feasiblity of offshore wind substructures in arctic environments

311 Pages·2015·15.86 MB·English

by  Steven Maier

FEASIBLITY OF OFFSHORE WIND SUBSTRUCTURES IN
ARCTIC ENVIRONMENTS

Steven Maier
August 2015

MSc. Thesis
Delft University of Technology
Faculty of Civil Engineering and Geosciences

Graduation Committee:

Prof. Ir. F.S.K. Bijlaard TU Delft - Faculty of Civil Engineering and Geosciences

Ir. R. Abspoel TU Delft - Faculty of Civil Engineering and Geosciences

Dr. Ir. M.B. Zaaijer TU Delft - Faculty of Aerospace Engineering

Ir. L.J.M. Houben TU Delft - Faculty of Civil Engineering and Geosciences
Author: Steven Maier

Email: [email protected]
Tel: [phone]

Cover photograph by Suomen Hyötotuuli OY.
Page | 2
MSc. Thesis
Delft University of Technology
Faculty of Civil Engineering and Geosciences

FEASIBLITY OF OFFSHORE WIND SUBSTRUCTURES IN
ARCTIC ENVIRONMENTS

Steven Maier
August 2015

Delft University of Technology
Faculty of Civil Engineering and Geosciences
Stevinweg 1
2628 CN Delft
The Netherlands
[phone]
Page | 3
Page | 4
ACKNOWLEDGEMENTS
This thesis report marks the end of my studies at the faculty of Civil Engineering at the
Technical University in Delft. With this report, I graduated for a degree as Master of Science,
within the department of Structural Engineering. The thesis report is written for anyone who
has an interest for the recent developments in offshore wind engineering and arctic
engineering. This report shows how those two fields of interest are connected, and provides
a glimpse of future developments that are on the verge of becoming reality.

This report is the end result of a long, educative and at times frustrating process, where I
came to learn important things about myself and the work I was doing. The motivation for
finishing this thesis work was not only found within myself, and I would like to thank all the
people that supported me along the way and that have in any way contributed to this work.

In particular I would like to thank my parents and my girlfriend for their unending support and
love in all things and for providing me with the motivation needed for me to complete this
thesis report. They have always allowed me to follow my own path in life, even though it must
have frustrated them to see me postpone the completion of my degree for so long.

I would like to extend my gratitude Prof. Ir. F.S.K. Bijlaard for his guidance and patience with
me, and for his understanding of my situation. That same gratitude is extended to the other
members of my graduation committee, Ir. R. Abspoel and Dr. Ir. M.B. Zaaijer.

Finally I would like to thank my employers at Seaway Heavy Lifting, and in particular my
manager J. Lodewijks, for allowing me to take the time I needed, for their understanding and
their support in order for me to complete this thesis.

For my grandfather, who would have been very proud to see me graduate

… PROMISES ARE KEPT WHERE PROMISES WERE MADE …
Page | 5
Page | 6
SUMMARY
Environmental research and in-situ measurements have shown that most regions of the
Baltic Sea harnesses a large wind energy potential. The Finnish government has outlined a
policy to generate a rapid increase in the use and production of renewable wind energy. In
order to achieve these goals, the need arises for the construction of full scale offshore wind
farms with a total capacity of around 2,500 MW. However, large parts of the northern basins
of the Baltic Sea are covered with ice during the winter season, which is something that to
offshore wind farms operational to this date have not yet been subjected to. The scope of
this thesis is to provide an insight into the overall feasibility and the preliminary design
process of an offshore wind substructure suitable for the conditions as found along to the
Northeastern coastline of the Bay of Bothnia.

Prior to the determination of the boundary conditions and design drivers for the case study,
an extensive literature study has been performed. The literature study is focused on the
recent development of offshore wind farms, offshore wind substructure concepts, the
environment of the Bay of Bothnia and on aspects related to arctic engineering.

Using the literature study as a reference, the case study was outlined in further detail, by
establishing the design basis regarding the NREL 5MW reference turbine and tower.
Governing environmental parameters such as the on-site wind speeds, wave heights and ice
conditions are determined, along with the on-site geotechnical parameters such as water
depth and soil composition.

Based on the case study, the various offshore wind substructure concepts are compared to
determine to most suitable concept for application in the Bay of Bothnia. This comparison is
based on a qualitative assessment of design drivers for each concept. Based on this
assessment, the monopile foundation was considered the most suitable for application within
the boundaries of the case study.

In order to examine the effects of ice loads on the overall feasibility of offshore wind monopile
foundations in arctic environments, the increase in structural dimensions and design weight
of the monopile foundation due to ice loading is analyzed. For this structural analysis, use
will be made of static load cases and a static analysis methodology. Making use applicable
offshore wind and arctic design codes, the load conditions and load cases regarding static
wind, hydrodynamic and global ice loading on offshore wind substructures are determined.

In order to establish a base line, a reference design is presented which is not subjected to
any form of ice loading. Based on this reference design, a preliminary design is presented
that is subjected to level ice loading. Another preliminary design has been drafted which is
subjected to ice ridge loading, which is considered to be the heaviest load condition for this
case study. Each design has been optimized regarding the structural dimensions required to
withstand the applicable load cases, by using allowable cross section stresses and
deflections as design criteria. Based on the structural analysis of all three designs and
loading conditions, it was determined that the effect of global ice loading on monopile
foundations can be considered significant.

In order to resist global ice loading, a monopile foundation designed for the Bay of Bothnia
has to increase its overall dimensions from a reference diameter of 5.20m to 7.20m, together
with a wall thickness increase from 60mm to 120mm. This results in an increase in structural
weight of 97.1% between the reference design and the design optimized to resist ice ridge
loading. This increase in structural weight will in turn result in a significant increase off the
overall development costs for an offshore wind farm in the Bay of Bothnia using monopile
foundations, which puts feasibility of such developments under pressure.
Page | 7
Page | 8
TABLE OF CONTENTS
Acknowledgements ....................................................................................................................... 5
Summary ....................................................................................................................................... 7
Table of Contents .......................................................................................................................... 9
Chapter 1 - Introduction .............................................................................................................. 18
1.1 Research Objective ...................................................................................................... 19
1.2 Scope of the Research ................................................................................................. 19
1.3 Thesis Outline .............................................................................................................. 21
Chapter 2 – Offshore Wind Farms............................................................................................... 24
2.1 Background to Offshore Wind Farms .......................................................................... 24
2.2 Offshore Wind Farm Layout ........................................................................................ 26
2.3 Offshore Wind Turbine Components .......................................................................... 28
2.3.1 Rotor .................................................................................................................... 29
2.3.2 Nacelle ................................................................................................................. 30
2.3.3 Tower................................................................................................................... 31
2.3.4 Substructure ........................................................................................................ 33
2.3.5 Environmental Protection Systems ..................................................................... 33
2.4 Offshore Wind Farm Feasibility ................................................................................... 34
2.4.1 Offshore Wind Energy Economics ....................................................................... 34
2.4.2 Offshore Wind Substructure Costs ...................................................................... 35
Chapter 3 – Offshore Wind Substructures .................................................................................. 36
3.1 Substructure Design Drivers ........................................................................................ 36
3.1.1 Geotechnical Site Conditions .............................................................................. 37
3.1.2 Environmental Site Conditions ............................................................................ 37
3.1.3 Offshore Wind Substructure Costs ...................................................................... 38
3.1.3.1 Fabrication Costs ............................................................................................. 38
3.1.3.2 Transportation, Installation and Removal Costs ............................................. 39
3.1.3.3 Maintenance Costs .......................................................................................... 39
3.1.4 Environmental Impact ......................................................................................... 39
3.2 Monopile Foundations ................................................................................................ 40
3.2.1 Transportation and installation ........................................................................... 42
3.2.2 Advantages .......................................................................................................... 44
3.2.3 Disadvantages ..................................................................................................... 45
Page | 9
3.2.4 Future Developments .......................................................................................... 45
3.2.4.1 X-Large Monopiles ........................................................................................... 45
3.2.4.2 Guyed Monopiles ............................................................................................ 46
3.2.4.3 Drilled Concrete Monopiles ............................................................................ 47
3.3 Gravity Based Foundations ......................................................................................... 48
3.3.1 Transportation and Installation ........................................................................... 51
3.3.2 Advantages .......................................................................................................... 53
3.3.3 Disadvantages ..................................................................................................... 53
3.3.4 Future Developments .......................................................................................... 54
3.3.4.1 Anchored Gravity Based Foundations ............................................................. 54
3.4 Tripod Foundations ..................................................................................................... 55
3.4.1 Transportation and Installation ........................................................................... 56
3.4.2 Advantages .......................................................................................................... 58
3.4.3 Disadvantages ..................................................................................................... 58
3.4.4 Future Developments .......................................................................................... 58
3.4.4.1 De-Centralized Tripods .................................................................................... 59
3.4.4.2 Tripod Foundations with Suction Buckets ....................................................... 60
3.4.4.3 Tri-Pile Foundations ........................................................................................ 61
3.4.4.4 Concrete Tri-Pile Jack-Up Foundations ........................................................... 62
3.5 Jacket Foundations ...................................................................................................... 64
3.5.1 Transportation and Installation ........................................................................... 66
3.5.2 Advantages .......................................................................................................... 68
3.5.3 Disadvantages ..................................................................................................... 68
3.5.4 Future Developments .......................................................................................... 68
3.5.4.1 Variobase Jacket Foundation .......................................................................... 68
3.5.4.2 Gravity Based Jacket Foundation .................................................................... 70
3.5.4.3 Twisted Jacket Foundation .............................................................................. 71
3.5.4.4 Jacket Foundation with Suction Buckets ......................................................... 72
3.6 Suction Bucket Foundations ........................................................................................ 73
3.6.1 Transportation and Installation ........................................................................... 74
3.6.2 Advantages .......................................................................................................... 75
3.6.3 Disadvantages ..................................................................................................... 75
3.6.4 Future Developments .......................................................................................... 76
3.6.4.1 Prototypes ....................................................................................................... 76
Page | 10