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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Whole-life foundation and structure integrity' taking place on Wednesday, 12 March 2014 at 14:15-15:45. The meet-the-authors will take place in the poster area.

Alvaro Hernando MS-ENERTECH, Spain
Co-authors:
Jesus Minguez (1) F P Enrique Martinez (1) Alvaro Hernando (1)
(1) MS-Enertech SL, Burgos, Spain

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Alvaro is a Master in Civil Engineering who has been working for wind power more than 8 years. He is currently the responsable of the Project Management Office and Geotechnical Manager of MS Enertech. Prior to this position, he worked as concrete tower process engineer in Gamesa and as Geotechnical and Structural engineer in Eptisa. He is one of the inventors and developers of the presented post-tensioned cable interface. He has also researched and developed innovative wind turbine foundation concepts and carried out several R&D programs in regards with concrete behaviour under fatigue loads.

Abstract

Innovative interface solution using post-tensioned cables to connect the tower and the foundation in wind turbines

Introduction

One of the most important issues in the design and construction of wind tower turbines is the connection interface between the tower and the foundation.
This interface is the structure part where the higher concentration of efforts occurs, because it is the force transmission path from the tower to the foundation. Thus its design needs special care. Also in the construction stage the assembly of existing solutions requires several operations with minimum leveling tolerances allowed.


Approach

Existing connection solutions between wind turbine tower and foundation are the steel tube interface system and the bolt cage system. The main systems characteristics are described below.
Steel Tube interface: this system is composed of two steel annular flanges (lower and upper) and a rolled steel plate welded between them.
These three pieces form a rigid solid heavily able to transmit forces from the base of the tower to the foundation in the underlying supporting soil. Its drawbacks are presented below:
­ Solid rigid: opaque. Total impermeability of the connecting structure: due to the existence of a continuous piece of steel sheet, a barrier is created on the foundation avoiding the concreting continuity
­ Critical joint concrete-steel
­ Need to be leveled before pouring
­ Heavy, difficult to handle because is a continuous solution in steel. Also because of the large thickness required in the flanges to avoid stress concentrations at the junction with the vertical plate. The overall weight (and thus the amount of steel used in the manufacture of parts) is very important.
­ Transport limitations limitations for wind turbines with increasingly powers, the needed diameter makes unviable road transport of a hole body, whose maximum size allowed is 4.5 m.
­ Expensive

Figure 1 Steel Tube interface
Bolt cage interface: it consists of two symmetric steel flanges (lower and upper) bolted between them with a network of bolts (steel bars with high yield limit). Its drawbacks are:
­ Need of tooling to preassembly
­ Need to be leveled before concreting
­ Heavy, difficult to handle
­ Flanges to form the cage
­ Grout conditionings


Figure 2 Bolt cage interface

Main body of abstract

Post-tensioned cable interface description
With the aim of reducing the main drawbacks associated with both methods which have been described in the previous section which are the currently used connection between turbine tower and foundation. The main drawbacks can be summarized as:
• high amount of material used
• high weight and need for cranes
• times and leveling assembly and considerable
An innovative concept based on an interface integrated with the rest of the structure has been developed.
This innovative system of connection between tower and foundation is designed as an indivisible part of the wind turbine support structure as in the design phase and in the construction phase, giving the tower and the foundation system greater continuity strength in the stresses flow to the underlying terrain.

The innovative concept of tower-foundation interface presented is a post-tensioned cables system. This solution, well known in civil engineering, consists of several vertical ducts where the cables (with higher yield limit than the bolts) are allocated. This system also includes commercial bearing plates and anchor heads.

Figure 3 Pos-tensioned cables interface

Subsequently the concreting of the foundation the cables are tensioned to connect the tower with the foundation.
Tolerance for tilt sheaths increases to approximately 7 mm (75% increase compared to existing solutions), being just necessary for the location within the foundation plastic templates or similar (economical, lightweight, modular and manually mountable) that would be subsequently embedded in the concrete.
The foundation may have associated or not, depending on the needs of each case, a central part withoit concrete which allows tensioning of the interface assembly from the botton of the tower. This can generate associated advantages: in the case of concrete towers, the frictional losses in the lower base of the tower where the stresses can be eliminated. in the case of steel towers the available space for placement the tensioning tool is greater
The most important features of the materials used are:
• Reduction of approximately 40% of the steel cable area required
• Outer diameter steel ducts: 70 mm approximately
• Yield Limit of the steel cables: 1860 MPa
• Tensile Limit of steel cable: 1670 MPa
• The cables will consist of approximately 5 to 7 strands of 0.6'' and will depend on the tensioning tool type and loads, always leaving a gap on either side to the inner diameter of the duct of approximately 10 mm.
• The number of ducts per interface may vary depending on the type of machine loads, approximately, between 65 and 85.


The benefits generated by the proposed solution are that top and bottom flanges are not necessary. Therefore there is a reduction of material and time in assembling. Other benefit is that the assembly can be performed manually without cranes, also leveling operations are reduced due to the clearance between the duct and the cable, which allows the cable to acquire perfect verticality when is tensioned. Transport operations are easier. The combination of all these features generates a significant reduction in the cost of the foundation.
In this paper a complete structural analysis is described to demonstrate that this system fulfills the wind power requirements for foundations. The design is performed using a solid FE model.

Figure 4 Fe model

Conclusion

In this paper an innovative tower to foundation interface solution is proposed. A complete analysis of the system is performed and it is compared with the existing systems nowadays. Existing connection solutions between wind turbine tower and foundation are the steel tube interface system and the bolt cage system.
The most important features of the materials used are:
• Reduction of approximately 40% of the steel cable area required
• Outer diameter steel ducts: 70 mm approximately
• Yield Limit of the steel cables: 1860 MPa
• Tensile Limit of steel cable: 1670 MPa
• The cables will consist of approximately 5 to 7 strands of 0.6'' and will depend on the tensioning tool type and loads, always leaving a gap on either side to the inner diameter of the duct of approximately 10 mm.
• The number of ducts per interface may vary depending on the type of machine loads, approximately, between 65 and 85.
• Tolerance for tilt sheaths increases to approximately 7 mm
The benefits generated by the proposed solution are that top and bottom flanges are not necessary. Therefore there is a reduction of material and time in assembling. Other benefit is that the assembly can be performed manually without cranes, also leveling operations are reduced due to the clearance between the duct and the cable, which allows the cable to acquire perfect verticality when is tensioned. Transport operations are easier. The combination of all these features generates a significant reduction in the cost of the foundation.
This innovative interface system shows more benefits than the current interface systems. Therefore it could be interesting to be applied to the future wind turbine tower interface.


Learning objectives
This innovative interface system shows more benefits than the current interface systems. Therefore it could be interesting to be applied to the future wind turbine tower interface.


References
[1] Chris Horgan “Using energy payback time to optimise onshore and offshore wind turbine foundations.“ Renewable Energy 53 (2013)
[2] "EN 61400-1 2005: Wind turbine generator systems - Part 1: Safety requirements.", European Standard, (2005)
[3] "Eurocode 2: ENV1992-1-1 Design of concrete structures. Part 1-1: General Rules and Rules for Buildings", European Committee for Standardization, (2004)
[4] “Guidelines for Design of Wind Turbines” Det Norske Veritas (DNV) (2002)
[5] "Model Code 2010 ", Comité Euro-International du Béton CEB, (2010)
[6] Mohammad AlHamaydeh, Saif Hussain “Optimized frequency-based foundation design for wind turbine towers utilizing soil–structure interaction” Journal of the Franklin Institute 348 (2011)
[7] "Rules and Guidelines: IV-1 Guideline for the Certification of Wind Turbines", Germanischer Lloyd (GL), (2010)