Structural Integrity Evaluation of Offshore Wind Turbines

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Wind turbines are complex structures that should deal with adverse weather
conditions, are exposed to impacts or ship collisions and, due to the strategic roles in
the energetic supplying, can be the goal of military or malevolent attacks.
Even if a structure cannot be design to resist any unforeseeable critical event
or arbitrarily high accidental action, this kind of systems should be able to maintain
integrity and a certain level of functionality also under accidental circumstances,
which are not contemplated or cannot be considered in the usual design verification.
According to a performance-based design view, the entity of actions to be resisted
and the services levels to be maintained are the design objectives, which should be
defined by the stakeholders and by the designer in respect of the regulation in force.
For what said above, the structural integrity of wind turbines is a central issue
in the framework of a safe design: it depends on different factors, like exposure,
vulnerability and robustness. Particularly, the requirement of structural vulnerability
and robustness are discussed in this paper and a numerical application is presented,
in order to evaluate the effects of a ship collision on the structural system of an
offshore wind turbine.
The investigation resorts nonlinear dynamic analyses performed on the finite
element model of the turbine and considers three different scenarios for the ship
collision. The review of the investigation results allows for an evaluation of the
turbine structural integrity after the impact and permits to identify some
characteristics of the system, which are intrinsic to the chosen organization of the
elements within the structure.

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Structural Integrity Evaluation of Offshore Wind Turbines

  1. 1. Structural Integrity Evaluation of Offshore Wind TurbinesLuisa Giuliani Franco Bontempiluisa.giuliani@uniroma1.it franco.bontempi@uniroma1.itStructural and Geotechnical Engineering DepartmentUniversity of Rome “La Sapienza”
  2. 2. Presentation outlineEARTH&SPACE 2010STRUCTURAL INTEGRITY OF OFFSHORE WIND TURBINESWhat is it and why to care about itSTRATEGIES AND MEASURE OF ACHIEVEMENTRobustness and vulnerabilityA CASE STUDYInvestigation of an offshore turbine response to a ship collisionCONCLUSIONSConclusive evaluations on application and methodologyL. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 2/26A CASE STUDYInvestigation of an offshore turbine response to a ship collisionSTRATEGIES AND MEASURE OF ACHIEVEMENTRobustness and vulnerabilityCONCLUSIONSConclusive evaluations on application and methodologySTRUCTURAL INTEGRITY OF OFFSHORE WIND TURBINESWhat is it and why to care about it
  3. 3. Why care about structural integrity?EARTH&SPACE 2010MIDDELGRUNDENS VINDMØLLELAUGOffshore wind farm in Øresund, outside Copenhagen harbor, 2000)Operator: Dong EnergyOwner: 50% investor cooperative50% municipalityOfficial website: http://www.middelgrunden.dkL. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 3/26
  4. 4. L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind TurbinesWhy care about structural integrity?4/26EARTH&SPACE 2010RUNAWAY EVENT(Jutland, 2008)1. High wind and breaking system failure 2. Blades spin out of control and fail3. Blade debris collided with the tower4. Turbine tower collapses to the ground.
  5. 5. Why care about structural integrity?EARTH&SPACE 2010RUNAWAY EVENT(Jutland, 2008)1. High wind and breaking system failure 2. Blades spin out of control and fail3. Blade debris collided with the tower4. Turbine tower collapses to the ground.DISPROPORTION BETWEENCAUSE AND EFFECTL. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 4/26
  6. 6. Disproportionate collapse in standardsEARTH&SPACE 2010ASCE 7ASCE 7--02, 200202, 2002The structural system shall be able tosustain local damage or failure with theoverall structure remaining stable and notbe damaged to an extend disproportionateto the original local damageGSA guidelines, 2003GSA guidelines, 2003the building must withstand as a minimum,the loss of one primary vertical load-bearingmember without causing progressivecollapseUnified facilities criteriaUnified facilities criteriaUFC 4UFC 4--023023--03, DoD 200503, DoD 2005All new and existing buildings with threestories or more in height must be designedto avoid progressive collapseModel code 1990Model code 1990Structures should withstand accidentalcircumstance without damage disproportionateto the original events (insensitivity requirement)ISO/FDIS 2394, 1998ISO/FDIS 2394, 1998Structures and structural elements shouldsatisfy, with proper levels of reliability:-exercise ultimate state requirements- load ultimate state requirements- structural integrity state requirementsEN 1991EN 1991--11--7:20067:2006Structures should be able to withstandaccidental actions (fires, explosions, impacts) orconsequences of human errors, withoutsuffering damages disproportionate to thetriggering causesCODESAMERICAN EUROPEANL. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 5/26
  7. 7. Structural integrityEARTH&SPACE 2010STIFFNESSService limitstates (SLS)RESISTANCESTRUCTURALSAFETYUltimate limitstates (ULS)SECTIONSSECTIONSOR ELEMENTSOR ELEMENTS?R > Sf < ff < fadmadmRESISTANCETOEXCEPTIONALACTIONStructuralintegrity limitstate (SILS)STRUCTURALSYSTEMverification onFAILURE IS PREVENTEDFAILURE IS PRESUMEDL. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 6/26
  8. 8. OFFSHORE STANDARD, DNVOFFSHORE STANDARD, DNV--OSOS--J101, 2004J101, 2004The structural system shall be able to resists accidental loads andmaintain integrity and performance of the structure due to local damageor flooding.CONSIDERED LIMIT STATESCONSIDERED LIMIT STATESACCIDENTAL ACTIONSACCIDENTAL ACTIONSStructural integrity for OWTEARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 7/26ULSUltimate LimitStatesALSAccidental LimitStatesFLSFatigue LimitStatesSLSServiceabilityLimit Statesmaximum loadcarrying resistancefailure due to theeffect of cyclicloadingdamage tocomponents due toan accidental eventtolerance criteriaapplicable to normaluse
  9. 9. Presentation outlineEARTH&SPACE 2010STRUCTURAL INTEGRITY OF WIND TURBINEWhat is it and why to care about itSTRATEGIES AND MEASURE OF ACHIEVMENTRobustness and vulnerabilityA CASE STUDYInvestigation of an offshore turbine response to a ship collisionCONCLUSIONSConclusive evaluations on application and methodologyL. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 8/26STRUCTURAL INTEGRITY OF WIND TURBINEWhat is it and why to care about itSTRATEGIES AND MEASURE OF ACHIEVMENTRobustness and vulnerability
  10. 10. Different factors affecting structural integrityEARTH&SPACE 2010P(F) = P(D|H) P(F|DH)P(H) x xoccurrenceof collapseVULNERABILITY ROBUSTNESSEXPOSURE VULNERABILITY ROBUSTNESSEXPOSURE[Faber,2006][Ellingwood,1983]STRUCTURALNON STRUCTURALMEASURESavoid dispropor. collapselimit initial damageevent control2) reducethe effects of theaction3) reducethe effects of afailure1) reducethe actiondamage is caused inthe structurecritical event occursnear the structuredamage spreads inthe structureL. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 9/26
  11. 11. EVENT CONTROLNon structural measuresEARTH&SPACE 2010Malfunctioning and fireNatural actionsShip collisionMalevolent attackSystem controlProtective barriersSacrificial structuresSurveillanceL. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 10/26a. Difficult to prevent every possible accidental eventb. Difficult to protect WT (exposed to natural action)c. Difficult to surveille WT (wide and isolated area)reduce the occurrenceof the actionreduce the exposureof the structure1)1) REDUCE THE ACTIONREDUCE THE ACTION
  12. 12. Structural measuresEARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 11/26TOPDOWNIdentification of failures at meso-level(intermediate components)Identification of failures at micro level(basic components)BOTTOMUPDeductive (top-down):critical event is modeledInductive (bottom-up):critical event is irrelevantROBUSTNESS2) REDUCE THE EFFECTS2) REDUCE THE EFFECTSOF THE ACTIONOF THE ACTION3) REDUCE THE EFFECTS3) REDUCE THE EFFECTSOF A FAILUREOF A FAILUREVULNERABILITYSHIP COLLISION DAMAGED COMPONENTS
  13. 13. d)d(RI∆∆=Structural robustnessEARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 12/26structure BdPsSTRUCTURE B:PsROBUSTNESS CURVESP (performance)structure ASTRUCTURE Adamagedinteger∆Pdamagedmore performant, less resistantinteger(damage level)∆P∆Pmore performant, less robust less performant, more robustPERFORMANCEultimate resistanceDAMAGE LEVEL# removed elementsSTRUCTURAL ROBUSTNESS: Insensitivity to local failure(ASCE/SEI-PCSGC, 2007 – Betonkalender, 2008)Proposed robustness measure:decrement of resistance that correspondsto an increment of damage in the structure
  14. 14. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 13/26280 1 2 --- 8 dλExact maxima and minima curvesEXHAUSTIVE INVESTIGATIONEXHAUSTIVE INVESTIGATION1CdEX.81Exhaustive combinationsCd =E!d! × (E-d)!Ctot = Σd Cd = 2E0for D = EDa structure of 8 elements is considered as exampleRobustness curves
  15. 15. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 13/260 1 2 --- 8 dApproximated maxima and minima curvesHEURISTIC OPTIMIZATIONHEURISTIC OPTIMIZATIONERROR!(non cons.)14 2Reduced combinations= 1d>1Cd == 2 ×[E-(d-1)]= Ed=1d=00DCtot = Σd Cd = E2 + 1for D = E128Exhaustive combinationsCd =E!d! × (E-d)!Ctot = Σd Cd = 2E0for D = EDexponentialexponentialpolynomialpolynomial8CdEX.CdRED.a structure of 8 elements is considered as exampleRobustness curves
  16. 16. 8/3111/31Stiffbeams(flex.behaviour)Stiffcolumns(shear-type)STRUCTURALBEHAVIOUR16 17 1813 14 159 10 12115 6 871 2 4319 20 21λλλλλλλλgg16 17 1813 14 159 10 12115 6 871 2 4319 20 21λλλλλλλλggSHEAR-TYPE FRAME ROBUSTNESS00,5117 1 2 3 4 5 6 7 8 9 10Damage LevelPU[ad]MAX MINFLEX-TYPE FRAME ROBUSTNESS00,250,50,7510 1 2 3 4 5 6 7 8 9 10Damage LevelPU[ad]MAX MINλg5 6 879 10 1213 14 1516 17 1819 20 2141 2 311λg1417151318195 6 87121141 2 39 101620 211 2 435 6 879 10 121113 14 1516 17 1819 20 21λgλg1417201513181621195 6 8741 2 39 10 1211EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 14/26Comparison between different design solutions
  17. 17. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 15/26Robustness applied to OWTOWT ROBUSTNESS COMPARISONbetween two jacket structures between two support types
  18. 18. Presentation outlineEARTH&SPACE 2010STRUCTURAL INTEGRITY OF WIND TURBINEWhat is it and why to care about itSTRATEGIES AND MEASURE OF ACHIEVMENTRobustness and vulnerabilityA CASE STUDYInvestigation of an offshore turbine response to a ship collisionCONCLUSIONSConclusive evaluations on application and methodologyL. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 16/26STRUCTURAL INTEGRITY OF WIND TURBINEWhat is it and why to care about itA CASE STUDYInvestigation of an offshore turbine response to a ship collisionSTRATEGIES AND MEASURE OF ACHIEVMENTRobustness and vulnerability
  19. 19. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 17/26OWT ship collision investigation
  20. 20. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 18/26OWT ship collision investigationOWTSTRUCTURE MODELINGPointed mass formodeling rotor andnacelle.One-dimensional elementsfor leg and tower withelastic-plastic behavior(spread plasticity).Soil interactionaccounted with 3Dfinite elements,which behaveelastically. Zoneextension calibratedin order to minimizeboundary effects.Typical OWT:5-6 MW power36 m water depth.S355 steel monopile withhollow circular section;diameter and thicknessvary along the tower.4 diagonal legsconnected to 445 m longfoundation piles,40 m deepenedinto the ground.
  21. 21. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 19/26OWT ship collision investigationOWTLOAD SCENARIOS SHIP IMPACT MODELINGOnly self-weight is assumed to act onthe turbine at the moment of impact.Three different scenarios areconsidered for ship collision point:A. Impact on onediagonal leg underthe seabed (modelnode #17);700 ton700 tonAA
  22. 22. 700 ton700 tonAAEARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 19/26OWT ship collision investigationOWTLOAD SCENARIOS SHIP IMPACT MODELINGOnly self-weight is assumed to act onthe turbine at the moment of impact.Three different scenarios areconsidered for ship collision point:A. Impact on onediagonal leg underthe seabed (modelnode #17);B. Impact at theseabed (modelnode #38);700 ton700 tonBB
  23. 23. 700 ton700 tonBBEARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 19/26OWT ship collision investigationOWTLOAD SCENARIOS SHIP IMPACT MODELINGOnly self-weight is assumed to act onthe turbine at the moment of impact.Three different scenarios areconsidered for ship collision point:A. Impact on onediagonal leg underthe seabed (modelnode #17);B. Impact at theseabed (modelnode #38);C. Impact on thetower above theseabed (modelnode #548).t [s]F [MN]0.5 1.5 2.00.07Ship impact is modeled by meansof an impulsive force acting onthe collision point.The value of the force is 7 MN (ca.700 ton) and the total length ofthe impulsive function is 2 s.Nonlinear dynamics analyses arecarried on the structure.700 ton700 tonCC
  24. 24. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 20/26Nonlinear dynamic investigationsSCENARIO Atime: 0.025 s
  25. 25. time: 0.0 sEARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 20/26Nonlinear dynamic investigationsSCENARIO Atime: 0.3 s
  26. 26. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 20/26Nonlinear dynamic investigationsSCENARIO Atime: 0.3 stime: 0.5 s
  27. 27. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 20/26Nonlinear dynamic investigationsSCENARIO Atime: 0.5 stime: 0.8 s
  28. 28. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 20/26Nonlinear dynamic investigationsSCENARIO Atime: 0.8 stime: 1.5 s
  29. 29. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 20/26Nonlinear dynamic investigationsSCENARIO Atime: 1.5 stime: 3.0 s
  30. 30. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 21/26Nonlinear dynamic investigationsSCENARIO Btime: 0.0 s
  31. 31. time: 0.0 sEARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 21/26Nonlinear dynamic investigationsSCENARIO Btime: 1.0 s
  32. 32. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 21/26Nonlinear dynamic investigationsSCENARIO Btime: 1.0 stime: 3.0 s
  33. 33. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 22/26Nonlinear dynamic investigationsSCENARIO Ctime: 0.0 s
  34. 34. time: 0.0 sEARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 22/26Nonlinear dynamic investigationsSCENARIO Ctime: 1.0 s
  35. 35. time: 1.0 sEARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 22/26Nonlinear dynamic investigationsSCENARIO Ctime: 3.0 s
  36. 36. CONCLUSIONSConclusive evaluations on application and methodologyCONCLUSIONSConclusive evaluations on application and methodologyPresentation outlineEARTH&SPACE 2010STRUCTURAL INTEGRITY OF WIND TURBINEWhat is it and why to care about itSTRATEGIES AND MEASURE OF ACHIEVMENTRobustness and vulnerabilityA CASE STUDYInvestigation of an offshore turbine response to a ship collisionL. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 23/26STRUCTURAL INTEGRITY OF WIND TURBINEWhat is it and why to care about itA CASE STUDYInvestigation of an offshore turbine response to a ship collisionSTRATEGIES AND MEASURE OF ACHIEVMENTRobustness and vulnerability
  37. 37. SCENARIO EFFECT OF ACTION EFFECT OF DAMAGEAIrreversible direct damage ofimpacted leg:VULNERABLE TO ACTION(not disproportionate, localresistance may be increased)Overloading of adjacent legsand part of monopile,but no damage propagation:ROBUST BEHAVIOR(other damages to bestudied)BElastic deformation:NOT VULNERABLE TOCONSIDERED ACTION---CElastic deformation:NOT VULNERABLE TOCONSIDERED ACTION---EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 24/26Nonlinear dynamic investigation results700 ton700 ton700 ton700 ton700 ton700 ton700 ton700 ton
  38. 38. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 25/26FAILURECAUSES TYPESACCIDENTALACTIONSHUMAN ERRORS§ DESIGN§ EXECUTION§ MAINTENANCE§ IMPACTS§ COLLISIONS§ FIRESUNFAVORABLECOMBINATIONSof usual load values orcircumstances:SWISS CHEESE THEORYBLADES§ OVER SPEED§ FATIGUE FAILURE§ LOCAL BUCKLINGHandling exceptions of OWT
  39. 39. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 25/26FAILURECAUSES TYPESACCIDENTALACTIONSHUMAN ERRORS§ DESIGN§ EXECUTION§ MAINTENANCE§ IMPACTS§ COLLISIONS§ FIRESUNFAVORABLECOMBINATIONSof usual load values orcircumstances:SWISS CHEESE THEORYBLADES§ OVER SPEED§ FATIGUE FAILURE§ LOCAL BUCKLINGTOWER§ SHAFT CRACKS§ WELDING FAILURE(FATIGUE ORFAULTY DESIGN)Handling exceptions of OWT
  40. 40. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 25/26FAILURECAUSES TYPESACCIDENTALACTIONSHUMAN ERRORS§ DESIGN§ EXECUTION§ MAINTENANCE§ IMPACTS§ COLLISIONS§ FIRESUNFAVORABLECOMBINATIONSof usual load values orcircumstances:SWISS CHEESE THEORYBLADES§ OVER SPEED§ FATIGUE FAILURE§ LOCAL BUCKLINGTOWER§ SHAFT CRACKS§ WELDING FAILURE(FATIGUE ORFAULTY DESIGN)FOUNDATION§ MOSTLY FOR OWTIN CONSTRUCTIONHandling exceptions of OWT
  41. 41. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 26/26PreventionPreventionIndirect design Direct designTop-downmethodsBottom-upmethodsCollapseresistanceStructuralrobustnessPresumptionPresumptionEvent controlSpecificSpecificanalysesanalysesStructuralStructuralmeasuresmeasures performedavoidedCriticalCriticaleventevent modeledno yesInvulnerabilityUnaccountedmay happen!No hazardscan occurHazards don’tcause failureProgressivecollapsesusceptibility?Effects areEffects areuncertainuncertain?Behavior followingBehavior followingother hazardsother hazardsremains unknown!remains unknown!irrelevantFAULTFAULTFAILUREFAILURESECURITYSECURITY INVULNERABILITYINVULNERABILITYHandling exceptions of OWT
  42. 42. Structural Integrity Evaluation of Offshore Wind TurbinesLuisa Giuliani Franco Bontempiluisa.giuliani@uniroma1.it franco.bontempi@uniroma1.itStructural and Geotechnical Engineering DepartmentUniversity of Rome “La Sapienza”
  43. 43. 10/311 2S1432 4S1S1243 4S23S1244S32134123414131242324342321 1 143423d=0d=2d=1d=32134d=434S123S12344S134S134S134S234S134S124S4S0Computational tree of anon-deterministic Turingmachine:all possible configurationsare computed inpolynomial time(# computational steps s =# system elements E).State of acceptance:damagedconfigurationInitial state:nominalconfigurationDamaged configurationsEARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 13/26
  44. 44. DI1Di1,D/dr1maxI aiia <<→<<∀ −=EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 27/21Additional slides0 1 2 3 4 5 6dλ0 1 2 3 4 5 6dλa. Maximum inclination of all the secant linesthat connect the first point of the curveswith the points pertaining to greaterdamage levels.
  45. 45. DI1Di1,D/dr1maxI aiia <<→<<∀ −=EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 27/21Additional slides0 1 2 3 4 5 6dλ0 1 2 3 4 5 6dλa. Maximum inclination of all the secant linesthat connect the first point of the curveswith the points pertaining to greaterdamage levelsb. Highest variation of secant lines betweentwo subsequent points (critical elementsare most relevant)( ) ( ) Di2rrrrmaxI 1iii1ib <∀ −−−= +−)isostatic(5.0I0)linear( b ≤≤←
  46. 46. DI1Di1,D/dr1maxI aiia <<→<<∀ −=EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 27/21Additional slidesc. Area subtended by the curve of minima weighted by the difference between that area andthe area subtended from the curve of maxima (representing the scattering)a. Maximum inclination of all the secant linesthat connect the first point of the curveswith the points pertaining to greaterdamage levelsb. Highest variation of secant lines betweentwo subsequent points (critical elementsare most relevant)[ ] [ ]−⋅+= ∑∑==D0dLOWUPD0dLOWc )d(r)d(rk)d(rDI ( ) 1k0for,1D2I5.0 c <<−⋅<≤→0 1 2 3 4 5 6dλ0 1 2 3 4 5 6dλ( ) ( ) Di2rrrrmaxI 1iii1ib <∀ −−−= +−)isostatic(5.0I0)linear( b ≤≤←
  47. 47. ( ) ( ) Di2rrrrmaxI 1iii1ib <∀ −−−= +−)isostatic(5.0I0)linear( b ≤≤←DI1Di1,D/dr1maxI aiia <<→<<∀ −=EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 27/21Additional slides0 1 2 3 4 5 6dλ0 1 2 3 4 5 6dλc. Area subtended by the curve of minima weighted by the difference between that area andthe area subtended from the curve of maxima (representing the scattering)a. Maximum inclination of all the secant linesthat connect the first point of the curveswith the points pertaining to greaterdamage levelsb. Highest variation of secant lines betweentwo subsequent points (critical elementsare most relevant)d. Scattering from linear trend, calculated as the area subtended between the curves and thestraight line that connect the point of the integer structure with that one of the null one.[ ] [ ] 2D)1k()d(rk)d(rID0dUPD0dLOWd ∑∑==+−+= EDand1k0for,1I1 d =<<<≤−→( ) 1k0for,1D2I5.0 c <<−⋅<≤→[ ] [ ]−⋅+= ∑∑==D0dLOWUPD0dLOWc )d(r)d(rk)d(rDI
  48. 48. ( ) ( ) Di2rrrrmaxI 1iii1ib <∀ −−−= +−)isostatic(5.0I0)linear( b ≤≤←[ ] [ ]−⋅+= ∑∑==D0dLOWUPD0dLOWc )d(r)d(rk)d(rDI ( ) 1k0for,1D2I5.0 c <<−⋅<≤→[ ] [ ] 2D)1k()d(rk)d(rID0dUPD0dLOWd ∑∑==+−+= EDand1k0for,1I1 d =<<<≤−→DI1Di1,D/dr1maxI aiia <<→<<∀ −=EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 27/21Additional slidesc. Area subtended by the curve of minima weighted by the difference between that area andthe area subtended from the curve of maxima (representing the scattering)a. Maximum inclination of all the secant linesthat connect the first point of the curveswith the points pertaining to greaterdamage levelsb. Highest variation of secant lines betweentwo subsequent points (critical elementsare most relevant)e. Upper and lower bound for maximal damage that makes the structure unstableUPLOWe DkDI ⋅+= E2I0and1k0andDDD1 cUPLOW≤≤<<≤≤≤←d. Scattering from linear trend, calculated as the area subtended between the curves and thestraight line that connect the point of the integer structure with that one of the null one.0 1 2 3 4 5 6dλ0 1 2 3 4 5 6dλDLOW DUP
  49. 49. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 27/21Additional slidesSTARTd := 0for e =1 to Ed := 1NL static analysis λu0λu,e1Remove element ePushover analysisRestore element efor d = 2 to Dλu1MAX, e1MAXλu1MIN, e1MINRemove element e(d-1)MINfor e =1 to Eλu,edRemove element ePushover analysisRestore element eCALLCALL ““CURVE MAXCURVE MAX””CALLCALL ““CURVE MINCURVE MIN””Robustness quantificationif e <> edMINλudMIN,edMIN““CURVE MINCURVE MIN““ MACROMACROCurve of minimaENDRestore integer structureCalculate area or derivativesExtrapolate equationsCalculate corresponding ∆RFix an admitted ∆dIdentify critical elementsSpot abrupt decrementROBUSTNESS QUANTIFICATIONROBUSTNESS QUANTIFICATIONALGORITHM FOR HEURISTIC OPTIMIZATIONALGORITHM FOR HEURISTIC OPTIMIZATION
  50. 50. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 27/21Additional slidesPushover on integer structureResulting robustness histogramPlastic strain development Axial stress developmentMeshed elements Plastic moment developmentResponse curveMax and min robustness histogramsPlasticization developmentALGORITHM FOR REDUCED ANALYSESALGORITHM FOR REDUCED ANALYSESMax resistance
  51. 51. STAR STRUCTURE ROBUSTNESS00,510 1 2 3 4 5 6 7 8Damage LevelPU[ad]MAX MINtdRRDdcos25.0max,...,1←=∂∂=′=EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 27/21Additional slides# of elements: E = 8static indetermin. level: i = 21lower max. damage: Dmin = 8upper max. damage: Dmax = 8fixed max. damage: D = 8reduced combination: Cred = 65exhaust. combination: Cex= 256λλgg1 2435867STATIC INDETERMINANCY:high restrain gradeDamaged El. ID Pumin El. ID Pumax0 0 1 0 11 1 0,831745 3 0,8961852 5 0,648331 7 0,8320963 4 0,523591 2 0,6848494 8 0,376882 8 0,5809285 2 0,269581 6 0,4297336 6 0,16434 4 0,3206327 7 0,042504 1 0,2619568 3 0 5 0MIN CFG MAX CFGIb ≈ 0Ia ≈ 1ROBUSTNESS INDICATOR:( ) ( ) Di2rrrrmaxI 1iii1ib <∀ −−−= +− )isostatic(5.0I0)linear( b ≤≤→Ed0Ed)d(rmaxIa ≤≤∀= )isostatic(EI1 b ≤≤→Ib maximum slope pt. tangent (measure the abrupt decrementdue to the removal of critical element)Ia maximum secant starting from the 1st pt. (account for thelateness of an abrupt decrement)
  52. 52. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 27/21Additional slides# of elements: E = 21static indetermin. level: i = 4lower max. damage: Dmin = 2upper max. damage: Dmax = 9fixed max. damage: D = 9reduced combination: Cred = 286exhaust. combination: Cex= 695860λ15 16 141711 9 121018 20 1921137 5 3 1 2 4 6 8
  53. 53. TRUSS STRUCTURE ROBUSTNESS00.250.50.7510 1 2 3 4 5 6 7 8 9Damage LevelPU [ad] MAX MINEARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 27/21Additional slides# of elements: E = 21static indetermin. level: i = 4lower max. damage: Dmin = 2upper max. damage: Dmax = 9fixed max. damage: D = 9reduced combination: Cred = 286exhaust. combination: Cex= 695860Damaged El. ID Pumin El. ID Pumax0 0 1 0 11 9 0,471351 1 0,8889412 3 0 2 0,8889413 10 0 11 0,6286254 17 0 12 0,6286255 16 0 5 0,6286256 1 0 6 0,6286257 5 0 13 0,6286258 11 0 14 0,6286259 6 0 17 0MIN CFG MAX CFG121 211 5 6 13 141793λ15 16 141711 9 121018 20 1921137 5 3 1 2 4 6 8MAX115 16 1411 9 121018 20 1921137 5 3 2 6 84170,26 > Ib > 0,13MIN15 16 141711 9 121018 20 1921137 5 3 1 2 4 6 84,76 > Ia > 1,11
  54. 54. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 27/21Additional slides# of elements: E = 13static indetermin. level: i = 12lower max. damage: Dmin = 2upper max. damage: Dmax = 10fixed max. damage: D = 10reduced combination: Cred = 158exhaust. combination: Cex= 810058 6 9712 10 13114 2 31λ
  55. 55. VIERENDEEL STRUCTUREROBUSTNESS00,250,50,7510 1 2 3 4 5 6 7 8 9 10Damage LevelPU [ad]MAX MINEARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 27/21Additional slides# of elements: E = 13static indetermin. level: i = 12lower max. damage: Dmin = 2upper max. damage: Dmax = 10fixed max. damage: D = 10reduced combination: Cred = 158exhaust. combination: Cex= 8100Damaged El. ID Pumin El. ID Pumax0 0 1 0 11 6 0,187621 1 0,9380972 10 0 2 0,6253173 1 0 3 0,3752424 2 0 6 0,1876215 3 0 7 0,1876216 4 0 8 0,1876217 5 0 9 0,1876218 7 0 4 0,1876219 8 0 5 0,18762110 9 0 10 0MIN CFG MAX CFG61237 8 9 4 5 1061058 6 9712 10 13114 2 31λMIN58 6 9712 13114 2 31100,31 > Icr > 0,09MAXλ58 6 9712 13114 2 31108,12 > Ia > 2,03
  56. 56. TRUSS STRUCTURE ROBUSTNESS00,250,50,7510 1 2 3 4 5 6 7 8 9Damage LevelPU [ad] MAX MINEARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 27/21Additional slides58 6 9712 10 13114 2 31λVIERENDEEL STRUCTURE ROBUSTNESS00,510 1 2 3 4 5 6 7 8 9 10Damage LevelPU [ad] MAX MIN661237 8 9 4 5 1010High element connectionHigh element number145 3 1 2 4 6 8λ7 5 3 1 2 4 6 81411 9 121018 20 192113 15 1617λSTATIC INDETERMINANCYi = 4 i = 120,26 > Ib > 0,13 0,31 > Ib > 0,0993121 211 5 6 13 14178,12 > Ia > 2,034,76 > Ia > 1,11
  57. 57. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 27/21Additional slides314 25 λλggSTATICINDETERMINANCYrestraintI3V STRUCTURE ROBUSTNESS00,510 1 2 3 4 5Damage LevelPU[kN]MAX MINI3C STRUCTURE ROBUSTNESS00,250,50,7510 1 2 3 4Damage LevelPU[kN]MAX MIN312 4λλggconnection1413 15161012 1196 51 27843λλggI3E STRUCTURE ROBUSTNESS00,250,50,7510 1 2 3 4 5Damage LevelPU[kN]MAX MINelement
  58. 58. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 27/21Additional slidesHIGH CONNECTIONContinuityCompartmentalizationIsolationElementnumberElementconnectionLOCAL REDUCTIONOF CONTINUITYLOCAL MECHANISMDEVELOPMENTRedundancyFragilityLOW CONNECTIONLOCAL REDUCTION OFDUCTILITY(inhomogeneous stiffeningof predetermined sections)EARLY RUPTURE ANDDETACHMENTElementductilityDuctilityExternal(restraints)STRESS IS NOT TRANSMITTEDCOLLAPSESTANDSTILLSTRESS REDISTRIBUTIONHIGH STRESS TRANSMISSIONon adjoining elements after a localized failureTRIGGERING OF CHAINRUPTUREPROGRESSIVE COLLAPSEDISPROPORTIONATE COLLAPSE SUSCEPTIBILITYSUDDEN/EARLYCOLLAPSEInternal(constraints)*Starossek &Wolff, 2005*FEASIBLE ALTERNATE LOADPATHROBUSTNESS
  59. 59. EARTH&SPACE 2010L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind Turbines 14/26
  60. 60. L. Giuliani, F. Bontempi - Structural Integrity Evaluation of Offshore Wind TurbinesWhy caring for structural integrity?2EARTH&SPACE 2010LILLEGRUND offshore wind farm(Øresund between Malmö and København, Siemens, June 2008)

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