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Dependability of Offshore Wind Turbines
 

Dependability of Offshore Wind Turbines

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In recent years more and more demanding structures are designed, built and operated ...

In recent years more and more demanding structures are designed, built and operated
to satisfy the increasing needs of the Society. This kind of structures can be denoted
as complex ones. Among large constructions arrangements, Offshore Wind Turbines
(OWT) are definitely complex structural systems, being this complexity related to
different aspects such as hard nonlinearities, wide uncertainties and strong
interactions, either among the single parts or between the whole structure and the
design environment.
On the whole, the quality of a complex system is denoted by the idea of
dependability, while for a structure the performances are connected to the property of
structural integrity, considered as the completeness and consistency of the structural
configuration. Even if these concepts have been originally developed, respectively, in
computer science and for aerospace applications they can be applied to other high
performance systems as OWT.
The present paper will show some specific aspects of the modern approach
for the design and the analysis of complex structural systems. In the first part of the
paper, the general aspects are recalled like the System Engineering approach and the
Performance-based Design. Attention is devoted to some important aspects, such as
the structure breakdown and the safety and performance allocations. In the second
part of the paper, a basic application of the concepts introduced is presented.

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    Dependability of Offshore Wind Turbines Dependability of Offshore Wind Turbines Document Transcript

    • Dependability of Offshore Wind TurbinesF. Bontempi1, M. Ciampoli2, S. Arangio31Professor, University of Rome La Sapienza, School of Engineering, Via Eudossiana18, 00184 Rome, ITALY; email: franco.bontempi@uniroma1.it2Associate Professor, University of Rome La Sapienza, School of Engineering, ViaEudossiana 18, 00184 Rome, ITALY; email: marcello.ciampoli@uniroma1.it3Associate Researcher, University of Rome La Sapienza, School of Engineering, ViaEudossiana 18, 00184 Rome, ITALY; email: stefania.arangio@uniroma1.itABSTRACTIn recent years more and more demanding structures are designed, built and operatedto satisfy the increasing needs of the Society. This kind of structures can be denotedas complex ones. Among large constructions arrangements, Offshore Wind Turbines(OWT) are definitely complex structural systems, being this complexity related todifferent aspects such as hard nonlinearities, wide uncertainties and stronginteractions, either among the single parts or between the whole structure and thedesign environment.On the whole, the quality of a complex system is denoted by the idea ofdependability, while for a structure the performances are connected to the property ofstructural integrity, considered as the completeness and consistency of the structuralconfiguration. Even if these concepts have been originally developed, respectively, incomputer science and for aerospace applications they can be applied to other highperformance systems as OWT.The present paper will show some specific aspects of the modern approachfor the design and the analysis of complex structural systems. In the first part of thepaper, the general aspects are recalled like the System Engineering approach and thePerformance-based Design. Attention is devoted to some important aspects, such asthe structure breakdown and the safety and performance allocations. In the secondpart of the paper, a basic application of the concepts introduced is presented.INTRODUCTIONIn recent years more and more demanding structures are designed, built and operatedto satisfy the needs of the Society. This kind of structures can be denoted as complexsystems. OWT arranged in offshore wind farms are examples of such systems (Hau,2006).A roadmap for the analysis and the design of complex structural systems isshown in Figure 1 and in this section one will explain the terms there introduced.2001Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
    • STRUCTURALSYSTEMINTERACTION AMONGDIFFERENTSTRUCTURAL PARTSINTERACTIONBETWEEN THE WHOLESTRUCTURE AND THEDESIGN ENVIRONMENTInteractions arecharacterized by strongcharacter, nonlinearityand uncertaintyQUALITYON THE WHOLEFOR THESTRUCTURALSYSTEM:DEPENDABILITYATTRIBUTESTHREATSMEANSSTRUCTURAL INTEGRITYCOMPLEXITYDECOMPOSITIONSTRATEGYPERFORMANCEBASED DESIGNSYSTEMAPPROACHFigure 1. Roadmap for the analysis and the design of complex structuralsystems.Among other definitions, with the term system one can consider an organizedassembly of elements or components united and regulated by interaction orinterdependence to accomplish a set of specific functions. Speaking about structuralsystems, elements can be imagined, for example, as beams or columns whilerestraints or control devices can be addressed as structural components: it is apparentthat the distinction resides in the fact that the first ones can be closely handled bystructural theories while for the second ones only approximated or phenomenologicallaws can be used (Bontempi, 2006).2002Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
    • The adjective complex in general usage tends to be used to characterizesomething with many parts with an intricate arrangement: more precisely, there is adistinction between a system composed by many elements that can eventually resultssimple, and then non complex, and a system composed with a relatively low numberof parts but connected in a non simple way: in this case it is just the kind ofrelationships among components that furnishes the complexity character to thesystem. Finally, the emergence of not obvious behavior at a whole system level isgenerally believed as a hint of complexity (Bontempi et al., 2007)Among large constructions, OWT are definitely complex structural systemsfor which the ambient load can significantly influence the structural behavior. Theircomplexity is related to different aspects such as hard mechanical and geometricalnonlinearities, wide characteristics and behavior uncertainties and strong interactioneither among the parts below the complete structure level or between the structure asa whole and the design environment.Only considering these aspects, a consistent evaluation of the structuralperformance can be obtained and then a suitable design achieved. This requiresevolving from a simplistic idealization of the structure as device for channeling loadsto the idea of the structural system, intended as a set of interrelated componentsworking together toward a common purpose (NASA System Engineering Handbook,2007), and acting according to the techniques of System Engineering, that is thetough approach to the creation, the design, the realization and the operation of anengineered system.If the previous reflections can be considered, in some sense, extraneous to theCivil Structural Engineering tradition, it is interesting to observe that a matchingapproach has been formalized in recent years for civil engineering demands: one isthinking to the so called Performance-based Design (Smith, 2001), by which thestructural performance during the whole service life of the constructions areexplicated and assessed. For this purpose, the advanced technologies for processingdata collected on site on real structures shall be properly taken into account, both forchecking the accomplishment of the expected performance during the service life,and for validating the original design (Berthold & Hand, 1999).On the whole, all the performance of a structure can be connected to theproperty of structural integrity, considered as the completeness and consistency ofthe structural configuration. Even if this concept has been developed in aerospaceapplications (Grandt, 2004) its principle can be applied to other high performancesystems as OWT systems. In particular, structural integrity monitoring represents anessential tool for the assessment of existing structural systems because it integrates,in a unified perspective, advanced engineering analyses and experimental data: it isbased on the structural identification concepts, employing instrumented monitoringas principal experimental tool (Bontempi et al, 2008).Coupled with the idea of integrity monitoring are the concepts of faultdetection and diagnosis. After the detection that something is changed in thestructural behavior, the diagnosis is a process that identifies cause-effectrelationships through a combination of simulations and heuristics, to understand howdefects, damage and deterioration mechanisms may affect structural performance andreliability at different limit states (Arangio and Beck, 2009). A thorough2003Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
    • understanding of the concepts of structural identification, fault detection anddiagnosis, hold the key to the maintenance of structural dependability, intended asthe overall good performance of the structure.The original definition of dependability is the ability to deliver service thatcan justifiably be trusted (Avizienis et al, 2004). This definition stresses the need forjustification of trust. The alternate definition considers dependable a system that hasthe capability to avoid service failures which are more frequent and more severe thanacceptable. Of course, the dependability of a structural system must be considered inthe design phase: it is starting from the design problem formulation and passing tothe conceptual design that the structural quality can be addressed and the properresources allocated.The present paper will show some specific aspects of this modern approachfor the design and the analysis of complex structural systems. In the first part of thepaper, the general aspects are recalled. Specific attention is devoted to the SystemEngineering approach and to the Performance-based Design. The second part of thepaper is related to the dependability assurance for an example of OWT structuralsystems with reference to the investigation of loss of structural integrity withincreasing level of demand.COMPLEX DESIGN PROBLEMUniversally, design of structural systems requires three dominant aspects to beoptimized, generally described as the Cost, Time and Performance factors (CPT).Attempting to optimize all three factors simultaneously is a very difficulttask; however, the adoption of improved system processes seems to significantlyimprove all three at the same time. In fact, the objective of a System Approach is thatthe system is designed, built and operated accomplishing its purpose in the mostcost-effective way possible. It means that a cost-effective system must provide aparticular kind of balance between effectiveness and cost: the system must providethe most effectiveness for the resources expended or, equivalently, it must be theleast expensive for the effectiveness it provides. This condition is a weak one,because there are usually many designs that meet the constraints (NASA SystemEngineering Handbook 2007).Each possible design can be represented as a point in the trade-off spacebetween effectiveness and cost. A graph, plotting the maximum achievableeffectiveness of available design with current technology as a function of cost, wouldin general yield a curved line such as the one shown in the Figure 2: the curved linerepresents the envelope of the currently available technology in terms of cost-effectiveness. In addition, this curve shows the saturation effect that is usuallyencountered approaching the highest levels of performances. Points above the linecannot be achieved with currently available technology and they represent currentlyunachievable designs: some of these points may be feasible in the future whenfurther technological advances will be made.Points inside the envelope are feasible, but are dominated by designs whosecombined cost and effectiveness lie on the envelope: in fact, considering the startingpoint D0 for the design inside the envelope, there are alternatives which reduce costs2004Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
    • without decreasing any aspect of effectiveness (design point D1) or that increasesome aspects of effectiveness without decreasing other or without increasing costs(design point D2). For these reasons, the projects represented by points on theenvelope are called cost-effective solutions.The process of finding the most cost-effective design is additionallycomplicated by the influence of uncertainty: the exact outcomes achieved by aparticular system design cannot be surely known in advance, so the cost and theeffectiveness of a design are better described by a probability distribution than by apoint. With reference to Figure 1, distributions resulting from a simple design whichhas little uncertainty are dense and highly compact, as is shown for concept A, whiledistributions associated with risky designs may have significant probabilities ofproducing highly undesirable outcomes, as it is suggested by the presence of anadditional low effectiveness/high cost cloud for concept C.CostEffectivenessABCAll possible designs produce resultsin this portion of the trade spaceThere are no designs that produce resultsin this portion of the trade spaceD2D0D1Figure 2. Uncertainty in the cost-effective solutions (adapted from NASASystem Engineering Handbook, 2007)DEPENDABILITYFor complex structural systems, as here in the case of large scale projects like OWTfarms, where there are significant dependencies among elements or subsystems, it isimportant to have a solid knowledge of both how the system works as a whole, andhow the elements behave singularly.In this contest, dependability is a global concept that describes the aspectsassumed as relevant with regards to the quality performance and its influencingfactors (Bentley, 1993).The original definition of dependability is the ability to deliver service thatcan justifiably be trusted. This definition stresses the need for justification of trust.2005Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
    • The alternate definition considers dependable a system that has the capability toavoid service failures that are more frequent and more severe than acceptable.Also if this concept was initially developed in the realm of the ComputerScience, it may be translated into the Structural Engineering field with the aid ofFigure 3: here it is shown that system dependability can be thought of as beingcomposed of three elements: attributes, threat, and means (Avizienis et al, 2004).Briefly,a) attributes can leads to objective measure of the dependability of thestructures;b) threats are things that can undermine the dependability of thestructures;c) means represent ways to increase the dependability of the structures.The main attributes can be subdivided into high level, or active, performance(reliability, availability, maintainability) and low level, or passive, (safety, securityand integrity) (Petrini et al, 2008).The threats for system dependability can be subdivided into failure, errorsand faults. The failure represents a permanent interruption of a system ability toperform a required function under specified operating conditions. In case of error, thesystem is in an incorrect state: it may or may not cause failure. On the other hand, afault is a defect and represents a potential cause of error, active or dormant. In caseof civil structures, the possible faults are damage, considered as a specific event, anddeterioration, considered as a continuous process. Within this framework, effectivemethods for fault detection and diagnosis are essential as means to assure thedependability assessment (Isermann, 2006; Nelles, 2001).These aspects are strictly related to the integrity monitoring of the structuralsystem. An efficient integrity monitoring system is expected to be able to proactivelypreserve the structural dependability, diagnosing deterioration and damage at theironset (Li & Ou, 2006). The circumstances that may eventually lead to deterioration,damage and unsafe operation may be diagnosed and mitigated in a timely manner, sothat costly replacement can be avoided or delayed by effective preventivemaintenance.STRUCTURAL PROBLEM BREAKDOWNGenerally speaking about the process of searching the solution of the structuralproblem, it is important to recognize that the way in which one describes the objectof investigation influences how one organizes the knowledge and the decision aboutthe object itself (Simon, 1998). As anticipated by the top-down approach, Figure 4shows how to deploy a system: one starts from the definition of the global structureand then, subsequently and orderly, develops further magnification of thedescription.The differentiation of the modeling level is adopted to reduce theuncertainties. The level of a generic model of the structure is here identified bymeans of two parameters: the maximum degree of detail and the scale of the model;if the finite element method is adopted, at each model level it is possible to associatea certain typology of finite element which is mainly used to build the model.2006Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
    • Figure 3. The overall framework for dependability and its components.ATTRIBUTESTHREATSMEANSRELIABILITYFAILUREERRORFAULTFAULTTOLERANTDESIGNFAULTDETECTIONFAULTDIAGNOSISFAULTMANAGINGDEPENDABILITYofSTRUCTURALSYSTEMSAVAILABILITYSAFETYMAINTAINABILITYpermanentinterruptionofasystemabilitytoperformarequiredfunctionunderspecifiedoperatingconditionsthesystemisinanincorrectstate:itmayormaynotcausefailureitisadefectandrepresentsapotentialcauseoferror,activeordormantINTEGRITYwaystoincreasethedependabilityofasystemAnunderstandingofthethingsthatcanaffectthedependabilityofasystemAwaytoassessthedependabilityofasystemthetrustworthinessofasystemwhichallowsreliancetobejustifiablyplacedontheserviceitdeliversSECURITYHighlevel/activeperformanceLowlevel/passiveperformanceATTRIBUTESTHREATSMEANSMEANSRELIABILITYRELIABILITYFAILUREERRORFAULTFAULTTOLERANTDESIGNFAULTTOLERANTDESIGNFAULTDETECTIONFAULTDETECTIONFAULTDIAGNOSISFAULTDIAGNOSISFAULTMANAGINGFAULTMANAGINGDEPENDABILITYofSTRUCTURALSYSTEMSAVAILABILITYSAFETYMAINTAINABILITYpermanentinterruptionofasystemabilitytoperformarequiredfunctionunderspecifiedoperatingconditionsthesystemisinanincorrectstate:itmayormaynotcausefailureitisadefectandrepresentsapotentialcauseoferror,activeordormantINTEGRITYwaystoincreasethedependabilityofasystemAnunderstandingofthethingsthatcanaffectthedependabilityofasystemAwaytoassessthedependabilityofasystemthetrustworthinessofasystemwhichallowsreliancetobejustifiablyplacedontheserviceitdeliversSECURITYHighlevel/activeperformanceLowlevel/passiveperformance2007Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
    • Figure 4. Hierarchical breakdown of a system.In general, with reference to Table 1, four model levels are defined for the OWTsystems (Bontempi et. al, 2008):1) System level (S): the model scale comprises the whole wind farm and can beadopted for evaluating the robustness of the overall plant; highly idealized modelcomponents are used in block diagram simulators.2) Macro level or Global modeling (G): in these models the scale reduces to thesingle turbine structure, neglecting the connections among different structuralparts. The component shapes are modeled in an approximate way, the geometricratios among the components are correctly reproduced; beam finite elements aremainly adopted;3) Meso-level or Extended modeling (E): these models are characterized by thesame scale of the previous level but with a higher degree of detail: the actualshape of the structural components is accounted for and the influence ofgeometrical parameters on the local structural behavior is evaluated. Shellelements are adopted for investigating the internal state of stress and strain (e.g.for fatigue life and buckling analysis) inside the structure extrapolated fromprevious models;4) Micro level or Detail modeling (D): this kind of models are characterized by thehighest degree of detail and used for simulating the structural behavior of specificindividual components, including connecting parts, for which a complex internalstate of stress has been previously pointed out e.g. due to the presence ofconcentrated loads. Shell or even solid finite elements are used.2008Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
    • Table 1. Definition of the model levels.Model level ScaleMaximum detaillevelMain adoptedfinite elementsSystem level wind farmapproximate shape ofthe structuralcomponentsBLOCK elementsMacro-level single turbineapproximate shape ofthe structuralcomponents, correctgeometryBEAM elementsMeso-level single turbinedetailed shape of thestructural componentsSHELL & SOLIDelementsMicro levelindividualcomponentsdetailed shape of theconnecting partsSHELL & SOLIDelementsPROGRESSIVE LOSS OF STRUCTURAL INTEGRITYOne will considers the OWT support structure shown in Figure 5 with thefollowing main information:− water level: 35 m;− height of the structure above water level: 105 m;− pile length under sea bed: 40 m;− steel: S355;− turbine: 5/6 MW.For this structure, as usual, ULS (Ultimate Limit State) requirements are verified:this situation, shown on the left of Figure 5, is conventionally associated here with aload multiplier λ=1.00. In consideration of the economic and strategic value of awind farm facility made by a large number of these kinds of structures, it isinteresting to assess the ability of the system to sustain further levels of demand up toextreme loading conditions. The survivability of the structure is then investigated,allowing large damage developing inside the structural system: it means that thespread of the plasticity inside the structure is allowed, until the last configuration ofequilibrium is reached. Of course, nonlinear analysis that accounts for materialplasticity and large displacements is needed.For the structure under examination, the last equilibrium configuration forλ=1.44 is shown on the right of Figure 5, where the spread of plasticity is marked bya dark color. Figure 6 shows more details in the bottom part of the support structure.Besides these qualitative pictures, it is interesting to obtain some quantitive measureof the progressive loss of integrity. This is obtained, in the most basic way,considering the modal behavior of the damaged structure. In Table 2, the first threenatural frequencies of the support structure are considered for load multiplier λ2009Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
    • ranging from 1.00 (at the ULS configuration) to the value of 1.44 that characterizesthe last obtained equilibrium situation under imposed load. These data are alsorepresented in the diagram of Figure 6.By these results, one can judge the structural safety with regards to extremeor abnormal situation, assuring the dependability of the system as extension of theusual conventional safety assessment considered (Starossek, 2009).Figure 5. Support structure for OWT considered: left, reference baselineconfiguration under ULS loading system (λ=1.00); right, last obtainedequilibrium configuration (λ=1.44).Table 2. Natural frequencies as function of the load multiplierλ.load multiplier λ 1st freq. (Hz) 2nd freq. (Hz) 3rd freq. (Hz)1.000 0.152 0.198 0.8711.100 0.123 0.191 0.8181.200 0.101 0.179 0.7201.220 0.098 0.176 0.6791.320 0.093 0.170 0.6041.380 0.087 0.158 0.5411.420 0.086 0.153 0.5182010Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
    • λ = 1.00λ = 1.00 λ = 1.10λ = 1.10λ = 1.32λ = 1.32 λ = 1.44λ = 1.44Figure 6. Increase of damage from the reference baseline ULS configuration(load multiplier λ=1.00) to the last equilibrium configuration (λ=1.44).2011Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
    • 0.1520.1230.1010.0980.0930.0870.0860.1980.1910.1790.1760.1700.1580.1530.8710.8180.7200.6790.6040.5410.5180.0000.1000.2000.3000.4000.5000.6000.7000.8000.9001.0001.000 1.250 1.500load multiplierfrequencies(Hz)1st2nd3rdFigure 7 Natural frequencies as function of the load multiplier λ.CONCLUSIONThe main ideas about complex structural systems, the connected design strategy andassessment procedures are presented in this paper. A simple application shows howthe concept of dependability can be applied to OWT support structure to investigatethe survivability of the system in the presence of extreme actions.ACKNOWLEDGEMENTSThe present work has been developed within the research project “SICUREZZA EDAFFIDABILITA DEI SISTEMI DELLINGEGNERIA CIVILE: IL CASO DELLETURBINE EOLICHE OFFSHORE", C26A08EFYR, financed by University ofRome La Sapienza.2012Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
    • REFERENCESHau, E., Wind Turbines: Fundamentals, Technologies, Application, Economics, 2ndedn., Springer-Verlag Berlin, Heidelberg, 2006.Bontempi, F., Basis of Design and expected Performances for the Messina StraitBridge, Proceedings of the International Conference on Bridge Engineering –Challenges in the 21st Century, Hong Kong, 1-3 November, 2006.Bontempi, F., Giuliani, L. and Gkoumas, K., Handling the exceptions: dependabilityof systems and structural robustness, Proceedings of the 3rdInternationalConference on Structural Engineering, Mechanics and Computation, AlphoseZingoni (Ed.), Millpress, Rotterdam, 2007.National Aeronautics and Space Administration (NASA), 2007. Systems EngineeringHandbook.Smith, I., 2001. Increasing Knowledge of Structural Performance, StructuralEngineering International, 12 (3), 191-195.Berthold, M & Hand, D.J., 1999. Intelligent Data Analysis, Springer.Grandt A. F. Jr, 2004. Fundamentals of Structural Integrity: Damage TolerantDesign and Nondestructive evaluation, John Wiley & Sons.Bontempi, F., Gkoumas, K. and Arangio, S., Systemic approach for the maintenanceof complex structural systems, Structure and infrastructure engineering, 2008,4, 77-94.Arangio, S., and Beck, J.L., Bayesian neural networks for bridges integrityassessment, sub-mitted to Structural Control and Health Monitoring, 2009.Avižienis, I., Laprie, J.C. and Randell, B., Dependability and its threats: a taxonomy.18th IFIP World Computer Congress, Building the Information Society,Kluwer Academic Publishers, 12, 91-120, 2004.Bentley, J.P., 1993. An Introduction to Reliability and Quality Engineering,Longman, Essex.Petrini, F., Ciampoli, M. & Augusti, G., 2008. Performance-based wind engineering:assessment of a long-span suspension bridge, Proceedings of IFIP WG 7.5,Toluca, Mexico.Isermann, R., Fault-Diagnosis Systems. An Introduction from Fault Detection toFault Tolerance, 2006 (Springer- Verlag Berlin Heidelberg).Nelles, O., 2001, Nonlinear System Identification, Springer.Li H. & Ou J., 2006. The intelligent health monitoring system for Shandong BinzhouYellow River Highway Bridge, Computer-Aided Civil and InfrastructureEngineering, 21 (4).Simon, H.A, The Sciences of the Artificial, The MIT Press, Cambridge, 1998.Bontempi, F., Li, H., Petrini, F. and Gkoumas, K., Basis of Design of Offshore WindTurbines by System Decomposition, Proceedings of the 4thInternationalConference on Advances in Structural Engineering and Mechanics, Jeju,Korea, 26-28 May, 2008.Starossek, U., Progressive Collapse of Structures, Thomas Telford Publishing,London, 2009.2013Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE