widac abbreviated foundation coffman golder

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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


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)


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


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


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)


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


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)


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

widac abbreviated foundation coffman golder

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