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.

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