1. Technical Field
The subject matter described here generally relates to static structures including a rigid member having a limited closed periphery and which is greatly elongated relative to any lateral dimension, and, more particularly, to joints for wall sections of wind turbine towers.
2. Related Art
A wind turbine is a machine for converting the kinetic energy in wind into mechanical energy. If that mechanical energy is used directly by machinery, such as to pump water or to grind wheat, then the wind turbine may be referred to as a windmill. Similarly, if the mechanical energy is further transformed into electrical energy, then the turbine may be referred to as a wind generator or wind power plant.
Wind turbines use one or more airfoils in the form of a “blade” to generate lift and capture momentum from moving air that is them imparted to a rotor. Each blade is typically secured at its “root” end, and then “spans” radially “outboard” to a free, “tip” end. The front, or “leading edge,” of the blade connects the forward-most points of the blade that first contact the air. The rear, or “trailing edge,” of the blade is where airflow that has been separated by the leading edge rejoins after passing over the suction and pressure surfaces of the blade. A “chord line” connects the leading and trailing edges of the blade in the direction of the typical airflow across the blade. The length of the chord line is simply the “chord.”
Wind turbines are typically categorized according to the vertical or horizontal axis about which the blades rotate. One so-called horizontal-axis wind generator is schematically illustrated in FIG. 1 and available from GE Energy of Atlanta, Ga. USA. This particular configuration for a wind turbine 2 includes a tower 4 supporting a drive train 6 with a rotor 8 that is covered by a protective enclosure referred to as a “nacelle.” The blades 10 are arranged at one end of the rotor 8, outside the nacelle, for driving a gearbox 12 connected to an electrical generator 14 at the other end of the drive train 6 arranged inside the nacelle along with a control system 16. An anemometer 18 is also provided on the nacelle for providing information to the control system 16.
The wind turbine tower 4 is typically formed from several tubular, frustro-conical sections that are stacked on top of each other. For example, commonly-owned co-pending U.S. patent application Ser. No. 11/399,829 by Ronald Cairo discloses “Methods and Apparatus for Assembling Wind Turbine Towers.” In that publication, the members include a protrusion that extends radially and circumferentially outward at a bottom end, defining an annular notch for receiving the top end of a second tower member. At least one bolt may be extended through second tower member protrusion and first tower member top end in order to increase the structural integrity of the joint formed by interlocking first tower member and the second tower member. The tower members may also be provided with vertical flanges that are substantially aligned to form a joint.
Other wind turbine tower configurations have been provided with inner or outer flanges that that are normally welded to the peripheral wall portions. For example, commonly-owned and co-pending U.S. patent application Ser. No. 10/517,585 by Wolfgang Fuelhaas and Holger Lühn discloses a “Method for Generating a Substantially Uninterrupted Connection of the Peripheral Wall Portions,” and an English-language abstract of German Patent Publication No. DE10152018 by Marc Seidel discloses “Component arrangement for manufacturing tower for wind power system has components with boundary surfaces bounding test channel enabling testing of contact between components.”
However, wind turbine towers with such flanged, connections suffer from a variety of drawbacks. For example, the flanges require additional material, welding, inspection and labor resources to install and inspect. Flanges can also cause positioning and bending problems, particularly during transportation and storage where protruding flanges can interfere with stacking and impinge upon other structures.