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229 Pages·2016·4.49 MB·English

CRANFIELD UNIVERSITY

ESTIVALIZ LOZANO MINGUEZ

FATIGUE & FRACTURE MECHANICS
OF OFFSHORE WIND TURBINE SUPPORT STRUCTURES

SCHOOL OF ENERGY, ENVIRONMENT AND AGRIFOOD
Energy Division

Thesis Submitted for the Degree of Doctor of Philosophy
Academic Year: 2015

Supervisor: Professor Feargal Brennan
Doctor Athanasios Kolios
July 2015

CRANFIELD UNIVERSITY

SCHOOL OF ENERGY, ENVIRONMENT AND AGRIFOOD
Energy Division

Thesis Submitted for the Degree of Doctor of Philosophy

Academic Year 2015

ESTIVALIZ LOZANO MINGUEZ

FATIGUE & FRACTURE MECHANICS
OF OFFSHORE WIND TURBINE SUPPORT STRUCTURES

Supervisor: Professor Feargal Brennan
Doctor Athanasios Kolios
July 2015

© Cranfield University 2015. All rights reserved. No part of this
publication may be reproduced without the written permission of the
copyright owner.

ABSTRACT
Wind power, especially offshore, is considered to be one of the most promising
sources of ‘clean’ energy towards meeting the EU targets for 2020 and 2050.
However, its popularity has always fluctuated with the price of fossil fuels since
nowadays wind electricity production cannot compete with nuclear or coal
electricity production. Support structures are thought to be one of the main drivers
for reducing costs in order to make the wind industry more economically efficient.
Foundations and towers should be fit for purpose, extending their effective
service life but avoiding costs of oversizing.
An exhaustive review of the background and state of the art of the Fatigue-Life
assessment approaches has been carried out, combining analysis of the
gathered experimental data and the development of Finite Element models based
on contemporary 3D solid models with diverse Regression Analyses, in order to
identify their weakness and evaluate their accuracy. This research shows that the
guides and practices currently employed in the design and during the operation
of the offshore wind turbine support structures are obsolete and not useful for
optimisation, which generally leads to conservationism and an unnecessary
increase in costs.
The basis for a comprehensive update of the Girth Weld and Tubular Joint S-N
curves and the Stress Concentration Factors of Tubular Joints has been set out.
Furthermore, a reliable methodology for deriving the Stress Intensity Factor at the
deepest point of a semi-elliptical surface saddle crack in a tubular welded T-joint
has been proposed.

Keywords:
Tubular Joint, Offshore Wind Support Structure, Stress Concentration Factor,
Stress Intensity Factor, S-N curve, Semi-elliptical crack, Tubular Joint Weld
Profile
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ACKNOWLEDGEMENTS
The research period has been more challenging than I or anyone could imagine.
I have learned much more than I expected and have needed more support than
I would have wished; therefore, it is not possible to individually thank all the
people who have helped me.
I grant the deepest gratitude to my main supervisor, Prof. F.P. Brennan, for his
invaluable supervision, exceptional guidance, and continuous encouragement. I
am also extremely grateful to my second supervisor, Dr. A. Kolios, for
encouraging me to do a PhD and for his excellent advice. Their faith in me has
given me the strength to complete this research.
I would like to express my appreciation to all current and past staff and my
colleagues in the Department of Offshore Process & Energy Engineering,
especially to Mr. E. Pérez-López and Mrs. S. Skears, for their help.
I would also like to express my sincere thanks to Prof. E. Giner-Maravilla for giving
me the opportunity to do a Research Stay at the Department of Mechanical and
Materials Engineering of the Universidad Politécnica de Valencia (UPV) and for
his numerous and masterful suggestions. Special thanks are also due to my
colleagues in the UPV who have made this stay a constructive and pleasant
experience.
Last but not least, I would like to acknowledge my family and my fiancé, Carlos,
because I would not have been able to carry on without their support, sacrifices,
patience, and love.

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Dedicated to my parents, Ernesto and María José

“The greatest glory in living lies not in never
falling, but in rising every time we fall.”
Ralph Waldo Emerson
American essayist, lecturer, and poet
(1803-1882)

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