ANCHOR GEOTECHNICS IN FLOATING

OFFSHORE WIND

SEMINAR REPORT

submitted by

PAVITHRA ANIL
in partial fulfilment of the requirements for the award of degree of

BACHELOR OF TECHNOLOGY

in

CIVIL ENGINEERING

DIVISION OF CIVIL ENGINEERING

SCHOOL OF ENGINEERING
COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY
Cochin, Kerala, India PIN 682022
2024
 CERTIFICATE

This is to certify that this is a bonafide record of the seminar report entitled “ANCHOR

GEOTECHNICS IN FLOATING OFFSHORE WIND” submitted by PAVITHRA

ANIL in partial fulfillment of the requirements to the award of Master of Technology Degree in
Civil Engineering of Cochin University of Science and Technology.

Course coordinator
DR BINDU CS Head
DR GLORY JOSEPH Civil Engineering Division
DR DEEPA BALAKRISHNAN
 Abstract

Floating offshore wind has the potential to unlock vast expanses of

previously untapped wind energy resources in deep offshore locations. Here a
detailed exploration of the dynamic and evolving field of floating offshore wind
technology, anchor geotechnics, and the critical consideration of liquefaction’s
impact on drag-embedded anchors. With a growing emphasis on renewable energy
sources, offshore wind power has emerged as a transformative solution. Floating
offshore wind systems, capable of harnessing wind resources in deeper waters, stand
at the forefront of this transition.The first section provides an in- depth overview of
floating offshore wind technology, highlighting its potential to unlock previously
untapped wind resources and expand the reach of sustainable energy generation.
Anchor geotechnics, the focus of the second section, is pivotal in ensuring the
stability and reliability of these offshore installations. We delve into the complexities
of anchor systems, from design principles to installation techniques.The final
section scrutinizes the effects of liquefaction on drag-embedded anchors, a pressing
concern in the industry. Liquefaction’s potential to compromise anchor performance
necessitates robust mitigation strategies. In conclusion, this paper underscores the
importance of continued research and innovation in these areas to advance the
floating offshore wind sector’s resilience and efficiency, ultimately contributing to
a greener and more sustainable energy landscape.
 Table of Contents

Title Page No.

List of Figures
List of Symbols
Acknowledgements
1 INTRODUCTION
1.1 General
2 FLOATING OFFSHORE WIND
2.1 General
2.2 Floating offshore wind turbine
2.3 Components
2.4 Types of floating offshore wind turbines
2.4.1 Floating spar buoy type offshore wind turbines
2.4.2 Semi submersible platform floating offshore wind turbines
2.4.3 Barge platforms
2.4.4 Tension leg platforms (TLP)
2.5 Summary
3 ANCHOR GEOTECHNIQUES
3.1 General
3.2 Anchors
3.2.1 Types of anchor loading condition
3.2.1.1 Catenary Mooring
3.2.1.2 Taut Mooring
3.2.1.3 Tension Leg Mooring (TLP)
3.2.2 Anchor design specifications
3.2.3 Types Of Anchors
3.2.3.1 Gravity Type Anchors
3.2.3.2 Pile type anchors
3.2.3.3 Plate Type Anchors
3.2.3.4 Composite Type Anchors
3.2.3.5 Group Type Anchors
3.2.4 Innovations In Anchor Technology
3.3 Summary
4 EFFECT OF LIQUEFACTION FOR DRAG EMBEDMENT ANCHOR
4.1 General
4.2 Seabed liquefaction
4.2.1 Momentary Liquefaction
4.2.2 Residual Liquefaction
4.2.2.1 Mechanism Of Residual Liquefaction
4.3 Sinking failure of DEA’s due to wave induced liquefaction
4.3.1 A mathematical model for wave induced residual liquefaction.
4.3.2 A mathematical Model For Sinking Of DEA In Liquefied Soil.
4.3.3 A mathematical Model For Upward Progression Of Compaction Front.
4.3.4 Implementation of integrated model: A numerical example
4.3.5 Inference
4.4 Summary
5 CONCLUSION
REFERENCES
 List of Figures

Figure 1.1 Floating offshore wind turbines

Figure 2.1 Components of floating offshore wind
Figure 2.2 Spar Buoy Type Offshore Wind Turbines
Figure 2.3 Semi Submersible Type Offshore Wind Turbines
Figure 2.4 Barge Platform
Figure 2.5 Tension Leg Platforms (TLP)
Figure 3.1 Example Of Loading Conditions Of Various Mooring Systems
Figure 3.2 Gravity anchors
Figure 3.3 Drag embedment
Figure 3.4 Stevshark Rex
Figure 4.1 The mechanism of wave-induced oscillatory pore pressure
Figure 4.2 Elastic deformation of seabed soil under a progressive wave
 List of Symbols

ρliq density of liquefied soil

ø angle of repose
ѵs poisson’s ratio
AA area of DEA
C concentration of sediment of liquefied soil
CD drag coefiicient
d50 grain size
d soil depth
DA size of DEA
Dn gross nominal spherical diameter
Dr density index
emax maximum void ratio
emin minimum void ratio
ev void ratio of soil
G shear modulus
h water depth
Hrms significant wavelength
k soil permeability
k0 coefficient of lateral earth pressure
L wave length
n1 porosity
s specific gravity of sediment grains
ReA Reynold’s number
tult time for sinking of DEA
Tz zero-up crossing period
UA sinking velocity
UC compaction front
v kinematic velocity
VA volume of DEA
WA weight of DEA
z0 Initial position of DEA
zult ultimate sinking depth of DEA
 Acknowledgements

This seminar work is the product of hard work and experience and it goes a long way in shaping
a person in his respective profession. If words can be considered as a token of acknowledgment
and symbols of love, then these words play a vital role in expressing my gratitude.

First of all, I’m thankful to God Almighty, for his choicest blessings for the successful
completion of my seminar. I would also like to express my sincere thanks to Dr. Sobha Cyrus,
The Principal, CUSAT for the kind support throughout the completion of this venture. With
greater respect, I express my sincere thanks to Dr. Deepa Balakrishnan, HOD in charge,
Department of Civil Engineering, and Dr. Bindu C S, Department of Civil Engineering and Dr.
Glory Joseph, Department of Civil Engineering, Seminar Coordinators, for providing me an
opportunity to present this seminar.Last but not least, I am grateful to my friends and parents
for their valuable motivation and support in making this seminar a success.
 CHAPTER 1
INTRODUCTION

1.1 GENERAL

Many countries have set net zero emissions goals as part of their efforts to
combat climate change. These goals typically reducing greenhouse gas emissions
to a level where they are balanced by removals or offsets, effectively achieving a
state of no net increase in atmospheric CO2 achieving net zero emissions requires
a significant shift away from fossil fuels, increased use of renewable energysource,
energy efficiency improvements and carbon capture and storage technologies .

Floating offshore wind is of paramount importance in the global effort to

achieve net zero emissions. This technology offers a unique solution to the
challenges of transitioning to a sustainable energy future. One of its primary
advantages is the access it provides to strong and consistent wind resources. Unlike
onshore wind turbines or fixed bottom offshore turbines, floating platforms can be
situated in deeper waters, where the wind blows consistently. This means they can
generate a more reliable and substantial amount of electricity, making them a
valuable source of renewable energy.

Even though floating offshore wind technology holds tremendous promise for
expanding the reach of renewable energy generation into deep offshore waters, faces
several challenges and stability is one of them. Floating offshore wind turbines operate
in deep waters where they are subjected to strong winds, high waves and unpredictable
sea conditions. These environmental forces can lead to platform motion, which, if not
effectively managed, can affect the structural integrity of the turbines and hamper
their energy generation capacity. Ensuring stability under these conditions requires
advanced engineering solutions. The selection of anchor types and the geotechnical
assessment of the sea bed must be accurate and site specific to provide the
necessary stability. Poorly designed mooring system can result in platform
movement and even potential risks to thesafety of offshore personnel.

Figure 1.1 Floating offshore wind turbines

 CHAPTER 2

FLOATING OFFSHORE WIND TURBINES

2.1 GENERAL

The pursuit of sustainable energy solutions has led to remarkable

advancements in the renewable energy sector, with offshore wind power emerging
as a promising frontier in this endeavor. Among the innovative developments in
offshore wind technology, floating offshore wind systems have garneredsignificant
attention for their potential to harness wind resources in deeper and more remote
ocean locations. A deeper knowledge about them is necessary as they are the
stepping stone towards a sustainable future. This chapter gives a detailed
explanation about floating offshore wind turbines and its types.

2.2 FLOATING OFFSHORE WIND TURBINES

Floating offshore wind represents a ground breaking advancement in the

renewable energy sector, offering a unique solution to harnessing wind energy in
deeper offshore waters. This technology has the potential to significantly expand
our capacity to generate clean electricity, reduce carbon emissions, and transition
towards a more sustainable energy future.

Traditional offshore wind turbines are firmly anchored to the seabed in

relatively shallow waters. While this approach has been successful in harnessing wind
energy in certain regions, it presents limitations when it comes to areas with deeper
waters. This is where floating offshore wind steps in. Floating platforms, equipped with
towering wind turbines, are designed to operate in water depths of hundreds of meters.
This capability unlocks vast expanses of untapped wind resources located farther
from the shore, where winds tend to be stronger and more consistent.
 One of the primary advantages of these turbines is its potential to access
abundant wind resources in deep water. As the world seeks to transition away from
fossil fuels and reduce greenhouse gas emissions, tapping into these offshore wind
resources becomes crucial. Traditional fixed bottom offshore wind installations are
limited by water depths, eliminate these limitations, making it possible to harness
wind energy in locations that were previously inaccessible.

FOWT also have the advantage of being less intrusive to the marine
environment compared to their fixed bottom counterparts. Traditional installations
often require substantial sea bed disruption during construction, which can harm
local ecosystems and marine life. In contrast, floating platforms have a smaller
environmental footprint since they do not necessitate extensive seabed anchoring.
This makes them a more environmentally friendly option and reduces the potential
for negative ecological impacts.

Another benefit of FOWT is its adaptability. These systems can be relocated

to different areas, allowing for better utilization of wind resources and the ability to
respond to changing wind patterns. This flexibility enhances the reliability and
efficiency of energy production, a crucial aspect of integrating renewable energy
into the grid.

Despite these advantages, floating offshore wind faces several challenges

that need to be addressed for widespread adoption. One of the primary challenges
is the development of reliable and cost-effective floating platforms. Unlike fixed-
bottom structures that are firmly anchored to the seabed, floating platforms must
contend with the dynamic forces of waves and currents. Designing stable and
resilient platforms that can withstand these conditions while supporting massive
wind turbines is difficult.

Cost is another significant challenge. At present, floating offshore wind is

more expensive than onshore or fixed-bottom offshore wind installations. The
development, manufacturing, and deployment of floating platforms require
substantial investments. However, as the technology matures and economies of
 scale are realized, costs are expected to decrease, making floating offshore wind
more competitive.

2.3 COMPONENTS

The floating offshore wind turbine system is functionally divided into four
parts: wind turbine, tower and transition piece, floating foundation, and mooring
system (see Figure.2.1).

Figure 2.1 Components of floating offshore wind

 Wind turbine: The wind turbine is mainly composed of a rotor system, nacelle, and
yaw system, among which the internal components of nacelle are crucial and
complicated.
 Tower and transition piece: The tower is used to support the wind turbine
components. The transition piece performs the function of maintaining an accurate
position of the tower. The supporting role, tower, and transition piece are also
expected to resist force and moment produced by the variable wind, wave, and
current loads.
 Floating foundation: The main types of floating offshore wind turbines under
development are the Semi-Submersible, Spar, and Tension Leg Platform (TLP).
 Mooring system: The mooring systems for floating offshore wind turbines have
benefited from the offshore oil and gas experience. The main mooring
configurations are the Catenary system and the taut leg system.

2.4 TYPES OF FLOATING OFFSHORE WIND TURBINES

Based on the floating platform used, floating offshore wind turbines are
classified as: (a) floating spar buoy type offshore wind turbines(FSOWT) (b) semi
submersible type offshore wind turbines (c) barge type (d) tension leg platform
(TLP).

2.4.1 Floating Spar Buoy Type Offshore Wind Turbines

Spar platforms are a type of offshore structure primarily stabilized by gravity.

They are characterized by a single vertical cylinder design with ballast at the
bottom. This ballast provides the necessary stability by lowering the center of
gravity. The wind turbine tower is directly connected to this vertical cylinder. Spar
platforms are effective in deep waters, as their design allows them to remain
relatively stable even in challenging offshore conditions. Their simplicity and
stability make them a preferred choice for some offshore wind installations.
Figure.2.2 shows a spar buoy type floating offshore wind turbine.

Figure 2.2 Spar Buoy Type Offshore Wind Turbines

2.4.2 Semi Submersible Type Offshore Wind Turbines

Semi-sub platforms employ a combination of water plane and gravity

stabilization mechanisms. They typically consist of three to five vertical cylinders
connected together, forming a buoyant structure. The wind turbine can be located
in the center or above one of the columns. This design provides stability through
buoyancy (water plane) and additional stability from the weight of the structure
(gravity). Semi-subs are versatile and can operate in various water depths, offering
a good compromise between stability and adaptability. Figure.2.3 shows a semi
submersible type offshore wind turbine.

Figure 2.3 Semi Submersible Type Offshore Wind Turbines

2.4.3 Barge Platforms

Barge platforms rely on water plane stabilization. They feature a floating,

shallow, and wide platform design that utilizes the buoyancy of the platform's large
water plane area to counteract forces and maintain stability. Barge platforms are
typically used in shallower waters and offer a cost-effective solution for offshore
wind installations. Their relatively simple design and ease of construction make
them a practical choice for certain locations and applications. Figure 2.4 shows a
barge platform.
 Figure 2.4 Barge Platforms

2.4.4 Tension Leg Platforms(TLP)

Tension Leg Platforms achieve stability through mooring. These platforms

consistof a submerged body or buoyant hull connected to the seafloor by tensioned
mooring lines. The tension in these lines keeps the platform in place, effectively
countering wave and wind forces. A central column connects the submerged body
to the wind turbine tower above the water's surface. TLPs are suitable for deepwater
locations and are known for their stability and minimal motion, making them ideal
for sensitive equipment like offshore oil rigs or wind turbines. Figure.2.5 shows a
tension leg platform floating offshore wind turbine.

Figure 2.5 Tension Leg Platforms (TLP)

 2.5 SUMMARY
Floating offshore wind represents a game-changing technology in the
renewable energy landscape. Its ability to harness wind resources in deep waters,
coupled with its environmental benefits and adaptability, makes it a compelling choice
for sustainable energy generation. As costs continue to decrease and technology
advances, floating offshore wind has the potential to revolutionize the way we
generate clean energy, contributing significantly to a greener and moresustainable
future for our planet.
 CHAPTER 3
ANCHOR GEOTECHNIQUES

3.1 GENERAL

Anchor geotechnics holds paramount importance in floating offshore wind

turbines. It is critical for safety. Understanding seabed conditions and anchor
systems is crucial to prevent accidents during installation and maintenance. It also
ensures structural integrity. Knowledge in this field enables the design of anchor
systems capable of withstanding dynamic forces, safeguarding against damage
and costly failures. It also enhances cost efficiency by optimizing anchor designs,
reducing unnecessary expenses during installation and maintenance. It aids in
minimizing environmental impact, helping to protect marine ecosystems and
habitats. Anchor geotechnics is pivotal for optimizing turbine performance,
ensuring maximum energy production.

3.2 ANCHORS
Floating offshore wind turbines rely on anchors as crucial components of
their mooring systems. Unlike traditional fixed turbines, which use foundations like
monopiles or jackets, floating turbines are deployed in deep waters where anchoring
directly to the seabed is impractical. Anchors in this context are massive structures
strategically positioned on the seabed. Their primary purpose is to prevent the
floating platform from drifting due to the dynamic forces of wind, waves, and
currents. These anchors connect to the floating platform throughrobust mooring
lines, typically thick cables or chains. These mooring lines transmit tension,
ensuring the turbine remains securely in place. Anchors are vital for stability and
operational efficiency, allowing the floating turbines to function effectively. In
some cases, dynamic positioning systems with thrusters further aid in maintaining
position. Nevertheless, anchors are the foundation of this offshore energy solution,
providing the initial stability necessary to harness wind power in deep offshore
locations, contributing to the growth of sustainable energy production.

3.2.1 Types Of Anchor Loading Condition

The magnitude and inclination of the load transferred to an anchor depends

on the mooring type, which generally falls into three categories: (i) catenary, (ii)
taut (or semi-taut) and (iii) tension leg.

The load on a mooring system is crucial for design and safety considerations.
The design mooring load is determined through simulations of the floating system.
In these simulations, the anchor is usually modeled as a fixed point, neglecting
potential loss of pretension due to anchor displacement.

3.2.1.1 Catenary Mooring`

Catenary mooring is the most common type used. It relies on the weight of
additional chain laid on the seabed to generate the restoring force to keep the turbine
in position. The mooring line follows a catenary curve along the seabed, and the
anchor connection point is often buried below the seabed. The shape of the inverse
catenary in the soil causes an inclined load on the anchor. This load can vary
depending on the interaction between the chain and seabed.

3.2.1.2 Taut Mooring

Taut mooring systems use synthetic rope instead of chains. The restoring
force in this case comes from the non-linear stiffness of the rope. These lines are
pre-tensioned to intercept the seabed at an angle, resulting in a more inclined load
compared to catenary mooring. Taut mooring systems are of interest for floating
wind turbines because they can potentially reduce the length and footprint of the
mooring system.

3.2.1.3 Tension Leg Mooring (TLP)

Tension leg moorings consist of a vertical taut line with

significant pre-tension.They apply a nearly vertical and substantial load to the
anchor. TLPs are often used for larger wind turbines, and their maximum mooring
load can be significantly higher than that of taut or catenary moorings.

Figure 3.1Example Of Loading Conditions Of Various Mooring Systems.

Fig.3.1 shows an example of loading conditions of various mooring systems. For 5

MW floating turbines, catenary and taut mooring systems typically experience
maximum loads in the range of 2–6 MN. Load inclination at the anchor pad eye
varies, with catenary moorings typically having an inclination between 0 and 15
degrees, while taut line moorings have inclinations between 30 and 45 degrees.
Tension-leg moorings apply larger and nearly vertical loads to the anchor, with
maximum loads for 5 MW turbines reaching up to 16 MN, along with a constant
 pre-tension of 8 MN. For larger turbines like 10 MW, tension-leg moorings can
experience maximum loads of up to 35 MN.

3.2.2 Anchor Design Specifications

An anchoring system for floating offshore wind has the following twoprimary
performance requirements.
 Capacity: the anchor design capacity must exceed the design value of the mooring
load applied via the attached mooring line throughout its entire design life. The
design capacity may be influenced by cyclic and other effects that depend on time
and the applied loading history and may also include an additional component from
the seabed resistance (‘friction’) against the attached embedded mooring chain.
Load and resistance factors are applied to the design loads and capacity according
to the relevant design codes.
 Installability : the anchor must be reliably installable in the local seabed conditions,
to the embedment depth at which the required capacity is available.

3.2.3 Types Of Anchors

Many anchor types, which differ in shape and installation method, exist for
offshore applications. Despite their differences, they share some similarities and

can broadly be classified into three families as a function of the volume of soil
they mobilise to resist loading from the mooring line: (a) gravity-type, (b) pile- type
or (c) plate-type anchors.

3.2.3.1 Gravity Type Anchors

Gravity-type anchors, also known as surface anchors, play a crucial role in

mooring systems for various offshore structures, including floating wind turbines.
They are heavy weight laid onto the seabed and have zero or minimal penetration
into the ground due to their self weight. This design makes them quickly
operational. When subjected to vertical uplift forces, the holding capacity of a
 gravity anchor is determined by the buoyant weight of the foundation and any
ballast added to it. However, this method is material-intensive and less efficient in
terms of tonnage of material used compared to other anchor options. For horizontal
loading, gravity anchors resist movement due to the combination of vertical buoyant

self-weight and horizontal forces from the mooring lines. Lighter anchors may
experience limited resistance and slide along the anchor-soil interface. To enhance
their performance, short ribs or grilles can be added underneath the anchor to
promote soil-soil shearing. Gravity anchors come in various types, such as large
precast concrete or steel elements, ballasted boxes, or grillages. Large precast
elements are quicker to install but heavy due to being pre- cast onshore. Box anchors
comprise an empty box that is filled with heavy ballast after being lowered to the
seafloor, reducing crane lift capacity requirements. An alternative is the grillage and
berm or ballast gravity anchor, consisting of a flat grillage placed on the seafloor
and buried by a rock-fill or iron ore berm. Grillagesuse less steel than other options
and leverage slight embedment to mobilize soil- soil friction at failure, enhancing
their overall performance.

Figure 3.2 Gravity anchors (a) Precast concrete anchor, (b) box anchor, and (c)grillage and berm
anchor
3.2.3.2 Pile type anchors

Pile-type anchors are embedded into the seabed but extend to the
surface. They resist axial load via interface shear mobilization and lateral load by
soil bearing mobilization. Anchor piles are critical components in offshore
foundation systems, commonly used to secure various structures, including wind
turbines, to the seabed. These piles are hollow, cylindrical steel tubes that come in
different sizes and are installed through various methods, such as driving, pushing,
drilling, grouting, or suction. For vertical uplift forces, anchor piles rely on their
buoyant weight, and their dimensions (diameter, wall thickness, length) are
carefully chosen to provide the required capacity. The installation method and
seabed conditions influence these dimensions. Repetitive driving of piles can lead
to friction fatigue but typically recovers over time. In layered soil or rock
conditions, a combination of driving and drilling may be used, sometimes requiring
grouting.In rock, piles can be grouted into drilled holes. Suction piles, alternatively
called suction caissons, are another type of anchor. They have large diameters and
are installed by creating a pressure difference beneath them, allowing them to
penetrate the seabed. Piles can be axially loaded in tension, utilizing the shear
strength along the shaft interface, and longer piles benefit from increased soil
strength with depth. Horizontally loaded piles mobilize a large volume of soil
against lateral displacement. Caissons also mobilize soil but are optimized for
lateral translation or rotation based on the depth of their attachment point. In recent
projects, there have been concerns about anchor capacity reduction due to soil
erosion caused by anchor chain movement, particularly in the vicinity of the pad-
eye attachment point.

3.2.3.3 Plate Type Anchors

Plate-type anchors are engineered to be buried in the seabed and use the
surrounding soil to resist applied forces. The mechanism they employ depends on
their depth and shape. When these anchors are placed close to the seabed's surface,
they tend to wedge the soil upward, extending to the surface. This is referred to as
a shallow mechanism. Alternatively, when plate anchors are embedded deeper, they
create a flow-around mechanism, and this depends on their specific shape and is
termed as a deep mechanism. Drag Embedment Anchors (DEA) consists of a plate,
often called a fluke, and attached to a shank where the mooring line is connected.
DEAs are commonly used for catenary moorings. Theyare dragged into the seabed,
and their depth of penetration and holding capacity are determined by factors such
as their geometry and the interaction between the anchor, the mooring line, and the
soil. Vertically Loaded Anchors (VLA) share similarities with DEAs, but they can
adapt to carry inclined or vertical loads through a release mechanism. This feature
makes them suitable for applications like taut or tension-leg moorings. Directly
Embedded Plate Anchors consist of a single plate, typically rectangular or circular,
that is embedded at a specific depth in various ways, such as via a suction caisson,
driven pile, or dynamic embedding. These anchors need to rotate, or key, to align
them with the direction of the mooring line to maximize their holding capacity. A
significant challenge with plate anchors is ensuring they achieve sufficient
embedment during installation or predicting the potential loss of embedment and
remolding due to the rotation process, which can reduce their ability to resist forces.
Embedment loss can vary but may range from 0.5 to 2.0 times the height of the
plate, depending on several factors.

Figure 3.3 Drag embedment

3.2.3.4 Composite Type Anchors

Composite-type anchors are a combination of plate and pile elements. Screw

piles consist of a steel shaft with helices and are installed by screwing them into the
seabed, reducing underwater noise compared to hammering. They resist tension by
mobilizing soil bearing resistance above the top helix, similar to plate anchors.
When subjected to lateral loads, screw anchors behave like straight- shafted piles
with some enhancement due to helical rotation. The plate resistance can be fully
utilized if the pile's rotation aligns with the loading direction. One challenge for
offshore use is up scaling screw piles, requiring tools capable of applying large
torques, which can be overcome by using groups of smaller screw piles.
Dynamically embedded torpedo or fish-like anchors also exhibit a combination of
pile and plate behaviors due to added fins. This design provides versatility in
responding to different load directions and seabed conditions.

3.2.3.5 Group Type Anchors

Group-type anchors consist of multiple plate or pile-type elements working

together to provide anchoring capacity. Small groups of pile-type foundations offer
high capacity without the need for large installation equipment. In some soils, pin
piles can be installed using dead weights and connected via surface foundations or
locating sleeves. Design methods exist to estimate the combined capacity of these
hybrid foundations. The anchoring capacity of pile groups depends on the level of
moment fixity at the pile tops and group interaction effects. Interaction can be
positive, where neighboring piles reinforce each other or negative, involving
overlapping soil zones or block failure mechanisms. Small- scale pile group tests
help identify these interactions and optimize group arrangements. Group-type plate
anchors have also been explored. Multiple drag anchors attached in series along a
mooring line, known as a piggyback approach, can be highly efficient. When spaced
2-5 anchor lengths apart, the combined capacity exceeds double that of a single
anchor due to interaction effects. These group type anchors offer versatility and
enhanced anchoring performance.
3.2.4 Innovations In Anchor Technology

The embedded ring foundation is a novel approach derived from the suction
caisson, designed for shared anchor applications. After installation, the upper
section of the caisson is removed through pumping, leaving the lower section
embedded at depth. Multiple pad eyes with anchor line stubs encircle the ring,
allowing it to withstand loads from any direction due to its symmetrical design. This
innovative anchor combines plate-like resistance for lateral loads and pile like
resistance for vertical loads, making it versatile and well-suited for a variety of
offshore anchoring needs.

To enhance their effectiveness in challenging seabed conditions, variants of

drag embedment anchors have been developed. In regions where hard or cemented
layers are near the surface, anchors like the 'Stevshark Rex' have emerged. These
anchors feature a serrated front on the shank, improving their ability to cut through
and penetrate such challenging soil layers. This serrated design enhances their
anchoring capacity in adverse conditions, making them more versatile and capable
of securely holding structures in areas where traditional anchors might struggle due
to the seabed's hardness or compacted nature.

Micropiles, consisting of high-strength steel rods grouted into drilled shafts,

are commonly employed onshore and are now being considered for offshore
applications, typically in groups connected by a pile cap. In rocky sea beds,
micropiles are particularly appealing due to their efficiency. This efficiency arises
from the inverse relationship between their diameter and unit shaft resistance, a
result of the dilatant interface behavior. In simpler terms, smaller micropiles can
effectively resist forces in rocky sea beds, making them an attractive choice for
anchoring offshore structures in such conditions, where larger traditional piles
may be less efficient due to the sea bed's rocky nature.

Rocky sea beds offer unique opportunities to enhance the performance of

gravity foundations, which are commonly used for fixed wind turbines in such
conditions. Under base grouting, a technique used with these foundations, improves
their sliding capacity on rocky substrates. Additionally, cables on rocky sea beds
benefit from the rugged seafloor, enhancing their stability. This suggests the
potential to harness similar dilatant interface behavior for gravity-based anchors.
The naturally robust characteristics of rocky sea beds can be leveraged to improve
the effectiveness of various offshore structures, including both foundations and
mooring systems, ultimately contributing to their stability and reliability in
challenging environments.

Various anchor technologies have been explored in the past, with limited
commercial adoption. However, the emerging offshore renewable market may lead
to a renaissance in these technologies. These include inflatable anchors,
mechanically expandable anchors, active suction-enhanced foundations, and
anchors designed like fish. Additionally, new installation methods for existing
anchor types are being developed to address previous constraints. These methods
aim to reduce underwater noise during pile installation through techniques like
sequentially installed piles, axial and torsional vibration, or axial and rotary jacking.
Innovations like self-drilling and grout less anchors for rocky sea beds aim to
minimize environmental impact and expand anchor options for offshore
applications.

Figure 3.4 Stevshark Rex

3.3 Summary

This chapter gives a clear idea of the role of anchor geotechnics in the
deployment of floating offshore wind turbines. Anchoring these massivestructures
in deep waters demands innovative solutions. It delves into various anchor types,
such as the anchor piles; drag embedded anchors etc, tailored for the unique
challenges of offshore environments. This chapter also gives an overview about
innovations in anchor technology.
 CHAPTER 4

EFFECT OF LIQUEFACTION FOR DRAG

EMBEDMENT ANCHOR

4.1 GENERAL

In the dynamic realm of renewable energy, floating offshore wind turbines stand as
beacons of innovations. But beneath the waves, these engineering marvels face an
intricate challenge that is liquefaction. As the demands for clean energy surge,
understanding how liquefaction affects the stability of anchors becomes paramount.
This chapter deals with the effect of liquefaction on drag embedment anchors in the
context of floating offshore wind turbines. Understanding these complexities paves
the way for a safer, more resilient offshore renewable energy infrastructure.

4.2 SEABED LIQUEFACTION

In geotechnical terms, seabed liquefaction is a phenomenon that occurs

when the state of soil beneath the seabed reaches a condition where the effective
normal stresses between individual soil particles essentially disappear. This often
happens during cyclic loading induced by waves and currents in cohesive less sea
beds. When liquefaction occurs, the seabed’s soil-water mixture becomes fluid-like,
and the structural integrity of the soil matrix is compromised. It is a phenomenon
where the soil on seabed loses its ability to bear loads and behaves like a liquid due
to vanishing of effective stresses between individual soil grains when subjected to
wave forces. There are two mechanisms that explain how seabed liquefaction
occurs. The first mechanism suggests that liquefaction isgenerated by oscillatory
pore pressure, which exhibits noticeable periodicity. This form of liquefaction is
referred to as “momentary liquefaction.” In essence, as waves and currents cycle,
they induce changes in pore pressure within the seabed, leading to the temporary
liquefaction of the soil. The second mechanism proposes that liquefaction is caused
by the accumulation of excess pore pressure over several wave cycles. This type of
liquefaction is known as “residual liquefaction.” In this case, the seabed soil
experiences gradual changes in pore pressure with each cycle, and the excess pore
pressure accumulates over time. This accumulated excess pore pressure eventually
leads to the liquefaction of the seabed. Understanding these mechanisms of seabed
liquefaction is crucial for the design and maintenance of marine structures, as it
helps engineers and researchers predict and mitigate the risks associated with soil
instability in underwater environments.

4.2.1 Momentary Liquefaction

Momentary liquefaction is characterized by the periodic changes in pore

pressure as it oscillates in tandem with the alternating peak and trough values caused
by wave action. These changes in pore pressure also exhibit amplitude attenuation
and response delay as one move deeper into the seabed. This unique behavior makes
it particularly well-suited for capturing and studying wave- induced liquefaction
events. Momentary liquefaction tends to occur near the wave troughs, particularly in
unsaturated seabed soils. During this phase, the excess pore pressure in the seabed
becomes negative. Conversely, near wave crests, the dynamic wave pressure acts
vertically downward, leading to soil compaction inthe seabed. However, near wave
troughs, the dynamic wave pressure movesupward, which can trigger liquefaction in
the seabed soil. The critical factor hereis the vertical gradient of oscillatory pore
pressure, which generates an upward seepage force. When this excess pore pressure
surpasses the effective weight of the soil at a specific point, it can result in
momentary liquefaction. Momentary liquefaction is limited to the top layer of the
seabed and is mainly relevant to small structures like pipelines and protection
blocks.
 Figure 4.1 The mechanism of wave-induced oscillatory pore pressure

4.2.2 Residual liquefaction

Residual liquefaction is a phenomenon associated with the seabed response

to wave loading. When waves exert pressure on the seabed, this pressure varies in
a periodic manner, causing shear stresses to develop within the seabed sediment
(as shown in Fig. 6). These shear stresses, in turn, change over time as waves
propagate through the water. Over a more extended period, these changing shear
stresses can lead to the rearrangement of soil particles in the seabed. This
rearrangement reduces the pore volume between these particles. In the case of
undrained soil types like silt, the pore pressure within the sediment accumulates over
time due to the influence of these shear stresses. When this accumulated pore pressure
surpasses the initial stress within the soil, a critical point is reached where the soil
starts behaving like a liquid. This state is referred to as residual liquefaction.
This mechanism shares similarities with earthquake-induced liquefaction,
where soil temporarily loses its strength and behaves like a liquid during seismic
 events. Residual liquefaction poses a threat to the stability and integrity of marine
structures. The consequences of RL are significant. For instance buried pipelines with
a lower specific gravity than the liquefied soil may float to the seabed’s surface. In
contrast, marine structures with higher specific gravities including anchors, amor
blocks etc would sink into the seabed.

4.2.2.1 Mechanism Of Residual Liquefaction

When a progressive wave passes over the seabed, it exerts compression under
the wave crest and expansion under the wave trough as in figure 4.2. This cyclic
loading generates shear stresses within the seabed soil. These cyclic shear stresses
gradually rearrange the soil grains, compressing the pore volume and increasing
pore-water pressure, especially in undrained soils like fine sand or silt. As pore
water pressure builds up, it may surpass the overburden pressure, causing the soil
grains to become unbound and free floating, effectively turning the seabed into a
liquid like state known as residual liquefaction.

Figure 4.2 Elastic deformation of seabed soil under a progressive wave.

4.3 SINKING FAILUREOF DEA’S DUE TO WAVE INDUCED

LIQUEFACTION

For describing the sinking failure of DEA’s an integrated mathematical model

was presented by Kirca and Sumer. Three components of the integrated model:

1. A mathematical model for wave induced residual liquefaction.

2. A mathematical model for sinking of DEA in the liquefied soil.

 3. A mathematical model for post liquefaction compaction process with the
focus of determining theupward progression of the compaction front.

4.3.1 A mathematical model for wave induced residual liquefaction.

The seabed soil is subject to a progressive wave with a wave height and a
wave period. The conditions are that the pore-water pressure builds up under the
cyclic action of the wave. With the introduction of waves, the pore-water pressure
builds up, attains a maximum value, subsequently falls off, and is eventually
completely dissipated.

The present model is concerned with the initial stage where the pore-water
pressure builds up until the point where the onset of liquefaction is reached. The
model consists of the following five elements: (1) An equation for accumulated
pore-water pressure; (2) A source term in the equation for accumulated pore-water
pressure; (3) Number of cycles to cause liquefaction; (4) Shear stress in soil by
waves (5) The solution of equation for the accumulated pore-water pressure The
fourth element involves the analytical solution was obtained from Biot’s Equation.

When the wave properties (wave height, wave period, and water depth) and
soil characteristics are given the model returns: (1) whether soil is liquefied; and (2)
the liquefaction depth sly if soil is liquefied. In shallow soils (a small soil depth
Dim figure 4.2), it may turn out that zL = d, meaning that liquefaction spreads
across the entire soil depth down to the impermeable base in such cases.

4.3.2 Mathematical Model For Sinking Of DEA In Liquefied Soil bed

When soil is liquefied, it behaves like a very dense liquid. The specific
gravity of this dense liquid is around 1.8- 2.0, depending on the initial soil specific
gravity and the coefficient of lateral earth pressure. As the soil is liquefied, DEA
starts to sink due to its large specific gravity of around 7.5, a value considerably
larger than that of liquefied soil. The force balance equation for a sinking DEA.
 -----------(1)

Where WA, VA, and AA are the weight, volume, and projection area of DEA,
respectively, ρliq is the density of liquefied soil, Cd is the drag coefficient
associated with the sinking of DEA in liquefied soil, and UA is the sinking velocity
of DEA. The equation in the left hand side is the weight of DEA minus the buoyancy
force on DEA and the right hand side is the drag force due to sinking motion of
DEA in liquefied soil. The tension force on DEA prior to sinking is assumed to be
nearly horizontal. This assumption is reasonable because the high holding capacity
anchors, such as DEA, cannot allow any significant vertical uplift forces without
reducing their efficiency. The model essentially solves the above equation for UA,
the sinking velocity of DEA. But in order to solve the equation the drag coefficient
Cd.

4.3.3 A mathematical Model For Upward Progression Of Compaction Front

Liquefaction is followed by the compaction process, which starts from the

bottom of the liquefied soil column and progresses upwards. The sinking of DEA
terminates when DEA in its downward motion meets the compaction front. Sumer's
compaction model gives the expression for the compaction front velocity Uc as
follows:

(4)

Where c is the concentration of sediment of liquefied soil, n1 is the porosity

of compacted soil, and w0 is the fall velocity of soil grains in clear water. n1 is
taken as approximately related to minimum void ratio emin.

(5)
 W0 calculated from the classic fall velocity expression. The value of Uc
appears between the range 1x10^-4 -1x10^-3 m/s.

4.3.4 Implementation of integrated model: A numerical example

The properties of DEA are given as (1) DEA model: Stevris Mk6-6 ton; (2)
material: cast iron; (3) weight: WA = 58.9 kN; (4) projection area: AA = 8.5 m2 ;
(5) gross nominal spherical diameter: Dn = 1.2 m; (6) volume: VA = 0.77 m3 ;and
(7) penetration depth: z0 = 4.0 m.
As for wave climate, the waves in the return period of 250 years are taken in the
present numerical example. The properties of the waves are (1) significant wave
height: Hrms = 7.3 m; (2) zero-up crossing period: Tz = 12.5; and (3) water depth:
h = 60 m.
The soil parameters used in the numerical example are (1) soil depth: d =7.5 m
(Figure. 4.2);(2) density index: Dr = 0.35, Dr=(emax-ev)/emax-emin) with ev denoting the
void ratio of soil; (3) grain size: d50 = 0.070 mm; (4) minimum void ratio of soil: emin
= 0.6; (5) maximum void ratio of soil: emax = 1.2; (6) specific gravity of sediment
grains: s = 2.65; (7) angle of repose of the soil: ø = 38 (8) coefficient of lateral earth
pressure: k0 = 0.384; (9) Poisson ratio: ѵs = 0.278; (10)shear modulus of elasticity: G
= 104 kPa; and (11) soil permeability: k = 105 m/s.
For a given set of wave and soil properties, the first component of the integrated
model, namely the mathematical model for residual liquefaction, is utilized. The
results indicate that the soil is liquefied, with the liquefaction depth equal to the entire
soil depth, meaning that liquefaction spreads over the entire soil depth.
The implementation of the rest of the model may be summarized as follows:
(1) work out the time series of the liquefaction front progressing downwards,
using the mathematical model for wave-induced residual liquefaction, the first
component of the present integrated model (Section 4.3.1)
(2) likewise, work out the time series of the sinking of the DEA anchor, utilizing the
mathematical modelfor sinking of DEA, the second component of the integrated model
(Section 4.3.2)
(3) work out the time series of the compaction front, using the mathematical model
for upward progression of compaction front, the third component of the integrated
model (Section 4.3.3) and
(4) determine where and when the latter two time series meet, thereby determining the
ultimate sinking depth of DEA (zult) and its associated time (tult).
The calculations show that DEA starts to sink once the liquefaction arrivesat z = z0
= 4 m, the initial position of DEA (z0). The structure (DEA) continues to sink until it
meets the compaction front, which is moving upwards. The calculations indicate that
this happens 11 min after DEA starts to sink.

4.3.5 Inference

From the above model researchers noted that the motion of a Deep
Embedment Anchor (DEA) doesn't remain steady throughout its entire sinking
process. Instead, it reaches a steady state with a nearly constant velocity after
initiating the sinking. Similarly, this steady-state motion persists until sometime
before it encounters the compaction front. This behavior suggests that the DEA's
motion is not steady near the beginning and the end of its descent. Here heavy
objects sinking in liquefied seabed soil can be approximated as moving steadily
from the onset of sinking until the termination of the downward motion. This
finding simplifies the analysis of anchor behavior in these conditions.

Another point was when dealing with very large water depths and thepresence
of deep-water waves, a critical relationship is defined by h = L^ (1/2), where h
represents the water depth, and L is the wave length. Under such circumstances, the
pressure generated by the waves does not penetrate all the way down to the seabed.
Consequently, the cyclic shear stresses and strains in the soil, typically induced by
wave-induced pressure, cease to exist. In this scenario, the soil does not experience
wave-induced liquefaction. However, it’s important to note that other types of
liquefaction, such as those triggered by seismic events like earthquakes, may still
affect the seabed even in these deep- water wave conditions.

These observations highlight the complex interplay of factors that influence

anchor motion in liquefied sea bed soil. The transition to a steady-state sinking
motion, the proximity of the anchor’s ultimate location to the seabed base, and the
dependence on water depth and wave characteristics all contribute to our
understanding of how heavy objects, like anchors, behave in these challenging
marine environments. This knowledge is crucial for the design and engineering of
anchoring systems and offshore structures to ensure their stability and integrity in
regions prone to seabed liquefaction.

4.4 SUMMARY

This chapter gives a clear idea about the effect of liquefaction on drag
embedment anchors with help of an integrated mathematical model. Implementation
of the model using a numerical example is also reviewed in this chapter. As the
floating offshore wind turbines are emerging day by day, It's crucial to
understand and address the risks associated with seabed liquefaction, especially
when designing and installing marine structures. Residual liquefaction can have
detrimental effectson the stability of these structures, potentially leading to costly
damage or failure if not properly accounted for in engineering and construction
practices. Therefore, researchers and engineers in the offshore industry must
consider these factors to ensure the safety and longevity of Marine installations in
areas prone to seabed liquefaction.
 CHAPTER 5

CONCLUSION

This paper has provided a comprehensive overview of key aspects within

the field of floating offshore wind and anchor geotechnics. This paper gives an idea
of the promising realm of floating offshore wind technology, highlighting its
potential to revolutionize renewable energy generation by harnessing wind
resources in deep- sea locations. Additionally, it also provides an investigation into
anchor geotechnics in floating offshore wind systems which has emphasized the
critical role of anchor systems in ensuring the stability and safety of these
installations. Furthermore, the examination of the effect of liquefaction on drag-
embeddedanchors has shed light on a crucial concern in the industry. Liquefaction
poses a significant challenge to the performance of anchors, potentially
compromising their ability to secure floating offshore wind structures. This
highlights the need for ongoing research and innovation in anchor design and
installation methods to mitigate liquefaction-related risks effectively. In essence,
this paper underscores the importance of continued research and development
efforts in the floating offshore wind sector, with a specific focus on anchor
geotechnics and strategies to address liquefaction concerns. As this field is
advancing day by day, there will be deployment of more resilient and efficient
floating offshore wind farms, contributing to a cleaner and more sustainable energy
future.
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