1. Institute for Steel Construction – Leibniz University of Hannover 2. 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

2. Institute for Steel Construction – Leibniz University of Hannover 2. PhD Seminar on Wind Energy in Europe October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark Agenda Introduction Influence of wave directions Structural modeling

3. Institute for Steel Construction – Leibniz University of Hannover 2. PhD Seminar on Wind Energy in Europe October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark 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

4. Institute for Steel Construction – Leibniz University of Hannover 2. PhD Seminar on Wind Energy in Europe October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark (Quelle: Google Earth) FINO Scatter diagram Based on 12896 30-minute-intervalls Long-term directional spread Relative number of occurrences

5. Institute for Steel Construction – Leibniz University of Hannover 2. PhD Seminar on Wind Energy in Europe October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark 0,0 10,0 20,0 30,0 40,0 50,0 60,0 SimulationMessungSimulationMessung  N BDSW,DEL [kN] Seegangzustand 1: H s =1m, T z =4s Seegangzustand 2: H s =3m, T z =6s BDSW 225° BDSW 315° BDSW 225° BDSW 315° 0,0 10,0 20,0 30,0 40,0 50,0 60,0 SimulationMeasurementSimulationMeasurement  N BDSW,DEL [kN] Seastate 1: H s =1m, T z =4s Seastate 2: H s =3m, T z =6s BDSW 225° BDSW 315° BDSW 225° BDSW 315° Influence of wave / sea state direction Damage Equivalent Load (axial force) in the diagonal bracing Database: Simulation: 5 realizations per sea state Measurements: mean values of 8 to 38 10-minute-intervalls, n eqv = 2·10 8 What is the reason for this discrepancy?  Wave Spreading?

6. Institute for Steel Construction – Leibniz University of Hannover 2. PhD Seminar on Wind Energy in Europe October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark Wave Spreading Linear, regular waves:

7. Institute for Steel Construction – Leibniz University of Hannover 2. PhD Seminar on Wind Energy in Europe October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark Wave Spreading Irregular sea state without wave spreading:

8. Institute for Steel Construction – Leibniz University of Hannover 2. PhD Seminar on Wind Energy in Europe October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark Wave Spreading Irregular sea state with wave spreading:

9. Institute for Steel Construction – Leibniz University of Hannover 2. PhD Seminar on Wind Energy in Europe October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark Mittendorf, Zielke (GIGAWIND Symposium ´03):  expensive calculation n Jonswap x n spreading partial waves! with, e.g.: Simulation of sea states considering spreading Seastate 1: H s = 1m; T z = 4s Seastate 2: H s = 3m; T z = 8s Simu w/o spread Simu w/ spread Meas Simu w/o spread Simu w/ spread Meas

10. Institute for Steel Construction – Leibniz University of Hannover 2. PhD Seminar on Wind Energy in Europe October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark Application to a Monopile 4 possible cases: (with increasing calculation cost) Long-term distributionShort-term distribution Case #1  Case #2  Case #3  Case #4

11. Institute for Steel Construction – Leibniz University of Hannover 2. PhD Seminar on Wind Energy in Europe October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark Application to a Monopile (2) Relative Damage: D max = 100 % D max = 37 %D max = 35 % D max = 46 %

12. Institute for Steel Construction – Leibniz University of Hannover 2. PhD Seminar on Wind Energy in Europe October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark Application to a Monopile (2) Relative Damage: D max = 100 % D max = 37 %D max = 35 % D max = 46 %  Spreading should be considered, at least for monopiles  Long-term distribution strongly site- dependant  For jacket or tripod structures more investigations necessary

13. Institute for Steel Construction – Leibniz University of Hannover 2. PhD Seminar on Wind Energy in Europe October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark 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

14. Institute for Steel Construction – Leibniz University of Hannover 2. PhD Seminar on Wind Energy in Europe October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark 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)

15. Institute for Steel Construction – Leibniz University of Hannover 2. PhD Seminar on Wind Energy in Europe October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark 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 properties determined by para- meterized formulae

16. Institute for Steel Construction – Leibniz University of Hannover 2. PhD Seminar on Wind Energy in Europe October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark Local Joint Flexibilities – first results LJF included in structural model FFT of the global bending moment at mudline H s = 3m, T z = 6s, dir = 290 deg 0.570.621.231.451.68 1.75 w/o LJF w/ LJF Statical excitation due to wave loading

17. Institute for Steel Construction – Leibniz University of Hannover 2. PhD Seminar on Wind Energy in Europe October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark Local Joint Flexibilities - Outlook Sub-structuring approach: Use detailed models needed for fatigue analysis Beam Elements Superelement: K, M, C 18 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)

18. Institute for Steel Construction – Leibniz University of Hannover 2. PhD Seminar on Wind Energy in Europe October 4th and 5th 2006 at Risø National Laboratory, Roskilde, Denmark Thanks for your attention!