DESIGN CHALLENGES FOR LARGE OWTSWITH FOCUS ON SUPPORT STRUCTURES

Science Meets Industry Bergen

By Jørgen R. Krokstad

(with contributions from Loup Suja Thauvin and

Lene Eliassen – and others)

Content

Design trends – turbines and support structures

Standards

Design loads and design basis

Integrated analysis and design

Soil stiffness and capacity

2

The bottom fixed offshore wind turbineAN INTEGRATED structure

3

The “cut”

Outlook OWF Foundations

Gravity-based

foundations

Monopiles

(incl. XL)

Tripod/Tripile

Jacket

Floating

< 20

10 - 40

25-50

35-60

> 50

21%

75%

2%

2%

< 1%

• XL pipes will pot. replace

Jackets < 40 m depth

• Known technology

• High fabrication costs

• Too heavy

• Expensive logistics

• Stiffer structure/less steel

• Higher installation efforts

• Higher fabrication costs

• Commercial realisation

long term only

• Logistical challenges

• Environmental restrictions

• Complicated logistics

• Suitable only for lower

water depth

Source: Roland Berger Presentation: Offshore Wind towards 2020

Monopiles remain the dominant foundation concept, but trend

toward deeper water is shifting to jacket foundations

Foundation Depth (m) Trend 2020 CommentsCum 2012

Development of MP‘s

Source: A2Sea News - Winter 2013 and EEW SPC

20022008

2012 20142015

2018

Horns Rev 1

2.0 MW

Water depth up to 14 m

Lynn

3.6 MW

Water depth up to 18 m

London Array

3.6 MW

Water depth up to 25 m

Baltic II

3.6 MW

Water depth up to 27 m

Gode Wind II

6 MW

Water depth up to 35 m

Future MP‘s

8+ MW

Water depth up to 40 m

L 34 m

Ø 4 m

160 tL 45 m

Ø 4.7 m

350 tL 68 m

Ø 5.7 m

650 tL 73.5 m

Ø 6.5 m

930 tL 80 m

Ø 8.5 m

1050 t L >80 m

Ø >9 m

>1050 t

EEW SPC/Bladt EEW SPCSIF MT Hojgaard EEW SPC/Bladt

Jacket development from pre-piled to suction bucket.

6

Design trends on bottom fixed turbines

Large turbines (6-10 MW, 150 - 200 meter diameter)

Simple substructures – mono-columns, jackets

Possible integrated installation (foundation, tower, nacell and rotor in one

piece) but has not shown to be economical so far

INTEGRATED design – optimize tower and foundation design

7

8

Some important considereations DNV-OS-J101

9

Combination of wind and wave loads a huge challenge for the offshore wind industry.

Why? Consequence?

10

11

Example of use of load cases

12

Note! A large number

of simulations with only

10 min duration. Consequence

on extreme values?

Design basis – metocean requirements

Wave growth by the action of wind

Nonlinear wave-wave interaction

Dissipation due to white-capping, bottom friction and depth-induced wave

breaking.

Refraction and shoaling due to depth variations.

If deemed relevant, wave-current interactions

13

Design basis – metocean requirements

hub height wind velocity

dependence

misalignment dependence

Joint probabilities of Water Level

and Hs

Spectral shape and short term

directional distribution

Time duration – 1 hour (not 10 min

or 3 hours)

14

Inconsistent wave theories due to shallow water – affect strongly statistical method

15

Hmax given as deterministic input –

destroy statistic

Challenges – Dynamic Analysis

Offshore wind turbines – highly dynamic – integrated

How to cut a dynamic structure in two parts – contractual issues

How to optimize foundation (different designs for a selection of

park locations) with tower (wants to keep one design)

The troublesome top mass (nacelle and rotor)

Waves and wave loads are non-linear in extreme weather and at

shallow water

Design consequences (dimension selection)

1. Ensure no or limited frequency interaction between turbine and structural frequencies

2. Document sufficient design life (FLS)

3. Ensure sufficient structural capacity or integrity in storm conditions (ULS)

16

Eigenmodes and eigenfrequencies

where K is stiffness matrix, M is structural mass and is the

added mass (zero in air – significant in water).

17

𝐊 − ω2 𝐌+𝐦𝐚 𝚽 = 0

𝐦𝐚

The “cut”

Qualification of Design Assumed 5 MW NREL

turbine(External and Internal use of Fedem Windpower Software package)

Status integrated analysis – real ongoing projects

Operator is defining load cases together with turbine supplier

Research institute is producing wave load data

Turbine contractor is running integrated aerodynamic loads (including

controller) and calculates global load responses for tower and foundation

Foundation engineer do independent calculations for their foundation contract

based on input from turbine contractor – sequential approach

The design situation is truly not INTEGRATED – plausible cause LACK OF

ACCESS TO CONTROLLER – and INDUSTRY ESTABLISHED PRACTICE

Aerodynamics loads (nP – frequencies) illustrated on a blade. Changing distance

20

By Lene Eliassen

NTNU

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7

Pow

er

spe

ctr

al de

nsity

Frequency [Hz]

Wavespectrum

5MW

10 MW

By Loup/Lene

Statkraft/NTNU

Design challenge of support structure with increasing rotor diameter

21

Lower rotational speed of

large turbines give lower

1P and 3P regions

Reflection on integrated design

Future designs – in what frequency range do we want to design our support

structure to reduce cost? Consequence?

22

www.statkraft.com

THANK YOU

Support structure types

Offshore Oil&Gas versus Offshore WindSupport structure challenges

The soil stiffness models

2

6

Simplified: p-y curves

and springs

More complicated:

FEA models.

Design considerations

Foundation Design & Soil Conditions

• Foundation design and selection influenced

by chalk, or the «absence» of it (within

foundation depth), i.e. when chalk is:

• Shallow -> MPs

• Deep -> Jacket piles (in Swarte Bank)

• Uncertainty relates to Swarte Bank

dominated infill (blue and green areas)

Park effects on aerodynamic loading –Interaction between turbines

29

Standard IEC 61400-1 says:

“The increase in loading generally assumed to result from wake effects may be

accounted for by the use of an effective turbulence intensity, which shall include

adequate representation of the effect on loading of ambient turbulence and

discrete and turbulent wake effects.

For fatigue calculations, the effective turbulence intensity, Ieff, may be derived

according to Annex D.

For ultimate loads, Ieff, may be assumed to be the maximum of the wake

turbulence intensity from neighbouring wind turbines as defined in Annex D.”

•This refers to the model of Frandsen (2005)

Park effects on aerodynamic loading –Interaction between turbines

30

Is dist. > 5*D sufficient?

Is turb class C sufficient?