[pt] A redução de espessura de parede de dutos, causada por corrosão, é um dos defeitos que mais afetam a integridade dos mesmos. Este defeito pode ocorrer no metal base, nas soldas longitudinais e circunferenciais. Com a expansão do número de dutos rígidos instalados no fundo do mar e o avançado tempo de uso, a indústria de petróleo tem desenvolvido diversas ferramentas de reparo desses dutos, sem comprometer a segurança e a perda da produção. O objetivo preliminar desse trabalho foi o estudo e a geração de modelos computacionais, os quais uma vez parametrizados, possam servir como ferramenta para análise de conexões de reparo de dutos submarinos. No decorrer desse estudo, sobre dois tipos de conexões de reparo, foi percebido que o comportamento na falha, descrito pelos modelos numéricos das mesmas, possui detalhes que precisavam ser esclarecidos em complementação ao programa experimental adotado (Alves,1995). Uma fase dessa pesquisa constou o trabalho de laboratório no qual repetiu-se a confecção da mistura de resina epóxi e cimento, segundo as referencias adotadas no programa experimental de Alves e realizaram-se ensaios de compressão em corpos-de-prova cilíndricos moldados com essa mistura visando o levantamento de curva tensão versus deformação e determinação do coeficiente de Poisson. Outra fase dessa pesquisa foi a consulta de diversos outros trabalhos que indicassem o comportamento dessa mistura, quando submetida à compressão. Por fim, foram gerados modelos analíticos e numéricos da conexão de reparo. Seus resultados foram comparados com os resultados obtidos no programa experimental de Alves. Com base nessa comparação foram levantadas as discrepâncias dos resultados, bem como as possíveis causas dessas discrepâncias e suas possíveis razões e sugestões para se alcançar o objetivo inicial foram indicadas. / [en] The reduction of pipeline wall thickness caused by corrosion is one of the defects that more affect pipeline structural integrity. These defects can take place in the metal base, in the longitudinal and circumferential welding. With the expansion of the rigid submarine pipelines mesh already installed and considering the advanced time of using, the oil industry has been developing several tools for pipeline repair without compromising the safety and avoiding production stops. The preliminary objective of this work was the study and the generation of computer models, which once parameter, they could be a tool for analyze the repair connection of submarine pipeline. During this study, about two types of repair connection, it was realized that the behaviour at failure described by the connection numerical models has details which needed to be explained in addition to the considered experimental program (Alves). One phase of this research was the laboratory works in which it was possible to repeat the mixture of epoxi resin and cement, according to the references adopted in the experimental program (Alves). Compression test were conducted on cylindrical specimens made of this mixture for determining the stress-strain curve and the Poisson coefficient. Another phase of this research was the consulting of other works which could indicate the expected behaviour of the mixture subjected to the compression load. Finally, analytical and numerical models of repair connection were generated the results were compared with those obtained on experimental program (Alves) Based on comparison, discrepancies were detected as well as they possible causes. Suggestions to obtain the initial objective were indicated.

[pt] A avaliação estrutural de dutos com defeitos de corrosão vem sendo estudada desde o final da década de 60. A partir dos conceitos da Mecânica da Fratura, foram elaboradas expressões semi-empíricas que permitiram estimar a pressão de ruptura de defeitos de corrosão. Desde então, essas expressões foram sendo ajustadas e aprimoradas por testes de ensaios destrutivos e análise de elementos finitos. Os principais métodos desenvolvidos são o ASME B31G, 085dL, Effective Area, DNV RP F-101 (defeitos isolados) e DNV RP F-101 (defeitos complexos). Esta tese foi elaborada utilizando alguns dos ensaios programados para o projeto Produt 600536, e parte dos seus resultados foi aproveitada neste projeto. Para o trabalho de tese, foram utilizados cinco espécimes tubulares de aço API 5L X46, com 3,0 m de comprimento aproximado, diâmetro nominal de 457,2 mm e espessura nominal de 6,35 mm. Estes espécimes continham defeitos reais de corrosão interna, do tipo longo, localizados na geratriz inferior, e foram retirados do oleoduto Orbel I, pertencente à Petrobras, durante a sua obra de reabilitação, em 2001. Os defeitos de corrosão foram mapeados com medições manuais por ultra-som espaçadas em 20 mm e com medições mecanizadas CSCAN espaçadas em 5mm. Para cada espécime, foram realizados ensaios de tração em 4 corpos de prova, sendo 2 corpos retirados transversalmente e 2 longitudinalmente. Estes espécimes foram instrumentados com extensômetros de resistência elétrica e pressurizados até a ruptura. Para cada espécime, foram levantados diversos perfis de corrosão em função do comprimento estabelecido para o defeito e do tipo de medição (manual ou mecanizada). A pressão de ruptura foi estimada pelas equações dos métodos ASME B31G, 085dL, Effective Area, DNV RP F-101 (defeitos isolados) e DNV RP F-101 (defeitos complexos), utilizando planilha Excel e/ou os programas computacionais RSTRENG e DNV RP F-101. Os valores de pressão de ruptura estimados para os espécimes, utilizando os métodos acima relacionados, foram comparados com as pressões de ruptura reais, obtidas nos ensaios de pressão. Os resultados confirmaram o conservadorismo embutido no método ASME B31G e comprovaram que os métodos Effective Area e DNV RP-F101 (complexo), que utilizam o perfil de corrosão, apresentam resultados melhores que os métodos ASME B31G e 085dL e podem ser considerados uma boa ferramenta para avaliar defeitos de corrosão, considerando somente carregamento de pressão interna. / [en] Structural assessment of pipelines with corrosion defects has been studied since the late 1960s. From the principles of fracture mechanics, semi-emprical mathematical expressions have been developed for predicting burst pressure of corroded pipes. Subsequently, these expressions have been modified and calibrated based on the results from finite element analyses and laboratory burst tests. The main methods are ASME B31G, Modified B31G (version 0.85 dL), Effective Area and DNV RP-F101 for single and complex shaped defects. This thesis was done utilizing some laboratory tests from Produt 600536 Project. For this study, five specimens with 3.0 meters each were removed from Petrobras pipeline (Orbel I), during its rehabilitation. The pipe material was API 5L X46, 18 diameter and 0.25 wall thickness. These specimens had real internal corrosion with very long defect length, at the 6:00 o`clock position. The corrosion defects were mapped by manual ultrasound, at 20 mm intervals, and mecanized ultrasonic measurements CSCAN at 5 mm intervals. For each pipe, the yield strength and ultimate strength were determined by tension tests of 4 specimens, 2 removed from transverse and 2 longitudinal. Every pipe specimen was monitored by strain gages and pressurized up to the point of rupture. For each specimen different profiles were determined depending on the length defined for the defect and for the intervals of measurements. The burst pressure was predicted by the following methods: ASME B31G, Modified B31G (version 0.85 dL), Effective Area and DNV RP-F101 for single and complex defects. These predicted rupture pressures were compared with the real burst pressures. The results confirmed the conservatism of the ASME B31G method and demonstrated that the Effective Area and DNV RP-F101 for complex shaped defects methods can be considered good for evaluating longitudinal corrosion defects, considering only internal pressure load.

[pt] A redução de espessura de parede causada por corrosão é um dos defeitos que mais afetam a integridade dos dutos. Estes defeitos podem ocorrer no metal base, nas soldas longitudinais ou circunferenciais, bem como nas zonas afetadas pelo calor. Os métodos de avaliação da resistência remanescente introduzem ressalvas ou proíbem o tratamento de defeitos de corrosão coincidentes com as juntas soldadas. No presente trabalho foram avaliados os níveis de segurança dos métodos usuais de avaliação da resistência de dutos com defeitos introduzidos na região da solda longitudinal de tubos soldados por arco submerso. Os tubos testados eram de aço C-Mn fabricados na década de 60 e foram retirados de operação após uma campanha superior a 30 anos. Com estes tubos foram fabricados 5 espécimes, cada qual com 1 defeito externo produzido por eletro-erosão, sendo posteriormente submetidos a testes de pressão monitorados. Foram realizados ensaios de tração, ensaios de impacto Charpy e ensaios de dobramento, para determinação das propriedades mecânicas do metal de base, do metal de solda e da zona termicamente afetada (ZTA). Foram realizados também ensaios metalográficos do metal de base e do metal de solda, análise química do metal de base e do metal de solda e medição das tensões residuais no metal de solda. A aplicação dos métodos de nível 1 (ASME B31G, 085dL, RPA, DNV isolado e PCORRC) resultou em pressões previstas inferiores às pressões reais de ruptura para defeitos de formato esférico. Para defeitos de seção retangular a aplicação dos métodos de nível 1 resultou em pressões superiores às pressões reais. A aplicação dos métodos de nível 2 (Effective Area e DNV geometria complexa) resultou em pressões previstas superiores às pressões reais de ruptura para defeitos de formato esférico e de seção retangular. As análises das fraturas indicaram que não ocorreu falha por deficiência de tenacidade em nenhum espécime. A corrosão alveolar leve pré-existente na superfície interna dos espécimes foi apontada como causa mais provável das pressões previstas superiores às pressões reais de ruptura. / [en] One of the most important issues that affects pipeline integrity is corrosion-caused metal loss. This type of defect can occur over the pipe body, seam or girth welds or even on heat affected zones. Pipeline remaining strength criterions are restricted or even prohibited for assessing corrosion defects coincident with weld regions. This present work investigated the reliability of the most common assessment criteria for corroded pipes when defects were coincident with seam weld region in submerged arc welded pipes. The tested specimens were C-Mn steel pipe, from a pipeline manufactured in the 60´s and had operated for over 30 years. Five external defects were created on five pipe specimens and pressure tested until rupture. Stress, charpy V-notch and bending tests were carried out to find the mechanical properties of the welded joints. Microstructure and chemical composition from base metal and weld metal were also analyzed. Residual stresses on welds were measured. The level 1 criterion (ASME B31G, 085dL, RPA, DNV single and PCORRC) applications have underestimated failure pressure for spherical shaped defects. On the other hand, the same criterions have overestimated failure pressure for rectangular shaped defects. The level 2 criterions (Effective Area e DNV complex geometry) application overestimated failure pressure for all 5 specimens with both spherical and rectangular shaped defects. Fracture analyses have indicated that no specimen had a toughness controlled failure. Internal pit corrosion that was found after pressure testing is recognized as the most probable cause of underestimated pressure forecasts.

The transportation of oil and gas and their products through the pipelines is a safe and economically efficient way, when compared with other methods of transportation, such as tankers, railroad, trucks, etc. Although pipelines are usually well-designed, during construction and later in service, pipelines are subjected to a variety of risks. Eventually, some sections may experience corrosion which can affect the integrity of pipeline, which poses a risk in high-pressure operations. Specifically, in pipelines with long history of operation, the size and location of the corrosion defects need to be determined so that pressure levels can be kept at safe levels, or alternatively, a decision to repair or replace the pipe section can be made. To make this decision, there are several assessment techniques available to engineers, such as ASME B31G, MB31G, DNV-RP, software code called RSTRENG. These assessment techniques help engineers predict the remaining strength of the wall in a pipe section with a corrosion defect. The corrosion assessment codes in the United States, Canada and Europe are based on ASME-B31G criterion for the evaluation of corrosion defects, established based on full-scale burst experiments on pipes containing longitudinal machined grooves, initially conducted in 1960s. Because actual corrosion defects have more complex geometries than machined grooves, an in-depth study to validate the effectiveness of these techniques is necessary. This study is motivated by this need. The current study was conducted in several stages, starting with the deformation behavior of pipe steels. In Phase 1, true-stress-true plastic strain data from the literature for X42 and X60 steel specimens were used to evaluate how well four commonly used constitutive equations, namely, those developed by Hollomon, Swift, Ludwik and Voce, fit the experimental data. Results showed that all equations provided acceptable fits. For simplicity, the Hollomon equation was selected to be used in the rest of the study. In Phase 2, a preliminary finite element modeling (FEM) study was conducted to compare two failure criteria, stress-based or strain-based, performed better. By using data from the literature for X42 and X60 pipe steels, experimental burst pressure data were compared with predicted burst pressure data, estimated based on the two failure criteria. Based on this preliminary analysis, the stress-based criterion was chosen for further FEM studies. In Phase 3, failure data from real corrosion pits in X52 pipe steels with detailed profiles were used to develop a FEM scheme, which included a simplified representation of the defect. Comparison of actual and predicted burst pressures indicated a good fit, with a coefficient of determination (R2) level of 0.959. In Phase 4, burst pressure levels were estimated for real corrosion pits for the experiments from the same study as in Phase 3, but only with corrosion pit depths and length and without corrosion widths. Widths were estimated from the data used in Phase 3, by using an empirical equation as a function of pit length. There was significant error between experimental and predicted burst pressure. Errors in Phases 3 and 4 were compared statistically. Results showed that there is a statistically significant difference in the error when the width of the corrosion pit is unknown. This finding is significant because none of the assessment techniques in the literature takes width into consideration. Subsequently, a parametric study was performed on three defect geometries from the same study in Phase 3. The pit depths and lengths were held constant but widths were changed systematically. In all cases, the effect of the pit width on burst pressure was confirmed. In Phase 5, the three assessment techniques, ASME B31G, MB31-G and DNV-RP were evaluated by using experimental test results for X52 pipe. Synthetic data for deeper pits were developed by FEM and used along with experimental data in this phase. Two types of the error were distinguished to classify defects. Type I errors (α) and Type II errors (β) were defined using Level 0 evaluation method. Results showed that although ASME B31G is the most conservative technique, it is more reliable for short defects than MB31G and DNV-RP. The least conservative technique was DNV-RP but it yielded β error, i.e., the method predicted a safe operating pressure and pipe section would fail. Therefore, DNV-RP is not recommended for assessment of steel pipes, specifically for X52 pipes.