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

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 GIGAWIND plus:

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

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

  2. Agenda • Introduction • Influence of wave directions • Structural modeling

  3. 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. FINO Scatter diagram Relative number of occurrences (Quelle: Google Earth) Long-term directional spread Based on 12896 30-minute-intervalls

  5. Influence of wave / sea state direction 60,0 60,0 50,0 50,0 315 315 315 315 ° ° ° ° BDSW BDSW BDSW BDSW [kN] [kN] 40,0 40,0 225 225 225 225 ° ° ° ° BDSW BDSW BDSW BDSW BDSW BDSW BDSW BDSW 30,0 30,0 BDSW,DEL BDSW,DEL N N 20,0 20,0 D D 10,0 10,0 0,0 0,0 Simulation Simulation Measurement Messung Simulation Simulation Measurement Messung Seegangzustand 1: Seastate 1: Seegangzustand 2: Seastate 2: H H =1m, T =1m, T =4s =4s H H =3m, T =3m, T =6s =6s s s z z s s z z • 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 = 2·108

  6. Wave Spreading • Linear, regular waves:

  7. Wave Spreading • Irregular sea state without wave spreading:

  8. Wave Spreading • Irregular sea state with wave spreading:

  9. Simulation of sea states considering spreading Seastate 2: Hs = 3m; Tz = 8s Seastate 1: Hs = 1m; Tz = 4s Simu w/o spread Meas Simu w/ spread Simu w/o spread Meas Simu w/ spread • Mittendorf, Zielke (GIGAWIND Symposium ´03):  expensive calculation nJonswap x nspreading partial waves! with, e.g.:

  10. Application to a Monopile

  11. Application to a Monopile (2) Dmax = 46 % Dmax = 37 % Dmax = 35 % Relative Damage: Dmax = 100 %

  12. Application to a Monopile (2) Relative Damage: Dmax = 100 % Dmax = 46 % Dmax = 37 % Dmax = 35 % • Spreading should be considered, at least for monopiles • Long-term distribution strongly site-dependant • For jacket or tripod structures more investigations necessary

  13. 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. 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. Local Joint Flexibilities Beam Elements “Rigid link” “Flex Element” Stiffness properties determined by para- meterized formulae • Parameterized formulae acc. Buitrago et al. (e.g. in DNV OS-J101) • Modeling of LJF using flex-elements:

  16. Local Joint Flexibilities – first results w/o LJF w/ LJF • 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

  17. Local Joint Flexibilities - Outlook Beam Elements Superelement: K, M, C 18 DOF in the example (6 per masternode) 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)

  18. Thanks for your attention!

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