Dependability of Offshore Wind Turbines

Dependability of Offshore Wind Turbines
F. Bontempi1
, M. Ciampoli2
, S. Arangio3
1
Professor, University of Rome La Sapienza, School of Engineering, Via Eudossiana
18, 00184 Rome, ITALY; email: franco.bontempi@uniroma1.it
2
Associate Professor, University of Rome La Sapienza, School of Engineering, Via
Eudossiana 18, 00184 Rome, ITALY; email: marcello.ciampoli@uniroma1.it
3
Associate Researcher, University of Rome La Sapienza, School of Engineering, Via
Eudossiana 18, 00184 Rome, ITALY; email: stefania.arangio@uniroma1.it
ABSTRACT
In recent years more and more demanding structures are designed, built and operated
to satisfy the increasing needs of the Society. This kind of structures can be denoted
as complex ones. Among large constructions arrangements, Offshore Wind Turbines
(OWT) are definitely complex structural systems, being this complexity related to
different aspects such as hard nonlinearities, wide uncertainties and strong
interactions, either among the single parts or between the whole structure and the
design environment.
On the whole, the quality of a complex system is denoted by the idea of
dependability, while for a structure the performances are connected to the property of
structural integrity, considered as the completeness and consistency of the structural
configuration. Even if these concepts have been originally developed, respectively, in
computer science and for aerospace applications they can be applied to other high
performance systems as OWT.
The present paper will show some specific aspects of the modern approach
for the design and the analysis of complex structural systems. In the first part of the
paper, the general aspects are recalled like the System Engineering approach and the
Performance-based Design. Attention is devoted to some important aspects, such as
the structure breakdown and the safety and performance allocations. In the second
part of the paper, a basic application of the concepts introduced is presented.
INTRODUCTION
In recent years more and more demanding structures are designed, built and operated
to satisfy the needs of the Society. This kind of structures can be denoted as complex
systems. OWT arranged in offshore wind farms are examples of such systems (Hau,
2006).
A roadmap for the analysis and the design of complex structural systems is
shown in Figure 1 and in this section one will explain the terms there introduced.
2001
Earth and Space 2010: Engineering, Science, Construction,
and Operations in Challenging Environments © 2010 ASCE

STRUCTURAL
SYSTEM
INTERACTION AMONG
DIFFERENT
STRUCTURAL PARTS
INTERACTION
BETWEEN THE WHOLE
STRUCTURE AND THE
DESIGN ENVIRONMENT
Interactions are
characterized by strong
character, nonlinearity
and uncertainty
QUALITY
ON THE WHOLE
FOR THE
STRUCTURAL
SYSTEM:
DEPENDABILITY
ATTRIBUTES
THREATS
MEANS
STRUCTURAL INTEGRITY
COMPLEXITY
DECOMPOSITION
STRATEGY
PERFORMANCE
BASED DESIGN
SYSTEM
APPROACH
Figure 1. Roadmap for the analysis and the design of complex structural
systems.
Among other definitions, with the term system one can consider an organized
assembly of elements or components united and regulated by interaction or
interdependence to accomplish a set of specific functions. Speaking about structural
systems, elements can be imagined, for example, as beams or columns while
restraints or control devices can be addressed as structural components: it is apparent
that the distinction resides in the fact that the first ones can be closely handled by
structural theories while for the second ones only approximated or phenomenological
laws can be used (Bontempi, 2006).
2002

The adjective complex in general usage tends to be used to characterize
something with many parts with an intricate arrangement: more precisely, there is a
distinction between a system composed by many elements that can eventually results
simple, and then non complex, and a system composed with a relatively low number
of parts but connected in a non simple way: in this case it is just the kind of
relationships among components that furnishes the complexity character to the
system. Finally, the emergence of not obvious behavior at a whole system level is
generally believed as a hint of complexity (Bontempi et al., 2007)
Among large constructions, OWT are definitely complex structural systems
for which the ambient load can significantly influence the structural behavior. Their
complexity is related to different aspects such as hard mechanical and geometrical
nonlinearities, wide characteristics and behavior uncertainties and strong interaction
either among the parts below the complete structure level or between the structure as
a whole and the design environment.
Only considering these aspects, a consistent evaluation of the structural
performance can be obtained and then a suitable design achieved. This requires
evolving from a simplistic idealization of the structure as device for channeling loads
to the idea of the structural system, intended as a set of interrelated components
working together toward a common purpose (NASA System Engineering Handbook,
2007), and acting according to the techniques of System Engineering, that is the
tough approach to the creation, the design, the realization and the operation of an
engineered system.
If the previous reflections can be considered, in some sense, extraneous to the
Civil Structural Engineering tradition, it is interesting to observe that a matching
approach has been formalized in recent years for civil engineering demands: one is
thinking to the so called Performance-based Design (Smith, 2001), by which the
structural performance during the whole service life of the constructions are
explicated and assessed. For this purpose, the advanced technologies for processing
data collected on site on real structures shall be properly taken into account, both for
checking the accomplishment of the expected performance during the service life,
and for validating the original design (Berthold & Hand, 1999).
On the whole, all the performance of a structure can be connected to the
property of structural integrity, considered as the completeness and consistency of
the structural configuration. Even if this concept has been developed in aerospace
applications (Grandt, 2004) its principle can be applied to other high performance
systems as OWT systems. In particular, structural integrity monitoring represents an
essential tool for the assessment of existing structural systems because it integrates,
in a unified perspective, advanced engineering analyses and experimental data: it is
based on the structural identification concepts, employing instrumented monitoring
as principal experimental tool (Bontempi et al, 2008).
Coupled with the idea of integrity monitoring are the concepts of fault
detection and diagnosis. After the detection that something is changed in the
structural behavior, the diagnosis is a process that identifies cause-effect
relationships through a combination of simulations and heuristics, to understand how
defects, damage and deterioration mechanisms may affect structural performance and
reliability at different limit states (Arangio and Beck, 2009). A thorough
2003

understanding of the concepts of structural identification, fault detection and
diagnosis, hold the key to the maintenance of structural dependability, intended as
the overall good performance of the structure.
The original definition of dependability is the ability to deliver service that
can justifiably be trusted (Avizienis et al, 2004). This definition stresses the need for
justification of trust. The alternate definition considers dependable a system that has
the capability to avoid service failures which are more frequent and more severe than
acceptable. Of course, the dependability of a structural system must be considered in
the design phase: it is starting from the design problem formulation and passing to
the conceptual design that the structural quality can be addressed and the proper
resources allocated.
The present paper will show some specific aspects of this modern approach
for the design and the analysis of complex structural systems. In the first part of the
paper, the general aspects are recalled. Specific attention is devoted to the System
Engineering approach and to the Performance-based Design. The second part of the
paper is related to the dependability assurance for an example of OWT structural
systems with reference to the investigation of loss of structural integrity with
increasing level of demand.
COMPLEX DESIGN PROBLEM
Universally, design of structural systems requires three dominant aspects to be
optimized, generally described as the Cost, Time and Performance factors (CPT).
Attempting to optimize all three factors simultaneously is a very difficult
task; however, the adoption of improved system processes seems to significantly
improve all three at the same time. In fact, the objective of a System Approach is that
the system is designed, built and operated accomplishing its purpose in the most
cost-effective way possible. It means that a cost-effective system must provide a
particular kind of balance between effectiveness and cost: the system must provide
the most effectiveness for the resources expended or, equivalently, it must be the
least expensive for the effectiveness it provides. This condition is a weak one,
because there are usually many designs that meet the constraints (NASA System
Engineering Handbook 2007).
Each possible design can be represented as a point in the trade-off space
between effectiveness and cost. A graph, plotting the maximum achievable
effectiveness of available design with current technology as a function of cost, would
in general yield a curved line such as the one shown in the Figure 2: the curved line
represents the envelope of the currently available technology in terms of cost-
effectiveness. In addition, this curve shows the saturation effect that is usually
encountered approaching the highest levels of performances. Points above the line
cannot be achieved with currently available technology and they represent currently
unachievable designs: some of these points may be feasible in the future when
further technological advances will be made.
Points inside the envelope are feasible, but are dominated by designs whose
combined cost and effectiveness lie on the envelope: in fact, considering the starting
point D0 for the design inside the envelope, there are alternatives which reduce costs
2004

without decreasing any aspect of effectiveness (design point D1) or that increase
some aspects of effectiveness without decreasing other or without increasing costs
(design point D2). For these reasons, the projects represented by points on the
envelope are called cost-effective solutions.
The process of finding the most cost-effective design is additionally
complicated by the influence of uncertainty: the exact outcomes achieved by a
particular system design cannot be surely known in advance, so the cost and the
effectiveness of a design are better described by a probability distribution than by a
point. With reference to Figure 1, distributions resulting from a simple design which
has little uncertainty are dense and highly compact, as is shown for concept A, while
distributions associated with risky designs may have significant probabilities of
producing highly undesirable outcomes, as it is suggested by the presence of an
additional low effectiveness/high cost cloud for concept C.
Cost
Effectiveness
A
B
C
All possible designs produce results
in this portion of the trade space
There are no designs that produce results
in this portion of the trade space
D2
D0
D1
Figure 2. Uncertainty in the cost-effective solutions (adapted from NASA
System Engineering Handbook, 2007)
DEPENDABILITY
For complex structural systems, as here in the case of large scale projects like OWT
farms, where there are significant dependencies among elements or subsystems, it is
important to have a solid knowledge of both how the system works as a whole, and
how the elements behave singularly.
In this contest, dependability is a global concept that describes the aspects
assumed as relevant with regards to the quality performance and its influencing
factors (Bentley, 1993).
The original definition of dependability is the ability to deliver service that
can justifiably be trusted. This definition stresses the need for justification of trust.
2005

The alternate definition considers dependable a system that has the capability to
avoid service failures that are more frequent and more severe than acceptable.
Also if this concept was initially developed in the realm of the Computer
Science, it may be translated into the Structural Engineering field with the aid of
Figure 3: here it is shown that system dependability can be thought of as being
composed of three elements: attributes, threat, and means (Avizienis et al, 2004).
Briefly,
a) attributes can leads to objective measure of the dependability of the
structures;
b) threats are things that can undermine the dependability of the
structures;
c) means represent ways to increase the dependability of the structures.
The main attributes can be subdivided into high level, or active, performance
(reliability, availability, maintainability) and low level, or passive, (safety, security
and integrity) (Petrini et al, 2008).
The threats for system dependability can be subdivided into failure, errors
and faults. The failure represents a permanent interruption of a system ability to
perform a required function under specified operating conditions. In case of error, the
system is in an incorrect state: it may or may not cause failure. On the other hand, a
fault is a defect and represents a potential cause of error, active or dormant. In case
of civil structures, the possible faults are damage, considered as a specific event, and
deterioration, considered as a continuous process. Within this framework, effective
methods for fault detection and diagnosis are essential as means to assure the
dependability assessment (Isermann, 2006; Nelles, 2001).
These aspects are strictly related to the integrity monitoring of the structural
system. An efficient integrity monitoring system is expected to be able to proactively
preserve the structural dependability, diagnosing deterioration and damage at their
onset (Li & Ou, 2006). The circumstances that may eventually lead to deterioration,
damage and unsafe operation may be diagnosed and mitigated in a timely manner, so
that costly replacement can be avoided or delayed by effective preventive
maintenance.
STRUCTURAL PROBLEM BREAKDOWN
Generally speaking about the process of searching the solution of the structural
problem, it is important to recognize that the way in which one describes the object
of investigation influences how one organizes the knowledge and the decision about
the object itself (Simon, 1998). As anticipated by the top-down approach, Figure 4
shows how to deploy a system: one starts from the definition of the global structure
and then, subsequently and orderly, develops further magnification of the
description.
The differentiation of the modeling level is adopted to reduce the
uncertainties. The level of a generic model of the structure is here identified by
means of two parameters: the maximum degree of detail and the scale of the model;
if the finite element method is adopted, at each model level it is possible to associate
a certain typology of finite element which is mainly used to build the model.
2006

Figure 3. The overall framework for dependability and its components.
ATTRIBUTES
THREATS
MEANS
RELIABILITY
FAILURE
ERROR
FAULT
FAULTTOLERANT
DESIGN
FAULTDETECTION
FAULTDIAGNOSIS
FAULTMANAGING
DEPENDABILITY
of
STRUCTURAL
SYSTEMS
AVAILABILITY
SAFETY
MAINTAINABILITY
permanentinterruptionofasystemability
toperformarequiredfunction
underspecifiedoperatingconditions
thesystemisinanincorrectstate:
itmayormaynotcausefailure
itisadefectandrepresentsa
potentialcauseoferror,activeordormant
INTEGRITY
waystoincrease
thedependabilityofasystem
Anunderstandingofthethings
thatcanaffectthedependability
ofasystem
Awaytoassess
thetrustworthiness
ofasystemwhichallows
reliancetobejustifiablyplaced
ontheserviceitdelivers
SECURITY
Highlevel/active
performance
Lowlevel/passive
performance
ATTRIBUTES
THREATS
MEANSMEANS
RELIABILITYRELIABILITY
FAILURE
ERROR
FAULT
FAULTTOLERANT
DESIGN
FAULTTOLERANT
DESIGN
FAULTDETECTIONFAULTDETECTION
FAULTDIAGNOSISFAULTDIAGNOSIS
FAULTMANAGINGFAULTMANAGING
DEPENDABILITY
of
STRUCTURAL
SYSTEMS
AVAILABILITY
SAFETY
MAINTAINABILITY
permanentinterruptionofasystemability
toperformarequiredfunction
underspecifiedoperatingconditions
thesystemisinanincorrectstate:
itmayormaynotcausefailure
itisadefectandrepresentsa
potentialcauseoferror,activeordormant
INTEGRITY
waystoincrease
Anunderstandingofthethings
thatcanaffectthedependability
ofasystem
Awaytoassess
thetrustworthiness
ofasystemwhichallows
reliancetobejustifiablyplaced
ontheserviceitdelivers
SECURITY
Highlevel/active
performance
Lowlevel/passive
performance
2007

Figure 4. Hierarchical breakdown of a system.
In general, with reference to Table 1, four model levels are defined for the OWT
systems (Bontempi et. al, 2008):
1) System level (S): the model scale comprises the whole wind farm and can be
adopted for evaluating the robustness of the overall plant; highly idealized model
components are used in block diagram simulators.
2) Macro level or Global modeling (G): in these models the scale reduces to the
single turbine structure, neglecting the connections among different structural
parts. The component shapes are modeled in an approximate way, the geometric
ratios among the components are correctly reproduced; beam finite elements are
mainly adopted;
3) Meso-level or Extended modeling (E): these models are characterized by the
same scale of the previous level but with a higher degree of detail: the actual
shape of the structural components is accounted for and the influence of
geometrical parameters on the local structural behavior is evaluated. Shell
elements are adopted for investigating the internal state of stress and strain (e.g.
for fatigue life and buckling analysis) inside the structure extrapolated from
previous models;
4) Micro level or Detail modeling (D): this kind of models are characterized by the
highest degree of detail and used for simulating the structural behavior of specific
individual components, including connecting parts, for which a complex internal
state of stress has been previously pointed out e.g. due to the presence of
concentrated loads. Shell or even solid finite elements are used.
2008

Table 1. Definition of the model levels.
Model level Scale
Maximum detail
level
Main adopted
finite elements
System level wind farm
approximate shape of
the structural
components
BLOCK elements
Macro-level single turbine
approximate shape of
the structural
components, correct
geometry
BEAM elements
Meso-level single turbine
detailed shape of the
structural components
SHELL & SOLID
elements
Micro level
individual
components
detailed shape of the
connecting parts
SHELL & SOLID
elements
PROGRESSIVE LOSS OF STRUCTURAL INTEGRITY
One will considers the OWT support structure shown in Figure 5 with the
following main information:
− water level: 35 m;
− height of the structure above water level: 105 m;
− pile length under sea bed: 40 m;
− steel: S355;
− turbine: 5/6 MW.
For this structure, as usual, ULS (Ultimate Limit State) requirements are verified:
this situation, shown on the left of Figure 5, is conventionally associated here with a
load multiplier λ=1.00. In consideration of the economic and strategic value of a
wind farm facility made by a large number of these kinds of structures, it is
interesting to assess the ability of the system to sustain further levels of demand up to
extreme loading conditions. The survivability of the structure is then investigated,
allowing large damage developing inside the structural system: it means that the
spread of the plasticity inside the structure is allowed, until the last configuration of
equilibrium is reached. Of course, nonlinear analysis that accounts for material
plasticity and large displacements is needed.
For the structure under examination, the last equilibrium configuration for
λ=1.44 is shown on the right of Figure 5, where the spread of plasticity is marked by
a dark color. Figure 6 shows more details in the bottom part of the support structure.
Besides these qualitative pictures, it is interesting to obtain some quantitive measure
of the progressive loss of integrity. This is obtained, in the most basic way,
considering the modal behavior of the damaged structure. In Table 2, the first three
natural frequencies of the support structure are considered for load multiplier λ
2009

ranging from 1.00 (at the ULS configuration) to the value of 1.44 that characterizes
the last obtained equilibrium situation under imposed load. These data are also
represented in the diagram of Figure 6.
By these results, one can judge the structural safety with regards to extreme
or abnormal situation, assuring the dependability of the system as extension of the
usual conventional safety assessment considered (Starossek, 2009).
Figure 5. Support structure for OWT considered: left, reference baseline
configuration under ULS loading system (λ=1.00); right, last obtained
equilibrium configuration (λ=1.44).
Table 2. Natural frequencies as function of the load multiplierλ.
load multiplier λ 1st freq. (Hz) 2nd freq. (Hz) 3rd freq. (Hz)
1.000 0.152 0.198 0.871
1.100 0.123 0.191 0.818
1.200 0.101 0.179 0.720
1.220 0.098 0.176 0.679
1.320 0.093 0.170 0.604
1.380 0.087 0.158 0.541
1.420 0.086 0.153 0.518
2010

λ = 1.00λ = 1.00 λ = 1.10λ = 1.10
λ = 1.32λ = 1.32 λ = 1.44λ = 1.44
Figure 6. Increase of damage from the reference baseline ULS configuration
(load multiplier λ=1.00) to the last equilibrium configuration (λ=1.44).
2011

0.152
0.123
0.101
0.098
0.093
0.087
0.086
0.198
0.191
0.179
0.176
0.170
0.158
0.153
0.871
0.818
0.720
0.679
0.604
0.541
0.518
0.000
0.100
0.200
0.300
0.400
0.500
0.600
0.700
0.800
0.900
1.000
1.000 1.250 1.500
load multiplier
frequencies(Hz)
1st
2nd
3rd
Figure 7 Natural frequencies as function of the load multiplier λ.
CONCLUSION
The main ideas about complex structural systems, the connected design strategy and
assessment procedures are presented in this paper. A simple application shows how
the concept of dependability can be applied to OWT support structure to investigate
the survivability of the system in the presence of extreme actions.
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.
2012

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