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Delegates are invited to meet and discuss with the poster presenters during the poster presentation sessions between 10:30-11:30 and 16:00-17:00 on Thursday, 19 November 2015.

Lead Session Chair:
Stephan Barth, ForWind - Center for Wind Energy Research, Germany
Giovanni Nappi DNV GL, Denmark
Co-authors:
Giovanni Nappi (1) F
(1) DNV GL, Hellerup, Denmark

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Presenter's biography

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

Giovanni Nappi is Senior Engineer at DNV GL Renewables Certification since 2010, managing type and component certification projects. In 2007 he attained a M.Sc. cum laude in Mechanical Engineering from the University of Perugia, Italy, and also took up a position as Structural Design Engineer at Siemens Wind Power in Denmark, responsible for Finite Element Analyses of mechanical and structural components for wind turbine hubs and nacelles.
His technical competence areas include design and testing of mechanical and structural components and methodologies for fatigue assessments.


Poster

Poster Download poster (9.08 MB)

Abstract

A common framework for validation of "zero-maintenance"Ě bolted joints

Introduction

Ensuring that bolts maintain their design preload is a typical task which takes a large share of the O&M efforts for wind turbines. At the same time, loss of preload is behind the majority of bolted joints failures. Wind turbine OEMs strive therefore for developing designs and practices aimed at limiting retightening efforts in bolted joints and thereby reducing their OPEX. Validation of these concepts on a larger scale is however still problematic and untrustworthy, since no common framework for validation is currently available.


Approach

Typical approaches adopted by OEMs to reduce retightening efforts will be presented. Each approach will be described together with its respective pros and cons and possible validation plans will be outlined.
A trustworthy validation protocol allows extrapolating results from small scale experiences, typical of prototype or laboratory tests, to the larger scales typical of serially produced equipment destined to widely different installation conditions, with a large degree of confidence. The engineering basis for implementing and validating ‚Äúzero-maintenance‚ÄĚ bolted joints will be finally outlined.


Main body of abstract

Common requirements from wind turbine OEMs are that ‚Äúzero-maintenance‚ÄĚ design of bolted joints should be developed based on short and limited test campaigns.
Alternative approaches would consist in skipping the test campaigns and directly equipping selected bolted joints with Condition Monitoring Systems, in order to remotely control the preload in the bolt and implement predictive maintenance plans.
However wind turbine OEMs are still not quite keen on this solution, since the large number of bolts in a wind turbine would require larger and more complicated CMSs. Applying CMSs only on selected bolts within a single joint appears to be a technically viable compromise, although still complicated and not at all risk-free.

Several OEMs therefore have over the years collected significant amount of data on loosened bolts on their equipment. The first attempts in this sense were quite unstructured, discontinuous and not directly aiming at a statistical control of the bolted joint performance.
In few other cases more rigorous approaches have been put into practice to monitor the evolution of bolt preload over time. However the collected data turned out to be quite affected by the particular wind turbine model.
Extrapolation of results from such tests to other turbine models or even other wind farms might therefore bring several risks, since effects such as deformation of the flanges under loading, imperfections, surface setting as well as environmental actions might differ quite a lot depending on the turbine design and installation conditions.


Conclusion

The described shortcomings pose several challenges to ‚Äúzero-maintenance‚ÄĚ bolted joints with regards to validation and certification, which can hinder the market acceptance of these design solutions.
A set of methodologies and requirements to validate "zero-maintenance" bolted joints is therefore presented.
Implementation of such designs, along with their validation plans, leads to a consistent reduction of the OPEX related to bolted joints retightening without compromising their structural safety.



Learning objectives
Getting acquainted with the common framework for validation of such bolted joints will allow delegates to understand the different value proposals of the concepts available on the market, it being understood the fulfillment of basic requirements on the joint’s structural safety.