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.