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Challenges in Shallow Water Riser Design for FPSOs Installed in Campos Basin

Roberto Alvim 2H Offshore

Rio de Janeiro, RJ, Brazil

Elton J. B. Ribeiro OGX Petróleo e Gás Ltda


Edson L. Labanca OGX Petróleo e Gás Ltda


Otavio Veras MBA Student at INSEAD formerly 2H Offshore Senior Engineer

Singapore

ABSTRACT

The selection of flexible riser configurations for production systems

consisting of a turret moored Floating Production Storage and

Offloading (FPSO) vessel for shallow waters can be as challenging as

for deep waters. OGX is currently developing the Waimea and the

Waikiki fields in shallow waters in the Campos Basin, offshore Brazil,

using the turret moored FPSO’s OSX-2 and OSX-3, respectively. Both

fields are identical in relation to the quantity of flexible lines: up to 26

lines, including umbilicals and flexible risers, with outer diameters

ranging from 4inch to 9inch. This paper describes the steps followed to

determine feasible riser configurations. The main challenges are

discussed while different configurations are compared and evaluated.

KEY WORDS: Flexible Risers, Shallow Waters, FPSO, Campos

Basin.

INTRODUCTION

OGX is a private Brazilian operator currently exploring hydrocarbons

in shallow waters in the Campos basin, offshore Brazil. FPSO’s have

been employed in all its field developments so far.

The most recent field developments are located in the Waimea and

Waikiki fields, whose FPSO’s are named OSX-2 and OSX-3

respectively and are located in water depths of 105m and 135m. Both

platforms are VLCC’s (Very Large Crude Carrier) and are kept in

position by a single point mooring system (Turret). OSX-2 makes use

of an internal turret, while OSX-3 adopts an external turret solution,

Figure 1. At the time of writing, they are both under construction and

are planned to be installed in 2013. Each FPSO will gather the oil and

gas production from four subsea satellites wells and from nearby wells

through dry completion Wellhead Platforms (WHP). Water injection

systems to increase the oil production at a later stage are also planned

to be installed from the WHP. Both OSX-2 and 3 are similar in many

aspects including the flexible pipe properties adopted for the flowline

and riser systems.

This paper describes the challenge of finding feasible riser

configurations for the field developments of OSX-2 and OSX-3. The

extensive study includes analysis of three different types of riser

arrangements: free hanging catenary, pliant-wave and lazy-S. The

intricacy of this work was complicated by the quantity of flexible lines

each field is designed for: up to 26 lines, including umbilicals and

flexible risers and flowlines with outer diameters ranging from 4inch to

9inch. Significant interference issues followed the whole process of the

study. The relatively light weight of the flexible lines, geometrical

limitations due to the shallow water depth and the highly dynamic

vessel response added to the complexity of the task and made the riser

system selected for the OSX-2 and OSX-3 vessels unique in the world.

Figure 1 – OSX-3 FPSO External Turret

Learn more at www.2hoffshore.com

PRELIMINARY FPSO SURVEY

A survey of the existing FPSOs in water depths below 200m was

conducted at the early stage of the project in order to identify standard

solutions previously considered for different fields. The riser

configuration types and the quantity of flexible structures attached to

each FPSO were investigated.

Figure 2 shows the quantity of flexible structures attached to the

different FPSO’s and their respective water depths (Wilhoit and Supan,

2010). The points lying on the horizontal and/or vertical axes mean that

either the information regarding the water depth and the number of

risers is missing.

Figure 2 - Number of Risers installed at FPSO’s by Water Depth

A brief description providing the riser configuration types adopted for

some key FPSO’s is also given in Figure 2. For example, the FPSO

OSX-1 is designed with 10 risers in pliant-wave configuration and

other 3 risers in lazy-S configuration through a single Mid-Water Arch

(MWA).

With a total of 26 risers, the Challis Venture, in the Challis and Cassini

fields, offshore Australia, decommissioned in January 2011, in a water

depth of approximately 100m was identified with the maximum

number of risers in shallow waters. However, this FPSO was connected

to a 10.5m diameter SALM riser via a yoke. Other FPSO’s installed in

shallow waters were identified with more than 15 risers, but using

spread moored systems: the Etame, offshore Gabon, and the

Okono/Okpono, offshore Nigeria.

The largest number of risers connected to a turret moored FPSO in

shallow water was identified as the Alvheim FPSO, which has been

designed to accommodate 14 risers and umbilicals. The lazy S

configuration was chosen for all risers on this field using a total of 3

Mid-Water Arch’s.

The conclusion of this survey is that a shallow water field comprised of

such a large number of risers attached to a turret moored FPSO has

never been developed, demonstrating that the riser system designs for

the OSX-2 and OSX-3 are firsts of their kind.

SYSTEM DESCRIPTION

The FPSO OSX-2 is an internal turret moored system and will be

located at the extreme South of the Campos Basin in 135m water depth

and the FPSO OSX-3 is an external turret moored system and will be

located in the Southern region of Campos Basin in 105m water depth.

Both systems will gather the oil production from satellites wells and a

WHP through dry completion wells. The satellite well bundles will be

comprised of: 6” flexible production lines, 4”/6” flexible gas lift/

service lines and electrical/ hydraulic control umbilicals.

The produced oil from the WHP will be exported to the FPSO by an 8”

flexible production line and will import electrical power (three x 5”

power cables), gas for injection and lift method (two 6” flexible lines),

diesel for service (one 4” flexible lines) and water for reservoir

injection (one 8” flexible line).

Figure 3 shows the OSX-2 turret arrangement and the Figure 4 shows

the OSX-3 turret arrangement. It can be observed that a large number

of risers are planned for both fields. It should be noted that the slots #4

and #20 of the OSX-2 FPSO and the slots #8, #15 and #19 of the OSX-

3 FPSO are spare. The properties of the flexible risers and umbilicals

used in the riser analyses of OSX-2/3 are presented on Table 1.

Figure 3 - OSX-2 Turret Arrangement with 25 Risers

Figure 4. OSX-3 Turret Arrangement with 25 Risers

0

20

40

60

80

100

120

140

160

180

200

0 5 10 15 20 25 30

Wa

ter

De

pth

(m

)

Number of Risers

OGX - Conceptual Riser design for OSX-2 and OSX-3

WATER DEPTH VS NUMBER OF RISERSFPSO Survey

North America South America Europe Africa Asia Oceania OSX1 OSX2 OSX3

Okono/Okpoho (NIG)Spread Moored

Alvhein (Norway)3 MWAs

Etame (Gabon)

Spread Moored

Tui (NZ)

4 MWA (3)

Fife, Fergus, Flora, Angus (UK)

8 Pliant + 1 MWA (6 risers)

OSX-2

OSX-3

OSX-1 Waimea

1 MWA (3 risers) +

10 Pliant Wave

Gryphon (UK)

10 risers

Challis/Cassini (AUS)

SALM

Guillemot (UK)

11 risers

Teal (UK)

12 risers

Chestnut and

Shelley (UK)

Monohull


Table 1 – Riser and Umbilical Structural Properties

FEASIBILITY ASSESSMENT METHODOLOGY

The feasibility assessment was conducted in accordance with the

sequence shown in Figure 5. A different configuration is investigated if

the design criteria are not satisfied in any of the steps described below.

Figure 5 – Feasibility Analysis Sequence

Preliminary Configuration Assessment

Static analysis is conducted in order to identify feasible configurations

for all flexible risers and umbilicals attached to each FPSO. Due to

geometrical restraints to assemble the I-Tubes at the OSX-2 turret and

the riser order previously defined for both OSX-2 and OSX-3 turrets,

the same top angle was adopted for all flexible structures attached to

the same FPSO. The top angle of 7 degrees was adopted for the OSX-2

risers and 9 degrees for the OSX-3 risers. The configuration is

considered valid for further analysis if the minimum resultant bending

radius (MBR) along the structure is above the operating bending radius

specific for each structure type. Preliminary interference issues are also

assessed in this phase, such as sag-bend interference with the mudline

and hog-bend interference with the FPSO for the compliant

configurations.

Extreme Storm Analysis

The configurations selected in the preliminary assessment are evaluated

considering a load case matrix developed in accordance with the

environmental data available for the field and vessel headings expected

for the turret moored FPSO’s. Regular wave approach is adopted for

the analysis.

A total of 8 loading directions are considered, with respect to the riser

plane: Near, Cross Near 1, Transverse 1, Cross Far 1, Far, Cross Far 2,

Transverse 2 and Cross Near 2. Each loading direction corresponds to a

vessel offset direction. Directional mean vessel offsets are considered

for the analysis in accordance with the information provided by the

designer of the mooring system.

100year return current loading together with 10year return wave

loading (and vice-versa), applied at the offset direction, or 22.5deg off

offset direction are considered as summarized in Table 2, for a given

Offset direction. A total of 64 loading conditions is analyzed by

multiplying the 8 loading conditions summarized in Table 2 by the 8

loading directions.

Table 2 – Extreme Storm Load Case Summary for Each Loading

Direction

Load

Case # Current Direction

Current

Return

Period

(yrs)

Wave Direction

Wave

Return

Period

(yrs)

1 Aligned with offset dir. 10 Aligned with offset dir. 100

2 Aligned with offset dir. 100 Aligned with offset dir. 10

3 +22.5deg off offset dir. 10 -22.5deg off offset dir. 100

4 +22.5deg off offset dir. 100 -22.5deg off offset dir. 10

5 -22.5deg off offset dir. 10 +22.5deg off offset dir. 100

6 -22.5deg off offset dir. 100 +22.5deg off offset dir. 10

7 Aligned with offset dir. 100 +90deg off offset dir. 1

8 Aligned with offset dir. 100 -90deg off offset dir. 1

The minimum and maximum content densities expected for the life of

both fields are considered in the analysis as well as the minimum and

maximum FPSO drafts. Marine growth is taken into account in this

assessment.

Abnormal conditions are also evaluated, considering a 25deg vessel list

together with 1yr return current and wave loadings.

Marine growth is considered for the analysis in all loading conditions.

In addition to the design criteria established for the preliminary

configuration assessment, compression in the flexible structures is

evaluated and compared against their limits. When applicable,

minimum tether tension is also assessed.

Interference Analysis

Due to the nature of the field layout, with a large number of structures

attached to each FPSO, interference is the key challenge to designing

the riser system. In some cases, 9 risers are present in a single turret

sector. A load case matrix similar to the one considered for extreme

storm analysis is considered for interference.

A finite element model with all risers, umbilicals and mooring lines is

constructed and preliminary quasi-static analysis considering 100yr

return current loading and directional extreme offsets is conducted in

order to identify if contact is likely to occur between any of the subsea

structures. Depending on analysis results, further investigations are

conducted on the configurations, including wave loadings.

During this analysis phase, the minimum clearance between the risers

and the mooring lines or any other subsea structure is assessed. Each

structure pair must present in all loading conditions a minimum

clearance larger than the sum of their outer diameters, as recommended

by DNV-RP-F203. Interference between the risers and the umbilicals is

left to be checked during the detailed design phase, but are assumed to

be acceptable provided that the contact energy obtained from the

dynamic simulations is shown to be within safe levels.

FREE HANGING CATENARY CONFIGURATION

ASSESSMENT

Typically, a free hanging catenary configuration is not recommended

for risers and umbilicals operating in shallow waters. This is because

the dynamic motions induced at the top of the risers by wave loading

are not sufficiently damped along the structure to avoid high curvatures

at the touchdown zone. This violating the operating bending radius of

the structure and produces high levels of compression at the TDP.

Function

Ou

ter

Dia

met

er (

mm

)

Insi

de

Dia

met

er (

mm

)

Eff

ecti

ve

Insi

de

Dia

met

er

(mm

)

Wei

gh

t E

mp

ty i

n A

ir (

kg

f/m

)

Wei

gh

t S

eaw

ater

Fil

led

in

Air

*

(kg

f/m

)

Wei

gh

t S

eaw

ater

Fil

led

in

Sea

wat

er*

(k

gf/

m)

Ben

din

g S

tiff

nes

s (k

N.m

2)

To

rsio

nal

Sti

ffn

ess

(kN

.m2

)

Ax

ial

Sti

ffn

ess

(kN

)

Op

erat

ing

Ben

din

g R

adiu

s (m

)

Fai

lure

Ten

sio

n (

kN

)

4” Service/G.L 164 102 106.1 44.3 53.4 31.6 3.64 240 1E+05 1.85 1395

5” PU 126 # # # 35.2 22.5 4 107 3E+05 1.9 1369

4” EHU 153 # # 34.8 40.2 23.8 8 237 4E+05 2.3 1987

6” EPHU 153 # # 34.8 40.2 23.8 8 237 4E+05 2.3 1987

6” G.L/ Gas Injection 250 152 159 111 131 80.4 25.7 1787 3E+05 2.63 1938

6” Production/ Test 247 152 157 114 134 84.9 33.6 2745 3E+05 2.63 2524

8” Production 316 203 209 182 217 137 83.7 5916 4E+05 3.45 3523

8” Water Injection 283 203 203 89.2 122 58.1 30.3 2380 2E+05 2.77 2973

9.13” Gas Export 348 232 238 209 255 157 124 8203 5E+05 3.91 7452

* for umbilical the hoses are full of hydraulic fluid and interstices.

Preliminary Configuration Assessment

Extreme Storm Analysis



Furthermore, the vessel offsets in terms of percentage of water depth

are significantly higher than the offsets usually considered in deepwater

applications, increasing the top angle range around its nominal value.

Nevertheless, a preliminary assessment is conducted for the 6inch

production and 6inch gas lift risers in order to assess the response of the

risers and confirm or not that this is unacceptable. This assessment is

conducted only for the OSX-2 development as it shows less severe

dynamic response than the OSX-3, mainly due to differences related to

the turret location with respect to the vessel center of motion.

Both structures analyzed presented minimum bending radii below the

operating bending radius at the touchdown region under 100yr return

wave loading coming from the Southwest, which is the most critical

wave loading present in the Campos Basin, confirming the unfeasibility

of this type of configuration for such applications.

PLIANT-WAVE CONFIGURATION ASSESSMENT

As the preliminary studies confirmed that the free hanging catenary

configuration does to not work in such water depths, it was necessary to

consider compliant configurations for the OSX-2 and OSX-3 risers.

The lazy-wave configuration was disregarded due to the large number

of risers attached to the FPSO and the proximity between the TDP’s

and buoyancy modules expected for both fields which would lead to

severe interference issues.

The pliant-wave configuration was selected because the suspended riser

region close to TDP is tethered, restraining considerably the lateral

motions in this region.

The first challenge of this is study is to find feasible configurations for

diverse flexible structure types considering similar top angles. A

preliminary study is conducted, varying several parameters such as net

buoyancy, length of the buoyant section, suspended length of the riser

and tether positioning. The same configuration is analyzed under 100yr

return period currents applied to the near and far offset directions with

respect to the riser plane. The minimum and maximum fluid densities

are also applied in order to verify if the resulting catenary profiles are

acceptable in terms of interference with the vessel and the seabed.

Once the preliminary configurations are selected, extreme storm

analysis is conducted in order to verify if the proposed configurations

are valid under combined extreme wave, current and offset loading. If

not, a different configuration is selected following an iterative

approach. When all configurations are considered valid, interference

between the adjacent structure pairs is conducted. Due to proximity of

the subsea structures, as shown in Figure 6, it is considered that contact

between two lines is allowable if the contact occurs at the sections

without buoyancy of both structures. The interference analysis

classifications are summarized in Table 3, and the results summarized

in Figure 7, considering the color scheme established in Table 3. The

analysis results show that almost 60% of the structure pairs are found to

present un-permissible clashing for the OSX-3 configurations. The

same trend was found for the pliant-wave configurations selected for

the OSX-2 development. The current loadings are found to be the main

issue to obtain feasible pliant-wave configurations from interference

standpoint.

In summary, interference is a complex problem to be solved due to the

large number of flexible structures planned for both fields. Interference

between the buoyant sections of the pliant-wave risers with adjacent

structures and interference between the risers and the mooring lines

were identified for several structures under normal operating and

extreme loading conditions. For this reason, the pliant-wave

configuration is not recommended to be adopted for the OSX-2 and the

OSX-3 risers.

Figure 6 – OSX-2 Interference Analysis Model

Table 3 – Interference Analysis Result Classification

Figure 7 – OSX-3 Interference Analysis Qualitative Results Summary

LAZY-S CONFIGURATION ASSESSMENT

The lazy-S configuration is identified as an alternative to mitigate the

severe interference issues. In the lazy-S configuration there is a subsea

buoy, also known as Mid Water Arch (MWA), which consists of a

subsea buoyancy tank assembly, two tethers and a gravity base. The

buoyancy tank assembly provides net buoyancy that withstands the

forces imposed to the structure by the risers, as shown in Figure 8. The

main advantage of using the Lazy-S configuration in comparison with

the pliant wave configuration is that a number of risers can be grouped

and attached to a single MWA, giving more room between the mooring


Result Classification Criteria

No Clashing (Minimum

Clearance is reported)

The lines do not touch each other although

they may be very close to.

Allowable Clashing (AC) Contact between two lines occurs along the

bare part of the line.

Impermissible Clashing

(C)

Clashing occurs between line and anchor,

or between lines along the buoyant section

of at least one of the lines.


lines and adjacent risers. Another advantage is that the TDP response is

significantly improved with the addition of the buoy, while absorbs the

tension variation induced by the FPSO motions.

Each of the three sectors delimited by the mooring lines are planned to

have 3 Mid-Water Arches (MWAs), giving a total of 9 MWAs for both

the OSX-2 and OSX-3 fields. The proposed arrangements consist of a

number of flexible structures, varying from 2 to 4, connected to a single

MWA. The reason for adopting the use of 3 MWAs on each sector is to

minimize the buoyancy requirements and subsequent increased tank

sizes, affecting the selection of installation resources.

The Mid-Water Arches are assumed with a net upthrust of

approximately 76Tonnes.

Figure 8 – Lazy-S Riser Configuration

Preliminary static analysis is conducted in order to select the lazy-S

configurations for both fields, following the same approach considered

for the pliant-wave configurations. In order to achieve the required top

angle of 9 degrees and improve the interference and extreme storm

response, some light structures attached to the OSX-3 FPSO, such as

umbilicals and service lines, require ballast modules.

The lazy-S flexible structure distribution per each MWA and the

configuration summary after the iterative analysis cycles considering

extreme storm and interference analysis are given in Table 4 for OSX-2

and Table 5 for OSX-3, in accordance with the schematic shown in

Figure 9.

The configurations selected are analyzed dynamically in order to

evaluate the riser system response under extreme and accidental

conditions.

The effective tension envelopes at the hang-off are shown in Figure 10

for the FPSO OSX-2 and Figure 11 for the FPSO OSX-3. Although the

riser structural properties and the internal fluids considered for both

developments are the same, the maximum effective tension and the

overall ranges found for the FPSO OSX-3 are significantly higher than

the FPSO OSX-2. Furthermore, the environmental conditions

considered for the analysis are the same.

The main reason for this is because the FPSO OSX-2 is designed with

an internal turret located approximately 110m from the vessel CoG.

The FPSO OSX-3 is designed with an external turret, located

approximately 190m from the CoG, i.e. 72% further from the FPSO

centre of gravity than OSX-2.

Figure 9 – Lazy S Riser Configuration Schematic

Table 4 – Lazy S Configuration Summary for OSX-2

Sec

tor

MW

A #

Slo

t #

Azi

mu

th

Str

uct

ure

Ty

pe

L1

L2

H1

H2

V1

NE

1

1 18 4” Service 173 70 80 24 60

2 18 6” Umbilical 176 71 80 25 60

3 18 6” Production 170 71 80 25 60

2

4 42 Spare - - - - -

5 42 4” Service 172 69 80 23 60

6 42 4” Power Umbilical 176 70 80 24 60

3

7 69 4” Power Umbilical 171 71 80 26 60

8 69 4” Power Umbilical 174 71 80 26 60

9 69 8” Production 170 71 80 26 60

S

4

10 124 6” Production Test 160 93 70 46 70

11 124 6” Gas Injection 164 93 70 56 70

12 124 8” Water Injection 161 93 70 56 70

5

13 160 6” Gas Lift 152 83 70 31 70

14 160 4” Service 149 82 70 31 70

15 160 4” Umbilical 153 83 70 31 70

6

16 188 6” Umbilical 154 92 70 52 70

17 188 6” Umbilical 158 92 70 51 70

18 188 6” Production 154 91 70 51 70

W

7

19 247 6” Production 155 53 75 28 40

20 247 Spare - - - - -

21 247 8” Water Injection 155 54 75 28 40

8

22 272 6” Gas Lift 159 53 75 27 40

23 272 9.13” Gas Export 154 54 75 28 40

24 272 4” Umbilical 159 54 75 28 40

9

25 299 4” Service 156 54 75 29 40

26 299 6” Umbilical 159 54 75 29 40

27 299 6” Production 155 53 75 28 40

Table 5 – Lazy S Configuration Summary for OSX-3

Sec

tor

MW

A #

Slo

t #

Azi

mu

th

Str

uct

ure

Ty

pe

L1

L2

H1

H2

V1

NE

1

4 27 4” Service 184 70 99 20 60

22 28 6” Umbilical 173 71 99 21 60

2 30 6” Production 176 70 99 20 60

2

5 44 4” Service 179 83 99 48 60

7 45 4” Umbilical 189 83 99 48 60

8 46 6” Umbilical 186 84 99 49 60

9 47 6” Umbilical 188 83 99 48 60

3 13 69 6” Production 161 83 89 50 60

16 70 6” Production 173 83 89 50 60


S

4

15 145 Spare - - - - -

19 146 8” Water Injection 168 71 95 48 47

23 146 6” Gas Lift 171 71 95 48 47

5

3 165 4” Service 177 76 100 39 58

21 167 6” Umbilical 186 77 100 40 58

1 167 6” Production 178 76 100 39 58

6 26 183 4” Umbilical 170 77 92 53 50

25 184 9.13” Gas Export 174 76 92 52 50

W

7

6 270 4” Service 195 81 101 46 60

12 271 4” Power Umbilical 197 81 101 46 60

11 272 4” Power Umbilical 193 82 101 47 60

8

10 287 4” Power Umbilical 183 62 95 32 47

14 288 8” Production 166 63 95 33 47

18 289 6” Production Test 177 64 95 34 47

9

17 308 6” Gas Injection 170 65 95 38 47

20 309 8” Water Injection 168 65 95 38 47

24 310 6” Gas Lift 180 65 95 38 47

Hence, although the RAOs for the both platforms are similar, the

dynamic amplification of the pitch is considerably higher for the FPSO

OSX-3, as illustrated in Figure 12.

Figure 10 – Effective Tension Ranges at the Hang-Off – OSX-2

Figure 11 – Effective Tension at the Hang-Off – OSX-3

Figure 12 – Timetrace Comparison of Hang-off Vertical Motions

The considerably higher dynamic effects at the top of the risers and

umbilicals for the OSX-3 are found to affect significantly the overall

response of the system, as seen for example, on tether tension

envelopes for OSX-2, Figure 13 and OSX-3 Figure 14. Note that

almost all OSX-3 tethers present minimum effective tensions below

100kN.

Both examples show that finding valid configurations for the OSX-3

development is an even harder task than finding the OSX-2 riser

configurations.

The minimum bending radius along the risers and the umbilicals are

verified in order to confirm the fitness-for purpose of the flexible

structures proposed for both fields.

Figure 13 – Effective Tension at the MWA Tethers – OSX-2

Figure 14 – Effective Tension at the MWA Tethers – OSX-3

After confirming that the flexible structures attached to all MWAs and

their selected configurations are valid under extreme storm conditions,

interference analysis is conducted to verify if they are fit to work

together. Orcaflex models are built considering all subsea structures

attached to the turret, including the mooring lines, as shown in Figure

15 for OSX-2 and Figure 16 for OSX-3.

The minimum clearance between the flexible structures and the other

structures (such as mooring lines, MWAs and tethers) within the same

sector is assessed. As a means to optimize the Lazy-S riser

configurations of the FPSO OSX-3 for interference issues, most of the

umbilicals and 4inch Service flexible risers and the 8inch Water

Injection of MWA-9, consider ballast modules installed along the line

in the region between the bellmouth and the MWA, as previously

mentioned. Different distances from MWA to the vessel turret for

0

100

200

300

400

500

600

4”

Serv

ice

6”

Um

bili

cal

6”

Pro

duct

ion

Spare

4”

Serv

ice

4”

Pow

er

Um

bili

cal

4”

Pow

er

Um

bili

cal

4”

Pow

er

Um

bili

cal

8”

Pro

duct

ion

6”

Pro

duct

ion

6”

Gas

Inje

ctio

n

8”

Wate

r In

ject

ion

6”

Gas

Lift

4”

Serv

ice

4”

Um

bili

cal

6”

Um

bili

cal

6”

Um

bili

cal

6”

Pro

duct

ion

6”

Pro

duct

ion

Spare

8”

Wate

r In

ject

ion

6”

Gas

Lift

9.1

3”

Gas

Export

4”

Um

bili

cal

4”

Serv

ice

6”

Um

bili

cal

6”

Pro

duct

ion

18 18 18 42 42 42 69 69 69 124 124 124 160 160 160 188 188 188 247 247 247 272 272 272 299 299 299

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

MWA1 MWA2 MWA3 MWA4 MWA5 MWA6 MWA7 MWA8 MWA9

Northeast Sector South Sector West Sector

Eff

ecti

ve

Te

nsio

n R

an

ge

(k

N)

OGX - Riser System Conceptual Design for OSX-2

EFFECTIVE TENSION RANGE AT THE FPSO ENDExtreme Storm - Case 1

Riser Type

Riser Azimuth

Slot Number

MWA Number

0

100

200

300

400

500

600

4”

Serv

ice

6”

Um

bili

cal

6”

Pro

duct

ion

4”

Serv

ice

6”

Um

bili

cal

4”

Um

bili

cal

6”

Um

bili

cal

6”

Pro

duct

ion

6”

Pro

duct

ion

Spare

8”

Wate

r In

ject

ion

6”

Gas

Lift

4”

Serv

ice

6”

Um

bili

cal

6”

Pro

duct

ion

4”

Um

bili

cal

9.1

3”

Gas

Export

4”

Serv

ice

4”

Pow

er

Um

bili

cal

4”

Pow

er

Um

bili

cal

4”

Pow

er

Um

bili

cal

8”

Pro

duct

ion

6”

Pro

duct

ion

6”

Gas

Inje

ctio

n

8”

Wate

r In

ject

ion

6”

Gas

Lift

27 28 30 44 45 46 47 69 70 145 146 146 165 167 167 183 184 270 271 272 287 288 289 308 309 310

4 22 2 5 7 8 9 13 16 15 19 23 3 21 1 26 25 6 12 11 10 14 18 17 20 24



Eff

ecti

ve

Te

nsio

n R

an

ge

(k

N)


EFFECTIVE TENSION RANGE AT THE FPSO ENDExtreme Storm - Case 1

Riser Type

Riser Azimuth

Slot Number

MWA Number

OGX Conceptual Ryser System Design for OSX-2 & OSX-3

TIMETRACE OF VERTICAL MOTIONS AT THE TOP OF THE RISERSW 100yr Return Wave - H = 15.28m, T = 14.86s

-16

-14

-12

-10

-8

-6

-4

-2

0

2

4

6

8

10

12

14

16

80 85 90 95 100 105 110 115 120

Time (s)

Ve

rtic

al

Po

sit

ion

at

the

To

p o

f th

e R

ise

r

(m)

OSX-3 OSX-2 Generic Semi-Sub

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

400.0

450.0

500.0

550.0

600.0

650.0

700.0

Tether01

Tether02

Tether01

Tether02

Tether01

Tether02

Tether01

Tether02

Tether01

Tether02

Tether01

Tether02

Tether01

Tether02

Tether01

Tether02

Tether01

Tether02



Eff

ecti

ve

Te

nsio

n R

an

ge

(k

N)


EFFECTIVE TENSION RANGE AT THE MWA TETHERSExtreme Storm - Case 1

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

400.0

450.0

500.0

550.0

600.0

650.0

700.0

Tether01

Tether02

Tether01

Tether02

Tether01

Tether02

Tether01

Tether02

Tether01

Tether02

Tether01

Tether02

Tether01

Tether02

Tether01

Tether02

Tether01

Tether02



Eff

ecti

ve

Te

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n R

an

ge

(k

N)


EFFECTIVE TENSION RANGE AT THE MWA TETHERSExtreme Storm - Case 1


adjacent MWAs and different heights for each MWA are some

additional measures considered to mitigate the interference issues.



One of the critical interference issues is to avoid clashing of any

flexible structure and umbilicals against the mooring lines. The lightest

structures such as umbilicals and the 4” Service risers present large

lateral displacements under extreme transverse current loading.

Additionally, the effect of wave loading at the top section of the riser is

found to reduce significantly the minimum clearances with the mooring

lines.

The sagbend displacements are also found to be an issue, due to the

proximity between the 3 MWAs of each sector. Interference between

the light structures and the MWAs and/or tethers are found to be

critical.

The interference analysis results are summarized in Table 6 for OSX-2

and Table 7 for OSX-3. Both results are for the Northeast Sector of

each field. This assessment only considers the evaluation of

impermissible clashing between the flexible structures and the mooring

lines or any other subsea structure. The analysis results show that the

interference response for the FPSO OSX-3 field is also more critical

than the FPSO OSX-2, mainly due to the more severe dynamic

behavior imposed by the OSX-3 vessel motions.

As identified for the pliant-wave configurations, the current loadings

are found to be the main issue for interference. However, wave loading

is found to affect significantly the interference response, especially for

the OSX-3, where the risers are connected to an external turret.

Table 6 – Interference Analysis Summary for Sector NE – OSX-2

*Minimum clearance values given in meters

Table 7 – Interference Analysis Summary for Sector NE – OSX-3

*Minimum clearance values given in meters

CONCLUSIONS

From the studies conducted the key conclusions are:

Never before has such large number of risers proposed for the

OSX-2 and OSX-3 developments have been used in shallow

waters using a turret moored FPSO;

Free hanging catenary riser configurations are not feasible for

such water depths as the curvatures at the TDP are severe due

to large FPSO offsets and wave induced motions;

Pliant-wave configurations are feasible in terms of extreme

storm response, however interference could not be solved for

the proposed layouts of both the OSX-2 and OSX3 FPSOs;

Lazy-S configuration concept is feasible for both fields;

The development of the OSX-3 riser configurations is

significantly more complicated than OSX-2 mainly because

of the external turret concept which is found to increase

significantly the dynamic response of the riser systems;

The results obtained from the conceptual studies and summarized in

this paper indicate that the lazy-S concept is technically feasible from

the stand point of operation of the flexible pipe, but there are still

interference issues to be verified, such as the interference between the

flexible lines and umbilicals.

For this reason, it is recommended that further analysis shall be

conducted in order to evaluate the contact energy levels and verify its

acceptability. Other essential structural verification to be assessed is the

evaluation of the fatigue response of the flexible structures.


Further improvements for the OSX-3 FPSO riser system may also be

adopted, such as an increase of the MWA net buoyancy in order to

increase its stability and reduce the dynamics along the riser sagbend

and TDP.

ACKNOWLEDGEMENTS

We would like to thank the 2H Offshore Brazil technical manager Pete

Simpson for the valuable guidance and advice and also we would like

to thank the 2H Offshore Brazil director Julio Ribeiro for providing us

helpful assistance for this project. Special thanks also for the project

group members: Marcio Silva, Vinicius Silva, Ana Carolina Aguiar,

Luiz Henrique Silva, Fernanda Bernardes, Thiago Almeida and

Marcelo Lopes for their invaluable assistance.

REFERENCES

DNV-RP-F203 (2009) “Recommended Practice for Riser Interference”

Wilhoit, L and Supan, C (2010). "2010 Worldwide Survey of Floating

Production, Storage and Offloading (FPSO) Units," Offshore

Magazine and Mustang Engineering.

Orcina - OrcaFlex 9.4d – Marine Dynamics Finite Element Program.


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