riopipeline2007_ipb1156

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______________________________1 LAMCSO Laboratory of Computational Methods and Offshore Systems PEC/COPPE/UFRJ2 PETROBRAS

IBP1156_07

ANALYSIS OF A LATERAL DEFLECTION METHOD FOR THE

INSTALLATION OF AN OFFSHORE PIPELINEDanilo M. L. da Silva1, Rodrigo A. Bahiense1,Breno P. Jacob1,

Fernando G. S. Torres2, Antonio R. Medeiros2, Marcos N. V. da Costa2

Copyright 2007, Instituto Brasileiro de Petrleo e Gs - IBP

Este Trabalho Tcnico foi preparado para apresentao naRio Pipeline Conference & Exposition 2007, realizada no perodo de 2 a4 de outubro de 2007, no Rio de Janeiro. Este Trabalho Tcnico foi selecionado para apresentao pelo Comit Tcnico do evento,seguindo as informaes contidas na sinopse submetida pelo(s) autor(es). O contedo do Trabalho Tcnico, como apresentado, nofoi revisado pelo IBP. Os organizadores no iro traduzir ou corrigir os textos recebidos. O material conforme, apresentado, nonecessariamente reflete as opinies do Instituto Brasileiro de Petrleo e Gs, seus Associados e Representantes. de conhecimento eaprovao do(s) autor(es) que este Trabalho Tcnico seja publicado nos Anais da Rio Pipeline Conference & Exposition 2007.

Abstract

The BGL-1 (a pipeline launching barge owned by the Brazilian state oil company - Petrobras) is used to performpipeline installation operations. The BGL-1 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 themooring line positions and induce the laybarge movement as it lays the pipeline. Basically, tug boats drop anchors atsome predefined positions; then the barge winches release the stern mooring cables, and collect the mooring cableslocated at the bow.However, the procedure described above has some limits, such as: i) it has a very restrictive limitation according to theweather conditions; ii) the procedure is extremely complex when performed in congested areas.Therefore, the objective of this work is to present a numerical simulation of a pipeline installation using a procedurecalled Lateral Deflection. The Lateral Deflection procedure basically consists of performing the pipeline assembly onshore and deflecting it to the sea using a tug boat.The computational tool used is able to incorporate the correct definition of the seabed and shore from bathymetriccurves. This is performed through a specialized interface with the SGO (Obstacles Management System) databasesystem. This system, developed by Petrobras, contains frequently updated information about the bathymetry andposition of subsea obstacles, gathered by a special vessel equipped with a ROV Remote Operated Vehicle).An actual pipeline installation by lateral deflection procedure is analyzed and discussed.The characterization of the procedure passes through the determination of the better velocity and direction of the tugboat in way to minimize the efforts on the pipeline (especially due the curvatures).

1. Introduction

Usual pipelaying operation procedures in offshore Brazil employ the BGL-1 barge owned by Petrobras. Thisbarge performs installation operations by moving forward using its own mooring lines. This involves the definition of acomplex mooring procedure, as a sequence of operations that determine the mooring line positions and induce thelaybarge movement as it lays the pipeline. Basically, tug boats drop anchors at some predefined positions; then thebarge winches release the stern mooring cables, and collect the mooring cables located at the bow.

Since the laybarge usually operates at locations where the sea bottom is congested with other pipelines,

Christmas trees, manifolds, or fiber optics, the definition of the mooring procedure (leading to the motion of the barge)must take into account not only the desired pipeline route, but also possible interferences between the mooring linesand subsea obstacles.

In order to allow the correct representation of the seabed, Petrobras developed a database system named SGO(Obstacles Management System) (SGO, 2002). This system contains frequently updated information about thebathymetry and position of subsea obstacles, gathered by a special vessel - RSV Salgueiro, equipped with a ROV(Remote Operated Vehicle). In order to avoid interference and accidents with such obstacles, the definition of themooring procedure includes the placement of buoys attached to the cables, with the purpose of lifting the mooring linesand increasing the distance between cables and subsea obstacles (Masetti et al., 2004). The definition of the mooringprocedure should also provide information about the amount and length of buoys' cables, pendants, and all relatedmaterial such as shackles, swivels, cables, etc.


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Rio Pipeline Conference & Exposition 2007

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However, the procedure described above has some limitations, such as: i) It has a very restrictive limitationaccording to the weather conditions; ii) The procedure is extremely complex when performed in congested areas.

Therefore, the objective of this work is to present results of numerical simulations of a pipeline installationusing a procedure called Lateral Deflection. This procedure basically consists of performing the pipeline assembly onshore, and deflecting it to the sea using a cable connected to a tug boat. The characterization of the deflection procedure

involves the determination of the better velocity and direction of the tug boat in order to minimize the efforts on thepipeline (especially due to the curvatures).

To perform the analyses of such procedure, Petrobras considered the use of the SITUA-Prosim system. Inorder to illustrate some steps of the lateral deflection procedure, Figure 1 presents snapshots of results obtained withthis system, for the scenario and numerical model that will be presented in the following sections of this work. Apicture of the operation is also shown in Figure 1.

Figure 1. Lateral Deflection Procedure

The SITUA-Prosim system has been developed since 1997 (Jacob and Masetti, 1997), in cooperation by

Petrobras and LAMCSO (Laboratory of Computational Methods and Offshore Systems, at the Civil Eng. Dept. ofCOPPE/UFRJ, Federal Univ. of Rio de Janeiro). It performs all the mooring calculations with an interface with theSGO system. In other words, the software reads the SGO seabed data and simulates the pipeline launching operationconsidering the actual bathymetric information and obstacle positions (Jacob and Masetti, 2005).

The following sections of this work describes the actual pipeline installation operation considered, employingthe lateral deflection procedure, and discusses the results of the analyses.

2. Scenario and characteristics of pipeline

PETROBRAS intends to rehabilitate a 10 pipeline that is in the end of its lifetime. The pipeline is located atthe Xaru field, interconnecting the PXA-1 platform to the buoy frame, with the basic purpose of transporting the oilproduction of Xaru, Atum and Curim fields, in Cear State (northeast of Brazil), to the NT ALIANZA Ship.Therefore, a new pipeline will be installed on the left side of the present pipeline. The new pipeline has a total length of

721m. The pipeline will be assembled at Canto Beach, in Paracuru city, deflected from shore to the sea, andtransported with buoys to the installation location, where it will be positioned on the guideline and sunk by flooding thebuoys.

2.1. Bathymetry of the beach

A photo of Canto beach is presented in Figure 2. Information about level curves, obstacles and tide lines aresupplied by bathymetric map shown in Figure 2. With those data the SITUA-Prosim system generates a bathymetricmesh for the analyses.


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Figure 2. Canto Beach; Bathymetry of Canto Beach

During assembly, the pipeline will be positioned on "big-bags" (bags of sand with 1m of volume) asillustrated and shown in Figure 3.

Figure 3. Pipeline Assembly scheme

The soil-resistance coefficients for the axial and lateral directions of the pipeline were supplied byPETROBRAS: lateral = 1.0; axial = 0.5.

2.2. Characteristics of Pipeline and Buoys

The physical and geometric properties of the pipeline and of the buoys are presented in the following tables.Three-dimensional frame elements are employed in the generation of the numerical model for the pipeline.

The buoys (shown in Figure 4) are fastened to the pipeline at each 8m measured from the center of each buoy.


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Tabela 1. 10 Pipeline data

Parameter Value UnitOutside Diameter 0.27305 mInside Diameter 0.2445 mYield Stress of steel 414000 KN/m2

Modulus of Elasticity of steel 207000 MPaAxial Stiffness (EA) 2402252.49 KNFlexional Stiffness (EI) 20169.39 KN*m2

Poisson Coefficient 0.3 -Density of steel 77 KN/m3

Corrosion Protection 0.0027 mCorr. Protection Specific Mass 9.32 KN/m3Hydrodynamic Diameter 0.27875 mTube Length 12 mWeight in Air 0.91099 KN/mWeight in Water 0.32220 KN/m

Figure 4. Buoy

Tabela 2. Buoy Data

Parameter Value UnitDiameter 0.762 mLength 1.129 m

Weight in Air 1.2851 KNBuoyancy 3.4138 KN

Three-dimensional frame elements are employed also for the representation of the pipe segments with buoys.An equivalent element represents both the pipeline physical properties and the buoy hydrodynamic properties. Thecharacteristics of the equivalent pipeline+buoy element are shown in Table 3.

Tabela 3. Pipeline + Buoy data

Parameter Value UnitOutside Diameter 0.27305 mInside Diameter 0.2445 mAxial Stiffness (EA) 2402252.49 KNFlexional Stiffness (EI) 20169.39 KN*m2

Hydrodynamic Diameter 0.762 mWeight in Air 2.23530 KN/mWeight in Water -3.06225 KN/m

2.3. Visualization of Numerical Model

Some views of the numerical model thus generated are presented in Figure 5. The water surface is indicated inblue; the beach is indicated in green.


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Figure 5. Details of the Numerical Model

3. Lateral Deflection: Parametric Studies

As mentioned before, the pipeline is assembled on shore and deflected to the sea using a tugboat. According tothe predominant effort in the pipeline, the lateral deflection procedure can be classified in two ways, as shown in Figure6.

Figure 6. Lateral Deflection: Compression (left) and Tension (right)

When compression predominates, the tugboat force needed to displace the pipeline from the beach is smaller;however stresses in the pipeline due to curvatures are larger. On the other hand, when tension predominates, thetugboat force is larger while stresses due curvatures are smaller.

Basically these two forms of deflection are differentiated by the angle between the axial pipeline axis, in thetowing extremity (axis X in Figure 6), and the tugboat route. Angles smaller than 90 o, in general, implicate incompression, while angles larger than 90o, in general, implicate in tension.

Therefore, the objective of the parametric studies presented here is to define adequate combinations of tugboatroute and velocity for the lateral deflection procedure. Several nonlinear dynamic analyses were performed:

Tugboat route: -5, 0, 5, 10, 15 e 20 (angles measured from axis Y, Figure 6, clockwise). ThePETROBRAS background in such operations suggested an angle of 10o as a starting point.

Tugboat velocity: 1km/h, 2km/h e 3km/hIn each analysis, the tugboat is modeled by prescribing its motion at the end of the tug cable, varying in time

according to the specified velocity. Since the operation is performed in a sheltered scenario, with benign environmentalconditions, no environmental loading of wave or current is applied to the pipeline.

The dynamic analyses are performed using a time step value Dt = 0,01s. The total simulation time T varieswith the tugboat velocity: T = 3610s, 1810s and 1210s respectively for velocities of 1km/h, 2km/h and 3km/h (asexpected, smaller velocities require a larger simulation time).

The results of the parametric studies are presented in the Figures that follow. Initially, Figures 7 illustrate oneinitial step of the deformed configuration, for three different tugboat routes.


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Figure 7. General view and detail of -5o, 10o and 20o configurations

In order to define an adequate combination tugboat route-velocity, that leads to small values of forces in thetugboat cable and Von Mises stresses in the pipeline, the results of the sequence of analyses comprising the parametricstudies are summarized in the graphs that follow. Initially, Figure 8 summarizes, for each tugboat velocity, the time-history of the tugboat forces for different tugboat routes. The maximum forces are summarized in Table 4.

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 1500Ti me ( s)

Traction

(KN)

Di recti on ( - 5)

Di rect i on ( 0)

Di rect i on ( 5)

Di rect i on ( 10)

Di rect i on ( 15)

Di rect i on ( 20)

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 1500Ti me ( s)

T

raction

(KN)

Di recti on ( - 5)

Di rect i on ( 0)

Di rect i on ( 5)

Di rect i on ( 10)

Di rect i on ( 15)

Di rect i on ( 20)

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 1500Ti me ( s)

Traction

(KN)

Di recti on ( - 5)

Di rect i on ( 0)

Di rect i on ( 5)

Di rect i on ( 10)

Di rect i on ( 15)

Di rect i on ( 20)

Figure 8. Tension in the cable 1 Km/h; 2 Km/h and 3 Km/h


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Tabela 4. Maximum Tension in the cable (KN)

Direction/Velocity

1 Km/h 2 Km/h 3 Km/h

-5 145.59 209.18 281.990 131.48 201.94 268.795 118.97 195.54 254.6410 111.89 190.07 242.9615 98.46 179.16 227.2020 88.18 167.92 214.20

If only tension responses were considered, one could deduce that it would be more suitable to deflect thepipeline with a tugboat route of 20o, and velocity 1km/h, leading to a maximum tension value of 88.18 kN. However,the behaviour of the pipeline stresses must also be considered, as shown in the Figure that follow, containing themaximum values of Von Mises stresses along the pipeline during the analysis. The red line indicates the allowablestress.

0

100000

200000

300000

400000

500000

600000

700000

800000

0. 0 100. 0 200. 0 300. 0 400. 0 500. 0 600. 0 700. 0Pi pel i ne Lengt h ( m)

Von

Mises

(KN/m2) Di rect i on ( - 5)

Di rect i on ( 0)

Di rect i on ( 5)

Di rect i on (10)Di rect i on (15)

Di rect i on (20)

Yi el d St r ess

0

100000

200000

300000

400000

500000

600000

700000

800000


Von

Mises

(KN/m2) Di rect i on ( - 5)

Di rect i on ( 0)

Di rect i on ( 5)

Di rect i on (10)

Di rect i on (15)

Di rect i on (20)

Yi el d St r ess

0

100000

200000

300000

400000

500000

600000

700000

800000


Von

Mises

(KN/m2) Di recti on (- 5)

Di rect i on ( 0)

Di rect i on ( 5)

Di rect i on (10)

Di rect i on (15)

Di rect i on (20)

Yi el d St r ess

Figure 9. Von Mises stress in the pipeline 1 Km/h; 2 Km/h and 3 Km/h

Tabela 5. Maximum Tension in the cable (KN)

Direction/Velocity


-5 333896.6 442877.0 538018.30 356715.5 466120.9 577679.85 397294.4 502232.4 622467.810 397184.4 548049.9 677823.515 436822.4 578775.1 715493.620 462776.2 623533.7 762678.4

The results above indicate that the 10o

direction is the most suitable, mainly at the first 10 minutes. At this timeof operation, when approximately 250m of pipeline has already left the beach, the maximum values of Von Mises stressare reached.


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The minimum values of curvature radius are shown in figure below.

0

25

50

75

100

125

150

0. 0 10. 0 20. 0 30. 0 40. 0 50. 0 60. 0 70. 0 80. 0 90. 0 100. 0Pi pel i ne Lengt h ( m)

Curvature

Radios

(m)

Di rect i on ( -5) Di rect i on (0) Di rect i on (5)Di recti on (10) Di recti on (15) Di recti on (20)

0

25

50

75

100

125

150

0. 0 10. 0 20. 0 30. 0 40. 0 50. 0 60. 0 70. 0 80. 0 90. 0 100. 0Pi pel i ne Lengt h ( m)

Curvature

Radios

(m)

Di rect i on ( -5) Di rect i on (0) Di rect i on (5)Di recti on (10) Di recti on (15) Di recti on (20)

0

25

50

75

100

125

150

0. 0 10. 0 20. 0 30. 0 40. 0 50. 0 60. 0 70. 0 80. 0 90. 0 100. 0Pi pel i ne Length ( m)

Curvature

Radios

(m)

Di recti on (- 5) Di recti on (0) Di recti on (5)Di recti on (10) Di recti on (15) Di recti on (20)

Figure 10. Curvature Radius 1 Km/h; 2 Km/h and 3 Km/h

Tabela 6. Curvature Radius (m)

Direction/Velocity


-5 86.3610 67.7688 54.42010 79.3458 61.9464 50.52005 71.7635 57.7244 46.6055

10 71.9409 52.3393 42.477215 65.7438 49.4071 40.202020 61.2436 45.9056 37.5399

As expected, smallest curvature radius was found for largest velocity and for models in which tugboat routetends to compress the pipeline.

4. Transport

The objective of the analyses presented in this section is to verify the pipeline behavior under environmentalloadings during its transport from Canto beach to PXA-1 platform.

Tabela 7. Environmental loads

Load Azimuth ValueCurrent 315o 1.18 m/sWave 30o Hs = 1.85m; Tp = 9.7s

As the pipeline remains totally submerged and the buoys at least 50% submerged, wind effect was notconsidered.

During transport, the pipeline may be connected to two tugboats, usually aligned as illustrated in Figure 11.Alternative models were analyzed, with different tugboat velocities, ranging from 5m/s to 9.26m/s. Also, in somemodels the back tugboat is disconnected, and only the front tugboat is pulling the pipeline. Finally, different headingswere considered for the system, relative to the environmental conditions.

The results of the analyses indicated that, for all cases, the maximum values of Von Mises stresses are not anissue, always staying well below the yield stress of the material. The objective then is to minimize tugboat forces.

It was observed that, in configurations with two tugboats such as the illustrated in Figure 11, smaller values ofcable tensions are obtained when the pipeline is nearly aligned to the resultant direction of the environmentalconditions. However, the cable tensions are still relatively high during the whole operation. In such cases, maximumtensions in the tugboat cable are approximately 51.1KN (5m/s transport velocity) and 223.9KN (9.26m/s velocity).


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Figure 11. Pipeline Transport typical configuration

The smaller values of cable tensions were found in configurations where the back tugboat is disconnected. Insuch cases, tensions in the cable are approximately 19.9KN (5m/s velocity), 42.4KN (7m/s velocity) and 61.0KN(9.26m/s velocity). Therefore, significant reductions were obtained in the cable tension: 61% for the velocity of 5m/s,and 72.8% for the velocity of 9.26m/s.

Therefore, the results of the analyses indicated that the best situation occurs when the back tugboat does nottension the pipe; it needs only to accompany the transport operation, for safety reasons, and then to perform themaneuvers needed for the subsequent pipeline launching process.

6. Final Remarks

This work presented the results of numerical simulations and parametric studies on the pipeline behaviorduring some stages of the installation procedure for the pipeline that will interconnect Xaru-1 (PXA-1) platform to thebuoy frame near the coast of Cear state, Brazil.

Such analyses are intended to verify the pipeline behavior during the lateral deflection (when the pipelineleaves the beach), and during transport to the installation site. The results were presented in terms of forces in thetugboat cable, and Von Mises stresses in the pipeline.

The results of the parametric studies allowed the definition of the most suitable conditions for each stage of theoperation.

During the lateral deflection procedure, it was verified that the tugboat velocity should not exceed 1km/h(mainly during the first 10 minutes of operation), and that the better route would make an angle of 10 o with the

perpendicular to the pipeline axis at its connection with the cable (Figure 6).Of course, these results should be viewed with care, due to the uncertainties in the supplied friction coefficientvalues for the beach sand. Further studies should include a parametric evaluation of the influence of frictioncoefficients in the pipeline behavior. Anyway, the results provide valuable information regarding the influence oftugboat route and velocity on the pipeline behavior.

Regarding the transport stage, it was noticed that the best configuration to transport the pipeline, wheretensions in the tugboat cable are minimized, occurs when the pipeline direction is close to the resultant ofenvironmental loads and the back tugboat is disconnected. Therefore, it may be positioned only to accompany thetransport for safety reasons, and to help performing the necessary maneuvers during the pipeline installation.

7. AcknowledgementsThe authors wish to acknowledge the participation of Drs. C.D. Padovezi and C.H. Umeda from IPT, Prof. CelsoP.Pesce from USP Univ. of So Paulo, and Eng. A.C. Pimenta Ferreira, in the technical meetings where the idea of

studying the transport condition described here was first proposed.

8. References

JACOB, B.P., MASETTI, I.Q., PROSIM: Coupled Numerical Simulation of the Behavior of Moored Units (inPortuguese), COPPETEC-Petrobras Internal Report, Rio de Janeiro, 1997.

__, SITUA-Prosim Program: Coupled Numerical Simulation of the Behavior of Moored Floating Units UserManual, ver. 3.0 (in Portuguese), LAMCSO/ PEC/COPPE, Rio de Janeiro, 2005.

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 23rdInternational Conference on Offshore Mechanics and Arctic Engineering OMAE, June 20-25, Vancouver,

Canada, 2004.SGO User Manual (in Portuguese) Petrobras, Rio de Janeiro, 2002.

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