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Conference programme 

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Poster session

Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Anita Buxton TWI Ltd, United Kingdom
Co-authors:
Anita Buxton (1) F Chris Punshon (1) P Ian Sewell (2) Giorge Koulin (2) Matt Liddle (3) Richard Eakin (4) Chris Leach (4) Miguel Delgado (4) Richard Cooper (6) Paul Towse (6)
(1) TWI Ltd, Great Abington, Cambridge, United Kingdom (2) Newcastle University, Newcastle, United Kingdom (3) BSP International Foundations Ltd, Ipswich, United Kingdom (4) Scottish Power Renewables, Glasgow, United Kingdom (5) Tata Steel, Scunthorpe, United Kingdom

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Poster
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Presenter's biography

Biographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited

Dr Anita Buxton (BEng, PhD, CEng, MIMMM) joined TWI in 2000 with an honours degree in Materials Technology from the University of Surrey and a PhD from the University of Sydney. As a Principal Project Leader in the Electron Beam Section, Anita leads projects for individual clients and consortia across a broad range of industry sectors and is experienced in understanding client requirements. She has successfully managed a number of large TSB and EU collaborative projects concerning the adoption of emerging electron beam technologies, including projects for the offshore wind industry.

Abstract

A multi-facetted design concept for offshore foundations

Introduction

It is anticipated that the market for offshore turbines will significantly outstrip the current manufacturing capacity. This work examines an innovative fabrication process for foundations which has the potential to allow the necessary production capacity to be met. The novel multi-facetted design proposed and investigated in this work is fabricated from long strips of flat plate welded together longitudinally. This allows sub-assemblies of greater than 10m in length to be manufactured, reducing the number of circumferential welds necessary in the foundation. The approach is enabled through the use of rapid, thick-section welding for the longitudinal seams.

Approach

This work examines the geotechnical and structural aspects of the multi-facetted design concept. The entire production route is examined in terms of performance, logistics and economics, in order to determine the viability of the approach. A 1:5 scale model has been fabricated and piled to demonstrate potential and assess advantages over conventional tubular monopoles in terms of:
• Geotechnical requirements
• Structural performance
• Fabrication route
• Production capacity and associated costs
• Design features.

This work was achieved through collaboration between TWI Ltd, Newcastle University, Gardline, BSP International Foundations Ltd, Scottish Power Renewables, Tata Steel and OGN, with support from the Regional Growth Fund and Narec.


Main body of abstract

Modelling of the foundation design has been carried out, using a typical circular foundation as a reference case. The structural performance, geotechnical requirements, installation implications and certification have all been considered resulting in a design specification for the multi-facetted foundation.
Electron beam (EB) welding is a fast process, producing single pass, high integrity welds in thick section material and hence is an ideal joining process for turbine foundations. Conventionally EB welding is carried out within a vacuum chamber, which restricts the size of component that can feasibly be welded using this process. Local vacuum EB welding with a mobile sliding seal (to provide a local vacuum atmosphere for the beam) has been developed and successfully demonstrated on corner welds in C-Mn steel of the thickness to be used for a full-scale foundation.
A 1:5 scale model was fabricated to demonstrate the potential of the fabrication route and to assess the behaviour of the multi-facetted design under piling. The scale model was instrumented and performance data was collected as the model foundation was piled into the ground.
Benefits and limitations of the multi-facetted approach have been identified and consideration has been given to the applicability of the concept for fabricating emerging foundation designs and more widely within the offshore wind industry.
Attention has been given to the industrial feasibility of fabricating the multi-facetted foundation system. Proposed production facilities have been modelled and the overall viability of the process has been determined.


Conclusion

A multi-facetted design for the fabrication of structural steel is showing promise for the manufacture of offshore wind turbine foundations. A 10-sided design has been developed and detailed calculations have shown satisfactory performance in the Ultimate Limit State and Serviceability Limit State and a suitable fatigue performance. The feasibility of the fabrication route has been demonstrated by making a 1:5 scale model and successfully piling it into the ground. A route for the fabrication of full-scale piles has been outlined; this involves local vacuum EB welding which has been demonstrated to produce welds of the required thickness and geometry.


Learning objectives
A novel multi-facetted pile concept is showing promise for a range of applications in the next generation of offshore structures. The fabrication route addresses the shortfall in capacity for the anticipated future market of offshore wind turbines and offers a number of product advantages including a reduction in the number of circumferential welds, allowing weld placement away from points of maximum stress.