Wind Turbine Foundation Design by Nishkian EngineersNov 06 2013 · 0 comments · Industrial, NISHKIAN MONKS ·0
Pembina, ND – Nishkian Monks, PLLC was proud to be a part of a prototype green energy project in North Dakota. Using $2.96 Million in funds provided by the American Recovery and Reinvestment Act, the U.S. General Services Administration (GSA) Rocky Mountain Region selected Colstrip Electric Inc. (CEI), a Montana based company, to construct a 1-megawatt wind turbine at the Pembina Land Port of Entry Border Station, in Pembina, N.D. The Land Port of Entry (LPOE) operates on a 24-hour-a-day, seven-day-per-week schedule, consuming approximately 1,635,000 kilowatt hours of electrical power annually. The border station is the largest in the State of North Dakota and services a major interstate trucking route between Winnipeg, Canada and the U.S. The wind turbine is expected to reduce electrical power consumption at the facility to zero, and conforms to the Energy Policy Act 2005. The project was to serve as a model for future renewable energy projects at other border stations across the country. The wind turbine project was a design-build project. CEI hired Nishkian Monks as the design team manager and as the project structural engineer. Nishkian Monks assembled a Montana based design team to come up with solutions to the challenges presented for this project. Nishkian Monks employed the geotechnical and civil engineering services of Allied Engineering Services, Inc., from Bozeman, MT, and electrical engineering services by Electrical Consultants, Inc. from Billings, MT.
The selected turbine site at the border station is located in wetlands. The area surrounding the border station is part of the floodplain of the Red River which flows nearby. Typically each year the river floods around the border station and the site of the turbine. The foundation design and equipment supports had to take into consideration the 100 year maximum expected high water level at the site. Access roads to the site had to be constructed along with careful consideration of how to bring the turbine components and erection crane to the site within the existing border station facilities, where border crossing traffic routinely backs up a mile or more several days a week. Delivery trucks were bringing turbine and crane components to the sites that were upwards of 112 ft long and 12 ft in diameter. Additionally, suitable materials for building the road and erection pad site were scarce and had to be trucked in from long distance; therefore, designing the access road and pad site required careful optimization of the geometry to minimize material costs.
The soils beneath the turbine site consist of approximately 120 ft of soft clays and deemed inadequate to support the loads imposed by the wind turbine in operation. Unique to wind turbine foundation design, are the requirements that turbine foundation/soil system be significantly stiff to greatly limit the rotation and horizontal movement of the foundation supporting the operating turbine. Small movements of the foundation translate into large movements and vibrations at the top of the tower, which may impact the strength of the tower or the life of the turbine. The wind turbine for this project is approximately 230 ft tall and weighs approximately 252,000 lbs. Overturning moments on the foundation were 15,400 ft-kips.
A deep foundation system was selected for this project to mitigate the soft soil support, which is relatively uncommon for turbine foundations. Turbine foundations are typically constructed on large concrete spread footings buried 4-6ft beneath the ground surface.
The Pembina wind turbine foundation consists of twelve 14×73 H-Piles equally spaced in a 34ft diameter array centered beneath the tower. The piles are embedded approximately 180ft into the soil and are designed for a 300,000 lbs. axial force per pile. The tops of the piles are embedded 3ft into a 5ft-6in thick x 38ft diameter round concrete pile cap. The pile cap in turn supports an 8ft-6in tall x 12ft-6in diameter concrete pedestal, which elevates the base of the tower above the 100 year flood event.
Nishkian Monks collaborated with the project geotechnical engineer to design the foundation system to meet the stringent foundation stiffness criteria by the turbine manufacturer. Numerous design iterations were required to obtain a computer model of the foundation system which matched the predicted pile-soil interaction behavior by the geotechnical Completed Wind Turbine engineer, and achieved the proper stiffness criteria by the manufacturer. Since the piles were so long, axial shortening of the piles under design turbine loads created difficulties in controlling the tilting of the foundation.
Additional challenges to the foundation design were controlling the construction cost of the foundation design. A careful balance of increasing the pile array diameter and number of piles to lower stresses and displacements on the foundation had to be balanced with the cost of the concrete pile cap utilized to connect the piles to the tower pedestal. Concrete costs for the foundation were abnormally high due to aggregate availability and the need for high sulfate resistant concrete. The final design maximized the strength of the piles while minimizing the size of the pile cap to the greatest extent possible, to achieve the proper foundation stiffness and strength required by the turbine manufacturer.
The large pile cap required for this turbine foundation is unique for turbine foundations. Careful consideration had to be given to the internal stresses of the concrete to ensure adequate strength and fatigue resistance. Extreme load cycling of vertical and bending loads from the tower pedestal had to be resisted by the pile cap, such that all imposed loads on the pile cap had to be transferred directly to the support piles embedded in the oil. No soil support beneath the pile cap could be utilized for supporting loads from the wind turbine.