Luận văn Phân tích và so sánh kết cấu jacket 3 và 4 chân cho turbine gió ngoài khơi dưới sự ảnh hưởng của xói mòn.

Escuela Técnica Superior de Ingenieros de Caminos,
Canales y Puertos.

UNIVERSIDAD DE CANTABRIA

Analysis and Comparison of The Behavior
Between 3-Legged and 4-Legged Jacket Structure
for Offshore Wind-turbine Influenced by The
Scouring Effect

Author:

CAO ANH VU
Supervisor:

FRANCISCO BALLESTER
Co-supervisor:

JOKIN RICO
Universitary Degree:
Master in Construction Research, Technology and
Management in Europe – Máster en Investigación,
Tecnología y Gestión de la Construcción en Europa

Santander, September 2017


MASTER IN COSTRUCTION RESEARCH, TECHNOLOGY AND MANAGEMENT IN EUROPE

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LỜI CẢM ƠN
Qua luận văn này, những dòng đầu tiên tác giả muốn được bày tỏ lòng biết ơn sâu sắc
đến những người đã nhiệt tình giúp đỡ, chỉ dạy, tạo mọi điều kiện tốt nhất suốt những buổi
đầu của khóa học đến những ngày hoàn thiện luận văn.
Tác giả muốn được bày tỏ lòng biết ơn chân thành đến thầy Francisco Ballester Muñoz
và thầy Jokin Rico Arenal, giáo sư hướng dẫn cũng như giám đốc phòng nghiên cứu phát triển
công nghệ xây dựng INGECID – Khoa Xây Dựng – Đại học Cantabria (Tây Ban Nha), người đã
định hướng và đưa ra những góp ý kịp thời cho tác giả trong suốt bốn tháng thực hiện luận
văn tại đây.
Lời cảm ơn chân thành xin được gửi đến các thầy trong ban điều hành khóa học, thầy
giáo Daniel Castro, thầy giáo Jorge Rodriguez và thầy giáo Pablo Pascual đã luôn động viên và
giúp đỡ tác giả trong suốt quá trình học tập tại Tây Ban Nha và Đan Mạch.
Tác giả cũng không quên công ơn của đội ngũ hơn hai mươi giảng viên đến từ các quốc
gia như Đức, Pháp, Đan Mạch, Bồ Đào Nha, Ý và Tây Ban Nha, những con người đã tạo nên
một môi trường học tập và làm việc chuyên nghiệp trong suốt khoảng thời gian một năm vừa
qua.
Tác giả cũng xin được dành những tình cảm chân thành gửi đến anh chị em lớp Cao học
khóa 2016-2017 cũng như những kỹ sư/kiến trúc sư/nghiên cứu sinh đang công tác tại phòng
nghiên cứu INGECID, đặc biệt là hai người đồng nghiệp Silvia Suarez và Marcos Cerezo, những
người luôn sát cánh, động viên, hướng dẫn, chỉ bảo tận tình cho tác giả trong quá trình công
tác tại trung tâm INGECID.
Cuối cùng xin được dành lời cảm ơn sâu sắc tới gia đình, bạn bè cùng bạn gái của tác giả
tại quê hương Việt Nam, những người đã luôn dõi theo và là nguồn động viên cả về mặt vật
chất lẫn tinh thần cho tác giả trong suốt quá trình học tập xa nhà.

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ACKNOWLEDGEMENTS
I owe my deepest gratitude to my supervisors Professor Francisco Ballester Muñoz –
Director of Engineering department - University of Cantabria (Spain) and Professor Jokin Rico
Arenal – Director of INGECID Company – for all of the useful comments, leading ideas through
the process of my final thesis.
I wish to thank, Professor Daniel Castro, Professor Jorge Rodriguez and Professor Pablo
Pascual, the Master’s director and coordinators, who gave me continuous encouragement
throughout one year of study in Spain and Denmark.
I would like to thank all Professors and Lectures, who are giving lectures in twenty
modules of the “Construction research, technology, and management in Europe” master
course for creating a professional working environment during the period of this master
course also for all their favors given to me.
I also share the credit of my work with all my colleagues in the Master class as well as in
INGECID Company, especially Silvia Suarez and Marcos Cerezo, because of their supports and
advice while I was doing my internship.
Last but not least, I must express my very profound gratitude to my parents and my
girlfriend in Viet Nam for all their unconditional love they gave to me. Without them, I would
not be able to achieve or enjoy these successes.

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TÓM TẮT NỘI DUNG LUẬN VĂN
Phân Tích Và So Sánh Ứng Xử Của Hệ Giàn Thép 3 Chân Và 4 Chân Cho Tháp Turbine Gió
Ngoài Khơi Dưới Sự Ảnh Hưởng Của Xói Mòn
Trải qua quá trình ba mươi năm phát triển, ngành công nghiệp năng lượng đang được
biết đến là một trong những mũi nhọn của các nền công nghiệp tiên tiến trên thế giới. Trong
đó, công nghệ năng lượng xanh, cụ thể là năng lượng gió ngoài khơi đã và đang mang đến
một nguồn năng lượng sạch, dồi dào và an toàn cho nhân loại. Tuy nhiên, khi các kỹ sư muốn
đưa những turbine gió ra xa ngoài khơi, họ phải đối mặt phải rất nhiều thử thách từ thiên
nhiên. Do đó, việc nghiên cứu đầy đủ về cách kết cấu ứng xử trước những điều kiện tự nhiên
khắc nghiệt là một vấn đề cấp thiết. Trong khuôn khổ của luận án, hệ thống móng cọc thép
dài 35m, đường kính 2.5m, chiều dày 5cm được lựa chọn để truyền toàn bộ tải trọng của hệ
thống jacket và wind turbine cao 161.6m xuống lòng biển. Luận văn này tập trung nghiên
cứu và phân tích ứng xử của hai loại jacket 3 chân và 4 chân trong trạng thái cực hạn cùng
với hiện tượng xói mòn và sự biến đổi của lòng biển trong suốt quá trình hoạt động của hệ
thống. Sự chuyển vị và ứng suất Von-Mises của mỗi kết cấu sẽ được đem ra so sánh, nhằm
đưa ra cái nhìn khái quát cho mỗi loại jacket. Kết quả của bài nghiên cứu này sẽ có ích cho
việc lựa chọn sơ bộ hệ jacket và phương án phòng ngừa thích hợp cho hiện tượng xói mòn
và sự biến động của lòng biển trong tương lai.

ABSTRACT
Analysis and Comparison of The Behavior Between 3-Legged and 4-Legged Jacket Structure
for Offshore Wind-turbine Influenced by The Scouring Effect
Throughout the period of thirty years, the energy industry is known as one of the most
developing fields in the world. Moreover, the wind energy industry in objective and the
offshore wind power in subjective is one of the main eco-friendly sources of energy for
humankind. Due to the needs of applicability and economic efficiency, the larger size of
offshore wind turbine structure needs to go to the deeper water. However, the further we

went to the ocean, the more complicated states of environment we got so that we need to
fully analyze and comprehend the behavior of the support structures against the severe or
extreme weather conditions. During this study, steel pile foundation, which has a
penetration length of 35m, the diameter of 2.5m and 5cm of wall thickness, is a primary
choice to anchor the jacket structure and wind turbine with 161.6m total height to the sea
floor. This study analyzed the offshore jacket’s behavior within the Ultimate limit state (ULS),
the scour and sand waves in general, supports for the 5MW offshore wind turbine. These
result will provide an overall view between 2 different types of the structure against the
scour and uneven sea bed level caused by sand waves. The deformations and the Von-Mises
stresses of the 3-legged and the 4-legged jacket were compared, in order to fulfill the gap of
understanding these two types of support structures. The study will be useful for considering
a suitable jacket and optimal scouring prevention methods to be executed for the future
project.

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TABLE OF CONTENTS

CHAPTER 1.

INTRODUCTION ................................................................................................ 9

1.1

Overall Review of Offshore Wind Turbines ................................................................. 9

1.2

Motivation and Objective of the study ...................................................................... 10

1.3

Tasks of the Thesis ..................................................................................................... 10

1.4

Structure of the Document ........................................................................................ 11

1.5

The Regulation and Software used ............................................................................ 11

CHAPTER 2.

THE BASIC OF OFFSHORE WIND TURBINE STRUCTURES ................................ 12

2.1

Introduction ............................................................................................................... 12

2.2

Support Structure of Offshore Wind Turbine ............................................................ 12

2.2.1

Monopile .......................................................................................................... 12

2.2.2

Gravity-Based Structure (GBS).......................................................................... 13

2.2.3

Tripod................................................................................................................ 14

2.2.4

Jacket Structure ................................................................................................ 14

2.2.5

Floating Structure ............................................................................................. 15

2.3

The Scouring Predictions and Prevention Methods for Jacket Structure ................. 16

2.3.1

The scouring phenomenon ............................................................................... 16

2.3.2

The Method of Predicting the Scouring Effect ................................................. 17

2.3.3

The Consideration of Sand Wave ..................................................................... 19

2.3.4

The Method of Preventing the Scouring Effect ................................................ 20

2.4

Wind Conditions ........................................................................................................ 24

2.4.1

Mean Wind Speed ............................................................................................ 25

2.4.2

Wind Speed Profiles ......................................................................................... 25

2.4.3

Distribution of Wind Pressure on the Structure............................................... 26

2.5

Waves ......................................................................................................................... 28

2.5.1

Basic Waves Characteristic ............................................................................... 28

2.5.2

Wave Modeling................................................................................................. 29

2.6

Currents ..................................................................................................................... 31

2.7

Combine Wave and Current by Morison’s Load Formulas ........................................ 31

2.8

Sand Wave ................................................................................................................. 32

2.9

Marine Growth........................................................................................................... 33

2.10

Ultimate Limit States (ULS) and Load Combinations ............................................. 33

2.11

Soil-Structure Interaction ....................................................................................... 34

2.11.1

Soil reaction for piles under axial compression ............................................... 34

2.11.2

Soil reaction for piles under lateral loads......................................................... 35

CHAPTER 3.

GENERAL INFORMATION OF THE JACKET STRUCTURES................................. 39

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3.1

Introduction ............................................................................................................... 39

3.2

Specific Environmental Conditions and Design Load Cases ...................................... 39

3.2.1

Wind, Wave, and Current Conditions ............................................................... 39

3.2.2

Soil Conditions .................................................................................................. 42

3.2.3

Scouring Conditions .......................................................................................... 42

3.2.4

Sand Wave Model............................................................................................. 43

3.2.5

The Meteorological – Oceanographic Parameter ............................................ 44

3.2.6

ULS Load Combinations .................................................................................... 44

3.2.7

The Wind-Wave Misalignment Models ............................................................ 44

3.3

Geometry of the specific 3-Legged Jacket and the 4-Legged Jacket Structure ......... 46

3.3.1

The Chosen Wind Turbine ................................................................................ 46

3.3.2

The Model of 3-Legged Jacket .......................................................................... 47

3.3.3

The Model of 4-Legged Jacket .......................................................................... 48

3.3.4

The Model of Steel Pile Foundation ................................................................. 49

3.3.5

The Initially Analysis of Von-Mises Stress ........................................................ 51

CHAPTER 4.
RESULT OF THE ANALYSIS AND THE DISCUSSION BETWEEN SCOURING AND
SAND WAVE TO THE NATURAL FREQUENCY OF JACKET STRUCTURES .................................... 53
4.1

Introduction ............................................................................................................... 53

4.2

The Result from the Natural Frequency Analysis ...................................................... 53

CHAPTER 5.
RESULT OF THE ANALYSIS AND DISCUSSION BETWEEN SCOURING AND SAND
WAVE TO THE BEHAVIOUR AND THE STRESSES OF THE JACKET STRUCTURES........................ 55
5.1

Introduction ............................................................................................................... 55

5.2

The Relationship between Scouring Effect, Stresses and Displacement of Jackets .. 55

5.2.1

Comparing the Stresses of the Jackets under the the Scouring ....................... 55

5.2.2

Comparing the Displacement of the Jackets under the Scouring .................... 57

5.3

The Sand Wave .......................................................................................................... 58

CHAPTER 6.

CONCLUSIONS AND FUTURE WORKS ............................................................. 60

6.1

Conclusions ................................................................................................................ 60

6.2

Future Works ............................................................................................................. 60

References…………….…………….…………….…………….…………….…………….…………….…………….………61
APPENDIX A .............................................................................................................................. 63
APPENDIX B............................................................................................................................... 67

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LIST OF FIGURES
Figure 1-1: Cumulative and annual offshore wind installation (MW) - (WindEurope, 01/2017)
.................................................................................................................................................... 9
Figure 1-2: Support structure for offshore wind turbine (WindEurope, 01/2017) .................... 9
Figure 1-3: Offshore wind substructure designs for varying water depths (Dvorak, 2017) .... 10
Figure 2-1: Monopile structure
(Garrad Hassan and Partners Ltd) ......... 12
Figure 2-2: Monopile installation (Lapke, 2015) ...................................................................... 13
Figure 2-3: Gravity-based foundation
(Garrad Hassan and Partners Ltd) ......... 13
Figure 2-4: Tripod Structure (Garrad Hassan and Partners Ltd) .............................................. 14
Figure 2-5: Pre-fabricate and transportation
of tripod structures ................ 14

Figure 2-6: Jacket support structure ........................................................................................ 15
Figure 2-7: The main construction stages ................................................................................ 15
Figure 2-8: Floating support structure for offshore wind turbine ........................................... 16
Figure 2-9: Scouring effect around a vertical pile .................................................................... 16
Figure 2-10: Scouring effect ..................................................................................................... 17
Figure 2-11: Scour formation around jacket foundation G2: August 2011 to February 2012 ....
(BOLLE, et al., 2012).................................................................................................................. 19
Figure 2-12: The phase, amplitude and wave length of natural sand waves vary in space (Berg
& Damme, 2004)....................................................................................................................... 20
Figure 2-13: The long lasting protection system (DHI team) ................................................... 20
Figure 2-14: Typical scour protection design (PETERS & WERTH, 2012) ................................. 21
Figure 2-15: The Geotextile containers solution ...................................................................... 21
Figure 2-16: The installation of scour protection system in Ireland (2011) ............................. 22
Figure 2-17: Filter Unit protects wind turbine foundation (KYOWA) ....................................... 22
Figure 2-18: Fauna and flora around RFU system .................................................................... 22
Figure 2-19: Scour Control System (SSCS) ................................................................................ 23
Figure 2-20: Concrete mattress installation ............................................................................. 23
Figure 2-21: The scour prevention system by using recycled rubber tire ................................ 24
Figure 2-22: The installation of rubber derivative system ....................................................... 24
Figure 2-23: Influence of the wind turbine to wind speed and air pressure (Veritas &
Laboratory, 2002) ..................................................................................................................... 26
Figure 2-24: Definition of water levels ..................................................................................... 28
Figure 2-25: Regular traveling wave properties ....................................................................... 29
Figure 2-26: Ranges of validity for various wave theories (Chakrabarti, 1987) ....................... 29
Figure 2-27: Characteristics of offshore sand bed forms ......................................................... 33
Figure 2-28: The data indicate for marine growth profile........................................................ 33
Figure 2-29: Partial safety factors for loads γf .......................................................................... 34
Figure 2-30: Design load cases ................................................................................................. 34
Figure 2-31: The p-y curves applied at nodal point in beam-column representation of pile .. 36
Figure 2-32: Coefficient as functions of friction angle ............................................................. 37

Figure 2-33: Initial modulus of subgrade reaction k as function of friction angle ................... 37
Figure 3-1: The equivalent wind force from the tower and blades ......................................... 40
Figure 3-2: The wave and current force for 4-legged jackets exported from EXCEL ............... 40
Figure 3-3: The wave and current moment for 4-legged jacket exported from EXCEL ........... 40
Figure 3-4: Wave and current load table of 4-legged jacket exported from Midas Software . 41
Figure 3-5: Wave and current load table of 3-legged jacket exported from Midas Software . 41
Figure 3-6: The Soil Properties ................................................................................................. 42
Figure 3-7: Example of scouring models .................................................................................. 42
Figure 3-8: The sand wave model type 1 ................................................................................. 43
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Figure 3-9: The sand wave model type 2 ................................................................................. 43
Figure 3-10: The sand wave model type 3 ............................................................................... 43
Figure 3-11: The direction of the wind, wave for 4-legged jacket structures .......................... 44
Figure 3-12: The direction of the wind, wave for 3-legged jacket structures .......................... 44
Figure 3-13: The Models of the Study taking into account the Wind-Wave Misalignment,
Scouring and Sand Wave Effect ............................................................................................... 46
Figure 3-14: The Gross Properties Chosen for the NREL 5-MW Baseline Wind Turbine ......... 47
Figure 3-15: The Basic Estimation for Jacket Level................................................................... 47
Figure 3-16: The Transition and Tower Base of 3-Legged Jacket ............................................. 48
Figure 3-17: The Lateral View of 3-Legged Jackets Offshore Wind Turbine Structure ............ 48
Figure 3-18: The Top View of 3-Legged Jackets Offshore Wind Turbine Structure ................. 48
Figure 3-19: The Lateral View of 4-Legged Jackets Offshore Wind Turbine Structure ............ 49
Figure 3-20: The Top View of 4-Legged Jackets Offshore Wind Turbine Structure ................. 49
Figure 3-21: The Modulus of Subgrade Reaction ..................................................................... 51

Figure 3-22: The Comparison of subgrade modulus reaction between clay and sand ............ 51
Figure 3-23: Comparison of maximum Von-Mises stresses between non-scour models
of
3-legged and 4-legged jackets .................................................................................................. 52
Figure 4-1: The allowable range for structure natural frequencies supports 5MW wind turbine
.................................................................................................................................................. 53
Figure 4-2: The natural frequency of jackets under scouring effect ........................................ 54
Figure 4-3: The effect of sand wave to the natural frequency................................................. 54
Figure 5-1 Maximum Von-Mises Stress of 3-Legged Jackets ................................................... 55
Figure 5-2: Maximum Von-Mises Stress of 4-Legged Jackets .................................................. 56
Figure 5-3: The Comparison of Maximum and Minimum Von-Mises Stress between 3-Legged
and 4-Legged Jackets ................................................................................................................ 56
Figure 5-4: Maximum Deformation of 3-Legged Jacket during the Scouring .......................... 57
Figure 5-5: Maximum Deformation of 4-Legged Jacket during the Scouring .......................... 57
Figure 5-6: Comparing the Maximum and Minimum Deformation between 2 Types of Jacket
under Scouring Effect ............................................................................................................... 58
Figure 5-7: Von-Mises Stresses and Deformations of 3-Legged Jackets within the Sand Waves
.................................................................................................................................................. 58
Figure 5-8: Von-Mises Stresses and Deformations of 4-Legged Jackets within the Sand Waves
.................................................................................................................................................. 59
Figure 5-9: The Maximum of Von-Mises Stresses and Deformations between 3-Legged and
4-Legged Jackets………………………………………………………………………………………………………………….59

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CHAPTER 1. INTRODUCTION
1.1

Overall Review of Offshore Wind Turbines

Throughout the period of thirty years, the energy industry is known as one of the most
developing fields in the world. Moreover, one of the eco-friendly methods of generating
energy is the offshore wind turbine. Based on the annual report The European Offshore
Wind Industry in January 2017 showed that the Europe’s additive installed offshore wind
capacity at the end of 2016 claimed to 12631 MW with the number of wind turbine
overcome 3589 wind turbines. There is a total of 81 offshore wind farms in 10 different
countries in Europe (WindEurope, 01/2017).

Figure 1-1: Cumulative and annual offshore wind installation (MW) - (WindEurope, 01/2017)
Also, the pie graph (Figure 1-2) besides
showed that the majority of the support structure
for offshore wind turbines are monopile
structures with 80.8% - 3354 foundations. It can
be seen that the market of wind energy is
increasing rapidly. However, the deep water
locations are not invested as much as other
foundation types. Thus, this thesis will study
further for the jacket structures. Before going
further into the study, there is a need of overall
review between each support structures in order
to provide a wide scope of the offshore wind
turbine industry. In general, the principal function
of supporting structure is to hold the wind
turbine in balance during every state of
circumstances. Moreover, there are several types

of foundation concepts as well as the ways to
distinguish each structure.
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Figure 1-2: Support structure for
offshore wind turbine (WindEurope,
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In Figure 1-3, five different types of main supporting structure for an offshore wind
turbine are listed below in the following order: from shallow to deeper sea level. For each
location with a level of water, the suitable structure types are recommended in term of cost
efficiency, fabrication and installation methods. Furthermore, the DNV and API standard for
offshore wind turbine explain the design methods, the manufacture, transportation,
installation process, also the analysis and checking procedure for each support structure.

Figure 1-3: Offshore wind substructure designs for varying water depths (Dvorak, 2017)
1.2

Motivation and Objective of the study

Nowadays, the wind energy in objective and the offshore wind power in subjective is
one of the main eco-friendly sources of energy for humankind. Due to the needs of
applicability and economic efficiency, the larger size of offshore wind turbine structure
needs to go to the deeper water. However, the further we went to the ocean, the more
complicated states of environment we got so that we need to fully analyze and comprehend
the behavior of the support structures against the severe or extreme weather conditions.

The pile foundations are mainly the first choice to anchor the jacket substructures to the sea
floor, due to the significant increases in base shear and overturning moment from the
combinations of hydraulic and aerobic loads. This study will analyze the offshore jacket’s
behavior in general, based on the East Anglia project in the North Sea (because of the
confidential information, the researchers have to change the parameter of jacket’s
geometry, the location, the environmental and geotechnical information). Also, there are
only a few studies analyzed the different of behavior between 3-legged and 4-legged jackets
but have not mentioned the effect of scouring and sand waves to these structures. The
reactions and the behaviors of the 3-legged and the 4-legged jacket will be compared, in
order to fulfill the gap of understanding these two types of support structures. These result
will provide an overall view between 2 different types of the structure against the scour and
uneven sea bed level caused by sand waves. The study will be useful for considering a
suitable jacket and optimal scouring prevention methods to be executed.
1.3

Tasks of the Thesis

Having the title: “Analysis and Comparison of The Behavior Between 3-legged and 4legged Jacket Structure for Offshore Wind-turbine Influenced by The Scouring Effect”, this
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thesis will concentrate on analyzing and comparing the natural frequency as well as the
deformations, stress/strain of the 3-legged and 4-legged jacket structures and its elements:
- Literature reading: ultimate strength analysis, the DNV design standard for an offshore
wind turbine, the wind turbine analysis, the environment theories, the soil-pile reaction and
any other previous study related to the offshore jacket and scouring effect.

- Calculate the environment loads and the soil-pile reaction based on DNV and API Standard.
- Modeling the whole structure in Midas Civil software: the wind turbine structure, the jacket
structure, and the piles.
- Create a variety of model in term of the changing scour and sand wave depth, types of sand
wave.
- Performing the Eigenvalue analysis and The ULS analysis for each model.
- Discuss and compare the result between the 3-legged and 4-legged jacket models.
- Reporting
1.4

Structure of the Document

In the first chapter: the overall scope of the project, offshore wind turbine and the aim
of the thesis, are pointed out. Chapter 2 provides the general theories and design
methodology applied to the project from the environment loads calculation, the method to
model the structures in Midas software, the method of predicting the sand wave and
scouring effect. Also the soil-pile interaction model has been investigated which is following
the API and DNV standard.
In the following chapter, the general information of the environment, jacket models
will be shown. Chapter 4 and 5 analyze the natural frequency, the displacements, also the
Von-Mises stresses of both jacket structures.
Chapter 6 shows the summaries of the conclusions and the recommendation for any
further study of offshore wind turbine jacket structures.
1.5

The Regulation and Software used
The following regulations and computer programs were used in this thesis:

- DNVGL Standard.
- MIDAS CIVIL 2018 (VER 1.1), bridges and civil structures analyze software, MIDAS

Information Technology Co., Ltd.
- EXCEL, spreadsheet program, Microsoft Inc.

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CHAPTER 2. THE BASIC OF OFFSHORE WIND TURBINE STRUCTURES
2.1

Introduction

In this chapter, the essential theories/formulas will be introduced, from the basic
information of wind turbine support structures and scouring effect to the knowledge of the
environmental phenomenon. There are so many phenomenon/load cases that could be
relatively important for some cases, for example wind, wave, current, soil condition,
scouring, tides, ice, earthquake, temperature, boat impact, construction load case, and so
on. However, due to the scope of this work, this report will analyze the five main aspect of
the environment, which are the most basic and essential values for the offshore projects,
including wind, wave, current, scouring, soil condition. Also, the DNV and API standard will
be applied to fulfill the tasks of the projects.
2.2

Support Structure of Offshore Wind Turbine
2.2.1

Monopile

Nowadays, one of the most well-known offshore wind turbine structure is monopile
foundation structure, and it is suitable for shallow water depth.
This support structure is a simple
and efficient design which the wind
turbine structure is carried by the
monopile structure, either directly
or through a transition piece. The
cylindrical steel tube is usually
chosen to be the material of the
monopile structure due to the ease
of fabrication, the bearing capacity
of the material, the methods of
installation in shallow to medium
sea level (the range is from 0 to 30meter depth).

Figure 2-1: Monopile structure
(Garrad Hassan and Partners Ltd)

The monopile foundation has the
diameter ranges up to 6-meter,
and the wall thickness is 150mm
maximum.

The weight of the monopile could be up to 650 Ton and the transition pieces of 580
Ton according to London Array offshore wind farm (4COffshore, 2013) could carry the wind
turbine of 6-8MW.

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The main elements of the monopile
structure can be seen in Figure 2-1 with
the installation procedure from the
foundation to the substructure. Firstly,
the steel pile will be fabricated onshore,
then be transported to the construction
site by the specific vessel. Secondly, by
using hydraulic or steam hammer, the
pile is driven into the seabed until it
reaches to the design depth (Figure 2-2).
Then, the scouring protection system
could be set up around the pile (If
needed). Lastly, the upper parts are
delivered to the right position and
grouted/joined with the pile.

2.2.2

Figure 2-2: Monopile installation (Lapke, 2015)

Gravity-Based Structure (GBS)

The gravity-based foundation is
usually made from concrete which
provided a high gravity force in order

to resist the overturning loads from
the environment. This foundation
demanded a nearly flat sea bed to
anchor the structure properly in the
balance position
(Figure 2-3).
According to Sanjeev Malhotra in
2010, the weight of gravity foundation
vary from 500 ton to 1000 ton and the
proper water depth is under 15 meter.
Furthermore, the outer diameter of
the base could reach to over 15
meters.
Moreover,
the
scour
protection is a necessary element of
this structure.

Figure 2-3: Gravity-based foundation
(Garrad Hassan and Partners Ltd)

The gravity-based structure is to be considered in shallow water depth by several
following reasons. At first, the material made the structure is concrete which is cheap and
brings no damaged to flora and fauna. Second of all, the cost of installation and maintenance
are lower than using the steel structure. Lastly, the concrete structure has a high resistance
strength against salt-water and compression force.
During the installation process, there is some required equipment to fulfill the task: a
floating crane, a special barge, a group of the tugboat to set the foundation in place.
Throughout 50 years of development, from the oil-gas industrial to the offshore wind

turbine market, the gravity based has been evolved to some variations design such as: Crane
free gravity base (Seatower), GBF® gravity base (Vinci Construction UK/Freyssinet
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International), Consortia of Skanska, Smit Marine Projects and Grontmij, STRABAG gravity
base, etc. (4COffshore, 2013)
2.2.3

Tripod

The tripod structure consists of a
lightweight three-legged jacket (around
150 ton) with a smaller diameter of the
steel tube – diagonal elements, compared
to the traditional lattice steel structure
(Figure 2-4). Three steel legs transfer the
forces from the tower to the foundation
below which are piles or caissons
structure. The tripod structure could take
place in the average water depth
locations, between 20 and 40-meter
depth.

Figure 2-4: Tripod Structure (Garrad Hassan
and Partners Ltd)

Furthermore, the tripod structure has a higher resistance to scour effect than the
monopile structure. The steel pile system is the common choice for this support structure,
the diameter of the pile could be 0.9m and anchored 10-20-meter to the seabed. On the
other hand, tripod structure needs to be placed in the flat sea beds which require an
additional leveled process. Besides, the tripod structure especially the complex joints of
each leg to the main structure has a high risk of fatigue failure compared to the other
support structures.
Like any other support structure
for an offshore wind turbine, the
tripod structure has to be
prefabricated onshore, then carried
to the construction site by a huge
barge. Following that, the structure is
slowly leveled to the seabed by the
floating crane; the steel pile has been
driven through the pile sleeve of the
structure by using a submersible
hammer. (Figure 2-5: Pre-fabricate
and transportation
of
tripod structures)
Until 2016, there is only 3.2
percentages of the offshore wind
turbines are using tripod structure.
2.2.4

Figure 2-5: Pre-fabricate and transportation
of tripod structures

Jacket Structure

Jacket or lattice structures also known as space-frames or truss-towers, have a variety
of modifications between three-legged and four-legged, as well as a diversified of
Foundation’s type: steel pile, suction bucket, and so forth. (Figure 2-6)
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According to (DET NORSKE VERITAS, 2013),
the water depth ranging from 20 to 50 meters is
the most favorable for the jacket structures. The
weight of jacket structure could be around 900
ton for the 30m water depths.
The
environmental loads are mainly transferred as
axial loads as well as shear forces, bending
moments and torsion. “Shear forces and torsion
moments are transferred as lateral loads into the
seabed. Axial loads and overturning moments
are mainly transferred as axial compressive and
tensile forces” – (Passon, et al., 2015). Because
of the small diameter steel tube, the jacket
structure has a lower wave and current loads
than the other support structures. Besides, this
structure has a higher stiffness to mass ratio and
less sensitive to the soil conditions compared to

other structures (Passon, et al., 2015).

Figure 2-6: Jacket support structure

In contrast, the jacket support structure is prone to scour and local dynamic effect
from the hydraulic loads. Moreover, the cost of manufacturing is relatively high due to the
complicated structure as well as the high cost of maintenance and transportation.
Nowadays, there are two main methods for jacket Foundation’s installing: pre-pilling
or post-pilling. Each method has the advantages and disadvantages, but the pre-pilling
method has been used mostly in the jacket structure, it can be seen in Figure 2-7 is the main
steps of constructing an offshore jacket wind turbine. Because the mud mats and pile
sleeves are separated from the jacket. Furthermore, the prefabricated of the jacket could be
operated in parallel with the pilling process.

Figure 2-7: The main construction stages
2.2.5

Floating Structure

Based on the annual report (WindEurope, 01/2017), the number of floating wind
turbine is only 0.02 percent of the total wind turbine in Europe. The floating structures could
be spar-bouy, tension leg platform or semi-submersible from the left to the right of Figure
2-8, respectively. Most of the floating concepts are during the experimental stage because
the stability of the structure and the wind turbine structure as a whole is hard to maintain
during the extreme conditions, but some experts have a strong belief in the bright future of
the floating structures. This structure could be installed in the locations where the fixedstructure can not be viable due to the technically and economically point of view.

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Figure 2-8: Floating support structure for offshore wind turbine
2.3

The Scouring Predictions and Prevention Methods for Jacket Structure
2.3.1

The scouring phenomenon

Based on the DNV Standard (Det Norske Veritas Germanischer Lloyd, 04/2016), “Scour
is the result of erosion of soil particles at and near a foundation and is caused by waves and
current.”

Figure 2-9: Scouring effect around a vertical pile
Figure 2-9 explained how the scouring effect takes shape around a vertical pile. At first,
the horseshoe vortex will be formed at the base at the front side of the pile. Following that, a
vortex flow pattern in the shape of vortex shedding will be formed on the side of the pile.
After that, the streamlines will contract at the side edges of the pile. The changing in the
stream will raise the bed shear stress, and the sediment transport capacity will increase
accordingly. This phenomenon is known as the local scour around the pile (Figure 2-10).

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Figure 2-10: Scouring effect
There are two different types of scour: local scour and global scour. Both types of
scouring depend on the geotechnical data, the diameter of the pile, the velocity of wave and
current, the time scale, the sea bed figures. However, the global scour only happened when
the space between each pile is over the top compared to the pile diameter (more detail will
be explained in the following section). Besides, it is necessary to distinguish two type of local
scour between clear-water scour and live-bed scour. This well-defined is important because
the behavior and the scour depth related with time will be different when each scours types
occur.
In (Det Norske Veritas Germanischer Lloyd, 04/2016), the standard pointed out the
main strategies for non-protected structures and scour protection structures. “If the
substructure is placed without protection the scour has to be considered in the design. If the
substructure is placed with scour protection, the scour protection stability has to be
documented”.
2.3.2

The Method of Predicting the Scouring Effect

According to the standard DNV-ST-0126 (Det Norske Veritas Germanischer Lloyd,
04/2016), in Appendix D: Scour at a vertical pile, the equilibrium scour depth S can be used
as a basis for structural design. The following formulas determined the empirical expression
for the equilibrium scour depth, S due to waves may be used:

S
 1,3.{1  exp[0.03(KC  6)]} while KC  6
D

(Eq. 1)

However, for steady current, which implies KC=h, the equation will be S/D=1,3. For
the conditions have KC below the value of 6, no scour hole is formed because there is no
horseshoe vortex develops for KC<6.
The time scale of souring effect which means the time development of the scour
t
depth, S, can be followed as: S1  S(1  exp( )) (Eq. 2)
T1
Which:

t: the time.

T1: the time scale of the scour process.

The time scale T1 of the scour process can be found from the nondimensional time
scale T* by the following relationship:
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T* 

g(s  1)d 3
T1
D2

(Eq. 3)

Which:

T* 

1 h 2,2

For steady current
2000 D

T *  106 (

KC



)3

For waves

(Eq. 4)

(Eq. 5)

The Shields parameter  is defined by:



U2f
g(s  1)d

(Eq. 6)

Where: s: the specific gravity of the sediment.
D: the grain diameter for the specific grain.
Uf: the bed shear velocity.
Besides, there are some different methods of estimating the scour depth from other
studies, such as the study from BOLLE, et al., in 2012 or the report of D. RUDOLPH, et al., n.d.
Those researchers provide a basis methods obtain from the previous studies (which will be
shown below):
The global scour depth (due to a 2x2 pile group): is defined by SG=0,37.Dcal
(B.M. & J, 2002). The global scour extent is equal to rG=SG/tan(/2), which  is the friction
angle of the soil. Nonetheless, the engineers have to remember to consider the distance
between each pile centers (L), if L is higher than the value of 6.Dcal then the global scour has
not to be taken into account (S. & K., 1982).
The local scour depth (SL): with the expected value SL,e=1,3.Dcal (DET NORSKE
VERITAS, 2013), the maximum value SL,m=2.Dcal (Considered the standard deviation of the
measurements, also taking into account some joints are situated between 2,5 and 5,0m
above the sea floor) – (B.M. & J, 2002). The local scour extent (rL): with the expected radius
rL,D=0,5.Dcal + SL,e/tan(/2), the maximum radius rL,D=0,5.Dcal + SL,m/tan(/2).
-

The total scour depth:

 The expected total scour depths: ST,e = SG + SL,e
 The maximum total scour depths: ST,m = SG + SL,m
-

The total scours extent:

 The expected radius: rT,e = 0,5.Dcal + ST,e/tan(/2)

 The maximum radius: rT,m = 0,5.Dcal + ST,m/tan(/2)
Inside the report from 2012, the researchers mentioned the considerations of jacket
structure is far more complex than the pile group (BOLLE, et al., 2012). Because of the
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horizontal and diagonal elements from the jacket structures could contribute to turbulence
effect, which increases the scour depth and extent. The impact from the upper element to
the global scour depends on the distance from its to the sea floor. The higher the distance is,
the lower contributions to the scouring effect. The estimations of the scour (mentioned
above) are the simple configuration, independent of the water depth, trustworthy value due
to the comparisons with observed data from the C-power wind farm Thornton Bank project
(Figure 2-11).

6

Figure 2-11: Scour formation around jacket foundation G2: August 2011 to February 2012
(BOLLE, et al., 2012)
2.3.3

The Consideration of Sand Wave

Basically, the sea bed is not totally flat; there are some different form of the bed form
for offshore locations and sand wave is one of the typical features of changing form’s sea
bed (Figure 2-12). They are nature migrating, long spatial and temporal scales may interfere
with offshore activities (Morelissen, et al., 2003). In the engineer’s point of view, the sand

wave is kind of interesting phenomenon, the sand wave could cause some problems to the
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pipe line systems or the unstable of the offshore structure, but it can be predicted and
anticipated the failure to structures.

Figure 2-12: The phase, amplitude and wave length of natural sand waves vary in space
(Berg & Damme, 2004)
2.3.4

The Method of Preventing the Scouring Effect

Due to the effect of scouring, many researchers and organizations invested in
developing the scour protection systems for offshore structure. There are several types of
the protection system, but the main purpose is to prevent the erosion of the soil from
seabed’s surface, by installing another layer of material (gravel, rock, asphalt, concrete, and
so on) on-top of sea bed level. However, the cost of dumping materials, as well as the
installation process, are relatively expensive. Thus, the designers and the contractors have to
consider carefully when choosing the solution based on the economic and time-consuming
aspect.
Several projects have been executed without the scouring protections system, for
example, the scour depth increased up to 6-7m with the tripod center pile after one calm
winter period (PETERS & WERTH, 2012). Therefore, the scour protection solutions are the
necessary object for offshore structures.
a. Gravel Scour Protection

One of the common strategies for
protecting the structure against scour is to set up
a layer of stone/gravel on the sea floor around
the foundation (Figure 2-13).
For some situations, a variety of
rock/gravel layers (could be the mix of different
size of components) have to be placed on top of
each other, in order to prevent the smaller
material has been washed out. However,
remember to keep in mind that there are no
solutions to avoid this phenomenon properly.

Figure 2-13: The long lasting protection
system (DHI team)

In addition, the development of scouring effect cannot be held off because the
granular filter layers can not work the same ways like onshore structures, due to Therzagi
criteria. Also, inside the study in 2012, Peters and Werth pointed out this solution also has
some disadvantages: the surface layer of material could be dispersed, the soil from the
bottom would be soaked up through the granular layer. Furthermore, the rock material
dumping around the pile caused the high risk to other sensitive elements (for example the
electricity cables or corrosion protection layer of structures). (Figure 2-14)
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Figure 2-14: Typical scour protection design (PETERS & WERTH, 2012)

b. Geotextile Sand Bags (GSB)
The study from Peters and Werth from 2012 showed that there are some advantages
by using geotextile sand bags for scour protection. Firstly, the whole system needs only two
layers and does not require an additional layer of granular filter or cover layer. Thus, the
prefabricated and installing the prevention system is simplified and causes no damages to
the foundation during the constructing process (Figure 2-15). Secondly, the GCB is an flexible
system connects each sand bag by the interlocking effect. Furthermore, the recommended
fill volume for each sand bag is between 1m3 and 1,5m3, contained in “the mechanically
bonded staple fiber non woven fabric.” Besides, the GBS system is installed to the whole
area before pile installation stage, protects the structure and the foundation area from the
very beginning of service life.

Figure 2-15: The Geotextile containers solution
Nowadays, there are a variety of projects successfully uses the GBS system (Figure
2-16), for example, 200km of the Mittellandkanal project, the project at Ley of List (Island
Sylt, North Sea, Germany), the Amurumbank West offshore project, and so on.

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Figure 2-16: The installation of scour protection system in Ireland (2011)
c. Rock Filled Filter Bags (RFU)
The mesh net makes the rock filled
Filter bags system (RFU) then filled with
stones; this solution protects marine cable,
pipeline, and monopile. RFU system has

been developed by a Japanese company
named KYOWA, which got several
achievements throughout the period from
1995 until now. (KYOWA, 2014).
There are some unique characteristics
of this system followed by KYOWA’s
website:

Figure 2-17: Filter Unit protects wind turbine
foundation (KYOWA)

- The net made from the synthetic fiber which is durable, against corrosion.
- The RFU could use for scour protection as well as cable and pipeline, leveling the seabed
and so on.
- The non-contaminated material of RFU system provides the eco-friendly habitat for aquatic
fauna and flora in wind farm. (Figure 2-18)
- The recommendation for filling stone’s diameter is between 50mm and 75mm, but 200mm
diameter stone could possibility used.

Figure 2-18: Fauna and flora around RFU system
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d. Frond Mats (FM) and Articulated Concrete Mattresses (ACM)
Frond mats includes the continuous lines
of overlapping floating polypropylene fronds,

when the systems activated, create a barrier
that relatively decreases the velocity of the
current. (Seabed Scour Control Systems,
2013). The weighted edges are constructed
from polyester sleeving filled with a standard
filling of gravel, as you can see in Figure 2-19.
The frond mats system has been developed
for around 30 years by SSCS company. It
causes no damage to the wind turbine
structure during the installing stages.
Furthermore, the FM system provides natural
habitat which is the homes for a huge amount
of sea creatures.

Figure 2-19: Scour Control System (SSCS)

The FM system does not require maintenance because they are “seft-maintaining.”
Furthermore, the frond could be tied with the concrete mattress, grout bag or anchored to
the sea bed by using duckbill anchors.
The other options are using the concrete mattresses in order to change the sea floor’s
surface (Figure 2-20). The ACM system could provide the protection and stabilization of the
protected objects, scour protection, being a supports or the foundation for the subsea
activities, and so on. However, this option is a relatively high cost due to the prefabricated
and construction process.

Figure 2-20: Concrete mattress installation

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e. Rubber Derivatives (RD)
One of the most advanced solutions
for scouring effect is rubber derivatives
system (Figure 2-21). The RD system
composes from the recycled tires and the
polypropylene rope, which are durable and
low-cost material. The Scour Protection
group is developing this solution also the
supply chain program for the rubber
derivatives system. The benefits of the RD
programs is undeniable; because of the lowcost material has been used, the
Figure 2-21: The scour prevention system by
environmentally friendly product for flora
using recycled rubber tire
and fauna around structures, the ease of
installation, and so on. (Scour Prevention,
2013)
After placing the RD system on top of
the sea bed, the tires adding friction to the
sea floor which decreasing the velocity of
current. Furthermore, the tires trap the
sediments inside tire spaces, creates a
reinforcement layer on top of the sea bed
and thus precluding the scouring effect
(Figure 2-22).
At some specific period of serving-lifetime of the wind farm, the rubber mattress

have to be moved in order to access the
cable, or they have to be removed
permanently when decommissioning the
site.

Figure 2-22: The installation of rubber
derivative system

The report from 2013 of Scour prevention group showed that the rubber derivative
works more efficient and easier to lift compared to the concrete mattresses.
2.4

Wind Conditions

Wind speed will change with time, as well as depends on the air density, the height
above the ground or mean sea level. Thus, the 10-minute mean wind speed U10 at the height
10m and the standard deviation 10 of the wind speed at 10m above the ground/sea surface
are the basic commonly reference to introduce a wind model (the wind speed profile,
turbulence, wind spectrum, and so on) – (DET NORSKE VERITAS, 2010). Besides, there are
two different type of wind conditions: normal wind conditions and extreme wind conditions.
The normal wind conditions focus on the repeated load models for fatigue limit designs,
while the extreme wind conditions consider the unusual/extraordinary environment
conditions. This report will concentrate on the extreme wind load case in order to analyze
the maximum displacement and stress/strain of the support structure.

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