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

Load Cases and Theory Validity

Wave period

- Can Morison’s equation be applied to our semi-sub?

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

Regular Wave Analysis – RAOs

Irregular Wave Analysis

Hs = 6.0 m Tp = 15.0 s V = 16 m/s

Turbulent Wind and Irregular Waves

Turbulent Wind and Irregular Waves

Hs = 6.0 m Tp = 15.0 s V = 16 m/s

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

Thank you!