Martin Valdez

Modeling the Effect of Material Behavior and Mild Thermal Treatments on Collapse Resistance of UOE Pipes

Oil exploration and production of offshore sources is continuously shifting towards increasing depths and more severe environmental conditions. Ultra deep waters are an objective in, e.g., the pre-salt layer off the Brazilian coast and in the Gulf of Mexico. Under these conditions, resistance to collapse of pipelines is a main concern. Increasing the collapse pressure pc is thus a primary objective, which would lead to a reduction of material and installation costs.To increase pc, it is fundamental to understand which variables affect it, and how to control these variables. For instance, it is well known that ovality, residual stresses, and material constitutive behavior have a direct effect on pc. Current efforts for improving pc of large diameter UOE pipes include an increase in flow stress by the application of a thermal cycle, similar to those typical of coating processes. These thermal treatments recover at least part of the early yielding due to the Bauschinger effect that develops during the collapse test, after the expansion stage.Predictive modeling of pc, based on an appropriate set of input variables, allows for an adequate design of deep- and ultra-deep water projects. In the present work, an assessment by finite element analysis of the requirements on material characterization tests for a reliable prediction of pc has been performed. The most appropriate testing direction is the transverse compression. Moreover, since for large diameter pipes the plastic strain levels attained at collapse are often below 0.2%, the sample should allow for an accurate determination of compression behavior in this very low deformation range. This is particularly relevant for cold-formed pipes, as with the UOE process. Based on these guidelines, a testing sample geometry and compression data processing methodology has been designed.The methodology has been applied to a series of UOE processed pipes that had been thermally treated. On one hand, compression samples were extracted and used for the FE calculation of pc. On the other hand, collapse tests were performed on the same pipes. Both the absolute values of pc, and the enhancement of pc due to thermal cycling, were accurately predicted.In addition, both the flow stress after thermal cycling, and the measured pc values, clearly show that the fabrication factor αfab used in the standard DNV OS-F101 should be set to αfab≥1 for an adequate rating of the pipes.Copyright

Modeling of the Collapse and Propagation Behavior of UOE SAW Pipes Under External Pressure: Influence of Thermal Treatments for Typical Coating Applications

Large diameter UOE pipes are being increasingly used for the construction of offshore pipelines. Since oil discoveries are moving towards ultra deep water areas, collapse resistance is a key factor in the design of the pipelines. It has been demonstrated in previous works that the application of typical coating thermal treatments increases the collapse resistance of the pipes recovering the original strength of the plate. To improve the understanding of these effects, the Tenaris has embarked on a program of both, experimental testing and finite element modeling.Previous phases of this work formulated the basis for model development and described the 2D approach taken to model the various stages of manufacture, from the plate to the final pipe and the collapse test. More recent developments included some modeling enhancements, sensitivity analyses, and comparison of predictions to the results of full scale collapse testing.In the present work, 3D finite element analyses of collapse were performed and compared with the latest collapse and propagation tests performed by Tenaris, where the effect of typical coating thermal treatments was studied and significant increments in the collapse pressure of pipes were obtained. The numerical results show a good agreement with the experimental ones and could predict the increment produced in the collapse pressure by the effect of the thermal treatments. Comparison of the results with the predictions from API RP 1111 and DNV OS-F101 equations was also performed.The outcomes of this study will be employed to further optimize the collapse resistance of subsea linepipe in order to reduce material and offshore installation costs through the increment of the fabrication factor as stated in the DNV OSF101 standard.Copyright

Effect of Forming on Behavior of UOE Pipe Material

The worldwide share of oil&gas produced from offshore sources is constantly increasing. Accordingly, deep and ultra-deep water projects go to ever increasing depths. Large diameter pipes for this type of projects are often manufactured by the UOE process. After the cold work associated with UOE forming, mechanical properties of pipe material are different from those of the original plate. In particular, the circumferential compression behavior is markedly affected by the Baushcinger effect which develops after the last expansion step, and this is a key property for the resistance to collapse under external pressurization. Standard formulas for the assessment of the collapse pressure pc variedly account for this effect. For instance, DNV OS-F101 penalizes the SMYS of the pipe with a fabrication factor αfab that reduces the pc rating of UOE pipes.Understanding the effect of deformation history on final material properties becomes desirable for a proper identification of processing strategies. A testing program was developed aimed at evaluating the effect of UOE forming on final transverse compression behavior, as it is relevant for collapse resistance. Work softening (i.e., the Bauschinger effect) and hardening were quantified under a variety of deformation operations.Copyright

Material Testing for the Prediction of the Effect of Deformation and Aging Thermal Treatments on Collapse Resistance of UOE Pipes

Oil & Gas offshore exploration and production increase continuously in deep waters. This trend requires pipes with increasing collapse pressure (pc), which is the primary design variable. The prediction of pc, based on an appropriate set of input variables, allows for the appropriate design of deep and ultra-deep water projects.Elastic and plastic material behavior is one of the main factors affecting pc. International application codes (e.g., DNV OS-F101) incorporate the yield strength into their formulas for pc. In the present work, an assessment of the requirements on material characterization tests for a reliable prediction of pc has been performed. The most appropriate testing direction is the transverse compression. Moreover, since for large diameter pipes the plastic strain levels attained at collapse are often below 0.2%, the sample should allow for an accurate determination of compression behavior in this very low deformation range. This is particularly relevant for cold-formed pipes, as with the UOE process.Based on these guidelines, a testing procedure has been designed. This analysis has been applied to the prediction of the effect of thermal cycles on pc. Calculated values show a very good agreement with experimental pc values determined for a series of UOE processed pipes that had been thermally treated and collapsed.Copyright