Wind turbine foundation and method of constructing a wind turbine foundation

A wind turbine foundation and method for making a wind turbine foundation. The wind turbine foundation preferably includes a core member including a substantially cylindrically-shaped main body, a first outer flange extending out from the main body along an upper section of the core member, and a second outer flange extending out from the main body along a lower section of the core member, and a plurality of radial girders connected to the first outer flange and the second outer flange and radiating out from the core member.

FIELD

This disclosure relates to the field of construction related to wind turbines or other tower-like structures. More particularly, this disclosure relates to a foundation for a wind turbine.

BACKGROUND

The wind energy generation market has experienced tremendous growth over the past decade with wind energy currently recognized as the lowest cost source of renewable energy generation. The key driver of this growth has been the advancements made in wind turbine technologies, with wind turbines growing in capacity, size and height every year. The advancements in wind turbine technologies has placed increasing strains on the other classical approaches to wind project design and construction, and as a result several of the classical, brute force approaches to wind project design and construction are reaching their limits of effectiveness and cost efficiency. Change in wind project design and construction is required to complement the turbine technology changes being experienced in the industry.

In 2018 US wind energy generation capacity grew by over 8%, and the installed capacity of wind generation is anticipated to exceed that of hydro by the end of 2020. This growth has been driven by the reductions in cost of the wind generation technologies. The key driver of these reductions in cost have been the advancements in wind turbine technologies. Wind turbines have grown consistently in the past few years with turbine size, weight and tower heights increasing significantly every year. In 2018 the largest turbines installed in North America had capacities on the order of 3.6 MW, with tower heights of 110 m. In 2020 wind turbine installations will include 4.8 MW turbines with tower heights exceeding 140 m.

While turbine sizes are growing every year, turbine logistics remain constrained with road, rail and truck transport limiting tower base dimensions. These increases in turbine size combined with limit on growth of the tower base dimensions has resulted in significant growth in the load demands being placed on the wind turbine foundations. In contrast to the technology improvements seen on the turbines, wind turbine foundation technologies have not advanced significantly over the past 20 years. Today's predominant wind turbine foundations are the traditional concrete raft foundation, with minor variations being applicable for unique ground conditions (shallow bedrock situations, etc.). While the concrete raft foundation was a good solution for the turbines installed in 2016, with capacities of 2.6 MW and tower heights of 70 m, they are now approaching their limits of applicability. Increasing the size and strength of the concrete raft foundation is not a simple matter, with rebar and anchor bolt cage densities reaching the limits of constructability, the complexities of very large concrete pours creating significant logistics issues and quality risks. What is needed therefore are new approaches to wind turbine foundations to meet the needs of the continuing advancements in wind turbine technologies.

SUMMARY

The above and other needs are met by a wind turbine foundation comprising a core member which may include, for example, a metal base can or a metal spool. In some embodiments wherein the core member comprises a metal base can, the metal base can further comprises a substantially cylindrically-shaped main body, a first outer flange extending out from the main body along an upper section of the base can, a second outer flange extending out from the main body along a lower section of the base can, and a tower flange including a plurality of apertures for attaching a wind turbine tower to the base can; and a plurality of metal radial girders connected to and radiating out from the base can wherein each of the plurality of radial girders are connected to the first outer flange and the second outer flange. Preferably, the wind turbine foundation of claim1wherein the wind turbine foundation is located in an excavated hole in the ground, wherein the hole in the ground is created by removing soil, and wherein at least some of the removed soil is laid over at least a portion of the plurality of metal girders. The wind turbine foundation preferably further includes an underlying slab and a layer of rebar located above the underlying slab. The wind turbine foundation preferably further includes a base layer of concrete poured along the underlying slab and the layer of rebar.

In some embodiments, the plurality of radial girders includes an upper girder flange and a lower girder flange wherein each upper girder flange is connected to the first outer flange and each lower girder flange is connected to the second outer flange.

In some embodiments, the wind turbine foundation includes an inner shell of concrete lining an inside surface of the base can. In similar embodiments, the wind turbine foundation may further include concrete substantially filling the base can.

In some embodiments, the wind turbine foundation includes a reinforced concrete base slab supporting the metal base can and the plurality of radial girders, wherein the excavation under the slab is tapered so that a bottom side of the slab filling the excavation is tapered and bulges along a middle portion of the base slab.

In some embodiments, the wind turbine foundation includes a reinforced concrete base slab wherein the excavation under the slab is in a stepped configuration so that a bottom side of the slab filling the excavation is in a stepped configuration.

In some embodiments, the wind turbine foundation includes a plurality of first transverse girders wherein individual members of the plurality of first transverse girders are located between and connected to pairs of the plurality of radial girders. The wind turbine foundation may further include a plurality of second transverse girders wherein individual members of the plurality of second transverse girders are located between and connected to pairs of the plurality of girders at distal ends of the radial girders.

In some embodiments, the wind turbine foundation includes a perimeter grade beam of concrete and a mid-grade beam of concrete beneath a reinforced concrete base slab.

In some embodiments, at least a first portion of the upper girder flanges are substantially parallel with a portion of the lower girder flanges. In some embodiments, the first portion of the upper girder flanges comprises most of the upper girder flanges.

In some embodiments the plurality of radial girders comprises a plurality of truss girders.

In some embodiments the wind turbine foundation includes a plurality of piles supporting the plurality of radial girders at distal ends of the plurality of radial girders.

In some embodiments the wind turbine foundation further includes a core column inside the base can and a plurality of stiffener plates connected to and radiating out from the core column wherein distal edges of the stiffener plates are connected to an interior surface of the base can. The wind turbine foundation may further include a first plurality of rock anchors connected to the plurality of radial girders wherein there is at least one rock anchor per radial girder extending into bedrock. The wind turbine foundation may further include a plurality of transverse girders wherein individual members of the plurality of transverse girders are located between and connected to pairs of the plurality of radial girders. The wind turbine foundation may further include a second plurality of rock anchors connected to the plurality of transverse girders wherein there is at least one rock anchor per transverse girder extending into bedrock. In some embodiments the base can further comprises a plurality of vertical flanges wherein individual vertical flanges of the plurality of vertical flanges are connected to individual radial girders of the plurality of radial girders.

In some embodiments the wind turbine foundation includes a plurality of vertically oriented beams connected to an interior surface of the base can to stiffen the base can.

In another aspect, a wind turbine foundation is disclosed comprising a metal spool; a plurality of metal radial girders connected to and radiating out from the metal spool; and a ring girder connected above the plurality of radial girders wherein the ring girder further comprises a tower flange including a plurality of apertures for attaching a wind turbine tower to the ring girder. The ring girder may further include a composite ring girder comprising a plurality of ring girder sections forming the composite ring girder wherein ring girder sections are individually connected to the plurality of radial girders with one ring girder section per radial girder. The spool may further include a substantially cylindrically-shaped main body, a first outer flange extending out from the main body along an upper section of the spool, and a second outer flange extending out from the main body along a lower section of the spool wherein each of the plurality of radial girders are connected to the first outer flange and the second outer flange. The spool may further include a plurality of vertical flanges wherein individual vertical flanges of the plurality of vertical flanges are connected to individual radial girders of the plurality of radial girders. The spool may further include a plurality of pairs of vertical flanges located in an area between the first outer flange and the second outer flange wherein individual pairs of vertical flanges of the plurality of pairs of vertical flanges are connected to individual radial girders of the plurality of radial girders.

In another aspect, a method of erecting a wind turbine foundation is disclosed, the method comprising the steps of excavating a foundation area in the ground by removing excavated soil from the ground; pouring a mud slab in the excavated foundation area to create a level work surface; placing a metal core member in the excavated foundation area; and attaching a plurality of metal radial girders to the core member. The core member may include, for example, a metal base can or a metal spool. The core member preferably includes a substantially cylindrically-shaped main body, a first outer flange extending out from the main body along an upper section of the core member, and a second outer flange extending out from the main body along a lower section of the core member wherein each of the plurality of radial girders are connected to the first outer flange and the second outer flange.

The summary provided herein is intended to provide examples of particular disclosed embodiments and is not intended to cover all potential embodiments or combinations of embodiments. Therefore, this summary is not intended to limit the scope of the invention disclosure in any way, a function which is reserved for the appended claims.

The figures are provided to illustrate concepts of the invention disclosure and are not intended to embody all potential embodiments of the invention. Therefore, the figures are not intended to limit the scope of the invention disclosure in any way, a function which is reserved for the appended claims.

DETAILED DESCRIPTION

An example of a wind turbine foundation100and its components is shown inFIGS. 1, 2, 2A, 2B, 2C, 2D, 3A and 3B.FIG. 1shows a base can102—a central component of the wind turbine foundation100shown more completely in the plan view ofFIG. 3Aand a side view ofFIG. 3B. The base can102, which comprises a metal shell, is referred to as a “can” because of its preferred cylindrical shape which looks like a traditional can as well as its preferred composition (i.e., including mostly or completely metal or metal alloy, hereinafter collectively referred to as “metal”). A rounded cylindrically-shaped base can is preferred but other shapes would work including a polygonal base can with multiple faces. A plurality of radial girders104are connected to the base can102. The base can102preferably includes a first outer flange106A and a second outer flange106B. The plurality of girders104are preferably connected to the base can102by bolting the girders104to the first outer flange106A and the second outer flange106B. Although bolting is specifically described in this example, other devices and/or methods of attachment may be used such as, for example, welding.

The plurality of girders104preferably includes twelve substantially similar girders of the same size and shape. In other examples, the plurality of girders104can include more than twelve or fewer than twelve girders. The girders104and other similar objects described herein are preferably made of steel but other metals or metal alloys could be used instead of or in addition to steel. The girders104are preferably made using traditional steel plate girder design used in bridge girders and existing steel bridge design codes and associated manufacturing methods. Each of the girders104is preferably tapered as shown and preferably has a length ranging from about 8 meters (m) to about 14 m and a height at the highest point ranging from about 2.5 m to about 5 m.

FIG. 2shows a closer view of the base can102connected to a first girder104A. A first close-up view of the connection between the first outer flange106A and the first girder104A is highlighted and shown inFIG. 2A. A second close-up view of the connection between the second outer flange106B and the first girder104A is also highlighted and shown inFIG. 2B. A plan view of the connections between the base can102and the plurality of girders104is shown inFIG. 2C. The base can102also preferably includes a tower flange108for attaching a first tower piece110to the base can102. A close-up view of a preferred connection between the tower flange108and the first tower piece110is highlighted and shown inFIG. 2Ashowing a preferred embodiment using bolts on the inside of the base can102and the first tower piece110but not on the outside. Other embodiments may include a two-sided tower flange for using both internal and external bolting to hold the base can102to the first tower piece110. Use of only internal bolts and only an inward facing tower flange is preferred because the mechanical connection is protected from the elements, thereby reducing corrosion or other deterioration of the connection between the base can102and the first tower piece110. The base can102preferably has a diameter that substantially matches the diameter of the first tower piece110. The wall thickness of the base can is preferably at least as thick as the wall thickness of the first tower piece110.

The wind turbine foundation100preferably includes a mud slab112on which the base can102rests. The mud slab preferably comprises concrete with a level top surface and is preferably about 100 millimeters (mm) to 150 mm thick in some embodiments. Rebar114and a base slab layer116(preferably made of concrete) is preferably located above the mud slab112inside and outside of the base can102. The base slab layer116(or “base slab” or “base layer”) is designed at a nominal thickness that is much less than the mass concrete of a traditional raft foundation, thus avoiding prevalent heat-of-hydration and associated cracking and performance concerns. The thickness of the base slab layer116is selected such that the required strength is achieved with a nominal, lower reinforcement ratio suitable to handle punching shear at the edges of the girders104. In some embodiments, the thickness of the base layer116preferably ranges from about 300 mm to about 600 mm.

The girders104preferably include downward facing studs118(e.g., Nelson™ studs) that are enmeshed with the rebar114and the base slab layer116and that are sized and spaced to provide sufficient steel to limit the stress range to meet fatigue design requirements. Each of the plurality of girders104preferably includes upper girder flanges120A and lower girder flanges120B as shown, for example, inFIG. 2. Preferably, the upper girder flanges120A are bolted to the first outer flange106A of the base can102and the lower girder flanges120B are bolted to the second outer flange106B of the base can102. Each of the plurality of girders104also preferably includes a solid girder web122and a plurality of stiffener plates124. Crushed gravel126is preferably placed directly adjacent to an upper section128A of the base can102at surface level, covering backfill130which is preferably placed along and/or above the girders104and base slab layer116. The backfill130will principally be the excavated in situ materials excluding topsoil. Only in instances of saturated soils or unusual soil composition would imported material be required. Backfill will be placed in standard 200 mm to 300 mm lifts compacted to about 95% standard proctor maximum dry density or better to achieve a dense soil ballast over the entire foundation. The top of the base slab layer116is screeded to the second outer flange106B for convenience and to provide assurance of complete contact between the concrete and the underside of the second outer flange106B. The lower girder flanges120B preferably include a plurality of “bleed holes” used to observe concrete flow under the girders104for this purpose. Corrosion protection will vary based on soil types but typically includes full epoxy coating of all steel components and galvanized bolts, as well as a site-specific designed impressed current grounding and monitoring system.

The radial girders104are proportioned at the base can102connection based on strength or stiffness. The girder104geometry is tapered towards the outside perimeter to maintain a relatively constant section capacity to resistance demand ratio. The can-ends of the girders104(the ends of the girders104closest to the base can102) have a short and preferably substantially horizontal sections of the top flange to facilitate the bolted connection to the can. This type of connection is selected because the first outer flange106A (or “bolting ring”) on the base can102also facilitates circumferential load distribution and ring stiffness acting as Tee Ring Beams. The connection is preferably designed as a “slip-critical” connection because shifting of the joint could lead to incremental tower misalignment. The structural design of the girders104preferably follows typical practice for traditional plate girders for bridges. In fact, in preferred embodiments, the girders104and base slab layer of concrete116act as a Composite Radial Inverted Bridge Section (CRIBS). The ends of the radial girders104are preferably fitted with a support leg111including levelling bolt positioned over a steel plate on the mud slab112to facilitate level installation prior to concreting.

The embodiment of the wind turbine foundation100shown inFIG. 2throughFIG. 3Bincludes an inner layer ring of concrete132located inside the base can102. The ring of concrete132may further include rebar134included therein. Granular fill136(preferably compacted to at least or about 98% standard proctor maximum dry density) may be added inside the ring of concrete132. An example of the average size of granular fill that can be used in some embodiments is 40 mm. Additionally or alternatively, gravel could be added inside the ring of concrete132for added weight and to discourage water retention. Another layer of rebar138and concrete140may be added above the ring of concrete132and the granular fill136. In a different embodiment shown inFIGS. 4A and 4B, a wind turbine foundation141includes a full concrete core142located inside the base can102. In the embodiments shown inFIG. 2throughFIG. 4B, inward facing studs144(e.g., Nelson™ studs) along the base can102are preferably included extending inside the base can102enmeshed with concrete. Corrugated Steel Pipe (csp)146is used to act as sacrificial steel form for the concrete.

FIGS. 5A and 5Bshow an embodiment of a wind turbine foundation150including the base can102and the plurality of girders104but not including a concrete ring or concrete core inside the base can102. In certain embodiments, it may be preferably to minimize the use of concrete inside the base can102.

FIG. 6shows an embodiment of a wind turbine foundation152wherein ground excavation154for the overall apparatus152is tapered. The wind turbine foundation152preferably includes a tapered mud slab156having a thickness preferably ranging from about 150 mm to about 300 mm. Above the mud slab156is a tapered base slab158which is preferably made of concrete and preferably reinforced with rebar. The base slab158is thickest beneath a base can102which is attached to a plurality of girders104in similar fashion to the wind turbine foundation100described above with reference toFIG. 2throughFIG. 3B. The thickness of the base slab158beneath the base can102preferably ranges from about 500 mm to about 1500 mm. The girders104include downward facing studs118which are enmeshed with the base slab158. Although tapered along a peripheral section160, a first portion of the base slab162is preferably substantially flat beneath the base can102.

FIG. 7shows an embodiment of a wind turbine foundation164wherein ground excavation166for the overall apparatus164is in a tapered stepped pattern. The wind turbine foundation164preferably includes a tapered stepped mud slab168having a thickness preferably ranging from about 150 mm to about 300 mm. Above the mud slab168is a tapered stepped base slab170which is preferably made of concrete and preferably reinforced with rebar. The base slab170is thickest beneath a base can102which is attached to a plurality of girders104in similar fashion to the wind turbine foundation100described above with reference toFIG. 2throughFIG. 3B. The thickness of the base slab170beneath the base can102preferably ranges from about 500 mm to about 1500 mm. The girders104include downward facing studs118which are enmeshed with the base slab170. In a preferred embodiment, the tapered step pattern includes three steps from the periphery of the tapered stepped base slab to its center as shown inFIG. 7.

FIG. 8AandFIG. 8Bshow views of a wind turbine foundation200including a mud slab202, a base can102on or otherwise above the mud slab202, a base slab204preferably made of concrete, and a plurality of girders104connected to the base can102. A peripheral section206of the base slab204preferably extends deeper into the ground than a central section208of the base slab204. Additional features include a plurality of inner transverse girders210connecting midsections212of adjacent radial girders104together and a plurality of outer transverse214connecting outer sections216of adjacent radial girders104together. The inner transverse girders210and outer transverse girders214are preferably steel I-beams which are preferably bolted or welded to adjacent girders104providing further structural support to the base can102and girders104. Use of the transverse girders in some circumstances could allow for a thinner mud slab or an alternative type of slab such as, for example, corrugated or ribbed steel panels or composite rigid panels. Granular backfill218preferably covers the girders104and the base slab204.

FIG. 9AandFIG. 9Bshow views of a wind turbine foundation300including a mud slab302, a base can102on or otherwise above the mud slab302, a base slab304preferably made of concrete, and a plurality of girders104connected to the base can102. The base slab304preferably includes an inner beam306and an outer beam308which both extend deeper into the ground than the surrounding portions of the base slab304. The inner beam306is preferably beneath midsections310of the girders104and the outer beam is preferably beneath outer sections312of the girders104.

FIG. 10AandFIG. 10Bshow views of a wind turbine foundation400that includes a mud slab202, a base can102on or otherwise above the mud slab202, a base slab204preferably made of concrete reinforced with rebar, and a plurality of radial girders406connected to the base can102. The plurality of girders406are like the plurality of radial girders104described above except for profile shape. The girders406are preferably tapered as shown and the length of each of the girders406preferably ranges from about 8 m to about 15 m and the height of each of the girders406at the highest point ranges from about 2.5 m to about 5 m. In the example shown inFIG. 10AandFIG. 10B, rectangular sections408of the plurality of girders406extend out substantially horizontally from about 20% to about 50% the length of each of the girders406before angling downward along tapered sections410. In another example, a wind turbine foundation412shown inFIG. 11AandFIG. 11Bincludes a plurality of radial girders414wherein rectangular sections416of the plurality of girders414extend out substantially horizontally from about 50% to about 80% the length of each of the girders414before angling downward along tapered sections418of the girders414. The girders414are preferably tapered as shown and the length of each of the girders414preferably ranges from about 4 m to about 12 m and the height of each of the girders414at the highest point ranges from about 2 m to about 5 m.

FIG. 12shows a side view of a wind turbine foundation500that includes a mud slab202, a base can102on or otherwise above the mud slab202, a base slab204preferably made of concrete reinforced with rebar, and a plurality of radial girders502connected to the base can102. The plurality of girders502are like the plurality of girders104described above; however, the plurality of girders502shown inFIG. 12include girder trusses (open web girders) which allows for the plurality of girders502to be lighter than the formerly described plurality of girders104but maintain substantially the same level of strength. Each of the girders502is preferably tapered and preferably has a length ranging from about 8 m to about 15 m and a height at the highest point ranging from about 2.5 m to about 5 m.

FIG. 13Ashows a plan view of a wind turbine foundation600including a leveling slab602, a base can102, and a plurality of radial girders604connected to the base can102. The plurality of girders604are supported at distal ends606by a plurality of piles608extending into the ground. Each of the girders604is preferably tapered and preferably has a length ranging from about 5 m to about 15 m and a height at the highest point ranging from about 2 m to about 5 m.FIG. 13Bshows an example in which the plurality of piles608include helical piles610.FIG. 13Cshows an example in which the plurality of piles include concrete bell piles612. In cases where there are soft soils near the surface and stiffer soils or bedrock at depth using piles608to provide support will sometimes be advantageous as opposed to making the overall foundation much larger in diameter. Types of piles608that can be used include, without limitation, pipe piles, H piles, helical screw piles and concrete bell piles depending on the soil properties, groundwater depth and depth to the firm soils or bedrock. The piles608may be used alone with just the radial girders (buried or not buried) such that no concrete base slab is used. However, the piles may also be used in combination with a concrete base slab and buried as usual depending on the soil characteristics, groundwater depth and load requirements. In the case of expanding clay soil in the upper soil strata, piles608may be used in combination with a compressible foam panel or similar void form placed under the mud slab, base slab or girders to prevent the soil expansion from imposing uplift forces on the foundation.

The base can generally requires increased shear stiffness relative to the towers above to provide overall rotational stiffness. In some embodiments, this is achieved by a combination of inner radial stiffeners702connected (preferably by welding) to the inside of a base can704as required by site conditions and turbine manufacturer requirements. For additional strength and support, concrete can be added in the base can704between the radial stiffeners702. An example of a wind turbine foundation700including these features is shown inFIGS. 14A-14G.FIG. 14Ashows a plan view of the wind turbine foundation700including the base can704, inner radial stiffeners702connected to a central core member708(e.g., a steel pipe) along proximal edges and connected to the inside surface of the base can704along distal edges. The radial stiffeners702preferably include steel stiffener plates which can be connected to the base can704by, for example, welding or using bolts.

FIG. 14Bshows a side view,FIG. 14Cshows a closer partial plan view, andFIG. 14Dshows a closer partial side view of the wind turbine foundation700.FIG. 14Eshows a plan view of the base can704by itself.FIG. 14Fshows a close-up side view of the wind turbine foundation700cut along a line revealing what is shown inFIG. 14G. In these various figures, different features are shown including a first outer flange710A near the top of the base can704and a second outer flange710B near the bottom of the base can704. A plurality of radial girders712are connected to the base can704. Each of the girders712includes upper girder flanges714A, lower girder flanges714B, and girder webs716. Each of the girders712is preferably tapered as shown and preferably has a length ranging from about 8 m to about 15 m and a height at the highest point ranging from about 2.5 m to about 5 m The base can704further includes a plurality of vertical flanges718which extend between the first outer flange710A and the second outer flange710B. The vertical flanges718are preferably situated directly adjacent to the girder webs716and first vertical plates720A and second vertical plates720B are preferably situated on either side, overlapping the vertical flanges718and the girder webs716such that, for example, bolts can be used to connect the vertical flanges718, girder webs716, first vertical plates720A and second vertical plates720B together. A close-up view of this is shown inFIG. 14G. In addition to this connection, the first outer flange710A is preferably connected to the upper girder flanges714A using, for example, bolts tightened through first upper horizontal plates722A and second upper horizontal plates722B as shown inFIG. 14F. Similarly, the second outer flange710B is preferably connected to the lower girder flanges714B using, for example, bolts tightened through first lower horizontal plates724A and second lower horizontal plates724B as shown inFIG. 14F.

The base can704further includes a tower flange726which preferably extends inward and outward (like a “T”), preferably with at least two rows of apertures728through which bolts can be inserted to attach a first tower piece730to the wind turbine foundation700. The base can704preferably includes upper stiffener plates732which preferably extend from the first outer flange710A to or near the tower flange726and alternating partial stiffener plates733which alternate between inner radial stiffeners702. The upper stiffener plates732are preferably dispersed in line with girder webs716as well as spaces in between where girder webs716are angled toward the base can704as shown, for example, inFIG. 14E. The base can704and girders712are preferably placed on support legs734including leveling bolts for leveling the base can704and girders712above rebar736on a mud slab738. After leveling is completed, a base layer740of concrete can be poured. The girders preferably include downward facing studs742(e.g., Nelson™ studs) that are enmeshed with the rebar736and the base layer base layer740and that are sized and spaced to provide sufficient steel to limit the stress range to meet fatigue design requirements.

FIG. 15AandFIG. 15Bshow an embodiment of a wind turbine foundation800including a base can802and a plurality of girders712connected to the base can802. Inside the base can802, metal beams804(e.g., H beams) are connected (preferably by welding or field bolted) to an inside surface806of the base can802at locations adjacent to where girders712are connected to the base can802. The base can802further includes a first outer flange808A near the top of the base can802and a second outer flange808B near the bottom of the base can802. Each of the girders712includes upper girder flanges714A, lower girder flanges714B, and girder webs716. The base can802further includes a plurality of vertical flanges810which extend between the first outer flange808A and the second outer flange808B. The vertical flanges810are preferably situated directly adjacent to the girder webs716and first vertical plates720A and second vertical plates720B are preferably situated on either side, overlapping the vertical flanges810and the girder webs716such that, for example, bolts can be used to connect the vertical flanges810, girder webs716, first vertical plates720A and second vertical plates720B together. A close-up view of this type of connection in a previous related embodiment is shown inFIG. 14G. In addition to this connection, the first outer flange808A is preferably connected to the upper girder flanges714A using, for example, bolts tightened through first upper horizontal plates722A and second upper horizontal plates722B. Similarly, the second outer flange808B is preferably connected to the lower girder flanges714B using, for example, bolts tightened through first lower horizontal plates724A and second lower horizontal plates724B. An example of these types of connections is shown in a previous embodiment shown inFIG. 14F.

The base can802further includes a tower flange812which, in this embodiment, extends inward and outward (like a “T”), preferably with at least two rows of apertures through which bolts can be inserted to attach a first tower piece730to the wind turbine foundation800. The base can802and girders712are preferably placed on support legs734including leveling bolts for leveling the base can802and girders712above rebar736on a mud slab738. After leveling is completed, a base layer740of concrete can be poured. The girders preferably include downward facing studs742(e.g., Nelson™ studs) that are enmeshed with the rebar736and the base layer740and that are sized and spaced to provide sufficient steel to limit the stress range to meet fatigue design requirements.

FIG. 16AandFIG. 16Bshow a different embodiment including a wind turbine foundation900which would typically be used when suitable bedrock is close or at the surface at a location where a wind turbine is to be built. Depending on the rock characteristics and the degree of rock weathering or quality, the foundation900may be either placed on the surface without backfill or excavated and backfilled as per previously described foundation installations with a concrete base slab or not. Given the stronger rock qualities for bearing and support, the diameter of the overall foundation900would typically be smaller and the forces would project out to the ends of a plurality of radial girders902. As such, in preferred embodiments, the girders902would not be tapered like those of the pure gravity base versions described above. The foundation900preferably includes the base can704including radial stiffeners702, central core member708, first outer flange710A, second outer flange710B, and vertical flanges718. The girders902are preferably connected to the base can in the same manner as the connection between the girders712and the base can704shown inFIGS. 14A-14G.

The girders902include rock anchors904at distal ends906of the girders904wherein the anchors904penetrate into surrounding bedrock. The rock anchors904will be drilled in place to a depth suitable to meet the uplift force requirements according to the rock mechanics and bonding design, and some consolidation grouting of the surrounding rock also may be required. Typically, a double corrosion protected grouted bar anchor will be used in this application with post tensioning. However, a multi-strand cable anchor or multiple bar anchor with some canting could also be deployed. Rock anchor heads908at the top of the rock anchors904preferably would be designed to be accessible to check their post tensioning from time to time and the anchor heads908preferably will be corrosion protected with removable caps and grease or a similar system. The wind turbine foundation900also preferably includes a plurality of transverse girders910preferably connected between at the ends906of the girders902. Preferably, one or more rock anchors904are also connected to the transverse girders between the radial girders902.

In another aspect, an embodiment of a wind turbine foundation1000and associated parts is shown inFIGS. 17A-17G. Instead of a wide base can with a diameter substantially the same a bottom tower piece, the wind turbine foundation1000has a narrower spool1002which preferably includes a metal cylindrical pipe including a top horizontal flange1004A, a bottom horizontal flange1004B, and a plurality of vertical flanges1006. The top horizontal flange1004A and the bottom horizontal flange1004B preferably extend out from the spool1002from about 2 m to about 6 m. The spool height preferably ranges from about 2.5 m to about 5 m. A plurality of girders1008are connected to the spool1002preferably using bolts along the top horizontal flange1004A, a bottom horizontal flange1004B, and a plurality of vertical flanges1006. The girders1008preferably have a length ranging from about 9 m to about 18 m and a maximum height ranging from about 2.5 m to about 5 m. The tower is mounted directly above the top of the girders themselves. The girders include upper girder flanges1010A, lower girder flanges1010B, and girder webs1012. The upper girder flanges1010A are connected to the top horizontal flange1004A, the lower girder flanges1010B are connected to the bottom horizontal flange1004B, and the girder webs1012are connected to the vertical flanges1006. As one example, the vertical flanges1006are preferably situated in pairs defining a plurality of slits1014wherein each pair includes a slit between each of the vertical flanges making up that particular pair of vertical flanges1006. Portions of the girder webs1012along proximal ends1016of the girders are slid into the slits1014and the girder webs1012are connected to the pairs of vertical flanges1006preferably using bolts. The upper girder flanges1010A along proximal ends1016of the girders1008are preferably tapered so that the girders1008can be connected to the spool1002in radial fashion as shown, for example, inFIG. 17C.

The girders1008preferably include curved flanges1018which are preferably an extension of the upper girder flanges1010A at a location along the girders1008above which a first tower piece1019would rest. The curved flanges1018together form a circle as shown, for example, inFIG. 17C. In one embodiment, a ring girder1020is placed above and connected to the curved flanges1018as shown, for example, inFIGS. 17B and 17F-17I. In this embodiment, the ring girder1020—preferably a short cylinder of metal including an upper ring girder flange1021and a lower ring girder flange1022—is bolted to the curved flanges1018along the lower ring girder flange1022. The first tower piece1019is connected to the ring girder1019along the upper ring girder flange1021. In an alternative embodiment shown inFIGS. 17H-17I, curved ring girder subsections1023are welded directly to the upper girder flanges1010A instead of using the curved flanges1018. In this alternate embodiment, there is preferably one ring girder subsection welded to each girder (one ring girder subsection per girder). When all girders1008are assembled in place (i.e., connected to the spool1002), the curved ring girder subsections1023form a composite ring girder1024similar to the ring girder1020. The curved ring girder subsections1023include upper ring girder subsection flanges1025for connection with the first tower piece1019.FIG. 17Jshows a plan view of the plurality of girders1008connected to the spool1002and including curved ring girder subsections1023connected to the radial girders1008to form the composite ring girder1024including the upper ring girder subsection flanges1025for connecting a tower piece to the composite ring girder1024.

In these examples, the girders1008include tapered stiffener plates1026. The stiffener plates1026are wide and are added to support the curved flange1018(if present), the ring girder1020, and/or the composite ring girder1024and distribute the tower forces to the full girder1008height.

During installation, the spool1002and girders1008are supported by support legs1027including leveling bolts. The support legs1027rest on a mud slab1028. A base layer1030, preferably of reinforced concrete, is laid above the mud slab1028, beneath the spool1002and girders1008. The girders1008preferably include downward facing studs1032(e.g., Nelson™ studs) that are enmeshed with the base layer1030and that are sized and spaced to provide sufficient steel to limit the stress range to meet fatigue design requirements. The wind turbine foundation1000provides a stiffer direct connection between the tower piece1019and the radial girders1008with the center girder connection done in a lower stress location reducing bolting and plate thicknesses as well as lessening fatigue issues. This configuration also provides a more direct load flow along each radial girder1008set from the compression side to the tension side of the foundation1000.

An example of a construction sequence for certain embodiments described herein is as follows:

1. Excavate foundation area (e.g., ˜3 m×20 m diameter), verify in situ ground conditions and improve as per normal foundation preparation.

2. Pour concrete mud slab to protect the exposed ground and create a level work surface.

3. Install base slab reinforcing over entire area noting the pattern for foundation orientation.

4. Install base can or spool on support legs which include leveling bolts.

5. Install all radial girders by bolting to the base can or spool. Final levelling of the tower flange is conducted by adjustment of the levelling bolts on the girder ends and base can or spool perimeter. The perimeter levelling bolts are used to perform final levelling adjustments to the tower flange. The levelling bolts on the base can or spool are raised during this process and re-lowered once the final levelling is complete.
6. Pour the base slab concrete and screed to the top of the second outer flange (girder base flange) ensuring full concrete contact to underside of flange by watching the air bleed holes in the flanges.
7. Install electrical conduits and grounding cables.
8. Pour concrete fill in base can (if applicable) and trowel finish top surface.
9. Backfill foundation with excavated soil stockpiled adjacent to the area.
10. Grade area for drainage, install gravel surfacing and install precast stairs foundation.

In one specific nonlimiting example, the wind turbine foundation100is preferably housed in a hole dug in the ground with a preferred height of from about 2 meters to about 4 meters and diameter of from about 15 meters to about 25 meters for use with a 3.5 megawatt (MW) wind turbine. Although specific preferred dimensions are provided herein for an example of a foundation for use with a 3.5 MW wind turbine, it should be understood that the technology described herein can be scaled with different dimensions to accommodate different sized wind turbines. Digging the hole is a first step (A1) in building the wind turbine foundation100. An additional step (B1) includes pouring an underlying mud slab112in the hole wherein, in this specific example, the underlying mud slab112is preferably from about 50 mm to about 200 mm thick. The base can102and girders104are preferably situated on the mud slab112that, in this specific example, is preferably round with a diameter of from about 15 meters to about 25 meters and most preferably about 20 meters. In this specific example, the girders104are preferably about 2 meters tall along the tallest edge of the girders104where the girders104attach to the base can102, however other sizes are contemplated for different embodiments.

An additional step (C1) in making the wind turbine foundation100includes placing the base can102in the hole on the underlying mud slab112. The base can102is preferably placed at the approximate center of the underlying mud slab112. In this specific example, the base can102is preferably from about 2 meters to about 6 meters high and most preferably about 3.5 meters high. In this specific example, the base can102is preferably about 4 meters to about 6 meters in diameter and most preferably about 5 meters in diameter. The base can102preferably includes a collector port172through which electrical and potentially other connections can be made to a wind turbine resting on the wind turbine foundation100.

An additional step (D1) includes placing rebar114in the hole along the underlying mud slab112. The rebar114preferably has a diameter of about 20 mm, a linear mass density of about 2.4 Kg/m and a cross-sectional area of about 300 mm2but other rebar sizes may be used. In this specific example, the total weight of rebar used per wind turbine foundation should be from approximately 10,000 Kg to about 16,000 Kg and most preferably less than about 13,000 Kg.

Another step (E1) includes attaching the girders104to the base can102preferably using bolts. In a following step (F1), a base layer of concrete116is poured beneath the girders104and beneath the base can102. In this specific example, preferably from about 100 m3to about 120 m3of concrete is used to form the base layer116.

A next step (G1) includes placing a mass of material (e.g., backfill128from the excavation to dig the hole) above the base layer116and preferably up to the collector port172. Another step (H1) includes installing a collector conduit bundle174preferably through a culvert.

Another step (I1) includes adding additional backfill to the hole and inside the base can102. Preferably, the backfill is added to a consistent depth across the hole with a slight slope of from about 2% to about 5% away from an upper section128A of the base can102that remains exposed. In another step (J1), additional rebar is added inside the base can102. A following step (K1) includes pouring concrete into the base can102to form a base can slab176wherein, in this specific example, from about 1 m3to about 3 m3of concrete is used. Another step (L1) includes attaching the first tower piece110to the tower flange108along the upper section128A of the base can102. The first outer flange106A extends out from a main body178of the base can102and is also located along the upper section128A of the base can102. The second outer flange extends out from the main body178of the base can102and is located along a lower section128B of the base can102. As an example, the first outer flange106A and the second outer flange106B may be welded to the main body178or may be formed as a part of the base can102.

In an embodiment where the base can is replaced by a spool, the placement of the spool, the installation of rebar, the attachment of the girders to the spool, the pouring of the concrete base layer, the placing of the mass of material including installation of a collector conduit and additional backfilling of the spool follow similar steps as outlined above with the base can being replaced by the spool.

The previously described embodiments of the present disclosure have many advantages. As described in the Background section, current methods of making foundations for wind turbines of the 3.5 MW size typically use about 400 m3of concrete, 83,000 Kg of steel, require a 5 week build cycle, and, depending on the geographic location, can only be built for certain months out of the year. For example, in many parts of Canada, construction can only be best carried out for about 8 months out of the year. Some of the embodiments described herein relating to 3.5 MW turbines typically use about 140 m3of concrete and 70,000 Kg of steel. Some of these embodiments described herein have a three week build cycle, and can be built all twelve months of the year regardless of geography since many of the components including the girders can be made offsite during colder or otherwise inclement months. Current methods for making foundations for 3.5 MW wind turbines require on average about 80 truckloads of material. Some of the embodiments described herein relating to 3.5 MW turbines require approximately 20 truckloads of material since much of the ballast used is backfill from the initial excavation (which, therefore, does not need to be hauled away).

The various embodiments preferably use pre-fabricated structural steel components for efficient load transfer and distribution as part of the foundation. Such embodiments maximize use of natural in-situ materials (e.g., excavated soil) to provide stability. The embodiments described herein do not use a pre-tensioned anchor bolt cage embedded in concrete for transferring load from the tower to the foundation. Eliminating the anchor bolt cage eliminates a major construction step and makes rebar placement easier. A bolted flange connection eliminates the entire anchor cage typically consisting of about 180 4 m long×40 mm bolts and associated steel anchor rings. Embodiments described herein have a design type that is a raft foundation, like a traditional concrete raft foundation, however instead of concrete providing part of the bending and shear resistance and most of the ballast, the embodiments described herein use radial girders connected to a base can or spool for primary load transfer and use mostly backfill as ballast over the thin concrete base slab. The loads transferred to the girders are distributed into the proximal parts of the concrete slab. The slab is held in place by bearing on the subgrade below and the weight of the backfill on top of it. Similar to traditional concrete raft foundations, in medium to low strength soil conditions, the design is typically governed by rotational stiffness, depending on the turbine manufacturer requirement for stiffness. In stronger soils and on bedrock, the foundation size tends to be governed by overturning stability and sometimes bearing capacity.

New 4+ MW wind turbines forces and diameters are causing design limits to be reached for traditional concrete raft foundations so an alternative to such foundations is becoming more necessary. High shear, rebar spacing issues and high-strength concrete are now common. Site conditions are dictating multiple traditional foundation solutions that increase cost and logistical challenges. For example, high groundwater and shallow weak bedrock are often found. One foundation type—a universal solution—is better than two or three different foundation types on one site from an economies-of-scale and simplicity-of-construction perspective. Pre-fabricated foundations offer year-round construction opportunity which decreases build time and reduces constraints. Shop manufactured components can be built and shipped any time of the year. In embodiments described herein, the tower to foundation joint is a bolted steel flange connection instead of a grouted base and anchor bolt connection. As turbine sizes increase, grouted connections are now reaching their maximum capacity. Bolted steel flanges offer much higher capacity that are in concert with the other tower connections above and have much better fatigue performance.

The foregoing description of preferred embodiments of the present disclosure has been presented for purposes of illustration and description. The described preferred embodiments are not intended to be exhaustive or to limit the scope of the disclosure to the precise form(s) disclosed. Different features of some embodiments can be substituted for other features of other embodiments to arrive at different embodiments of the concepts described herein. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of the principles of the disclosure and its practical application, and to thereby enable one of ordinary skill in the art to utilize the concepts revealed in the disclosure in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the disclosure as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.