1. Introduction
1.1 Background on wind energy
With climate change becoming an increasingly worrying threat to our plant, and
with ever growing global demand for energy, the need for clean and renewable sources
is of paramount importance. A large volume of research on renewable energy throughout
modern history has uncovered a wide range of possible energy sources.
Although the idea of harnessing power from wind has been around for centuries,
recent investment and research into wind energy has made it a reliable and relatively
mature technology when compared to other forms of renewable energy. In order for
wind energy to compete with, and even replace, fossil fuels, it has to undergo a lot of
technological development to overcome current issues facing it in order to reduce the
cost of energy production.
1.2 Wind energy
Some of the main issues facing onshore wind power nowadays are its adverse
environmental impacts such as visual and noise pollution and its need for large areas of
land, preferably far away from people. These factors limit the potential of wind power
significantly.
Placing wind farms offshore mitigates the visual and environmental impacts and
improves their social acceptance (Aghaalikhani et al, 2016). In addition, placing wind
farms offshore provides access to continuous higher speed, unobstructed wind that
results in greater power generation when compared to their onshore counterparts.
Typical offshore wind turbines use a fixed base foundation, such as monopiles or
gravity bases. This means that they are usually limited to depths of around 20 m
(Butterfield et al, 2005). This limits the availability of potential sites for wind farms.
Going further offshore provides access to even higher wind speeds and more
potential sites for establishing larger wind farms capable of producing even more energy
than current shallow offshore sites. A result of going further offshore is the increasing
water depths. This creates the need to develop new technologies to support these
turbines in deeper water.
1.3 Floating offshore wind turbines
Floating platforms are attracting a lot of interest from the industry as they allow
access to waters with greater depths. The first full scale floating farm to begin operation
is the Hywind wind farm of the coast of Scotland back in 2017. It is comprised of 5 full
scale 6 MW turbines that can power around 36,000 households. This project proves the
feasibility of this new technology at a full working scale and provides the industry with
much needed knowledge on the challenges faced during the construction and operation
of such farms.
Floating offshore platforms can be divided into 3 groups, characterized by the method
used to provide stability, these are: mooring line tension support (tension leg platform),
ballast support (spar-buoy), and buoyancy support (barge). These platforms are
anchored to the seabed using several different anchoring methods, such as driven pile
anchors, suction caissons and drag anchors.
Figure 1 Floating support structures. Right-TLP, Middle-Spar buoy, Left-Barge (Aghaalikhani et al., 2016)
Tension leg platforms provide the most stable base for wind turbines, against wave and
wind forces. It is comprised of a floating platform anchored to driven piles or suction
caissons embedded into the seabed by vertically tensioned mooring lines.
Anchor piles are subjected to large vertical and inclined loads from the mooring lines. If
these forces are larger than the uplift capacity of the anchors, they will be pulled out of
the soil bed and lose their capacity. If that were to happen, the platform will lose its
stability and the whole structure will fail catastrophically. For that reason, proper
foundation design is an extremely important factor in floating wind platform design.
Designing economical, and more importantly safe foundations is key in making floating
offshore wind a feasible and practical source of clean energy. If the current issues are
solved floating offshore wind will become a major industry providing much needed clean,
renewable energy.
1.4 Aims and objectives
the following paper aims to investigate the influence of pile length and loading angle on the ultimate
uplift capacity of piles embedded in sand. Using a finite element model produced in PLAXIS3D, loaddisplacement curves for each pile under different loading cases will be produced. Using the resulting
curves, the ultimate loads for each pile length loaded vertically will be compared and tabulated. For
each pile length the ultimate uplift load at various loading angles will be plotted against
displacement and the results will be tabulated. The results will then be analysed and discussed. Any
relationships found between the parameters and the ultimate uplift loads will be presented and
discussed.
1.5 Basic investigation methodology
To determine the influence of these factors on the pullout capacity of the piles, a finite element
model will be created in PLAXIS 3D to simulate a volume of soil with a driven pile. The soil properties
will be defined to model a typical sand. Different pile lengths will be tested under incrementally
increasing loads for each type of soil. Each load will be applied at angles varying between 0° and 90°
to investigate the effect of load inclination on the pullout capacity of the piles. For each loading case
a load-displacement curve will be plotted to show the effect of each parameter from which
conclusions can be drawn.
1.6 Overview of report structure
The report will begin with a review of existing literature on the subject to highlight important
existing research, and to discuss its shortcomings. After that, the methodology and assumptions
used to develop the finite element model will be discussed in detail. An estimate of the pile
capacities from existing literature will be presented. The results will then be presented and
discussed in a separate section. Finally, conclusions of the findings will be reported in the final
section.
Literature review
Floating wind turbines experience a wide range of dynamic loads that arise from
wind, waves, and currents. During storms, the structures are subjected to extreme
loading conditions that onshore turbines never experience. Keeping the turbines stable is
essential for making sure they work properly. This poses a unique set of challenges when
designing safe and efficient mooring systems.
Butterfield et al. presented the main engineering challenges that must be
overcome to make floating offshore wind turbines economically viable. According to the
U.S. Department of Energy estimates, it was concluded that if platform cost can be kept
at around 25% of the total system cost, then the price goal of $0.05/kWh can be
attained (Butterfield et al., 2007). The paper goes on to discuss 3 basic types of floating
platforms, the spar-buoy platform, the barge platform, and tension leg platform (TLP),
and highlights the main challenges for each.
The barge and spar-buoy platforms are anchored to the seabed by relatively long
catenary mooring lines attached to drag embedded horizontally loaded anchors. While
the TLP uses relatively shorter taut mooring lines attached to embedded piles or
anchors. The TLP design is concluded to be the most stable platform against large wind
and wave forces, having the smallest effect on turbine dynamics.
These anchors need to have sufficient capacity to withstand the large, upward
buoyant forces acting on them, as well as reserve capacity to prevent the loss of tension
in the mooring lines. As previously mentioned, efficient design of these anchors
contributes greatly to the cost reduction of the system.
In order to understand the magnitude of forces acting on floating offshore
turbines as a result of extreme wind and wave loading, Zwicki et al (2017) performed a
simple parametric analysis to define the optimal geometry of a tension leg platform
supporting a 6 MW turbine. The study also involved the determination of loads acting on
the length of the structure. Although the scope of the following paper will not include any
detailed calculation or discussion of the forces acting on the TLP structure, the research
undertaken by Zwicki et al serves as guide for understanding the conditions that the
structure experiences.
The shift towards deeper water has presented the industry with a new and unique
set of challenges that set it apart from onshore geotechnical practice. Anchors and
foundations for offshore structures experience very different conditions when compared
to conventional foundations. This along with the fact that offshore site investigation
tends to be expensive and time consuming, means that special considerations must be
taken into consideration when designing offshore foundations.
Conventional methods used to evaluate shaft resistance of onshore piles rely
heavily on empirical data which is limited to 1m pile diameters. In contrast, offshore
piles that are subjected to much larger loads can have diameters in excess of 2m, this
means that conventional empirical methods have to extrapolated to account for the
different conditions. This proves to be one of the major issues facing offshore
geotechnical design (Randolph et al., 2005). In their paper Randolph et al. presented the
design guidelines set out by the American Petroleum Institute (API) to calculate the shaft
friction capacity of pile in sand and discussed the shortcoming of those methods. API
design guidelines give the following formula for calculating the shaft friction resistance
(𝜏𝑠 ).
𝜏𝑠 = 𝐾𝜎′𝑣𝑜 𝑡𝑎𝑛 𝛿 ≤ 𝜏𝑠−𝑚𝑎𝑥
Where K is the soil stress ratio, 𝜎′𝑣𝑜 is the initial effective vertical stress and 𝛿 is
the interface friction angle. According to Randolph et al. this method does not account
for the effect of friction fatigue. Friction fatigue occurs as result of the driving forces
applied during pile installation, which causes the sand particles around the pile to densify
and fine particles to move away from the interface. This in turn causes relaxation of
radial stress. This causes the pile capacity to decrease. Several methods to account for
the effects of friction fatigue have been suggested and presented but are outside the
scope of this investigation, and so will not be mentioned in detail. From the work of
Randolph et al. it can be concluded that for the offshore wind industry to mature,
continued research is still needed.
Chattopadhyay and Pise, (1986) proposed that traditional methods for estimating
pile uplift capacity in sands were not in great agreement with scale model tests
performed, as these models assume that the failure takes place along the interface
between the soil and the pile. When in reality the failure surface is complex and depends
on a number of factors such as the pile embedment depth, pile roughness, soil friction
angle and installation method.
They presented an analytical model that assumes a curved failure surface passing
through the soil body around the pile. When uplift forces are applied, a soil disc is
assumed to move upwards with the pile. The uplift capacity is resisted by the shear
strength mobilised along the failure line as well as the weight of the pile and soil disc.
Applying principles of equilibrium, they were able to formulate a theoretical method to
calculate the uplift capacity. Charts were produced to relate the slenderness ratio of the
pile to the ultimate uplift capacity. through their theoretical results they were able to
conclude that below a critical depth the average skin friction reaches a practically
constant value. Using scale model tests from various sources they were able to verify
that their analytical model gives accurate estimates. The work done by Chattopadhyay
and Pise, (1986) will be used in the following investigation to estimate uplift capacities of
the piles, and to validate the results of the investigation.
To test the effect of soil type on the pullout capacity of Owino et al (2018)
performed a plane-strain finite difference analysis using FLAC2D software. The analysis
included three different soil types. These are a silty soil, clayey soil, and sandy soil.
Displacement load curves were plotted for each test. A 1.4 m pile was first modelled and
tested in each soil type. It was shown that the sandy soil resulted in the largest applied
axial load of 94 kN at a head displacement of 10 mm. While the pile in the silty soil
reached an axial load of 90 kN at 10 mm displacement. With the clayey soil reaching a
load of 80 kN at 10 mm displacement. The investigation also included a parametric
study to determine the effect of pile depth and soil internal friction angle on the pullout
capacity. It was found that there is a linear relationship between pile length (i.e.
embedment depth) and pullout capacity. This relationship was confirmed by the scale
tests done by Rao et al (2005). Varying the friction angles for the 3 soil types showed
that there was a curvilinear relationship between friction angle and pullout capacity for
both sandy and silty soils. For both sandy and silty soil peak force was reached at an
angle of 20°. For clayey soil there was a decrease in axial load capacity as the friction
angle was increased.
The paper presented by Owino et al (2018) involved the use of a 2D finite
difference analysis which provides less accurate results than the finite element method.
For the following investigation, a 3D finite element analysis (FEM) will be performed to
give more accurate results.
The previous conclusions made by Chattopadhyay and Pise were further
confirmed by the work of Owino et al. Owino et al used a finite difference model (FDM) in
FLAC2D to test the effect of several parameters on the uplift capacity of piles in different
soil types. Viewing the resulting shear strains in FLAC2D a failure surface, following the
shape of the proposed failure plane presented by Chattopadhyay and Pise, can be seen
clearly forming around the piles.
Gaaver (2013) conducted a number of scale model tests on single piles and pile
groups in sand to test the effect of pile embedment ratio (L/d) and soil properties on the
ultimate uplift capacity. It was found that both parameters have a significant effect on
the ultimate capacity. As the pile embedment ratio increases the uplift capacity
increases. This is due to the increased effective stress acting at the pile mid height and
the greater contact area between the pile and the soil. The relative density of the soil
was also found to have a significant impact on the ultimate load capacity, as an increase
in relative density results in an increase of the friction angle of the soil. A general
equation for predicting the upward displacement of piles in sand was proposed. It was
also concluded that an upward displacement of 1.4-2.5% times the diameter of the pile
is needed to reach the net uplift capacity of piles.
The work of owino et al can be validated by the scale tests done by gaaver. As
both investigations concluded that the ultimate uplift capacity of pile is directly related to
both the soil properties and the pile length.
Achmus and Thieken highlighted the gap in current literature on the interaction
effects of combined horizontal and vertical loads (inclined loading) and proposed an
analytical model to study this interaction. It was concluded that inclined loading results
in complex interaction effects due to the mobilisation of passive earth pressures and the
pile skin friction. For a pile loaded in tension at an angle, where the vertical component
dominates, an increase in the uplift capacity can be seen. This is due to greater normal
stresses applied by the horizontal component of the load on the pile shaft. In other
cases, a reduction in the uplift capacity is observed. The reduction is a result of the
upward movement of the passive earth pressure wedge which leads to a reduction in the
negative skin friction.
Reviewing existing literature, it can be seen that there is a large volume of
research on both floating offshore wind platforms and pile capacities in different soils and
under different conditions. But there is a lack of studies that relate pile capacities in the
context of floating wind platforms. The following investigation will link existing research
on pile pullout capacity with the technology of floating offshore wind turbines. The work
presented above will be used to predict, analyze and validate the results of the FEM
analysis performed in PLAXIS3D.
Pile capacity estimates
Using the method put forward by Chattopadhyay and Pise, the ultimate uplift capacity
for the 3 piles will be estimated. The method started out with the assumed failure
mechanism shown in figure x
Applying vertical equilibrium and simplifying resulted in the equation shown below
Integrating and introducing a constant termed the gross uplift capacity factor A 1, the
ultimate uplift load can be given by
𝑃𝑢 = 𝐴1 𝛾𝜋𝑑𝐿2
Where 𝛾 is the unit weight of the soil, d is the diameter of the pile and L is the length of
the pile.
Chattopadhyay and Pise plotted the variation of the gross uplift capacity A1 for a soil with
a friction angle of 40° and different interface angles with slenderness ratio.
Using the proposed investigation parameters and given that the friction angle for the soil used in the
investigation (42.2°) is similar to the value of friction angle used to formulate the graph for A1 a
rough estimate of the pile uplift capacity can be calculated. The results of the load estimates are
provided in the table below
Estimated failure load
(kN)
Pile 1 (λ=10)
1155
Pile 2 (λ=15)
2227
Plie 3 (λ=20)
3300
Results
Pile length
To investigate the effect of pile length on the uplift capacity of piles, 3 piles were
modelled in PLAXIS3D under incrementally increasing loads. The results of the
investigations are presented in the following section.
Variation of L/d
2500
2000
Load (kN)
1500
L/d=10
L/d=15
1000
L/d=20
500
0
-5,00
0,00
5,00
10,00
15,00
20,00
25,00
30,00
35,00
Displacement (mm)
Ultimate load (kN)
Pile 1
Pile 2
Pile 3
720
1840
2080
22.8
31.8
16.4
Pile head
displacement at
failure (mm)
From the results it is evident that there is a direct relationship between the pile length
and the ultimate uplift load.
Comparing the ultimate loads from the plaxis model with the estimated