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.

1. DEPENDABILITY OF OFFSHOREDEPENDABILITY OF OFFSHORE
WIND TURBINESWIND TURBINES
Franco Bontempi, Marcello Ciampoli, Stefania Arangio
University of RomeUniversity of Rome ““La SapienzaLa Sapienza””
Department ofDepartment of
Structural and Geotechnical EngineeringStructural and Geotechnical Engineering
Honolulu, March 17th 2010

2. OUTLINEOUTLINE
Conclusions
ConclusionsConclusionsConclusionsConclusions
System approach for structural engineering applications
IntroductionIntroductionIntroductionIntroduction
The concept of dependability
PartIPartIPartIPartIPartIIPartIIPartIIPartII
Application to an offshore wind turbine support structure
Bontempi F., Ciampoli M., Arangio S. – Dependability of offshore wind turbines

3. Definition of structure
Stefania Arangio - Structural integrity monitoring of long span bridges using adaptive models
“device for channeling loads that results from the use and/or presence of the
building to the ground”
STRUCTURE
This definition does not consider some important aspects:
INTERACTION AMONG
DIFFERENT
STRUCTURAL PARTS
INTERACTION
BETWEEN THE
WHOLE STRUCTURES
AND THE DESIGN
ENVIRONMENT
Interactions are
characterized by
nonlinearities and
uncertainties
COMPLEXITY

4. Structure vs. Structural System
STRUCTURE
SYSTEM APPROACH
STRUCTURAL
SYSTEM
“a set of interrelated
components working
towards a common
purpose”
INTERACTION AMONG
DIFFERENT
STRUCTURAL PARTS
INTERACTION
BETWEEN THE
WHOLE STRUCTURES
AND THE DESIGN
ENVIRONMENT
Interactions are
characterized by
nonlinearities and
uncertainties
COMPLEXITY
Bontempi F., Ciampoli M., Arangio S. – Dependability of offshore wind turbines

5. NASA System complexity
Bontempi F., Ciampoli M., Arangio S. – Dependability of offshore wind turbines
IntroductionIntroductionPartIPartIPartIIPartIIConclusionsConclusions

6. Structure vs. Structural System
Stefania Arangio - Structural integrity monitoring of long span bridges using adaptive models
STRUCTURAL
DECOMPOSITION
Bontempi F., Ciampoli M., Arangio S. – Dependability of offshore wind turbines
SYSTEM APPROACH
STRUCTURAL
SYSTEM
INTERACTION AMONG
DIFFERENT
STRUCTURAL PARTS
INTERACTION
BETWEEN THE
WHOLE STRUCTURES
AND THE DESIGN
ENVIRONMENT
Interactions are
characterized by
nonlinearities and
uncertainties
COMPLEXITY

7. Structural decomposition
Bontempi F., Ciampoli M., Arangio S. – Dependability of offshore wind turbines
IntroductionIntroductionPartIPartIPartIIPartIIConclusionsConclusions

8. OWT Structural decomposition
SCALE
DETAIL
LEVEL
FINITE
ELEM.
SYSTEM LEVEL MACRO LEVEL MESO-LEVEL MICRO-LEVEL
Wind farm Single turbine Single turbine
Individual
components
Idealized model
components
Approximate
shape of the
components
Detailed
shape of the
components
Detailed shape of
the connections
BLOCK elements BEAM elements SHELL and
SOLID elements
SHELL and
SOLID elements

9. OUTLINEOUTLINE
Bontempi F., Ciampoli M., Arangio S. – Dependability of offshore wind turbines
System approach for structural engineering applications
IntroductionIntroductionIntroductionIntroduction
The concept of dependability
PartIPartIPartIPartIPartIIPartIIPartIIPartII
Application to an offshore wind turbine support structure

10. How is possible to define (and measure ) the quality of complex structural
systems as the OWT farms?
We need a concept able to take properly into account the different aspects
related to conceptual and structural design, construction and maintenance
during the whole lifetime.
DEPENDABILITY
It is a global concept that describes the aspects assumed as relevant with
regards to the quality performance and its influencing factors (Bentley,1993)
In rigorous terms, the dependability of a system reflects the user’s degree of
trust in that system, i.e. the user’s confidence that the system will operate as
expected and will not “fail” in normal use (Sommerville, 2000): the system
shall give the expected performance during the whole lifetime
Dependability (1)
Bontempi F., Ciampoli M., Arangio S. – Dependability of offshore wind turbines

11. Dependability (2)
DEPENDABILITY
OF STRUCTURAL
SYSTEMS
MEANS
THREATS
ATTRIBUTES
Objective measure of the
dependability
Things that can undermine
the dependability
Ways to improve the
dependability
Bontempi F., Ciampoli M., Arangio S. – Dependability of offshore wind turbines
IntroductionIntroductionPartIPartIPartIIPartIIConclusionsConclusions

12. Availability
Integrity
Safety
Reliability
Maintainability
Security
High level/ Active
performance
Low level/ Passive
performance
Attributes of the dependability
ATTRIBUTES
Bontempi F., Ciampoli M., Arangio S. – Dependability of offshore wind turbines

13. ReliabilityReliability: the system capacity of failure-free operation over a
specified time in a given environment for a given performance
(can be expressed quantitatively by a probability … )
AvailabilityAvailability: the system capacity (or readiness) at a point in time
of being operational and able to perform as required (can be
expressed quantitatively by a probability … )
MaintainabilityMaintainability: the system attribute concerned with the ease of
repairing the system after a failure has been discovered or
improving the system to include new features
Active performance
Bontempi F., Ciampoli M., Arangio S. – Dependability of offshore wind turbines
IntroductionIntroductionPartIPartIPartIIPartIIConclusionsConclusions

14. IntegrityIntegrity: absence of alterations of structural response (related to
the completeness and consistency of the structural configuration)
SafetySafety: is a property of the system that reflects its ability to
operate, normally or abnormally, without danger of causing
human injury or death and without damage to the system’s
environment (safety-related prescriptions usually exclude
undesirable situations, rather than specify required
performances)
SecuritySecurity: The system property that reflects the system’s ability to
protect itself from accidental or deliberate external attack
(robustnessrobustness)
Passive performance
Bontempi F., Ciampoli M., Arangio S. – Dependability of offshore wind turbines
IntroductionIntroductionPartIPartIPartIIPartIIConclusionsConclusions

15. Reliability and safety are related but distinct
o In general, reliability (and availability) is necessary but not sufficient
condition for system safety
o Reliability is concerned with conformance to a given specification for a
given performance
o Safety is concerned with ensuring system cannot cause damage
irrespective of whether or not it conforms to its specification
ReliabilityReliability SafetySafety
SecuritySecurity
It is an essential pre-requisite for availability, reliability and safety.
Bontempi F., Ciampoli M., Arangio S. – Dependability of offshore wind turbines
IntroductionIntroductionPartIPartIPartIIPartIIConclusionsConclusions

16. Failure
Fault
Error
Permanent interruption of the system
ability to perform a required function
under assigned operating conditions
THREATS The system is in an incorrect state: it
may or may not cause failure
Defect that represents a potential
cause of error, active or dormant
Fault managing
Fault detection
Fault diagnosis
MEANS
Fault tolerant
design
Inverse problems

17. ATTRIBUTES
THREATS
MEANS
RELIABILITY
FAILURE
ERROR
FAULT
FAULT TOLERANT
DESIGN
FAULT DETECTION
FAULT DIAGNOSIS
FAULT MANAGING
DEPENDABILITY
of
STRUCTURAL
SYSTEMS
AVAILABILITY
SAFETY
MAINTAINABILITY
permanent interruption of a system ability
to perform a required function
under specified operating conditions
the system is in an incorrect state:
it may or may not cause failure
it is a defect and represents a
potential cause of error, active or dormant
INTEGRITY
ways to increase
the dependability of a system
An understanding of the things
that can affect the dependability
of a system
A way to assess
the dependability of a system
the trustworthiness
of a system which allows
reliance to be justifiably placed
on the service it delivers
SECURITY
High level / active
performance
Low level / passive
performance
ATTRIBUTES
THREATS
MEANSMEANS
RELIABILITYRELIABILITY
FAILURE
ERROR
FAULT
FAULT TOLERANT
DESIGN
FAULT TOLERANT
DESIGN
FAULT DETECTIONFAULT DETECTION
FAULT DIAGNOSISFAULT DIAGNOSIS
FAULT MANAGINGFAULT MANAGING
DEPENDABILITY
of
STRUCTURAL
SYSTEMS
AVAILABILITY
SAFETY
MAINTAINABILITY
permanent interruption of a system ability
to perform a required function
under specified operating conditions
the system is in an incorrect state:
it may or may not cause failure
it is a defect and represents a
potential cause of error, active or dormant
INTEGRITY
ways to increase
the dependability of a system
An understanding of the things
that can affect the dependability
of a system
A way to assess
the dependability of a system
the trustworthiness
of a system which allows
reliance to be justifiably placed
on the service it delivers
SECURITY
High level / active
performance
Low level / passive
performance
Dependability
Bontempi F., Ciampoli M., Arangio S. – Dependability of offshore wind turbines

18. Analysis and design of complex structural systems
Stefania Arangio - Structural integrity monitoring of long span bridges using adaptive models
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
COMPLEXITY
DECOMPOSITION
STRATEGY
SYSTEM
APPROACH
QUALITY
ON THE WHOLE
FOR THE
STRUCTURAL
SYSTEM:
DEPENDABILITY
ATTRIBUTES
THREATS
MEANS
STRUCTURAL INTEGRITY
PERFORMANCE
BASED DESIGN
IntroductionIntroductionPartIPartIPartIIPartII
ConclusionConclusion
ss

19. OUTLINEOUTLINE
Bontempi F., Ciampoli M., Arangio S. – Dependability of offshore wind turbines
1/31
System approach for structural engineering applications
IntroductionIntroductionIntroductionIntroduction
The concept of dependability
PartIPartIPartIPartIPartIIPartIIPartIIPartII
Application to an offshore wind turbine support structure

20. Progressive loss of structural integrity
105m
35m
• Water level: 35 m
• Heigth of the structure
above water level:
105 m
• Pile length under sea bed
• Steel 355
•Turbine 5/6 MW
For wind farms with a lot of
structures it is interesting to
investigate the ability of the
system to sustain further levels of
demand after the ULS up to
extreme loading conditions
Bontempi F., Ciampoli M., Arangio S. – Dependability of offshore wind turbines
IntroductionIntroductionPartIPartIPartIIPartIIConclusionsConclusions

21. Description of the analysis (1)
The survivability of the system is investigated allowing large damage developing
inside the structural system: the spread of the plasticity is allowed until the last
configuration of equilibrium is reached
Non linear analysis:
-Material plasticity
-Large displacements
Integer structure (ULS) Loss of structural integrity
λ = 1.00 λ = 1.44

22. Increase of damage from the reference baseline ULS configuration to
the last equilibrium configuration
λ = 1.44λ = 1.00 λ = 1.32λ = 1.10
Description of the analysis (2)
Bontempi F., Ciampoli M., Arangio S. – Dependability of offshore wind turbines

23. Measuring the loss of integrity
1st freq
2nd freq
3rd freq
A quantitative measure can
be obtained considering the
modal behavior
Load multiplier
Frequencies

24. OUTLINEOUTLINE
Bontempi F., Ciampoli M., Arangio S. – Dependability of offshore wind turbines
System approach for structural engineering applications
IntroductionIntroductionIntroductionIntroduction
The concept of dependability
PartIPartIPartIPartIPartIIPartIIPartIIPartII
Application to an offshore wind turbine support structure

25. Conclusions
Offshore wind turbines (OWT) are complex structural systems
Their complexity is related to:
- nonlinearities
- uncertainties
- interaction between the parts
- interaction between the whole structure and the environmental
design
University of Rome “La Sapienza”
A global concept is needed to the define the quality of the OWT
in a comprehensive way
DEPENDABILITY

26. Conclusions (2)
The concept of dependability has been applied to OWT support
structures to investigate the survivability of the system in
presence of extreme actions
University of Rome “La Sapienza”
λ = 1.44λ = 1.00
The increasing of damage from
the reference ULS configuration
has been considered by means of
the load multiplier and considering
the modal behavior of the structure