Structural Design of HybridTowers for Wind Energy Converters

1
Structural Design of Hybrid-Towers for Wind
Energy Converters
Prof. Peter SchaumannChristian Keindorf
Leibniz Universität Hannover Institute for Steel
Construction Prof. Dr.-Ing. P. Schaumann
2
Motivation

• New tower concepts for the next generation of WEC
• Increase of ultimate limit state (shell
  stability)
• Increase of fatigue limit state (welding details)
• Check the use of high strength steels
• New types of connections
• New materials

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Ultimate limit state (shell buckling)

• Shell buckling due to axial compression DIN
  18800-4, EC 3 Part 1-6

Axial compression
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Comparison of tower sections with various steel
grades

• Geometry H 30 m / D 5.5 m / t const.

ST 235 t50
ST 355 t35
ST 460 t24
ST 690 t16
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A new concept for tower sections

• The objective is to increase the shell stability
  simultaneously with reduction in overall masses

outer steel face
core material
inner steel face
SCS
ST
SGS
SES
Steel
Steel-Elastomer-Steel
Steel-Grout-Steel
Steel-Concrete-Steel
Idea Sandwich shells with a core material as
full space stiffener, for

• increasing the shell stability
• using high strength steels
• assembly thinner steel plates
• additional load capacity due to the core
• decreasing weld deposit and pre heating -gt
  faster welding processes
• new hybrid tower variants and new type of
  connections possible

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Hybrid- and Sandwichtowers

• Parameter study

steel grades S 235 S 355 S 460
tower section L 30000 mm R0
2750 mm t-1/1 8 - 50 mm
core materials grout elastomer t0 0 - 80 mm
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Optimization of shell stability
Start the optimization process with buckling
reduction curve
1.
3.
Get the critical buckling stress
for S 235
for S 460
4.
Thickness of core material
0.25
2.
Get the limit value depending on the relative
slenderness of shell
5.
68 mm
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Buckling loads and tonnage
the objective was increase the shell stability
simultaneously with reduction of overall masses
is possible in theory! but in practice?
tonnage
buckling loads for axial compression
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Shell buckling tests
Questions for the buckling tests with
sandwich-cylinders

• Increase in shell stability?
• Additional load capacity due to the core
  material?
• Remain stable up to the yield stress of steel
  faces?
• Bonding between core and steel faces sufficient?
• Which failure modes occur?

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Test Setup (model scale tests)

• Shell buckling tests for axial compression of
  sandwich cylinders

SES
SGS
processes of injection - vertical or
horizontal position - prefabrication or
in-situ - full or parts of core volume - with
or without top plate
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Buckling curves
Npl,ST_1 Npl,ST_2 178 kN
(only inner steel shell tested)
(only outer steel shell tested)
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Axial load capacity
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Failure modes
shear crimping
general buckling
face dimpling
face wrinkling inside and outside
local impact
face yielding or fracture
core shear failure
Source Dan Zenkert
SES
SGS
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Fatigue

• increased stress ranges Ds for steel faces with
  high strength steels, but no reduction in fatigue
  resistance due to the thickness effect!

with

• example circular butt weld between segments
  (FAT 90)

• increase in fatigue strength necessary
• using methods for post weld treatment

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Post weld treatment
as welded (Dsc 95 MPa)

• Enhancing fatigue resistance by high frequency
  needle peening (Ultrasonic Impact Treatment, UIT)

UIT
weld
pin
weld toe
with UIT (Dsc 205 MPa)
fatigue crack
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A new type of connection
steel section

• advantages
• - double shear tube-in-tube connection
• - lower overlapped length as for grouted joints
• - no eccentricities
• - compensation of imperfection possible
• no ring flanges
• no bolts
• fewer hot spots than for ring flanges and bolts
  (fatigue)
• - lower costs for inspection and maintenance
• stiff connection due to three parallel shells

high performance grout
overlapped length
sandwich section
pre-cast core material
spacer
no ring flanges

• disadvantages
• welding of additional shear keys
• more erection techniques and work

base section
foundation
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Conclusions

• New tower concept with sandwich shells
• Increase in shell stability up to the plastic
  range
• Additional load capacity due to stiff core
  materials
• Using S 460 for steel faces, but not S 690
• Enhanced fatigue strength of welds necessary
• New type of connection without bolts and ring
  flanges

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Structural Design of Hybrid-Towers for Wind
Energy Converters
Prof. Peter SchaumannChristian
Keindorf keindorf_at_stahl.uni-hannover.de www.stahl
bau.uni-hannover.de