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Wind Turbine
Foundation Design
by
Tony SlatonBarker, MS, PE, LEED AP
Coffman Engineers
and
Travis Ross, PE
Golder Associates
International Wind-Diesel WorkshopMarch 2011, Girdwood, AK
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Many of AVECsvillages are inWestern Alaskahave Class 4 or
better windregimes.
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Toksook Bay
Foundation Design Overview
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Wind Turbine Locations
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Toksook Bay Foundation
Design CriteriaTurbine: NorthWind 100 (100 kW Turbine)
Tower: Danwin Tower (108 Feet, 32 meter)
Design Parameters:
Class 6 Wind Regime
Maximum Wind Speed = 130 mph, 58 m/s (50 year)
Overturning Moment = 1,830,000 ft lb (2,481 kNm)
Total Tower/Turbine Weight = 42,000 lb (187 kN)
Maximum Rotor Frequency = 60 rpm (1.00 Hz)
System Frequency >= 1.05 Hz (Includes 5% Safety Factor)
4th tower installed in 2010
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Toksook BayGeotechnical Conditions & Design
Tundra Mat / Organics, Ice-Rich Silty Permafrost
Frozen Siltstone below ~18 ft average depth
Six 20 Pipe Piles, driven into Rock
Drilled 20 ft Concrete Socket into Rock
Rock Socket needed to develop Uplift
(in addition to Adfreeze in Permafrost)
Pile Point of Fixity at Future Thawed Active Layer
Special Consideration for Concrete
Curing in 31 F Frozen Siltstone.
Ice-RichPermafrost
FrozenSiltstone
FutureThawedActiveLayer
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Toksook Bay Foundation
Analysis
Coffman completed static analysis of foundation to
determine number/depth of piles for foundation
Determining the system natural frequency required finite
element analysis
RISA was originally chosen as the modeling software, but
was deemed inadequate to resolve the long slender
elements and differences in material stiffness
SAP software has been used to conduct analysis since
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The Toksook Bay wind
turbine foundations
were based on a steel
frame embedded within
a 2.5 foot (762 mm)thick concrete
foundation supported
by piles.
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Winter construction
Holes pre-drilled
Piles driven to refusal Piles later cut
Toksook Bay, Alaska
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Drilling out center of piles
20ft (6.1m) below end ofpile to install reinforced
concrete.
Toksook Bay, Alaska
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Rebar Cage to be Installedin Drilled-out Pile
Toksook Bay, Alaska
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Installing Rebar
Cage Inside 20
(510mm) Pile in
Preparation for
Concrete
Toksook Bay, Alaska
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Six piles for a single tower foundation
Piles shown here with rebar cage installed andconcrete poured
Toksook Bay, Alaska
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Pile Verification Testing
Pile design uplift
load 63 Kips(280kN)
Tested up to 2times load (560kN) with 0.019(0.48mm)movement
Tested up to210 Kips
(934kN) withless than 0.25(6.3mm)movement
Toksook Bay, Alaska
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Steel Foundation Star
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Drain
Conduit
Bolts
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Toksook Bay, Alaska
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Wind Turbine Foundation
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Toksook BayTurbine/Tower/Foundation
System FrequencyResponse:
Calculated Freq. 1.051 Hz *
Measured Freq. 1.07 Hz **
* Calculated frequency determined throughdynamic modeling of turbine, tower, andfoundation system.
** Measured frequencies determined through
accelerometer measurements performed duringthe August service by Northern Power.
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Nacelle at NPS production
facility in Barre, Vermont
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i
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Kasigluk
Foundation Design Overview
Kasigluk
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Wind Turbine Locations
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Kasigluk Foundation
Design CriteriaTurbine: NorthWind 100 (100 kW Turbine)
Tower: Danwin Tower (108 Feet, 32 meter)
Design Parameters:
Class 6 Wind Regime Maximum Wind Speed = 130 mph, 58 m/s (50 year)
Overturning Moment = 1,830,000 ft lb (2,481 kNm)
Total Tower/Turbine Weight = 42,000 lb (187 kN)
Maximum Rotor Frequency = 60 rpm (1.00 Hz)
System Frequency >= 1.05 Hz (Includes 5% Safety Factor)
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KasiglukGeotechnical Conditions
Ice-Rich, Over-Saturated Silty Sand
Marginally Frozen and Discontinuous
Permafrost (close to 32F)
Located in a Region of Degrading
Permafrost (Y-K Delta).
Unique Foundation Conditions
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Remote Alaska Wind Towers
Require Unique Foundations Wind Towers are uniquely subjected to dynamic wind and vibration loading
Manufactures require that foundation systems meet certain stiffness requirements,
which ensures longevity of the turbines and prevents resonance
Special Soil considerations related to cyclic weakening or degradation in strength
as a result of dynamic loading
Concrete is commonly used in wind tower foundationsproviding mass and dampening
However, the ground in most of Western Alaska (where most of the installations are)
is often not suitable for shallow bearing foundations
Remote locations, lack of local aggregate, and cold-climate make concrete challenging &
expensive.
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Pile Design in PermafrostAdfreeze Strength as function of temp
- Most of Y-K Delta is Warm Permafrost
Adfreeze Stength also a function of Load Duration
- Favorable for transient short term wind loads
- Creep settlement in ice much less of a concern
Passive Refrigeration sometimes needed to:
- Preserve & Aggrade Permafrost (in a changing world)
- Increase Adfreeze Strength by Chilling Permafrost
Thermosyphons: Thermo-Piles vs. Thermo-Probes
Passive Refrigeration not only increases axial resistance
it Provides Lateral Stiffness by limiting thawed active layer
Installation: Driven (not always practical), Thermal
Modification, Drill & Slurry, Pre-Drill, Battered if possible Photo Credit:STG, Incorporated
14 psi
29 psi
52 psi
32 F28 F25 F
Thermo-Probes Thermo-Piles
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KasiglukGeotechnical Design
Foundation Caps were identical to Toksook Bay
(steel frame / concrete cap supported by piles).
Six Helical Piles were screwed into the Ground
about 40 ft (design by HDL, DMA, & Almita).
Passive Refrigeration needed to Preserve
Permafrost (which Pile Capacities relies upon), AND
To Provide Lateral Stiffness
(by Restricting Thawed Active Layer).
First Large Diameter Screw Piles Installed
in Permafrost in AK (by STG, Inc. & Almita)
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KasiglukTypical Section
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The foundations were
identical to Toksook Bay
(steel frame embedded
within a thick concretefoundation supported by
piles).
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Turbine/Tower/Foundation
System FrequencyResponse:
Calculated Freq. 1.045 Hz *
Measured Freq. 1.07 Hz **
* Calculated frequency determined throughdynamic modeling of turbine, tower, andfoundation system.
** Measured frequencies determined through
accelerometer measurements performed duringthe August service by Northern Power.
Kasigluk
H B
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Hooper Bay
Foundation Design Overview
Hooper Bay
H B F d i
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Hooper Bay Foundation
Design Same turbine and tower configuration as Toksook Bay and Kasigluk
Prior to beginning design, a thorough review of Toksook Bay andKasigluk models was conducted by Coffmans modeling experts basedin Los Angeles
Minor modeling errors/inconsistencies were noted and corrected
Based on construction managers recommendation, AVEC requestedthat the concrete encasement be eliminated to reduce constructioncost
Steel beams were increased in size to maintain foundation stiffness Steel Helical Piles anchored into Permafrost
Old M d l R i d M d l
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Old Model Revised Model
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Gambell
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Gambell
Foundation Design Overview
Gambell
G b ll F d ti
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Gambell Foundation
DesignTurbine: NorthWind 100 (100 kW Turbine)
Tower: Nordtank Tower (98 Feet, 30 meter)
Design Parameters:
Class 7 Wind Regime Maximum Wind Speed = 134 mph, 60 m/s (50 year)
Maximum Rotor Frequency = 60 rpm (1.00 Hz)
System Frequency >= 1.10 Hz
Concrete foundation (avoided piles as could hit seawater permeated soil)
Size foundation to have acceptable vibrational frequency
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Gambell, AK
G b ll
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GambellGeotechnical Conditions
Well Rounded Beach Gravel
Permafrost below about 9 ft depth,
- but found to be discontinuous
Sporadic Un-Frozen Zones
- Complex mix of Bering Sea Influence
(salt water lowers the freezing temp)
- High flow of GW from Lake to the Sea
Frozen gravel considered thaw-stable
Long-term Preservation of Permafrost
is uncertain
G b ll
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GambellGeotechnical Design
Concrete Mat Foundation was chosen as best option
- Good bearing capacity in the gravel, with no
degradation in strength under cyclic / dynamic loads
- Piles not practical in frozen gravel
- Concrete Mat Foundation can better accommodate
settlement resulting from thawing
Dynamic Soil properties provided as part of dynamic
modeling
STG, Incorporated utilized on-island
concrete aggregate source.
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Kokhanok
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Kokhanok
Foundation Design Overview
Kokhanok
Kokhanok
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KokhanokFoundation Design
Turbine: Vestas V17 (90 kWTurbine)
Tower: 80 foot lattice (24 meter)
Design Parameters: Class 6 Wind Regime
Maximum Wind Speed = 90 mph, 40 m/s(measured 2004)
Maximum Rotor Frequency = 51 rpm (0.85 Hz)
Concrete foundation (gravel available on site, widetower base, lower turbine freq)
No uplift test required as based on gravity load thatcan be calculated
Weight turbine?? Photo Credit: John Lyons, March Creek, LLC
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Turbine
location
s
Aerial Photo Credit:Aerometric, Inc.
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Photo Credit: John Lyons, March Creek, LLC
Kokhanok
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KokhanokGeotechnical Conditions
Beach Gravel (Qb) prevalent throughout
Limited Area of Shallow Bedrock(Bx),
w/ Small Pocket of Glacial Drift (Ggd)
Groundwater at 5 ft depth (i.e. Lake Iliamna)Turbine
locations
Aerial Photo Credit: Aerometric, Inc.
Turbine
location
s
Kokhanok
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KokhanokGeotechnical Conditions (cont.)
Qb Beach Gravel
Qgd Glacial Drift
One TowerSite SelectedOn a BedrockShelf,underlyingBeach Gravel
Second TowerSite SelectedBearing onGlacial Drift
Kokhanok
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KokhanokGeotechnical Design
Clean Sand & Gravel Borrow Pit
made Shallow Concrete Foundations Feasible
Great Bearing Materials for Shallow Foundations
Base of the Concrete was Raised to avoid
having to pump down Lake Iliamna during
pour
Bearing Material NOT susceptible to cyclic
weakening or dynamic strength loss
Photo Credit: John Lyons, March Creek, LLC
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Typical Small Turbine Foundations
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yp
Foundation Design Summary
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Foundation Design Summary
Foundation SummaryToksook Bay - Driven 25 steel piles with reinforced concrete socket to 40. Steel/concrete pile cap
Kasigluk - Screwed Helical piles to 40 . Steel/concrete pile cap system
Hooper Bay - Screwed Helical piles to 40 . Steel pile cap system
Gambell - Concrete mat 4 foot thick x 24 square (7 below grade). 4 foot tall concrete pier.
Kokhanok - Concrete mat 2 foot thick x 20 square (6 below grade). 6 foot tall concrete piers for 4
leg lattice tower
Quinhagak - Thermopiles with helicies to 25. Pre-cast reinforced concrete cap. Smaller systems: mat foundations, single concrete piers poured into CMP, steel piles, etc
Arctic Design Considerations
Thermosyphons may be required in warm permafrost: 1) to aggrade or preserve permafrost,
2) to enhance adfreeze bond by lowering the temperature, and 3) to increase lateral rigidity
Insulation may be required to: 1) reduce frost jacking around foundation, 2) offset heat gained by fill
pad, and 3) reduce thawed active layer.
Seasonal Frost depth greater in Alaska, requiring deeper foundations
Rock sockets or anchors in frozen rock require cold-climate concrete or grout (i.e. Fondu grout)
Cold temp steel and anchor bolts may be required where applicable
Installation can only occur in winter in some cases (tundra areas susceptible to damage in
summer)
Remote access mobilization issues and availability of concrete aggregate
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Questions???
Photo Credit: STG, Incorporated
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