Patrick & Henderson Tensionless Pier
By Shelton L. Stringer, PE, GE, PG, EG Hongbin Huo, PhD, PE Earth Systems Southwest
The Patrick and Henderson Tensionless Pier (P&H Pier) The P&H Pier consists of a large, cast-in-place pier foundation to support wind turbines on a monopole tower.
Design Team
Patrick and Henderson, Inc.
(P&H)
Bakersfield, California Allan Henderson, PE, GE proprietor of patented design, US Patent No. 5,586,417
Earth Systems Southwest (ESSW) Bermuda Dunes, California Shelton L. Stringer, PE, GE, PG, EG Hongbin Huo, PhD, PE engineering design and analysis
Construction of the P&H Pier
Construction of the pier begins by digging a hole with an excavator or drill rig. Rock sites require controlled pre-blasting.
Typical depth 25 to 32 feet
Cranes set an outer corrugated metal can (CMP) in the hole.
Construction of the P&H Pier
Sand-cement slurry is placed as backfill between the outer CMP and the excavation sides.
Threaded steel rods (encased in PVC sleeves) are arranged with a template that matches the base flange of the tower.
These rods are set and bolted to an embedment ring within the annular space between CMP cans.
Construction of the P&H Pier
A smaller, inner CMP is set concentric within the hole.
A lower plug of concrete and the excavated spoils are placed within the inner can.
Foundation concrete is placed between the two CMP cans, forming a hollow cylinder. A concrete floor slab and top collar is cast.
Construction of the P&H Pier
The tower is bolted to the threaded rods extending above the concrete.
The grout trough beneath the base flange is filled.
The rods are post-tensioned to keep the concrete in compression (hence tensionless) during loading.
Advantages of the P&H Pier for Wind Turbine Foundations
Most Economical Foundation Available – Less Concrete (50-90 cy versus 300 to 500 cy for spread footing) – Uses less materials – Tailored to fit most geologic site conditions.
Proven Track Record for over 4500 Wind Turbines including: – – – – – – – –
Nordex 1000 kW, 1300 kW Vestas 660kW, 1.65MW, 1.8MW, 3 MW Mitsubishi 600 kW, 1 MW GE Wind 1.5 MW NEG Micon 750 kW, 950 kW, 1.5 MW Siemens (Bonus) 1300 kW, 2.3 MW Suzlon 1.2 MW Gamesa 2.0 MW
Project Experience Select Wind Energy Projects Built with Patrick & Henderson Foundations Year Built Project 2006 Horse Hollow II 2001 King Mountain 2001 Stateline 2006 Wildhorse 2003 Taiban Mesa 2000 Indian Mesa I & II 2000 Woodward Ranch 2001 Trent Mesa 2005 Hopkins Ridge 2003 High Winds I 2003 Evanston 2006 Forest Creek 2005 Buffalo Gap 2001 Gray County 2005 Weatherford 2005 Bison Wind Power Project 2006 Sand Bluff 2006 JD4 2002 McBride Lake-Fort Macleod 2006 Soderglen 2004 Summerview 2003 Waymart 2002 Whitewater Hill (WECS 3) 2003 Edgele y Kulm 2004 Oasis 2005 Cresent Ridge
Location Abilene, TX McCamey, TX Walla/Umatilla Co., WA/OR Kittitas Co., Vantage,WA Taiban, NM McCamey, TX McCamey, TX Sweetwater, Texas Dayton, WA Rio Vista, Solano Co., CA Evanston, WY Big Spring, TX Buffalo Gap, TX Montezuma, KS Weatherford, OK St. Leon, Manitoba Big Spring, TX Bernstein, TX Pincher Creek, Alberta Fort MacLeod, Alberta Summerview, Alberta Waymart, PA Riverside County, CA Edgley-Kulm, ND Mojave, CA Tiskilwa, IL
State Prov TX TX WA/OR WA NM TX TX TX WA CA WY TX TX KS OK MB TX TX AB AB AB PA CA ND CA IL
Turbine Type Siemens 2.3MW Bonus 1,300KW Vestas V-47 660kW Vestas V-80 GE Wind 1.5 MW Enron Z70.5, 1.5MW Vestas V-47 660kW Enron Z70.5, 1.5MW Vestas V-80 1.8 MW Vestas V-80 1.8 MW Vestas V-80 1.8 MW Siemens 2.3MW Vestas 1.8MW Vestas V-47 660kW GE Wind 1.5 MW NEG-Micon 72C 1650kW Gamesa G87 2.0MW Suzlon S-88 2.1MW Vestas V-47 660kW GE 1.5MW Vestas V-80 1.8 MW GE Wind 1.5 MW Enron Z70.5, 1.5MW GE Wind 1.5 MW Mitsubishi MH1 1MW NEG-Micon 1.65MW
No of Turbines 128 214 399 127 136 107 242 100 83 81 80 54 67 170 71 63 45 38 114 47 38 43 41 41 60 33
Total MW 294.4 278.2 263.3 228.6 204.0 160.5 159.7 150.0 149.4 145.8 144.0 124.2 120.6 112.2 106.5 104.0 90.0 79.8 75.2 70.5 68.4 64.5 61.5 61.5 60.0 54.5
Geotechnical Analyses
Analysis of the stability and external forces acting on the pier (soil-structure interaction).
Required to demonstrate overturning moment stability and deflections are within acceptable limits for design loads.
How the P&H Pier works
The lateral and moment capacity is developed by side bearing as the rigid pier is free to rotate within the earth.
The ultimate passive resistance is dependent on the shear strength of the surrounding soil or rock (friction angle, and cohesion, c).
Pier rotation and deflection are dependent on compressibility of the soil or rock, expressed as a non-linear, load-deformation (p-y) curve.
Key Geotechnical Issues
The geotechnical report for the project is the basis for the properties of the soil or rock in analyses. Geotechnical engineer required to verify condition of excavations. The analysis to indicate overturning stability with a global safety factor of at least 2. Pier rotations and deflections should remain within a tolerable range – typically, 3 to 6 mm operational, – 10 to 25 mm extreme – 1 mm/m rotation – operational
Proper drainage is key to maintain performance Foundation rotational stiffness to avoid resonance and excessive vibrations
Design Loads
Loads come from the wind turbine manufacturer based on IEC.
Typical Extreme Wind Loads: – Axial 140 – 575 kips (700 - 2550 kN) – Lateral 70 – 190 kips (300 – 800 kN) – Moment 10,000 to 52,000 ft-kips (14 - 70 MN-m)
Seismic loads, even in moderate seismic regions, are generally less than design wind loads – Exception: 2001 CBC, Type A faults, <15km, non-building structure, minimum design force - San Andreas & Garlock faults .
Post-tensioning Requirement
Set to prevent liftoff and tension in concrete to extreme load Check by elongation of rod,
= PL/AE
Requires maintenance program to check and retension if > 10% loss P = 4M/nD – 0.9W/n – Where P = post tension, – M = unfactored extreme overturning moment – n = number of anchors in circle – D = anchor circle diameter – W = weight of foundation
Geotechnical Data
The project geotechnical report is the basis for the properties of the soil or rock selected for the analysis.
Sand: The shear strength is expressed as a friction angle,
Clay: The undrained shear strength (cohesion) is used.
Rotational (Rocking) Stiffness
Important factor for performance of wind turbine foundations Rotational Stiffness, K θ = ∆M/∆θ, where M = moment, θ
= rotation
Normal requirement: K θ = 20 to 60 GN-m/radian
θ Greatly exaggerated rotation from FEM
M
3D FEM using COSMOS Half Model
Loading at Top of Tower Tower
P&H Pier
Ground
FEM Results Lateral Deformation of Pier
Tower Frequency Verification Testing
We use Dr. Kevin Jackson to conduct testing of tower frequency and rotational stiffness to verify FEM results
Tower frequency changes when considering SoilStructure Interaction
The wind turbine natural frequency including foundation response should be a margin away from the rotor rotation frequency to avoid dynamic amplification.
Testing Setup
Sensors consists of: – Accelerometers – LVDT – Strain gages mounted on tower wall
Shutdown
Strongest response comes during shutdown of turbine
Tower Frequency Response
Rotational Stiffness, K = M/ – M calculated from strain in gages = /D, – D = diameter of foundation between sensors
Frequency response of tower recorded by accelerometer Verification testing confirms FEM analysis of foundation stiffness
Contacts
Contact Allan Henderson to discuss using the P&H Piers – @ (661) 391-9854 –
[email protected]
Contact Shelton Stringer or Hongbin Huo to discuss analyses for the P&H Pier or other Earth Systems – @ (760) 345-1588 or 1-800-924-7015 –
[email protected] –
[email protected]
