OTC-25039-MS Welding Residual Stress and Pipeline Integrity R.Törnqvist, J. Wang, J. P. Tronskar, and A. Mirzaee-Sisan, DNV GL Copyright 2014, Offshore Technology Conference This paper was prepared for presentation at the Offshore Technology Conference Asia held in Kuala Lumpur, Malaysia, 25–28 March 2014. This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

Abstract Understanding welding-induced residual stresses can help pipeline industry produce and install safer, more reliable and potentially more cost-efficient pipeline designs. Welding residual stresses are may have a detrimental effect on the fatigue life and fracture capacity of the pipeline. Welding residual stresses magnitude and through-thickness distribution may be affected by several factors during welding, such as properties of the base and filler materials, welding process parameters, the number of passes, joint groove parameters, weld joint restraint, and geometry effects such as the thickness and the thickness over diameter (t/D) ratio. In addition, post-weld plastic deformation may also interact with welding residual stress and affect the magnitude and distribution of the final residual stress state through-thickness and around the pipe’s circumference. Consequently, it is crucial to understand how these factors may influence welding residual stress. With the latest development in Computational Welding Mechanics (CWM), it is today possible to estimate the residual stresses and distortions due to welding more accurately, and the results from which can be used for optimisation and determination of the structural life time of pipelines. Moreover, recent advances in analytical solutions have also enabled efficient estimation of the effects of plasticity on welding residual stresses. This paper briefly reviews published analytical, experimental and numerical studies on the influence of welding residual stress on pipeline integrity in open literature. A review of the different aspects regarding the effects of welding residual stress on pipeline integrity is illustrated through a case study. Another case study is also presented to highlight the influence of post-welding plasticity on final residual stress magnitude and distribution. Introduction to Residual Stress Residual stress is defined as any stress which exists in the bulk of a material without application of an external load, such as applied force displacement or thermal gradient. The cause of residual stresses in structural components is often complex and may be a result of the manufacturing process, fabrication or assembly, where the classification of residual stresses is often made based on their type, origin or scale. Residual stress can be further be both tensile and compressive, and residual stress fields can be one, two or three-dimensional, and where the performance of component or a structure due to residual stresses depends highly on its material and application. Regardless of the differences in their origins, residual stresses are always the result of some form of miss-match, e.g. between different parts, different regions within the same part, or even different phases within the same microstructure (Withers and Bhadeshia, 2012; Francis, et al., 2007). Residual stresses may be classified as macro residual stresses that develop in a component on a scale larger than the grain size of the material, micro residual stresses that vary on the scale of an individual grain or micro residual stresses that exist within a grain. Macroscopic residual stresses (often split into short and long range residual stresses) are often generated in heat treatment, machining, secondary processing and assembly processes, whereas microscopic residual stresses are often generated due to mismatch between phases and constituents or phase transformations. All types of residual stresses can co-exist in the same component. Residual stresses are for example unavoidable in welded structures and components, and where the residual stresses affect the fracture, fatigue, and stress corrosion cracking behaviour in welded joints. Often, the welded structures and components suffer from restrictions that are determined by the residual stresses. Residual stresses may be determined analytically, numerically (Finite Element Analysis or Computational Welding Mechanics) or

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experimentally (i.e. measurement). However, there is still a lot of ongoing fundamental research on how to accurately quantify the residual stresses and assessing their effect on structural integrity. Today there are rather limited accurate ways to characterise and quantify residual stresses, and the current simulation tools are often found too complex to use or they are too simplified thus often giving inaccurate results. The residual stress can be measured using a variety of methods, where these methods may be classified as destructive or non-destructive, and each has its advantages and disadvantages. Destructive measurement techniques include slitting, the Contour method, splitting and layering (BRSL) and block removal. There are also a group of measurement techniques, including Deep Hole Drilling (DHD), ring core and centre hole-drilling that are considered semi-destructive, as most of the component remains intact although locally these methods are destructive. Non-destructive measurement techniques include X-ray, Synchrotron X-ray diffraction, neutron diffraction, magnetic and ultrasonic methods. The choice of one measurement method over another should be based on the sampling volume characteristic of the technique, the types (macro/micro) of residual stresses, the component geometry, materials properties and cost. Welding-induced Residual Stress in Pipeline Welds Welding is a metal-joining technique which involves addition of molten filler metallic material and localized application of heat. The welded components are subject to complex transient temperature fields and associated transient thermal stresses, where the stresses are often high in magnitude to cause yielding, and occur around the deposited weld metal and across the structure as a result of differential thermal expansion/contraction. Hence, misfits are introduced into the welded structures which in turn generate residual stress when the structure has cooled to ambient temperature. Materials within the Heat Affected Zone (HAZ) in close proximity to the weld are often subject to cyclic yielding, and the cooling rates associated with welding cycles affect the microstructure of the HAZ. Furthermore, additional misfits may be introduced into Ferritic weldments during the cooling rate- and deformation-controlled phase transformation in the HAZ, redistributing the residual stress in the joined structure. The welding process is highly complex as discussed by Francis, et al., 2007. Pipelines can have both girth welds and seam welds. Pipeline girth welds have been given more attention due to criticality of the acceptance criteria in pipeline Engineering Critical Assessment, whereas pipeline seam welds in UOE pipes are generally cold expanded which significantly reduces the residual stresses. Computational Welding Mechanics of Pipeline Girth Welds As discussed earlier, residual stress may significantly affect fracture and fatigue behaviour of a structure and may be either detrimental or beneficial to a component’s structural performance. For example, compressive residual stresses might be considered beneficial if the applied load is mainly in tensile, and vice versa. However, so far it has remained difficult to accurately include residual stresses in integrity assessments as the measuring techniques for such are complex and may be cumbersome and expensive. In addition, many of the easier and affordable measurement equipment are mainly limited to point values on the surface and the through thickness residual stress distribution remains unknown. Also, since the residual stress levels and distributions are highly dependent on the welding procedures, the material as well as the geometry, it is difficult to establish residual stress databases that give accurate residual stress distributions. Hence, simplified conservative assumptions are often used, where the residual stresses are assumed to be equal to the yield stress, uniform through-thickness, and superimposed as a static load on any other applied loading. This may lead to overly conservative integrity assessments. Weld process modelling by FE simulations therefore seems attractive for quantifying residual stresses and where whole residual stress field for the actual geometry will be established and parametric studies can easily be performed. To study the effect of residual stresses on structural integrity, FE simulations with models that include the whole residual stress field together with applied external loading can be used to study local stress/strain fields where crack initiation and growth might appear. With the finite element analyses relaxation of residual stresses due to their redistributions due to plastic flow can also be established. As mentioned earlier, the welding simulation is highly complicated to simulate, where most existing simulation tools have many simplifications that the results become very uncertain and sometimes inaccurate. Therefore, it is vital that the welding simulation methods are validated against reliable measured data, so that welding-induced residual stresses distribution can be accurately analysed using computational and analytical methods. Yet, there is a lack of reliable weld experiments both in the industry and academia for the calibration and validation of FEM-weld simulations.

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FE-based, thermo-mechanical simulations to determine the temperature and stress fields present during welding have been performed for about forty years mainly in academia. The FE-technology and the computer capacity (computations and storage) have now developed to a state that Computational Welding Mechanics, CWM, is an established and becoming a mature process (Mirzaee-Sisan and Motarjemi, 2009) and CWM can be used as reliable tool in structural design, for example in optimization and parametric studies (Goldak and Akhlaghi, 2005). Recent examples of simulating welding with experimental verification (Lindgren, 2007; Pakhamaa et al., 2012) are, however, areas where improvement may still be needed: 1) material modelling in particular at high temperatures and at solid state phase transformations 2) accurate modelling of the welding process and possibly 3) geometric modelling of complex welds including contacts. Axisymmetric Analysis – A Case Study An axisymmetric model was developed to predict the welding residual stress in a girth weld where the outer diameter of the pipe was 323.9mm and the wall thickness was 24.3mm, matching the measured pipe geometry from the recently completed DNV GL joint industry project (JIP) where a number of detailed measurements of residual stress were carried out. However the measured data are confidential to the JIP sponsors. The weld in the axisymmetric model was divided into several regions to represent the addition of material each weld pass, Figure 1. Sequential thermal-stress finite element analysis was used to evaluate the welding residual stress. Linear elements were used for the thermal analysis and quadratic elements were used for the stress analysis. A fine mesh was used in the weld region to capture the large thermal gradients which develop during welding. The mesh consisted of 8345 elements and 8525 nodes.

Figure 1: Schematic showing the weld passes used in the case study Temperature dependent material properties were used for the analysis, and the same properties were assumed for the weld and parent metal. Temperature-dependent stress-strain behaviour of the weld and parent metal used in the simulation is summarised in Figure 2.

(a) (b) Figure 2: Temperature dependent plastic true stress-strain curves for (a) the weld, and (b) the parent metal

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The modelling of welding residual stress consisted of a heat transfer analysis and a subsequent thermal-stress analysis. The purpose of the thermal analysis was to obtain the spatial and transient temperature distribution in the pipe. The nodal temperatures calculated in the thermal analysis were then read into the stress analysis as a thermal loading. The element birth technique was used to simulate the addition of the weld metal. The weld shape was therefore built-up pass by pass. Table 1: Heat input used in the model and welding parameters

Process Weld Pass

Current (A)

Volt (V)

Wire Dia. (mm)

Wire feed speed (m/min)

Interpass Temperature

(°C)

Travel Speed (cm/min)

Theoretical Heat input (kJ/mm)

Model Heat input

(kJ/mm) PGMAW 1 166 17.94 1 5.6 75 42.4 0.421 0.421 PGMAW 2 217 22.59 1 9.5 110 41.8 0.703 0.9 PGMAW 3 241 22.93 1 10 130 39.3 0.843 0.843 PGMAW 4 243 23.04 1 10 158 37.8 0.888 1.2 PGMAW 5 240 22.93 1 10 165 35.5 0.930 1.2 PGMAW 6 240 23.07 1 10 180 36.6 0.907 1.3 PGMAW 7 251 23.48 1 10 205 41.2 0.858 1.3 GSFCAW 8 204 22.49 1.2 7 160 16.5 1.668 2.1 GSFCAW 9 200 22.49 1.2 7 190 15.7 1.718 2.1

In the heat transfer analysis, the heat flux was calculated using the welding parameters provide in the WPQR. The heat from the moving welding arc was applied as a volumetric heat flux using a double ellipsoid heat flux distribution as proposed by Goldak et al., (1984). In the Goldak method, the heat flux is distributed in a Gaussian manner throughout the volume. The weld heat input used in the analysis is listed in Table 1. The heat input was slightly increased in the model to ensure that volume of metal was above the melting temperature of 1500 °C. In addition, a welding efficiency of 0.8 was used for both the PGMAW and GSFCAW welding processes. The heat convection coefficient was assumed to be 10 W/m2K and the emissivity was 0.4 to represent the heat losses to air. After each pass the pipe was allowed to cool down until the inter-pass temperature was reached. In the stress analysis, the temperature history was read as thermal loads. The same mesh design as in the heat transfer was used. A linear kinematic hardening model was used. The model was constrained in the axial direction to restrict rigid body motion. Figure 3 shows the axial and hoop residual stress in the axisymmetric pipe mesh model using kinematic hardening material models for the base metal and weld material as described in Figure 2.

Figure 3: Axial and hoop residual stress in pipe based on kinematic hardening material model

Residual Stress and ECA of Pipes Fatigue and fracture assessment of pipeline girth welds are usually referred to as Engineering Critical Assessment (ECA), where welding-induced residual stress forms a crucial input parameter for the ECA. Residual stress is a tensor quantity which varies spatially throughout a weldment in three dimensions. Nonetheless, it is common practice to use simplified residual stress profiles in either the weld longitudinal or transverse directions in the ECA procedures. Generally, there are three options for estimating as-welded residual stress distribution in ECA of girth welds, with the first option being the simplest and the last most realistic:

• Uniform yield magnitude • Upper bound profiles, recommended by various codes • Some form of best-fit to FEA or measurements data, or ideally a combination of both

Several standards/guidelines such as R6 (2001), BS 7910 (2005), FITNET (2008) and SINTAP (1999) have provided various upper bound residual stress profiles to be considered in pipe girth weld ECA. The residual stress

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profile recommended by BS 7910, (2005) is one of the simplest and often leads to the most conservative results. R6 (2001) suggests three levels for classifying of welded residual stresses, of which Level 3 is expected to lead to the most realistic assessment results. In order to produce more realistic ECA results, both strength mismatch condition and welding residual stress effects should be addressed thoroughly as they are inherent properties of a girth weld. Dong and Zhang (1999) points to the fact that over-matched welds may cause high residual stresses, and that very limited published research work addressing the effect of mismatch on welding-induced residual stress is available. Dong (2008) suggested that more accurate and less conservative assumption should be used in ECA by consideration of both load-controlled and displacement-controlled conditions and by recognizing the effect of length scales. However, no specific ECA guideline addressing the treatment of residual stress in displacement controlled, i.e. strain-based, conditions is currently available. Some experimental work has shown that residual stress effects could be negligible under large primary loads. Still, the interaction between residual stress and primary loads can be complex. It is well-known that residual stress may affect the fatigue behaviour of structure (DNV-RP-C203, 2012). There are some processes (thermal or mechanical) for producing beneficial compressive surface residual stresses for enhancing fatigue life of a structure. Influence of residual stress on fatigue is addressed quantitatively by superposition of applied stress intensity factors with secondary stress intensity factors. In order to allow for the potential presence of highly tensile welding residual stresses, the fatigue crack growth curves for R≥0.5 (R = σmin/σmax is the load ratio) are recommended (DNV-OS-F101, 2012, and DNV-RP-C203, 2012). However, there is not any detailed guideline to estimate residual stress re-distribution due to crack growth. Plasticity Residual Stress in Pipes – A Case Study Besides welding, there are several processes that results in plastic deformation, such as rolling, extrusion, pipe bending and installation of offshore pipelines (e.g. reeling installation). In the offshore oil and gas industry, pipelines could experience displacement-controlled boundary conditions during installation and operation, such as during pipe reeling, snaking, earthquake and ice impact, giving rise to different levels of primary loads which may induce plasticity and associated residual-stress in the pipe steel structure. A particularly note-worthy source of plasticity-induced residual stress is from the reeling process, which is the most rapid installation method compared to S-lay and J-lay. The plastic strains experienced by the pipe due to bending cycles in the reeling process can be very high.

Figure 4: Idealised schematic moment-curvature response of a pipeline during bending-reverse bending-straightening By ignoring the tensile force in the pipeline maintained during installation and the additional bending cycle over the aligner, the reel-lay process may be simplified to a bending-reverse bending-straightening sequence, and the moment-curvature response of the pipeline may be idealized to that shown in Figure 4. The residual stress distribution at the end of one bending-reverse bending-straightening cycle is shown schematically in Figure 5 (Zhou and Mirzaee-Sisan, 2012). Zhou and Mirzaee-Sisan, (2012) and Wang et al., (2013) derived simplified equations to predict the magnitude of σ1 and σ2 as in Figure 5. The stresses shown as σ1 and σ2 correspond to peak residual stresses near the neutral axis (3 o’clock position) and at the apex (12 o’clock position) respectively. Further study was carried out to predict how a welding-induced residual stress in pipes girth welds change when the pipe is subject to a bending-reverse bending-straightening loading cycle.

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Figure 5: Plasticity-induced residual stress distribution at the end of one bending-reverse bending-straightening cycle, after Zhou and Mirzaee-Sisan, (2012) In this case study, a specific example has been analysed using the ABAQUS Finite Element Analysis (FEA) to compare the results with analytical approaches. The pipe properties and loading considered are summarised in Table 2. An initial axisymmetric linear residual stress profile, increasing from –0.5σy at the pipe’s inner surface to +0.5σy at the pipe’s outer surface, is considered for this case study. To investigate the influence of strain-hardening on the results, a separate model with exactly the same geometry, boundary conditions and loading but Ramberg-Osgood material stress-strain behavior simulating that of API 5L X65 steel is also run in ABAQUS. Table 2: Pipe parameters and loading considered in FEA validation

Property Symbol Value Unit Outer diameter OD 273.15 mm Wall thickness WT 12.7 mm Length:Diameter Ratio L/OD 6 - Yield Strength σy 448 MPa Young’s Modulus E 207 GPa Poisson’s Ratio ν 0.3 - Elastic-plastic bending radius R 15 m

Figure 6: Comparison of final longitudinal residual stress profiles around pipe’s circumference The analysis results are summarised in Figure 6 which plots the σ33 (i.e., longitudinal stress) component of the residual stress against y (defined as distance to pipe’s centroidal axis during bending, see Figure 5) for the pipe’s inner and outer surface after the bending-reverse bending-straightening process. “ABAQUS EP” are the ABAQUS FEA results using elastic-perfectly plastic material model; “ABAQUS RO” are the ABAQUS FEA results using Ramberg-Osgood material model similar to that of API 5L X65 steel; and “Analytical” are the results obtained from the spreadsheet. Good correlation was observed between the “Analytical” and “ABAQUS EP” results throughout the pipe. Similar validation processes have also been performed for other pipe geometries and initial residual stress profiles, and similar degree of conformity is observed for all cases considered. Figure 6 suggests that incorporation of strain hardening does not seem to have a significant effect on the resulting residuals stress distribution for the cases studied in this work. This is demonstrated by the proximity of the “ABAQUS RO” line to the other two lines in Figure 6. Measurement on the FEA pipe model also reveals that the post-cycle residual curvature of the pipe is typically less than 1×10-3 m-1, suggesting that the reverse bending

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radius may be less affected by strain-hardening. In order to evaluate the influence of different welding residual stress profiles on the final residual stress distribution, sensitivity studies have also been performed in this case study with three different initial longitudinal residual stress profiles applied as shown in Figure 7. An additional base case (Profile O) is also considered where the pipe is free from any initial residual stress. The yield stress and Young’s Modulus for all analysis cases are kept constant and at 448MPa and 207GPa respectively.

Figure 7: Initial longitudinal residual stress profiles considered Figure 8 plots the longitudinal residual stress profiles around the pipe’s outer diameter for Cases 1 to 4 (corresponding to initial residual stress profiles O, A, B and C respectively) after one cycle of bending-reverse bending-straightening. It is evident that for |y| > 52 mm, i.e. 52 mm away from the pipe’s centroidal axis during bending, the different initial residual stress profiles have almost no effect on the final residual stress distribution. This is confirmed by Figure 9 which plots the through-thickness final residual stress profile at the pipe’s 12 o’clock position. For the 3 o’clock position, on the other hand, strain levels have not reached the plastic region throughout the deformation sequence, and hence the initial residual stress profile has been retained, as illustrated by Figure 9. It is anticipated that, if high tension is introduced to the model, the region that are heavily influenced by initial residual stress profile will move away from the 3/9 o’clock positions, but will not disappear entirely. It is further believed that the residual stresses relaxes to some extent by operational loads as temperature changes and mechanical loads, however this effect is not documented and cannot be accounted for in this case study. Based on the results from this case study, it is evident that analytical and numerical simulations have confirmed that the welding-induced residual stresses at locations far away from the neutral axis can be reduced by plastic deformations such as reeling. However, the peak residual stress in the pipe can occur near the neutral axis with magnitude close to yield.

Figure 8: Final longitudinal residual stress profile around pipe’s outer diameter for Cases 1 to 4

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Figure 9: Final longitudinal residual stress profile through thickness at 12 o’clock position (left) and 3 o’clock (right) for Cases 1 to 4 Concluding Remarks Large variations in welding residual stress profiles are known to exist. Scatter of available welding residual stress data is significant, which prompts for further study to increase the confidence level. There have been many examples where residuals stresses have a detrimental effect on most failure mechanism (e.g. hydrogen cracking and stress corrosion). It is commonly understood that the residual stresses may have significant impact on the structural integrity and component life. The extent of such influence is also affected by the material property of the pipeline, the manufacturing method and thermal history. This paper briefly reviews published analytical, experimental and numerical studies on the influence of weld residual stress on pipeline integrity in open literature. The two case studies included in this paper demonstrate how welding residual stress can be simulated using Finite Element Method, and the latest developments in analytical approaches respectively. Acknowledgment Our colleague, Rajil Saraswat’s contribution in axisymmetric simulation of the case study presented in this paper is highly appreciated. References API RP1111. Design, construction, operation and maintenance of offshore hydrocarbon pipelines (limit state design). Code of

practice, American Petroleum Institute (API), 2009 (Errata 2011). BS 7910:2005. Guide to methods for assessing the acceptability of flaws in metallic structures. Code of practice. British

Standards Institution (BSI), 2005. DNV JOINT INDUSTRY PROJECT (JIP), Treatment of residual stress in the ECA of pipeline girth welds under high plastic

deformation, DNV Report No. PD-125UIKT-1-2009, 2009. DNV-OS-F101. Offshore standard: submarine pipeline systems. Code of practice. Det Norske Veritas (DNV), 2012. DNV-RP-C203. Recommended practice: fatigue design of offshore steel structures. Code of practice. Det Norske Veritas

(DNV), 2012. DONG, P. ZHANG, J. Residual stresses in strength-mismatched welds and implications on fracture behavior. Engineering

Fracture Mechanics, vol. 64, n.4, p.485-505, 1999 DONG, P. Length scale of secondary stresses in fracture and fatigue. Int. Journal of Pressure Vessels and Piping, vol. 85,

p.128-143, 2008. FITNET. FITNET Fitness-for-service (FFS) procedure. 2008. FOCKE, E.S., GRESNIGT, A.M., BIJLAARD, F.S.K. A theoretical model of straightening without an aligner during the reeling

installation process. Proc. 13th Int. Offshore and Polar Eng. Conf. Honolulu, Hawaii, USA, May 25-30, 2003. FRANCIS, J.A., BHADESHIA, H.K.D.H., WITHERS, P.J. Welding residual stresses in Ferritic power plant steels, Materials

Science and Technology, vol. 23, n. 9, p.1009-1020, 2007. GOLDAK, J., AKHLAGHI, M., Computational Welding Mechanics, ISBN: 9780387232874, Springer-Verlag New York Inc., USA, 2005. LINDGREN, L.E., Computational Welding Mechanics, ISBN: 1845692217, Woodhead Publishing, UK, 2007. MIRZAEE-SISAN, A., MOTARJEMI, A. The effect of strength mis-match and residual stress in ECA of girth welds with internal

circumferential cracks. Proc. ASME Pressure Vessels and Piping Division Conf. Prague, Czech Republic, July 26-30, 2009. PAKHAMAA, A., WÄRMEFJORD, K., KARLSSON, L., SODERBERG, R., AND GOLDAK, J., Combining variation with welding

simulation for prediction of deformation and variation of a final assembly, International Journal of Computing and Information Science in Engineering, Vol. 12, 2012.

R6-CODE. Software for assessing the integrity of structurescontaining defects. Code of practice. British Energy Generation Ltd, 2001.

S.I.N.T.A.P. Structural Integrity Assessment Procedure. Code of Practice. 1999. WANG J., SARASWAT, R., MIRZAEE-SISAN, A. Simplified equations for predicting secondary stress around pipe’s

circumference. Proc. ASME Pressure Vessels and Piping Division Conference, June 14-18, Paris, France, PVP2013.

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WITHERS, P. J., BHADESHIA, H.K.D.H. Residual stress Part 2 – Nature and origins. Materials Science and Technology, 17. vol. 17, n. 4, p. 366-375, 20012.

ZHOU, D., MIRZAEE-SISAN, A. Plasticity induced residual stress in pipes. Proc. 31st Int. Conference on Ocean, Offshore and Arctic Engineering, June 10-15, Rio de Janeiro, Brazil, OMAE2012.

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