transfer functions for the floating wind turbine with site-specific long-term environmental conditions. Environmental conditions characterized by each of four proposed sites are four random variables, representing the wind velocity, significant wave height, peak period of the wave process, and the angle between the wind and waves. One step in the overall method is development of modified two-dimensional wind-power curves, which can be used to show the relative importance of consideration of the wave properties on overall energy harvest.
Wind-energy is increasingly gaining acceptance as an economically viable and environmentally friendly method of energy harvest. At the same time, local permitting and environmental issues associated with considerations such as noise emissions and interruption of the view-scape are becoming critical problems as desirable space for on-shore wind turbines becomes increasingly scarce. European countries, which are relatively densely populated compared with the United States, began siting offshore wind turbines in shallow waters near shore, most notably in Denmark and Germany in the 1990’s. Today, the growth of offshore wind energy in European countries is significant, with projected growth rates of 1700 to 3000 MW per year (Snyder and Kaiser, 2009). In the Far East, China is beginning installation of offshore wind, including the first offshore wind farm in East China Sea, which will which will produce 267 GWh per year for the energy market in Shanghai (Chen, 2011). In very deep waters, the bottom-founded support towers may prove cost-prohibitive, but the cost of floating offshore systems is relatively insensitive to water depth, and may prove to offer a viable way to develop wind energy beyond the sight of land.
The relatively small profit margins and high costs of floating systems make design optimization critically important. Unfortunately, use of computer-based time-domain numerical simulations for long-term prediction of energy harvest in site-specific conditions requires so much elapsed time as to be impractical. For example, direct simulation of ten years of wind turbine performance for one case would require over 70,000 20-minute cases, which would require almost 10 weeks running on a modern personal computer. Here, a new method is presented that enables accurate prediction of energy harvest for a specific design in site-specific wind-wave conditions but that requires substantially less computer time.
A new numerical methodology is presented for simulation of dynamic behavior of floating offshore wind turbines. Wind forces are computed from wind velocities, blade pitch angles, and the resulting rotor speeds. No small-angle assumptions are required in the solution of the equations of motion; vessel motions are included in wind and wave force calculations. Aero-elastic effects are quantified using the industry-standard subroutine Aerodyn, with blade pitch-angles computed by the “discon” subroutine, both open-source and publicly available from the National Renewable Energy Lab (NREL).
Effectiveness is demonstrated for small-angle cases by comparison with results from an industry standard simulation tool for the spar-based NREL OC3-Hywind with variable wind speed and no waves. The method is further demonstrated by application of the same environmental conditions to a smaller spar-based floater for which standard simulation tools would not be applicable. Finally, a case is presented including irregular winds and waves.
Future designers can use the method to analyze smaller alternative conceptual designs that are likely to experience large angular motions.
Future programmers facing legacy software issues can use similar programming techniques. Here, mex files are used to interface a new large-angle simulation method recently developed using MATLAB with well-established aerodynamic and control routines previously implemented in FORTRAN.
Results, Observations and Conclusions:
Simulation results for a small-angle case are shown to be equivalent to those of a well-established industry-standard simulator, but with enhanced numerical efficiency. Hull motions, rotor speed, blade pitch angle, and electrical power output are critically compared.
The smaller spar-based floater is found to have large-angle motions far exceeding the theoretical and practical limits of industry-standard software. The two floater designs are compared in terms of dynamic behavior and electrical output.
1. The methodology enables efficient simulation of large-angle floater motions, which is necessary to assess cost-effectiveness of innovative designs.
2. The method demonstrates successful integration of an Euler-angle-based wind turbine simulator with existing aero-elastic and blade pitch control routines.
3. The special-purpose method is substantially more computationally efficient than a general-purpose finite-element tool.
A new numerical methodology is presented for simulation of dynamic behavior of floating offshore wind turbines. Wind forces are computed in the time domain through application of blade element momentum theory using the instantaneous wind velocities, blade pitch angles, and resulting rotor speed. Equations of motion are developed and solved in Euler-space, such that no small-angle assumptions are required in the solution; vessel motions are included in wind and wave force calculations. Aero-elastic effects are quantified using the industry-standard subroutine AeroDyn, with blade pitch-angles computed by the "DISCON" subroutine, both open-source and publicly available from the National Renewable Energy Lab (NREL). Effectiveness is demonstrated through a series of examples, first a case for small-angle motion is compared with results from an industry standard simulation tool for the spar-based NREL OC3-Hywind with constant wind speed and no waves; next, the same environmental conditions are applied to a smaller spar-based floater for which standard simulation tools would not be applicable. Finally, a case is presented including irregular winds and waves.
The airgap of a specific semisubmersible platform subjected to irregular waves is considered. Detailed model test results for both motions and airgap time histories are used to verify analysis results. The effects of various methods of including second-order diffraction contributions are demonstrated. A new method is proposed for use in post-processing second-order hydrodynamic transfer functions in which those transfer functions that are unavailable or believed to be unreliable are replaced with those of an undisturbed second-order Stokes wave. INTRODUCTION AND BACKGROUND Airgap modeling is of concern for both fixed and floating structures, but it is particularly challenging in the case of floating structures because of their large volumes and the resulting effects of wave diffraction and radiation. Standard airgap response prediction uses linear theory, which generally does not effectively reproduce measurements from model tests. First-order diffraction is considerably less demanding than second-order, so use of only first-order diffraction merits some consideration. Second-order diffraction effects are expected to better reflect observed data. However, these radiation /diffraction panel calculations are very sensitive to the numerical modeling. The cost of increasing the airgap for a semisubmersible is considerably higher than that for a fixed structure. Hence, instead of increasing the still-water airgap, it may be less expensive to strengthen the underside of the deck to withstand rare negative airgap events. In order to know the wave load and location of impact, reliable prediction tools are important. Model tests are often performed as part of the design of a new semisubmersible. If so, these calculations are needed to determine the locations for placement of airgap probes on the model. Here, the numerical impact of modeling second-order diffraction effects is assessed by comparing various predictions of the statistical behavior of the free surface with model test results.