ABSTRACT Offshore wind turbines are relatively complex structural and mechanical systems located in a highly demanding environment. Boundary conditions are intrinsically time-variable and space-dependent, both as loads and as constraints. Furthermore, different structural configurations must be handled: in fact, one has to pass from complete functionality to rotor stop. In consideration to the fact that in Italy, the construction of offshore wind farms for power production is currently under consideration, the aim of this paper is to corroborate the basis of design of offshore wind turbines, as a support to the decision making, having as a specific objective the structural design of the structure. In doing so, a systemic decomposition of the relevant elements, both physical related (e.g. the constituting parts) and due to the external conditions (that lead to the identification of the structural loads) is performed. A necessary reference to the Codes and Standards is coherently given, to develop a sound basis of design. The 4th International Conference on Advances in Structural Engineering and Mechanics (ASEM'08) Jeju, Korea, May 26-28, 2008

1. XPGw™–Œšš–™GYPGw™–Œšš–™GZPGwkGz›œ‹Œ•›G[PGwkG
Basis of Design of Offshore Wind Turbines by System Decomposition
Franco Bontempi1)
, Hui Li2)
, Francesco Petrini3)
, Konstantinos Gkoumas4)
1,3,4)
GUniversity of Rome “La Sapienza”, Via Eudossiana 18, Rome, Italy.
2)
Harbin Institute of Technology,GNo.92, West Da-Zhi Street, Harbin, Heilongjiang,
China
3)
francesco.petrini@uniroma1.it
ABSTRACT
Offshore wind turbines are relatively complex structural and mechanical systems located
in a highly demanding environment. Boundary conditions are intrinsically time-variable and
space-dependent, both as loads and as constraints. Furthermore, different structural
configurations must be handled: in fact, one has to pass from complete functionality to rotor
stop. In consideration to the fact that in Italy, the construction of offshore wind farms for power
production is currently under consideration, the aim of this paper is to corroborate the basis of
design of offshore wind turbines, as a support to the decision making, having as a specific
objective the structural design of the structure. In doing so, a systemic decomposition of the
relevant elements, both physical related (e.g. the constituting parts) and due to the external
conditions (that lead to the identification of the structural loads) is performed. A necessary
reference to the Codes and Standards is coherently given, to develop a sound basis of design.
INTRODUCTION
Offshore wind turbines are formed both by mechanical and structural elements. As a
consequence, an offshore wind turbine is not a “common” civil engineering structure; it behaves
differently according to different conditions (idle, power production etc), and is subject to highly
variable loads (wind, waves, sea currents etc.). With these considerations, a structure is better
defined as a physical entity having a unitary character that can be conceived of as an
organization of positioned constituent elements in space in which the character of the whole
dominates the interrelationship of the parts.
Moreover, since the structural behavior of offshore wind turbines is influenced from
nonlinearities, uncertainties or interactions, they can be defined as complex.
This definition of “complex” highlights that a modern approach in Structural Engineering
has to evolve from the idea of “Structure”, as a simple device for channeling loads, to the idea of
“Structural System”, as “a set of interrelated components which interact one with another in an
organized fashion toward a common purpose” (NASA, 1995): this systemic approach includes a
set of activities which lead and control the overall design, implementation and integration of the
complex set of interacting components. In this sense, it is important to identify that the way in
which the object of investigation is described has a direct impact on the organization of the
knowledge and the future decisions about the object itself (Simon, 1998).
45
The 4th International Conference on Advances in
Structural Engineering and Mechanics(ASEM'08)
Jeju, Korea, May 26-28, 2008
M2A

2. SYSTEM APPROACH TO STRUCTURAL DESIGN AND COST EFFECTIVENESS
For a complex system such as the one considered, a System Engineering approach should
be used in its design, built and management. System Engineering is a robust approach to the
design, creation and operation of systems. It focuses on the precise specification and goals of the
system structure and behavior, the activities required in order to develop an assurance that those
specifications and goals have been met, and the evolution of the system over time. As a result,
starting from the identification and the quantification of system goals and requirements and by
fixing the performances, the correct and robust design can be implemented.
Some aspects regarding offshore wind turbines are similar to on-shore wind turbines,
while some other entirely different, mostly due to the different design environment (Hau, 2006).
In addition, contrary to onshore wind turbines which have been studied and tested since many
years, with thousands of practical applications around the world, only recently the focus has
been to studying and developing offshore wind turbines. In this sense, while onshore wind
turbines are verified in real conditions and represent the currently available technology, the
advantages and the cost effectiveness of offshore wind turbines (better power production due to
higher wind speeds, less turbulence and lower wind shear) have yet to be proven.
In general demand for structural systems requires three dominant aspects to be optimized
(Bontempi et. al, 2008). These are 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 approach seems to significantly improve all three at the same
time. In fact, the objective of a systemic approach is that the system is designed, built and
operated 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. 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 Figure 1.
Fig. 1. Uncertainty in the cost-effective solutions

3. 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
as the highest levels of performances are approached. Points above the line cannot be achieved
with currently available technology and they represent currently unachievable designs, although
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.
Considering the starting point D0 for the design inside the envelope, there are alternatives
that reduce costs without decreasing any aspect of effectiveness (design point D1) or that
increase some aspects of the effectiveness without decreasing others or without increasing costs
(design point D2). For these reasons, the projects represented by the points on the envelope are
called the 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
known in advance with certainty, so the cost and the effectiveness of a design are better
described by a probability distribution than by a point. Again with reference to Figure 1,
distributions resulting from a 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 is suggested by the
presence of an additional low effectiveness/high cost cloud for concept C. Concept B represents
an intermediate situation.
STRUCTURAL DECOMPOSITION
The offshore wind turbine structure is organized hierarchically, considering the structural
parts categorized in three levels:
1. MACROSCOPIC, related to geometric dimensions comparable with the whole
construction or with general role in the structural behavior; the parts so considered are
called structural systems: one has essentially three systems, as can be seen in Figure 2:
- the main structural system, connected with the main resistant mechanism;
- the secondary structural system connected with the structural part directly loaded by
the energy production system; and
the auxiliary structural system related to specific operations that the turbine can normally
or exceptionally face during its design life: serviceability, maintainability and emergency.
The main structural system, consists in all the structural elements that form the offshore
wind turbine. In general, the following segments can be identified:
a. support structure; the segment of an offshore wind turbine consisting of the following
parts:
i. foundation; the part which transfers the loads acting on the structure into the
seabed;
ii. substructure; the part which extends upwards from the seabed and connects the
foundation to the tower;
iii. tower; the part which connects the sub-structure to the rotor-nacelle assembly;
b. rotor-nacelle assembly; the segment of the main structural system carried by the
support structure.
The secondary structural system consist in all the structural elements related to the
production and transfer of energy, that don’t belong to the main structural system.
The auxiliary structural system, consist in all the structural elements related to the

4. operation, maintenance and emergency. Although non directly influencing the load
bearing capacity of the main structural system, parts of the auxiliary structural system
have an influence to the structural loads (e.g. loads vary depending on the operational
conditions).
2. MESOSCOPIC (Meso level), related to geometric dimensions still relevant if compared
to the whole construction but connected with specialized role in the structural system;
the parts so considered are called structures or substructures;
3. MICROSCOPIC (Micro level), related to smaller geometric dimensions and specialized
structural role: these are components or elements.
Main structural system
Rotor-nacelle assembly
Tower
SubstructureSupport structure
Nacelle
Blades
Rotor
Auxiliary structural system
Operation
Maintenance
Emergency
Secondary structural system
Energy production
Energy transfer
Foundations
Macro-level Meso-level Micro-level
G
Fig. 2. Structural decomposition of an offshore wind turbine
The meaning of this subdivision is manifold (Bontempi 2006):
• the organization of the structure is first of all naturally connected with the load paths that
must be developed by the structure itself; in this way, this subdivision can clear the vision
of the design team about the duties of each part of the structure; this identification is
essential in the Conceptual Design, and it is implicitly a precondition for the
accomplishment of the so-called Performance Based Design (P.B.D.), where the
importance of form is strongly emphasized;
• parts belonging to different levels of this organization require different dependability
provisions (where dependability is defined as the holistic and comprehensive measure of
the quality, that can be synthetically defined as the grade of confidence on the safety and on
the performance of a structural system); with regard to structural failure conditions, this
decomposition allows single critical mechanisms to be ranked in order of risk and

5. consequences of the failure mechanism. These qualitatively assumed requirements can be
quantitatively translated defining different levels of stress in the different structural parts;
all these considerations lead to the so-called crisis canalization;
• there are strong relationships with the life cycle and maintenance of the different parts: with
reference to their structural function, the safety required levels and their reparability,
structures and sub-structures are distinguished in primary components (critical, non-
repairable or which require long repairing times), and secondary components (repairable
with minor restrictions on the operation).
The previous scheme of Figure 2 can be appreciated with reference to Figure 3, where the
main parts of an offshore wind turbine structure are shown, with explicit reference to the
foundation structure. Some of the substructures have been further developed in numerical
(F.E.M.) models, as a consequence to the inquiry of an optimal structural scheme for an offshore
wind farm project in Italy (Figure 4).
Support structure
The support structure consists in the tower, the substructure and the foundations.
• Tower; in general, the tower for an offshore turbine consists in a steel tube, although a
concrete tower or concrete/steel hybrid construction are also possible. The typical tower
dimensions vary with the hub height and the nacelle-motor assembly. In general, the tower
height is determined as a tradeoff between the cost for an increase in tower height and an
eventual additional gain in energy production, as a consequence to an increase in wind
speed with the altitude. The tower weight is strongly influenced by the height and the
optimization adopted (e.g. division in more sections of different thickness). As a
consequence, weights may vary significantly: typical towers for 3 MW power plants weight
108 tons (www.vattenfall.com) and 153 tons (http://www.bowind.co.uk).
• Substructure; the substructure consists in the part of the structure connecting the tower
(which is entirely exposed to the air) with the foundations (which are generally underwater).
• Foundations; with reference to Figure 3, four major types of foundations can be identified.
1) Gravity based. The gravity based foundation is a gravity base serving as the foundation
of the tower. The gravity base is designed with the objective of avoiding lifting between
the bottom of the gravity base and the seabed. This is achieved by providing sufficient
ballast such that the bottom plate of the gravity base always remains in compression
under all environmental conditions.
2) Mono-pile. The mono-pile foundation consists of a welded steel pile which transfers the
loading on the wind turbine to the supporting soils by means of lateral earth pressure. As
a consequence, a certain depth is required to achieve the required capacity. Depending
on the specific site conditions, the mono-pile is either driven (when the soil conditions
allow it) or drilled in the sea-bed (when a rock is encountered).
3) Tripod. The tripod foundation consists of a 3-leg structure, made of cylindrical steel
tubes with driven steel piles. With respect to the mono-pile, it ads stiffness and strength.
Currently, a known application of a tripod foundation is on a demonstrator offshore wind
turbine project.
4) Jacket. The jacket foundation consists of a 3-leg or 4-leg structure, made of cylindrical
steel tubes with driven steel piles, with either vertical or inclined pile sleeves.
The decision on what type of foundations to use is based on technical (primary the water
depth and the soil condition) and economical factors. The DNV Offshore Standard, on the basis
of economical considerations, indicates the water depth ranges of Table 1.

6. G
Fig. 3. The main parts of an offshore wind turbine structure for different foundations (partially
adapted from IEC 61400-3)
G
Fig. 4. Basic Macroscopic F.E.M. models of different offshore wind turbines

7. Table 1: Water depth range for offshore structures foundations
Water depth (m) Foundation type
0-10 Gravity based
0-30 Mono-pile
>20 Tripod/Jacket
>50 Floating
Zaaijer and Henderson (2004) suggest that lack of knowledge of pile behavior when
diameters exceed 5 meters and wall thicknesses are between 70 and 100 millimeters, as well as
the difficulties of handling and obtaining the right equipment, are arguments put forward by the
developers against the use of mono-pile foundations at lengths of 20 meters, even if they are
advantageous from an economical point of view.
The foundations of Figure 3 cover a water depth range, in real word applications, from a
few meters and up to 45 meters, on a tripod or jacket quattropod foundations
(www.beatricewind.co.uk). For deeper waters other solutions such as floating support platforms
may be considered. Floating support platforms are mainly of two kind (Westgate and DeJong,
2005):
• tension-leg platforms, which are submerged using tensioned vertical anchor legs with or
without ballast tanks, can be floated to a site in fully-commissioned condition and simply
connected to the moorings or anchors;
• low-roll floaters, are stabilized by mooring chains and anchors which dampen the motions
of the platform; there is a stabilizer installed at bottom of floater to reduce roll while the
anchors may be fluke anchors, drag-in plate anchors, suction anchors, or pile anchors.
Jonkman and Buhl (2007) cross-studied in simulation the response of a floating support
platform (a barge with catenary moorings) having a turbine installed on land as a reference,
concluding that the barge was susceptible to excessive pitching during extreme wave conditions,
yet the load excursions reduce with decreasing severity in the waves.
Finally, a relatively novel type of foundation called bucket foundation (Ibsen and Brincker,
2004) is currently under testing in which the stability of the foundation is ensured by a
combination of earth pressures on the skirt and the vertical bearing capacity of the bucket.
Among the advantages indicated by the authors is that the steel weight is about half as compared
to a traditional pile foundation, it is much easier to install and it can easily be removed when the
wind turbine is taken down.
Rotor-nacelle assembly
Rotor-nacelle assembly consists in the nacelle, the rotor and the blades. Generally, these
elements are standardized and manufactured (optimized) as a whole and tested in demonstrator
projects before their commence in projects. The three-bladed concept is the most common
concept for modern wind turbines, although concepts based on one and two blades are still
present. However, they are not considered for mainstream applications due to specific
disadvantages (e.g. higher required speed for the same power output, presence of unbalanced
loads, need of a counterweight for the one-bladed turbines).
From a structural point of view, the rotor-nacelle assembly is beside the point of the
offshore wind structure design, however certain characteristics (e.g. weight, induced loads,
induced vibrations) have a strong influence on it.
At present commercial power plant turbines vary in power output, with the upper end
consisting of units from 3 to 5MW (see Table 2).

8. Table 2: Principal characteristics of current offshore power plant
Wind speed (m/s)Power
plant*
Rated
power
(kW)
Rotor
diameter
(m)
Rotor
speed
(rpm)
Weight
(nacelle+rotor)
(t)
Cut-in Cut-out Rated
REpower®
5M
5,000 126.0 6.9 –
12.1
290+120 3.5 30.0 13.0
GE Wind®
3.6MW
3,600 104.0 8,5 –
15,3
290 3.5 27.0 14.0
VESTAS®
V90
3,000 90.0 8.6-
18.4
70+41 4.0 25.0 15.0
* www.repower.de, www.gewindenergy.com, www.vestas.com
The power plant choice has to be founded on reliability issues as well. In this sense, a
power plant already tested in the field presents the most likely choice. In one case (Gerdes et al.,
2006), due to the failure of a significant number of individual turbine transformers, the wind
farm availability dropped to 50%, while the consequent prolonged maintenance period for their
replacement led to the running of an inadequate number of power plants.
EXPECTED PERFORMANCE, SAFETY AND ROBUSTNESS REQUIREMENTS
The basis of design of the offshore wind turbines has to be founded on principles imposed
by renowned structural standards. The performance requirements of safety and robustness, are
identified as follows:
a. assurance of the serviceability and 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;
b. assurance of an elevated lever 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;
c. 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
(e.g. transportation and assembly), and that have to be verified as well;
d. assurance of sufficient robustness to the structural system, that is to a assure the
proportionality between an eventual damage and the resistance capacity, independent to
the triggering cause, assuring at the same time an eventual endurance of the structure in
the hypothetical extreme conditions.
For the structural system identified the following performance criterions can be identified
and, eventually, the 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; in particular, in the dynamic
analysis the water mass has to be accounted for, while, in the case structural elements are
oversized to compensate the eventual loss of thickness due to corrosion, the dynamic
characteristics during the life-cycle have to be calculated
- compatibility of the intrinsic vibration characteristics of the structural system with those
of the agent forces and loads; this compatibility has to be extended not only to the
natural loads (wind and wakes), but to the dynamic loads as well, related to the operation
of the motor and including the vibrations due to control;

9. - compatibility assessment for the movement and the accelerations of the support system
for the functionality of the turbine; the maximum deflection of the blades has to be
assessed, that has to be compatible with the tower position;
• 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 with respect to the maintenance
strategy adopted (constructive corrosion protection, active protection, passive
protection);
- structural behavior with respect to fatigue (FLS- Fatigue Limit State); of particular
importance is the aeroelastic and hydroelastic interaction with the structure, an
interaction which requires suitable dynamic modeling of the structure; specific
solicitation mechanisms due to rotor rotational sampling effects have to be taken into
account; in the above stochastic tridimensional modeling of the wind velocity is advised;
• 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 phenomenon;
• Structural behavior in presence of accidental scenarios (ALS- Accidental Limit State)
- decrease in the load bearing capacity proportional to the damage;
- survival 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.
In fulfilling the above, the conception and design of offshore wind turbines, in particular
the structural design, has to be framed within rules, dictated by International Codes and
Standards.
Generally speaking, these documents, originate from different entities, identified as
activities of:
a. manufacturers (or groups of manufactures) with industrial prerogatives;
b. governmental entities;
c. certification commissions;
d. international commissions.
The most significant of documents related to the structural design of offshore wind
turbines are indicated below:
• BSH – Design of Offshore Wind Turbines, 20 December 2007.
• GL-OWT, Germanischer Lloyd Wind Energie GmbH: Guideline for the Certification of
Wind Turbines. Edition 2003 with supplement (2004).
• GL-OWT, Germanischer Lloyd Wind Energie GmbH: Guideline for the Certification of
Offshore Wind Turbines. Edition 2005.
• IEC 61400-1 Wind Turbines - Part 1: Design Requirements. Third edition 2005.
• IEC 61400-3 Wind Turbines - Part 3: Design Requirements for Offshore Wind Turbines,
Committee Draft, December 2005.
• DNV-OS-J101 Design of Offshore Wind Turbine Structures. Det Norske Veritas. October
2007.
Among the above Codes and Standards the GL Wind offshore guideline provides
requirements for structures, machinery, safety and electrical system and condition monitoring

10. systems, thus covering in depth all aspects of the structural safety for offshore wind turbines
(Dalhoff and Argyriadis, 2004).
These Codes and Standards are combined with area or country-specific structural codes,
for the design and verification of the structural parts.
DESIGN ENVIRONMENT
In order to identify the loads agent on the structural system, it is important to perform a
decomposition of the different structural parts subject to the different loads, relevant to different
environmental conditions, that are identified with reference to their spatial position.
Therefore, the following sections can be identified:
a. Seabed section; consists in the parts of the structural system in the seabed. The soil
characteristics have an influence to the foundation system, that in accordance to the
vertical and horizontal extension, has an influence on its part to the underwater
segment, described in sequence;
b. Underwater section; this section is formed by the part of the substructure exposed to
the water;
c. Air exposed section; this section is formed by:
i. the part of the substructure exposed in the air;
ii. the tower; and
iii. the rotor-nacelle assembly.
In Figure 5, the most relevant loads agent on the offshore wind turbine are shown, as
relevant to the different sections.
Foundation
Immersed
Pressure
Connection
z
y
x
z
y
x
Wind
Fluid-dynamic
Geotechnical G
Fig. 5. Representation of the external loads agent on an offshore wind turbine

11. The definition of the design environment, that is the external conditions, is important
since it has a direct influence on the loads agent on the main structural system. As said before,
the most complete guidelines regarding the external conditions is the GL Wind offshore
guideline which subdivides external conditions in two categories:
• normal conditions, which generally concern recurrent structural loading conditions; and
• extreme conditions, which represent rare external design conditions.
With reference to the diagram in Figure 6, the external conditions can be divided in
environmental (wind, marine and other) and electrical.
The environmental conditions are site-specific and in general, need to be assessed, along
with the seismic, topographic and soil conditions. In general, the major concern is the wind and
the marine conditions, since they contribute the most to the loads agent on the structural system.
Wind is most relevant to the structural integrity of the motor-nacelle assembly.
The electrical conditions refer to the network conditions. IEC 61400-3 indicates the
necessary checks to be performed on the electrical network, which include grid compatibility
conditions of a wind farm.
External conditions
(adapted from IEC 61400-3, draft 2005)
Wind conditions
Marine conditions
Waves
Sea currents
Water level
Marine growth
Environmental conditions
Air temperature
Solar radiation
Humidity
Rain, hail, snow and ice
Chemically active substances
Salinity causing corrosion
Lighting
Seismicity causing earthquakes
Water density
Water temperature
Traffic
Electrical conditions
Normal wind conditions
Extreme wind conditions
Seabed movement and scour
Other conditions
Mechanically active substances
GGGG
Fig. 6. External conditions [adapted from IEC 61400-3]

12. Wind Conditions
The wind induced stress analysis require the characterisation of statistical parameters
specific to the wind turbine installation site. The required data are those obtained by
anemometric measurements, usually synthesised in a wind-rose diagram, which shows the
frequency of winds blowing from particular directions. Starting from these data, it is possible to
define the design wind force for the structural analysis.
In particular, it is required to estimate the maximum average wind speed with a prefixed
return period (50 years). This can be obtained by means of extreme values analysis and the
consequent estimation of a p.d.f.- probability density function parameters (usually Gumbel), in
order to determine the best fitting to the extreme wind values by means of a linear regression.
In addition to obtaining the extreme values, it is necessary to evaluate the long term wind
conditions, useful for fatigue and deformation analysis (other than the efficiency assessment of
the eventual wind farm). For the above mentioned reasons, and aiming at the statistical
quantification of the turbulence fluctuation, along with models providing the vertical profile of
the mean wind speed, an eligible spectral model is implemented. In the end, the induced forces
on the structural system are estimated on the basis of appropriate formulas. Two aspects have to
be accounted for:
- the wind current value is null near the sea surface, while it increases monotonically
with the altitude;
- the wind current is characterised by turbulence
Regarding the first, IEC 61400-3 prescribes a formulation for the average wind speed,
U(z), as a function of the height (z) above the mean sea level, which for standard turbine classes,
is given by:
α
¸¸
¹
·
¨¨
©
§
=
hub
hub
z
z
UzU )( (1)
being Uhub the reference velocity at the height of the hub (zhub); for normal wind conditions the
power exponent Į is 0,14.
The same Standard recommends the following equation for estimating the extreme wind
speed with a recurrence period of 50 years:
0.11
50 1.3e ref
hub
z
U U
z
§ ·
= ⋅ ¨ ¸
© ¹
(2)
The turbulence components of the velocity may be modelled as stationery Gaussian
stochastic processes with null average value. Among the various analytic-numerical techniques
to represent these components, the most used in structural design is the one based on a spectral
model.
Another aspect to be accounted for, not only for power production but for structural -
specially fatigue- analysis (Veldkamp, 2007) as well, is the “wake effect” phenomenon. The
wake effect is relevant when considering a wind farm, consisting in many turbines in several
rows. In this case, the presence of a wind turbine will influence the wind flow locally, and the
turbulence in the wake behind the turbine will be different from that in front of the turbine. The
DNV- Guidelines for Design of Wind Turbines (2002) state that wake effects need to be
considered, for wind turbines installed behind other turbines with a distance of less than 20 rotor
diameters.

13. Marine Conditions
Marine conditions include waves, sea currents, water level and marine growth. Marine
conditions, similar to wind, are divided into normal and extreme.
• Waves Waves concern the structural elements in contact with water or in the transition zone
in correspondence to the water surface. The action of the wave motion affects the structure in
contact with water, as a consequence of the alternative motion of fluid particles, induced by the
fluctuating perturbation of the liquid surface, or, in shallow water conditions, as a consequence
of the breaking waves.
In order to determine the wave loads relevant to the structural analysis, three phases can
be identified:
a. statistical elaboration of the wave motion;
b. determination of the design wave;
c. identification of the appropriate theory, related to the relative depth, to calculate the
fluid kinematics.
Regarding the point (a.), the water surface height, in respect to the mean sea level, is a
time-dependent stochastic variable, and can be described by means of statistical parameters:
- the significant wave height HS; it is defined as four times the standard deviation of
the sea elevation process. The significant wave height is a measure of the intensity of
the wave climate as well as of the variability in the arbitrary wave heights.
- the spectral peak period TP; it is related to the mean zero-crossing period of the sea
elevation process.
For extreme events analysis, in general, a sea state with return period TR=50 years is
considered, and the significant wave height is defined as (DNV-OS-J101, 2004):
¸¸
¹
·
¨¨
©
§
−=
−
R
yearHTS
T
FH SR
1
1
1
1max,,,
G G G G G G G (3)
where FHS,max,1year represents the maximum annual significant wave height, that can be deduced
by means of a Weibull distribution.
Regarding the point (b.), the wave characteristics required in order to define the wake
loading on the structure, are represented by the wave height HS and the wave period TP.
For the fatigue analysis of the structure subject to the wave action, it is necessary to define
an appropriate spectral density of the surface elevation. The characteristic spectral density of the
specific sea-state S(f) can be defined by means of the parameters HS and TP, after selecting an
appropriate mathematical model for the S(f) function. IEC 61400-3, 2005 indicates two spectral
types:
- the Jonswap spectrum for a developing sea; and
- the Pierson-Moskowitz (PM) spectrum for a fully developed sea.
The spectral density of the first one, is given by the analytic formulation:
( )
( )
»
»
¼
º
«
«
¬
ª
¸
¸
¹
·
¨
¨
©
§ −
−−
−
»
»
¼
º
«
«
¬
ª
¸¸
¹
·
¨¨
©
§
−=
2
5.0exp4
5
4
2
4
5
exp
2
P
P
f
ff
Pf
f
f
g
fS
σ
γ
π
α (4)
where f=2π/T is the frequency, f=2π/T is the peak frequency, Į and g constants, ı and Ȗ
parameters dependent from HS e TP.

14. The analytical formulation of the Pierson-Moskowitz spectrum is similar to the first one,
differing only for the absence of the peak amplification factor:
»
»
»
¼
º
«
«
«
¬
ª
¸
¸
¹
·
¨
¨
©
§ −
−
2
5.0exp
p
p
f
ff
σ
γ (5)
and for the presence of a different normalisation coefficient.
In general, the sea state is characterised by a distribution of the energy spectral density,
depending on the geographic direction of the wave components: this is obtained by multiplying
the one-dimensional spectrum S(f) by a function of directional dispersion, symmetric to the
principal direction of the wave propagation
Finally (point c.), DNV-OS-J101, 2004 identifies the analytical or numerical wave
theories, and their range of validity, which may represent the kinematics of regular waves:
- linear wave theory (Airy theory) for small-amplitude deep water waves; by this
theory the wave profile is represented by a sine function
- Stokes wave theories for high waves
- stream function theory, based on numerical methods and accurately representing the
wave kinematics over a broad range of water depths
- Boussinesq higher-order theory for shallow water waves
- solitary wave theory for waves in particularly shallow water.
• Sea currents Sea currents induced by the tidal wave propagation in shallow water are
characterised, in general, by a velocity range practically horizontal, while their intensity
decreases slowly with the depth. The vertical profile of the current can be obtained, in absence
of site-specific measurements, by the following equations (IEC 61400-3, 2005):
( ) ( ) ( )
( )
( ) ¸¸
¹
·
¨¨
©
§ +
=
¸
¹
·
¨
©
§ +
=
+=
0
0
0
71
0
h
zh
VzV
h
zh
VzV
zVzVzV
windwind
tidetide
windtide
(6)
where z indicates the height above sea water level, Vtide0 and Vwind0 are tide and wind induced the
current velocities on the surface, and h0 is a reference depth (which typically is assumed of 20
meters).
The wind velocity generated sea surface current velocity is determined on the basis of
appropriate site-specific measurements. The above mentioned Standard, indicates that it may be
estimated from:
)10(01.0 10 mzVV hourwind =⋅= G G G G G G G (7)
• Water level The oscillation of the water level, both in the short term and in the long term,
has an influence on the hydrodynamic loading of the structure. This is apparent from the fact
that the transition zone changes, and consequently, the loads agent on the structure vary. IEC
61400-3, indicates that a constant water level equal to the mean sea level (the level of the sea
taken over a long period, taking account of all tidal effects but excluding meteorological effects)
may be assumed for ultimate load cases in normal wave conditions.

15. • Marine growth For the underwater structural elements the marine growth has to be
considered. The marine growth, which is site specific and depends on conditions such as salinity,
oxygen content, pH value, current and temperature, adds weight to the structural components
and may increase the hydrodynamic forces on the components. Therefore, the potential for
marine growth has to be addressed by increasing the outer diameter of the structural member in
question in the wave load calculations. DNV-OS-J101, 2004, indicates, in absence of more
precise data, that the marine growth profile in Table 3 may be used for design:
Table 3: Design marine growth profile [DNV-OS-J101, 2004]
Depth below MWL (m) Marine growth thickness (mm)
0-10 50
10-20 45
20-25 65
25-35 90
>35 80
with the further prescription that:
- the outer diameter of a structural member subject to marine growth shall be increased
by twice the thickness specified in this Table;
- the type of marine growth may have an impact on the values of the hydrodynamic
coefficients that are used in the wave load calculations.
Seabed movement and scour
Seabed movement is mostly present in the form of sand waves. Sand waves are caused by
tidal currents in marine non-cohesive bottoms and generally form regular patterns, formed after
a slow process that may cover several years. They can be found in places where the upper soil
layer consists of loose material that can be transported by sea currents.
Scour is the phenomenon where a hole around the structure, in correspondence to the
seafloor, is created by the sand particles transported away by the flowing water. Scour is
strongly affected by the presence of the structures, since the latter cause local increase of the
current and wave motions. Scour can be divided in (Sumur and Fredsøe, 2002; Tempel et al.,
2004) local scour (in the form of a steep scour pit of conical shape, caused by alternating
currents of waves washing ashore at low tide, significant to mono-pile foundations) and global
scour (represented by the lowering of a large area around the entire structure, particularly
relevant to more complex structures, e.g. jacket or tripod foundations). Scour is caused by
currents, waves or their combination, as well as from sea screws (caused by ship manoeuvres).
Seabed movement and scour have an affect the lateral loading capacity of the foundation,
the natural vibration of the structure, and ultimately, it can be the cause of power production
disruption (being the power cable from the structure over to the seafloor).
Other environmental conditions
Other environmental conditions can affect the integrity and safety of an offshore wind
turbine, by thermal, photochemical, corrosive, mechanical, electrical or other physical actions,
while their effects may increase when combined.
IEC 61400-3, 2005 indicates the normal environmental condition values that should be
taken into account (Table 4):

16. Table 4: Normal environmental conditions [IEC 61400-3, 2005]
Condition Value
ambient air temperature range -10 °C to +40 °C
relative humidity up to 100 %
solar radiation intensity 1,000 W/m2
air density 1,225 kg/m3
water density 1025 kg/m3
water temperature range 0 °C to +35 °C
• Air density Air density has a direct influence on the aerodynamic loads and the power
output. Regarding the power output, a change in power is linear to the air density, thus, in order
to avoid losses in the energy supply, a different fixed blade pitch angle must be selected at lower
air density, and the rotor speed may also have to be adapted, something that has a direct
consequence on the loads from the rotor. Air density depends fundamentally on two factors: the
air temperature (including the differences between summer and winter) and the altitude. The
latter has practically no influence, since offshore wind turbines are placed at more or less
standard altitudes.
• Humidity The humidity has an influence on the atmospheric corrosion and the ice build-up
on the turbine (when relevant). It should be evaluated the possibility to control the humidity for
internal zone (compartments) for corrosion protection.
• Solar radiation Solar radiation is significant for partial stresses on the metallic surfaces, and
for this it has to be accounted for in the design process. Furthermore, the intensity of solar
radiation should be considered for components, which are sensitive to ultraviolet radiation
and/or temperature (e.g. secondary parts in thermoplastic). DNV-OS-J101 and IEC 61400-3
require that the structure be designed for a solar radiation intensity of 1000w/m2
, which
coincides with the Central European conditions.
• Rain, hail, snow and ice In general protection against weather effects by means of
provisional coverings is necessary during the assembly. Regarding ice and snow accumulation,
DNV OS-C101, 2007 requests that ice accretion from sea spray, snow, rain and air humidity
shall be considered, where relevant, while snow and ice loads may be reduced or neglected if
snow and ice removal procedures are established. Furthermore, when determining wind and
hydrodynamic loads, possible increases of cross-sectional area and changes in surface roughness
caused by icing shall be considered, where relevant.
• Chemically substances and mechanically active particles The presence of chemically active
substances (e.g. hydraulic fluids) and mechanically active particles (e.g. sand) should be
considered, since their leakage or outflow may alter the characteristics of the structural parts. So
may occur from waste in-sea waste materials (e.g. petroleum).
• Salinity Water salinity causes corrosion and should be evaluated in consideration of the
corrosion protection system.
• Lighting Lightning strikes are unavoidable on large wind turbines (Hau, 2006). Most of the
lightning strikes on the rotor blades hit the area of the blade tip, resulting in considerable

17. damage. Modern offshore wind turbines are equipped with outer lightning protection by means
of multiple receptors located in the rotor blades and the lightning rod at the weather mast.
• Seismicity Seismic activity is explicitly accounted for as a loading condition, as prescribed
from local Norms and Standards. Underwater seismic activity, together with volcanic eruption,
may lead to seismic sea waves (tsunamis).
• Water density The water density influences the intensity of hydrostatic and hydrodynamic
(current and wave) loads, including breaking waves loads when relevant. In absence of site-
specific data, the sea water density (salt water) is assumed as equal to 1025 kg/m3
.
• Water temperature The water temperature has an influence on the cinematic viscosity of the
water, and thus to the loads. It has to be assessed also in relevance to the underwater joints (RP
2A-LRFD).
• Traffic Two kinds of traffic should be considered: marine traffic (both dedicated service
vessels and normal traffic) and air traffic (again in this case, due to service helicopters and to
normal air traffic). Risk analysis for the evaluation of the occurrence of an extreme event should
be carried out, as indicated in a next paragraph.
LOADS DECOMPOSITION AND CONTINGENCY SCENARIOS
The site-specific external conditions lead to the identification of the loads agent on the
structural system (Figure 7).
Loads
Gravitational/Inertial
Loads decomposition
(adapted from IEC 61400-3, draft 2005)
gravity
vibrations
seismic activity
rotation
gravity
vibrations
seismic activity
rotation
Aerodynamic
Actuation
Hydrodynamic
Other
wake loads
impact loads
sub-sea earthquakes
wake loads
impact loads
sub-sea earthquakes
torque control
mechanical braking loads
yaw and pitch actuator loads
torque control
mechanical braking loads
yaw and pitch actuator loads
wave
current
wave
current
G
Fig. 7. Loads decomposition for the design of an offshore wind turbine

18. In this section the forces agent on the main structural system are identified, forces that
lead to the equivalent loads needed in order to perform the structural analyses. It is anticipated
that the design values of the external forces (e.g. wind, wakes, currents) are given by relevant
studies, or defined by effective Codes.
For further specifications on the identification and application of the loads, contingency
scenarios and load cases, together with a concise application, the reader is directed to Bontempi
et al., 2008, presented in the same conference.
Contingency scenarios are defined, that account for the various situations in which the
turbine can be found during its lifetime.
In general, these contingency scenarios for an offshore wind turbine are obtained by
combining:
• nominal conditions (turbine integer and functioning) with extreme external conditions;
• nominal conditions (turbine integer and functioning) with normal external conditions;
• damaged conditions with appropriate external conditions;
• transient conditions (transport, assembly, maintenance and repair) with appropriate external
conditions.
Other situations to be accounted for are for the normal start up, normal/emergency shut
down and loss of electrical network, again combined with normal or extreme external conditions.
In the above defined contingency scenarios, appropriate coefficients for the combination
of loads and materials are introduced. In general, partial safety factors for loads vary with the
design situation (e.g from 1.1 for Normal Situations, to 1.3 for Extreme Situations and up to 1.5
for Transport and Erection), while when they are favourable, are taken equal to 0.9. The partial
safety factors for the materials are also defined (varying from 1.0 in the SLS to 1.25 for fatigue
analysis)
SPECIAL ISSUES
It is worth mentioning some of the special issues, in no sense comprehensive due to lack
of space, relevant to the offshore structure design.
Vessel impact loads
These loads account for the load from a vessel impacting the structural system. Vessel
impact loads can be divided in two classes:
a. Normal impact loads. These account for the impact of the support structure by the
dedicated service vessel, considered in the appropriate load combinations (transport,
assembly, maintenance and repair, see IEC 61400-3, 2005 and DNV-OS-J101, 2004)
b. Extreme impact loads. These account for the collision of a vessel due to an accidental
(unaccounted) situation.
For the extreme impact loads risk analysis has to be carried out (GL, 2002), considering
all the possible adverse effects for the environment (e.g. leak of contaminating fluids from the
ship) and to the humans (e.g. nacelle falling onto the ship’s deck), accompanied by numerical
tests to corroborate highly possible events (Biehl and Lehmann, 2006). From a design point of
view, the structural system should be regarded as collision-friendly.
Corrosion protection
Frandsen et al. (2003), make the following considerations regarding the corrosion
protection of offshore wind turbines:

19. • The offshore wind energy converter systems are designed for a lifetime of 20 years.
• The saline seawater is a corrosion accelerating medium.
• The mean corrosion rate for unprotected steel is about 0.3mm-6mm in the design lifetime
(strongly dependent on the specific site salinity).
In the same paper, three different levels of corrosion protection are proposed, namely:
• Constructive corrosion protection; includes all corrosion protection measures that can be
accounted for during the design (e.g. preventing or limiting the number of gaps between
metals or the contact between different metals, etc.)
• Passive corrosion protection; it consist of coating systems which have the properties of a
barrier and shield the steel from oxidation.
• Active corrosion protection; electrochemical protection by using a sacrificial anode or by
impressing external voltage.
CONCLUSIONS
In this paper, the structural decomposition of offshore wind turbines, has been performed,
with a view on the structural analysis and performance. Furthermore, the essential performances
have been identified, along with the external conditions. The latter have a direct influence on the
structural design of these structures, since they identify loads.
All these considerations have as an aim the organization of the framework for the basis of
design of offshore wind turbines, as a support to the decision making, with specific reference to
the structural safety and reliability for the entire lifespan.
A practical exemplification of the above is given in the paper “Numerical modeling for
the analysis and design of offshore wind turbines” presented in the same conference, where
numerical analyses are performed by means of F.E.M models.
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