Structural Integrity Evaluation of Offshore Wind Turbines

Structural Integrity Evaluation of Offshore Wind Turbines
L. Giuliani1
, F. Bontempi2
,
1
Structural Engineer, Ph.D., Structural and Geotechnical Engineering Department,
University of Rome ”La Sapienza”, Italy (on leave for the Technical University of
Denmark, Lyngby, Denmark); e-mail: luisa.giuliani@uniroma1.it
2
Full Professor, Structural and Geotechnical Engineering Department, University of
Rome “La Sapienza”, Italy; e-mail: franco.bontempi@uniroma1.it
ABSTRACT
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.
INTRODUCTION
Wind turbines are built in order to exploit the wind energy available in the
considered location in the most efficient and economical way. The leading design
parameters could thus be expected to depend on the external loading and on the
material strengths according to the design service and ultimate limit state
verifications. However, wind turbines have to deal often with extreme loading and
exceptional events not included in the above mentioned verifications. Protection
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Earth and Space 2010: Engineering, Science, Construction,
and Operations in Challenging Environments © 2010 ASCE

against extreme loading is provided by a control system that, among others, brings
the turbine to calm in case of high winds that could determine the turbine over-speed
and cause the failure of some components. The control system may however also be
subjected to failures or malfunctioning (Tarp-Johansen 2005) in case of improper
maintenance or system faults. Furthermore, other extreme events like impacts or ship
collisions cannot be managed by some active defense like an automatic protection
system and call instead on the passive defense of the turbine, i.e. the resistance to
failures of its structural system.
Recent codes and guidelines for onshore and offshore turbines account
therefore, even if with different approaches and terminology, for the structural
behavior of the structure under rare but extreme events and require to maintain a
certain level of structural integrity, as in accordance to most American and European
regulations for building and general structures: structural systems cannot be designed
to resist fully undamaged exceptional loads or accidents, but major damages and
disproportionate collapse should be prevented in any case (ASCE 7-02; GSA
Guidelines 2003; EN 1991-1-7 2006). With respect to wind turbines, the verification
of accidental limit state (ALS) is required in (Offshore Standard 2004), where the
turbine response consequent to the damage of one or more components has to be
evaluated, due to an accidental event or operational failure. Furthermore, the risk of
ship collision for offshore wind turbines should be specifically evaluated.
A failure mode and effects analysis (FMEA) or an equivalent analysis shall
be conducted according to (OCT Guidelines 2005) for the safety systems and for the
auxiliary and control systems needed to operate it. Aim of the analysis is to verify
that a single failure would not lead to any major damage to the structure. The
identification of possible failure conditions with a common cause is also required
together with a check on possible redundancy decrement that could imperil the
structure.
STRUCTURAL INTEGRITY OF WIND TURBINES
Old codes and regulations used to deal with the problem of structural safety
of building and other constructions, requiring the prevention of any possible
structural failure. This aim can be easily achieved by means of local resistance
verifications (e.g. at a sectional level). Nowadays instead, as previously mentioned,
also the presumption of the failure is considered necessary, in order to assure a
limited 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 it
impossible to prevent every single failure in a structure (let think to human error in
design or execution phases, as well as extreme events that could not be directly
considered in a common design), but also even a small initial failure can result in a
disproportionate structural damage. This is generally true for several types of
constructions, but is also a concern for wind turbines, as shown by past cases of
tower collapses or major turbine damage. Some of these cases are briefly reported in
the following as examples of disproportionate collapses of wind turbines.
These structures are particularly interesting from the point of view of
structural integrity, intended here as resistance to exceptional actions, due to the high
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exposure of these structure to extreme natural actions, impacts from debris or even
ship collisions in case of offshore installations. Depending on the particular system
chosen for the tower and also partly to the design optimization aimed at reducing
material costs and avoiding cumbersome sections, a high level of structural
redundancy, which could provide for alternate load path after the failure of some
structural parts and a redistribution of loads, could be difficult to obtain. On the other
hand, the intrinsic low connection of a lattice structural system for the tower could
avoid the transmission of high stresses from the initial overloaded elements to the
adjoining ones and maintain the damage limited to the zone of initial failure.
FAILURES: REAL CASES AND TYPOLOGY
Several documented cases of wind turbine accidents have been reported in the
past years, referring to different typology of failure and damages.
In the last decade almost 30 cases of blades failures and collapse have been
reported in Denmark and a law has been recently established that requires yearly
inspections 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-speed
triggered the failure of a one of the wing blades. The debris impacted on the other
wings and on the tower, which was almost sheared into half and collapsed
immediately afterwards.
Few days later, another in-land wind turbine collapsed in New York: the
company imputed the failure to the combination of power loss and the wiring
anomaly experienced by the turbine.
Again this year, wind turbines malfunction led to structural collapse in
Denmark and in Sweden within the same week (Copenhagen Post 2009). In the first
case a 120-foot turbine threw off all of its blades due to a defective axle and one of
them slammed into a power transformer. In the second case another turbine threw off
a blade that landed on a hiking trail. Afterwards, the wind turbine industry in Sweden
has 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 accidents
and damages could be useful in order to better understand and classify the failure
typologies and investigate possible countermeasures and design solutions. In the
following, 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 external
conditions. Structural failures may affect different parts of the wind turbines and
particularly blades, tower and foundations.
Blade failure can arise from a number of possible sources and results in either
whole blade or pieces of blade being thrown from the turbine. It represents one of the
most frequent turbine failures and can lead to the damage of other turbine
components impacted by the debris and eventually to the collapse of the tower, as in
the cases of the turbine collapse mentioned above. Blade failures are often imputed
to over-speed caused by control-system failure as well as fatigue failure or local
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buckling of the blade. The latter aspect is becoming particularly compelling, as blade
become larger and new materials are used (Overgaard 2005).
Tower failure may be caused by cracks in the shaft or welding failure that can
originate by faulty design or improper maintenance, especially when referring to
weld fatigue failure, and can lead to the tower buckling and collapse.
As mentioned before, the collapse of the tower is caused by indirect damage
of the shaft, hit by flying blade debris or, in case of off-shore turbine, resulting by the
impact of a ship collision. The prevention of all these kind of failures is practically
unfeasible and the resistance of the structural integrity of the tower should rely on the
low vulnerability or intrinsic robustness of the structural system. In this respect, the
different 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 impact
and resistance to local failure. Particularly interesting from this point of view is the
variation of pile section that is often obtained as result of a design optimization.
Typically, optimizations of the design are based on serviceability
requirements and material and construction costs, but do not usually consider
structural integrity issues. Even if it would probably unfeasible to account in the
design optimization for the structural response under extreme events and additional
costs for structural damages and repairs due to less robust design, still the material
optimization 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 of
the turbine (especially offshore) but is seldom reported as direct cause of tower
collapse when the turbine is in usage. Still the design of foundation of wind turbines
is becoming particularly challenging, due to the increasing hub heights and size of
turbines, which have been allowed by technological advancements in design and
execution and by the development of wind energy usage. Slab foundations of wind
turbines should therefore deal with extremely eccentric loads and overturning
moments. Design for stability usually considers a safety factor of 1.5 for both sliding
and overturning, but the occurrence of to extreme winds load is seldom accounted
and the response of the foundation to these events remains unknown.
Fire. Another type of accident that seems to affect wind turbine with a relatively
high frequency is fire that can arise from electrical failures as well as be triggered by
lightning strikes, which represent of course a peril for the high and slender
constructions of wind turbines.
A list of possible causes and examples of fire damage is reported in the
German guideline for fire protection (VdS 3523en: 2008-07), where also a review of
possible measure to reduce the risk or the consequence of onshore and offshore
turbine fire is presented. Fire may trigger in the nacelle, in the tower and in the
power 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 have
major consequence, due to the practical difficulties for firemen to reach the height of
the nacelle with ladder or water jet and extinguish the flame. In case of a total loss of
the nacelle, the restoration costs are very high and even comparable with the value of
the wind turbine. Furthermore, due to the high density of technical equipment and
combustible material in the nacelle, fire can spread rapidly and even lead to the
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collapse of the turbine, as for example occurred in the late 2005at the Nissan car park
of Sunderland, UK: a 60m tall wind turbines caught on fire, probably due to a fault
of the power pack in the concrete shaft (The northern Echo 2005) and the fire brigade
could 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 of
the fire in the area nearby the tower. A similar case seems to have occurred few years
ago in Spain, where a significant forest fire was claimed to be triggered by a burning
wind turbine (La voz de Galicia 2009). The authors couldn’t find confirmation of this
particular accident from other sources; nevertheless the risk of fires triggered as
consequence 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 in
the first phase of the fire. Particularly in this case, sparkles and hot carbons can be
thrown at a very long distance from the original flame and lead to other unexpected
fires.
MEASURES FOR STRUCTURAL INTEGRITY
As mentioned before, the resistance of structures to exceptional actions is
present in many recent regulations but is often not supported with a comprehensive
description of feasible methods to improve and verify this requirement. This is true
for all construction types in general but in particular concerns the design of wind
turbine, which is gaining only recently development and specific attention from the
point of view of design and regulations.
The assessment and verification of an acceptable level of structural integrity
of a system is a quite difficult task, since the response of a structure to an exceptional
action or to an abrupt failure depends on different properties of the action and of the
construction and can hardly be evaluated without a proper distinction of all the
different aspects. A further difficulty for engineers and practitioners arises from the
fact that the terms used for defining resistance to exceptional loads, resistance to
impacts and resistance to internal failures often differ or are not consistent among
different regulations.
An important task to be accomplished concerns therefore the conceptual
organization of all the properties that play a role in the structural integrity of a
structure, clearly distinguishing between the properties that depends on the action
and the properties of the structural system alone (Bontempi 2007). This is an
important distinction, since the measures to improve structural integrity are very
different when addressed to reduce the action, the effect of the action and the effect
of the failure.
The reduction of the action avails non structural measures as surveillance
system or protective barriers aimed at reducing the probability of occurrence of the
critical events itself as well as, respectively, the exposure of the structure to a critical
event. Neither the first or the second measures seems to be easily applicable in case
of wind turbines, that are exposed to natural actions as main scope of usage and are
placed in wide and isolated areas that can hardly be protected even against of
possible malevolent attacks aimed at damaging the energy supply of an urban area.
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The reduction of the effects of the action concerns instead structural measures
aimed at reducing the vulnerability of the construction, here intended as the structural
resistance to direct impacts and extreme loads that directly affect some of its
elements (Faber 2006).
A low vulnerability of wind turbine could be obtained by a proper design of
the structural parts: particularly, in case of a monopile or tripod system the walls of
the 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, where
the resistance is committed to several slender elements. In this case, the effort could
be aimed at reducing instead the effect of the initial failures, disregarding the
modeling of the actions the caused it and focusing on the response of the structure
consequent 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 as
the sensitivity to local failure (Starossek 2005). That requires also the employment of
structural measures, which cannot be limited to the design of single elements but
calls in question instead the behavior of the structural system as a whole.
Generally speaking, two alternate and somehow antagonist strategies are
considered for the robust design of structural system: the first strategy is aimed at
providing the system with an high redundancy, in order to allow for alternate load
paths and redistribution of stresses, that could avoid any further damage in the
structure after the initial one. The second strategy is aimed instead at creating some
predetermined sections in the structure, where the propagation of the collapse comes
to a halt (Starossek 2005). In this case the loss of a limited and predetermined area of
the construction is accepted, in order to avoid the propagation of stresses and
therefore possible ruptures to the elements adjacent to those initially damaged by the
action.
The compartmentalization of the damage can be achieved by insertion of low
connected joints or oppositely by strengthening of some sections, as in the fuselage
design of some plane: an example is the Aloha Boeing 737, which suffered in April
1988 a service-induced damage that led to explosive decompression and loss of large
portion of fuselage skin, when small fatigue crack suddenly linked together. The
subsequent fracture was eventually arrested by fuselage frame structure and the craft
landed safely (NTSB/AAR-89/031989). The strategy seems to be applicable also to
the monopile shaft of wind turbine, in order to avoid buckling or collapse of the
tower after a crack in the shaft wall caused by design error as well as debris impact.
STRUCTURAL INTEGRITY IN CASE OF SHIP IMPACT
As above mentioned, the event of a ship collision seems particularly
interesting from the point of view of structural integrity evaluation of offshore wind
turbines.
In the following, a monopole offshore wind turbine is chosen as example to
study the sensitivity of structural part to an abrupt impact and evaluate the
vulnerability and robustness of the structural system. In order to perform such study,
the investigation has followed a top-down approach, where the action has been
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modeled and the entity of a possible damage in the element caused by this action has
been investigated.
The structure considered in the investigation is a typical steel structure used
for 5-6 MW offshore wind turbine (OWT), with a monopile tower connected to four
foundation 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. The
section of the monopile is a hollow circular section of S355 steel, whose diameter
and thickness vary along the tower height, according to a design optimization in term
of stiffness and resistance. The foundation piles deepen 40 m under sea level, while
the upper 5 m of the piles extend over the water and provide the support for the
monopile struts.
A finite element model of the turbine has been developed in a current
commercial 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 soil
interaction has been accounted by means of three-dimensional finite elements, which
behave elastically and cover the zone represented in Figure 1 (left), whose extension
has 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).
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The turbine is considered to be loaded only with self-weight at the moment of
the impact, i.e. wind and possible wave overloading have been disregarded. This
assumption seems to be reasonable with respect to the specific investigation, which is
aimed at identifying some characteristics of the structural response and not a specific
resistance value, which also depends on the realistic resistance of the materials and
on the actual loads acting on the structure at the moment of the impacts.
The element material is modeled with an elastic-plastic behavior, which uses
a value of yielding stress equal to 355 MPa and an ultimate strength equals to 510
MPa for the considered S355 steel.
Performed investigations. In order to assess the vulnerability of the structure, the
impact of the ship is modeled by means of an impulsive force acting on the point
considered for the collision.
The value considered for the force is 7 MN (around 700 t) and the impulsive
function has a total length of 2 seconds, divided in an initial and final ramp of 0.5
seconds 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 displacements
and large deformations together with plastic material behavior.
The outcomes of the performed investigation show that the structure is
damaged by an impact on one of the legs (scenario A).
The trend of displacements during time is reported in Figure 2 with respect to
the horizontal direction for three nodes in the zone of the impact (top images) and for
the node at the top of the tower (central images). It can be seen that the displacement
of 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 elastic
behavior.
The maximum moment developed in the leg sections is also represented in
Figure 2 (bottom image) with respect to the curvature of the section. The elastic
moment resistance has been overcome and a final irreversible deformation is evident
for the considered section.
Main global results for scenario A are shown in Figure 3, where the nodal
displacement evolution with time is represented: considering that the deformed
structural configuration is represented in real scale in the images (i.e. no
displacement amplification has been used), it’s evident that after 3 seconds, the
deformations reached by the node of the impacted leg are very high and an
irreversible damage has developed in the leg.
Conversely, investigations carried on for scenarios A and B didn’t show any
damage in the structural system, as can be seen in Figure 4, where the nodal
displacements 3 seconds after the impact are reported for scenarios B and C. In both
cases an essentially elastic behavior, without evident structural damage, can be
recognized by the observation of the deformed configuration, which is always
represented in a unitary scale.
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Figure 2. Scenario A: displacement for the impacted zone and for the top tower node;
moment-curvature diagram for the impacted beam (element #89).
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SCENARIOA:Impanctonturbineleg
t = 0.025 s t = 0.300 s
t= 0.500 s t = 0.800
t =1.200 s t = 3.000 s
OBS: The scale used for representing displacements in the figures is unitary
Figure 3. evolution during time of structural damage for scenario A.
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SCENARIO B: impact on the tower SCENARIO C: impact on the tower
t = 3.000 s t = 0.300 s
OBS: The scale used for representing displacements in the figures is unitary
Figure 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, the
structure develops irreversible deformations in the impacted legs, which leads to an
overloading of adjacent structural elements of the support and the pile opposite to the
considered leg. The damage though seems to remain localized to the zone directly
affected by the impact and the global response of the tower remains essentially
elastic, as can be seen by observing the horizontal displacement of the node at the top
of the tower, reported in the central image of Figure 2.
The structure remains damaged after the impact and costs will be incurred for
repairing or substitution of the damaged parts, as well as for the interruption of
turbine operation. Still the rotor and the nacelle of the turbine are preserved integer
and could be immediately reused. This aspect is particularly important, considering
that these components represents the highest cost item on most machines and their
reliability is therefore very important.
This result is even more significant when considering that the structural
system is formed by a relative low number of elements. The damage of one of the leg
represents therefore a failure of a significant portion of the support system.
It has to be noticed that the elements composing the support system and
especially the four upper legs are highly exposed to collisions and other possible
impacts (e.g. fragments of blade failure). Therefore, even if the response of structural
system seems not to be disproportionate to the modeled impact, some proper
measures could be considered in order to further improve the structural integrity of
the turbine.
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In this respect, some structural design modification could be addressed to
increase the specific local resistance of the legs, while a lower exposure of these
elements could be obtained by moving the supporting sub-structure deeper in the
water or protecting the legs with barriers (non structural measures). Sacrificial
structures, properly designed to stop ships and protect main structural elements are
often used for protection of bridge piers and could be considered also in this case: for
example, the ship impact protection of the Inchon Bridge in Seoul, Korea, is
provided in the form of dolphin-shaped structures disposed around the piers and
design to stop a vessel by dissipating energy throughout various mechanisms (Kim
2007).
Further studies. Further studies could be address at investigating the sensitivity of
the system to other kind of local damage in the support substructure as well as in the
tower, that could be caused by impacts of different or greater intensity but also by
different cause, like for example the corrosion of the immersed tower wall, that can
reduce locally the resistance of the tower and lead to a degradation of the
performance 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 the
structural response: as better explained above, initial failures should be then assumed
in the structural system, disregarding the explicit modeling of the action that could
have caused those failures. If several initial damages are considered and the
structural response is separately evaluated in each case, a more quantitative
assessment of the structural robustness could be obtained, by comparing the
performance of the considered damaged structures. For example, a different
degradation of stiffness could be considered at several locations along the tower
height or in the supporting legs and the response of the tower in terms of load
bearing capacity of the whole system could be evaluated. A probabilistic
optimization, which avails e.g. simulating annealing techniques, could be used in
order to account for the high number of damage conditions and perform a feasible
number of analyses, as described in (Giuliani, 2009).
The comparison of the results in term of degradation of structural
performance corresponding to greater damage levels can provide for a direct measure
of the structural robustness and suggest possible design modifications aimed at
reducing the effects of local failures, to be considered in addition to those above
mentioned, which were instead aimed at reducing the effect of the action on the most
vulnerable structural parts.
CONCLUSION
In this paper the structural properties of wind turbine are discussed, which
affect the response of the system to exceptional actions such as the collision of a ship
on 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 for
the plastic behavior of the material and a full geometrically nonlinear formulation of
the structural elements. Three different positions have been considered for the ship
collision, which has been modeled by means of a pointed dynamic force. The
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response of the structure following this impact has been evaluated by means of
nonlinear dynamic analyses.
The outcomes of the performed investigation show a low vulnerability of the
tower, which resists elastically to the collision, and a satisfactory robustness of the
whole system, whose global behavior seems not significantly compromised by a
possible local damage in one of the 4 couple of legs that supports the tower.
ACKNOWLEDGEMENTS
The present work has been developed within the research project “SICUREZZA ED
AFFIDABILITA' DEI SISTEMI DELL'INGEGNERIA CIVILE: IL CASO DELLE
TURBINE EOLICHE OFFSHORE", C26A08EFYR financed by University of Rome
La Sapienza.
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Structural Integrity Evaluation of Offshore Wind Turbines

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