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

1. Advanced Topics in Offshore Wind Turbines Design
F. Bontempi1
1
Professor of Structural Analysis and Design, School of Engineering, University of
Rome La Sapienza, Via Eudossiana 18, 00184 Rome, ITALY; phone: +39-06-
44585265; email: franco.bontempi@uniroma1.it
ABSTRACT
Offshore Wind Turbine (OWT) is a relatively complex structural and mechanical
system located in a highly demanding environment. In this study the fundamental
aspects and the major issues related to the design of such structures are inquired. The
System Approach is proposed to carry out the design of the structural parts: in
accordance with this philosophy, decomposition of the system (environment,
structure, actions/loads) and of the structural performance is carried out in order to
organize the qualitative and quantitative assessments in various sub-problems. These
aspects can be faced by sub-models of different involvedness both for the structural
behavior and for the load models. Numerical models are developed accordingly to
assess safety, performance and robustness under aerodynamic and hydrodynamic
actions.
INTRODUCTION
Offshore Wind Turbine (OWT) emerges as an evolution of the onshore plants for
which the construction is a relatively widespread and consolidated practice providing
a renewable power resource (Hau, 2006). In order to make the wind generated power
more competitive with regards to conventional exhaustible and high environmental
impact sources of energy, the attention has turned toward offshore wind power
production (Breton and Moe, 2009).
Besides being characterized by a reduced visual impact, since they are placed
far away from the coast, OWT can take advantage from more constant and intense
wind forcing, something that can increase the regularity and the amount of the
productive capacity and make such a resource more cost-effective if the plant is
lifelong and operates with minimum interruption through its lifespan.
From a general point of view, an OWT is formed by both mechanical and
structural elements. As a consequence, it is not a “common” Civil Engineering
Structure: it behaves differently according to different circumstances related to the
specific functional activity (idle, power production, etc), and it is subjected to highly
variable loads (wind, wave, sea currents loads, etc.). In the design process, different
structural schemes for the supporting structure can be adopted (Figure 1), mainly
depending on the water depth which determines the hydrodynamic loads acting on
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2. the structure and drives the choice of the proper techniques for the installation and
maintenance of the support structure. Moreover, since the structural behavior of
OWTs is influenced by nonlinearities, uncertainties and interactions, they can be
defined as complex structural systems, as other recently designed constructions
(Bontempi, 2006).
seabed
transition
platform
foundation
tower
support
structure
sub-
structure
blades – rotor - nacelle
foundation
sea floor
water level
TRIPODMONOPILE JACKET
Figure 1. Main parts of OWT for different support structure typology.
The above considerations highlight that a modern approach to study such
structures has to evolve from the idea of “structure” itself, intended as a simple
device for channeling loads, to the one of “structural system”, intended as “a set of
interrelated components which interact one with the other in an organized fashion
toward a common purpose” (NASA, 2007). The consequent System Approach
includes a set of activities which lead and control the overall design, implementation
and integration of the complex set of interacting components (Simon, 1998;
Bontempi et al., 2008): here, the original definition by NASA has been extended in
such a way that the “structural system” organization contains also the actions and
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3. loads: the latter derives from, and is strictly related to, the environment (Figure 2).
A certain amount of complexity arises from the lack of knowledge and from
the modeling of the environment in which the OWT is located. In this context two
main design issues can be individuated: the consideration of the uncertainty deriving
from the stochastic nature of the environmental forces (in particular aerodynamic and
hydrodynamic) and the proper modeling of the possible presence of non linear
dynamic interaction phenomena among the different actions and between the actions
and the structure.
Figure 2. Structural system organization.
Generally speaking, the uncertainties can spread during the various analysis
phases that are developed in a cascade. The incorrect modeling of the involved
uncertainty can lead to an incorrect characterization of the structural response from a
stochastic point of view and, thus, to an improper quantification of the risk for a
given structure subjected to a specific hazard.
Having as goal the schematization of the problem and the individuation of the
uncertainty propagation mechanisms, reference can be made to the Figure 3, where
the process of the environmental actions generation is qualitatively represented also
with considerations on the involved uncertainties.
ENVIRONMENT ZONE
Structure
Non
environmental
solicitations
EXCHANGE ZONE
Wind and wave flow
Structural (non-
environmental)
system
Site-specific
environment
Wind site basic
parameters
Other
environmental
agents
Wave site basic
parameters
Wind, wave and
sea current
actions
Aerodynamic and
Aeroelastic
phenomena
Hydrodynamic
phenomena
1. Aleatoric
2. Epistemic
3. Model
Types of uncertainties
1. Aleatoric
2. Epistemic
3. Model
1. Aleatoric
2. Epistemic
3. Model
Propagation Propagation
Figure 3. Generic depiction of the uncertainties and the interaction mechanisms
in the design of OWT structure.
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4. In Figure 3, following the wind and the hydrodynamic flows impacting on the
structure, it is possible to distinguish two zones:
• Environment zone: it is the physical region, sufficiently close to the structure
to assume the same environmental site parameters of the structure, yet far
enough to neglect the flow field perturbations (in terms of particle’s
trajectories, pressure field, etc.) induced by the presence of the structure
itself. In the environment zone, the wind and the hydrodynamic flows can
interact with each other and with other environmental agents, changing their
basic parameters. The physical phenomena and uncertainties in the
environment zone propagate themselves in the neighborhood regions.
• Exchange zone: it is the physical region adjoining the structure. In this zone,
the structure itself, the wind and the hydrodynamic field experience the
mechanical interchange (aerodynamic and hydrodynamic phenomena) from
which the actions arise. In the exchange zone, some non-environmental
solicitations are present; these solicitations may change the dynamic or
aerodynamic characteristics of the original structure; so the actions are
generated considering this structural sub-system (original structure combined
with non-environmental solicitations) instead of regarding only the original
structure itself. By definition, physical phenomena and uncertainties cannot
propagate themselves from the exchange zone to the environment zone.
In general the uncertainties can be subdivided in three basic typologies:
• epistemic uncertainties (deriving from the insufficient information and the
errors in defining and measuring the previously mentioned parameters);
• aleatory uncertainties (arising from the unpredictable nature of the
magnitude, the direction and the variance of the environmental actions);
• model uncertainties (deriving from the approximations in the models).
Regarding for example the wind model and considering the turbulent wind velocity
field as a Gaussian stochastic process, an epistemic uncertainty related to the
hypothesis of gaussianity is introduced. For the sake of simplicity, the epistemic
uncertainties are not considered in this study.
The aleatoric uncertainties can be treated by carrying out a semi-probabilistic
(looking for the extreme response) or a probabilistic analysis (looking for the
response probabilistic distribution) analysis.
Finally, a possible way to reduce the model uncertainties is given by
differentiating the modeling levels. This can be carried out not only for the structural
models, but also for the action and interaction phenomena models; for this reason
different model levels are adopted.
In this way, a suitable strategy to govern the complexity is given from the
structural system decomposition, represented by the design activities related with the
classification and the identification of the structural system components, and by the
hierarchies (and the interactions) between these components. As mentioned before,
the decomposition regards not only the structure, but also the environment and the
actions and loads, and it is the subject of the first part of this study.
From the design point of view, due to the complexity of these structures one
has to adopt a Performance-Based Design philosophy: different aspects and
performance under different loading conditions (with reference to all possible system
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5. configurations that can be assumed by the blades and the rotor) have to be
investigated for this type of structures. In this framework, additional design issues
related to the structural aspects can be accommodated and are mentioned below with
some proper references:
− aerodynamic optimization (Snel, 2003);
− foundation design and soil-structure interaction (Westgate and DeJong,
2005; Ibsen and Brincker, 2004; Zaaijer, 2006);
− fatigue calculations (Veldkamp, 2007; Tempel. 2006);
− vessel impact and robustness (Biehl and Lehman, 2006);
− life cycle assessment (Martinez et al., 2009; Weinzettel et al., 2009);
− marine scour (Sumur and Fredsøe, 2002; Tempel et al., 2004);
− possible floating supports (Henderson and Patel, 2003; Jonkman and
Buhl, 2007);
− standards certification (API, 1993; BSH, 2007; DNV, 2004; GL, 2005;
IEC, 2009).
The most important issue for OWT is anyway the choice of the kind of the support
structure, related principally to the water depth, the soil characteristics and economic
issues. If the water depth (h) is considered as the major parameter, the following
rough classification can be made: monopile, gravity and suction buckets (h<25m);
tripod, jacket and lattice tower (20m<h<40÷50m); low-roll floaters and tension leg
platform (h>50m). Due to optimization processes, cross structural forms can emerge,
as the “strutted” system shown in Figures 4 and 5, with the ability of the structural
scheme to adjust different water levels (Polnikov and Manenti, 2009).
Figure 4. Scheme of strutted support structures for OWT positioned in sea with
water level ranging from 20 to 35 m.
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6. Figure 5. Main loading systems for strutted support structure for OWT.
STRUCTURAL SYSTEM DECOMPOSITION
As previously stated, the decomposition of the structural system is a fundamental
strategy for the design of complex structural systems, and it has to be performed
together with the decomposition of the performance the structure has to fulfill
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7. (Figure 6). The decomposition is carried out focusing the attention on different levels
of detail: starting from a macro-level vision and moving on towards the micro-level
details (for more details see Bontempi et al., 2008).
Figure 6. Structural system and performance decomposition of an OWT.
Decomposition of the environment. The first step of the structural system
decomposition concerns the environment. This is due to the fact that, in a global
approach, the structure is considered as a real physical object placed on an
environment where a variety of conditions, strictly related to the acting loads, should
be taken into consideration. The environment decomposition is performed in the first
column of Figure 6.
Decomposition of the structure. The second step is related to the OWT structure.
This is organized hierarchically, considering all the structural parts categorized in
three levels (second column of Figure 6):
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8. • Macroscopic (MACRO-LEVEL), related to geometric dimensions
comparable with the whole construction or parts with a principal role in the
structural behavior; the parts so considered are called macro components
which can be divided into:
− the main structure, that has the objective to carry the main loads;
− the secondary structure, connected with the structural part directly loaded
by the energy production system;
− the auxiliary structure, related to specific operations that the turbine may
normally or exceptionally face during its design life: serviceability,
maintainability and emergency.
Focusing the attention on the main structure, it consists in all the elements
that form the offshore wind turbine. In general, the following segments can
be identified:
− support structure (the main subject of this study);
− rotor-nacelle assembly.
• Mesoscopic (MESO-LEVEL), related to geometric dimensions still relevant
if compared to the whole construction but connected with specialized role in
the macro components; the parts so considered are called meso-components.
In particular the support structure can be decomposed in the following parts:
− foundation: the part which transfers the loads acting on the structure into
the seabed;
− substructure: the part which extends upwards from the seabed and
connects the foundation to the tower;
− tower: the part which connects the substructure to the rotor-nacelle
assembly.
• Microscopic (MICRO-LEVEL), related to smaller geometric dimensions and
specialized structural role: these are simply components or elements.
Decomposition of the actions and loads. The next step is the one of the actions
derived from the environmental conditions. These can be decomposed as shown in
the third column of Figure 6, from which the amount of the acting loads can be
comprehended. It is important to underline that, since the environmental conditions
in general are of stochastic nature, the magnitude of the actions involved is usually
characterized, from a statistical point of view, by a return period TR: lower values of
TR are associated with the so called “normal conditions”, while higher values of TR
are associated with “extreme conditions”.
Performance decomposition. As a final step, the performance requirements are
identified and decomposed as follows (lower part of Figure 6):
• Assurance of the serviceability (operability) of the turbine, as well as of the
structure in general. As a consequence, the structural characteristics
(stiffness, inertia, etc.) have to be equally distributed and balanced along the
structure.
• Safety assurance with respect to collapse, in probable extreme conditions; this
is applicable also to the transient phases in which the structure or parts of it
may reside (transportation and assembly), and that have to be verified as well.
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9. • Assurance of an elevated level of reliability for the entire life-span of the
turbine. As a consequence, a check of the degradation due to fatigue and
corrosion phenomenon is required.
• Assurance of sufficient robustness for the structural system, that is to assure
the proportionality between an eventual damage and the resistance capacity,
independently from the triggering cause, assuring at the same time an
eventual endurance of the structure in the hypothetical extreme conditions.
The following performance criterions can be identified for the structural system,
leading eventually to the selection of appropriate Limit States:
• Dynamic characterization of the turbine as dictated by the functionality
requirements:
− natural vibration frequencies of the whole turbine (compressive of the
rotor-nacelle assembly), the support structure and the foundations;
− compatibility of the intrinsic vibration characteristics of the structural
system with those of the acting forces and loads;
− compatibility assessment for the movement and the accelerations of the
support system for the functionality of the turbine.
• Structural behavior regarding the serviceability (SLS - Serviceability Limit
State):
− limitation of deformations;
− connections decompression.
• Preservation of the structural integrity in time:
− durability for what regards the corrosion phenomenon;
− structural behavior with respect to fatigue (FLS - Fatigue Limit State).
• Structural behavior for near collapse conditions (ULS - Ultimate Limit State):
− assessment of the solicitations, both individual and as a complex, to the
whole structural system, to its parts, its elements and connections;
− assessment of the global resistance of the structural system;
− assessment of the resistance for global and local instability phenomena.
• Structural behavior in presence of accidental scenarios (ALS - Accidental
Limit State)
− structural robustness: decrease in the load bearing capacity proportional to
the damage (see for example Starossek, 2009, and Bontempi et al., 2007);
− survivability of the structural system in presence of extreme and/or
unforeseen, situations; these include the possibility of a ship impacting
the structural system (support system or blades), with consequences
accounted for in risk scenarios.
CONCLUSION
In this paper, the System Approach has been proposed as a conceptual method for the
design of OWT structures. In this sense, structural system decomposition has been
performed, with a specific view on the structural analysis and performance. The
presented considerations aim to the organization of the framework for the basis of
design of offshore wind turbines, as a support to the decision making, with specific
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10. reference to the structural safety, serviceability and reliability for the entire lifespan.
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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