Ground Based Wind Turbine Blade Inspection

© 2016 Electric Power Research Institute, Inc. All rights reserved.
John Lindberg, PE
EPRI Program Manager – NDE Innovation
John W. Newman, President
Digital Wind Systems, Inc.
Sandia Wind Turbine Blade Workshop
August 31, 2016
Ground Based Inspection and Monitoring
of Wind Turbine Blades

2
Discussion
 Comprehensive Blade inspection – Drivers
 Ground based inspection overview
 Preliminary research
 SABRE™ data overview
– Infrared (IR) data
– Photography
– Acoustic spectral analysis data
 Summary

3
Blade Reliability, Statistics –
Why blades need inspection
 Blade Failure - No. 2 Problem for Reliability in Wind Turbines – Sandia
Surveys 2008, 2011 (update)
 Wind turbine rotor blades are failing at a rate of ~ 3,800 a year = 0.54% of the
700,000 or so blades in operation worldwide (WindPower Monthly)
 Annually, 1 to 3% of Turbines Require Blade Replacements with Spikes in
Years 1 and 5 (DNV GL)

4
 63 GW of installed wind capacity added globally in 2015 (Global Wind
Energy Council) to a total of 432.9 GW
– World's installed wind capacity is on track to reach 705.5 GW by 2020
(AWEA/Navigant Research)
– US – 8598 MW added in 2015; total – 74,471 MW (Global Wind
Energy Council);
 18 GW of new capacity under construction or in the advanced
stages of development (AWEA)
 With wind energy growth continuing at its current pace, there is a growing
need for safe, cost effective, comprehensive blade health monitoring to:
– Improve blade reliability and service life;
– Implement better, faster, cheaper blade inspection to better
manage blade life; and drive down blade maintenance costs
Ground Based Inspection of Wind Turbine Blades - Drivers

5
Visual examination and tap testing blades may be unrevealing
(limited to damage at the surface or within 3mm)
What flaws
are being
missed?
A more
efficient,
safer
inspection
method is
needed.
Photos – Courtesy of Digital Wind Systems

6
EPRI researched advanced inspection technologies for the assessment
of wind turbine blade integrity; and chose to develop, demonstrate, and
validate Digital Wind System’s (DWS) SABRE™ Ground Based
Inspection System.
SABRE™ features:
• Safer - No uptower access required - Wind turbine blades are inspected
from the ground, while operating, for detection of surface and subsurface
structural defects.
• Faster - Towers inspected in < 30 minutes (50 meter blades), in good weather.
• Incorporates multiple proprietary sensors technologies: Thermal Imaging,
Wide-Band Acoustic Spectral Analysis and High speed/resolution photography
• Detection and monitoring of blade anomalies such as: gel-coat and
propagating fatigue cracks; breaking, weak or missing adhesive bonds/joints;
leading edge erosion, fiber waves, delaminations, lightning damage,
poor repairs, pitch errors.
Ground Based Inspection of Wind Turbine Blades

7
SABRE™ - Closer Look
IR Camera
Acoustic Analysis
Microphone

8
SABRE™ System Elements and Validation
Copy Right © 2016 Digital Wind Systems Inc. All Rights Reserved
Prototype SABRE™ System
Mobile turbine rotor inspection Data Acquisition Analysis and Report
Generation
Digital Wind Systems, Inc., in collaboration with EPRI is completing a multi-year
project to validate the “SABRE™” ground based, high speed inspection technology,
for evaluating the structural integrity of wind turbine blades.
• Rotors at EPRI member wind farms were tested with results correlated with up
tower findings, telephotography and maintenance records wherever possible.
• In addition DWS has tested and has validated findings for an additional 634
turbine blade sets to build a detailed knowledge base.

9
Demonstrated SABRE™ Blade Evaluation Capabilities
Structural: Latent Manufacturing Flaws & Repairs
1. Resin Starved Areas -
2. Stress/Fatigue Cracking
3. Fiber Waves, Wrinkles
4. Trailing Edge Adhesive Bond Width
5. Composite Repairs
Operational Anomalies: Degraded Power Curve
1. Blade pitch errors – Major cause of vibration, wear
2. Drag inducing anomalies- Leading Erosion, Contamination

10
Latent blade manufacturing flaws may fail minutes
or many years after the blades enter service.
Or they may never fail.

11
Preliminary Research - Fiber Waves and Blade “Hot Spots”
• Fiber waves are a leading cause of WT blade
failure reduce the load bearing capacity of the
fiber matrix
• Generate heat due to thermo-elastic stress and
internal friction during cyclic stress
• Stress and friction induced temperatures exceed
the glass transition temperature, initiating
cracks, and propagating at crack tips.
Blade Test at NWTC

12
Sandia “Sensor” 9M Blade Tests
• 3 Fiber Wave Defects implanted in Spar Cap @
3.5m, 5m and 6m and fatigue test > 1M cycles
• 6 acoustic emission (AE) sensors on the spar cap
• Early experiments using IR for fiber wave detection.
Below, LIR scan of blade during fatigue tests
showing thermal emissions from the programmed
fiber wave defects.
DWS IR
Sensor
Sandia
AE Sensors

13
Multiple varying loads on blades
created generates internal heat
by the viscoelastic effect.
The temperature profile
corresponds to the sum of all
principal stresses. Flaws in
blades may generate heat if
propagating.
Lord Kelvin, 1853
SABRE IR Imaging – Stresses in Operating Blades

14
Fiber Wave and other indications on the HP side Carbon Spar Cap @ 32.1m and 37.8m
Evaluation:
Indication metrics:
1. location
2. dimensions, area
3. signal-to-noise ratio (S/N)
4. temperature
FW1 FW2Reflection of Spar
Tower/Nacelle
37.8m32.1m
SABRE DWS
Single
Frame
Multiple
Sequential
0.33msec
Frames
SABRE™ IR Imaging
Size is .2x.2m and .5x.5m

15
Defective Carbon Spar Cap Joint
• operational for 7 years
• failures in Type
Defective Carbon Spar Cap Joint @ 30m
DWS Proprietary Information
Copyright © 2015 Digital Wind Systems Inc. All Rights Reserved

16
IR Results
SABRE Tested : 12/15/2014 Result Rating: 5 Date of Failure: 3/20/2016
Indication at approximately 107 ft. LE crack at adhesive
bond line, span-wise length approx. 4 ft.

17
Possible Resin starved area on Blade A. This turbine had been
in service more than two years.
SABRE™ Imaging – Resin Starved Blade Root
Blade A Blade B Blade C

18
SABRE Laminar vs. Turbulent Air Flow
DWS Proprietary Information
Copyright © 2015 Digital Wind Systems Inc. All Rights Reserved
Laminar
Flow - Dark
Turbulent
Flow - Bright
VGs

No significant
Erosion or
Contamination

IR Result:
HP Side: Moderate LE erosion/contamination
inboard, severe outboard
LP Side: Severe LE erosion/contamination from the end of
the VGs outboard to the blade tip reducing power output.

HP Side Blade B Tip LP Side
SABRE Blade Imaging – IR & Visible Light

22
 Operating wind turbine blades generate up
to 3 psi at the blade tip due to centripetal
acceleration.
– Compressed air, escaping through
cracks, lighting strike holes generate
sonic and ultrasonic signals.
 Wide band acoustic sensor detects these
Doppler shifted signals
 Data is collected less than a minute, at
bottom dead center of rotor
 Digital spectral analysis software algorithms
determine the location of these (Doppler
shifted) acoustic sources on the blade.
 Detects: breaches in blade outer mold line
due to leading or trailing edge splits,
cracks, lightning strikes/damage;
aerodynamic changes due to improper
blade pitch angle/control, disruption of
airflow due to damage; and mechanical &
electrical noise
@17.4 rpm
SABRE™ - Acoustic Spectral Analysis (Patented)

23
Approaching
Blade
Receding
Blade
Time
Signal
Frequency
SABRE™ Acoustic Spectral Analysis

24
New Blades
10 Year Old Blades
SABRE™ Acoustic Spectral Analysis
• LE Erosion
• 1 lightning
• 2 noisy
Lightning
receptor

25
Acoustic Results – Same wind turbine as
previous slide
Acoustic response indicative of moderate to severe
leading edge erosion on all blades causing power loss.

26
Acoustic Spectral Analysis- Blade Tip TE Split
Acoustic spectrum data is collected by a sensor located at bottom center of wind turbine.
In the example shown below, a signal (arrow) is seen in the spectrum of blade pass (1).
The spectrum below shows three consecutive blade passes, labeled (1, 2 & 3), as
recorded at a location in the plane of the turbine disk at bottom dead center. The signal
parameters are measured and entered into the SABRE software to calculate the position
of the sound source anomaly on the blade.
Trailing Edge Split at Blade Tip
The SABRE calculation determined the source location, shown
above, to be 129 ft. from the hub. Up-tower rope access technicians
found the trailing edge split shown at left, 130.7 ft. from the hub.
Center of blade pass
Blade approaches Blade recedes
Photo – Courtesy NextEra

Acoustic Result: Blade B has a strong but intermittent indication at 39m (128 ft.) located at
lightning strike. Blade B also has acoustic signals showing LE erosion or LE tape degradation
(red arrow). Blade A has moderate LE erosion or LE tape degradation.
Blade B showing LE erosion
or LE tape degradation
A B C
Blade B HP showing candidate lightning strikes at approximately 39m.

28
Table 2. Defect Key:
FW = Fiber Wave (Wrinkle) Defect
DL = Delamination, Crush Damage
LG = Lightning Strikes
LE-E/C = LE Erosion/Contamination
CR = Crack
SC = Stress concentration
TES= Trailing Edge Split
LES= Leading edge Split
GC = Degraded Gel Coat
Flaw Evaluation and Reporting – Typical Evaluation Key
Rating Description Repair Priority Action
Level 1 No Indications NA Run, Re-inspect 1 yr.
Level 2 Minor cosmetic, GC, LG, LE-E/C NA Run, Re-inspect 1 yr.
Level 3 Major cosmetic, LE-E/C, LG Low Run, Re-inspect 1 yr.
Level 4 Minor structural, DL, LG, CR <200x200mm Low Run, Re-inspect 6 months
Level 5 Moderate structural: TES,LES, CR, DL <1000x1000mm Med Run, Repair ASAP
Level 6 Major structural: All splits>1000mm, open cracks, Hi Evaluate, Repair /Replace
fiber waves on spar cap or blade root
Defect Severity Guide - EPRI report 3002001502

29
Demonstrated SABRE™ Blade Evaluation Capabilities
1. Evaluation of drag inducing leading edge erosion, a cause of AEP degradation
2. Detection and characterization of both surface and subsurface damage &
defects that may be undetectable with drones, visual examination or tap testing
3. Propagating fatigue cracks, transverse shell cracks
4. Leading, trailing edge and tip splits
5. Blade pitch errors - cause vibration,
loss of AEP
6. Fiber waves, propagating damage, delaminations
7. Disbonds, voids, breaking or missing adhesive joints
8. Defective composite or bolted repairs (RUC repairs)

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Ground Based Inspection of Wind Turbine Blades
Supplemental Project Notice 3002009063
Objectives and Scope
 Refine, and demonstrate long wave infrared
(LWIR) thermography and acoustic spectral
analysis techniques for the inspection and
assessment of operating wind turbine blades
Value
 Ground based; non-intrusive approach to
inspection of rotating wind turbine blades
 Enhance capability for early detection and
monitoring of wind turbine blade damage
Performed at: Duke Energy, DTE Energy,
WE-Energies, and Iberdrola wind farms
Assess structural integrity critical to reliable long term operation
Details and Contact
• Qualifies for Member Self Directed Funds
John Lindberg
• Program Manager – NDE Innovation
• jlindberg@epri.com, 704.595.2625
SPN Number: 3002009063

31
Summary
SABRE™ provides wind farm owner / operators
valuable information to:
• Safely monitor blade structural integrity and degradation
online, under operating conditions;
• Provides recorded data that enables identification,
categorization, trending of
blade anomalies for maintenance and repair;
• Recorded and analyzed data assists with inspection
and maintenance planning, condition and performance
monitoring
• May be used to survey potential wind farm acquisitions,
or assess blade integrity for wind assets nearing end of
warranty period.

32
The authors would like to thank:
Jeff Hammitt, NextEra Energy
Renewables Fleet Team
Scott Hughes, Mike Desmond
National Wind Technology Center
David Coffey, AWS True Power
Scott Abramson, Duke Renewables
Dennis Buda, DTE Energy
Louis Caracci, WE Energies

33
Together…Shaping the Future of Electricity

Ground Based Wind Turbine Blade Inspection

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