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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
Andreas Makris Center of Renewable Energy Sources & Saving, Greece
Co-authors:
Denja Lekou (2) F P Andreas Makris (2) Theodore Philippidis (1) Theodore Kossivas (3)
(1) University of Patras, Patras, Greece (2) Centre of Renewable Energy Sources, Pikermi, Athens, Greece (3) Compblades, Athens, Greece

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Presenter's biography

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

Dr Andreas Makris is a mechanical engineer with a Phd in mechanics of composite materials from Free University of Brussels. After his studies he worked for three years in an industry on extrusion and injection of polyethylene products. He is currently a researcher in the Centre of Renewable Energy and Resources in Athens, focusing mainly on mechanical testing and FEM simulations of wind turbine blades.


Poster

Poster Download poster (13.09 MB)

Abstract

Verification of repair method for a damaged wind turbine blade

Introduction

Blade failures occur either due to non-wind related events, e.g. lightning strikes, or due to structural damages instigated from structural defects, fatigue and/or coating failures. These damages encumber WT operation and influence power performance. DNV GL estimates that up to 3% of WTs in North America require blade replacements annually during the first 10 years of their life. The high cost of replacing a damaged blade, considering also downtime of the WT, reflects in power production costs with direct implications on investment return period. Therefore, repair of rotor blades constitutes a crucial issue for WT operation, with great environmental impact due to lack of recyclability and increased material use as blades increase in size (more intense for new longer blades >40m). Yet, at present, there are no guidelines available for repairing load-bearing blade components, or for assessing structural integrity of damaged or repaired ones. Aiming to be a stepping stone in this direction, the present work addresses all required phases involved in the repair of a blade, including design via numerical simulation and focusing on concept verification through development of a test methodology specifically for this purpose.

Approach

Depending on the defect/damage repair might involve actions for surface restoration (coating, aerodynamic accessories), mostly called "cosmetic repair" or repair of structural components of the blade (e.g. trailing panels, spar caps) targeting to strength and stiffness restoration (load carrying capacity) of the blade. As the latter directly affect strength and stability of the structure and the need for verification is higher this work deals only with structural repairs. The methodology developed for repair of a blade structural part including the method for the verification of the repair approach is discussed in the paper. Starting from issues involved in the design of the repair patch all steps are analyzed providing a draft guideline on the procedure that need to be followed.

Main body of abstract

The application of the developed methodology was performed for a real case scenario involving repair of a 20m blade requiring cutting of the spar cap during repair. Repair of fiber-reinforced composite laminates is a challenge to the repair service providers since once the reinforcing fibers are cut, as e.g. in the case of the spar cap, fibers cannot be "restored" to bear load. This is dealt using laminated patches that are mostly adhesively bonded to the damaged structural element. All stages followed for the patch design including evaluation of joint concepts for the composite laminates are discussed, while emphasis is given to the verification of the repair using a sub-part of the blade, thus avoiding an expensive full scale test for that purpose. Although IEC 61400-23 allows the testing of the blade including a repaired part of the blade, usually experiments are performed on a single full-scale blade. This fact increases the cost for the validation of a repair methodology and actually reduces the number of repair procedures that can be verified. The design of the proposed test is described along with experimental data from the application performed revealing the potential of our proposed verification method, which significantly reduces experimental costs.

Conclusion

Design of the structural repair coming from a mixed numerical experimental procedure is complemented by the developed test configuration, suitable for experimental verification of the whole repair procedure. Therefore, an evolutionary method is presented that leads to rules for the design and verification of the structural repair of a blade part in the interest of manufacturers, as well as WT operators and maintenance teams.


Learning objectives
The implications involved in the design and verification of a repair procedure on a structural part of the WT blade are revealed and solutions are proposed. The approach followed for a real case can be used as a starting base for other situations of structural repair on blades, including repair verification.