Field of the Invention

The invention described herein relates to a support structure for a wind turbine tower, a tower having such a support structure making it possible to reach tall heights and use long blades, in order to optimize the exploitation of the available wind potential.

Background of the Invention

In our era of massive energy consumption, depletion of conventional energy sources and increased awareness of environmental parameters, alternative energy sources are constantly gaining ground. As an example, the EU has set a target to achieve at least 32% energy production from renewable sources by 2030. Among these sources, wind is becoming all the more popular as a potentially efficient and cost-effective energy source. Several aspects, such as the aerodynamic elements of the rotor, the electrical mechanisms for power extraction, and the areas where wind power plants are installed, are continuously optimized, to provide the highest possible power output. Towards that goal, major increases in the power output can mostly be achieved by increasing the swept area of the rotor (thus using longer blades) and by locating the wind turbines at taller heights from the ground where wind is stronger and steadier. Both requires taller wind turbine support structures.

As a result, structural actions on the towers and their foundation are also increased. Larger swept area and higher wind velocities cause larger wind-induced horizontal forces. Those, in turn, multiplied by the taller tower height yield very large bending moments, particularly on the lower parts of the tower and on the foundation.

Nowadays, the most common type of wind turbine tower in use is the free-standing tubular steel tower, composed of several cylindrical and/or conical shell segments. For onshore wind turbines, high bending moments near the tower’s base cannot be addressed by increasing the diameter, as would have been dictated by mechanics, because transportability constraints restrict tower diameters from exceeding a limit of approximately 4.5m. For shell diameters up to that limit, tubular towers seem to have exhausted their potential as far as their height is concerned. For taller heights other structural systems are necessary.

Segmental tubes consisting of smaller, thus transportable, shell segments jointed together by bolts along their vertical sides have been proposed and applied, providing an option to some extent. However, bolting and maintenance of the very large number of bolts is time consuming and costly.

The tripod concept is a very old, well-known solution to provide increased stability and stiffness to free standing objects and structures. Unsurprisingly, it has been used for offshore wind turbines, where fabrication can take place in factories located at the shore, while sea transportation by vessels allows for the large size of the pre-welded tripods. This is, however, not a viable concept for on-shore transportation. Concepts of multi-leg base sections consisting of individual inclined legs that are bolted together on the site have been proposed. However, they remain at an abstract level, very far from realistically satisfying the stiffness, strength, stability, resonance and fatigue verification criteria for any wind turbine tower of substantial height, not even for heights that are already feasible in existing tubular towers. Moreover, basic issues such as access inside the turbine and cabling path for grid connection are not foreseen. Modularity and scalability are not addressed and even realizability is doubtful.

Some tripod designs are employed in the prior art. A Multibrid M5000 turbine tower is known to be installed onshore with a tripod style foundation, as mentioned in a press release by the former Multibrid GmbH, titled “Errichtung der zweiten Multibrid M5000 im Grauwallring Bremerhaven erfolgreich abgeschlossen” (10 December 2006), essentially for testing purposes. However, this tripod structure requires legs with large diameters and has not found widespread use. It is noted that in this proposal the tripod legs are welded together. This requires that either onsite welding is employed, which is not recommended for reasons of welding quality, particularly in view of the strict fatigue requirements in wind turbine towers, or the tripod structure must be transported in a prewelded form, limiting its use to offshore applications but even then with small overall tripod size.

The document numbered CA2759979A1 discloses a wind turbine tower with a tripod shaped modular lower section composed of multiple segments. The tripod is essentially formed by three inclined legs, connected to each other and to the bottom of the main tower. The inclined legs are of an open semi-tubular shape providing ease of transport. While the upper section of the tower can be installed on top of the tripod for added height, the forces and moments transferred from the upper section are not wholly supported by the tripod due to the absence of a vertical leg and to the small bending and buckling capacity of the open section legs.

The document numbered EP2444663A2 discloses a wind turbine tower support with an annular support element attached to a main tower. The support element is in the form of a sleeve attached around the main tower with the help of an adhesive. The annular element is connected to a multitude of inclined legs. Both the annular element and the legs can have segmental structures serving to allow rail and truck transport. The support structure can be included during initial production or be added to an existing tower as an upgrade. The adhesive compound between vertical part and sleeve of inclined legs seems to be a weak part in terms of durability and fatigue.

The document numbered WO2011147477A1 discloses support structure for a wind turbine tower that is composed of a plurality of standard modular tubular elements. Emphasis is placed on how to connect individual tubular cans to each other to form longer sections. Use of concrete and steel tubular elements is mentioned. Various support structure shapes can be obtained by combining these tubular elements. Tripod shaped support structures are used among the examples.

Purpose of the Invention

Hence, objective of the invention is to propose realistic, modular and scalable base support structures for wind turbine towers of very large heights, relying on the tripod or multi-pod concept, and having suitable member sections and joint details that satisfy all necessary verification criteria related to safety and operation, while at the same time respecting fabrication, on-shore and off-shore transportation, erection, maintenance and dismantling constraints.

The limits on the feasibly achieved wind turbine tower height due to a high rate of increase in diameter and thickness of supporting structure members and the resulting high rate of increase in weight with respect to the increase in total height and rotor diameter, is avoided with this invention. Thanks to its scalability, the concept can also be used as an alternative to increasing power by replacing the towers and turbines of existing wind farms (repowering), by simply elevating them to exploit the improved wind conditions at taller heights and thus to increase power production.

Moreover, existing tubular towers have, in most cases, a very heavy reinforced concrete matt foundation in order to counterbalance the large imposed bending moments. A further objective of the invention is to propose a much lighter and more cost-effective foundation solution, based on reinforced concrete piles.

Both, base support structures and their pile foundation, will be adaptable, to fit any upper tubular section to be placed on top, whether new or existing.

Brief Description of the Invention

A wind turbine tower assembly is provided. It consists of an upper structure of tubular steel, corresponding to a conventional tower carrying a wind turbine, and an adaptable lower “tripod section”, that is a tripod or multi-pod shaped support structure that can fit any upper structure. The tripod section comprises a vertical leg and three pairs of upper and lower inclined legs, also made of tubular steel sections. Spirally welded tubes or tubes manufactured by any other technology may be adopted for the legs. In other variations, the number of pairs of legs may be different, such as in multi-pod arrangements. Each pair of upper/lower legs meet at their outer, bottom ends and are connected to an individual foundation consisting of a group of reinforced concrete piles with a common pile cap at their top, being substantially more cost-effective than massive mat foundations commonly used.

Special junction pieces at all interfaces between multi-pod legs, including their interfaces to the foundation and to the upper structure are provided in two alternatives. In one alternative each joint is fabricated in one piece and is connected to the adjoining legs by bolted flanges. In the other alternative each leg is pre-welded to a part of the junction and the individual junction parts are bolted together. The choice between the two alternatives is dictated by transportation constraints, with the first alternative being simpler and preferable but larger and more difficult to transport. The proposed junction pieces are crucial for making transportation and erection possible, thus ensuring feasibility of the wind turbine tower assembly. Moreover, connections are configured to allow easy access for maintenance.

A manhole at the bottom of the vertical leg is provided, avoiding the commonly encountered manhole on the side wall. Internal parts are provided through the vertical multi-pod leg, continuing into the upper tubular steel section. Moreover, any conventional transformer as well as other units that are needed for the turbine operation can be located below the tripod structure in a simple building, which in turn provides easy maintenance, flexibility of purchasing such components/system parts minimizing their design requirements and sizing. This will also ease the safe access to the aforementioned manhole.

The wind turbine tower assembly and in particular the lower “tripod section” and all individual parts and their connections satisfy all performance criteria related to operation and safety dictated by pertinent regulations. Moreover, they meet all fabrication, on-shore and off-shore transportation, erection, maintenance and dismantling constraints. The concept is realistic and realizable in terms of production, material thicknesses, flange diameters, logistics and handling. Moreover, it is scalable and adaptable, suitable for new wind turbines, particularly very tall ones for optimum exploitation of the wind potential, but also as an alternative solution to replacing turbines (repowering), by elevating already existing turbines for provision of increased production efficiency. The tubular approach avails a modular structure where the parts may be divided into smaller parts, which offers improvements in terms of production, transport, installation and maintenance costs and provision of shorter delivery terms. Erection, maintenance and dismantling methodologies of the wind turbine tower assembly and in particular the lower “tripod section” are also provided.

Detailed Description of the Invention

A support structure according to the invention has been shown in the listed figures.

Figure 1 is a view of a wind turbine tower assembly consisting of an upper structure of tubular steel carrying the wind turbine and a tripod shaped lower support structure.

Figure 2 is a view of an upper structure.

Figure 3 is a view of a support structure and foundation.

Figure 4 is a view of a unified upper junction piece.

Figure 5 is a view from above of a unified lower junction piece. Figure 6 is a view from below of a unified lower junction piece.

Figure 7 is a view of a unified foundation junction piece.

Figure 8 is a view of pile foundation below the unified foundation junction piece.

Figure 9 is a view of a support structure and foundation with ground tie rods.

Figure 10 is a view of a support structure and foundation with radially arranged ground tie rods.

Figure 11 is a view of a segmental upper junction piece welded at the top of an upper inclined leg.

Figure 12 is a view of an upper tubular shell of an upper inclined leg with a segmental junction piece welded at its top end and a ring bolted flange at its bottom end.

Figure 13 is a view of a tubular shell of an upper leg with a bolted ring flange at both ends.

Figure 14 is a view of a segmental lower junction piece welded at the top of a lower inclined leg.

Figure 15 is a view of an upper tubular shell of a lower leg with a segmental lower junction piece welded at its top and a bolted ring flange at its bottom.

Figure 16 is a view of a tubular shell of a lower inclined leg with bolted ring flanges at both ends.

Figure 17 is a view of a segmental foundation junction piece between an upper inclined leg, a lower inclined leg and foundation.

Figure 18 is an exploded view of a segmental foundation junction piece between an upper inclined leg, a lower inclined leg and foundation.

Figure 19 is a view of a support structure with a temporary support structure during erection.

The parts in the figures have been numbered and their references have been listed below.

100. Wind turbine tower

101. Upper structure

102. Support structure

103. Foundation

104. Individual cylindrical can

105. Prewelded upper structure section

106. Ring flange

107. Vertical leg 108. Upper inclined leg

109. Lower inclined leg

110. Prewelded vertical leg section

111. Upper junction piece

112. Lower junction piece

113. Foundation junction piece

114. Upper can of vertical leg

115. Upper can of upper inclined leg

116. Manhole

117. Lower can of vertical leg

118. Inner can of lower inclined leg

119. Stiffener

120. Outer can of upper inclined leg

121. Outer can of lower inclined leg

122. Auxiliary tube

123. Individual foundation section

124. Pile

125. Pile cap

126. Ground tie rod

127. Segment of upper junction piece

128. Vertical end plate

129. Partial ring flange

130. Internal ring stiffener

131. Internal vertical stiffener

132. Intermediate can of upper inclined leg

133. Segment of lower junction piece

134. Intermediate can of lower inclined leg

135. Segmental foundation junction piece

136. First segment

137. Second segment

138. Temporary support structure

139. Temporary foundation The following detailed description refers to a wind turbine tower (100) assembly consisting of a conventional upper structure (101) of tubular steel and an innovative adaptable lower support structure (102) in the form of a tripod or more generally a multi-pod and its foundation (103), which is scalable and adjustable so that it can fit any upper structure (101). The description enables one with ordinary skill in the art to make and use the disclosure, and the description encompasses several embodiments of the disclosure, including what is presently believed to be the best ways for fabricating, transporting, erecting, maintaining and dismantling the disclosure. The disclosure is described herein as being applied to an exemplary embodiment, namely an onshore wind turbine tower (100) carrying a horizontal axis wind turbine. However, it is contemplated that this disclosure has a wide range of application to other towers, including offshore wind turbines and a variety of applications other than wind turbines.

The multi-pod support structure (102) for a wind turbine tower (100), comprises, a tubular vertical leg (107) that is coaxial with and forming a base for the upper structure (101) to be supported, a multitude of tubular upper inclined legs (108) and tubular lower inclined legs (109) arranged in pairs around the vertical leg (107), with each inclined leg descending from the vertical leg (107) to a ground level, and a multitude of foundation junction pieces (113) connecting outer ends of each pair of upper inclined leg (108) and lower inclined leg (109) to each other and to a foundation (103). Said ground level may be determined by the seabed for offshore installations. The foundation (103) level may be the same for all pairs of upper inclined leg (108) and lower inclined leg (109), in cases of essentially flat terrains, or it may be different to adapt to the ground level in cases of inclined or rough terrains.

The upper inclined legs (108) and the vertical leg (107) form an upper junction piece (111) corresponding to the top end of the vertical leg (107), and the lower inclined legs (109) and the vertical leg (107) form a lower junction piece (112) corresponding to the bottom end of the vertical leg (107), with the lower junction piece (112) being above the ground level.

The vertical leg (107), the upper inclined legs (108) and the lower inclined legs (109) have lengths, diameter profiles and thickness profiles adapted to support said upper structure (ioi).

The upper inclined legs (108) and the lower inclined legs (109) comprise more than one tubular member conforming to said lengths, diameter profiles and thickness profiles. This allows said members to be easily transported and the support structure (102) to be easily assembled on site.

The upper inclined legs (108) and the lower inclined legs (109) have lengths at least one order of magnitude larger than their respective maximum diameters, thus allowing larger span to be achieved by the upper inclined legs (108) and the lower inclined legs (109) both laterally and vertically for a given vertical leg (107) length, in turn allowing the multi-pod support structure (102) supporting higher loads by the axial action of the inclined legs to be built using said members.

Fig. 1 is a schematic representation of a wind turbine tower (100). The wind turbine tower assembly consists of an upper structure (101) composed of a conventional tubular steel construction for carrying a wind turbine, a lower support structure (102) in the shape of a tripod and its foundation (103). The support structure (102) can be formed to be adapted to various sizes of upper structure (101).

The upper structure (101) may be any existing or future tower of conventional or innovative nature. As shown in Fig. 2, it may be comprised of any number of individual cylindrical cans (104) of cylindrical, conical or any other shape with overall dimensions and shell thickness depending on mechanical/structural requirements as well as fabrication and transportation constraints. The cans may be fabricated by cold curving of flat plates and subsequent longitudinal welding, or by any other method. Individual cans may be welded to adjacent cans in the factory in order to form longer, prewelded upper structure sections (105), having length that depends on transportation constraints. At their ends parts, prewelded upper structure sections (105) may be fitted with ring flanges (106) or any other means for enabling their connection to other prewelded upper structure sections (105). Such prewelded upper structure sections (105) may be lifted onsite by cranes or by any other method and connected to adjacent parts by prestressed bolts or by any other method that meets operational and safety verification criteria. Similarly, the connection of the upper structure (101) to the underlying support structure (102) may be realized in a similar manner as the connections between adjacent prewelded upper structure sections (105), or by any other method that meets operational and safety verification criteria.

The support structure (102) and its foundation (103) are described in more detail in Fig. 3. In this embodiment section, the support structure (102) consists of one vertical leg (107) and three pairs of upper inclined legs (108) and lower inclined legs (109). In other cases, the number of pairs may be different, leading to a multi-pod arrangement. This approach substantially enlarges the perimeter of support points of the tower on the ground, thus increasing stability against overturning. Moreover, the support structure (102) resists the large developing bending moments primarily by axial action instead of flexural action, thus exploiting material much better and requiring significantly smaller sections for the legs.

In this embodiment the vertical leg (107) is configured in a tubular, cylindrical arrangement, similar as the upper structure (101). It consists of individual cans of varying thickness to adjust to the needs imposed by developing mechanical actions, commonly requiring gradually smaller thickness from top to bottom. Individual cans may be welded to adjacent cans in the factory in order to form longer, prewelded vertical leg sections

(110), having length that depends on transportation constraints. At their ends prewelded vertical leg sections (110) may be fitted with ring flanges or any other means for enabling their connection to other prewelded vertical leg sections (110) or to an upper junction piece

(111) at the intersection of vertical leg (107) and upper inclined legs (108) or a lower junction piece (112) at the intersection of vertical leg (107) and lower inclined legs (109). Such prewelded vertical leg sections (110) may be lifted onsite by cranes or by any other method and connected to adjacent parts by prestressed bolts or by any other method that meets operational and safety verification criteria.

The vertical leg (107) will be fitted with internal equipment, such as ladders, platforms, elevators, cabling and everything else that is needed for the wind turbine operation. It is not structurally necessary to have a foundation below the vertical leg (107), therefore, its bottom will be a few meters above the ground. This will make it possible to provide a manhole (116) at the bottom of the vertical leg (107), specifically on the lower can of the vertical leg (117), to enable entrance and exit of personnel and equipment. The lower can of the vertical leg (117) may have an appropriate conventional ring or similar stiffener around the perimeter, still allowing enough space in between for the manhole (116). Thus, the commonly used side opening of tubular towers, requiring a very heavy and costly frame stiffener of complex shape, will be avoided, increasing the efficiency in terms of costs, production time and quality assurance.

As there is no need to cover the areas between tripod legs, it will be possible to locate the transformer and other potential units underneath the tower with simpler technical requirements. This will provide easy access inside the turbine and eliminate special design requirements that will reduce procurement costs of such parts, as well as their service and maintenance. Turbine designers will then be more flexible during design phase and provide cost effective solutions.

In other embodiments, the vertical leg (107) may have conical shape or any other shape that optimizes material and cost, while respecting operational and safety criteria and satisfying fabrication, transportation, erection, maintenance and dismantling constraints.

Each upper inclined leg (108) and each lower inclined leg (109) will have a circular hollow steel section, or other appropriate section that optimizes material and cost, while respecting operational and safety criteria and satisfying fabrication, transportation, erection, maintenance and dismantling constraints. It may be fabricated by cold curving flat plates, similarly as the vertical leg. Alternatively, it may comprise spirally welded tubes or tubes fabricated by any other method that meet the pertinent safety requirements. Considering that the mechanical action of inclined legs will be primarily axial, with substantial flexural action developing only near the ends, a constant tube thickness over most of their length will be cost effective, with possible thickness increase near the ends. Avoiding the need for gradually varying thickness, combined with relatively small diameter requirements because of the primarily axial function, will permit the adoption of long tube pieces manufactured with different methods, thus minimizing the needs for welding and bolting and permitting potentially significant fabrication time and cost reduction.

In this embodiment each inclined leg is fitted with three pairs of ring flanges (106); one near its junction with the vertical leg (107), one near its junction with its accompanying inclined leg and the foundation (103), and one near the middle, so that welded assemblies meet the transportation constraints. The leg cans above the upper and below the lower flange may be of different thickness to comply with increased requirements due to local flexure. In other embodiments different number of ring flanges (106) may be used, to better adopt to thickness requirements, or for transportation issues, or for any other reason. Depending on the diameter and the location, ring flanges (106) may be either internal or external, taking into account accessibility for initial bolting and for subsequent maintenance. External ring flanges (106) and accompanying bolts may require additional protection against corrosion and environmental factors. In case of internal ring flanges (106) and depending on the inclination of the legs, internal ladders may be necessary to allow access of personnel for bolting and maintenance. In case of external ring flanges (106) bolting and maintenance may be carried out using telescoping boom lifts or scissor lifts.

The upper junction piece (111) between vertical leg (107) and upper inclined legs (108), the lower junction piece (112) between vertical leg (107) and lower inclined legs (109), as well as the foundation junction pieces (113) between upper inclined legs (108), lower inclined legs (109) and the foundation (103), are very important for the successful realization of this invention. High local stress concentrations are encountered at these locations. In general, such stress concentrations may be addressed either by local shell wall thickening, or by arrangement of internal stiffeners or by a combination of the two. Further considerations to be taken into account are, among others, (i) to provide openings for access into the inclined legs, for bolt tightening and maintenance purposes, (ii) to respect fabrication limitations related to maximum plate thickness that can be cold curved, (iii) to minimize weld lengths for cost reduction, (iv) to respect size limitations for transportation, which may be different for onshore and offshore transportation and also depend on the specific factory and site locations and the alternative itineraries between them. To that effect, different alternatives for these special junction pieces are proposed herein, constituting major aspects of the present disclosure.

In Fig. 4 one embodiment of a unified upper junction piece (111) between vertical leg and upper legs is presented. It comprises the upper can of the vertical leg (114) and the upper cans of the upper inclined legs (115). Each of these four pieces may be either cold curved from a flat plate (most likely case for the can of the vertical leg, due to its larger diameter) or spirally welded or fabricated by any other means (most likely case for the pieces of the upper legs, due to their relatively smaller diameter). These four pieces are welded together in the factory and transported as one assembly.

The diameter of the upper can of the vertical leg (114) is dictated by the diameter of the bottom part of the upper structure (101), as the two must be connected together. In this embodiment this connection is realized by means of two ring flanges (106), one welded at the top of the upper can of the vertical leg (114), and the other welded at the bottom of the upper structure (101), with the ring flanges (106) bolted together by prestressed bolts. In other embodiments other types of connection may be adopted. A similar ring flange (106) is provided at the bottom of the upper can of the vertical leg (114), to enable the connection to the below part of the vertical leg. This upper can of the vertical leg (114) will also be fitted with internal equipment, such as ladders, platforms, elevators, cabling and everything else that is needed for the wind turbine operation, thus any stiffeners that may be necessary must take this into consideration.

The upper cans of the upper inclined legs (115) are pre-welded in the factory to the upper can of the vertical leg (114). At its bottom end, each upper can of the upper inclined leg (115) is fitted with a ring flange (106) or other appropriate connection, to connect to the adjacent part of the upper leg. Depending on the diameter of the upper leg, this flange may be internal, if access for initial bolting and subsequent maintenance is possible, or external otherwise. In the embodiment shown in Fig. 4, the ring flange (106) is internal, therefore manholes (116) are provided on the shell wall of the upper can of the vertical leg (114), centrally located with respect to each upper inclined leg (108), with sufficient size to enable access to personnel and equipment. Moreover, the upper inclined leg (108) will be provided with a ladder and platforms, as needed.

In this embodiment, the high local stress concentrations in upper can of vertical leg (114) and upper can of upper inclined leg (115) are addressed by means of sufficiently high shell thickness, without any stiffeners. In other embodiments, smaller shell thickness may be adopted, compensated by stiffeners. Due to welding cost, it is likely that higher thickness without stiffeners will be preferable, as long as it is feasible to curve plates of that thickness.

Overall dimensions of this unified upper junction piece (111) are critical for the realization of this alternative. Considering that the external diameter of the upper can of the vertical leg (114) is dictated by the diameter of the bottom part of the upper structure (101), it is likely that this diameter will already be near the transportation limitations. Considering also the three protruding upper cans of upper inclined legs (115), special transportation solutions may be necessary. In case transportation limitations cannot be overcome, the second alternative of a segmental upper junction piece (111) composed of multiple segments of upper junction piece (127) as described below, may be preferable.

In Fig. 5 a top view and in Fig. 6 a bottom view of an embodiment of a unified lower junction piece (112) between vertical leg and lower legs is presented, similar to the one of Fig. 4, of the corresponding junction between vertical leg and upper legs. It comprises the lower can of the vertical leg (117) and the inner cans of the lower inclined legs (118). Each of these four pieces may be either cold curved from a flat plate (most likely case for the piece of the vertical leg (107), due to its potentially larger diameter) or spirally welded or fabricated by any other means (most likely case for the pieces of the lower inclined legs (109), due to their relatively smaller diameter). These four pieces are welded together in the factory and transported as one assembly.

In this embodiment the connection between the lower can of the vertical leg (117) and the adjoining part of the vertical leg is realized by means of two ring flanges (106), one welded at the top of the lower can of the vertical leg (117), and the other welded at the bottom of the adjoining part of the vertical leg (107), with the ring flanges (106) bolted together by prestressed bolts. In other embodiments other types of connection may be adopted. At the bottom of the lower can of the vertical leg (117), a local strengthening by stiffeners (119) or other means may be provided, as needed, allowing sufficient space for a manhole (116) to enable entrance and exit to personnel and equipment. This lower can of the vertical leg will also be fitted with internal equipment, such as ladders, platforms, elevators, cabling and everything else that is needed for the wind turbine operation, to be continued through the entire length of the vertical leg into the overlying upper structure (101).

The inner can of the lower inclined legs (118) of each lower inclined leg (109) is prewelded in the factory to the lower can of the vertical leg (117). At its outer end, this inner can of the lower inclined leg (118) is fitted with a ring flange (106) or other appropriate connection, to connect to the adjacent can of the lower inclined leg (109). Depending on the diameter of the lower inclined leg (109), this ring flange (106) may be internal, if access for initial bolting and subsequent maintenance is possible. However, due to the relatively small actions of the lower inclined legs (109), diameter requirements will in most cases be modest, so that external ring flanges (106) are likely, as shown in Fig. 5 and Fig. 6. Adoption of external ring flanges (106) is also promoted by the lower height over the ground, which facilitates external access for bolting and maintenance. In such case, no manholes are necessary on the shell wall of the lower can of the vertical leg (117).

In this embodiment, the high local stress concentrations in lower can of the vertical leg (117) and inner cans of the lower inclined legs (118) are addressed by means of sufficiently high shell thickness, without any stiffeners. In other embodiments, smaller shell thickness may be adopted, compensated by stiffeners. Due to welding cost, it is likely that in this case also higher thickness without stiffeners will be preferable, as long as it is feasible to curve plates of that thickness. Considering also that the expected mechanical stresses on this lower can of the vertical leg (117) are expected to be lower than the corresponding ones on the upper can of the vertical leg (114), the required shell thickness to avoid stiffeners is expected to be manageable.

Overall dimensions of this unified lower junction piece (112) are critical for the realization of this alternative. Considering the reduced actions on the vertical leg towards its bottom, a conical arrangement may be adopted, yielding smaller diameter of the lower can of the vertical leg (117), thus facilitating transportation of the entire lower junction piece (112) in spite of the protruding inner cans of lower inclined legs (118). Alternatively, a lower segmental lower junction piece (112) composed of multiple segments of lower junction piece (133) as described below may be adopted.

In Fig. 7 an embodiment of a unified foundation junction piece (113) between one upper leg and the corresponding lower leg is presented. It comprises the outer can of the upper inclined leg (120) and the outer can of the lower leg (121). In order to accommodate the foundation junction piece (113), a short, vertical auxiliary tube (122) is also provided. This auxiliary tube (122) may be either cold curved from a flat plate or spirally welded or fabricated by any other means, depending on its diameter. These three pieces are welded together in the factory and transported as one assembly.

In this embodiment the connection between the outer can of the upper inclined leg (120) and the adjoining part of the upper inclined leg is realized by means of two ring flanges (106), one welded at the top of the outer can of the upper leg (120), and the other welded at the bottom of the adjoining part of the upper inclined leg (108), with the ring flanges (106) bolted together by prestressed bolts. Similarly, the connection between the outer can of the lower inclined leg (121) and the adjoining part of the lower inclined leg (109) is realized by means of two ring flanges (106), one welded at the top of the outer can of the lower inclined leg (121), and the other welded at the bottom of the adjoining part of the lower inclined leg (109), with the ring flanges (106) also bolted together by prestressed bolts. In other embodiments other types of connection may be adopted. Ring flanges (106) may again be external or internal, depending on diameter, accessibility and protection from environmental factors. Considering that these ring flanges (106) are very near the ground, external ring flanges (106) are likely to be preferable. In this embodiment, at the bottom of the auxiliary tube (122), a ring flange (106) is provided for anchor bolts to ensure connection to the foundation (103). The ring flange and anchor bolts may be external, internal or both, as dictated by strength requirements. Alternatively, other types of anchorage of the auxiliary tube (122) into the foundation (103) may be adopted.

In this embodiment, at the top of the auxiliary tube (122), a local strengthening by a ring shaped stiffener (119) or other means is provided, allowing sufficient space for a manhole (116), providing entrance and exit to personnel and equipment, in case internal bolts are used, either at the ring flanges of the inclined legs, or for the anchoring of the bottom of the auxiliary tube (122) into the foundation (103). Alternatively, in case no access into the auxiliary tube (122) is needed, a welded lid may be provided at its top, acting as a stiffener (H9).

In this embodiment, the high local stress concentrations of shells in this junction are addressed by means of sufficiently high shell thickness, without any stiffeners. In other embodiments, smaller shell thickness may be adopted, compensated by stiffeners. Due to welding cost, it is likely that, in this junction also, higher thickness without stiffeners will be preferable, as long as it is feasible to curve plates of that thickness.

Overall dimensions of this unified foundation junction piece (113) are critical for the realization of this alternative. The diameter of the auxiliary tube (122) will have to be somewhat larger than the larger among the two diameters of the outer can of the upper inclined leg (120) and the outer can of the lower inclined leg (121), for better welding between the three tubes, and this will ultimately dictate the overall dimensions of the unified foundation junction piece (113). In case transportation of the entire foundation junction piece (113) is not possible, the segmental foundation junction piece (135) may be adopted, described below.

The overall foundation (103) of the wind turbine tower (100) consists of individual foundation sections (123) below each foundation junction piece (113) of each pair between an upper inclined leg (108) and the corresponding lower inclined leg (109). One such individual foundation section (123) below a unified foundation junction piece (113) is illustrated in Fig. 8. It comprises of reinforced concrete piles (124) connected by a common reinforced concrete pile cap (125) at their top. The number, diameter, depth of embedment and reinforcement of the piles (124) depend on the soil conditions, and on the actions transferred to the foundation by the superstructure. In this embodiment, two piles (124) are arranged in radial direction, to resist the applied reaction forces more effectively. The inner pile (124), developing tension due the horizontal reaction force directed radially outwards, is placed directly below the auxiliary tube (122), in order to counterbalance some of this tension by the vertical reaction component. Moreover, in this arrangement the vertical reaction is transferred directly to the pile (124), without causing bending in the pile cap (125). Alternatively, other pile (124) number and arrangement may be adopted, but the above guiding principles will ensure an optimized foundation design.

In other embodiments, particularly in cases of poor soil conditions, pile caps (125) of individual foundation sections (123) of each pair of upper inclined legs (108) and lower inclined legs (109), may be connected to each other above or below ground by tie rods (126) (Fig. 9, Fig. 10), arranged either directly between individual pile caps (125), or oriented radially and connected to each other below the vertical leg (107) in a star-shaped arrangement, in order to better resist the applied horizontal reactions without compromising the overall stiffness of the wind turbine tower (100).

As an alternative to the unified upper junction piece (111) between vertical leg (107) and upper inclined legs (108), a segmental upper junction piece (111) having consisting of segments with smaller overall dimensions and thus facilitating transportation is provided. One segment of the upper junction piece (127) is shown in Fig. 11. It is part of a cylinder, with the same height as the unified upper junction piece (111), but corresponding to only one third of the cylinder’s circumference, thus having a central angle of 120°. This cylindrical segment of the upper junction piece (127) is welded in the factory to the top end of one upper inclined leg (108).

At its two vertical edges the segment of the upper junction piece (127) features two welded vertical end plates (128), arranged in radial direction. Through these end plates (128) segment of the upper junction piece (127) is bolted on-site to similar end plates (128) of the adjoining segments of the upper junction piece (127) via prestressed bolts. Thus, a full cylindrical piece is created. In other embodiments the connections between segments of the upper junction piece (127) may be achieved by other methods. In multi-pod arrangements the number of segments of the upper junction piece (127) will be different, equal to the number of pairs of upper inclined legs (108) and lower inclined legs (109), and the central angle of each such segment of the upper junction piece (127) will be equal to 360° divided by their number. Individual segments of the upper junction piece (127) may feature small temporary auxiliary bars to maintain their dimensional stability during transportation and erection, so as to facilitate assembling them together.

At its upper edge, each segment of the upper junction piece (127) features a welded partial ring flange (129), similar to the upper ring flange (106) of the unified upper junction piece (111), but only 120° in central angle. After the three segments of the upper junction piece (127) are bolted together via the vertical end plates (128), the three individual partial ring flanges (129) will form a full 360° circular ring flange, through which this segmental upper part of the vertical leg is bolted to the ring flange (106) welded at the bottom of the upper structure (101) by prestressed bolts. In other embodiments other types of connection may be adopted.

Similarly, at its lower edge, each segment of the upper junction piece (127) features also a welded partial ring flange (129), similar to the lower ring flange (106) of the unified upper junction piece (111), but only 120° in central angle. After the three segments of the upper junction piece (127) are bolted together via the vertical end plates (128), the three individual partial ring flanges (129) will form a full 360° circular ring flange, through which this segmental upper can of the vertical leg (114) is bolted to the ring flange (106) welded at the top of the below part of the vertical leg (107) by prestressed bolts. In other embodiments other types of connection may be adopted.

In this embodiment, the segment of the upper junction piece (127) is fitted with two internal ring stiffeners (130) and two internal vertical stiffeners (131). The number of stiffeners may vary, depending on the need to address local stress concentrations, as resulting from detailed finite element analyses of this area. In this embodiment, access into the upper inclined leg (108) through a manhole is not considered necessary. In case access into the upper inclined leg (108) through a manhole (116) is necessary, the locations of stiffeners must consider the location and dimensions of the manhole (116), and some of the stiffeners may also act as peripheral frame for the manhole (116). Alternatively to the use of stiffeners, local stress concentrations can be addressed by higher thickness of segment of the upper junction piece (127), as described for the unified upper junction piece (111) in Fig. 4. Considering the relatively small size of segment of the upper junction piece (127), the upper can of the upper inclined leg (115) to which segment of the upper junction piece (127) is welded may be of significant length (Fig. 12) still allowing onshore transportation by trucks. Thus, there is no need for a ring flange of the upper inclined leg (108) near the segments of upper junction piece (127) with the vertical leg (107), as was the case for the unified upper junction piece (111). The ring flange (106) can be located at the bottom of the upper can of the upper inclined leg (115), thus reducing the overall number of ring flanges (106) along each upper inclined leg (108). In such case, access to the ring flanges (106) from the interior of the upper inclined legs (108) may be more difficult due to the longer distance, thus external ring flanges (106) may be preferable, accessible for bolting and maintenance with the use of telescoping boom lifts or scissor lifts. In other embodiments, a ring flange (106) of the upper leg (108) near the segments of upper junction piece (127) with the vertical leg (107) may nevertheless be provided, in order to adjust wall thickness to bending moments that are expected to be locally higher near this junction.

Depending on its overall length the upper inclined leg (108) (Fig. 3) may comprise a varying number of intermediate cans of upper inclined leg (132) (Fig. 13), featuring ring flanges (106) at their two ends to connect to neighbouring intermediate cans of upper inclined leg (132) via prestressed bolts. The number of intermediate cans of upper inclined leg (132) per each upper inclined leg (108) depends on transportability constraints, desirable variation of thickness along upper inclined leg (108) to adjust to developing mechanical actions, and cost considerations in balancing weight optimization and weld length minimization. In other embodiments, connection between adjacent intermediate cans of upper inclined leg (132) may be realized by other methods instead of ring flanges (106) and prestressed bolts.

Similar considerations as for the upper legs are provided for the lower legs. Different choices may be preferable for the lower legs, due to their lower expected developing mechanical actions that lead to smaller section requirements.

As an alternative to the unified lower junction piece (112) between vertical leg (107) and lower inclined legs (109), a segmental lower junction piece consisting of segments with smaller overall dimensions and thus facilitating transportation is provided. One segment of the lower junction piece (133) of this segmental lower junction piece is shown in Fig. 14. It is part of a cylinder, with the same height as the unified lower junction piece (112), but corresponding to only one third of the cylinder’s circumference, thus having a central angle of 120°. This cylindrical segment of the lower junction piece (133) is welded in the factory to the inner end of one lower inclined leg (109).

At its two vertical edges, the segment of the lower junction piece (133) features two welded vertical end plates (128), arranged in radial direction. Through these end plates (128) segment of the lower junction piece (133) is bolted on-site to similar end plates (128) of the adjoining segments of the lower junction piece (133) via prestressed bolts. Thus, a full cylindrical piece is created. In other embodiments the connections between segments of the lower junction piece (133) may be achieved by other methods. In multi-pod arrangements the number of segments of the lower junction piece (133) will be different, equal to the number of pairs of upper inclined legs (108) and lower inclined legs (109), and the central angle of each such segment of the lower junction piece (133) will be equal to 360° divided by their number. Individual segments of the lower junction piece (133) may also feature small temporary auxiliary bars to maintain their dimensional stability during transportation and erection, so as to facilitate assembling them together.

At its upper edge, each segment of the lower junction piece (133) features a welded partial ring flange (129), similar to the upper ring flange (106) of the unified lower junction piece (112), but only 120° in central angle. After the three segments of the lower junction piece (133) are bolted together via the vertical end plates (128), the three individual partial ring flanges (129) will form a full 360° circular ring flange, through which this segmental lower can of the vertical leg (117) is bolted to the ring flange (106) welded at the bottom of the above part of the vertical leg (107) by prestressed bolts. In other embodiments other types of connection may be adopted.

Similarly, at its lower edge, each segment of the lower junction piece (133) features also a welded partial ring flange (129), also only 120° in central angle. After the three segments of the lower junction piece (133) are bolted together via the vertical end plates (128), the three individual partial ring flanges (129) will form a full 360° circular ring flange, through which this segmental lower can of the vertical leg (117) is bolted by prestressed bolts to a full 360° ring flange (106) constituting the bottom of the vertical leg (107), having larger width to act as stiffener (119), and allowing sufficient space for the manhole (116) to be provided there. In other embodiments other types of connection may be adopted. In this embodiment, the segment of the lower junction piece (133) is fitted with two internal ring stiffeners (130) and two internal vertical stiffeners (131). As in the upper junction, here also the number of internal stiffeners may vary, depending on the need to address local stress concentrations, as resulting from detailed finite element analyses of this area. In case access into the lower inclined leg (118) through an opening is necessary, the locations of internal stiffeners must consider the location and dimensions of this opening, and some of the internal stiffeners may also act as peripheral frame for the opening. It is however more likely that lower legs will feature external ring flanges (106), due to their smaller diameter and small distance from the ground, so that access into the lower legs will not be needed. Alternatively to the use of stiffeners, local stress concentrations can be addressed by higher thickness of the segments of the lower junction piece (133), as described for the unified lower junction piece (112) in Fig. 5 and Fig. 6.

Considering the relatively small size of the segments of the lower junction piece (133), the inner can of the lower inclined leg (118) to which a segment of the lower junction piece (133) is welded may be of significant length (Fig. 15), still allowing onshore transportation by trucks. Thus, there is no need for a ring flange of the lower inclined leg (109) near the segmental lower junction piece (112) with the vertical leg (107), as was the case for the unified lower junction piece (112). The ring flange (106) can be located at the bottom of the inner can of the lower inclined leg (118), thus reducing the overall number of ring flanges (106) along each lower inclined leg (109). Access to the ring flanges (106) from the interior of the lower inclined leg (109) will be unnecessary, as explained. External ring flanges (106) will be provided, accessible for bolting and maintenance with the use of telescoping boom lifts or scissor lifts. In other embodiments, a ring flange (106) of the lower inclined leg (109) near the lower junction piece (112) with the vertical leg (107) may nevertheless be provided, in order to adjust wall thickness to bending moments that are expected to be locally higher near the lower junction piece (112).

Depending on the overall length of the lower inclined leg (109) (Fig. 3), it may comprise a varying number of intermediate cans of lower inclined leg (134) (Fig. 16), featuring ring flanges (106) at their two ends to connect to neighbouring intermediate cans of lower inclined leg (134) via prestressed bolts. The number of intermediate cans of lower inclined leg (134) per each lower inclined leg (109) depends on transportability constraints, desirable variation of thickness along lower inclined leg (109) to adjust to developing mechanical actions, and cost considerations in balancing weight optimization and weld length minimization. In other embodiments, connection between adjacent intermediate cans of lower inclined leg (134) may be realized by other methods instead of ring flanges and prestressed bolts.

As an alternative to the unified foundation junction piece (113), in case its overall dimensions make its onshore transportation prohibitive, a segmental foundation junction piece (135) is provided (Fig. 17, Fig. 18). A cylindrical first segment (136) of the auxiliary tube (122) of this segmental foundation junction piece (135) is welded to the outer can of the upper inclined leg (120) and the outer can of the lower inclined leg (121). It has the same height as the auxiliary tube (122) of the unified foundation junction piece (113), but a smaller central angle, dictated by the need to accommodate the welds of the outer can of the upper inclined leg (120) and the outer can of the lower inclined leg (121). At its two vertical edges this piece has radial, vertical end plates (128), used to connect it via prestressed bolts to corresponding radial vertical end plates (128) of another cylindrical second segment (137) of the auxiliary tube (122). The first segment (136) and the second segment (137) complement each other to form a full cylinder. In other embodiments the connections between the first segment (136) and the second segment (137) may be achieved by other methods. The first segment (136) and the second segment (137) may feature small temporary auxiliary bars to maintain their dimensional stability during transportation and erection, so as to facilitate assembling them together.

Along their low edge, the first segment (136) and the second segment (137) feature welded partial ring flanges (129) similar to the lower ring flange (106) of the unified foundation junction piece (113). Through these partial ring flanges (129) anchor bolts are provided, either directly embedded into the foundation concrete, or using also an intermediate full 360° ring flange for better connection. The ring flanges and anchor bolts may be external, internal or both, as dictated by strength requirements. Alternatively, other types of anchorage of the auxiliary tube (122) consisting of first segment (136) and the second segment (137) into the foundation may be adopted.

Similarly, at their upper edge, the first segment (136) and the second segment (137) feature welded partial ring flanges (129). These two partial ring flanges (129) are bolted to a common ring stiffener (119), similar to the ring stiffener (119) of the unified foundation junction piece (113), allowing sufficient space for a manhole (116), allowing entrance and exit to personnel and equipment, in case internal bolts are used, either at the ring flanges (106) of the inclined legs, or for the anchoring into the foundation (103). Alternatively, in case no access into the auxiliary tube (122) is needed, a circular lid may be bolted to the two partial ring flanges (129), acting also as a stiffener (119).

Local stress concentrations in this segmental foundation junction piece (135) can also be addressed either by sufficiently high thickness of the first segment (136) and the second segment (137), or by arranging internal ring stiffeners (130) and/or internal vertical stiffeners (131), or by a combination of the two.

A methodology for the erection of the proposed wind turbine tower assembly is provided, consisting of the following steps:

Individual foundation sections (123) are constructed using conventional pile boring machines.

The foundation junction pieces (113) or the equivalent segmental pieces of Fig. 17 are anchored into the corresponding individual foundation sections (123).

The lower inclined legs (109) are assembled on the ground by bolting together their constituent parts.

A temporary support structure (138) is erected below the intended location of the vertical leg (107), comprising a temporary foundation (139) and a temporary scaffolding / truss system reaching from the ground to the bottom level of the vertical leg, equipped with jacks to control its vertical position (Fig. 19). A matt type of temporary foundation consisting of a grid of steel beams is provided, so that it is transportable and reusable.

The lower junction piece (112) between vertical leg (107) and lower inclined legs (109) is lifted from the ground by means of cranes and placed securely on the temporary support structure.

The lower inclined legs (109) are lifted from the ground by means of cranes and connected at their inner end to the lower junction piece (112) and at their outer end to the foundation junction pieces (113). Upon completion of this process for all lower legs and fixing of the bolts, the system becomes laterally stable. Vertical stability may not be secured yet due to the small inclination of the lower legs, thus the temporary support structure is retained in place. Alternatively to the two above steps, in case of segmental lower junction pieces, the erection and assembly of lower junction piece and lower legs takes place simultaneously.

Successive sections of the vertical leg (107), from bottom to top, are lifted from the ground by means of cranes and bolted together. This is continued until the upper junction piece (111) is erected. Up to that point the vertical leg acts in a cantilever manner. If necessary, its lateral stability is ensured by temporary supports or other means.

The upper inclined legs (108) are assembled on the ground by bolting together their constituent parts.

The upper inclined legs (108) are lifted from the ground by means of cranes and connected at their inner/top end to the upper junction piece (111) and at their outer/bottom end to the foundation junction pieces (113). Upon completion of this process for all upper legs and fixing of the bolts, the system becomes vertically and laterally stable.

Alternatively, in case of segmental upper junction pieces (111), the erection and assembly of upper junction piece (111) and upper inclined legs (108) takes place simultaneously.

The temporary support structure (138) below the vertical leg (107) and its temporary foundation (139) are removed by relaxing the jacks. They are moved to the next location and are used for the erection of the next wind turbine tower (100) assembly.

The lower part of the upper structure (101) is lifted from the ground by means of cranes and bolted at the top of the upper junction piece (111).

Successive sections of the upper structure (101), from bottom to top, are lifted from the ground by means of cranes and bolted together, as in conventional tubular wind turbine towers. The upper structure (101) acts in a cantilever manner, for which it has been properly designed.

The nacelle, rotor and blades are lifted from the ground by means of cranes and are put in place, as in conventional tubular wind turbine towers.

For increasing the efficiency and total power output of an already existing wind turbine tower (100), the second last step of the above method may be performed on sections of said already existing wind turbine tower (100) whose wind turbine will be changed. Alternatively, the last two steps may be performed on sections and wind turbine of said already existing wind turbine tower (100).

A methodology for dismantling the proposed wind turbine tower (100) assembly is also provided, consisting of the same steps as the erection methodology, executed in inverse order. All steel parts of the wind turbine tower (100) assembly are reusable and recyclable.

Acceptable performance of the proposed wind turbine tower (100) assembly in all pertinent operation and extreme cases is ensured by a comprehensive design methodology according to current standards and making use of state-of-the-art computational methods. Loads in extreme, fatigue and serviceability limit states are obtained by state-of-the-art aeroelastic codes.

Of particular importance for the appropriate performance of the proposed wind turbine tower (100) assembly is the design of the three proposed junction pieces, the upper junction piece (111), the lower junction piece (112) and the foundation junction piece (113), either in their unified or in their segmental version. Due to the complex shape of the junction pieces, their design is carried out adopting advanced numerical tools, such as nonlinear finite element analyses taking into account material nonlinearity, geometric nonlinearity and imperfections. Alternatively, other numerical methods of same level of reliability may be used. The results of any numerical analyses must be calibrated and verified by comparison to experimental results from tests of properly scaled specimens.