Effects of Hydrodynamic Modelling in Fully Coupled ... · field by BEM or GDW Tool for fully...
Transcript of Effects of Hydrodynamic Modelling in Fully Coupled ... · field by BEM or GDW Tool for fully...
Marit Kvittem (NOWITECH/NTNU)
Erin Bachynski (CeSOS/NTNU)
Torgeir Moan (CeSOS/NTNU)
Effects of Hydrodynamic Modelling in
Fully Coupled Simulations of a
Semi-submersible Wind Turbine
DeepWind Jan 2012, Trondheim
Agenda
1. Tool for fully coupled analysis of floating
wind turbines
• Riflex + AeroDyn
2. Comparison of Morison’s equation and
potential theory for a semi-submersible
wind turbine
Program Shortcomings
FAST+HydroDyn • No mooring elements
• No horizontal Morison elements
• No twist dof on blades
• Modal theory
HAWC2 Only slender body theory
USFOS + VpOne Only slender body theory
SIMO+RIFLEX
windturbine
No spatial wind field
Motivation for Linking Riflex and AeroDyn
RIFLEX windturbine (w control)
Structural analysis of slender, flexible
beams (mooring lines, tower, blades)
SIMO
Motion analysis of floating structures (hull)
AeroDyn + TurbSim
Aerodynamic forces from turbulent wind
field by BEM or GDW
Tool for fully coupled analysis
of floating wind turbines
SIMO/RIFLEX + AeroDyn
FEATURES
► Powerful hydrodynamics
► Non-linear FEM for blades, tower and mooring lines
► Internal or user defined control
► Verified and well tested aerodynamics (AeroDyn)
► Turbulent wind field through TurbSim
► Generalized dynamic wake option for high wind speeds
► Eccentric aerodynamic centre
► Tower shadow for upwind turbine
► Wind on tower
MISSING FEATURES
► Eccentric blade element mass
A powerful analysis
tool for floating
wind turbines!
Land based case in
good agreement with
FAST and HAWC2
Semi-submersible Wind Turbine
Similar to WindFloat
Column diameter 10 m
Column cc 46 m
Draft 17 m
Displacement 4640 tonnes
Mooring lines 4
Turbine NREL 5 MW
Courtesy of Principal Power
Morison vs Potential theory
For a single DOF system:
Linear potential theory with quadratic drag
Morison
Diffraction for
small
wavelength-to-
diameter ratios
ma is calculated based on A() from potential
theory, for columns and heave plates
Morison vs Potential – Four models
Potential theory • M, A(), B(), C and force
transfer functions
• Quadratic drag
Pure Morison (z= or z=0) • Inertia terms
• Quadratic drag
Morison with dynamic
pressure (z=0) • Inertia terms
• Quadratic drag
• Correction for dynamic pressure
under columns
Morison updated pos. (z=) • Inertia terms
• Quadratic drag elements
• Calculates forces in updated
position of the platform
Conclusions
• Diffraction effects are important for heave motions for
wave periods below 7 s
• Morison can be applied for this structure, but stretching
and coefficients must be chosen with care
• Effect of updated position is small
• Pitch motions are important to power production and
blade root bending moment, so correct preditcion of
motions is important