FEA-Wind Turbine Tower
Transcript of 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
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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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Need for using a FE model
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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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Need for using a FE model
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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Need for using a FE model
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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Need for using a FE model
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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Modelling strategies 1:Linear model
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.
Modelling strategies 1:Linear model
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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
Modelling strategies 1:Linear model
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Modelling the foundation by linear elements
(grid of beams on unilateral elastic support)
to simulate soil-structure interaction
Modelling strategies 1:Linear model
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
Modelling strategies 3:Detailed Fe model
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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
Modelling strategies 3:Detailed Fe model
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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)
Modelling strategies 3:Detailed Fe model
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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
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