FEA-Wind Turbine Tower

7/30/2019 FEA-Wind Turbine Tower

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What FEA can be used in:The whole tower analysisDetailed analysis of opening

Detailed analysis of the connections

Prof. Ch. Baniotopoulos

I. Lavassas, G. Nikolaidis, P.Zervas

Institute of Steel Structures

Aristotle Univ. of Thessaloniki, Greece

HISTWIN-High strength steel tower for wind turbines - RFSR-CT-2005-00031


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Need for using a FE model

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A wind turbine tower is a simple cantilever.

Assuming uniform wind pressure along its height, moment &

shear force at any point can be calculated directly regardlessof the tower configuration

Even when tower wind pressure is a function of height

[ p=p(h) ] , moment & shear at the base can be calculated

also by simple integrations of the load function



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Meridional stress for local buckling check can also be

easily calculated by hand

Tower displacements & eigenmodes cant be calculated

so easily because of the change of the elastic

characteristics of the cantilever along its height.For this calculation a computational model is needed



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Computational model (Linear):

All sections of the tower are simulated via linearbeam elements.

Rotor & blade system is simulated as a mass atthe top of the tower placed with eccentricity

Soil-structure interaction can be simulated by theuse of a rotational spring at tower base

By the use of a linear model we can calculatetower displacements and perform an eigenvalueanalysis with accurate results.

So whats the need for a Finite Element model?

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Cross section of the tower is deformed due tothe wind loads

Stress concentration at the door positioncannot be estimated by a linear model

Bolt forces at flange positions cannot be

calculated from the linear model. A specialFE model is needed.

Anchor forces and the stress state of the

concrete on anchoring position need to bedetermined.

Soil-structure interaction affects the towerdynamic characteristics, and itsdisplacements for wind loading.



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Wind pressure distribution over the circumferenceof the tower causes ovalling of the circular towersection

But when it comes to the positions where the towersection is much stiffer (flange & support positions) ,the section shape is forced to be circular.

This causes circumferencial stresses to the shellnear flange positions

In buckling check (EC 3-1-6) not only meridionalstress but circumferencial & shear stresses areused in combination as well



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Modern Finite Element Software

A few years ago, developing of a complex Finite Element model

was very difficult due to:

Need of expensive big computers (and expensive software) to run a non-linear analysis

Too much effort for developing the FE model geometry and loading

To much effort by hand to interpret the results (stress integrations etc).

Modern Finite Element software that can run on a simple PC offer:

Direct input of the geometry from CAD software

CAD capabilities inside the FEM software

Geometrical & material non-linear analysis capabilities

Unilateral contact with friction capabilitiesNon-linear analysis now can run on a personal computer

Automatic procedures for results interpretation

Interfaces to join with design software



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Modelling strategies

Modelling Strategies can be described by the chart below:

Overall model

Linear FE model

Foundation included Separate model for the Foundation

Included to the general model Separate models for the details

Foundation

Details (Flanges, Door position etc) : Finite Elements

Foundation model

Linear(grid on elastic support) FE model (including anchoring detail)

Three different strategies will be presented



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Modelling strategies 1:Linear model

1: Linear model for the tower, andanalytical detailed FE models to the doorand flange positions



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FE model for the flanges and for theanchoring system



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Flanges are modelled using plate elements

Bolts are placed as linear elements connecting the

flanges active only in tension

All other nodes are connected thru unilateral

contact elements.

Moment & shear force is applied distributed

to the circumference.




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Advantages:

Easy developing of the general model and the part models

Fast calculation (on a simple PC: instant calculation for the overall model & 10-15 minutes for thenon-linear models of the flanges)

Easy change of the model configuration

Disadvantages:

Circumferencial variation of the tower loads cannot be introduced

Boundary conditions for the part models need to be estimated

Only meridional stress can be calculated

Circumferencial & shear stresses are neglected

EC 3-1-6 requires the complete stress-state (meridional, circumferencial & shear) for the bucklingcheck

Only Axial & Shear forces and Bending moment are available to be applied to the part models. In-

plane deformation of the tower due to the wind load distribution is neglectedSoil-structure interaction is neglected or need to be estimated by introducing a rotational spring on

tower support




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Modelling the foundation by linear elements

(grid of beams on unilateral elastic support)

to simulate soil-structure interaction


Simulates soil-structureinteraction

Good for the design of thefoundation

It doesnt give an answer tothe stress state of theanchoring system



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Modelling strategies 2:Simple FE model

Simple FE model for the tower, andcorresponding analytical FE models to thedoor and flange positions



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Modelling strategies - 2:Simple FE model

Advantages:

Fast calculation in PC (about 1-2 minutes a run for the general model and about 10-15 minutesfor the non-linear analysis of the flange models)

Estimation of the full stress-state on any point

Disadvantages:

Boundary conditions for the part models need to be estimated

Much effort is needed for the transfer by hand the stress-state of a specific cross-section to thecorresponding cross-section of the part model.

Soil-structure interaction is neglected or need to be estimated by introducing springs on towersupport



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Modelling strategies - 3:Detailed FE model

1: Complete detailed model forthe tower and the foundation

Shell skirts are modelled usingshell elements

Flanges are modelled usingbrick elements

Foundation is modeled usingbrick elements

Unilateral contact to the ground



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Model details to the flange positions

Modelling Strategies 3: Detailed FE model

Connection type for the flanges



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Simulation of the eccentricity of mass at the top of the tower

Top-flange is undeformed in-plane (rigid links connection).

Modelling strategies 3:Detailed FE model



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Modelling of the tower- foundation

anchoring

Embedded flange to the concrete

Partially prestressed anchors inside (but notconnected to) the concrete

Unilateral contact between flanges & concrete

Collapse due to anchoring system failure

Nanamata, Japan

Modelling strategies 3: Detailed FE model



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Foundation Modelling:

Concrete & shrink-mortar : Brick elements

Washer plates : Plate elements

Anchors : cable type elements active only in tension

Unilateral contact to the ground

Modelling strategies 3:Detailed FE model



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Cross-section to the foundation

Modelling strategies 3:Detailed Fe model



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Anchoring Modelling detail

Each anchor is a single element

connecting only the washer plates (active intension)

Unilateral contact conditions betweenthe base washer plate and the shrinkmortar & between the embeded washerplate and the concrete




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Design of the foundation-anchoring system

Washer & base plates (von Mises stresses)

Prestressed anchors (tensile forces)

Non-shrink mortar (compressive & shear stresses) Concrete (compressive,shear & punching shear stresses)

Reinforcing bars




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Reinforcing bars check points

1,2 : Footing, bottom mesh

3,4 : Footing, top mesh5 : Top of pedestal mesh

6 : Pedestal vertical rebars (transfer of tensile anchor forces)

7 : Circumferencial reinforecement (split-up forces)




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Modelling strategies - 3:Detailed FE model

Advantages:

Estimation of the full stress-state on any point

Best possible approximation to the real-world situation

No transfer of loads is needed from one model to another

No need to estimate any partial model boundary conditions

Disadvantages:

Big effort for developing the FE model

Changes to the model are difficult

Non-linear calculation needs time to run on a personalcomputer (can take 4-5 hours a run on a modern PC)



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Tower loads

Vertical loads

Self mass & weight is estimated directly by the FE software

The rotor & nacelle mass is applied to the top of the tower distributed to the nodes of

the upper flange taking into account the eccentricity

(mass instead of loads, to be used in spectrum analysis as well)

Wind loads

At the top of the tower, rotor forces and moments are

applied

At tower stem wind pressure is calculated

acc. EC1-1-4 as a logarithmic function of z.



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Types of analysis

Types of analysis for buckling check

EC 3-1-6, LS3 (Buckling limit state):

LA (Linear analysis)

MNA & LBA (Material non-linear analysis & Linear buckling analysis)

GMNIA (Geometric & material non-linear analysis with imperfections)



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Types of analysis

Seismic loading:

Response spectrum analysis for the seismic loading must be performed

Additional time history harmonic response analysis for the seismic loading

(only in extreme cases)

Due to the distributed mass of the tower itself two eigenmodes are participating.

Equivalent lateral load method cannot be used

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Comparison of results Wind loading

Tower displacements for the linearmodel vs FE models

Linear model is accurate to theestimation of displacements forwind loading.



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Comparison of the results Eigenvalue analysis

1st & 3rd mode shapes for the three models



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Comparison of results

Comparison of the results for the three models



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Conclusions:

Although the use of an overall complex Finite Element model needsmore calculation effort, it is necesary in order to establish a betterapproach the stress state on the structure

With modern Finite Element software such a nonlinear analysis can be

run on a simple personal computer

The cost of developing such a model is extremely small compared withthe budget of a single aeolic park installation

Simplified models (linear model, even hand-calculation) are alsonecessary to develop in paralell, for initial design, and for checking theresults of the FE model


Documents

FEA-Wind Turbine Tower