Advanced Topics in Offshore Wind Turbines Design

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Offshore Wind Turbine (OWT) is a relatively complex structural and mechanical
system located in a highly demanding environment. In this study the fundamental
aspects and the major issues related to the design of such structures are inquired. The
System Approach is proposed to carry out the design of the structural parts: in
accordance with this philosophy, decomposition of the system (environment,
structure, actions/loads) and of the structural performance is carried out in order to
organize the qualitative and quantitative assessments in various sub-problems. These
aspects can be faced by sub-models of different involvedness both for the structural
behavior and for the load models. Numerical models are developed accordingly to
assess safety, performance and robustness under aerodynamic and hydrodynamic
actions.

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Advanced Topics in Offshore Wind Turbines Design

  1. 1. Advanced Topics in Offshore Wind Turbines DesignF. Bontempi11Professor of Structural Analysis and Design, School of Engineering, University ofRome La Sapienza, Via Eudossiana 18, 00184 Rome, ITALY; phone: +39-06-44585265; email: franco.bontempi@uniroma1.itABSTRACTOffshore Wind Turbine (OWT) is a relatively complex structural and mechanicalsystem located in a highly demanding environment. In this study the fundamentalaspects and the major issues related to the design of such structures are inquired. TheSystem Approach is proposed to carry out the design of the structural parts: inaccordance with this philosophy, decomposition of the system (environment,structure, actions/loads) and of the structural performance is carried out in order toorganize the qualitative and quantitative assessments in various sub-problems. Theseaspects can be faced by sub-models of different involvedness both for the structuralbehavior and for the load models. Numerical models are developed accordingly toassess safety, performance and robustness under aerodynamic and hydrodynamicactions.INTRODUCTIONOffshore Wind Turbine (OWT) emerges as an evolution of the onshore plants forwhich the construction is a relatively widespread and consolidated practice providinga renewable power resource (Hau, 2006). In order to make the wind generated powermore competitive with regards to conventional exhaustible and high environmentalimpact sources of energy, the attention has turned toward offshore wind powerproduction (Breton and Moe, 2009).Besides being characterized by a reduced visual impact, since they are placedfar away from the coast, OWT can take advantage from more constant and intensewind forcing, something that can increase the regularity and the amount of theproductive capacity and make such a resource more cost-effective if the plant islifelong and operates with minimum interruption through its lifespan.From a general point of view, an OWT is formed by both mechanical andstructural elements. As a consequence, it is not a “common” Civil EngineeringStructure: it behaves differently according to different circumstances related to thespecific functional activity (idle, power production, etc), and it is subjected to highlyvariable loads (wind, wave, sea currents loads, etc.). In the design process, differentstructural schemes for the supporting structure can be adopted (Figure 1), mainlydepending on the water depth which determines the hydrodynamic loads acting on1981Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  2. 2. the structure and drives the choice of the proper techniques for the installation andmaintenance of the support structure. Moreover, since the structural behavior ofOWTs is influenced by nonlinearities, uncertainties and interactions, they can bedefined as complex structural systems, as other recently designed constructions(Bontempi, 2006).seabedtransitionplatformfoundationtowersupportstructuresub-structureblades – rotor - nacellefoundationsea floorwater levelTRIPODMONOPILE JACKETFigure 1. Main parts of OWT for different support structure typology.The above considerations highlight that a modern approach to study suchstructures has to evolve from the idea of “structure” itself, intended as a simpledevice for channeling loads, to the one of “structural system”, intended as “a set ofinterrelated components which interact one with the other in an organized fashiontoward a common purpose” (NASA, 2007). The consequent System Approachincludes a set of activities which lead and control the overall design, implementationand integration of the complex set of interacting components (Simon, 1998;Bontempi et al., 2008): here, the original definition by NASA has been extended insuch a way that the “structural system” organization contains also the actions and1982Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  3. 3. loads: the latter derives from, and is strictly related to, the environment (Figure 2).A certain amount of complexity arises from the lack of knowledge and fromthe modeling of the environment in which the OWT is located. In this context twomain design issues can be individuated: the consideration of the uncertainty derivingfrom the stochastic nature of the environmental forces (in particular aerodynamic andhydrodynamic) and the proper modeling of the possible presence of non lineardynamic interaction phenomena among the different actions and between the actionsand the structure.Figure 2. Structural system organization.Generally speaking, the uncertainties can spread during the various analysisphases that are developed in a cascade. The incorrect modeling of the involveduncertainty can lead to an incorrect characterization of the structural response from astochastic point of view and, thus, to an improper quantification of the risk for agiven structure subjected to a specific hazard.Having as goal the schematization of the problem and the individuation of theuncertainty propagation mechanisms, reference can be made to the Figure 3, wherethe process of the environmental actions generation is qualitatively represented alsowith considerations on the involved uncertainties.ENVIRONMENT ZONEStructureNonenvironmentalsolicitationsEXCHANGE ZONEWind and wave flowStructural (non-environmental)systemSite-specificenvironmentWind site basicparametersOtherenvironmentalagentsWave site basicparametersWind, wave andsea currentactionsAerodynamic andAeroelasticphenomenaHydrodynamicphenomena1. Aleatoric2. Epistemic3. ModelTypes of uncertainties1. Aleatoric2. Epistemic3. Model1. Aleatoric2. Epistemic3. ModelPropagation PropagationFigure 3. Generic depiction of the uncertainties and the interaction mechanismsin the design of OWT structure.1983Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  4. 4. In Figure 3, following the wind and the hydrodynamic flows impacting on thestructure, it is possible to distinguish two zones:• Environment zone: it is the physical region, sufficiently close to the structureto assume the same environmental site parameters of the structure, yet farenough to neglect the flow field perturbations (in terms of particle’strajectories, pressure field, etc.) induced by the presence of the structureitself. In the environment zone, the wind and the hydrodynamic flows caninteract with each other and with other environmental agents, changing theirbasic parameters. The physical phenomena and uncertainties in theenvironment zone propagate themselves in the neighborhood regions.• Exchange zone: it is the physical region adjoining the structure. In this zone,the structure itself, the wind and the hydrodynamic field experience themechanical interchange (aerodynamic and hydrodynamic phenomena) fromwhich the actions arise. In the exchange zone, some non-environmentalsolicitations are present; these solicitations may change the dynamic oraerodynamic characteristics of the original structure; so the actions aregenerated considering this structural sub-system (original structure combinedwith non-environmental solicitations) instead of regarding only the originalstructure itself. By definition, physical phenomena and uncertainties cannotpropagate themselves from the exchange zone to the environment zone.In general the uncertainties can be subdivided in three basic typologies:• epistemic uncertainties (deriving from the insufficient information and theerrors in defining and measuring the previously mentioned parameters);• aleatory uncertainties (arising from the unpredictable nature of themagnitude, the direction and the variance of the environmental actions);• model uncertainties (deriving from the approximations in the models).Regarding for example the wind model and considering the turbulent wind velocityfield as a Gaussian stochastic process, an epistemic uncertainty related to thehypothesis of gaussianity is introduced. For the sake of simplicity, the epistemicuncertainties are not considered in this study.The aleatoric uncertainties can be treated by carrying out a semi-probabilistic(looking for the extreme response) or a probabilistic analysis (looking for theresponse probabilistic distribution) analysis.Finally, a possible way to reduce the model uncertainties is given bydifferentiating the modeling levels. This can be carried out not only for the structuralmodels, but also for the action and interaction phenomena models; for this reasondifferent model levels are adopted.In this way, a suitable strategy to govern the complexity is given from thestructural system decomposition, represented by the design activities related with theclassification and the identification of the structural system components, and by thehierarchies (and the interactions) between these components. As mentioned before,the decomposition regards not only the structure, but also the environment and theactions and loads, and it is the subject of the first part of this study.From the design point of view, due to the complexity of these structures onehas to adopt a Performance-Based Design philosophy: different aspects andperformance under different loading conditions (with reference to all possible system1984Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  5. 5. configurations that can be assumed by the blades and the rotor) have to beinvestigated for this type of structures. In this framework, additional design issuesrelated to the structural aspects can be accommodated and are mentioned below withsome proper references:− aerodynamic optimization (Snel, 2003);− foundation design and soil-structure interaction (Westgate and DeJong,2005; Ibsen and Brincker, 2004; Zaaijer, 2006);− fatigue calculations (Veldkamp, 2007; Tempel. 2006);− vessel impact and robustness (Biehl and Lehman, 2006);− life cycle assessment (Martinez et al., 2009; Weinzettel et al., 2009);− marine scour (Sumur and Fredsøe, 2002; Tempel et al., 2004);− possible floating supports (Henderson and Patel, 2003; Jonkman andBuhl, 2007);− standards certification (API, 1993; BSH, 2007; DNV, 2004; GL, 2005;IEC, 2009).The most important issue for OWT is anyway the choice of the kind of the supportstructure, related principally to the water depth, the soil characteristics and economicissues. If the water depth (h) is considered as the major parameter, the followingrough classification can be made: monopile, gravity and suction buckets (h<25m);tripod, jacket and lattice tower (20m<h<40÷50m); low-roll floaters and tension legplatform (h>50m). Due to optimization processes, cross structural forms can emerge,as the “strutted” system shown in Figures 4 and 5, with the ability of the structuralscheme to adjust different water levels (Polnikov and Manenti, 2009).Figure 4. Scheme of strutted support structures for OWT positioned in sea withwater level ranging from 20 to 35 m.1985Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  6. 6. Figure 5. Main loading systems for strutted support structure for OWT.STRUCTURAL SYSTEM DECOMPOSITIONAs previously stated, the decomposition of the structural system is a fundamentalstrategy for the design of complex structural systems, and it has to be performedtogether with the decomposition of the performance the structure has to fulfill1986Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  7. 7. (Figure 6). The decomposition is carried out focusing the attention on different levelsof detail: starting from a macro-level vision and moving on towards the micro-leveldetails (for more details see Bontempi et al., 2008).Figure 6. Structural system and performance decomposition of an OWT.Decomposition of the environment. The first step of the structural systemdecomposition concerns the environment. This is due to the fact that, in a globalapproach, the structure is considered as a real physical object placed on anenvironment where a variety of conditions, strictly related to the acting loads, shouldbe taken into consideration. The environment decomposition is performed in the firstcolumn of Figure 6.Decomposition of the structure. The second step is related to the OWT structure.This is organized hierarchically, considering all the structural parts categorized inthree levels (second column of Figure 6):1987Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  8. 8. • Macroscopic (MACRO-LEVEL), related to geometric dimensionscomparable with the whole construction or parts with a principal role in thestructural behavior; the parts so considered are called macro componentswhich can be divided into:− the main structure, that has the objective to carry the main loads;− the secondary structure, connected with the structural part directly loadedby the energy production system;− the auxiliary structure, related to specific operations that the turbine maynormally or exceptionally face during its design life: serviceability,maintainability and emergency.Focusing the attention on the main structure, it consists in all the elementsthat form the offshore wind turbine. In general, the following segments canbe identified:− support structure (the main subject of this study);− rotor-nacelle assembly.• Mesoscopic (MESO-LEVEL), related to geometric dimensions still relevantif compared to the whole construction but connected with specialized role inthe macro components; the parts so considered are called meso-components.In particular the support structure can be decomposed in the following parts:− foundation: the part which transfers the loads acting on the structure intothe seabed;− substructure: the part which extends upwards from the seabed andconnects the foundation to the tower;− tower: the part which connects the substructure to the rotor-nacelleassembly.• Microscopic (MICRO-LEVEL), related to smaller geometric dimensions andspecialized structural role: these are simply components or elements.Decomposition of the actions and loads. The next step is the one of the actionsderived from the environmental conditions. These can be decomposed as shown inthe third column of Figure 6, from which the amount of the acting loads can becomprehended. It is important to underline that, since the environmental conditionsin general are of stochastic nature, the magnitude of the actions involved is usuallycharacterized, from a statistical point of view, by a return period TR: lower values ofTR are associated with the so called “normal conditions”, while higher values of TRare associated with “extreme conditions”.Performance decomposition. As a final step, the performance requirements areidentified and decomposed as follows (lower part of Figure 6):• Assurance of the serviceability (operability) of the turbine, as well as of thestructure in general. As a consequence, the structural characteristics(stiffness, inertia, etc.) have to be equally distributed and balanced along thestructure.• Safety assurance with respect to collapse, in probable extreme conditions; thisis applicable also to the transient phases in which the structure or parts of itmay reside (transportation and assembly), and that have to be verified as well.1988Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  9. 9. • Assurance of an elevated level of reliability for the entire life-span of theturbine. As a consequence, a check of the degradation due to fatigue andcorrosion phenomenon is required.• Assurance of sufficient robustness for the structural system, that is to assurethe proportionality between an eventual damage and the resistance capacity,independently from the triggering cause, assuring at the same time aneventual endurance of the structure in the hypothetical extreme conditions.The following performance criterions can be identified for the structural system,leading eventually to the selection of appropriate Limit States:• Dynamic characterization of the turbine as dictated by the functionalityrequirements:− natural vibration frequencies of the whole turbine (compressive of therotor-nacelle assembly), the support structure and the foundations;− compatibility of the intrinsic vibration characteristics of the structuralsystem with those of the acting forces and loads;− compatibility assessment for the movement and the accelerations of thesupport system for the functionality of the turbine.• Structural behavior regarding the serviceability (SLS - Serviceability LimitState):− limitation of deformations;− connections decompression.• Preservation of the structural integrity in time:− durability for what regards the corrosion phenomenon;− structural behavior with respect to fatigue (FLS - Fatigue Limit State).• Structural behavior for near collapse conditions (ULS - Ultimate Limit State):− assessment of the solicitations, both individual and as a complex, to thewhole structural system, to its parts, its elements and connections;− assessment of the global resistance of the structural system;− assessment of the resistance for global and local instability phenomena.• Structural behavior in presence of accidental scenarios (ALS - AccidentalLimit State)− structural robustness: decrease in the load bearing capacity proportional tothe damage (see for example Starossek, 2009, and Bontempi et al., 2007);− survivability of the structural system in presence of extreme and/orunforeseen, situations; these include the possibility of a ship impactingthe structural system (support system or blades), with consequencesaccounted for in risk scenarios.CONCLUSIONIn this paper, the System Approach has been proposed as a conceptual method for thedesign of OWT structures. In this sense, structural system decomposition has beenperformed, with a specific view on the structural analysis and performance. Thepresented considerations aim to the organization of the framework for the basis ofdesign of offshore wind turbines, as a support to the decision making, with specific1989Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  10. 10. reference to the structural safety, serviceability and reliability for the entire lifespan.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.REFERENCESHau, E., Wind Turbines: Fundamentals, Technologies, Application, Economics, 2ndedn., Springer-Verlag Berlin, Heidelberg, 2006.Breton, S.-P. and Moe, G., Status plans and technologies for offshore wind turbinesin Europe and North America, Renewable Energy, 2009, 34 (3), 646-654.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.National Aeronautics and Space Administration (NASA), 2007. Systems EngineeringHandbook, available online at:http://education.ksc.nasa.gov/esmdspacegrant/Documents/NASA%20SP-2007-6105%20Rev%201%20Final%2031Dec2007.pdfSimon, H.A, The Sciences of the Artificial, The MIT Press, Cambridge, 1998.Bontempi, F., Gkoumas, K. and Arangio, S., Systemic approach for the maintenanceof complex structural systems, Structure and infrastructure engineering, 2008,4, 77-94.Snel, H., Review of Aerodynamics for Wind Turbines, Wind Energy, 2003, 6 (3),203–211.Westgate, Z.J. and DeJong, J.T., Geotechnical Considerations for Offshore WindTurbines, 2005, Report for MTC OTC Project, Available online on 10/2009at: http://www.masstech.org/IS/Owec_pdfs/GeotechOffshoreFoundations-MTC-OWC.pdfIbsen, L. B. and Brincker R., Design of a New Foundation for Offshore WindTurbines, Proceedings of the IMAC-22: A Conference on StructuralDynamics, Michigan, 26 – 29 January, 2004.Zaaijer, M. B., Foundation modelling to assess dynamic behaviour of offshore windturbines, Applied Ocean Research, 2006, 28 (1), 45–57.Veldkamp, D., A probabilistic approach to wind turbine fatigue design, Proceedingsof the European wind energy conference and exhibition, Milan, 7-10 May,2007.Tempel, J. van der, Design of support structures for offshore wind turbines, PdDThesis, Technische Universiteit Delft, 2006.Biehl, F. and Lehmann, E., Collisions of Ships and Offshore Wind Turbines:Calculation and Risk Evaluation, Proceedings of the International Conferenceon Offshore Mechanics and Arctic Engineering, Hamburg, 4-9 June, 2006.Martìnez, E., Sanz, F., Pellegrini, S., Jiménez, E. and Blanco, J., Life cycle1990Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  11. 11. assessment of a multi-megawatt wind turbine, Renewable Energy, 2009, 34(3), 667-673.Weinzettel, J., Reenaas, M., Solli, C. and Hertwich, E.G., Life cycle assessment of afloating offshore wind turbine, Renewable Energy, 2009, 34 (3), 742-747.Sumur, B.M. and Fredsøe, J., The mechanics of scour in the marine environment,World Scientific Publishing Co., Singapore, 2002.Tempel, J. van der, Zaaijer, MB. and Subroto, H., The effects of scour on the designof offshore wind turbines, Proceedings of the 3rd International conference onmarine renewable energy, Blyth, July 6-9, 2004.Henderson, A.R. and Patel, M.H., On the modeling of a floating Offshore WindTurbines, Wind Energy, 2003, 6 (1), 53–86.Jonkman, J.M. and Buhl Jr., M.L., Loads Analysis of a Floating Offshore WindTurbine Using Fully Coupled Simulation, Proceedings of the WindPowerConference and exhibition, Los Angeles: California, June 3–6, 2007.Polnikov, V.G., Manenti S., Study of relative role of nonlinearity and depthrefraction in wave spectrum evolution in shallow water, Journal ofEngineering Applications of Computational Fluid Mechanics, 2009, 3 (1),42–55.API (American Petroleum Institute), Recommended Practice for Planning, Designingand Constructing Fixed Offshore Platforms –Load and Resistance FactorDesign (RP 2A-LRFD), 1993 (suppl. 1997).BSH (Bundesamt für Seeschifffahrt und Hydrographie), Standard: Design ofOffshore Wind Turbines, 2007.DNV (Det Norske Veritas), Offshore Standard: Design of Offshore Wind TurbineStructures (DNV-OS-J101), 2004.GL (Germanischer Lloyd) Wind Energie GmbH, Richtlinie zur Erstellung vontechnischen Risikoanalysen für Offshore-Windparks, 2002.GL (Germanischer Lloyd) Wind Energie GmbH, Guideline for the Certification ofOffshore Wind Turbines, 2005.IEC (International Electrotechnical Commission), Wind Turbine Generator Systems– part 3: Design requirements for OWT (IEC 61400-3), 2009.Bontempi, F., Li, H., Petrini, F. and Gkoumas, K., Basis of Design of Offshore WindTurbines by System Decomposition, Proceedings of the 4th InternationalConference on Advances in Structural Engineering and Mechanics, Jeju,Korea, 26-28 May, 2008.Bontempi, F., Li, H., Petrini, F. and Manenti, S., Numerical modeling for theanalysis and design of offshore wind turbines, Proceedings of the 4thInternational Conference on Advances in Structural Engineering andMechanics, Jeju, Korea, 26-28 May, 2008.Starossek, U., Progressive Collapse of Structures, Thomas Telford Publishing,London, 2009.Bontempi, F., Giuliani, L. and Gkoumas, K., Handling the exceptions: dependabilityof systems and structural robustness, Proceedings of the 3rd InternationalConference on Structural Engineering, Mechanics and Computation, AlphoseZingoni (Ed.), Millpress, Rotterdam, 2007.1991Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE

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