Combined Wave and Wind Fatigue Damage for Offshore Structures Ronald Horn, S.M.I.L.E.-FEM GmbH, Heikendorf Hongxia Gu, IMPaC Offshore Engineering GmbH, Hamburg October, 19-21 th, 2011, Stuttgart 1
S.M.I.L.E.-FEM IMPaC's SERVICES GmbH IMPaC Offshore Engineering GmbH Integrated Project/Field Development Engineering Overall Consultancy Conceptual & Feasibility Studies Front End Engineering Design (FEED) Detailed Design Procurement & Logistics Construction Management Contract Preparation & Management Planning & Monitoring Construction / Installation Supervision & Management EPCM / EPC Project Management Research & Development 2
S.M.I.L.E.-FEM GmbH S M I L E Structural and thermal analysis Multiphysics solutions Interaction of fluid and structure Life cycle and fatigue analysis Explicit dynamics analysis Within the project, S.M.I.L.E.-FEM GmbH is subsidiary of IMPaC Offshore Engineering. 3
Scope Ultimate strength analysis of met mast tower Ultimate strength analysis of working platform Ultimate strength analysis of foundation Ultimate strength analysis of overall structure Fatigue strength analysis of overall structure Boat impact analysis Lifting analysis Installation analysis Driveability analysis of monopile Connection detail analysis 4
Project meteorological station (met mast) of wind farm typical monopile / transition piece structure foundation 95 m height above LAT LAT 25 m above sea bed 33 m monopile penetration in soil length of monopile/transion piece/tower: 62.0 m / 24.0 m / 74.5 m 35 m/s wind velocity, return period 50 years,10 min., LAT +10m 15,13 m maximum wave height, return period 50 years 13 s max. design wave period, return period 50 years 1,34 m current velocity, return period 50 years 5
Model No. Analysis Model Type Element Type global local detail 1D 2D 3D Lattice tower model Strength analysis Of tower X X Simplified tower model Overall Structure Analysis X X Working platform model Working Platform Stress Analysis X X Foundation model Foundation Analysis X X Overall structure model Strength Analysis, Fatigue Analysis X X Boat landing model Boat Impact Analysis X X Lifting model WP & TP Lifting Analysis X X Lifting model MP Lifting Analysis X X Flange model Flange Analysis X X Grout connection Grouted Connection Analysis X X Landing point model Installation Analysis X X 6
Fatigue analysis Fatigue strength analysis of overall structure Fatigue estimation due to wind Fatigue estimation due to wave Fatigue estimation due to pile driving Total fatigue life estimation 7
Wind conditions 8
Wave conditions Marine Condition Case 1 Case 2 Case 3 Case 4 DLC No. DLC 6.1b DLC 6.1c DLC 6.5 DLC 8.5 Code required Wave Height H red50 H max50 H red1 H st Recurrence Period 50 years 50 years 1 year Limiting Used Wave Height (m) 10.69 15.13 8.32 1.30 Significant Wave Height H s (m) 8.1 8.1 6.3 1.3 Design Wave Period T D1 (s) 10.09 10.09 8.9 4.04 Design Wave Period T D2 (s) 12.99 12.99 11.46 5.21 Ice Formations No no yes no Loads Factor 1.35 1.35 1.35 1.1 9
Fatigue analysis locations 10
Fatigue analysis method The fatigue analysis has been carried out according to GL Wind, by using cumulative damage ratio method (Palmgreen und Miner) The cumulative damage ratio D should not exceed the limit damage ratio of 1. where: l = total number of blocks of the stress range for summation, I = 28 n i = number of stress cycles in block i N i = number of induced stress cycles determined from the S-N curve 11
S-N curve The S-N curve for welded joints has been used in the analysis Log N = 6.69897+ m*q where: Q = log ( σ Rc / (γ M * σ i ) 0.3994/ m 0 m 0 = 3 (for welded joints) m = 5 σ Rc = corrected fatigue strength reference value γ M = partial material safety factor, γ M = 1.25 σ i = stress range of block i 12
Stress range Considering the axial stress and bending stress, the following stress range has been computed in all fatigue load cases Δσ = max(σ 1 ) min(σ 3 ) with σ 1 : maximum principal stress σ 3 : minimum principal stress 13
Wave load cases 45000000 40000000 35000000 30000000 25000000 20000000 15000000 10000000 5000000 0 Significant Wave Height Hs and 20 years Occurance 1 2 3 4 5 6 7 8 9 10111213141516171819202122232425262728 18 16 14 12 10 8 6 4 2 0 14
Wave load fatigue results 1,00E+12 1,00E+10 1,00E+08 1,00E+06 1,00E+04 1,00E+02 Endured stress cycles 1,00E+00 1 3 5 7 9 11 13 15 17 19 21 23 25 27 Stress range Δσ [N/m²] 140000 120000 100000 80000 60000 40000 20000 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 15
Fatigue damage calculation procedure S.M.I.L.E.-FEM GmbH 1. Use wave scatter diagram to obtain load classes for wave heights and occurances 2. Calculate wave fatigue damage 3. Use identical wind occurrences, gust is induced by wave (most conservative assumption) 4. Calculate Weibull parameter for N(Hs) wave height occurrences and N(v) wind speed occurrences from met data 5. Correlate Hs and v 6. Obtain significant wind speed per load class 7. Calculate wind fatigue damage 8. Combine fatigue results from wind / wave 9. Combine with pile driving fatigue damage 16
Wind load cases relationship between wave height and wind speed (Weibull distributions) 28 identical fatigue classes and number of stress cycles per class wave and wind conditions described by the Weibull distribution Weibull function parameters A (scale parameter) and k (shape parameter) are specified to correlated wave height and wind speed time-dependent gust is calculated for each class according to GL wind gust prescribed as transient force depending on height (and class). Wind Wave 17
Weibull distribution Wave: blue Wind: red 18
Wind speed / wave height correlation Vertical wind profile V 10m,3h = 0.737*v 80m,10min The profile given by GL Wind is used to calculate the wind speed at hub height (57 m) from the transferred scatter data (10 m) height: V(z) = v hub (z / z hub ) 0.14 19
Wind speed / gust v(z,t) = v(z) 0.37*v gustn * sin(3πt/t)*(1-cos(2πt/t) with v gustn as the maximum value of the wind speed for the extreme operating gust according to GL Wind, IV Part 2, 4.2.2.4.2 v gustn = β*(σ 1 /(1+0.1*(D/Λ 1 ))) with standard deviation σ 1, GL Wind, equation 4.2.5, β =6.4 (N=50), Λ 1 = 21 m and D= rotor diameter (corresponding to tower height) 20
Wind load cases 45.000.000 40.000.000 35.000.000 30.000.000 25.000.000 20.000.000 15.000.000 10.000.000 5.000.000 Wind speed / 10y-occurance vs load case 40 35 30 25 20 15 10 5 0 1 2 3 4 5 6 7 8 9 10111213141516171819202122232425262728 0 21
Wind load fatigue results Stress range / damage ratio vs load case 30000 25000 20000 15000 10000 5000 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 7,00E-03 6,00E-03 5,00E-03 4,00E-03 3,00E-03 2,00E-03 1,00E-03 0,00E+00 22
Combined wind and wave fatigue damage with D = combined in-place damage ratio due to wave and wind D 1 = calculated fatigue damage for the high frequency response D 2 = calculated fatigue damage for the low frequency response ν 1 = mean zero up crossing frequency for the high frequency response ν 2 = mean zero up crossing frequency for the low frequency response m = inverse slope of the S-N curve (=5) (index 1 refers to wind and index 2 to wave) 23
Results of combined wind and wave fatigue damage 1,00E-09 1,00E-08 1,00E-07 1,00E-06 1,00E-05 1,00E-04 1,00E-03 Wave (green), wind (blue) and combined (white) fatigue damage 1,00E-02 Life Fat = (1-D PD ) * Life in-place 1,00E-01 1,00E+00 1 2 3 4 5 6 7 8 9 10 24
Summary 1. Less complex structures and suitable environmental conditions require less complex methods 2. Influence of environmental conditions on the load generation must be treated very carefully 3. If applicable, identical load cases and occurrences can be chosen 4. Then, fatigue damage contributions can be calculated separately for identical load cases 5. If applicable, individual results of wave, wind and pile driving fatigue damage can be combined 25
Thank you for your attention! Please ask questions! 26