ibp1048_09

______________________________ 1 LAMCSO – Laboratory of Computational Methods and Offshore Systems – PEC / COPPE / UFRJ

IBP1048_09 COUPLED DYNAMIC ANALYSIS OF SUBSEA PIPELAYING

OPERATIONS

Danilo Machado Lawinscky da Silva1 Breno Pinheiro Jacob1

Copyright 2009, Brazilian Petroleum, Gas and Biofuels Institute - IBP This Technical Paper was prepared for presentation at the Rio Pipeline Conference and Exposition 2009, held between September, 22-24, 2009, in Rio de Janeiro. This Technical Paper was selected for presentation by the Technical Committee of the event according to the information contained in the abstract submitted by the author(s). The contents of the Technical Paper, as presented, were not reviewed by IBP. The organizers are not supposed to translate or correct the submitted papers. The material as it is presented, does not necessarily represent Brazilian Petroleum, Gas and Biofuels Institute’ opinion, or that of its Members or Representatives. Authors consent to the publication of this Technical Paper in the Rio Pipeline Conference Proceedings. Abstract

It is recognized that deepwater offshore oil exploitation activities requires the use of sophisticated computational tools to predict the behavior of floating offshore systems under the action of environmental loads. These computational tools should be able to perform coupled dynamic analyses, considering the non-linear interaction of the hydrodynamic behavior of the platform with the structural/hydrodynamic behavior of the mooring lines and risers, represented by Finite Element models.

The use of such a sophisticated computational tool becomes mandatory not only for the design of production platforms, but also for the simulation of offshore installation operations. For instance, in the installation of submarine pipelines, the wall thickness design may not be governed by the pressure containment requirements of the pipeline during the operation, but by the installation process, specifically the combined action of bending, tension and hydrostatic pressure acting on the pipeline, that is also submitted to the motions of the laybarge.

Therefore, the objective of this work is to present the results of numerical simulations of S-lay installation procedures using a computational tool that performs dynamic analysis coupling the structural behavior of the pipe with the hydrodynamic behavior of the vessel motions under environmental conditions. This tool rigorously considers the contact between the pipeline and its supports (laybarge, stinger, seabed).

The results are compared to traditional pipelaying simulations based on RAO motions.

1. Introduction

Traditional analysis methods for pipeline laying 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. In such analysis, the pipeline is represented by a Finite Element model and the lay vessel is represented by RAO (Response Amplitude Operator) motions.

For floating production systems (FPS) under the action of environmental loadings in deepwater scenarios, it has been recognized that the use of coupled dynamic analysis tools is mandatory for the accurate numerical simulation, analysis and design (Ormberg,1997; Wichers,Heurtier,2001; Senra,2002). Coupled analysis formulations consider the non-linear interaction of the hydrodynamic behavior of the FPS hull with the structural/hydrodynamic behavior of the mooring lines and risers, represented by Finite Element models. In the implementation of such analysis tools, the 6-DOF equations of motion of the platform hull are coupled with the equations of motion of the FEM model of the lines.

It can intuitively be seen that the use of coupled formulations is important not only for the design of production platforms, but also for the simulation of offshore installation operations. In the case of pipelines in S-Lay operations, even in shallow waters the motions of the laybarge can be significantly affected by the structural behavior of the pipeline. Also regarding pipelines in S-Lay installation operations, it should be considered that the contact mechanism between the pipeline and the launching structure is complex, specified only in some points of the ramp and stinger.

Therefore, the objective of this work is to present results of a coupled dynamic analysis of a S-lay pipeline installation procedure using the BGL-1 laybarge (Figure 2). The analysis was performed using an in-house computational tool, referred as SITUA-PetroPipe.

Rio Pipeline Conference and Exposition 2009

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2. SITUA-PetroPipe

The PetroPipe computational tool is a module incorporated into the SITUA-Prosim system, which is a computer program that performs the coupled static and dynamic analysis of floating offshore systems. The SITUA-Prosim system has been developed since 1997 (SITUA-Prosim,2005), in cooperation by Petrobras and LAMCSO (Laboratory of Computational Methods and Offshore Systems, at the Civil Engineering Department of COPPE/UFRJ, Federal University of Rio de Janeiro).

This specific module for simulation of pipeline installation integrates a graphic interface to the developed numerical tools, and can easily generate numerical models for pipeline installation procedures. Moreover, the PetroPipe module address the requirements regarding the analysis formulations previously mentioned, which includes the coupling of the structural behavior of the pipe with the hydrodynamic behavior of the vessel motions. Also, the contact of lines (mooring lines, risers, pipelines) with the platform can be rigorously modeled during a nonlinear dynamic analysis, as well as the contact involving different lines or even the contact of one line with itself. A detailed description of the PetroPipe capabilities can be found in reference (Silva et al, 2008c). Several small preliminary problems have been run to test the validity of the algorithms. A variety of examples involving complex configurations and nonlinear boundary conditions were also analyzed (Silva, 2009).

It should be pointed out that the traditionally used computational tool for pipelaying simulations (such as RIFLEX, OFFPIPE, ORCALAY, PIPELAY and SIMLA, see references) are not prepared to perform coupled dynamic analysis of pipelaying operations such as those that will be presented here.

3. Coupled Model for Pipelaying Simulations

Pipelaying installation analysis has been traditionally performed based on uncoupled models. In the case of moored lay vessels, preliminary analyses are performed focused on the mooring lines evaluation. This is the case of BGL-1 laybarge. Reference (Silva et al, 2008a) presents mooring line analyses in pipeline installation simulations; in those analyses the pipeline structural behavior was not taken into account.

The coupled model for pipelaying simulations considers the hydrodynamic behavior of lay vessel and the structural behavior of the pipeline in the same model, as is done in coupled analysis of moored floating production systems (the formulation of such coupled model is not presented here but it can be found in presented references). The difference here is that the pipeline/lay vessel interaction must be considered. This interaction occurs at the roller boxes in the ramp and stinger. The contact between the pipe and the roller must be considered and, moreover, the support reaction must be taken into account in the calculation of the lay vessel motions.

In Figure 1 it is shown the combination of the basic implementation of a coupled model for moored floating systems (which considers only the top tensions of the lines) with the pipeline/lay vessel interaction considered here, which occurs by means of the contact between the pipe and support rollers, and the pipeline tensioner.

Figure 1. Coupled Model for Pipelaying Simulations: Effects of mooring lines combined with the pipeline/lay vessel interaction.

3.1 Contact Model The contact model is a crucial point in pipeline installation simulations, especially for the coupled model

presented here. Traditional contact models consider for instance a generalized scalar element, consisting of two nodes linked by a non-linear gap spring (Grealish et. al, 2005). Here, the contact model consists of a generalized elastic surface contact algorithm. The contact is modeled by augmentation of the global stiffness matrix, based on the


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orientation and contact stiffness of the contact surfaces. The complete contact model formulation is presented in (Silva, 2009), details of this algorithm are also presented in (Silva et. al, 2008b).

The algorithm has been shown to be able of capturing the detailed characteristics of the interaction between mooring lines, risers, pipelines, hulls, in a sophisticated model such as the illustrated in Figures 2 and 3, depicting the contact between the pipeline and the rollers of the laybarge stinger.

Figure 2. BGL-1: Contact Model.

Important aspects of the contact between the pipeline and its supports are rigorously modeled, which includes the support separation and reactions:

Support separation: The distance measured between the pipeline and the roller of the support. It is calculated at the middle point of each element of all roller boxes on the laybarge and stinger as schematically shown in Figure 3.

Support reactions: The force exerted on the pipeline by the roller boxes in the laybarge and stinger. The horizontal, vertical and lateral support reactions are also calculated for each element of all roller boxes on the laybarge and stinger.

The reactions are the perpendicular components of the force on the roller box surface. Their values come from the contact model at the end of the iterative process in each time step of the nonlinear time-domain solution procedure. The resultants are printed at the same points as the support separation distance, Figure 3.

Note that in ideal situations all rollers components make contact with the pipe reducing/redistributing the applied local forces. In real situations, under dynamic loading conditions some of the rollers may miss the pipe contact, resulting in more concentrated forces on a fewer number of rollers, as schematically shown in Figure 3. These situations can be easily identified in the proposed model.

(a) (b)

Figure 3. (a) Points for Reactions and Separation Distance Output; (b) Reactions and Pipeline Support.

4. S-Lay Model

Whenever possible, pipelaying operation in offshore Brazil are performed by S-Lay procedures employing the BGL-1 barge (Figure 3) owned by Petrobras. The BGL-1 is a second-generation anchor positioned laybarge that performs installation operations by moving forward using its own 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


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located at the bow. Therefore, the basic operations of the BGL-1 laybarge during pipelaying can be outlined as it follows: (a) The laybarge is positioned on its 8 anchors holding it aligned with the pipeline route; (b) The anchors are progressively moved forward as the laying takes place. Each anchor is lifted clear of the bottom and set in its new position (Masetti et. al, 2004).

The laybarge is restrained from lateral motion by the mooring lines and it is moved periodically one pipe length ahead. The mooring lines are kept under tension by the winches. These tension varies cyclically due to the long-period sway plus surge built up by the waves, storing energy in the wire lines as the barge gradually moves to one extreme of its lateral range. The mooring lines must provide the horizontal restraint against wave drift, wind drift, and current drift. They also react against one another and especially must counter the tension on the pipe, which in effect is like a mooring line of relatively equal tension, leading directly astern.

The model generated for the coupled analyses with the SITUA/PetroPipe system in shown in Figure 4. The geometrical and hydrodynamics characteristics of BGL-1 can be found in reference (Silva et. al, 2008a). A typical mooring line system in an intermediate configuration was considered, which means that none of the lines have its total cable length collect or released (Masetti et. al, 2004). The main mooring line characteristics are shown in Tables 1 and 2, more details can be found in (Silva, 2009). The pipeline physical and geometric properties are presented in Table 3.

The environmental loads consist of a current profile, 1.0 m/s at water surface and 0 at seabed (89m water depth), and a regular, 4.0m height and 12.0s period. The current and wave are applied aligned.

Figure 4. BGL-1: S-Lay Model

Table 1. Characteristics of Mooring Line Segments

Segment Length (m) Material 1 (anchor) 150 R3S Stub Chain 3”

2 1780 (max) EEIPS Steel Wirerope 2.5”

Table 2. Mooring Lines Top Tension.

Mooring Line Tension (kN)1 545.1 2 618.9 3 601.3 4 508.8 5 620.3 6 698.1 7 692.4 8 579.3

Table 3. 16-in Pipeline data

Parameter Value Unit Parameter Value Unit Outside Diameter 0.40640 m Concrete Coating Thickness 0.0381 m Wall Thickness 0.011125 m Conc. Coat. Weight Density 21.974 kN/m3

Yield Stress 414000 kN/m2 Hydrodynamic Diameter 0.489 m Modulus of Elasticity of steel 207000 MPa Field Joint Length 0.6 mDensity of steel 77 kN/m3 Joint Fill Weight Density 10.065 kN/m3 Corrosion Coating Thickness 0.0032 m Weight in Air 2.25593 kN/m Corr. Coating Weight Density 9.32 kN/m3 Weight Submerged 0.36849 kN/m


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5. Analysis Results

Since the analysis considers a regular wave load, the total simulation time is defined as 25 times the wave period (300s). The ramp for loads application is equal to 5 times the wave period (60s). Results for two load cases will be presented here. The load cases directions, (30o) Load Case 1 and (120o) Load Case 2, are schematically shown in Figure 4.

Each figure compares results of uncoupled and coupled simulations. Two types of coupled results are presented: The first one consists of the basic coupled model as implement for coupled analysis of moored floating systems. In this case, the pipeline/laybarge contact is modeled but is not taken into account in the calculation of the laybarge motions. This means that the forces (or reactions) that the pipelines exert over its supports (ramp and stinger rollers) are not considered. The second result considers the whole pipeline/laybarge interaction, which means that those forces (or pipeline reactions) are taken into account in the calculation of the laybarge motions.

The BGL-1 displacements, at the center of gravity, are presented in Figures 5 and 6 for the two load cases (30o) and (120o), respectively. The displacements of the pipeline top connection for both load cases are shown in Figure 7. Observing these results, and those of several other load cases analyzed in (Silva, 2009), the following comments can be established:

• The coupled model had smaller surge amplitudes; • More significant differences occurred in the sway response. The responses clearly show the low-frequency

sway component, which is not captured by the uncoupled model; • The coupled model heave amplitudes are slightly higher than the uncoupled model; • The roll and pitch are also higher than the uncoupled model. The yaw behavior is similar to sway behavior. The pipeline Von Mises stresses are presented in Figure 8. The overbend and touch down point (TDP) regions are

shown in details in this figure. Note that the maximum Von Mises stress values in both coupled models are smaller than values in the uncoupled model. Another very important difference that should be noticed is the variation of the Von Mises stress values shown in the TDP region. These differences occur because the TDP position is changed in the coupled model, being moved far from the laybarge. In this example the Von Mises stress values are not an issue but it can have a significant effect over other design criteria, such as the collapse check in DNV-OS-F101 (2007).

Note that there is peak in Von Mises stress response for the Load Case 2 (120o). It occurs because the pipeline contacts the lateral roller at the last roller box in stinger because of the long-period sway movement and the associated yaw movement. Note also that the contact/impact between the pipeline and last roller boxes are the most severe to the pipeline coating and it is not captured by the uncoupled model.

6. Final Remarks

The presented results clearly show that there are differences between coupled and uncoupled analyses of pipeline installation procedures. The results indicate that the coupled model presents smaller values of Von Mises stresses. It should be noticed that, before more detailed parametric studies are performed, these conclusions (drawn from only one case was analyzed with a specific water depth, pipeline, wave height and period) should not be extrapolated to more general situations. In any case, the results indicate that the use of coupled numerical models can be very important for the simulation of installation procedures.

This work closes the first cycle of developments related to pipeline installation analysis that has been started by the authors about five years ago at LAMCSO (Laboratory of Computational Methods and Offshore Systems – COPPE/UFRJ). Other previous results can be found in references (Silva et. all, 2008b,c), and a detailed description of some aspects involved in the developments can be found in (Silva, 2009). Currently, a powerful computational tool has been developed and several research works related to pipeline installation are ongoing using this tool, including exhaustive evaluation of the coupled model in simulations of typical pipeline installation procedures. Several parametric studies are being performed for different scenarios including shallow to deep waters, and different pipeline sizes.

The results of these studies will also allow the precise assessment of the influence of the application of the coupled model (lay vessel + mooring lines or DP system + pipeline) on the dynamic pipeline-laybarge behavior in such different scenarios, indicating where a coupled pipelay analysis, rather than a traditional uncoupled analysis, is required.


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-2.0-1.5-1.0-0.5

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Figure 5. Laybarge displacements (30º).

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Figure 6. Laybarge displacements (120º).


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(30o)

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Figure 7. Pipeline Top Connection displacements: 30º and 120º.

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SAGBEND: 462.0SEABED: 977.1

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Figure 8. Von Mises Stress: 30º - Left; 120º - Right.

Tabela 4. Max. Von Mises Stress (MPa).

Model (30º) (120º) Von Mises Stress % Yield Stress Von Mises Stress % Yield Stress

É 360.15 87.0 360.11 87.0 É 343.50 83.0 355.85 86.0 É 343.60 83.0 356.25 86.1


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7. Acknowledgements

The authors would like to acknowledge the active support of Petrobras, the Brazilian state oil company. Petrobras is internationally acknowledged as pioneer and leader in deep water exploitation activities, and has been boosting research activities in this area and encouraging the use of innovative numerical tools in real-life design situations.

8. References

DNV-OS-F101, “Submarine Pipeline Systems”, Offshore Standard, Det Norske Veritas, October 2007. GREALISH, F., LANG, D., CONNOLLY, A., LANE, M., “Advances in Contact Modelling for Simulation of

Deepwater pipeline Installation”, Rio Pipeline Conference & Exposition, Rio de Janeiro, Brazil, 2005. HEURTIER, J.M., BUHAN, P.LE, FONTAINE, E., CUNFF, C.L., BIOLLEY, F., BERHAULT, C., 2001, Coupled

Dynamic Response of Moored FPSO with Risers. Procs. of the 11th Int. Offshore and Polar Engineering Conference, Stavanger, Norway.

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 23st Int. Conf. on Offshore Mechanics and Arctic Engineering, Vancouver, Canada, 2004.

OFFPIPE, "OffPipe User's Guide - Version 2.05", 1996. ORCALAY, “Software Documentation”, http://www.orcina.com, 2009. ORMBERG, H., FYLLING I. J., LARSEN K., SODAHL N., 1997, Coupled Analysis of Vessel Motions and Mooring

and Riser System Dynamics. Procs of the 16th Int. Conf. on Offshore Mechanics and Arctic Engineering, Yokohama, Japan.

PIPELAY, “Software Features”, http://www.mcs.com/, 2009. RIFLEX, Theory Manual. Version 3.4, Marintek, 2005. SENRA, S.F., CORRÊA, F.N., JACOB, B.P., MOURELLE, M.M., MASETTI, I.Q., 2002, Towards the Integration of

Analysis and Design of Mooring Systems and Risers: Part I — Studies on a Semisubmersible Platform. Procs of the 21st Int. Conf. on Offshore Mechanics and Arctic Engineering, Oslo, Norway.

SILVA, D.M.L., CORRÊA, F.N., JACOB, B.P., TORRES, F.G.S., MEDEIROS, A.R., “Numerical Simulation of a Pipeline Installation Procedure at the Negro River”, Procs of the 27th Int. Conf. on Offshore Mechanics and Arctic Engineering, Estoril, Portugal, 2008a.

SILVA, D.M.L., LIMA Jr, M. H. A., JACOB, B.P., “Pipeline-Laybarge Interaction Model for the Simulation of S-Lay Installation Procedures”, Procs of the 27th Int. Conf. on Offshore Mechanics and Arctic Engineering, Estoril, Portugal, 2008b.

SILVA, D.M.L., ALBRECHT, C.H., JACOB, B.P., MASETTI, I.Q., BARROS, C.R.M., SAAD, A.C., “Subsea Pipelaying Simulation by the “Situa-Petropipe” Software - A User Friendly Alternative”. Procs of the 7th International Pipeline Conference – IPC, September 29-October 3, Calgary, Alberta, Canada, 2008c.

SILVA, D.M.L., Computational Tools for Analysis and Design of Submarine Pipeline Installation (in Portuguese), D.Sc. Thesis, COPPE/UFRJ, Rio de Janeiro, RJ, Brazil, 2009.

SIMLA, “Simulation of Pipelaying” http://www.sintef.no/Home/Marine/ MARINTEK/MARINTEKs-activities-in-the-petroleum-sector/SIMLA---Simulation-of-Pipelaying/, 2009.

WICHERS, J.E.W., DEVLIN, P.V., 2001, Effect of Coupling of Mooring Lines and Risers on the Design Values for a Turret Moored FPSO in Deep Water of the Gulf of Mexico. Procs. of the 11th Int. Offshore and Polar Engineering Conference, Stavanger, Norway.

__, “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.

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