mooring system in floating wind turbines ... school of engineering 29. september 2017 christian...

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


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



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DEGREES OF FREEDOM

16 DOF:

• 6 DOF for rotor

• 2 DOF for drivetrain

• 2 DOF for tower

• 6 DOF for spar rigid body motions



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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



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VERIFICATION

Verification using FAST:

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


Steady wind:15 m/s


AARHUSUNIVERSITET

VERIFICATION

Turbulence

Adding other external loads, turbulence and waves

Waves

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


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



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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



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LUMPED MASS/FEM

• Seabed is modelled using springs

and dampers.

• Mooring line is modelled using

lumped mass

• Non-linear force-displacement

relation

• Dynamics included



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


Linear spring Quasi static Lumped mass/FEM

AARHUS UNIVERSITET

mooring system in floating wind turbines ... school of engineering 29. september 2017 christian...

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