Composite Floating 'Point Absorbers' for Wave - Universiteit Gent

344 Pages·2009·12.22 MB·English

Composite Floating 'Point Absorbers' for Wave Energy Converters:
Survivability Design, Production Method and Large-Scale Testing
Drijvende composiet 'point absorbers' voor golfenergie-converters:
overlevingsontwerp, productiemethode en grootschalige experimenten
Chris Blommaert
Promotoren: prof. dr. ir. W. Van Paepegem, prof. dr. ir. J. Degrieck
Proefschrift ingediend tot het behalen van de graad van
Doctor in de Ingenieurswetenschappen
Vakgroep Toegepaste Materiaalwetenschappen
Voorzitter: prof. dr. ir. J. Degrieck
Faculteit Ingenieurswetenschappen
Academiejaar 2009 - 2010
ISBN 978-90-8578-302-2
NUR 978
Wettelijk depot: D/2009/10.500/60
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Copyright © C. Blommaert
Oostende, Juni 2009

Promotors:

Prof. dr. ir. Wim Van Paepegem
Ghent University
Faculty of Engineering
Department of Materials Science and Engineering

Prof. dr. ir. Joris Degrieck
Ghent University
Faculty of Engineering
Department of Materials Science and Engineering
Examination Committee:

Prof. R. Verhoeven (Chair) Ghent University, Belgium
Prof. W. Van Paepegem (Promotor) Ghent University, Belgium
Prof. J. Degrieck (Promotor) Ghent University, Belgium
Prof. J. De Rouck Ghent University, Belgium
Prof. M. Vantorre Ghent University, Belgium
Prof. J. Vierendeels Ghent University, Belgium
Prof. S. Vanlanduit Free University of Brussels,
Belgium
Dr. K. C. Strømsem Noble Denton Consultants
Ltd., United Kingdom

Research Institute:

Ghent University
Department of Materials Science and Engineering
Sint-Pietersnieuwstraat 41
B-9000 Gent
Belgium
Tel. [phone]
Fax. [phone]
[email protected]

Acknowledgements

ACKNOWLEDGEMENTS
There are a number of people who have played an important role throughout my
research. Without them this work would not have been the same.

The first persons whom I would like to gratefully acknowledge are Prof. Dr. Ir. Joris
Degrieck, who gave me the opportunity to do a PhD on a very interesting subject,
and Prof. Dr. Ir. Wim Van Paepegem, for his continuous support and valuable
suggestions throughout the years.

A special thank you should go to Luc van den Broecke for his help with the
preparation of all laboratory experiments, with the guidance of six thesis students
and for the valuable input for the experiments at the ‘Watersportbaan’. Also, special
thanks should go to Dr. Ir. Ives De Baere and Dr. Ir. Joost Van Slycken for their
help and presence during these outdoor large scale experiments. Others that I should
remember for their input for these experiments are Tom Versluys, Dr. Ir. Hans
DeBacker and Sam Meurez.

Also, for the discussions about our work and several other things I would like to
thank Ir. Griet De Backer as for the nice week at the ‘Watersportbaan’. I believe she
is a great person to work with.

Pascal Dhondt from Spiromatic was a great colleague and the several times we
discussed about Seewec or life in general I will not easily forget. Also, his work for
the production of the buoys was an important input which would not have been
possible without him.

Next, I would like to thank Prof. Dr. Ir. Julien De Rouck to fulfil a remarkable job as
a coordinator of the European Seewec project.

Additionally, Fred Olsen Ltd. deserves all credit for the general concept of the wave
energy converter and therefore I would like to thank them for including Ghent
University in the European project.

Then, I should not forget to mention my former colleagues for the interesting and
fun conversations about almost everything where I immediately have to think at Ir.
Ebrahim Lamkanfi with his views on the world.

Also, each Thursday, during coffee break at 10 o’clock, Ir. Rudy Verhelst surprised
the rest of us with very good baking skills. Next to this, I would like to thank him for
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Acknowledgements

several new mp3’s; I think he is the only person that I have ever met who has a more
general knowledge about music and is more addicted to it than me.

Also, a special thanks to Martine Botte for being there whenever a talk was needed.

In fact, I would like to thank all my former colleagues for the 4 years at Ghent
University. The way the whole group stuck together was very important for me.
Because of them I do not consider those years as mere ‘work’.

Of course I would also like to thank my partner for being there during difficult
times.

Last but not least I would like to thank my parents for the support they have given
me during this Seewec project and especially to encourage me to transform this
work into a doctoral thesis.

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English summary

ENGLISH SUMMARY
This dissertation is situated within the ‘Seewec’ project funded by the sixth
framework program (FP6) of the European Union (EU). Seewec stands for
Sustainable Economically Efficient Wave Energy Converter which means that it
concerns a device for converting energy out of the sea waves. The Seewec structure
is an offshore structure with floating ‘point absorbers’ also called ‘buoys’. These
buoys move up and down and they are used to capture the energy from the waves.

The main objectives of this dissertation are the following:

• Establish a finite element model for a specific buoy/point absorber in
composite material consisting of several layers that withstands slamming.

• Choosing an effective production method for the buoys.

• Obtain a better understanding of ‘slamming’ on composite materials by
performing a literature survey, numerical methods and experiments.

• Comparison between experiments and analysis/literature.

To achieve this, research was necessary in different fields. Until present no research
has been carried out to investigate the use of composite materials for point
absorbers. In this dissertation Chapters 3, 4, 5, 7 and 9 contain the most relevant
issues while Chapters 2, 6 and 8 contain background information. Keeping this in
mind a short overview of the different Chapters is given and the main results are
mentioned.

In Chapter 2 background information was given about the global energy problem,
the use of wave energy converters and about the Seewec project. Today, the global
energy problem receives worldwide attention. The Kyoto agreement states that by
2012 an important decrease of greenhouse gasses (GHG), like CO , has to be
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accomplished. Also, further negotiations about global strategy are currently ongoing
for the ‘post-Kyoto’ era. Since the supply of fossil fuels is finite, by 2050, other
energy sources will have to be used. The reserves of oil are estimated to last for 40
years, the gas reserves for 60 years and coals for a bit more than 200 years. In any
case, the prices will rise simultaneously with decreasing resources. This makes the
European Union (EU) economically vulnerable and the development of renewable
energy quite urgent. Hence, the EU also ratified the agreement of Kyoto and decided
to increase its share in renewables to 12% by 2012. E.g. solar, biomass, wind and
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English summary

ocean energy can all give a contribution. There should not be any conflict between
those. Policy makers should search for the right application for the right purpose or
location.

Today, wave energy is a small player on the energy market. However, it has a lot of
potential. The European Atlantic coast wave climate is characterised by large
energy. Waves can travel for thousands of kilometres with almost no loss of energy.
Within the sixth framework program of the EU, the Seewec project has been
launched at the end of 2005. The general concept originates from the company Fred
Olsen Ltd. in 2001. This concept of the WEC looks like a traditional drill platform.
However, there are two factors that differentiate it.

The first one is the presence of point aborbers in the middle of the platform. Point
absorbers are defined as oscillating energy converters with horizontal dimensions
smaller than the wavelength. They are there to absorb energy out of the waves. At
start, the design of the point absorbers looked like buoys of about 4-6 meter height
and 3-5 meter diameter. Second, where traditional drill platforms are steel
constructions, this platform will be made out of light materials, such as composites
and light metals. Composite materials do not corrode like metals. Also, they are a lot
lighter than steel. In this way, maintenance or replacements at sea are easier to carry
out. The main task of our department within this Seewec project was the
survivability design of the point absorbers, their production method and large-scale
testing.

For the survivability design a very critical load is ‘slamming’. It is a phenomenon,
known in the Marine Engineering, as the short impact of water waves on a floating
or sailing structure which gives very high local peak pressures. A relevant literature
study concerning ‘slamming’ was done and can be found in Chapter 3. The upward
propelling of a structure and the fall back on the water surface, is called straight,
vertical or bottom slamming. Also, breaking waves can introduce slamming. This is
called lateral or breaking wave slamming. This wave impact can cause damage on
the bow of a ship above the waterline. Additionally, breaking wave slamming can
occur against a component of an offshore construction. These components have to
be designed for impact loads coming from breaking waves. This is quite similar for
the buoys within the Seewec project. Also, these buoys are suffering from bottom
slamming. In normal to high waves and small penetration depth of a buoy, it is
possible that the buoy goes out of the water. The slamming phenomenon was studied
in the Marine Engineering on rigid bodies but for deformable structures almost no
literature is available.

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