AARHUS SCHOOL OF ENGINEERING

29. SEPTEMBER 2017 CHRISTIAN ELKJÆR HØEGH, MSC STUDENT AND PALLE FLYDTKJÆR, MSC STUDENT

AARHUSUNIVERSITET

MOORING SYSTEM IN FLOATING WIND TURBINES

OCTOBER 3 2017

DEPARTMENT OF ENGINEERING

CHRISTIAN ELKJÆR HØEG, MSc STUDENT AND PALLE FLYDTKJÆR, MSc STUDENT

29. SEPTEMBER 2017 STUDERENDE DEPARTMENT OF ENGINEERING

AARHUSUNIVERSITET

PROJECT MOTIVATION

• New market for offshore wind

turbines

• Sensitivity study requested from the

industry.

• Needed level of details in the

mooring loads, to design the

turbines.

• Lower levelized cost of energy (LCOE)

Hywind Scotland, Statoil

CHRISTIAN ELKJÆR HØEG, MSc STUDENT AND PALLE FLYDTKJÆR, MSc STUDENTOCTOBER 3 2017

29. SEPTEMBER 2017 STUDERENDE DEPARTMENT OF ENGINEERING

AARHUSUNIVERSITET

PROJECT OVERVIEW

Development and verification of own

coupled floating turbine model

Implemented in model:

• Floating spar foundation and NREL 5MW

turbine

• Linear structural dynamics based on

modal method

• Blade pitch and generator torque

controller

• BEM model for aerodynamic loads

• Morison equation for hydrodynamic

loads

• Linear wave theory

• Mooring system modeled as linear

spring

CHRISTIAN ELKJÆR HØEG, MSc STUDENT AND PALLE FLYDTKJÆR, MSc STUDENTOCTOBER 3 2017

29. SEPTEMBER 2017 STUDERENDE DEPARTMENT OF ENGINEERING

AARHUSUNIVERSITET

DEGREES OF FREEDOM

16 DOF:

• 6 DOF for rotor

• 2 DOF for drivetrain

• 2 DOF for tower

• 6 DOF for spar rigid body motions

CHRISTIAN ELKJÆR HØEG, MSc STUDENT AND PALLE FLYDTKJÆR, MSc STUDENTOCTOBER 3 2017

29. SEPTEMBER 2017 STUDERENDE DEPARTMENT OF ENGINEERING

AARHUSUNIVERSITET

LOADS AND SIMULATION CONDITION

• Model can include external loads from wind, waves

and mooring system

• Delta connection is replaced by torsional spring

• Normal operating conditions at varying wind speeds

CHRISTIAN ELKJÆR HØEG, MSc STUDENT AND PALLE FLYDTKJÆR, MSc STUDENTOCTOBER 3 2017

29. SEPTEMBER 2017 STUDERENDE DEPARTMENT OF ENGINEERING

AARHUSUNIVERSITET

VERIFICATION

Verification using FAST:

Simple 16DOF model captures overall behaviour, with only small deviations

CHRISTIAN ELKJÆR HØEG, MSc STUDENT AND PALLE FLYDTKJÆR, MSc STUDENTOCTOBER 3 2017

Steady wind:15 m/s

29. SEPTEMBER 2017 STUDERENDE DEPARTMENT OF ENGINEERING

AARHUSUNIVERSITET

VERIFICATION

Turbulence

Adding other external loads, turbulence and waves

Waves

CHRISTIANELKJÆR HØEGH, MSc STUDENT AND PALLE FLYDTKJÆR, MSc STUDENTOCTOBER 3 2017

29. SEPTEMBER 2017 STUDERENDE DEPARTMENT OF ENGINEERING

AARHUSUNIVERSITET

LINEAR SPRING

• Currently implemented in model

• Position at rest determines stiffness

of mooring lines

• Cable length is driving factor

No dynamics is included

• No non-linearities is considered

CHRISTIAN ELKJÆR HØEG, MSc STUDENT AND PALLE FLYDTKJÆR, MSc STUDENTOCTOBER 3 2017

29. SEPTEMBER 2017 STUDERENDE DEPARTMENT OF ENGINEERING

AARHUSUNIVERSITET

QUASI STATIC

• Considering equilibrium state at

each new position, to update

mooring line forces

• Seabed and line friction

interactions is considered

• Non-linear force-displacement

relation

• No dynamics

CHRISTIAN ELKJÆR HØEG, MSc STUDENT AND PALLE FLYDTKJÆR, MSc STUDENTOCTOBER 3 2017

29. SEPTEMBER 2017 STUDERENDE DEPARTMENT OF ENGINEERING

AARHUSUNIVERSITET

LUMPED MASS/FEM

• Seabed is modelled using springs

and dampers.

• Mooring line is modelled using

lumped mass

• Non-linear force-displacement

relation

• Dynamics included

CHRISTIAN ELKJÆR HØEG, MSc STUDENT AND PALLE FLYDTKJÆR, MSc STUDENTOCTOBER 3 2017

29. SEPTEMBER 2017 STUDERENDE DEPARTMENT OF ENGINEERING

AARHUSUNIVERSITET

PROJECT OUTCOME

Initial findings from litterature review:

• Large effect on cable tension when including complete cable dynamics

• Dominating loads on structure comes from wind and hydro loads

• Structure is less sensitive for mooring loads.

Our focus:

• Which structural components are most influenced by

mooring dynamics

• What level of detail is needed and how does it affect

the computational cost

CHRISTIAN ELKJÆR HØEG, MSc STUDENT AND PALLE FLYDTKJÆR, MSc STUDENTOCTOBER 3 2017

Linear spring Quasi static Lumped mass/FEM

AARHUS UNIVERSITET