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Proceedings of the Marine Operations Specialty Symposium 2008
MOSS-26
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
Parameter Value Unit
Diameter 0.762 m
Length 1.129 m
Weight in Air 1.2851 kN
Buoyancy 3.4138 kN
TABLE 3. PIPELINE + BUOY DATA
Parameter Value Unit
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
FIGURE 4. VON MISES STRESS IN PIPELINE 2 KM/H
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 5. VON MISES STRESS IN PIPELINE 1 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 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
o
n
(
K
N
)
Direction (-5)
Direction (0)
Direction (5)
Direction (10)
Direction (15)
Direction (20)
FIGURE 7. TENSION IN THE CABLE 2 KM/H
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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
o
n
(
K
N
)
Direction (-5)
Direction (0)
Direction (5)
Direction (10)
Direction (15)
Direction (20)
FIGURE 8. TENSION IN THE CABLE 1 KM/H
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.