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Offshore Wind Turbine Towers – A Quicker, Cheaper Flange Supply Route

Final Report Summary - RINGMAN (Offshore Wind Turbine Towers – A Quicker, Cheaper Flange Supply Route)

Executive Summary:
Large diameter (>5m) wind tower flanges are typically forged products, often supplied from the Far East. These components have a high cost and are associated with long lead times. The increased market demand is expected to drive prices higher and lengthen lead times. The RingMan project has developed an alternative welded fabrication route for flanges (using lower-cost rolled rectangular sections) which will enable European SMEs to compete favourably with large companies.

The process of qualifying the new flange fabrication route for offshore wind structures has begun and initial feedback is encouraging. Steps have been identified to address issues necessary for the process to withstand further scrutiny with a view to the issue of a certificate of compliance.

The commercial viability of the proposed route has been evaluated against the current method of flange production. A cost saving of >17% has been predicted with no deficit in in-service performance. The prospects for cost savings in the manufacture of similar large scale fabrications is now a topic now of interest to the consortium as this project draws to a close.

Project Context and Objectives:
Introduction

There is a tendency to higher power offshore wind turbines, leading to larger turbine towers (5m diameter and greater) and a requirement for large diameter connection flanges between the tower sections and between the tower and foundation transition pieces. Currently, forged rings are specified, with inherently high cost and few suppliers worldwide. This leads to long lead times and higher costs for these strategic parts. The RingMan project has developed an alternative manufacturing route (using lower-cost rolled rectangular sections) which will enable the fabrication of large diameter wind tower flanges to be undertaken by our SME consortium members.

The project worked towards enabling the partners to commission:

• Development of high quality, low distortion, thick section electron beam welding fabrication and machining to produce flanges from readily available cold rolled steel plate material.
• Understanding of the flange property requirements and how these can be met by a fabricated flange leading to design tools to enable wind turbine tower designers to specify these products.
• Procedures for inspection that will aim to ensure that the parts manufactured meet or exceed the required performance properties.

Objectives

The RingMan project objectives were to:

• Produce a predictive model that quantifies the stresses and fatigue performance for a fabricated wind tower flange (containing experimental results to validate the model) and optimises the flange design.
• Produce a design of a high power EB gun capable of welding at least 200mm thick steel at a rate of >75mm/min.
• Verify that finite element electrostatic/magnetic modelling of electron guns at high space charge density can accurately predict beam profiles at beam powers of up to 100kW and focus spot sizes of less than 1mm diameter through designing and building a modelled gun.
• Build and commission a prototype high power (up to 100kW) EB gun and welding system.
• Manufacture and test a demonstrator flange component, with comparison to the modelled properties.
• Quantify the economic and technical issues of fabricated flanges vs. forged flanges.
• Disseminate the outputs of the project through SME training, a project web site, liaison with tower manufacturers and technical transfer by training of SMEs.

Work Performed

Determination of Flange Property Requirements

Flanges in wind turbines have traditionally been forged and consequently the design standards have provided guidelines for forged components. It is necessary to understand the loads on a flange in order to determine the design criteria for a fabricated flange (metal grades, weld properties, dimensions, allowable defect size etc).

Ring flange property requirements fabricated by EB welding have been analysed to determine the influence of out of plane tolerances, and the critical size and shape of a flaw either embedded or at a surface. A comprehensive two step procedure has been established and illustrated on a case study turbine flange design.

In the first step, a Finite Element model has been developed to accurately calculate the stress distribution in the ring flange and in the tower segment close to it. The model is beyond the state of the art because it considers the complete ring flange connection including all bolts and 500 mm of the adjacent segment of the tower. The model accurately predicts the effect of geometry imperfections on the behaviour of the connection, for example out of flatness. The ultimate load causing the fracture of the bolts and local buckling of the shell is predicted.

In the second step, an Engineering Critical Assessment concept is used. Fracture and fatigue assessment using Failure Assessment Diagram (FAD) and Paris law, respectively, have been performed to predict acceptance criteria for a flaw size. The calculations are based on the output stresses obtained in the first step. The lowest temperature of a construction site of -30C is assumed in the case study. Fracture assessment and fatigue crack growth has been considered for a range of flaw aspect ratios, with flaws located internally or externally in different locations of the flange.

This two stage approach has resulted in:

• The development of a design and acceptance criteria for a fabricated flange.
• Determination of the maximum acceptable flaw size within the fabricated flange.


High Power EB Gun Design

The RingMan project is aimed at lowering the cost for producing flanges intended for use for wind turbine towers and foundations. The project objective is to deploy high power reduced pressure electron beam (RPEB) welding as the joining method to join the ends of a rolled rectangular section ring to produce a continuous ring with base metal properties in the weld region.

In order to achieve this, the project in-part develops electron beam welding equipment in order to allow thick section flange welds to be made quickly and to a high integrity.

The consortium agreed an outline requirements specification for the electron beam welding machine as follows:

• 45kW to 60kW beam power.
• 200mm to 250mm penetration capability.
• Demonstrate 160mm deep welding within the project.
• 60kV preferred.
• Diode gun recommended.

The present TWI 150kV diode gun performance was examined and used as a benchmark to assess speculative 60kV gun designs. Using finite element analysis to simulate the diode guns, the 60kV design was optimised to generate comparable beam qualities at the required power. The functionality and practicality of the gun design was checked over its full working power range. Operating the gun at 60kV reduces the requirements for x-ray shielding making it simpler to deploy the process. It also reduces the risk of electric discharges across the electrode which can cause weld defects to be formed.

The electron gun has been designed, built and tested.

Prototype EB Welder

Wind turbine tower flanges are produced in a range of sizes and can reach 6-7m in diameter and over 200mm in thickness. This size and geometry precludes the use of a vacuum chamber on economic grounds and thus a local vacuum approach for generating an appropriate atmosphere for thick section welding has been proposed.

Following a series of discussions both internally at TWI and with the informed clients in the collaborative group, a number of outline machine concepts were proposed. After due consideration, an outline concept has been developed to provide a production friendly deployment route.

The prototype system places the weld joint line in the 2G or PC welding position (preferred for thick section EB welding). Using a box type vacuum chamber with an engineered sliding vacuum seal provides a high integrity vacuum engineering solution, as the sliding elements are flat and polished surfaces, remote from the weld region, the need to maintain a seal on a hot weld and prominent weld bead are avoided.

Following successful design, manufacture and test of the vacuum system it was approved for welding trials at higher powers. Welding capability has been demonstrated at 50mA beam current. As part of the commissioning tests it was also shown that a weldment (ring sub-element demonstrator section) could be inserted into the vacuum assembly and the system closed and made ready for welding in less than 30 minutes.

Further trials with the high power EB gun and local vacuum systems are ongoing.


Manufacturing Validation

The ultimate goal of the Ringman project is to demonstrate the equivalence and fitness for purpose of a circular flange produced by a welding fabrication route rather than ring forging, as is current practice. Electron beam welding was selected as the welding process to join the ends of a cold formed rectangular bar rolled into a ring section. This is on the grounds that it requires minimal preparation, a square close fitting butt joint is preferred, and can rapidly complete a butt joint of up to 200mm in thickness in a single pass. In addition, and more critically, it is an autogenous welding process and therefore requires no filler metal and produces weld chemistry near to matching the base materials composition. As a consequence, a normalising heat treatment applied after welding would be expected to restore the weld metal microstructure to that of the parent materials and yield near equivalent mechanical properties.

Test material was obtained in the form of a commercial grade hot rolled C-Mn steel plate of 135mm thickness supplied to EN ISO 100025 grade 275NL. A number of rectangular sections were cut from the plate and machined to produce butt weld trial pieces representative of a typical flange cross section. EB welding procedures were developed which were appropriate for producing nominally parallel sided welds of 130-160mm thickness in a single pass at a welding speed of 100mm/min. The weld profile and internal quality were initially assessed by using ultrasonic testing and sectioning.

Having established a suitable welding procedure capable of producing welds of appropriate profile and quality (as assessed by destructive examination) a normalising heat treatment procedure was developed with the objective of producing a fine grained pearlite/ferrite microstructure nominally indistinguishable from the base material.

Based on the critical flaw sizes predicted through engineering critical assessment (ECA) a series of weld inspection procedures were examined to assess the effectiveness of detecting and sizing critical flaws in different regions of the flange section. It was established that a combination of surface flaw detection and ultrasonic volumetric inspection methods would be sufficient to achieve full coverage of the proposed EB welded joints.

To summarise, the weld process was demonstrated at 130mm thickness to be capable of producing sound welds of good profile reliably. Following a normalising heat treatment it was observed that the welds were similar in terms of microstructure to the parent material and that the sensitivity of the inspection methods examined were improved by this operation such that significant weld flaws were reliably detected.


Component Validation

Mechnical Testing
Samples of the welded test pieces were extracted and the following mechanical tests were performed:

• Hardness HV10.
• Weld metal Charpy testing – transition curve determination.
• Fracture toughness testing – R-curve determination at -10 degrees Celsius and -50 degrees Celsius.
• Cross weld tensile testing.

The results of the tests were considered to be acceptable in view of the proposed application and the critical property levels defined in requirements report. In particular the level of toughness was shown to be within the range of acceptance for both impact toughness and CTOD at the service temperature.

The need to generate fatigue property data was negated on the basis that the weld beads would be machined flush and the structure would be fully normalised (therefore stress relieved) and on the grounds that the threshold initiation and crack growth data available in the literature could be used conservatively in the predictions of critical flaws for fatigue crack initiation and growth used in the ECA. Data was collated and presented by TWI illustrating that flush ground and heat treated EB welds without surface flaws could be considered to approach the behaviour of base metal in terms of fatigue performance.

The conclusions however for the material tested was that the minimum basic mechanical properties defined by the structural analysis modelling were met for EB weld metal subjected to a post weld normalising heat treatment and provided a strong position to begin discussions with the appropriate certifying body.

Fabrication of a demonstrator
To demonstrate the fabrication route, a C-Mn steel demonstrator flange sub-element has been welded using the RPEB process and local vacuum system developed. The section is 130mm in thickness and representative of a typical flange element identified during the course of the project in discussion with the end users. The demonstrator will be used as an exhibit for promotion of the RingMan approach, for cost effective flange manufacture, to industry. It is planned to fabricate a full flange ring demonstrator (of ~3 metres diameter and >200mm thickness) during 2014.

Assessment of the welded flange demonstrator
The demonstrator was fully non-destructively inspected to determine the suitability of the welds against the requirements determined in the acceptance criteria report. Assessment of the EB welded, PWHT and machined flange element illustrated that the weld quality defined by the structural analysis could be met. Through the use of PWHT the weld line became only just visible both ultrasonically and to the naked eye on final machining.



Exploitation
The process of qualifying the new flange fabrication route for suitability in offshore wind turbines has begun and initial feedback is encouraging. Steps have been identified to address issues necessary for the process to withstand further scrutiny with a view to the issue of a certificate of compliance.

A model of the new flange fabrication route has been generated, outlining the necessary production steps and the associated time and costs of each step. This has allowed the commercial viability of the proposed route to be evaluated against the current method of flange production. This functional production costing tool, taking into account consortium and third party cost and time factors, across Europe, has shown the viability of the proposed RingMan approach and the attractiveness of the forecast >17% production cost which may be attained through up-take of the EB welding technology, with no deficit in in-service performance. The prospects for cost savings in the manufacture of similar large scale fabrications is a topic now of interest to the consortium as this project draws to a close and further dissemination is planned.

Project Results:
The main results achieved are:

• A model has been developed to define the design criteria and mechanical properties of the fabricated flange.
• A model has been developed to identify the critical areas of a flange and the maximum defect size that can be tolerated in each region of the welded flange. This was necessary to set the threshold for weld quality and inspection resolution.
• A low voltage, high power EB welding gun has been designed (through a combination of analytical design, FEA modelling and optimisation), with the potential capability to weld steel up to 200mm in thickness. The EB gun has been built and performance testing is underway.
• A prototype welding system has been designed, built, commissioned and demonstrated, providing a production friendly deployment route for the manufacture of flanges.
• A demonstrator sub-element flange has been fabricated.
• The welded demonstrator flange section has been analysed and tested and shown to be of the required quality and meet the defined mechanical property requirements.
• NDT methods suitable for the detection of flaws, of the dimensions and locations that have been identified as being critical to the performance of a flange, have been defined.
• The fabrication route and resulting product have been viewed favourably by a certification body and work is underway to achieve full compliance to the regulatory constraints.
• The cost and time implications of the new fabrication route have been compared to current manufacturing processes. A cost saving of greater than 17% per flange is predicted over the traditional forged flange route.

Potential Impact:
The final results are:

• A predictive model.
• A high power EB welding gun design.
• A fabrication route.
• Verification test data.
• A techno-economic study.
• A demonstrator sub-element flange.

Large diameter (>5m) wind tower flanges are traditionally forged products, supplied from the Far East by large companies. These components have a high cost and are associated with long lead times. The increased market demand is expected to drive prices higher and lengthen lead times. The fabricated flange route developed within the RingMan project will provide a lower cost production method which will enable European SMEs to compete favourably with large companies.

List of Websites:
The project website (http://www.RingManProject.com) was established on 15/3/2012. The website consists of a public area to promote the project and a member’s only area (accessed via a user password) for the storage of project documentation.