Structural Design and Analysis of Offshore Wind Turbines from a System Point of View

Structural Design and Analysis of Offshore WindTurbines from a System Point of View

by

Francesco Petrini, Sauro Manenti, Konstantinos Gkoumas, Franco Bontempi

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WIND ENGINEERINGVOLUME 34, NO. 1, 2010

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Structural Design and Analysis of Offshore WindTurbines from a System Point of View

Francesco Petrini1, Sauro Manenti2, Konstantinos Gkoumas3, Franco Bontempi*,4

1Department of Structural and Geotechnical Engineering, University of Rome “La Sapienza”, viaEudossiana, 18 - 00184 Rome, Italy ([email protected])2Department of Hydraulics Transportation and Roads, University of Rome “La Sapienza”, viaEudossiana, 18 - 00184 Rome, Italy ([email protected])3Department of Hydraulics Transportation and Roads, University of Rome “La Sapienza”, viaEudossiana, 18 - 00184 Rome, Italy ([email protected])4Department of Structural and Geotechnical Engineering, University of Rome “La Sapienza”, viaEudossiana, 18 - 00184 Rome, Italy ([email protected])

WIND ENGINEERING VOLUME 34, NO. 1, 2010 PP 85–108 85

ABSTRACTOffshore wind turbines are relatively complex structural and mechanical systems located in

a highly demanding environment. In this study, the fundamental aspects and 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, a

decomposition of the system (environment, structure, actions/loads) and of the structural

performance is carried out, in order to organize the qualitative and quantitative assessment

in various sub-problems. These can be faced by sub-models of different complexity both for

the structural behavior and for the load models. Numerical models are developed to assess

the safety performance under aerodynamic and hydrodynamic actions. In the structural

analyses, three types of turbine support structures have been considered and compared: a

monopile, a tripod and a jacket.

*Corresponding author

1. INTRODUCTIONOffshore wind turbines (OWT) emerge as an evolution of the onshore plants for which the

construction is a relatively widespread and consolidated practice providing a renewable

power resource [1]. In order to make the wind generated power more competitive with

respect to conventional exhaustible and high environmental impact sources of energy, the

attention has turned toward offshore wind power production [2].

Besides being characterized by a reduced visual impact, since they are placed far away

from the coast, OWTs can take advantage from more constant and intense wind forcing,

something that can increase the amount and regularity 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. Therefore, 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 subject 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 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 from nonlinearities,

uncertainties and interactions, they can be defined as complex structural systems [3].

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 another in an organized fashion toward a common purpose” [4]. This system

approach includes a set of activities which lead and control the overall design,

implementation and integration of the complex set of interacting components [5,6].

In this study, the original definition by NASA [4] has been extended in such a way that the

“structural system” organization contains also the actions and loads. The latter derive from,

and are 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 turbine 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 forcings (in particular aerodynamic and hydrodynamic) and the proper

86 STRUCTURAL DESIGN AND ANALYSIS OF OFFSHORE WIND TURBINES

FROM A SYSTEM POINT OF VIEW

Seabed

Transitionplatform

Foundation

Tower

Supportstructure

Sub-structure

Blades – Rotor – Nacelle

Foundation

Sea floor

Water level

TRIPODMONOPILE JACKET

Figure 1: Main parts of an offshore wind turbine for different support structures.

modeling of the possible presence of non linear interaction phenomena between the different

actions and between the actions and the structure.

In general, 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 a 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.

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 in the neighborhood regions.

WIND ENGINEERING VOLUME 34, NO. 1, 2010 87

ENVIRONMENT STRUCTURE ACTIONSInteraction

Structural system

PERFORMANCE

Figure 2: Structural system organization.

Figure 3: Generic depiction of the uncertainties and the interaction mechanisms in the design of an

offshore wind turbine structure.

Wind and wave flow

Wind site basicparameters

Wave site basicparameters

Hydrodynamicphenomena

Otherenvironmental

agents

1. Aleatoric 2. Epistemic3. Model

1. Aleatoric 2. Epistemic3. Model

1. Aleatoric2. Epistemic3. Model

Aerodynamic andaeroelasticphenomena

Non-environmentalsolicitations

Structure

Wind, wave andsea currentactions

ENVIRONMENT ZONE EXCHANGE ZONE

Types of uncertainties

Site-specificenvironment

Structural (non-environmental)

system

Propagation Propagation

• 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 from the exchange zone

to the environment zone.

In general, the uncertainties can be subdivided in three basic typologies:

• aleatory uncertainties (arising from the unpredictable nature of the magnitude, the

direction and the variance of the environmental actions);

• epistemic uncertainties (deriving from the insufficient information and the errors in

measuring the previously mentioned parameters);

• 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 uncertainty related to the hypothesis of Gaussianity is

introduced.

The aleatory 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.

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 (for the

sake of simplicity, the epistemic uncertainties are not considered in this study).

A suitable tool 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.

Furthermore, due to the complexity, the design of these structures has to be carried out

under a Performance-Based Design philosophy: different aspects and performance under

different loading conditions (with reference to all possible system configurations that can be

assumed by the blades and the rotor) have to be investigated for this type of structures.

Additional design issues related to the structural aspects are mentioned below with some

proper references:

• Aerodynamic optimization [7].

• Foundation design and soil-structure interaction [8, 9, 10].

• Fatigue calculations [11, 12].

• Vessel impact and robustness [13].

• Life Cycle assessment [14, 15].

• Marine scour [16, 17].

• Possible floating supports [18, 19].

• Standards certification [20, 21, 22, 23, 24, 25].



Finally, an important issue for offshore wind turbines is the choice 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 principal parameter, according to the DNV-OS-J101 [22],

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). In this study focus is given to monopile, tripod and jacket support

structures.

The paper starts with the description of the system approach applied to OWT design: while

the system point of view is a consolidated practice in many engineering fields (e.g. aerospace

engineering), in the case of OWTs, it is not fully established and represents an ongoing process.

In the second part of the paper, the system point of view is applied to the numerical modeling

of a case study. More precisely, numerical analyses are carried out on different OWT support

structures. The obtained results justify the adoption of a jacket structure for the specific case

(Figure 4).

2. STRUCTURAL SYSTEM DECOMPOSITIONAs previously stated the decomposition of the structural system is a fundamental tool 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 (Figure 5). 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. [26]).

2.1. Decomposition of the EnvironmentThe 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. Their decomposition is performed in the first column of

Figure 5.


Figure 4: Different views of the jacket support structure adopted after the numerical analyses.

2.2. Decomposition of the StructureThe second step of the decomposition relates to the offshore wind turbine structure. This is

organized hierarchically, considering all the structural parts categorized in three levels

(second column of Figure 5):

• 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.



STRUCTURE

Main structure

Nacelle

Rotor–nacelle assembly

Operation

Maintenance

Emergency

Support structure

ACTIONS/LOADSENVIRONMENT

Junctions/bearings

Rotor

Junctions/bearings

Blades

Junctions/bearings

Tower

Junctions

Substructure

Junctions

Foundations

Junctions

Secondary structure

Energy production

Energy transfer

Auxiliary structure

Gravitational / Inertial

Gravity

Braking

Aviation

Seismic activity

Aerodynamic

Hydrodynamic

Actuation

Other

Wave

Current

Torque control

Mechanical breaking loads

Yaw and pitch actuator loads

Tsunami

Impact loads

Wake loads

Wind conditions

Marine conditions

Seabed movement and scour

Other conditions

Normal wind conditions

Extreme wind conditions

Waves

Sea currents

Water level

Marine growth

Air temperature

Humidity

Solar radiation

Rain, hail, snow, ice

Chemically active substances

Mechanically active substances

Environmental aggressiveness

Lighting

Seismicity

Water density

Water temperature

Maritime traffic

Normal wave conditions

Extreme wave conditions

Serviceability Safety Reliability Robustness

Service Limit States –SLS Ultimate Limit States –ULS_1 Accidental Limit States –ALSUltimate Limit States –ULS_2

Fatigue Limit States –FLS

Deflections/Displacements

Vibrations Strain limit

Stress limit Degradation effects

Buckling Fire

Impact

Explosion

PERFORMANCE

Figure 5: Structural system and performance decomposition of an offshore wind turbine.

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:

a. support structure (the main subject of this study);

b. 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:

a. foundation: the part which transfers the loads acting on the structure into the

seabed;

b. substructure: the part which extends upwards from the seabed and connects

the foundation to the tower;

c. 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.

2.3. Decomposition of the Actions and LoadsThe next step of the structural system decomposition is the one regarding the actions related

to the environmental conditions. These are decomposed as shown in the third column of

Figure 5, 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”.

2.4. Performance DecompositionAs a final step, the performance requirements are identified and decomposed as follows

(lower part of Figure 5).

• 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 plausible 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;

• 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 phenomena

is required;

• Assurance of sufficient robustness for the structural system, that is to ensure the

proportionality between an eventual damage and the resistance capacity,

independently from the triggering cause, ensuring at the same time the survival of

the structure under a hypothetical extreme condition.

The following performance criteria can be identified for the structural system, leading

eventually to the selection of appropriate Limit States:

• Dynamic characterization of the turbine as defined by the functionality

requirements, regarding the:

– natural vibration frequencies of the whole turbine, including the rotor-nacelle

assembly, the support structure and the foundations;


– compatibility of the intrinsic vibration characteristics of the structural system

with those of the applied forces and loads;

– compatibility of the displacement and the acceleration of the support system

with the functionality requirements of the turbine.

• Structural behavior with respect to serviceability (SLS- Serviceability Limit State),

regarding the:

– limitation of deformations;

– prevention of any loosening of the connections.

• Preservation in time of the structural integrity, regarding the:

– durability for corrosion;

– structural behavior with respect to fatigue (FLS-Fatigue Limit State).

• Structural behavior under near collapse conditions (ULS-Ultimate Limit State),

regarding the:

– assessment of the stress state for the whole structure, its parts, 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), regarding the:

– provisions for the decrease in the load bearing capacity proportional to the

damage (the concept of structural robustness- see for example Starossek [27]

and Bontempi et al. [28]);

– provisions for the 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 specific risk scenarios.

3. ENVIRONMENT AND ACTION MODEL From all the loads indicated in Paragraph 2.3, in this study attention is focused on the

aerodynamic and the hydrodynamic ones.

Typically, an environmental action, when observed during a short time period, is made of

two components: a mean (or slowly variable) component, and a stochastic one. For the

aerodynamic and the hydrodynamic actions, the mean component is generated respectively

by the mean wind velocity and by the sea current, while the stochastic component is

generated respectively from the turbulence wind velocity and from the linear waves.

The definition of “mean” has to be specified with reference to a specified “short time

period” (usually less than 1 hour); in contrast, the so called “mean component” varies in a

stochastic manner during long time periods. For this reason, in what follows the mean

components will be considered as constant only for short periods of analyses.

The generic environmental configuration is shown in Figure 6, where the macro-

geometric parameters defining the problem are also represented. These are the local mean

water depth (h), the hub height above the mean water level (H) and the blade length (or rotor

radius, R).

Correct prediction of the structural response under extreme and normal load

conditions requires the definition of their probability distribution and statistical



parameters; these are site specific, and have to be estimated by carrying out statistical

analyses of the measurements database. In particular two kinds of investigations are

usually carried out: short term statistics for fatigue analysis, and long term statistics, for

ultimate limit state analysis.

Finally, the definition of the extreme load cases requires an estimation of the probability

distribution for: (i) the extreme 10-min average wind velocity at the reference height, and (ii)

the significant wave height estimated in a 3-hour reference period along with the associated

spectral peak period.

When no information is available for defining the long term joint probability distribution of

extreme wind and waves, it shall be assumed that the extreme 10-min mean wind speed with

a 50-year recurrence period occurs during the extreme sea state with a 50-year recurrence

period (IEC 64100-3 [25]) adopting appropriate reduced values.

3.1. Aeolian and Hydrodynamic Fields ModelConcerning the wind modeling for the computation of the aerodynamic action, a Cartesian

three-dimensional coordinate system (x,y,z), with origin at the mean water level and the z-

axis oriented upward is adopted. Focusing on a short time period analysis, the three

components of the wind velocity field Vx(j), Vy(j), Vz(j) at each spatial point j (the variation

with time is omitted for the sake of simplicity) can be expressed as the sum of a mean (time-

invariant) value and a turbulent component u(j), v(j), w(j) with mean value equal to zero.


Turbulentwind

Water level (mean)

Mud line

P

P

uP (t)VP (t)

Vw (z′)

Vcur (z′)

Vm (zP)

WP (t)

y′y

z

Meanwind

Hub level

Water level (mean)

R

H

h

Mud line

x, x′

z′

Waves

Current

Figure 6: Problem statement and configuration of the actions.

Assuming that the mean value of the velocity is non-zero only in x direction, the three

components are given by:

Vx(j) = Vm(j) + u(j); Vy(j) = v(j); Vz (j) = w(j) (1)

The mean velocity Vm(j) can be determined by a database of values recorded at or near the

site, and evaluated as the record average over a proper time interval (e.g. 10 minutes), while

the variation of the mean velocity Vm with the height z over a horizontal surface of

homogeneous roughness can be described by an exponential law. Finally, the turbulent

components of the wind velocity are modeled as zero-mean Gaussian ergodic independent

processes. By using the proposed model, it is possible to generate samples of the wind action

exerted at each point j of the structure.

Concerning the hydrodynamic actions, as previously stated, they are due to currents and

waves. For what concerns the sea currents induced by the tidal wave propagation in shallow

water condition (i.e. the ratio between water depth h and wave length L is lower than 0.05), in

general they are characterized by a sub-horizontal velocity field, while their intensity

decreases slowly with the depth. Waves act on the submerged structural elements and on the

transition zone above the water-air interface; in the first case actions are the result of the

alternative motion of fluid particles, induced by the fluctuating perturbation of the liquid

surface; in the second case the action is the consequence of the breaking waves, which may

occur in shallow water condition. In general the short period water surface fluctuation, with

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 mean square root of the

sea elevation process. It represents a statistical 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 the significant wave height is defined with respect

to a proper return period TR (DNV-OS-J101 [22]).

For fatigue analysis the sea state is characterized through the wave energy spectral

density, defined upon the domain of frequency and geographic direction of the wave

components: usually this is obtained by multiplying the calculated one-dimensional spectrum

S(f) by a function of directional spreading, symmetric with respect to the principal direction

of the wave propagation [29].

3.2. Aerodynamic and Hydrodynamic Actions ModelIn general, the components of the actions could be calculated separately for all structural

elements adopting a common frame of reference and then superimposed by a vector sum in a

phase-correct manner.

The aerodynamic force can be decomposed, as usual, in a drag (parallel to the mean wind

velocity) and a lift (orthogonal to the mean wind velocity) component, while moments have

been neglected in the present paper. These can be computed for each structural component

from the specific wind velocity field and for each structural configurations (for example,

extreme wind and parked turbine configurations), by using well known expressions, as shown

in Bontempi et al. [30] and Petrini et al. [31]. The equivalent static load can be derived through

peak factors, based on the probabilistic characteristics of the wind velocity modeled as a

stochastic process [32].



Concerning the hydrodynamic loads on a structural slender cylindrical member (D/L < 0.2,

with: D member diameter normal to the fluid flow, L wave length), both wave and (stationary)

current generate the following two components:

• A force per unit length acting in the direction perpendicular to the axis of the

member and parallel to the orthogonal (with respect to the member) components

of the water particle velocity (wave vw plus current Vcur induced) and acceleration

(wave only); it can be estimated by means of Morison equation:

(2)

where ρwat is the water density, ci and cd are the inertia (including added mass for a

moving member) and drag coefficient respectively, which are related to structural

geometry, flow pattern and surface roughness: superposed dot indicates the time

derivate, in the Eq. (2). Periodic functions are adopted for both the wave velocities

and accelerations [33].

• A non-stationary (lift) force per unit length acts in the direction perpendicular both

to the axis of the slender member and to the water current. This component is

induced by vortex shedding past the cylinder and inverts its direction at the

frequency fl of eddies separation which is related to flow field and structural

geometry through Strouhal number St = Dfl /Vcur ; fl should be kept far from the

structure’s natural frequency to avoid resonances.

In the case of static analysis, equivalent static forces are applied considering the amplitude of

the fluctuating actions and, eventually, applying proper load amplification factors.

4. NUMERICAL MODELING OF THE STRUCTUREAs stated in Section 1, a 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.

In general, four model levels are defined for the structure:

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 between different structural parts.

The component shapes are modeled in an approximate way, the geometric ratios

between 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;

dF z t cD

v z t c D v ziwat

w d wat w( ', ) ( ', ) ( '= +ρ π ρ2

4

1

2

·,, ) ( ', ) ( ', ) ( ', )t V z t v z t V z tcur w cur+ ⋅ +

dz '


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.

The features for different structural model levels are resumed in Table 1; a similar

distinction can be made regarding the specification of the external loads.

According to what said above, at the initial stage of investigation structural analyses have

been carried out with macro-level and meso-level models of the three offshore wind turbine

structure types previously described.

With reference to Figure 7 some of the developed macro-level structural models are shown

for the monopile (left part), tripod (middle) and jacket (right part) support structure.



Table 1: Definition of the model levelsModel Level Scale Maximum Detail Level Main Adopted

Finite ElementsSystem level wind farm approximate shape of the BLOCK elements

structural componentsMacro-level single turbine approximate shape of the BEAM elements

structural components, correct geometry

Meso-level single turbine detailed shape of the SHELL, SOLID elementsstructural components

Micro level individual detailed shape of the SHELL, SOLID elementscomponents connecting parts

Foundation

Transition

(a) (b) (c)

Immersed

Emergent

Y

Z

X

Figure 7: Macro-level finite element models: monopile (a), tripod (b) and jacket (c).

The effect of foundation medium should be simulated with a full non-linear model in order

to account for possible plastic effects and load time-history induced variation of the

mechanical properties. At this level of investigation, an idealized soil has been simulated by

means of both:

• Linear springs: such technique has been adopted for the macro-level models.

Springs are applied at the pile surface and acts in the two coordinate horizontal

directions: the corresponding mechanical parameters have been set up on the basis

of available soil characteristic and simulates its lateral resistance at the pile

interface;

• Solid elements: used for meso-level models. These three-dimensional elements

simulate the linear mechanical behavior of the soil. The extension of the foundation

medium included in the model has been selected in order to minimize boundary

effects.

Both kinds of models have been used for evaluating the modal response of the structural

system.

The decomposition of both the structural system and the performance, and the

differentiation of the model levels can be used to guide and optimize the numerical analysis

efforts in this design phase. In this sense, focusing on a certain structural component and

selecting the specific performance, the choice of both model level and type of analysis to

adopt can be done, in such a way, to give the best efficiency of the analysis (deriving from a

suitable balance between the required detail level of outputs and the computational efforts

needed).

For example, focusing the attention on the tower with a steel tubular section, the

maximum stresses for Ultimate Limit State analysis can be preliminary obtained by adopting

a macro-level model and by carrying out a static extreme analysis (characterized by extreme

values of the environmental loads). However, if the local buckling phenomena need to be

assessed, a more detailed meso-level structural model and a static incremental analysis is

required. These considerations are summarized in Table 2.


Table 2: Model and analysis type selectionStructural Component Performance Model Level Analysis Type

MacroStress safety (ULS) � Static extreme

Meso

MacroGlobal Buckling (ULS) � Static incremental

MesoTower

MesoLocal Buckling (ULS) � Static incremental

Micro

Macro (poor)

Fatigue (FLS) � Meso Dynamic

Micro

5. NUMERICAL ANALYSES The numerical analyses have been conducted for three different support structures:

monopile, tripod and jacket. The principal geometrical and structural features adopted for the

analyses are as follows:

• hub height positioned 100 m above the mean sea level (m.s.l.);

• tower with a steel tubular section, with a diameter of 5 m and a thickness of 0.05 m;

• water depth of 35 m;

• foundations depth of 40 m;

• foundation diameter of 6 m (monopile), 2.5 m (tripod and jacket).

For the tripod substructure, the tubular tripod arm diameter and thickness is respectively

of 2.5 m and 0.05 m. For the jacket substructure, the diameter (thickness) of the vertical,

horizontal and diagonal tubular members, is respectively of 1.3 m (0.026 m), 0.6 m (0.016 m)

and 0.5 m (0.016 m). Finally, the tower supports a Vestas-V90 turbine [34] with a rotor diameter

of 90 m.

5.1. Modal AnalysisThe preliminary task of the dynamic analysis is to assess the natural modes of vibration in

order to avoid that non-stationary load (e.g. wind and wave induced) could cause the system

resonance when excitation and natural frequencies are closer.

Geometrical parameters of the three support structures have thus been selected with the

aim of maintaining the corresponding natural frequency far from that of the non-stationary

external forcing (wind and wave).

The finite element modal analysis provided the deformed shapes given in Figure 8, where

only odd modes are displayed since modes i and i+1 (with i = 1, 3) have the same frequency but

vibration occurs in orthogonal planes, according to the axial symmetry of the tower (the

eccentric mass of the blades is neglected).

In Figure 9, the two x-parallel dashed lines correspond, respectively, to the mean rotor

frequency (1P) and the frequency of a single blade passing (3P), which is triple with respect to

the former one for a three bladed turbine.

These frequencies determine the sampling period of the wind turbulent eddies and, as a

consequence, the characteristics of the induced non-stationary actions. Therefore, they



1st

(a) (b) (c)

M0 1st M01st M03rd M0 3rd M0

3rd M0

Z

YX

Z

YX

Figure 8: Modal analysis (macro-level models). Natural mode shapes for the monopile (a), tripod (b)

and jacket (c) support structures.

assume importance when performing dynamic analysis and are generally compared with

respect to the first natural frequency fnat in order to classify the structural behavior:

• if fnat falls below 1P the structure is called “soft-soft”; for this type of structure the

wave load could be dominant with respect to the wind load, and the fatigue effects

can be significant;

• if fnat is between 1P and 3P the structure is called “soft-stiff”; for this type of structure

the wind action frequency could be considerable higher than the one due to waves,

and the fatigue effects can be still significant;

• if fnat is greater than 3P the structure is called “stiff-stiff”; for this type of structure the

fatigue effects in general are not significant.

From the results plotted in Figure 9 it can be seen that the structural system falls in the soft-

stiff range only if the jacket support type is adopted. In the same figure, it can be noted that for

the first couple of modes the dynamic behavior of the jacket is stiffer than the one of the other

types, but the trend inverts from the third mode on.

5.2. Static Analysis Under Extreme LoadsSteady loads have been assumed for the principal environmental actions and no functional

loads are present (parked condition). The external forcing has been characterized by

assuming prudentially a return period larger than the one prescribed by Codes and Standards.

The numerical analysis for the selected support structure types has been carried out

considering the three load cases summarized in Table 3, where:

• Vhub represents the wind velocity at the hub height;

• VeN (with N = 1 or 100) represents the maximum wind velocity with a return period

TR equal to N years, derived from the reference wind velocity associated with the

same return period VrefN multiplied by a certain peak factor;

• VredN represents the reduced wind velocity with a return period TR equal to N years

and it is derived from the previous one by applying a reduction factor;


Figure 9: Comparison of the natural frequencies.

2.5

2.0

1.5

1.0

0.51P

3P

Fre

q. [H

z] Monopile

0.01 3 5

Mode number

Tripod Jacket



Table 3: Load casesDesign Combination Wind Marine Load Factors γF

Situation Name Condition Condition Environmental Gravitational Inertial6.1b Vhub=Ve100 H=Hred100 1.35 1.1 1.25

Parked(standstill 6.1c Vhub=Vred100 H=Hmax100 1.35 1.1 1.25or idling) 6.3b Vhub=Ve1 H=Hred100 1.35 1.1 1.25

In the same table HmaxN and HredN represent respectively the design maximum wave

height and the design reduced wave height associated whit a return period TR equal to N

years.

Steady wind field has been assumed along with stationary and regular wave actions; both

actions have been assumed to act in the same direction.

The design wind exerts a force distribution that is dependent on the undisturbed flow

pattern: the resultant action on the rotor blades has been concentrated at the hub height while

the drag forces acting on the support are distributed along the tower and the exposed piece of

the substructure (jacket type only). The immersed part of the support structure is subject to

combined drag and inertia forces induced by the undisturbed wave and the current induced

flow field.

In Figure 10, the calculated vertical profiles of the aerodynamic and hydrodynamic actions

induced per unit length on the tower and the substructure respectively are shown for the

monopile case. The analyses carried out through macro-level models allowed for evaluation

of both the reactions at the mud line (shear and overturning moment) and the induced

displacement at the hub height.

Results obtained with macro-level models are summarized in Figure 11. The maximum

shear stress at the mud line is reached for the load case 6.1c, i.e. the one characterized by

maximum wave height and reduced wind speed (see Table 3); on the other hand, the

combination giving the maximum bending moment at the mud line corresponds to extreme

wind and reduced wave height (combination 6.1b).

From what above follows that wave and current exert much more influence on the

resultant shear force, while the wind appears to be more critical for the overturning moment,

being distributed at a higher distance from the base.

0

20

40

60

80

100

120

0 5000 10000

Heightabovesealevel [m]

Aerodynamic

Action [N/m]

Hydrodynamic

0 100000 200000

Heightabovemud line [m]

Action [N/m]0

10

5

15

20

25

30

35

40

Action [N/m]0

10

5

15

20

25

30

35

40

0 100000 200000

Hydrodynamic

Heightabovemud line [m]

Comb 6.1b Comb 6.1c Comb 6.1b Comb 6.1c Comb 6.1b Comb 6.1c

Figure 10: Environmental actions (monopile type support).


Table 4: Applied loads and the numerical results (loads combination 6.1b)Monopile Tripod Jacket

Actions Wind on rotor [KN] 1663 1663 1663Wind on tower [KN] 740 740 428Wave and current [KN] 3372 3372 3500Overturning moment [KN*m] 350456 350456 337087

Reactions at Shear reaction at mud 5775 5775 5591mud line line [KN]

Vertical reaction at mud 10714 10356.3 13768 line [KN] (max in pile (max in pile

=15018) =9929)Structural Maximum stress in the 286 230 151checks tower [N/mm2]

Nacelle displacement [m] 4.66 3.72 1.82

Moreover, from the same figure it can be seen that the three structural types experience

approximately the same resultant shear and moment under each load combination.

Concerning the horizontal displacement at hub height, it can be seen an increasing stiffness of

the support structure moving from the monopile to the jacket type under each load

combination. Maximum displacement occurs always for load case 6.1b giving rise to the higher

overturning moment; for the jacket type it is almost one-third the one of the monopile.

In Table 4 the applied loads and the numerical results obtained for the more severe

combination (6.1b) are reported, where the maximum stress in the tower has been computed

by the combination of compression (or tension) and bending stresses.

400000

350000

300000

250000

200000

150000

100000

50000

0[KN*m] Monopile

6.1b

Tripod Jacket

6.1c 6.3b

7000

6000

5000

4000

3000

2000

1000

0[KN] Monopile

6.1b

Tripod Jacket

6.1c 6.3b

5.0

3.03.54.04.5

2.52.01.51.00.50.0[m] Monopile

6.1b

Tripod Jacket

6.1c 6.3b

Figure 11: Overturning moment, total shear reaction at the mud line and hub displacements, for three

different load cases.

From the previous results, it can be deduced that the jacket support type is the best choice

for what concerns the structural response under extreme loads (above all for the maximum

stress in the tower and for the nacelle displacement).

A meso-level model has been prepared for this type of support, after the exploration of a

number of tentative models (the model is shown in detail in Figure 12).

The meso-level model of the OWT structure is shown in Figure 13 (left part), while the right

part of Figure 13 shows the foundation medium (five substrates with different mechanical

characteristics), modeled using brick finite elements.

This level of detail allows the designer to investigate the internal state of stress for critical

parts (Figure 14). The connection between the tower (shell elements) and the jacket is

modeled using rigid beams elements (middle part of Figure 14). The meso-model is subjected

to the load case referred to as 6.1b in Table 3 (most severe); the result gives a nacelle

displacement equal to 2 m and a maximum stress in the tower equal to 178 MPa (at the jacket-

tower connection). This is in good agreement with the result of the macro-level. The small

differences are probably related to the variation in the tower diameter (ranging from

5.0 meters at the tower base to 3.4 meters at the top) along the vertical direction and to the



Figure 12: Detailed jacket support structure meso-level model.

varying thickness of the tubular member at a fixed transition section (right part of Figure 14).

These features are properly reproduced in the meso-level model, while in the macro-level

model they are set equal to their maximum values.

5.3. Buckling AnalysisAnother important aspect concerns the stability problem. A static incremental analysis has

been conducted in order to assess the buckling load; in this case, the hydrodynamic actions


Figure 13: Meso-level structural model of the jacket and corresponding deformed shape under static

aerodynamic and hydrodynamic loads.

Figure 14: Elastic internal state of stress at critical zones, jacket-tower connection and tower

thickness transition.

have been schematized by using of single force acting on the jacket at the mean water level

(Figure 15).

The analysis gives a multiple of 1.17 for the extreme load case referred to as 6.1b in Table 3.

It is important to outline that the first buckling mode shows a local instability of the tower

tubular section, an effect that cannot be accounted for with the macro-models.

6. CONCLUSIONSIn this paper, the system approach has been proposed as a conceptual method for the design

of offshore wind turbine structures. In this sense, a structural system decomposition has been

performed, with a specific view on the structural analysis and performance. The presented

considerations aim at 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, serviceability and reliability for the entire lifespan. Furthermore, numerical analyses

have been performed to compare the safety performance of three different support structure

types, generally adopted for a water depth lower than 50m: monopile, tripod and jacket

support structures. Extreme loads with a recurrence period of 100-years have been applied at

this stage of investigation.

Well-known analytical formulations have been summarized for correct characterization

of both the aerodynamic and hydrodynamic actions, whose contribution is crucial for

assessing the structural behavior. An early analysis has been carried out for the investigation

of the dynamic response for each one of the three support structures. Thus, the natural modes

of vibration have been determined in relation with the principal geometrical design

parameters. This is essential for avoiding the occurrence of resonance when the frequencies

of the external forces could excite the structure’s natural modes. A subsequent static analysis

has been carried out simulating three different load combinations as prescribed by

International Standards: the relative influence of aerodynamic and hydrodynamic loads has

been assessed, focusing on the resultant shear force and the overturning moment at the mud

line, and on the horizontal displacement at the hub height. This step is introductory for the

selection of the jacket structure as the appropriate support type.

Moreover, the internal state of stress under the abovementioned steady extreme loads has

been evaluated by means of two different levels of detail for the numerical models (macro-

and meso-level). The analyses have confirmed that macro-level model results can predict the

basic aspects of the structural response, yet the meso-level model provides an additional and

more detailed picture of the structural behavior due both to the major capabilities of the



Figure 15: Results of the buckling analysis.

adopted finite elements (shell and brick instead of beam elements) and to the higher

geometrical resolution of the models.

Finally, an incremental analysis has been carried out to assess the buckling load of the

examined offshore wind turbine: this occurs in the tower tubular section for a multiplier equal

to 1.17 for the more severe extreme loads.

Starting from the results presented here, future and more refined studies can take into

account for other relevant effects influencing the dynamic response of the structure (e.g.

scour, coupling with foundation medium, non-stationary loads, non-linear interactions etc.) by

performing transient analyses.

ACKNOWLEDGEMENTSThe present work has been developed within the Wi-POD Project (2008-2010) and other

research projects in the field of wind engineering, partially financed by the Italian Ministry for

Education, University and Research (MIUR). Fruitful discussions with Prof. Pier Giorgio

Malerba of the Politecnico di Milano, Prof. Marcello Ciampoli of the Sapienza – Università di

Roma, Professor Hui Li of the Harbin Institute of Technology and Dr. Ing. Gaetano Gaudiosi of

the OWEMES association are gratefully acknowledged. Finally, Prof. Jon McGowan is

acknowledged, for inspiring part of this work.

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