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A COMPUTATIONAL SYSTEM FOR SUBSEA PIPELAYING SIMULATION

Danilo Machado Lawinscky da Silva

Carl Horst Albrecht

Breno Pinheiro Jacob

danilo@lamcso.coppe.ufrj.br

carl@lamcso.coppe.ufrj.br

breno@lamcso.coppe.ufrj.br

LAMCSO Laboratory of Computational Methods and Offshore SystemsDepartment of Civil Engineering, COPPE/UFRJ, Rio de Janeiro, RJ, Brazil

Isaias Quaresma Masetti

Claudio Roberto Mansur Barros

Arthur Curty Saad

masetti@petrobras.com.br

claudio.mansur@petrobras.com.br

arthur.saad@petrobras.com.br

PETROBRAS Petrleo Brasileiro S.A.

Abstract. Traditional analysis methods for pipelaying simulation consider an uncoupled

model where the motions of the barge are previously determined, without taking into account

the influence of the pipeline, and are then prescribed at the top of the pipeline.

Currently, the analyses of pipelaying operations have been performed by commercial

softwares, such as OffPipe. However, such tools presents restrictions/limitations related to

the user interface, model generation and analysis formulations. These limitations hinder its

efficient use for analyses of installation procedures for the scenarios considered by

Petrobras, using the BGL-1 barge or other vessels.

Therefore, the objective of this work is to present a computational tool in which the modules

follow the Petrobras users specifications. The main objective of such tool is to overcome thelimitations for specific needs and particular scenarios in the simulation of several types of

pipeline procedures. Such tool, called SITUA-PetroPipe, presents an extremely friendly

interface with the user, for instance allowing the complete customization of the configuration

of laybarge and stinger rollers, and includes novel analysis methods and formulations, for

instance the ability of coupling the structural behaviour of the pipe with the hydrodynamic

behaviour of the vessel motions under environmental conditions.

Several simulations of actual operations are shown, in order to illustrate the application of

this new computational tool.

Keywords:Numerical Methods, Offshore Operations, Pipeline Installation Procedures

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1. INTRODUCTIONInstallation of pipelines and flowlines constitute some of the most challenging offshore

operations. The technical challenges have spawned significant research and developmentefforts in a broad range of areas, not only in studies regarding different installation methods,

but also in the formulation and implementation of new computational tools required to thenumerical simulation. This work addresses this latter issue.The most common installation methods are the S-Lay, J-Lay, and Reel-Lay methods,

schematically shown in Fig. 1, and Towing methods, schematically shown in Fig. 2 (Guo,Bai, 2005; Kyriakides,2007).

Figure 1 S-Lay, J-Lay and Reel-Lay Methods.

In the S-Lay method, as the laying barge moves forward, the pipe is eased off the stern,curving downward through the water until it reaches the touchdown point. After touchdown,as more pipe is played out, it assumes the S shaped curve. To reduce bending stress in the

pipe, a stinger is used to support the pipe as it leaves the barge. To avoid buckling of the pipe,a tensioner must be used to provide appropriate tensile load to the pipeline (Clauss,1998).This method is used for pipeline installations in a range of water depths from shallow to deep.

In the J-lay method, the pipe is dropped down almost vertically until it reachestouchdown; after that it assumes the J shaped curve. J-Lay barges have a tall tower on thestern to weld and slip pre-welded pipe sections. With the simpler pipeline shape, the J-Laymethod avoids some of the difficulties of S-Laying such as tensile load forward thrust, and

can be used in deeper waters.In the Reel-Lay method, the pipeline is installed from a huge reel mounted on an offshore

vessel. Pipelines are assembled at an onshore spool-base facility and spooled onto a reelwhich is mounted on the deck of a pipelay barge. Horizontal reels lay pipe with an S-Layconfiguration. Vertical reels most commonly do J-Lay, but can also S-Lay.

Towing methods basically consists in weld the pipeline onshore with an onshore pipelinespread. Once the pipeline is complete and hydrotested, the pipeline is dewatered and movedinto the water, while being attached to a tow vessel. It is then towed to an offshore locationwhere each end is connected to pre-installed facilities (Silva,2007b,2008).

There are four variations of the towing method: surface tow, mid-depth tow, off-bottomtow, and bottom tow (Fig. 2). In the surface tow approach, buoyancy modules are added to the

pipeline so that it floats at the surface. Once the pipeline is towed on site by one or twotugboats (Silva,2008), the buoyancy modules are removed or flooded, and the pipeline settles

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to the sea floor. The mid-depth tow requires fewer buoyancy modules, and the pipeline settlesto the bottom on its own when the forward progression ceases. The off-bottom tow involves

buoyancy modules and chain weights. In the bottom tow, primarily used for soft and flat seafloor in shallow water, the pipeline is towed along the sea floor

Towing could be cheaper than other methods that use laybarges. However, a case-by-case

analysis is required to determine the cost-benefit ratio.

Figure 2: Tow-in Methods.

2. PIPELAYING IN OFFSHORE BRAZILUsual pipelaying operation in offshore Brazil are performed by S-Lay procedures

employing the BGL-1 barge (Fig. 3) owned by Petrobras. The BGL-1 is a second-generationanchor positioned laybarge that performs installation operations by moving forward using itsown mooring lines. Basically, tug boats drop anchors at some predefined positions; then the

barge winches release the stern mooring cables, and collect the mooring cables located at thebow.

In order to prevent the pipe from buckling in the regions of maximum bending, the bendradius is controlled by keeping the pipe under tension, so that the pipe actually follows a lazyS shape. The tension is applied to the pipe by tensioners on the barge which are usuallyarrays of rubber wheels or belts which surround the pipe and apply an axial force to the pipethrough the friction generated between the tensioner and the pipe external coating as shown inFig. 4.

Figure 3 The BGL-1 Pipeline Launching Barge

The force on the pipeline is reacted at the seabed end of the pipeline by the dead weightof the pipeline and friction between it and the seabed. Obviously the larger the force applied

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pipeline, with different lengths and geometries adapted to certain laying conditions in S-Layprocedures.

Therefore, the objective of this work is to present the development of a in-housecomputational tool, referred as SITUA-PetroPipe, that overcomes the limitations for specificneeds and particular scenarios in the simulation of several types of pipeline procedures, and

addresses the requirements regarding the analysis formulations mentioned above.As will be described in the remainder of this work, such tool presents an extremelyfriendly interface with the user, allowing for instance the complete customization of theconfiguration of laybarge and stinger rollers.

4. SITUAPROSIMThe SITUA-PetroPipe tool may be seen as specialized modules of the SITUA-Prosim

system (Jacob,1997), which has been developed since 1997, in cooperation by Petrobras andLAMCSO (Laboratory of Computer Methods and Offshore Systems, at the Civil EngineeringDepartment of COPPE/UFRJ, Federal University of Rio de Janeiro)1. This system constitutes

a computational tool that performs coupled static and dynamic nonlinear analyses of a widerange of offshore operations.

The PetroPipe modules described here are based in the SITUA graphical interface, and inthe Prosim numerical solver (Jacob,2005). This numerical solver comprises a time-domainnonlinear dynamic analysis program, which has been employed by Petrobras since 1998 inseveral design activities related to floating production systems.

The coupled formulation of the Prosim program incorporates, in the same computationalcode and data structure, a hydrodynamic model to represent the hull and a finite elementmodel to represent the structural hydrodynamic behavior of the mooring lines, risers and

pipelines. This coupled formulation allows the simultaneous determination of the motions ofthe hull, and the structural response of the lines. Moreover, the results will be more accuratesince all dynamic and nonlinear interaction effects between the hull and the lines areimplicitly and automatically considered. Details of such coupled model are presentedelsewhere (Senra,2002;Jacob,2005), and will not be reproduced here

The original Prosim code was oriented towards the analysis and design of FPS,considering their installed and operational situations. Later, the SITUA-Prosim system wasdeveloped by incorporating a graphical interface and adapting / specializing the code for theanalysis of installation and damage situations (hence its name, from the PortugueseSITUaes de instalao e Avaria).

The SITUA interface (Fig. 5) is designed to work as a pre-processor and model generatorfor the Prosim finite-element based numerical analysis modules, and to provide facilities for

statistical and graphical post-processing and visualization of results. The model generationprocedures of the interface incorporate an analytical catenary solver, able to representcomplex configurations such as lines with multiple segments and different materials,connected to other lines or to platforms, and with flotation elements such as buoys orsegments with distributed floaters.

The interface allows a very simple and intuitive definition of the model of a line. Theuser needs only to specify the number, length and type of segments that comprise the line. Adatabase with several material types is incorporated in the system. Another enhanced facilityfor the definition of lines for actual operations of the BGL barge consists in the definition oftwo of the parameters that define a catenary (including anchor position, horizontal force, total

1 It should be pointed out that the SITUA-Prosim and the PetroPipe modules are not commercial programs; allrights are reserved to Petrobras.

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calculate the motions of the barge due to the operations performed with its mooring lines,leading to changes in their catenary configuration (including placement of buoys, variation ofthe onboard/released cable lengths, and relocating anchors).

During the simulation of such mooring operations by the SITUA interface, a specializedinterference management module can be employed to characterize interference situations.

Such situations are detected when an obstacle falls into an exclusion volume defined aroundsegments of a line laying on the seabed, and a vertical distance below suspended segments,with risks of collision and damage to the line and/or the obstacle (a manifold, another

pipeline, etc.).Figure 6 presents a 3D view where the exclusion region around one line is graphically

displayed, showing a possible interference situation with a previously installed pipeline. Amore detailed visualization, including the definition of the types of obstacles and distancesfrom the line, can be observed in 2D views such as the depicted in Fig. 7. In these views theinterferences are indicated by red arrows, with the corresponding distances, and a tag definingthe obstacle.

Once the possible interferences are identified, the BGL-1 operator can take measures to

avoid them, including the placement of buoys in given positions along the line. Figure 8shows a configuration of a mooring line with two buoys, to keep the line suspended well oversubsea obstacles.

Figure 6 3D View of Exclusion Region with Interference.

Figure 7 2D View Detailing Interference.

Figure 8 3D View of Mooring Lines with Buoys.

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5. SITUAPETROPIPEAs mentioned before, the PetroPipe modules include new tools developed following the

Petrobras users specifications. These tools are intended to automate the generation ofnumerical models for the simulation of pipeline installation procedures (for instance, allowing

the complete customization of the configuration of the laybarge and stinger rollers).Moreover, the PetroPipe modules address the requirements regarding the analysisformulations mentioned in a preceding section, including the coupling of the structural

behavior of the pipe with the hydrodynamic behavior of the vessel motions. Also, the contactof lines (mooring lines, risers, pipelines) with the platform can be rigorously modeled duringa nonlinear dynamic analysis, as well as the contact involving different lines or even thecontact of one line with itself.

5.1 Modeling of ContactTraditional contact models consider for instance a generalized scalar element, consisting

of two nodes linked by a non-linear gap spring (Grealish,2005). Here, the contact modelconsists of a generalized elastic surface contact algorithm. The contact is modeled byaugmentation of the global stiffness matrix, based on the orientation and contact stiffness ofthe contact surfaces. Details of this algorithm are presented in (Silva,2006a,2007a).

The algorithm has been shown to be able of capturing the detailed characteristics of theinteraction between mooring lines, risers, pipelines, hulls, in a sophisticated model such as theillustrated in Fig. 9, depicting the contact between the pipeline and the rollers of the laybargestinger. A more detailed example will be presented in the application presented later.

Figure 9 Contact Model.

5.2 Tensioner ModelAs mentioned before, the tensioner (Fig. 4) is intended to control the tension level in the

pipeline during the pipelaying operation, by keeping it within a feasible operational range.In the PetroPipe modules, the tensioner is represented by a specialized generalized scalar

element, automatically added to the pipeline top end, which consists of two nodes linked by anonlinear gap spring. Force-displacement or stiffness-displacement functions associated toeach local direction are defined, and the local coordinates systems can also be updated at eachsimulation step.

To simulate the tensioner behavior in keeping the tension level at the defined range, theaxial stiffness of this element continually varies, leading to changes in the element length asthe pipeline end moves back and forth. It should be recalled that the pipeline end motions areinduced by the tensioner behavior and by the barge motions applied at the tensioner. Thetensioner model is schematically shown in Fig. 10.

All main characteristics of the tensioner machine are incorporated in this model,including:

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Operational Range defines the range of desired tension values; the tensioner is notactivated whenever the pipeline end tension is within this range.

Response Delay Whenever the pipe tension leaves the operational range, thetensioner is activated but only after a given time delay, when it effectively startsworking.

Response Velocity After the tensioner effectively starts working, it does not restorethe tension level immediately, but after a certain period defined by its design responsevelocity.

Displacement Limit This defines the limit in which the tensioner can move thepipeline back and forth in order to compensate its tension level.

Figure 10 Tensioner Model.

6. MODELING OF PIPELINE INSTALLATION PROCEDURESIn the following sections, the facilities incorporated in the PetroPipe modules are

illustrated by their application to real-case pipeline installation scenarios.

6.1 Lateral Deflection ProcedureThe Lateral Deflection procedure, associated to towing methods, may be used to move

the pipeline into the sea. In this context, it consists basically on deflecting the pipeline to thesea (after assembled at the coastline) using a cable connected to a tug boat. Thecharacterization of the deflection procedure involves the determination of the better velocityand direction of the tug boat when the pipeline is leaving the shore in order to minimize itsefforts (especially due to the curvatures).

The PetroPipe modules have been employed to model such a procedure for an actualscenario, as presented in (Silva,2007b). Some steps of the results of numerical simulations forthis procedure are illustrated in Fig. 11: the pipeline is on shore (1), at the coastline, beforestarting towing (2), the pipeline leaves shore (3,4) and is towed to the installation site.

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Figure 11: Lateral Deflection Procedure6.2 Tow-in

As mentioned before, tow-in operations are performed in many situations to transportpipelines of several lengths. Usually, Petrobras performs these operations following a lateraldeflection procedure such as the previously described. In the typical configuration for surfacetow, the pipe is towed using a front and a back tugboat aligned at the transportation route, asshown in Fig. 12.

Numerical simulations of actual operations were performed using the SITUA-PetroPipesystem, in order to assess the pipeline behavior under environmental loadings. The studies

presented in (Silva, 2008) include an alternative configuration, shown in Fig. 13, where thetugboats are not aligned. Smaller values of cable tension were obtained when the pipeline isnearly aligned with the direction of the resultant of the environmental loadings.

Figure 12: Tow-in Typical Configuration.

Figure 13: Tow-in Alternative Configuration.

A contingency procedure was also analyzed in (Silva,2008), for a situation in which the

back tugboat is disconnected and only the front tugboat is pulling the pipeline. Thisconfiguration simulates a situation in which one of the tugboats loses control and its cable is

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disconnected. The results of the analyses indicated that the smaller values of cable tensionswere found in configurations where the back tugboat is disconnected, indicating that the bestsituation occurs when it does not tension the pipe, or simply when it is not connected to the

pipe.Therefore, from the results of the numerical simulations, the actual pipeline

transportation was performed by Petrobras using only one tugboat, employing a smaller boatonly to accompany the transport operation for safety reasons, and to perform the maneuversneeded for the subsequent pipeline launching process. During the operation, all numerical

predictions related to the pipeline behavior were confirmed.

6.3 Shore PullThe shore pull operation illustrated here consists in pulling the pipe from the BGL-1

barge onto the shore by a winch. The winch needs to keep adequate pulling force to ensurethat the pipe is maintained under controlled tension within the allowed stress/strain limits. Theforces applied must be controlled such that no damage to the pipeline anodes or coating

occurs. Buoyancy aids can be used if required to keep pulling tension within acceptablelimits.

During the numerical simulation by the SITUA-PetroPipe system, forces in the pipelineand cable are analyzed including any overloading, friction and dynamic effects that mayoccur. Figure 14 shows snapshots from the animation of the numerical results, as the pipelineis pulled from the barge and arrives on the shore.

Figure 14 Shore Pull Operation.

7. GENERATION OF A S-LAY MODELThe complete generation of an S-Lay model using the specialized interface of the

SITUA-PetroPipe is described in the following sections.

7.1 Laybarge CharacteristicsFigure 15 and Table 1 illustrate the main geometric characteristics of the BGL-1 barge.Detailed actual data, in terms of the geometrical and hydrodynamic characteristics of the

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BGL-1 hull, were provided by Petrobras and employed to generate the model of the bargehull, represented in Fig. 16. The geometric data are used in the definition of the contactsurface of the barge hull.

Figure 15 BGL-1 Geometry

Table 1 Main geometric characteristics of BGL-1

Propriety Values (real scale)Drought 5.182 mHeight 9 mBeam 30 m

Length 120 m

Figure 16 SITUA-PetroPipe BGL-1 Model

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7.2 Ramp and Stinger DataFigure 17 illustrates the configuration of the ramp and stinger considered for the

application described here. The local ramp-stinger coordinate system has its origin on thestern shoe, X-axis positive direction from bow to stern and Z-axis vertical with positive

direction upwards, as indicated in Fig. 18. The geometric data of ramp and stinger aresummarized in Tables 2 and 3, respectively.The geometric data of the stinger structure are also used in the definition of its contact

surface. During the finite element analysis the stinger is considered a rigid body connected tothe barge hull and all contact forces acting on it are transferred to the barge. Thehydrodynamic characteristics of the stinger are incorporated at the barge hull model by itshydrodynamic coefficients.

Figure 19 shows typical configurations for roller boxes on the laybarge stinger and ramp,respectively.

Figure 17 BGL-1, Ramp and Stinger Geometry

Figure 18 Ramp/Stinger, Local Coordinate System.

Table 2 Ramp radius 150 m

Element X (m) Z (m) Length (m)Tensioner -56.335 1.550 -

Roller Box 1 -38.905 1.094 2.75Roller Box 2 -26.574 0.768 2.75Roller Box 3 -18.078 0.034 2.75Roller Box 4 -9.292 -1.241 2.75Roller Box 5 -0.432 -3.157 3.00

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Table 3 Stinger radius 150 m

Element X (m) Z (m) Offset (m) Length (m)Roller Box 1 5.277 -4.632 0.687 4.00Roller Box 2 9.094 -5.825 0.687 4.00Roller Box 3 12.856 -7.156 0.694 4.00Roller Box 4 16.586 -8.555 0.714 4.00Roller Box 5 20.275 -10.099 0.748 4.00Roller Box 6 23.882 -11.770 0.793 4.00Roller Box 7 27.443 -13.581 0.850 4.00Roller Box 8a 29.361 -15.198 0.919 --Roller Box 8 30.883 -15.835 0.950 --

Figure 19 Configuration of Rollers (Stinger and Ramp)

As mentioned before, this configuration of the laybarge ramp and stinger roller boxes canbe easily and completely customized by the new modules of the graphical interface of theSITUA-PetroPipe system, as illustrated in Figs 20, 21 and 22. A general view of the generatedmodel for the BGL-1 is shown in Fig. 22.

Figure 20 Ramp Configuration.

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Figure 21 Stinger Configuration.7.3 Mooring Lines

The BGL-1 has eleven fairleads, but in usual operations only nine or ten mooring linesare connected. All mooring lines are composed by two segments, with characteristics

presented in Table 4. The length value for segment 2 corresponds to the total length availableon the winch drum; the released length varies during the mooring operations, as presented in(Masetti,2004).

The catenary solver provides the results defining the equilibrium configuration of the

mooring system, and the interference management module allows the identification of severalpossible interferences with obstacles. All interferences are successfully avoided by placingtwo buoys on most of the lines. Detailed tables indicating position in the line measured fromthe anchor, and the length of the pendant for each buoy, can be found in (Masetti,2004).

Figure 22 General View of the Generated Model

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Table 4 Characteristics of Mooring Line Segments

Segment Length (m) Material1 (anchor) 150 R3S Stub Chain 3

2 1780 (max) EEIPS Steel Wirerope 2.5

7.4 PipelineAll pipeline characteristics can be defined by the user. A database with common material

properties and usual pipeline characteristics, such as wall thickness and coating, isincorporated in the system. The model generated here considers a typical 16-in pipeline, with

physical and geometric properties presented in Table 5.

Table 5 16 Pipeline data

Parameter Value Unit

Outside Diameter 0.40640 mWall Thickness 0.011125 mYield Stress 414000 kN/m2

Modulus of Elasticity of steel 207000 MPaAxial Stiffness (EA) 2859694.14 kN

Flexional Stiffness (EI) 55894.90 kN*m2Poisson Coefficient 0.3 -

Density of steel 77 kN/m3Corrosion Coating Thickness 0.0032 mCorr. Coating Weight Density 9.32 kN/m3Concrete Coating Thickness 0.0381 m

Concrete Coating Weight Density 21.974 kN/m3Hydrodynamic Diameter 0.489 mTube Length 12 m

Field Joint Length 0.6 mJoint Fill Weight Density 10.065 kN/m3

Weight in Air 2.255935 kN/mWeight Submerged 0.368493 kN/m

7.5 Visualization of the Complete ModelThe initial equilibrium configuration of the pipeline is generated using dynamicrelaxation techniques as proposed in (Silva,2006b). The top tension in the pipeline is the

parameter that defines the S shape. The generated S-Lay configuration is shown in thefigures that follow.

The actual bathymetric data and soil properties are considered for the pipeline behavioron seabed. Information about free-spans is then available during analysis.

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Figure 23 SITUA-PetroPipe S-Lay Model

7.6 Typical ResultsBesides typical results in terms of tension and Von Mises stresses along the pipe length,

as shown in Figs 24 and 25, information about distances between the pipeline and its supportsas well as the reaction at each roller box are generated during static and dynamic analyses.

Specific reports are automatic generated for relevant information such as distance fromthe laybarge stern and the pipeline touchdown point. Reports for all relevant informationabout the mooring lines are also automatic generated.

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Figure 24 Von Mises Stress (static).

Figure 25 Von Mises Stress (dynamic).

8. FINAL REMARKSThe in-house computational system described in this work has already been employed by

the BGL-1 crew in the simulation and planning of actual mooring procedures for pipelinelaying operations in Campos Basin. The system has been shown to be able to calculate themotions of the barge due to the operations performed with its mooring lines (including

placement of buoys, and variation of the onboard/released cable lengths), taking into accountgeneral seabed data and interferences with subsea obstacles.

Regarding the simulation of the actual pipeline launching process, the Prosim finite-element numerical solver already included a 3D frame element that can account for all

material and geometrically nonlinear effects that arise in the pipeline behavior during thelaying operation. It was also able to couple the structural behavior of the pipe with thehydrodynamic behavior of the vessel motions under environmental conditions, considering allmooring lines also modeled by Finite Elements, which in itself is a step further overtraditional methods for the numerical simulation of pipelaying operations.

In order to comprise an accurate and user-friendly alternative for the analysis of pipelineinstallation procedures, some adaptations in the SITUA interface and in the Prosim numericalsolver were needed.

Therefore, this work described some of the recent implementations that comprise theSITUA-PetroPipe modules, including: a) Generation of initial finite-element meshes for theS-laying configuration of the pipeline by a dynamic relaxation procedure; b) Inclusion of

generalized scalar elements to represent the tensioner; c) Implementation of automaticcustomization facilities for the definition of the ramp and stinger rollers; d) Development of arigorous contact algorithm to represent the variable contact between the pipeline and therollers; e) Generation of finite-element models for other types of laying operations that mayeventually be considered for the BGL-1 or other laybarges, including J-lay and reelingmethods. Due to limitations in space, these latter facilities (regarding J-Lay and Reeling

procedures) could not be presented here, and will be demonstrated in future works.As the result of the recent implementations described in this work, the SITUA-PetroPipe

system now comprises a computational tool intended to improve the applicability andaccuracy of analysis of pipeline installation operations, making the simulations more realistic.

Several parametric studies are currently being performed considering the described

modeling facilities, for different scenarios including shallow to deep waters, and differentpipeline sizes. The results of these studies will also allow the precise assessment of the

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influence of the application of the coupled model (barge + mooring lines + pipeline) on thedynamic pipeline-laybarge behavior in such different scenarios, indicating where a coupled

pipelay analysis, rather than a traditional uncoupled analysis, is required.

Acknowledgements

The authors would like to acknowledge the members of the BGL-1 crew that activelycontributed with the development of the SITUA-PetroPipe software, with valuable commentsand suggestions.

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