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
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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