Folie 1

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Influences on the fatigue of offshore structures
at the example of the FINO 1 research platform
Cord Böker
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Agenda

• Introduction
• Influence of wave directions
• Structural modeling

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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

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FINO Scatter diagram
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Influence of wave / sea state direction

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

What is the reason for this discrepancy? ? Wave
Spreading?
Database Simulation 5 realizations per sea
state Measurements mean values of 8 to 38
10-minute-intervalls, neqv 2108
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Wave Spreading

• Linear, regular waves

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Wave Spreading

• Irregular sea state without wave spreading

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Wave Spreading

• Irregular sea state with wave spreading

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Simulation of sea states considering spreading

• Mittendorf, Zielke (GIGAWIND Symposium 03)
• ? expensive calculation
• nJonswap x nspreading partial waves!

with, e.g.
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Application to a Monopile
4 possible cases (with increasing calculation cost) Long-term distribution Short-term distribution
Case 1 ? ?
Case 2 ? ?
Case 3 ? ?
Case 4 ? ?
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Application to a Monopile (2)
Relative Damage
Dmax 100
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Application to a Monopile (2)

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

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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
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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)

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Local Joint Flexibilities

• Parameterized formulae acc. Buitrago et al. (e.g.
  in DNV OS-J101)
• Modeling of LJF using flex-elements

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Local Joint Flexibilities first results

• LJF included in structural model

0.57
0.62
1.23
1.45
1.68
1.75
Statical excitation due to wave loading
FFT of the global bending moment at mudline Hs
3m, Tz 6s, dir 290 deg
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Local Joint Flexibilities - Outlook

• Sub-structuring approach
• Use detailed models needed for fatigue analysis

• Advantage use of existing detail models allows
  integrated workflow in the design
• More accurate than simplified approach
• Arbitrary joint geometries possible (e.g. Tripod)

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Thanks for your attention!