Wind Turbines

Analysis of Floating Wind Turbines

Background:


Although land based wind turbine technology has been up and running for decades the wind power industry is quickly moving towards offshore technologies for several reasons. One large problem is the efficiency of land based systems. As shown below the majority of land based wind is in central U.S whereas a large portion of the US population is centered on coastal areas. Offshore sites on the other hand offer a closer source. This coupled with the fact that the largest wind resource is offshore rather than on land led to the development of shallow water turbines. Currently many offshore shallow water (under 30 meters deep) designs have been implemented to harvest this potential source of energy. The foundations of these turbines are drilled directly into the ground. This simple design works well, but the pressing fact is that most of the US wind resources offshore are found in deep water (more than 30 meters deep) where these typical boring foundation techniques won’t work. In order to tap into the vast amount of wind energy out at sea new designs must be made. We investigated one particular design in our research on floating wind turbines.


U.S. Wind Resource U.S. Population Density
U.S. Wind Resource U.S. Population Density

Design:


Qualitative Comparison:


Designs
Four possible designs for the floating wind turbine model

Comparison
Qualitative comparison of the four different floating wind turbine designs.

Loads to account for in the model:

    Wind-inflow:
  • Discrete events
  • Turbulence
    Waves:
  • Regular
  • Irregular
    Aerodynamics:
  • Induction
  • Rotational augmentation
  • Skewed wake
  • Dynamic stall
    Hydrodynamics:
  • Diffraction
  • Radiation
  • Hydrostatics
  • Viscous force
    Structural dynamics:
  • Gravity/inertia
  • Elasticity
  • Foundations/moorings
    Control system:
  • Yaw, torque, pitch

Modeling:


To numerically investigate the dynamic response of a floating wind turbine we developed a fluid-structure interaction model which included a boundary element model (BEM) for the wave-body interaction, a cable dynamics model for the mooring system, and a finite element model (FEM) for the structural response. Loads were applied using an Aero-Hydro-Elastic model described in more detail below. The BEM and the cable dynamics model were applied to calculate the frequency-dependent added mass, damping, and restoring coefficients that were used in the dynamic equations to predict the motion of the system, based on which the structural deformation was determined through the FEM. This hybrid approach was applied to simulate the dynamic response (including surging and pitching motions) and structural vibrations of a wind turbine based upon a 65KW design with a spar-buoy style mooring system.


Aerodynamic Loads:

Incoming Wind - Coherent structures and turbulence modeling (TURBSIM)



Wake Modeling - Blade Element Momentum (BEM)



Hydrodynamic Loads:

Irregular Sea - JONSWAP spectra (IEC61400-3designstandard)


Wave Structure Interaction - Boundary Element Method = Excitation with an added mass, damping, and restoring force.


Validation of the model - Wave interaction with floating sphere


U.S. Wind Resource U.S. Population Density

Mooring Lines:

Cable Modeling - Fully nonlinear cable dynamics equations with stretching, bending, and twisting stiffness.


Experiments:


Experimental and numerical study of responses of floating 65KW wind turbines responses to be added later.