Bladena has developed a new retrofit technology called a D-String that limits panel deformation and reduces stresses on trailing edge bonds. A D-String installation was piloted on turbines in Denmark and Texas, and measurements showed the D-String reduced stresses by 97%, corresponding to a 10-fold increase in blade lifetime. Bladena aims to use the D-String both reactively on damaged blades and proactively on fleets to prevent failures and extend blade lifetimes beyond 20 years.

1. Together, we make your blades stronger
Bladena’s new simple retrofit for increased
reliability and repair of the trailing edge bonds
Dr. Carsten Hein Westergaard
President, NextraTEC Inc.
Board member, Bladena Aps
Bladena representative
cw@nextratec.com
Sandia National Laboratories
Wind Reliability workshop 2013
August 13 – 14, 2013, Albuquerque
Bladena US based service partners

2. Together, we make your blades stronger
Agenda
• Short introduction to Bladena
• Trailing edge damages
• Effect of D-string
• Pilot projects
• Installation
• Recommendations
• Future projects
©Bladena 2013

3. Together, we make your blades stronger
• Spin out from Danish Technical
University, established 2011
• Seven patents making blades
stronger and more durable
developed at RISØ (DTU)
• Technology based on 10+ years
of research & experience at DTU
• Based in Denmark
• Two Strong Service Partners in
North America
.
©Bladena 2013
D-String
Simply effective!
Bladena – the company
Together, we make your blade stronger

4. Together, we make your blades stronger
Catastrophic trailing edge failures,
starts small
• Repairs can be costly and does not eliminate the
need for larger fleet repairs or replacement at a
later stage
• Significant risk for catastrophic failure remains,
when the small cracks propagate unnoticed
Unrepaired cracks lead to de-bonding
Blade broken near max chord Trailing edge splitting
©Bladena 2013

5. Together, we make your blades stronger
Root causes of trailing edge cracks are many
• Design system, verification system and standards applied:
– Non accommodating for critical errors; for example; lack of combined loads
verification during testing
– Exclusion of important failure mode combinations; for example, breathing of panels
which leads to accelerated peeling
• Manufacturing deficiencies:
– Tolerances, random and/or systematic; for example, incorrect application of fibers
– Quality deviations; for example, application of glue or curing of glue bond deviations
• Unforeseen blade loading events:
– Systematic; for example, by failure of sensors
– Random occurrence; for example, induced by extreme weather events
• Service methods
– It is very difficult to detect hairline cracks, service methods can influence the root
cause of failure mode development
• Almost always, failure is a sequence or combination of these root causes
©Bladena 2013

6. Together, we make your blades stronger
Blade panel deformations induced
by edgewise gravity induced loads
during any operation of any wind
turbine makes the panels breath
©Bladena 2013
Panel breathing

7. Together, we make your blades stronger
Panel breathing and peel stress
• Peel stresses at the bond lines,
peels the adhesive from the
shells by stress concentration
• Until peeling occurs, the
stresses will create surface
tension right at the trailing
edge bond line eventually
developing into (hairline)
cracks
• Trailing edge bonds are
designed for shear stress,
which occurs from blade
bending during wind loading.
• Peel stress is added to the
shear stress
©Bladena 2013

8. Together, we make your blades stronger
Panel breathing accelerates crack
propagation through peeling and
necessitates regular inspections and
preventative repairs
Hairline crack found at inspection
Uptower repairs of trailing edge cracks
©Bladena 2013
It starts with hairline cracks

9. Together, we make your blades stronger
D-String reduces breathing
(Deformation Limiting String)
Typically, ten strings
spaced along the trailing
edge limits the panel
breathing, sufficiently
©Bladena 2013

10. Together, we make your blades stronger
The D-String limits the
outward panels from
breathing, hence reducing
peel stresses in the bond
lines
©Bladena 2013
Without D-Strings With D-Strings installed
Typical 10 D-Strings installed in
max chord area
Typical 10 D-Strings per blade
Finite element model prepared for
a specific blade sized by calibration
with site measurements of the in-
situ panel deformations

11. Together, we make your blades stronger
First demonstration project
– rope access installation
Installed in Denmark in January 2013.
©Bladena 2013

12. Together, we make your blades stronger
First US Pilot Project
First pilot under installation in Texas
Platform access
Partners:
©Bladena 2013

13. Together, we make your blades stronger
First step: Size string by one time
measurement of panel deformation
Currently three blades are documented with D-
string implantation
For other blades:
A. Blade Survey to establish key dimensions of
blade, and determine needs for step B&C
B. Pilot, where loads and deformation levels are
measured and installation methods are tested
C. If needed: panel testing to establish panel
strength
D. Feasibility study establishes:
 Suitable strength and fatigue life of panel
and D-String
 FEM simulation
 Verify the effect of the D-String
 Variations from a standard installation
©Bladena 2013
Step B: Measurements with temporary
installed sensor from rope access, typically
there are three places used to qualify D-string
installation

14. Together, we make your blades stronger
Estimating impact through measurements
Without D-
Strings
With D-Strings
installedTest on turbine, results from
temporarily installed sensors
Resulting analysis
©Bladena 2013

15. Together, we make your blades stronger
Easy Installation from platform or rope access
©Bladena 2013

16. Together, we make your blades stronger
Static safety against damage
to blade skin from D-string
• A comprehensive test program of
the D-String has been performed at
DTU, demonstrating safety margins
against damage to the supporting
blade skin foam sandwich
• A string (fuse) is included for well
defined protection at breaking
point 1.5kN
• Field test has shown measured
string loads are very low, with
peaks up to 0.4kN during
emergency stops
©Bladena 2013

17. Together, we make your blades stronger
Fatigue safety against damage to blade
skin from D-string
Measured
extreme event
Typical normal
load situation
Fuse
©Bladena 2013

18. Together, we make your blades stronger
Increased reliability based on measured data
• Based on the in-situ measured deformation and stress, the lifetime stress
reduction can be calculated. By using Palgren-Miners, rule the life
extension can be calculated
• In the pilot projects, the measured improvement from the D-string is a
97% reduction in stress. This can be translated to:
“If the blade was prone to cracks after 4 years, the life expectancy would
be expanded to 40 years, i.e. a factor 10 in lifetime”
Without D-
Strings
With D-Strings
installed
Resulting analysis based on measurements
©Bladena 2013

19. Together, we make your blades stronger
Preventive or reactive use ?
Age Frequency of
damage
Action Result of D-string use
Reactive
A few damaged
blade(s)
1 to 4
years
One time isolated
event(s)
Add D-string to
blade repair
• Avoid scrap
• Additional safety in repair
• “New” healthy blades in
inventory
• Direct savings
Proactive
Multiple
damaged blades
2 to 10
years
Multiple events,
more than 1/200
rotors damaged
per year
Install D-string fleet
wide on turbine
type, preferably
combined with
blade inspection
• Life extension through reduced
stress in hairline cracks
• Life extension far beyond
20 years for un-cracked blades
• Preventive savings by reduced
(eliminated) failure rate
Preventive N/A No damage
documented
Install D-string fleet
wide on turbine type
• Strongly reduced sensitivity to
manufacturing deficiencies or
any other potential weaknesses
• Statistical mean value moved
far beyond 20 year life, so “no
failures” will occur
• Stronger blades
Improved new
built blades
N/A Statistical variance
of design
Include D-string in
new blade build
©Bladena 2013

20. Together, we make your blades stronger
Next project: Cross-sectional shear
distortion was observed in the field
Measurements on a MW turbine showed cross-sectional shear distortion under normal operation
Cross section shear, adds
loading to the leading edge
bond, and possible additional
failures
©Bladena 2013

21. Together, we make your blades stronger
Three new combined technologies making
blades stronger in the field
1) D-String
2) X-Stiffener3) Floor
1) Stronger trailing edges 2) Stronger leading edges3) Strong root panels
less prone to buckling
©Bladena 2013

22. Together, we make your blades stronger
©Bladena 2013
Thank you
Dr. Carsten Hein Westergaard
President, NextraTEC Inc.
Board member, Bladena Aps
Bladena representative
cw@nextratec.com
Bladena US based
service partners
Bladena US based
service partners