Repower_Offshore Foundations for Wind Turbines_Current Trends for Jacket Substructures

Conference

“Offshore Foundations for

Wind Turbines”

Current trends for jacket

substructures

Dr.-Ing. Marc Seidel

REpower Systems SE

2

Advantages of jacket substructures

Last year’s conference ….Last year’s conference ….

3

Project experience

Beatrice Windfarm Demonstrator (2)

ThorntonbankPhase 1, 2 & 3

(48 jackets)

alpha ventus (6)

Ormonde (30)

6 Projects with jackets: - 87 jackets installed

- additional 48 in fabrication

Nordsee Ost (48)

Bremerhaven (1)

4

Project experience

22m – 26m2012 / 2013Aker Solutions

Nordsee Ost

15m – 26m2011 / 2012SmuldersThorntonbank Phase 2 & 3

18m – 21m2010BiFabOrmonde

30m2009BiFabalpha ventus

Onshore2007WeserwindBremerhaven (onshore)

45m2006BiFabBeatrice

Water depth (LAT)

Year of jacket installation

Jacket supply

Project

Project overviewProject overview

5

Project experience – The very first offshore wind jacket

The Beatrice Offshore Wind Farm Demonstrator was part

of the DOWNVInD Project under the FP6 Programme of

the European Commission

The first offshore wind turbine installed on a jacket:

• 45m water depth

• REpower 5M turbine

• Jacket and turbine installed in 2006

6

Project experience – Jackets for “Thorntonbank Phase 2”

7

Project experience – Jackets for “Thorntonbank Phase 2”

8

Project experience – Jackets for “Nordsee Ost”

99

Jacket fabrication

Market overview – capacities and plansMarket overview – capacities and plans

8050Emden, GermanySIAG

15030Hadrian Yard, Wallsend,

Tyne&Wear, UK

Offshore Group

Newcastle Limited

5050Aalborg, DKBladt Industries

Company Location Capacity 2012 Capacity 2016

AMBAU GmbH Cuxhaven, Germany 0 40 - 60

Burntisland

Fabrications LimitedBurntisland, UK 80 150

Weserwind Bremerhaven, Germany 80 80

Base case:- Jacket with footprint of 25 x 25m

- Total height: 65m - Weight incl. secondary structures (excl. piles): 650t

Base case:- Jacket with footprint of 25 x 25m

- Total height: 65m - Weight incl. secondary structures (excl. piles): 650t

10

Jacket designs for large wind turbines

Influencing factors for large turbinesInfluencing factors for large turbines

Rotor diameter:

⇒ impact on aerodynamic loads⇒ increase of lever arm (longer tower)

Natural frequency requirements: Top mass, rotational speeds

Rotor diameter:

⇒ impact on aerodynamic loads⇒ increase of lever arm (longer tower)

Natural frequency requirements: Top mass, rotational speeds

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Impact of tower top mass

Natural frequencyNatural frequency

Some simple math …

Tower top stiffness:For a typical support structure (jacket + tower L=60m) this is:

Ktop = 2·106 N/m

Generalized tower top mass:Rotor-Nacelle-Assembly: MRNA = 460.000kgGeneralized tower mass: Mtwr = 100.000kg

Total generalized mass: M = 560.000kg

Natural frequency:

Some simple math …

Tower top stiffness:For a typical support structure (jacket + tower L=60m) this is:

Ktop = 2·106 N/m

Generalized tower top mass:Rotor-Nacelle-Assembly: MRNA = 460.000kgGeneralized tower mass: Mtwr = 100.000kg

Total generalized mass: M = 560.000kg

Natural frequency:

Hzf

sM

K

30.02

190.1

560000

102 6

==

=⋅

==

π

ω

ω

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Frequency tuningFrequency tuning

First natural frequency shall be between 1p (rotational speed) and 3p (blade passing frequency) excitations

For f=0.30 Hz, this is ideally fulfilled for the REpower 6M rotational speed range, as

f1P,max = 12 rpm / 60s = 0.200 Hz

f3P,min = 7.5rpm / 60s · 3

= 0.375 Hz

First natural frequency shall be between 1p (rotational speed) and 3p (blade passing frequency) excitations

For f=0.30 Hz, this is ideally fulfilled for the REpower 6M rotational speed range, as

f1P,max = 12 rpm / 60s = 0.200 Hz

f3P,min = 7.5rpm / 60s · 3

= 0.375 Hz

0

0.2

0.4

0.6

0.8

1

1.2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Frequency [Hz]

3p Range1p Range

13



What happens for a REpower turbine, if we would have a lower top mass?

Assume: 100t less top mass, i.e. 460.000kg total generalized mass

Change in natural frequency:

A lower top mass would bring us closer to the limits, whichwould not be favourablefor the design!

What happens for a REpower turbine, if we would have a lower top mass?

Assume: 100t less top mass, i.e. 460.000kg total generalized mass

Change in natural frequency:

A lower top mass would bring us closer to the limits, whichwould not be favourablefor the design!

Hzf

sM

K

33.02

109.2

460000

102 6

==

=⋅

==

π

ω

ω

0

0.2

0.4

0.6

0.8

1

1.2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Frequency [Hz]

3p Range1p Range

14



One of our competitors: M=460.000kg (generalized mass)

Rotor speed range: 5.9 … 14.8 rpm

Problem for low wind speeds created

Further decreasein top mass wouldmake the situation

even worse

One of our competitors: M=460.000kg (generalized mass)

Rotor speed range: 5.9 … 14.8 rpm

Problem for low wind speeds created

Further decreasein top mass wouldmake the situation

even worse

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8Frequency [Hz]

3p Range1p Range

15


Impact on loads / costs for the substructureImpact on loads / costs for the substructure

Higher mass = higher loads = higher costs?

Some simple considerations:

If the support structure is infinitely stiff, then the impact on loads depends on the ratio of wind loads to dead weight

For the 6M at tower bottom:Dead weight – 6%Wind load – 94%

Hence, tower top weight is not significant in this case.

For a flexible system, it gets slightly more complicated

Higher mass = higher loads = higher costs?

Some simple considerations:

If the support structure is infinitely stiff, then the impact on loads depends on the ratio of wind loads to dead weight

For the 6M at tower bottom:Dead weight – 6%Wind load – 94%

Hence, tower top weight is not significant in this case.

For a flexible system, it gets slightly more complicated

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Response under stochastic wind loading:

1) Energy spectrum of the wind

2) Mechanical transfer function- Natural frequency- Damping

3) Response

Response under stochastic wind loading:


2) Mechanical transfer function- Natural frequency- Damping

3) Response

Energy

spectrum

Mechanical transfer function

Response

17



Loads depend on:


about 5% lessenergy for f0=0.33 Hz compared tof0=0.30 Hz

Loads depend on:


about 5% lessenergy for f0=0.33 Hz compared tof0=0.30 Hz

Kaimal spectrum

0.1000

0.0948

0.08

0.09

0.10

0.11

0.12

0.2 0.25 0.3 0.35 0.4f

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Loads depend on:

2) Transfer function

For ω = Ω the transfer function just depends on the damping ratio, which does not vary significantly for the different eigenfrequencies

3) Response

Response (and hence loads) do not vary significantly depending on tower top mass!

Loads depend on:

2) Transfer function

For ω = Ω the transfer function just depends on the damping ratio, which does not vary significantly for the different eigenfrequencies

3) Response

Response (and hence loads) do not vary significantly depending on tower top mass!

( )2

22

2

1

1

Ω⋅

Λ+

Ω−

=Ω

ωπω

mH

19



Example calculation:North Sea project, 40m water depth, REpower 6M on a jacket

Governing extreme load cases for jacket extreme loads: DLC 6.1

DLC 6.1:460t tower top mass: 100%335t tower top mass: 97%


Governing extreme load cases for jacket extreme loads: DLC 6.1

DLC 6.1:460t tower top mass: 100%335t tower top mass: 97%

Conclusion:

There is an effect of tower top mass on extreme loads, but it is small to negligible.

Conclusion:

There is an effect of tower top mass on extreme loads, but it is small to negligible.

20




Fatigue loads at tower bottom

Fatigue loads (DELs) for m=4:460t tower top mass: 100%335t tower top mass: 110%


Fatigue loads at tower bottom

Fatigue loads (DELs) for m=4:460t tower top mass: 100%335t tower top mass: 110%

Conclusion:

Fatigue loads do INCREASE with lower tower top mass!

Conclusion:

Fatigue loads do INCREASE with lower tower top mass!

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BSH requirements

Key issues for BSH approvalKey issues for BSH approval

Static pile capacities

Requirement for dynamic pile tests (execution, evaluation) to “prove”static capacities

“BSH-Storm” for assessment of cyclic loading on foundations

Assessment procedures for performance

of piles under cyclic loading

Noise reduction during pile driving

Grouted connection between jacket and piles (“Zustimmung im Einzelfall”)

Static pile capacities

Requirement for dynamic pile tests (execution, evaluation) to “prove”static capacities

“BSH-Storm” for assessment of cyclic loading on foundations

Assessment procedures for performance

of piles under cyclic loading

Noise reduction during pile driving

Grouted connection between jacket and piles (“Zustimmung im Einzelfall”)

22

BSH requirements: 35h storm event / cyclic loading

IEC 61400-3 / BSHIEC 61400-3 / BSH

Basis for assessment of cyclic loading for piles is the “35h-storm” (adapted from Norsok standard N-003 “Action and action effects”, Ed. 2, Sept. 2007)

Basis for assessment of cyclic loading for piles is the “35h-storm” (adapted from Norsok standard N-003 “Action and action effects”, Ed. 2, Sept. 2007)

0.000

0.200

0.400

0.600

0.800

1.000

1.200

-17.50 -12.50 -7.50 -2.50 2.50 7.50 12.50 17.50

Wind

Wave

Wind (stepped)

Waves (stepped)

23


Consideration of wave nonlinearityConsideration of wave nonlinearity

Small waves can be approximated well by linear theory (grey lines)

For higher waves deviation from nonlinear theory (black lines) is very significant

Small waves can be approximated well by linear theory (grey lines)

For higher waves deviation from nonlinear theory (black lines) is very significant

24


Consideration of wave nonlinearityConsideration of wave nonlinearity

25


Surfaceelevation

Surfaceelevation

Factors for wave load at crest /

trough

Factors for wave load at crest /

trough

Resulting time series of

“nonlinear” loads

Resulting time series of

“nonlinear” loads

Consideration of wave nonlinearity - ExampleConsideration of wave nonlinearity - Example

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Determine peak skin friction (for tension or compression)

acc. to ICP method

Multiply peak skin friction with ageing factor (if

appropriate)

Import peak skin friction in

RATZ and run cyclic loading history

0

5

10

15

20

25

30

0 50 100 150 200 250 300

Peak shaft friction (kPa)

Dep

th (

m) Peak

-8000

-4000

0

4000

8000

12000

-0.002 0 0.002 0.004 0.006 0.008

Local pile displacement (m)

Pil

e h

ead

lo

ad

(kN

)

Process time series for assessment of cyclic loadingProcess time series for assessment of cyclic loading

27


Evaluate degradation of peak skin friction

Determine pile capacity

Shaft friction degradation (kPa)34

84

149163

234

286

21

89

191

212219

234

286

62

134

15

49

120

0

5

10

15

20

25

30

0 50 100 150 200 250 300

Dep

th (

m)


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BSH requirements: Assessment of cyclic loading


Validation of proposed method against HSE Dunkirk test Details see:

M. Seidel, M. Coronel: A new approach for assessing offshore piles subjected to cyclic axial loading, Geotechnik 34 (2011)

Validation of proposed method against HSE Dunkirk test Details see:

M. Seidel, M. Coronel: A new approach for assessing offshore piles subjected to cyclic axial loading, Geotechnik 34 (2011)

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Summary and conclusions

GeneralGeneral

Jackets have emerged as the preferred substructure solution for deeper waters and larger turbines

Significant experience with >100 jackets installed exists with REpower turbines

Further projects are in the planning phase with >200 jackets at the moment

Jackets have emerged as the preferred substructure solution for deeper waters and larger turbines

Significant experience with >100 jackets installed exists with REpower turbines

Further projects are in the planning phase with >200 jackets at the moment

30


Impact of increasing turbine sizeImpact of increasing turbine size

Tower top weight:

For a 6MW turbine, a weight of 450-500t is ideal to achieve a first natural period within the required limits.

Smaller top weights can create resonance problems which require non-optimal rotational speeds at some wind speeds.

Loads vs. top mass:

A smaller nacelle mass does not lead to smaller loads – on the contrary, governing fatigue loads do even INCREASE when top mass is reduced.

Tower top weight:

For a 6MW turbine, a weight of 450-500t is ideal to achieve a first natural period within the required limits.

Smaller top weights can create resonance problems which require non-optimal rotational speeds at some wind speeds.

Loads vs. top mass:

A smaller nacelle mass does not lead to smaller loads – on the contrary, governing fatigue loads do even INCREASE when top mass is reduced.

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BSH approval processBSH approval process

Assessment of piles:

Methods have evolved to evaluate cyclic loading, this is not the big issue any more

Bigger problems arise with “static” capacities, which form the basis for the assessment!

“Proving” capacities with dynamic pile tests is very problematic – more details about this later at this conference

Grouted connections are another difficult subject – will also be discussed later at this conference

Assessment of piles:

Methods have evolved to evaluate cyclic loading, this is not the big issue any more

Bigger problems arise with “static” capacities, which form the basis for the assessment!

“Proving” capacities with dynamic pile tests is very problematic – more details about this later at this conference

Grouted connections are another difficult subject – will also be discussed later at this conference

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REpower Offshore Engineering

Repower_Offshore Foundations for Wind Turbines_Current Trends for Jacket Substructures

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