influences on the fatigue of offshore structures at the example of the fino 1 research platform

Institute for Steel Construction – Leibniz University of Hannover2. PhD Seminar on Wind Energy in Europe

October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark

Influences on the fatigue of offshore structures at the example of the FINO 1

research platform

Cord Böker



Agenda

• Introduction

• Influence of wave directions • Structural modeling



Introduction

• Joint research project GIGAWINDplus:Validation and improvement of design methods and tools for support structures of Offshore Wind turbines

• Focusing on fatigue• Measurement data from the research platform FINO

1 strain gages at 11 locations

• Enhanced structural model



(Quelle: Google Earth)

FINO Scatter diagram

2.25 2.75 3.25 3.75 4.25 4.75 5.25 5.75 6.25 6.75 7.25 7.75 8.25 8.75 Sum0.25 1 18 28 19 10 5 1 1 830.75 8 58 90 59 29 18 4 2 2661.25 3 55 85 51 25 13 5 1 1 1 1 2401.75 1 35 73 46 20 7 1 1832.25 1 17 43 28 6 1 972.75 1 12 26 12 3 543.25 2 12 15 8 1 383.75 1 5 7 2 154.25 1 4 4 94.75 1 2 2 65.25 1 2 1 55.75 1 1 26.25 1 1 26.75 1 1

Sum 1 26 89 165 190 176 146 104 54 26 12 6 4 3 1000

Tz [s]

Hs [

m]

Based on 12896 30-minute-intervalls

Long-term directional spread

N

NE

E

SE

S

SW

W

NW

h=7m h=6m h=5m h=4m h=3m h=2m h=1m

Relative number of occurrences



0,0

10,0

20,0

30,0

40,0

50,0

60,0

Simulation Messung Simulation Messung

NB

DS

W,D

EL [k

N]

Seegangzustand 1:Hs =1m, Tz =4s

Seegangzustand 2:Hs =3m, Tz =6s

BDSWBDSW225°

BDSW

315°

BDSWBDSW225°

BDSW

315°

0,0

10,0

20,0

30,0

40,0

50,0

60,0

Simulation Measurement Simulation Measurement

NB

DS

W,D

EL [k

N]

Seastate 1:Hs =1m, Tz =4s

Seastate 2:Hs =3m, Tz =6s

BDSWBDSW225°

BDSW

315°

BDSWBDSW225°

BDSW

315°

Influence of wave / sea state direction

• Damage Equivalent Load (axial force) in the diagonal bracing

Database:Simulation: 5 realizations per sea stateMeasurements: mean values of 8 to 38 10-minute-intervalls,neqv = 2·108

What is the reason for this discrepancy?

Wave Spreading?



Wave Spreading

• Linear, regular waves:



Wave Spreading

• Irregular sea state without wave spreading:



Wave Spreading

• Irregular sea state with wave spreading:



• Mittendorf, Zielke (GIGAWIND Symposium ´03):

expensive calculationnJonswap x nspreading partial waves!

ij i jA 2 S D

22D cos ;

2 2with, e.g.:

Simulation of sea states considering spreading

0.0

10.0

20.0

30.0

40.0

50.0

60.0

Simulationw/o spread

Messung Simulationw/ spread

Simulationw/o spread

Messung Simulationw/ spread

N

BD

SW

,DE

L [k

N]

Seastate 1:Hs = 1m; Tz = 4s

Seastate 2:Hs = 3m; Tz = 8s

0,0

10,0

20,0

30,0

40,0

50,0

60,0


Seegangzustand 1:Hs=1m, Tz=4s


BDSWBDSW225°

BDSW

315°

BDSWBDSW225°

BDSW

315°

0,0

10,0

20,0

30,0

40,0

50,0

60,0




BDSWBDSW225°

BDSW

315°

BDSWBDSW225°

BDSW

315°

Simuw/o spread

Simuw/ spread

Meas Simuw/o spread

Simuw/ spread

Meas



Application to a Monopile

00,050,1

0,150,2

0,250,3

N

NE

E

SE

S

SW

W

NW

4 possible cases:(with increasing calculation cost)

Long-term distribution

Short-term distribution

Case #1 Case #2 Case #3 Case #4



Application to a Monopile (2)

00,050,1

0,150,2

0,250,3

N

NE

E

SE

S

SW

W

NW

N

NE

E

SE

S

SW

W

NW

Relative Damage:

Dmax = 100 %

00,050,1

0,150,2

0,250,3

N

NE

E

SE

S

SW

W

NW

N

NE

E

SE

S

SW

W

NW

Dmax = 37 %

00,050,1

0,150,2

0,250,3

N

NE

E

SE

S

SW

W

NW

N

NE

E

SE

S

SW

W

NW

Dmax = 35 %

00,050,1

0,150,2

0,250,3

N

NE

E

SE

S

SW

W

NW

N

NE

E

SE

S

SW

W

NW

Dmax = 46 %



Application to a Monopile (2)

00,050,1

0,150,2

0,250,3

N

NE

E

SE

S

SW

W

NW

N

NE

E

SE

S

SW

W

NW

Relative Damage:

Dmax = 100 %

00,050,1

0,150,2

0,250,3

N

NE

E

SE

S

SW

W

NW

N

NE

E

SE

S

SW

W

NW

Dmax = 37 %

00,050,1

0,150,2

0,250,3

N

NE

E

SE

S

SW

W

NW

N

NE

E

SE

S

SW

W

NW

Dmax = 35 %

00,050,1

0,150,2

0,250,3

N

NE

E

SE

S

SW

W

NW

N

NE

E

SE

S

SW

W

NW

Dmax = 46 %

Spreading should be considered, at least for monopiles Long-term distribution strongly site-dependant For jacket or tripod structures more investigations necessary



Structural modeling

EF 1:0.616 Hz

EF 2:0.635 Hz

EF 3:1.452 Hz

EF 4:1.746 Hz

EF 5:1.825 Hz



Local Joint Flexibilities

• In the FE model it is assumed that chords and braces are connected by rigid joints over-estimation of system stiffness!

• This has an influence on:– Structural dynamics– Fatigue (due to the distribution of

member forces)



Local Joint Flexibilities

• Parameterized formulae acc. Buitrago et al. (e.g. in DNV OS-J101)

• Modeling of LJF using flex-elements:

Beam Elements

“Rigid link”

“Flex Element”Stiffness propertiesdetermined by para-meterized formulae



1

10

100

1000

10000

100000

0 0.5 1 1.5 2

Frequency [Hz]

Glo

bal M

omen

t at m

udlin

e [k

N²m

²] )

Local Joint Flexibilities – first results

• LJF included in structural model

FFT of the global bending moment at mudlineHs = 3m, Tz = 6s, dir = 290 deg

0.57 0.62 1.23 1.45 1.68 1.75

w/o LJF

w/ LJF Sta

tical

exc

itatio

n du

e to

w

ave

load

ing



Local Joint Flexibilities - Outlook

Sub-structuring approach:• Use detailed models needed for

fatigue analysis

Beam Elements

Superelement: K, M, C18 DOF in the example

(6 per masternode)

• Advantage: use of existing detail models allows integrated workflow in the design

• More accurate than simplified approach

• Arbitrary joint geometries possible (e.g. Tripod)



Thanks for your attention!

influences on the fatigue of offshore structures at the example of the fino 1 research platform

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