Structural Integrity Evaluation of Offshore Wind TurbinesL. Giuliani1, F. Bontempi2,1Structural Engineer, Ph.D., Structur...
 against extreme loading is provided by a control system that, among others, bringsthe turbine to calm in case of high win...
 exposure of these structure to extreme natural actions, impacts from debris or evenship collisions in case of offshore in...
 buckling of the blade. The latter aspect is becoming particularly compelling, as bladebecome larger and new materials are...
 collapse of the turbine, as for example occurred in the late 2005at the Nissan car parkof Sunderland, UK: a 60m tall wind...
 The reduction of the effects of the action concerns instead structural measuresaimed at reducing the vulnerability of the...
 modeled and the entity of a possible damage in the element caused by this action hasbeen investigated.The structure consi...
 The turbine is considered to be loaded only with self-weight at the moment ofthe impact, i.e. wind and possible wave over...
 Figure 2. Scenario A: displacement for the impacted zone and for the top tower node;moment-curvature diagram for the impa...
 SCENARIOA:Impanctonturbinelegt = 0.025 s t = 0.300 st= 0.500 s t = 0.800t =1.200 s t = 3.000 sOBS: The scale used for rep...
 SCENARIO B: impact on the tower SCENARIO C: impact on the towert = 3.000 s t = 0.300 sOBS: The scale used for representin...
 In this respect, some structural design modification could be addressed toincrease the specific local resistance of the l...
 response of the structure following this impact has been evaluated by means ofnonlinear dynamic analyses.The outcomes of ...
 Turbine Structures, June 2004Overgaard L.C.T., Lund E.: “Structural Design Sensitivity Analysis andOptimization of Vestas...
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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 TurbinesL. Giuliani1, F. Bontempi2,1Structural Engineer, Ph.D., Structural and Geotechnical Engineering Department,University of Rome ”La Sapienza”, Italy (on leave for the Technical University ofDenmark, Lyngby, Denmark); e-mail: luisa.giuliani@uniroma1.it2Full Professor, Structural and Geotechnical Engineering Department, University ofRome “La Sapienza”, Italy; e-mail: franco.bontempi@uniroma1.itABSTRACTWind turbines are complex structures that should deal with adverse weatherconditions, are exposed to impacts or ship collisions and, due to the strategic roles inthe energetic supplying, can be the goal of military or malevolent attacks.Even if a structure cannot be design to resist any unforeseeable critical eventor arbitrarily high accidental action, this kind of systems should be able to maintainintegrity 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 resistedand the services levels to be maintained are the design objectives, which should bedefined 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 issuein the framework of a safe design: it depends on different factors, like exposure,vulnerability and robustness. Particularly, the requirement of structural vulnerabilityand 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 anoffshore wind turbine.The investigation resorts nonlinear dynamic analyses performed on the finiteelement model of the turbine and considers three different scenarios for the shipcollision. The review of the investigation results allows for an evaluation of theturbine structural integrity after the impact and permits to identify somecharacteristics of the system, which are intrinsic to the chosen organization of theelements within the structure.INTRODUCTIONWind turbines are built in order to exploit the wind energy available in theconsidered location in the most efficient and economical way. The leading designparameters could thus be expected to depend on the external loading and on thematerial strengths according to the design service and ultimate limit stateverifications. However, wind turbines have to deal often with extreme loading andexceptional events not included in the above mentioned verifications. Protection2116Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  2. 2.  against extreme loading is provided by a control system that, among others, bringsthe turbine to calm in case of high winds that could determine the turbine over-speedand cause the failure of some components. The control system may however also besubjected to failures or malfunctioning (Tarp-Johansen 2005) in case of impropermaintenance or system faults. Furthermore, other extreme events like impacts or shipcollisions cannot be managed by some active defense like an automatic protectionsystem and call instead on the passive defense of the turbine, i.e. the resistance tofailures of its structural system.Recent codes and guidelines for onshore and offshore turbines accounttherefore, even if with different approaches and terminology, for the structuralbehavior of the structure under rare but extreme events and require to maintain acertain level of structural integrity, as in accordance to most American and Europeanregulations for building and general structures: structural systems cannot be designedto resist fully undamaged exceptional loads or accidents, but major damages anddisproportionate collapse should be prevented in any case (ASCE 7-02; GSAGuidelines 2003; EN 1991-1-7 2006). With respect to wind turbines, the verificationof accidental limit state (ALS) is required in (Offshore Standard 2004), where theturbine response consequent to the damage of one or more components has to beevaluated, due to an accidental event or operational failure. Furthermore, the risk ofship collision for offshore wind turbines should be specifically evaluated.A failure mode and effects analysis (FMEA) or an equivalent analysis shallbe conducted according to (OCT Guidelines 2005) for the safety systems and for theauxiliary and control systems needed to operate it. Aim of the analysis is to verifythat a single failure would not lead to any major damage to the structure. Theidentification of possible failure conditions with a common cause is also requiredtogether with a check on possible redundancy decrement that could imperil thestructure.STRUCTURAL INTEGRITY OF WIND TURBINESOld codes and regulations used to deal with the problem of structural safetyof building and other constructions, requiring the prevention of any possiblestructural failure. This aim can be easily achieved by means of local resistanceverifications (e.g. at a sectional level). Nowadays instead, as previously mentioned,also the presumption of the failure is considered necessary, in order to assure alimited damage of the structure, in case the failure could not be prevented after all.This assessment is substantiated with two different considerations: not only is itimpossible to prevent every single failure in a structure (let think to human error indesign or execution phases, as well as extreme events that could not be directlyconsidered in a common design), but also even a small initial failure can result in adisproportionate structural damage. This is generally true for several types ofconstructions, but is also a concern for wind turbines, as shown by past cases oftower collapses or major turbine damage. Some of these cases are briefly reported inthe following as examples of disproportionate collapses of wind turbines.These structures are particularly interesting from the point of view ofstructural integrity, intended here as resistance to exceptional actions, due to the high2117Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  3. 3.  exposure of these structure to extreme natural actions, impacts from debris or evenship collisions in case of offshore installations. Depending on the particular systemchosen for the tower and also partly to the design optimization aimed at reducingmaterial costs and avoiding cumbersome sections, a high level of structuralredundancy, which could provide for alternate load path after the failure of somestructural parts and a redistribution of loads, could be difficult to obtain. On the otherhand, the intrinsic low connection of a lattice structural system for the tower couldavoid the transmission of high stresses from the initial overloaded elements to theadjoining ones and maintain the damage limited to the zone of initial failure.FAILURES: REAL CASES AND TYPOLOGYSeveral documented cases of wind turbine accidents have been reported in thepast years, referring to different typology of failure and damages.In the last decade almost 30 cases of blades failures and collapse have beenreported in Denmark and a law has been recently established that requires yearlyinspections to the turbine. Due to high winds experienced last winter in the country,two in-land wind turbines collapsed in February, 2008. In one case, the over-speedtriggered the failure of a one of the wing blades. The debris impacted on the otherwings and on the tower, which was almost sheared into half and collapsedimmediately afterwards.Few days later, another in-land wind turbine collapsed in New York: thecompany imputed the failure to the combination of power loss and the wiringanomaly experienced by the turbine.Again this year, wind turbines malfunction led to structural collapse inDenmark and in Sweden within the same week (Copenhagen Post 2009). In the firstcase a 120-foot turbine threw off all of its blades due to a defective axle and one ofthem slammed into a power transformer. In the second case another turbine threw offa blade that landed on a hiking trail. Afterwards, the wind turbine industry in Swedenhas proposed to create a commission in order to investigate incidents like this one.As a matter of fact, the draw up of a comprehensive list of turbine accidentsand damages could be useful in order to better understand and classify the failuretypologies and investigate possible countermeasures and design solutions. In thefollowing, the turbine damages and failures are differentiated into two main groups,depending on the cause that originated the damages.Structural failure. It is a failure of a component can be caused by overload,insufficient strength, failure of the control system, as well extreme externalconditions. Structural failures may affect different parts of the wind turbines andparticularly blades, tower and foundations.Blade failure can arise from a number of possible sources and results in eitherwhole blade or pieces of blade being thrown from the turbine. It represents one of themost frequent turbine failures and can lead to the damage of other turbinecomponents impacted by the debris and eventually to the collapse of the tower, as inthe cases of the turbine collapse mentioned above. Blade failures are often imputedto over-speed caused by control-system failure as well as fatigue failure or local2118Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  4. 4.  buckling of the blade. The latter aspect is becoming particularly compelling, as bladebecome larger and new materials are used (Overgaard 2005).Tower failure may be caused by cracks in the shaft or welding failure that canoriginate by faulty design or improper maintenance, especially when referring toweld fatigue failure, and can lead to the tower buckling and collapse.As mentioned before, the collapse of the tower is caused by indirect damageof the shaft, hit by flying blade debris or, in case of off-shore turbine, resulting by theimpact of a ship collision. The prevention of all these kind of failures is practicallyunfeasible and the resistance of the structural integrity of the tower should rely on thelow vulnerability or intrinsic robustness of the structural system. In this respect, thedifferent structural solutions that can be chosen for the tower, usually mono-pile,tripod or lattice systems, may perform very differently in term of resistance to impactand resistance to local failure. Particularly interesting from this point of view is thevariation of pile section that is often obtained as result of a design optimization.Typically, optimizations of the design are based on serviceabilityrequirements and material and construction costs, but do not usually considerstructural integrity issues. Even if it would probably unfeasible to account in thedesign optimization for the structural response under extreme events and additionalcosts for structural damages and repairs due to less robust design, still the materialoptimization leads often to more slender elements and lower redundant system,whose effect on the structural integrity of the constructions are seldom investigated.Foundation failure can be expected to occur mostly during the construction ofthe turbine (especially offshore) but is seldom reported as direct cause of towercollapse when the turbine is in usage. Still the design of foundation of wind turbinesis becoming particularly challenging, due to the increasing hub heights and size ofturbines, which have been allowed by technological advancements in design andexecution and by the development of wind energy usage. Slab foundations of windturbines should therefore deal with extremely eccentric loads and overturningmoments. Design for stability usually considers a safety factor of 1.5 for both slidingand overturning, but the occurrence of to extreme winds load is seldom accountedand the response of the foundation to these events remains unknown.Fire. Another type of accident that seems to affect wind turbine with a relativelyhigh frequency is fire that can arise from electrical failures as well as be triggered bylightning strikes, which represent of course a peril for the high and slenderconstructions of wind turbines.A list of possible causes and examples of fire damage is reported in theGerman guideline for fire protection (VdS 3523en: 2008-07), where also a review ofpossible measure to reduce the risk or the consequence of onshore and offshoreturbine fire is presented. Fire may trigger in the nacelle, in the tower and in thepower substation of the turbine. The risk of fire is particularly high in the nacelle,where switchgear, inverter, control cabinets and transformer are placed and may havemajor consequence, due to the practical difficulties for firemen to reach the height ofthe nacelle with ladder or water jet and extinguish the flame. In case of a total loss ofthe nacelle, the restoration costs are very high and even comparable with the value ofthe wind turbine. Furthermore, due to the high density of technical equipment andcombustible material in the nacelle, fire can spread rapidly and even lead to the2119Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  5. 5.  collapse of the turbine, as for example occurred in the late 2005at the Nissan car parkof Sunderland, UK: a 60m tall wind turbines caught on fire, probably due to a faultof the power pack in the concrete shaft (The northern Echo 2005) and the fire brigadecould just set up cordons to avoid injuries on the near motorway and let the turbine,which eventually fall into a nearby field, burn itself out.A further peril of onshore turbine fires is represented by a possible spread ofthe fire in the area nearby the tower. A similar case seems to have occurred few yearsago in Spain, where a significant forest fire was claimed to be triggered by a burningwind turbine (La voz de Galicia 2009). The authors couldn’t find confirmation of thisparticular accident from other sources; nevertheless the risk of fires triggered asconsequence of burning turbine seems reasonable, due to the duration of these fires,the high wind of locations and the possibility that blades remain in motion at least inthe first phase of the fire. Particularly in this case, sparkles and hot carbons can bethrown at a very long distance from the original flame and lead to other unexpectedfires.MEASURES FOR STRUCTURAL INTEGRITYAs mentioned before, the resistance of structures to exceptional actions ispresent in many recent regulations but is often not supported with a comprehensivedescription of feasible methods to improve and verify this requirement. This is truefor all construction types in general but in particular concerns the design of windturbine, which is gaining only recently development and specific attention from thepoint of view of design and regulations.The assessment and verification of an acceptable level of structural integrityof a system is a quite difficult task, since the response of a structure to an exceptionalaction or to an abrupt failure depends on different properties of the action and of theconstruction and can hardly be evaluated without a proper distinction of all thedifferent aspects. A further difficulty for engineers and practitioners arises from thefact that the terms used for defining resistance to exceptional loads, resistance toimpacts and resistance to internal failures often differ or are not consistent amongdifferent regulations.An important task to be accomplished concerns therefore the conceptualorganization of all the properties that play a role in the structural integrity of astructure, clearly distinguishing between the properties that depends on the actionand the properties of the structural system alone (Bontempi 2007). This is animportant distinction, since the measures to improve structural integrity are verydifferent when addressed to reduce the action, the effect of the action and the effectof the failure.The reduction of the action avails non structural measures as surveillancesystem or protective barriers aimed at reducing the probability of occurrence of thecritical events itself as well as, respectively, the exposure of the structure to a criticalevent. Neither the first or the second measures seems to be easily applicable in caseof wind turbines, that are exposed to natural actions as main scope of usage and areplaced in wide and isolated areas that can hardly be protected even against ofpossible malevolent attacks aimed at damaging the energy supply of an urban area.2120Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  6. 6.  The reduction of the effects of the action concerns instead structural measuresaimed at reducing the vulnerability of the construction, here intended as the structuralresistance to direct impacts and extreme loads that directly affect some of itselements (Faber 2006).A low vulnerability of wind turbine could be obtained by a proper design ofthe structural parts: particularly, in case of a monopile or tripod system the walls ofthe shaft could be designed to resist the impact of blade fragments or ship collisions,by providing these elements with high specific local resistance.This measure seems difficult to be attainable in case of a lattice tower, wherethe resistance is committed to several slender elements. In this case, the effort couldbe aimed at reducing instead the effect of the initial failures, disregarding themodeling of the actions the caused it and focusing on the response of the structureconsequent to the complete loss or partial strength reduction of some of its elements(Giuliani 2009). The aim is that of improving the system robustness, intended here asthe sensitivity to local failure (Starossek 2005). That requires also the employment ofstructural measures, which cannot be limited to the design of single elements butcalls in question instead the behavior of the structural system as a whole.Generally speaking, two alternate and somehow antagonist strategies areconsidered for the robust design of structural system: the first strategy is aimed atproviding the system with an high redundancy, in order to allow for alternate loadpaths and redistribution of stresses, that could avoid any further damage in thestructure after the initial one. The second strategy is aimed instead at creating somepredetermined sections in the structure, where the propagation of the collapse comesto a halt (Starossek 2005). In this case the loss of a limited and predetermined area ofthe construction is accepted, in order to avoid the propagation of stresses andtherefore possible ruptures to the elements adjacent to those initially damaged by theaction.The compartmentalization of the damage can be achieved by insertion of lowconnected joints or oppositely by strengthening of some sections, as in the fuselagedesign of some plane: an example is the Aloha Boeing 737, which suffered in April1988 a service-induced damage that led to explosive decompression and loss of largeportion of fuselage skin, when small fatigue crack suddenly linked together. Thesubsequent fracture was eventually arrested by fuselage frame structure and the craftlanded safely (NTSB/AAR-89/031989). The strategy seems to be applicable also tothe monopile shaft of wind turbine, in order to avoid buckling or collapse of thetower after a crack in the shaft wall caused by design error as well as debris impact.STRUCTURAL INTEGRITY IN CASE OF SHIP IMPACTAs above mentioned, the event of a ship collision seems particularlyinteresting from the point of view of structural integrity evaluation of offshore windturbines.In the following, a monopole offshore wind turbine is chosen as example tostudy the sensitivity of structural part to an abrupt impact and evaluate thevulnerability and robustness of the structural system. In order to perform such study,the investigation has followed a top-down approach, where the action has been2121Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  7. 7.  modeled and the entity of a possible damage in the element caused by this action hasbeen investigated.The structure considered in the investigation is a typical steel structure usedfor 5-6 MW offshore wind turbine (OWT), with a monopile tower connected to fourfoundation piles by means of four diagonal legs disposed as shown in Figure 1(right). The total height of the tower is 140 m, whose 104 m above the seabed. Thesection of the monopile is a hollow circular section of S355 steel, whose diameterand thickness vary along the tower height, according to a design optimization in termof stiffness and resistance. The foundation piles deepen 40 m under sea level, whilethe upper 5 m of the piles extend over the water and provide the support for themonopile struts.A finite element model of the turbine has been developed in a currentcommercial code, as shown in Figure 1. The turbine is modeled by means of one-dimensional elements both for the legs and the tower, which are properly meshed.The rotor and the nacelle have been modeled as a pointed mass while the soilinteraction has been accounted by means of three-dimensional finite elements, whichbehave elastically and cover the zone represented in Figure 1 (left), whose extensionhas been calibrated in order to minimize the boundary effects.Figure 1. OWT finite element model: whole model with soil explicit representation(left) and naked model with water level representation (right).2122Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  8. 8.  The turbine is considered to be loaded only with self-weight at the moment ofthe impact, i.e. wind and possible wave overloading have been disregarded. Thisassumption seems to be reasonable with respect to the specific investigation, which isaimed at identifying some characteristics of the structural response and not a specificresistance value, which also depends on the realistic resistance of the materials andon the actual loads acting on the structure at the moment of the impacts.The element material is modeled with an elastic-plastic behavior, which usesa value of yielding stress equal to 355 MPa and an ultimate strength equals to 510MPa for the considered S355 steel.Performed investigations. In order to assess the vulnerability of the structure, theimpact of the ship is modeled by means of an impulsive force acting on the pointconsidered for the collision.The value considered for the force is 7 MN (around 700 t) and the impulsivefunction has a total length of 2 seconds, divided in an initial and final ramp of 0.5seconds and a central constant phase of 1 second.Three different impact scenarios are considered:A. impact on one of the leg under the sea level (model node #17);B. impact at the sea level (model node #38);C. impact on the tower above the sea level (model node #548).The nonlinear dynamic analysis is developed considering large displacementsand large deformations together with plastic material behavior.The outcomes of the performed investigation show that the structure isdamaged by an impact on one of the legs (scenario A).The trend of displacements during time is reported in Figure 2 with respect tothe horizontal direction for three nodes in the zone of the impact (top images) and forthe node at the top of the tower (central images). It can be seen that the displacementof one node of the leg becomes abruptly very high few instants after the impact (ca.half second), while the other nodes monitored in the support maintain an elasticbehavior.The maximum moment developed in the leg sections is also represented inFigure 2 (bottom image) with respect to the curvature of the section. The elasticmoment resistance has been overcome and a final irreversible deformation is evidentfor the considered section.Main global results for scenario A are shown in Figure 3, where the nodaldisplacement evolution with time is represented: considering that the deformedstructural configuration is represented in real scale in the images (i.e. nodisplacement amplification has been used), it’s evident that after 3 seconds, thedeformations reached by the node of the impacted leg are very high and anirreversible damage has developed in the leg.Conversely, investigations carried on for scenarios A and B didn’t show anydamage in the structural system, as can be seen in Figure 4, where the nodaldisplacements 3 seconds after the impact are reported for scenarios B and C. In bothcases an essentially elastic behavior, without evident structural damage, can berecognized by the observation of the deformed configuration, which is alwaysrepresented in a unitary scale.2123Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  9. 9.  Figure 2. Scenario A: displacement for the impacted zone and for the top tower node;moment-curvature diagram for the impacted beam (element #89).2124Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  10. 10.  SCENARIOA:Impanctonturbinelegt = 0.025 s t = 0.300 st= 0.500 s t = 0.800t =1.200 s t = 3.000 sOBS: The scale used for representing displacements in the figures is unitaryFigure 3. evolution during time of structural damage for scenario A.2125Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  11. 11.  SCENARIO B: impact on the tower SCENARIO C: impact on the towert = 3.000 s t = 0.300 sOBS: The scale used for representing displacements in the figures is unitaryFigure 4. structural damage after 3 seconds for scenario B (left) and C (right).Consideration on the outcomes. In case of an impact on the support zone, thestructure develops irreversible deformations in the impacted legs, which leads to anoverloading of adjacent structural elements of the support and the pile opposite to theconsidered leg. The damage though seems to remain localized to the zone directlyaffected by the impact and the global response of the tower remains essentiallyelastic, as can be seen by observing the horizontal displacement of the node at the topof the tower, reported in the central image of Figure 2.The structure remains damaged after the impact and costs will be incurred forrepairing or substitution of the damaged parts, as well as for the interruption ofturbine operation. Still the rotor and the nacelle of the turbine are preserved integerand could be immediately reused. This aspect is particularly important, consideringthat these components represents the highest cost item on most machines and theirreliability is therefore very important.This result is even more significant when considering that the structuralsystem is formed by a relative low number of elements. The damage of one of the legrepresents therefore a failure of a significant portion of the support system.It has to be noticed that the elements composing the support system andespecially the four upper legs are highly exposed to collisions and other possibleimpacts (e.g. fragments of blade failure). Therefore, even if the response of structuralsystem seems not to be disproportionate to the modeled impact, some propermeasures could be considered in order to further improve the structural integrity ofthe turbine.2126Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  12. 12.  In this respect, some structural design modification could be addressed toincrease the specific local resistance of the legs, while a lower exposure of theseelements could be obtained by moving the supporting sub-structure deeper in thewater or protecting the legs with barriers (non structural measures). Sacrificialstructures, properly designed to stop ships and protect main structural elements areoften used for protection of bridge piers and could be considered also in this case: forexample, the ship impact protection of the Inchon Bridge in Seoul, Korea, isprovided in the form of dolphin-shaped structures disposed around the piers anddesign to stop a vessel by dissipating energy throughout various mechanisms (Kim2007).Further studies. Further studies could be address at investigating the sensitivity ofthe system to other kind of local damage in the support substructure as well as in thetower, that could be caused by impacts of different or greater intensity but also bydifferent cause, like for example the corrosion of the immersed tower wall, that canreduce locally the resistance of the tower and lead to a degradation of theperformance of the whole structure or maybe even to the propagation of failures.In this case, a bottom-up approach could be instead used for investigating thestructural response: as better explained above, initial failures should be then assumedin the structural system, disregarding the explicit modeling of the action that couldhave caused those failures. If several initial damages are considered and thestructural response is separately evaluated in each case, a more quantitativeassessment of the structural robustness could be obtained, by comparing theperformance of the considered damaged structures. For example, a differentdegradation of stiffness could be considered at several locations along the towerheight or in the supporting legs and the response of the tower in terms of loadbearing capacity of the whole system could be evaluated. A probabilisticoptimization, which avails e.g. simulating annealing techniques, could be used inorder to account for the high number of damage conditions and perform a feasiblenumber of analyses, as described in (Giuliani, 2009).The comparison of the results in term of degradation of structuralperformance corresponding to greater damage levels can provide for a direct measureof the structural robustness and suggest possible design modifications aimed atreducing the effects of local failures, to be considered in addition to those abovementioned, which were instead aimed at reducing the effect of the action on the mostvulnerable structural parts.CONCLUSIONIn this paper the structural properties of wind turbine are discussed, whichaffect the response of the system to exceptional actions such as the collision of a shipon an offshore turbine.A finite element model of an offshore wind turbine has been implemented,which accounts for an explicit modeling of the ground and foundations as well as forthe plastic behavior of the material and a full geometrically nonlinear formulation ofthe structural elements. Three different positions have been considered for the shipcollision, which has been modeled by means of a pointed dynamic force. The2127Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  13. 13.  response of the structure following this impact has been evaluated by means ofnonlinear dynamic analyses.The outcomes of the performed investigation show a low vulnerability of thetower, which resists elastically to the collision, and a satisfactory robustness of thewhole system, whose global behavior seems not significantly compromised by apossible local damage in one of the 4 couple of legs that supports the tower.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 of RomeLa Sapienza.REFERENCESASCE 7-02: “Minimum design loads for buildings and other structures”, AmericanSociety of Civil Engineers, Reston, VA, 2002Bontempi F., Giuliani L., Gkoumas K.: “Handling the exceptions: dependability ofsystems and structural robustness”(invited lecture), 3rd internationalconference on structural engineering, mechanics and computation (SEMC2007), Cape Town, South Africa, 10-12 September 2007.Copenhagen Post Online, Buisness section, 3 November 2009(http://www.cphpost.dk, last visited 16th Nov. 2009)EN 1991-1-7: 2006, “Actions on structures”, Eurocode 1, Part 1-7: General actions -Accidental actions., Comité Européen de Normalisation (CEN).Faber F.: “Robustness of structures: an introduction”, Structural Engineeringinternational, SEI Vol. 16, No. 2. May 2006Giuliani L.: “Structural integrity: robustness assessment and progressive collapsesusceptibility”, Ph.D. dissertation, University of Rome “La Sapienza”, Italy,April 2009GSA Guidelines - Progressive Collapse Analysis and Design Guidelines for NewFederal Office Buildings and Major Renovation Projects, June 2003.Kim J.H., Kim Z.C., Shin H.Y., Cho S.M., Schaminée P.E.L., Gluver H.: "Centrifugetesting for the design of ship impact protection of Incheon Bridge Project",Proc. of the 16 International Offshore and Polar Engineering Conference,Lisbon, Portugal, July 1-6, 2007La voz de Galicia: “Una avería en un aerogenerador originó unfuego forestal en Muros”, 20 Sept. 2009, in Spanish(http://www.lavozdegalicia.es/hemeroteca/2006/09/20/5125652.shtml, lastvisited 16th Nov. 2009)NTSB/AAR-89/03: “Aircraft accident report”, National Transportation Safety Board,Washington D.C., June 14, 1989OCT Guidelines “Rules and guidelines, IV Industrial Services, Part 1: Ocean CurrentTurbines Guideline for the Certification of Ocean Energy Converters”,Germanischer Lloyd WindEnergie 2005Offshore Standard, Det Norske Veritas, DNV-OS-J101, Design of Offshore Wind2128Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE
  14. 14.  Turbine Structures, June 2004Overgaard L.C.T., Lund E.: “Structural Design Sensitivity Analysis andOptimization of Vestas V52 Wind Turbine Blade”, 6th World Congress onStructural and Multidisciplinary Optimization, Rio de Janeiro, Brazil, 30 May- 03 June 2005Starossek U., Wolff M.: “Design of collapse-resistant structures”, presented at 2005Workshop “Robustness of Structures” organized by the JCSS & IABSE WC1, Garston, Watford, UK November, 28-29, 2005.Tarp-Johansen N.J., Kozine I., Rademarkers L., Dalsgaard Sørensen J., Ronold K.:“Structural and System Reliability of Offshore Wind Turbines: An account”,report of Risø National Laboratory, Roskilde, April 2005The Northern Echo: “Car plant windfarm fire forces motorists off A19”, This is theNorth East, Archive, Saturday 24th December 2005(http://archive.thisisthenortheast.co.uk, last visited 16th Nov. 2009)VdS 3523en: 2008-07 (01): “Wind turbines - Fire protection guideline” the GermanInsurance Association (GDV) and JJ Germanischer Lloyd Industrial ServicesGmbH, Business Segment Wind Energy (GL Wind), VdS Verlag, July 2007.2129Earth and Space 2010: Engineering, Science, Construction,and Operations in Challenging Environments © 2010 ASCE

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