offshore foundations: technologies, design and application · offshore foundations: technologies,...
Post on 03-Jul-2018
250 Views
Preview:
TRANSCRIPT
Offshore Foundations: Technologies, Design and Application
Master Student: Pedro Gomes Simões de Abreu
Supervisor: Dr Peter Bourne-Webb
Abstract:
The offshore oil industry started over 60 years ago, since then it evolved immensely. This
evolution was forced by the need of exploiting oil and gas reserves in more challenging regions.
The purpose of this study was to gather information about the foundation structures used in the
offshore industry, and to assess the applicability of two types of foundation in a real scenario. São
Tome & Principe was selected as the case-study for this paper because it is a member of the
Community of Portuguese Language Countries, and has recently been subjected to several studies
in its offshore region to evaluate its potential as an oil & gas supplier. This paper described the
geotechnical characterisation of the offshore of STP based on investigations performed in the
Gulf of Guinea (GoG) for more than 10 years. The results of the study were that the soil is
probably a highly sensitive clay (St=2 to 4), and the shear strength profile presents a gradient of
about 1.5 kPa/m. Another conclusion is that many sites in the GoG exhibit a greater resistance
(up to about 15 kPa) in the first 2 m, this phenomenon is called a “crust”. The paper also describes
design principles for two anchoring systems: the Torpedo Anchors and Suctions Embedded Plate
Anchors (SEPLA). For Torpedo, the results revealed that the pull-out resistance, after
reconsolidation, is expected to be 8.7 MN. Whereas, the results for SEPLA holding capacity is
expected to be 10 MN. For both systems the calculations were made for the largest of their
solutions available in the market.
1. Introduction:
The offshore oil industry started in 1947 with the
installation of the first oil rig in just 6 m depth of water,
off the coast of Louisiana in the United States.
Nowadays there are over 7000 offshore platforms
around the world located in a large range of water
depths, which are starting to exceed 2000 m. This
evolution forced a change in the concept of “deep
water”, as in the 1970s deep-water meant depths of 50
m to 100 m, now this concept refers to water depths
around 800 m. With this, a new concept was created to
refer to water depths starting from 1000 m, “ultra-
deep” water.
As this evolution made possible the exploration of
more challenging oil and gas fields, some countries
where Portuguese is the official language have gained
attention, and as a result some are currently under
investigation (e.g. Mozambique, Guinea Bissau and
São Tomé & Principe) , while in others investments
have already been made (e.g. Brazil and Angola).
São Tome & Principe is a member of the group of
countries which belong to the Community of
Portuguese Language Countries (CPLP) and over the
past five years has been subject to many field tests in
order to quantify the potential oil & gas reserves and
to evaluate the quality of the potential extractable
product (oil) of those reserves as well. The exclusive
economic zone (EEZ) of STP is now divided into
several blocks, which are licensed to Oil & Gas
companies, so they can develop investigation work
and evaluate the potential of the reserves, Figure 1.
Most of the recent developed projects in Brazil
and Angola are in deep and ultra-deep water,
therefore, the adopted foundation systems had to be
anchoring systems. The majority of the EEZ of STP is
also in ultra-deep waters, ranging from 1800 m to 3000
m.
This work has the purpose of gathering
information about the foundation structures used in the
offshore industry, and the assessment of the
application of two types of foundation in the offshore
of STP. The choice of these types of foundation
systems are based on their novelty and economic
aspects. Thus, the systems evaluated are: the torpedo
anchor which have been applied in Campos Basin in
Brazil, and the suction embedded plate anchors that
are currently in use in Angola.
Figure 1 - EEZ of STP and block division for licensing round.
1.1. Torpedo anchors
As offshore exploitation moves to water depths of
around 3000 m, new technologies have had to be
developed in order to reduce installation costs, and
facilitate construction. Moreover, the high number of
floating production and drilling units in operation may
provoke the congestion of the sea bottom due to the
high number of risers and mooring lines employed. In
this scenario, dynamically penetrated anchors (DPA),
and in particular torpedo anchors, have proven to be a
reliable alternative used in Brazilian offshore fields
(Aguiar et al., 2009). The reduced mooring line radius
employed on torpedo anchors relative to catenary
mooring systems with drag anchors, reduces sea
bottom congestion, Figure 2.
Figure 2 – Radius comparison between floating units linked to conventional drag anchors and torpedo anchors, from: http://www.hindawi.com/journals/jam/2012/102618.fig.002.jpg.
Torpedo anchors (TA) are the most applied type of
DPA and they have been developed by the Brazilian
oil company Petrobras. TAs are cone-tipped,
cylindrical steel pipes filled with concrete and scrap
metal. They penetrate the seabed relying on the kinetic
energy they acquire while free falling from heights of
between 30 m and 150 m above the seabed. Torpedo
anchors come in various sizes from 0.76 m to 1.07 m
in diameter, 12 m to 17 m in length, and 241 kN to 961
kN in weight. The inside of the anchor shaft is filled
with ballast to increase the weight and maintain the
centre of gravity below the centre of buoyancy for
stability. Some versions of the TA have been fitted
with 4 flukes at the trailing edge, ranging in width
from 0.45 m to 0.9 m, and 9 m to 10m long (Raie,
2009; Medeiros et al.,1997, 2001, 2002). Two
different DPAs, with and without fins are pictured in
Figure 3(a).
Torpedoes can easily reach velocities of 25 m/s to 35
m/s at the seabed after being released from a height of
20 m to 40 m above the seabed, allowing tip
penetrations up to 3 times the anchor length and
holding capacities after consolidation that are
expected to be in the range of 5 to 10 times the weight
of the anchor (Randolph et al., 2005).
Figure 3 – Dynamically penetrating anchors (a) Torpedo anchor with fins and without fins (Medeiros, 2002); (b) installation of 4 flukes torpedo anchor (Medeiros, 2002; O’Loughlin et al., 2004)
The installation procedure for DPA has developed
from its original method. Instead of using only one
anchor-handling vessel (AHV) to lower the anchor to
a predetermined height above the seabed, using the
permanent mooring line, now two AHV are used. The
installation process was modified to minimize the
effect of drag force on the mooring line on the free
falling motion of the anchor. Accordingly, the anchor
is lowered using an installation wire from the first
AHV while the second AHV holds the permanent
mooring line that is attached to the anchor and forms a
loop. A remote release system is used at the end of
installation wire to release the anchor (Araujo et al.,
2004). A chain segment is recommended for the lower
portion of the mooring line because model tests of the
anchor installation (Lieng et al., 2000) have shown
that chain drag does not reduce the velocity of the
anchor during free fall. Figure 3(b) demonstrates the
lowering of two model scale torpedo anchors to
position them before free-fall releasing. A full scale
torpedo pile and the situation immediately prior to TA
release, in which it is possible to see the loop between
the permanent mooring line and the installation line, is
illustrated in Figure 4.
Figure 4 – Full scale torpedo pile and releasing situation, Lieng et al. (1999).
The main reason for using this type of anchor solution
is its simplicity and speed of installation. With regard
to the equipment required for installation, the torpedo
anchor installation is depth-independent. Moreover,
torpedo piles are cost-effective throughout fabrication,
transportation, and installation. Fabrication is easy and
inexpensive due to the simple design of the torpedo
anchors. The cost of transportation is low because the
compact design of the torpedo anchor allows more
anchors to be transported per voyage of the AHV than,
for example, suction caissons. Also, the installation is
economical because an external source of energy is not
required for installation and a quick installation is
possible using one or two AHVs and limited use of
ROVs. Finally, the predicted holding capacity is less
dependent on the precise evaluation of the soil shear-
strength profile. Since higher strength profiles reduce
the penetration and lower strength profiles increase
penetration, therefore the holding capacity is mainly a
function of the kinetic energy obtained during free
falling. Nevertheless, torpedo anchors have the
disadvantage of the uncertainty in verticality of the
anchor, which affects the holding capacity
(O’Loughlin et al., 2013; Raie, 2009).
1.2. Suction embedded plate anchors
A new system, called a suction embedded plate anchor
(SEPLA), was developed to overcome the problems of
the conventional plate anchor (e.g. VLA), achieving
greater and more precise depth location below the
seabed (Dove et al., 1998; Wilde et al., 2001).
The SEPLA uses a suction caisson (or “follower”) to
embed a rectangular plate anchor, providing a known
initial penetration depth for the anchor, at a specified
geographical location. The components of a SEPLA
are illustrated in Figure 5.
Figure 5 – Components of a suction embedded plate anchor
(Gaudin et al., 2006).
SEPLA installation consists of 3 steps: caisson
penetration, caisson retraction, and anchor keying.
These steps are shown schematically in FIGURE 6.
First, the caisson with a plate anchor slotted vertically
in its base is lowered to the seafloor and penetrated
into the soil under its dead weight until the skin
friction and end-bearing resistance of the soil on the
caisson equal the caisson’s dead weight. The vent
valve on the top of caisson is then closed and the water
trapped inside is pumped out. The ensuing differential
pressure at the top drives the caisson to the design
depth. The plate anchor is then released and the water
is pumped back into the caisson, causing the caisson
to move upward, leaving the plate anchor in place in a
vertical orientation. The caisson is retracted from the
seabed and prepared for the next installation. As the
anchor chain is tensioned, it cuts into the soil.
Simultaneously, the anchor line applies a load to the
anchor’s offset padeye causing it to rotate or “key”. In
order to achieve the maximum mobilized capacity, the
plate must be as close to perpendicular to the direction
of loading as possible (Yang et al., 2011).
FIGURE 6 –SCHEMATIC OF SEPLA INSTALLATION (YANG ET AL., 2011).
SEPLA installation accuracy represents a great
improvement over that for drag embedment anchors,
however two questions emerge (these questions are
applied to all offshore plate anchors such as VLAs).
Firstly, the caisson penetration and anchor keying
provokes a disturbance in the soil mass around the
SEPLA, which leads to a decrease of the soil strength
in the region. Secondly, when keying is being initiated,
a loss of embedment depth occurs. While, the first
question can be solved as the soil strength is largely
recovered over time by soil reconsolidation, the
second problem cannot because loss of embedment
depth is permanent. This is a very important issue,
since SEPLA capacity significantly depends on its
embedment depth when the soil has increasing
strength with depth (which is a typical in the offshore
environment). Therefore, it becomes very important
to accurately estimate the loss of embedment depth
during the keying process. This estimate can then be
factored into the design; Wilde et al. (2001) report
upward movements ranging between 0.5 and 1.7 times
the plate height, which is a wide range when plate
heights of 2.5 m to 4.5m are used in practice.
Even though the undrained capacity of plate anchors
has been extensively investigated by means of
analytical and experimental methods; for SEPLA,
there are a limited number of reported studies and
therefore the keying process is not yet well
understood. However, Song el al. (2009) present a
theoretical model to predict the trajectory and
corresponding capacities of SEPLA during the keying
process based on empirical and plastic limit analysis.
2. Geotechnical site conditions
São Tomé & Principe is a group of islands situated on
the Gulf of Guinea, the island of Principe is the nearest
to the site where possible oil exploration is more
likely. Figure 7 shows the location of STP and the
surrounding geology, it is also possible to see two red
lines which one of them refer to the schematic cross
sections presented in Figure 8. The cross section
extend from Principe Island to Nigeria.
Figure 7 – São Tomé & Principe location and surrounding
geology, Courtesy of Agencia Nacional do Petroleo of STP
from: http://www.stp-
eez.com/DownLoads/Posters/3_STP_RegionalGeol.pdf .
Figure 8 – Cross section from Nigeria cost to Principe Island,
Courtesy of Agencia Nacional do Petroleo of STP from:
http://www.stp-
eez.com/DownLoads/Posters/3_STP_RegionalGeol.pdf .
2.1. Index Properties of GoG sediments
The sediments present in GoG deep-waters are
characterised by very high water contents, values
which can be between 150% and 250% at the seabed
and gradually decrease with depth, Figure 9(a). Puech
(2004) suggests soil unit weights starting from 12-13
kN/m3 at seabed and increasing to 13-15 kN/m3 below
6-8m, Figure 9(b).
Figure 9 - Deep-water sediments physical properties on Gulf of Guinea (Puech, 2004)
2.2. Shear strength profiles
Sultan et al. (2007) reported some studies from the
continental slope of Nigeria. Even though the water
depth where this study was executed ranged between
only 1100 m and 1250 m, the study area was actually
very close to Principe Island as shown in Figure 10.
The shear strength profile based on laboratory
geotechnical tests is represented in Figure 11. This
profile has a similar development to the Puech (2004)
proposal of a gradient of 1.5 kPa/m, therefore in future
calculations this is the gradient that will be used.
However, the shear strength profile does not start from
the zero, therefore it is assumed to be 5 kPa in the first
3 m, and after that assumes the proposed 1.5 kPa/m
gradient (blue line Figure 11).
Figure 10 - Nigerian continental slope study area, Sultan et al. (2007).
Figure 11 - Shear strength profile of Nigerian continental
slope obtained in laboratory geotechnical tests and its
comparison with shear strength profiles proposed by others.
Table 1 are summarized the adopted values for the
different soil characteristics that were used in the
evaluation of the SEPLA and torpedo anchors.
Table 1 – Adopted values for different soil characteristics and
references from which they were based on.
3. Design of anchor solutions
Since the water depths in the zone of proposed
offshore development near São Tomé & Príncipe
range from 1800 m to 3000 m, future installed
facilities must be floating platforms and hence the
foundation systems in the seabed will be resisting
tensile forces instead of compression. Therefore, the
only types of foundation solution suitable for this
region are anchoring systems.
3.1. Torpedo anchors
The design process for Torpedo anchors is rather
complex due to the difficulties of predicting the anchor
embedment and set-up after installation. These two
factors have a direct effect on the anchor capacity and
they depend on the geometry and characteristics of the
anchor, as well as the soil properties such as undrained
shear strength and coefficient of consolidation
(horizontal and vertical).
3.1.1. Adopted anchor geometry
The main problem with torpedo anchors is the lack of
field experience, especially outside Brazil. Therefore,
the geometry suggested in this text will be based on
that used in the Albacora Leste Field (FPSO P-50), a
FPSO unit in water depth of 1400 m with required
capacity of 7500 kN (Araujo et al., 2004).
The torpedoes used for the FPSO P-50 were type T-
98, which are illustrated in Figure 12 and Figure 13,
and had the following characteristics (Brandão et al.,
2006):
Total mass of 98 tons
Diameter of 1.07 meters
Length of 17 meters
Four stiffener wings (flukes): 0.9 m wide x
10m long.
Figure 12- Schematic longitudinal section drawing of the T-
98, Brandão et al. (2006).
Figure 13 – Photos of the T-98 body sections welding and its
final adjustments, Brandão et al. (2006).
3.1.2. Impact Velocity and free-fall Height
It is advised to use drop heights above the seabed
between 30 m and 150 m, which usually result in
impact velocities from 0.5 to 0.33 times the terminal
velocity (Medeiros, 2002). Thus, for this study, an
impact velocity of 40m/s is considered. Using 𝒂 =𝟏
𝟐. 𝑪𝑫. 𝝆𝒘. 𝑨𝒑. 𝒗𝟐. 𝒎 Eq. 1 and adopting 𝐶𝐷 = 0.33
as proposed by Fernandes et al. (2005) for torpedo
anchors, the acceleration needed to achieve an impact
velocity of 40m/s after free-release is 7.4 m/s-2.
𝒂 =𝟏
𝟐. 𝑪𝑫. 𝝆𝒘. 𝑨𝒑. 𝒗𝟐. 𝒎 Eq. 1
Using the equations for conservation of mechanical
energy, determine the height needed to achieve the
chosen impact velocity as the anchor reaches the
seabed:
𝐸𝑐 =1
2. 𝑚. 𝑣2 = 0.5 × 98000 × 402
Eq. 2
𝐸𝑝 = 𝑚. 𝑎. ℎ = 98000 × 7.4 × ℎ Eq. 3
𝐸𝑐 = 𝐸𝑝 ⟹ 𝒉𝒇𝒓𝒆𝒆−𝒇𝒂𝒍𝒍 = 𝟏𝟎𝟖. 𝟒 𝒎 Eq. 4
3.1.3. Tip Embedment Depth
O’Loughlin et al. (2013) after gathering penetration
data from worldwide field tests and comparing them
to centrifuge tests of equivalent prototype scale
models, were able to propose a relationship that
predicted penetration depth with reasonable accuracy
for this very large dataset that encompassed a wide
range of anchor masses, geometries and impact
velocities, Figure 14.
Figure 14 – Comparison of centrifuge and field test
embedment data, O’Loughlin et al. (2013).
From Figure 14 it is possible to see the good
agreement between the formulated curve and the
dataset. So, the prediction for STP using the
O’Loughlin et al. (2013) proposed relationship, would
be:
𝑧𝑒
𝑑𝑒𝑓𝑓≈ (
𝐸𝑡𝑜𝑡𝑎𝑙
𝑘. 𝑑𝑒𝑓𝑓4 )
1/3
Eq. 5
𝐸𝑡𝑜𝑡𝑎𝑙 = 𝐸𝑐 + 𝐸𝑝 = 1/2. 𝑚. 𝑣𝑖𝑚𝑝𝑎𝑐𝑡2 + 𝑚′. 𝑔. 𝑧𝑒 Eq. 6
𝑧𝑒+1 ≈ (0,5. 𝑚. 𝑣𝑖𝑚𝑝𝑎𝑐𝑡
2 + 𝑚′. 𝑔. 𝑧𝑒
1.5 × 1.174 )
13
× 1.17
= 39.24 𝑚
Eq. 7
The two grey lines traced in the graph (Figure 14)
bound almost every result, additionally the grey line
below the original equation line intersects the field test
result from T-98 (Brandão et al., 2006). Therefore, this
ratio may vary 13% from the real depth, so the
penetration achieved is expected to be between 36 m
and 43 m.
3.1.4. Pull-out capacity
The torpedo anchor resistance was determined by the
sum of three components: shaft resistance, reverse en
bearing resistance and weight of the torpedo pile.
3.2. SEPLA
The functional requirement of SEPLA is to resist the
specified maximum factored mooring line load, while
avoiding significant displacements, both in the
direction of the applied load or vertically. SEPLA
holding capacity is related primarily to three basic
aspects, which must be defined for the design of the
solution, those aspects are:
Anchor plate area
Undrained shear strength
Penetration depth
3.2.1. Anchor geometry
The components of a typical SEPLA are illustrated in
, the geometry and the characteristics proposed for
SEPLA applied offshore of STP are:
Plate area: 10 m length by 4.5 m width
Shank: 2.5 m high (padeye eccentricity) and
at an angle of 60ᵒ with the fluke.
Anchor thickness: 0.20 m
Anchor weight: 50 tons
Installation penetration: 24 m
Figure 15 – Typical SEPLA with keying flap, Wang et al. (2012).
3.2.2. Keying
The anchor keying process promotes two negative
effects in the plate holding capacity. First, it induces
an upward movement on the anchor during the plate
rotation, hence reducing the embedment depth; and
second, the soil in the immediate vicinity of the plate
anchor is remoulded, therefore reducing the soil
strength (Randolph et al., 2005). Even though, this
latter effect may be recovered as the soil
reconsolidates, the loss of embedment is permanent.
As clay deposits in Gulf of Guinea are typically
characterized by an increasing strength profile with
depth, any loss in embedment will correspond to a
non-recoverable loss in potential anchor capacity.
Song et al. (2009) ran several centrifuge tests and
developed large deformation finite-element (FE)
analyses. From the results of those studies, they found
that the loss in anchor embedment during anchor
keying may be expressed in terms of a non-
dimensional anchor geometry factor, which is a
Table 2 - Soil properties, penetration and vertical resistance of Campos Basin in Brazil and STP offshore.
Offshore Site São Tomé & Príncipe
Tip embedment depth (m) 40
Average undrained shear
strength gradient (kPa) 1.5z
Average undrained shear
strength on shaft (kPa) 47.25
Adhesion factor (α) 0.7
𝜸𝒔 (kN/m3) 14
Rs (kN) 4880
Rb (kN) 3470
Vertical pile resistance (kN) 8720
Offshore Site São Tomé & Príncipe
Tip embedment depth (m) 40
Average undrained shear
strength gradient (kPa) 1.5z
Average undrained shear
strength on shaft (kPa) 47.25
Adhesion factor (α) 0.7
𝜸𝒔 (kN/m3) 14
Rs (kN) 4880
Rb (kN) 3470
Vertical pile resistance (kN) 8720
function of the eccentricity of the padeye, angle of
loading, and the net moment applied to the anchor at
the stage where the applied load balances the anchor
weight. Using eq. (8) taking into account all the anchor
geometry measurements and anchor submerged unit
weight. The initial moment M0 corresponding to zero
net vertical load on the anchor is given by eq. (9).
∆𝑧𝑒
𝐵=
0.2
(𝑒𝐵
) (𝑡𝐵
)0.3
(𝑀0
𝐴𝐵𝑠𝑢)0.1
Eq. 8
𝑀0 = (𝑓 + 𝑊′𝑎)𝑒 − 𝑓𝑒𝑓 − 𝑊′𝑎𝑒𝑤 Eq. 9
∆𝑧𝑒 = 4.19 𝑚 ≈ 4 𝑚
Therefore, for this case study, it is plausible to say that
the SEPLA embedment depth after installation is
about 20 m.
3.2.3. Plate Holding capacity
There are several suggested procedures for evaluating
the plate holding capacity in clay, and in this work
this estimation will be made according to three
different procedures, which are:
Wilde et al. (2001)
Merifield et al. (2001)
DNV-RD-E302 (2002)
The predicted plate anchor capacity according to the
three different methods is expressed in Table 3. The
Merifield et al. (2001) procedure returned an ultimate
holding capacity 50% higher than the others, and this
may be explained by the two facts. Firstly, one
parameter of the evaluated situation is outside of the
range of the theoretical solutions. Secondly, Merifield
et al. (2001) do not use any empirical reduction factor
whereas the other two methods do; 0.70 is used by
Wilde et al. (2001) and 0.75 in DNV-RP-E302 (2002).
Procedure
suggested by:
Ultimate Holding
Capacity (MN)
Design Holding
Capacity (MN)
Wilde et al.
(2001) 13 9.2
Merifield et
al. (2001) 18 9.0
DNV-RP-
E302 (2002) 13.3 9.5
Table 3 - Plate anchor holding capacities in the idealized STP offshore conditions according to three different methods.
Wilde et al. (2001) and DNV-RP-E302 (2002) have
similar results, and the major source of their difference
is the usage of a partial safety factor by DNV-RP-E302
(2002), for that reason a partial factor of 1.4 is also
applied in Wilde et al. (2001) method to obtain a
design holding capacity. For the design holding
capacity in Merifield et al. (2001) method is used a
global safety factor of 2.0.
Contrary to Merifield et al. (2001), these two
procedures do not take into account the self-weight of
the plate anchor in their resistance. This conservative
decision means not taking into consideration 500 kN
of resistance, which represents 4 to 5% of the anchors
holding capacity.
As a result, is believed to be more reliable to
use DNV-RP-E302 (2002) method, since it seems to
have a better calibration of its empirical reduction and
safety factors. Thus, plate anchor capacity adopted is
9.5 MN, determined after DNV-RP-E302 (2002) plus
the anchor self-weight, 0.5 MN, therefore the
predicted plate anchor capacity in this case study is
predicted to be 10 MN.
4. Discussion of results
Both of the considered anchor solutions are likely to
be economically competitive alternatives to
conventional offshore anchors, for application in STP.
In Table 4 and Table 5 are enumerated, respectively,
the advantages and disadvantages of both SEPLA and
torpedo anchors.
SEPLA advantages Torpedo anchor (DPA)
advantages
Cost of anchor element
is the lowest of all the
deep-water anchors.
Uses proven suction
caisson installation
methods.
Provides an accurate
measure of embedment
and position of the
anchor.
Design based on well-
developed procedures
for plate anchors.
Experience in the Gulf
of Guinea
Simple and economic to
fabricate
Simple to design.
Accurate to position
with no requirements
for proof loading.
Rapid installation
Robust and compact
design makes handling
and installation simple
and economic with only
one Anchor Handling
Vessel (AHV) and no
ROV.
Table 4– SEPLA and torpedo anchor advantages.
SEPLA disadvantages Torpedo anchor (DPA)
disadvantages
Patented installation
method.
Installation time
greater than for a
caisson.
Requires keying and
proof loading.
Requires a ROV.
Limited field load tests.
Patented installation
method.
No experience outside
Brazil.
Lack of documented
installation and design
methods with
verification agencies.
Unknown orientation
once embedded.
Table 5 – SEPLA and torpedo anchor disadvantages
Finally, when comparing the resistance obtained by
each foundation element, the SEPLA has a greater
advantage. After applying safety factors in the
proposed SEPLA, solution the predicted resistance is
about 10 MN. For torpedo anchors the ultimate
resistance is predicted to be 8.7 MN, however the use
of a safety factor of 2 is advised (Eltaher et al., 2003),
which reduces the design resistance to about 4.4 MN.
Thus, double the number of torpedoes will be required
to provide the same load resistance as a single plate
anchor.
In other words, the resistance obtained per unit weight
of the anchor element is higher for SEPLA, 10000 𝑘𝑁
50 𝑡𝑜𝑛𝑛𝑒𝑠=
200 𝑘𝑁𝑡𝑜𝑛𝑛𝑒⁄ ; than for the torpedo anchor,
4350 𝑘𝑁
98 𝑡𝑜𝑛𝑛𝑒𝑠=
44 𝑘𝑁𝑡𝑜𝑛𝑛𝑒⁄ .
Even though, both torpedo and suction embedded
plate anchors have smaller unit costs in comparison
with the other types of anchoring systems, one of them
is more suitable than the other to be recommended for
use in this case-study. The best solution is the one
which has more unit resistance per foundation
element, the design and installation method is more
reliable, research has provided a more fundamental
basis for understanding the method, and if possible,
should already been used in the Gulf of Guinea or a
similar geotechnical region. Taking into account the
above criteria, the more appropriate solution for this
case study is considered to be the Suction Embedded
Plate Anchor.
5. Conclusion
This thesis has presented an outlined of existing
platform and associated foundation technologies used
in offshore developments, and the most important
aspects involved in their design. Also, geotechnical
characterization issues relating to the offshore
environment such as topography, seabed composition
and geohazards were discussed.
In order to relate the foundation technologies
discussed with a real scenario, a specific region was
chosen to assess the viability of two different
foundation solutions. Proposed offshore developments
near São Tomé & Príncipe were chosen for the case
study, because it is a region that has recently gathered
attention from the oil & gas industry, and has not been
subjected to any platform construction yet.
Water depths in the zone of proposed offshore
development near São Tomé & Príncipe range from
1800 m to 3000 m; thus, future installed facilities must
be floating platforms and hence the foundation
systems in the seabed will be resisting tensile forces
instead of compression. Therefore, the only types of
foundation solution suitable for this region are
anchoring systems. Anchoring systems that were
selected for evaluation in this case are those for which
fewer studies and investigation have been made, but
on the other hand are likely to be the most economic
systems – in this case, torpedo anchors and suction
embedded plate anchors were evaluated.
Geotechnical characterization of the zones offshore
from São Tomé & Príncipe was based on
investigations performed in the Gulf of Guinea over
more than 10 years. This large database on the
behaviour of deep-water sediments made possible the
assumption of several soil parameters. The soil in the
Gulf of Guinea typically is a highly sensitive clay (St
= 2 to 4), and one of the most important parameters is
the shear strength profile, it was apparent that it would
have a positive gradient with depth of about 1.5 kPa/m.
It also became clear that many sites in this region
exhibit a greater resistance (up to about 15 kPa) in the
first 2 m, this phenomenon is called a “crust”, and no
unique or convincing explanation has been proposed
for its existence so far. Other important properties,
which are not well defined yet, are the interface soil-
steel friction resistance and the set-up effects.
The selected torpedo anchor to be employed in this
case study would be the same that was used in
Albacora Lest Field in Brazil Basin by Petrobras, and
it is the T-98 torpedo. The design of the torpedo anchor
was based on the design of simple cylindrical pile. Its
pull-out capacity comes from three different sources:
shaft friction resistance, self-weight of the pile and
reverse end bearing capacity. The shaft friction
resistance is the fraction that gives the greatest
contribution, thus the correct assessment of the
adhesion factor (α) plays an important role.
The assessment of the torpedo anchor free penetration
into the soil is also important to recognise the torpedo
embedment, and therefore the soil shear strength along
the torpedo shaft. Following O’Loughlin et al. (2013),
after releasing the torpedo from 108 m above the
seafloor, it should reach a velocity of about 40 m/s
before impacting with the soil and the torpedo should
penetrate about 40 m into the soil. An adhesion factor,
α=0.7 was considered, which corresponds to full
reconsolidation of the soil in the vicinity of the pile
after being remoulded by the torpedo penetration,
consequently the pull-out resistance is expected to be
8.7 MN.
The SEPLA solution considered is a 4.5 m x 10 m
plate anchor, which is proposed by Wilde et al. (2001)
for permanent installations. Plate anchor keying
induced loss of embedment was calculated according
to Song et al. (2009) and is expected to be about 0.81B
(B is the anchor breadth), i.e. approximately 4 m.
The holding capacity of the SEPLA is provided by the
end bearing resistance plus the self-weight of the
anchor. This capacity was calculated according to
three different design procedures:
Wilde et al. (2001)
Merifield et al. (2001)
DNV-RP-E302 (2002)
The most conservative procedure proved to be from
DNV-RP-E302 (2002), with a holding capacity of
about 9.5 MN. However, this do not take into account
the self-weight of the anchor and includes a partial
resistance factor, therefore an additional 0.5 MN may
be added to provide a total resistance of 10 MN.
Based on a criteria that involved the resistance,
installation process, experience, knowledge and
reliability of both anchoring systems, is was
considered that the use of suction embedment plate
anchor systems would be more appropriate offshore
from São Tome & Principe.
6. Further Research
In terms of geotechnical issues associated with this
case study, there is still need for further studies, in
particular:
Site specific characterisation of the seabed in
the São Tomé & Principe region, instead of
the broad characterisation of the Gulf of
Guinea made in this study and based on the
amalgamation of various published data from
the Gulf generally.
Understanding of the origin and
characterisation of the near seabed “crust”
particular to this region and the effect this may
have on foundation installation processes.
The interface friction resistance between the
soil and the steel elements in the short and
long-term.
Since there is no experience of torpedo anchors in the
Gulf of Guinea, it would be of great interest to develop
in situ model scale tests to study the behaviour of the
torpedo during penetration of the soil, and its
resistance in the short and long-term as well. As this
in situ tests are very expensive, it would be very
interesting to evaluate the influence of the near seabed
“crust” on the torpedo penetration, using for that
computational programs or laboratorial tests.
A study to evaluate the forces that the platforms will
be subjected in the offshore of São Tomé & Principe
would be of great interest, because depending on those
forces the foundation solutions may be more or less
economic, i.e. number and size of the foundation
elements.
7. References
Aguiar, C. S., de Sousa, J.R., and Ellwanger, G. B.
(2009), “Análise da Interação Solo-Estrutura
de Âncoras do Tipo Torpedo para
Plataformas Offshore”, PhD Thesis, UFRJ.
Araújo, J. B., Machado, R. D., and Medeiros Jr., C.
P. (2004), “High Holding Power Torpedo
Pile – Results for the First Long Term
Application”, Proceedings of the ASME 23rd
OMAE Conference, 51201, Vancouver.
Brandão, F. E. N., Henriques, C. C. D., Araújo, J. B.,
Ferreira, O. C. G. & Amaral, C. D. S. (2006).
“Albacora Leste field development: FPSO P-
50 mooring system concept and installation”.
Proc. Offshore Technol. Conf., Houston, TX,
paper OTC 18243.
DNV-RP-E302 (2002). “Design and Installation of
Plate Anchors in Clay- Recommended
Practice”, Det Norsk Veritas, 43 pages.
Dove, P., Treu, H., and Wilde, B. (1998). “Suction
embedded plate anchor (SEPLA): A new
anchoring solution for ultra-deep water
mooring.” Proc., D.O.T. 10th Int. Conf. and
exhibition, Deep Offshore Technology,
London.
Eltaher, A., Rajapaska, Y., AND Chang, K. T.
(2003). “Industry trends for design of
anchoring systems for deepwater offshore
structures”. Proc., Offshore Technology
Conf., OTC No. 15265.
Fernandes, A. C., dos Santos, M. F., Araújo, J. B.,
Almeida, J. C. L., Diniz, R. and Matos, V.
(2005). “Hydrodynamic aspects of the
torpedo anchor installation”. Proc.
International Conference on Offshore
Mechanics and Artic Engineering, Halkidiki,
Greece, OMAE 2005-67201.
Lieng, J. T., Hove, F., Tjelta, T. I. (1999). “Deep
Penetrating Anchor: Subseabed Deepwater
Anchor Concept for Floaters and Other
Installations.” Proceedings of the 9th
International Offshore and Polar Engineering
Conference, Brest, France, 30 May- 4 June
1999, Vol. I, pp. 613-619.
Lieng, J. T., Kavli, A., Hove, F., Tjelta, T.I. (2000).
“Deep Penetrating Anchor: Further
Development, Optimization and Capacity
Verification.” Proceedings of the 10th
International Offshore and Polar Engineering
Conference, Seattle, Washington, 28 May- 2
June 2000, pp 410-416.
Medeiros Jr., C. J., Hassui, L. H. Machado, R. D.,
(1997). “Pile for Anchoring Floating
Structures and process for Installing the
Same.” United States Patent Number
6,106,199.
Medeiros, C.J. (2001). “Torpedo anchor for deep
water”. Proc. Deepwater Offshore
Technology Conf., Rio de Janeiro.
Medeiros, C.J. (2002). “Low cost anchor system for
flexible risers in deep waters”. Proc. Annual
Offshore Technology Conf., Houston, Paper
OTC 14151.
Merifield, R. S., Sloan, S. W. & Yu, H. S. (2001).
“Stability of plate anchors in undrained clay”
Géotechnique 51, No. 2, 141-153.
O’Loughlin, C. D., Randolph, M. F., Richardson, M.
(2004). “Experimental and Theoretical
Studies of Deep Penetrating.” Proceedings of
the 36th Annual OTC, Texas, May 3-6, 2004,
Paper nº. OTC 16841.
O’Loughlin, C. D., Richardson, M., Randolph, M. F.
& Gaudin, C. (2013). “Penetration of
Dynamically Installed Anchors in Clay”.
Géothecnique, 63(11), 909-919.
Puech, A., Dendani, H., Nauroy, J-F. and Meunier, J.
(2004). “Characterisation of gulf of guinea
deepwater soils for geotechnical engineering:
Successes and Challenges”. 21-22 Octobre
2004, Seatech week Colloque
“Caractérisation in situ des sols marins”
Brest, France.
Raie, M. S., & Tassoulas, J. L. (2009). “Installation
of torpedo anchors: numerical modeling”.
Journal of geotechnical and
geoenvironmental engineering, 135(12),
1805-1813.
Randolph, M., Cassidy, M., Gourvenec, S., &
Erbrich, C. (2005, September). “Challenges
of offshore geotechnical engineering”. In
Proceedings of the international conference
on soil mechanics and geotechnical
engineering (Vol. 16, No. 1, p. 123). AA
BALKEMA PUBLISHERS.
Song, Z., Hu, Y., O’Loughlin, C., & Randolph, M. F.
(2009). “Loss in anchor embedment during
plate anchor keying in clay”. Journal of
geotechnical and geoenvironmental
engineering, 135(10), 1475-1485.
Sultan, N., Voisset, M., Marsset, T., Vernant, A.M.,
Cauquil, E., Colliat, J.L. & Curinier, V.
(2007). “Detection of free gas and gas
hydrate based on 3D seismic data and cone
penetration testing: An example from the
Nigerian continental slope” .Marine Geology,
240, 235–255.
Wang, D., Gaudin, C., & Randolph, M. F. (2013).
“Large deformation finite element analysis
investigating the performance of anchor
keying flap”. Ocean Engineering, 59, 107-
116. ISO 690.
Wilde, B., Treu, H., and Fulton, T. (2001). “Field
testing of suction embedded plate anchors”.
Proc., 11th Int. Offshore and Polar
Engineering Conf., Int. Society of Offshore
and Polar Engineers, Mountain View, CA,
544–551.
Yang, M., Aubeny, C. P., & Murff, J. D. (2011).
“Behaviour of suction embedded plate
anchors during keying process”. Journal of
Geotechnical and Geoenvironmental
Engineering, 138(2), 174-183.
top related