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Proceedings of the Marine Operations Specialty Symposium 2008

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ANALYSIS OF AN ALTERNATIVE PIPELINE INSTALLATION PROCEDURE THATCOMBINES ONSHORE DEFLECTION AND OFFSHORE TRANSPORTATION

Danilo Machado Lawinscky da Silva1

Rodrigo Almeida Bahiense1

Breno Pinheiro Jacob1

Fernando Gomes da Silva Torres2

Antonio Roberto Medeiros2

1LAMCSO Laboratory of Computational Methods and Offshore Systems PEC/COPPE/[email protected] , [email protected] , [email protected]

2PETROBRAS Petrleo Brasileiro S.A.

[email protected] , [email protected]

ABSTRACT

Conventional offshore pipeline installation operations in

Brazil have been performed in an S-Lay procedure employing

the BGL-1 barge, owned by Petrobras. However, this procedure

has some limits, and may not be feasible in some particular

scenarios.An alternative procedure used by Petrobras is the so-called

lateral deflection procedure, which basically consists of

performing the pipeline assembly on shore, and then deflecting

it to the sea using a tugboat. After that, the pipeline is towed to

its installation area.

The objective of this work is to present numerical

simulations of both stages of this procedure (the lateral

deflection procedure itself and the pipeline transportation). The

simulations were performed to help planning an actual

operation that was scheduled to occur in the Xaru field, at the

state of Cear, northeast Brazil. These simulations employ the

SITUA-Prosim computational tool, which is able to incorporate

the correct definition of the seabed and shore from bathymetriccurves.

Regarding the transportation stage, typically it is

performed using a front and a rear tugboat aligned at the

transportation route. As a result of the simulations, a different

scheme was proposed, using only one tugboat.

INTRODUCTION

The installation of pipelines is among the most challenging

offshore operations. The most common method of pipeline

installation in shallow water is the S-Lay method. In this

method, the welded pipeline is supported on the rollers of the

vessel and the stinger, forming the over-bend. Then it is

suspended in the water all the way to seabed, forming the sag-

bend. The over-bend and sag-bend form the shape of an S.

The BGL-1 (a pipeline launching barge owned by the

Brazilian state oil company - Petrobras) is used to perform S-Lay pipeline installation operations. The BGL-1 is a second

generation lay barge that performs installation operations by

moving forward using its own mooring lines. This involves the

definition of a complex mooring procedure, as a sequence of

operations that determine the mooring line positions and induce

the lay barge movement as it lays the pipeline. Basically,

tugboats drop anchors at some predefined positions; then the

barge winches release the stern mooring cables, and collect the

mooring cables located at the bow. This is a delicate operation

essential to keeping the position and direction of the lay barge

in accordance with the planned route. The loss of a mooring

anchor during such operation can cause sudden yawing or

drifting of the barge, which in turn can result in buckling of thepipe at the end of the stinger due the excessive bending.

The procedure described above has some limits, such as: i)

it has a very restrictive limitation according to the weather

conditions; ii) the procedure is extremely complex when

performed in congested areas [1].

Therefore, Petrobras has considered an alternative

procedure that combines onshore lateral deflection and offshore

transportation. This work is focused in the numerical

simulation of this procedure. Several analyses are performed in

order to assess the behavior of this alternative pipeline

installation procedure.

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LATERAL DEFLECTION PROCEDURE

The lateral deflection procedure basically consists of

performing the pipeline assembly on shore and deflecting it tothe sea using a tugboat. The characterization of this procedure

involves the determination of the better velocity and direction

of the tugboat, in order to minimize the efforts on the pipeline

(especially due to the curvatures).

Scenario and characteristics of pipeline

The pipeline was installed to rehabilitate a 10 pipeline

that was in the end of its lifetime. The pipeline is located at the

Xaru field, interconnecting the PXA-1 platform to the buoy

frame, with the basic purpose of transporting the oil production

of Xaru, Atum and Curim fields, in Cear State (northeast of

Brazil), to the NT ALIANZA Ship.The pipeline has a total length of 721m and was assembled

at Canto Beach, in Paracuru city, deflected from shore to the

sea, and transported with buoys to the installation location,

where it was positioned on the guideline and sunk by flooding

the buoys.

During assembly, the pipeline was positioned on "big-

bags" (bags of sand) as shown in Figure 1.

FIGURE 1. PIPELINE ASSEMBLY SCHEME

The physical and geometric properties of the pipeline and

of the buoys are presented in the following tables. The buoys

were fastened to the pipeline at every 8m measured from the

center of each buoy.

Numerical Models

To perform the analyses of the lateral deflection procedure,

Petrobras considered the use of the SITUA-Prosim system.

This system has been developed since 1997 [2], in cooperation

by Petrobras and LAMCSO (Laboratory of Computational

Methods and Offshore Systems, at the Civil Eng. Dept. of

COPPE/UFRJ, Federal Univ. of Rio de Janeiro). It is a Finite-

Element based nonlinear dynamic solver, that performs the

calculations with an interface with the Petrobras SGO, that

comprises a database with the seabed bathymetry and obstacles.

Therefore, the simulation takes into account the actual

bathymetric information and obstacle positions [3,4].

Three-dimensional frame elements were employed in the

generation of the numerical model for the pipeline. Three-dimensional frame elements were employed also for the

representation of the pipe segments with buoys. An equivalent

element was used to represent both the pipeline physical

properties and the buoy hydrodynamic properties. The

characteristics of the equivalent pipeline+buoy element are

shown in Table 3.

TABLE 1. 10 PIPELINE DATA

Parameter Value Unit

Outside Diameter 0.27305 m

Inside Diameter 0.2445 m

Yield Stress of steel 414000 kN/m2

Modulus of Elasticity of steel 207000 MPaAxial Stiffness (EA) 2402252.49 kN

Flexional Stiffness (EI) 20169.39 kN*m2

Poisson Coefficient 0.3 -

Density of steel 77 kN/m3

Corrosion Protection 0.0027 m

Corr. Protection Specific Mass 9.32 kN/m3

Hydrodynamic Diameter 0.27875 m

Tube Length 12 m

Weight in Air 0.91099 kN/m

Weight in Water 0.32220 kN/m

TABLE 2. 10 BUOY DATA


Diameter 0.762 m

Length 1.129 m

Weight in Air 1.2851 kN

Buoyancy 3.4138 kN

TABLE 3. PIPELINE + BUOY DATA


Outside Diameter 0.27305 m

Inside Diameter 0.2445 m

Axial Stiffness (EA) 2402252.49 kN

Flexional Stiffness (EI) 20169.39 kN*m2

Hydrodynamic Diameter 0.762 mWeight in Air 2.23530 kN/m

Weight in Water -3.06225 kN/m

Performed Analyses

Several numerical simulations were performed to guide the

lateral deflection procedure. The objective of these parametric

studies was to define adequate combinations of tugboat route

and velocity for the lateral deflection procedure, Figure 2.

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FIGURE 2. DIFFERENT TUGBOAT ROUTES

Some typical results are shown in the figures that follow:

Figures 3, 4 and 5 show the maximum values of Von Mises

stresses along the pipeline for several directions and three

tugboat velocities (the red line indicates the allowable stress);

Figures 6, 7 and 8 show the time-history of the tugboat forces

for the same directions and velocities. The complete description

and results of these parametric studies is presented in reference

[5].

0

100000

200000

300000

400000

500000

600000

700000

800000

0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0

Pipeline Length (m)

V

o

n

M

i

s

e

s

(

K

N

/

m

2

) Direction (-5)

Direction (0)

Direction (5)

Direction (10)

Direction (15)

Direction (20)

Yield Stress

FIGURE 3. VON MISES STRESS IN PIPELINE 3 KM/H

0

100000

200000

300000

400000500000

600000

700000

800000

0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0

Pipeline Length (m)

V

o

n

M

i

s

e

s

(

K

N

/

m

2

) Direction (-5)

Direction (0)

Direction (5)

Direction (10)

Direction (15)Direction (20)

Yield Stress


0

100000

200000

300000

400000

500000

600000

700000

800000

0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0

Pipeline Length (m)

V

o

n

M

i

s

e

s

(

K

N

/

m

2

) Direction (-5)

Direction (0)

Direction (5)

Direction (10)

Direction (15)

Direction (20)Yield Stress


0.0

50.0

100.0

150.0

200.0

250.0

300.0

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

Time (s)

T

r

a

c

t

i

o

n

(

K

N

)

Direction (-5)

Direction (0)

Direction (5)

Direction (10)

Direction (15)

Direction (20)

FIGURE 6. TENSION IN THE CABLE 3 KM/H

0.0

50.0

100.0

150.0

200.0

250.0

300.0

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500Time (s)

T

r

a

c

t

i

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(

K

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)

Direction (-5)

Direction (0)

Direction (5)

Direction (10)

Direction (15)

Direction (20)


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0.0

50.0

100.0

150.0

200.0

250.0

300.0

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500Time (s)

T

r

a

c

t

i

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(

K

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)

Direction (-5)

Direction (0)

Direction (5)

Direction (10)

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Direction (20)


Lateral Deflection Operation

Later, when the actual lateral deflection operation was

performed, it could be observed that the pipeline behavior was

the same as predicted by the numerical simulations. Differentstages of this procedure are shown in Figures 9, 10 and 11.

FIGURE 9. LATERAL DEFLECTION: INITIAL STAGE

FIGURE 10. LATERAL DEFLECTION: INTERMEDIATE STAGE

FIGURE 11. LATERAL DEFLECTION: FINAL STAGE

OFFSHORE TRANSPORTATION

After the inicial lateral deflection operation was concluded,

the pipeline installation procedure proceeded by towing the

pipe using a front and a back tugboat aligned at the

transportation route, as shown in Figure 12.

FIGURE 12. TRANSPORT TYPICAL CONFIGURATION

The objective of the numerical simulations performed for

this transportation stage was to verify the pipeline behavior

under environmental loadings with the transport configuration

defined by Petrobras.

In this configuration, shown in Figure 12, two cables with

250m length connect the pipeline and the two tugboats. Thetugboats velocities are about 5km/h (aligned at 355o from

north). The environmental loads are shown in Table 4. As the

pipeline remains totally submerged and the buoys at least 50%

submerged, wind effect was not considered.

TABLE 4. ENVIROMENTAL LOADS

Load Azimuth Value

Current 315o

1.19m/s

Wave 30o Hs = 1.6m; Tp = 9.5s

The minimum and maximum velocities of tow were

defined by Petrobras as 5km/h and 9.26km/h. The maximum

velocity was defined to prevent buoy movement on thepipeline. It was verified in previews operations, under similar

conditions, that buoys can slip if the tow velocity exceed

9.26km/h. This movement of the buoys makes the pipeline lose

buoyancy, and may experience excessively higher curvatures at

these points.

After several parametric studies, a second configuration

was proposed. In this configuration the two tugboats are not

aligned, as indicates in Figure 13.

FIGURE 13. TRANSPORT - ALTERNATIVE

CONFIGURATION

It was observed that, in such configurations, smaller values

of cable tensions are obtained when the pipeline is nearly

aligned to the resultant direction of the environmental

conditions. However, the cable tensions are still relatively high

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during the whole operation. In such cases, maximum tensions

in the tugboat cable are approximately 51.1kN (5km/h velocity)

and 223.9kN (9.26km/h velocity).

It was also analyzed a situation in which the back tugboatis disconnected and only the front tugboat is pulling the

pipeline. This configuration simulates a situation in which one

of the tugboats loses control and its cable is disconnected.

The results of the analyses indicated that, for all cases, the

maximum values of Von Mises stresses are not an issue, always

staying well below the yield stress of the material. The

objective then was to minimize tugboat forces.

The smaller values of cable tensions were found in

configurations where the back tugboat is disconnected. In such

cases, tensions in the cable are approximately 19.9kN (5km/h

velocity) and 61.0kN (9.26km/h velocity). Therefore,

significant reductions were obtained in the cable tension: 61%

for the velocity of 5m/s, and 72.8% for the velocity of9.26km/h.

Therefore, the results of the analyses indicated that the best

situation occurs when the back tugboat does not tension the

pipe, or simply when it is not connected to the pipe. Another

smaller boat can accompany the transport operation for safety

reasons, and to perform the maneuvers needed for the

subsequent pipeline launching process.

The pipeline transportation was performed by Petrobras

using only one tugboat and all numerical predictions related to

the pipeline behavior were confirmed. Different stages of the

pipeline transportation are shown in Figures 14, 15, 16 and 17.

FIGURE 14. PIPELINE LEAVING BEACH

The pipeline assumes different configurations depending

on transport velocity. The pipeline configuration for the

minimum tugboat velocity is shown in Figure 15; the

configuration for the maximum tugboat velocity is shown in

Figure 16.

FIGURE 15. LOWER TRANSPORT VELOCITY

FIGURE 16. HIGHER TRANSPORT VELOCITY

The maneuvers at installation area are shown in Figure 17.

At this time an auxiliary boat is already connected to the

pipeline.

FIGURE 17. MANEUVERS AT INSTALLATION AREA

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FINAL REMARKS

This work presented the results of numerical simulations

and parametric studies on the pipeline behavior during somestages of the installation procedure for the pipeline that

interconnect Xaru-1 (PXA-1) platform to the buoy frame near

the coast of Cear state, Brazil.

Such analyses were intended to verify the pipeline

behavior during the lateral deflection (when the pipeline leaves

the beach), and during transport to the installation area.

The results of the parametric studies allowed the definition

of the most suitable conditions for each stage of the operation.

Regarding the transport stage, it was noticed that the best

configuration to transport the pipeline, where tensions in the

tugboat cable are minimized, occurs when the pipeline

direction is close to the resultant of environmental loads and

the back tugboat is disconnected. The pipeline transportationwas performed using only the front tugboat and all numerical

predictions related to the pipeline behavior were confirmed. A

small boat was used just for safety reasons and to help

performing the necessary maneuvers during the pipeline

installation.

REFERENCES

[1] Masetti, I.Q., Barros, C.R.M., Jacob, B.P., Albrecht, C.H.,

Lima, B.S.L.P., Sparano, J. V., Numerical Simulation of

the Mooring Procedures of the BGL-1 Pipeline Launching

Barge. Procs of the 23rd International Conference onOffshore Mechanics and Arctic Engineering OMAE,

June 20-25, Vancouver, Canada, 2004.

[2] Jacob, B.P., Masetti, I.Q., PROSIM: Coupled Numerical

Simulation of the Behavior of Moored Units (in

Portuguese), COPPETEC-Petrobras Internal Report, Rio

de Janeiro, 1997.

[3] __, SITUA-Prosim Program: Coupled Numerical

Simulation of the Behavior of Moored Floating Units

User Manual, ver. 3.0 (in Portuguese), LAMCSO/

PEC/COPPE, Rio de Janeiro, 2005.

[4] SGO User Manual (in Portuguese) Petrobras, Rio de

Janeiro, 2002.

[5] Silva, D.M.L., Bahiense, R. A., Jacob, B.P., Torres, F.G.S.,Medeiros, A.R., Costa, M.N.V., Numerical Simulation of

Offshore Pipeline Installation by Lateral Deflection

Procedure. Procs of the 26rd International Conference on

Offshore Mechanics and Arctic Engineering OMAE,

June 10-15, San Diego, USA, 2007.

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