In the development of oil fields, submarine pipelines are used in various applications. These pipelines and risers are subject to accidents that may occur during operation, such as shocks between risers or shocks between a riser and an anchor, rock, or any equipment or heavy object, which may cause mechanical failure, such as dents. The objective of this work is to study of the effect of the introduction of plain dents on the structural integrity of rigid risers under fully reversed bending. A three dimensional finite element model was developed to estimate the stress concentration on dented risers under bending. Several numerical simulations were carried out to evaluate stress concentration factors (SCFs) for varying dimensions of dents and risers, in a parametric study. These SCFs can be used in the prediction of the remaining fatigue life of dented rigid risers.

The hardness of a material shows its ability to resist to microplastic deformation caused by indentation or penetration and is closely related to the plastic slip capacity of the material. Therefore, it could be significant to study the resistance to microplastic deformations based on microhardness changes on the surface, and the associated accumulation of fatigue damage. The present work is part of a research study being carried out with the aim of proposing a new method based on microstructural changes, represented by a fatigue damage indicator, to predict fatigue life of steel structures submitted to cyclic loads, before macroscopic cracking. Here, Berkovich indentation tests were carried out in the samples previously submitted to high cycle fatigue (HCF) tests. It was observed that the major changes in the microhardness values occurred at the surface of the material below 3 μm of indentation depth, and around 20% of the fatigue life of the material, proving that microcracking is a surface phenomenon. So, the results obtained for the surface of the specimen and at the beginning of the fatigue life of the material will be considered in the proposal of a new method to estimate the fatigue life of metal structures.

The aim of this work is to evaluate the residual fatigue life enhancement of damaged pipelines after the execution of composite material repairs made of laminates of epoxy matrix reinforced with glass fibers. In view of structural performance and cost concerns, the more suitable repair thickness should be proposed. The work comprises a numerical and experimental study on the stress concentration of damaged pipes under internal pressure before and after repair. A numerical model is developed, based on the finite element method, to provide stress concentration factors of damaged pipes (plain dent defect), under cyclic internal pressure, before and after applying glass fiber and epoxy matrix laminate repairs with varying thicknesses. Small-scale steel pipe samples are submitted to denting and the resulting stress concentration in the damaged region is estimated under cyclic internal pressure, before and after repair execution. From correlation between numerical and experimental results, the finite element model is calibrated and validated. A parametric study is carried out to evaluate stress concentration factors of dented pipes repaired with varying laminate thickness. Stress concentration factors of dented pipes under internal pressure after repair can be used in a fatigue assessment methodology from correction of S-N curves. The effect of repair thickness on the reduction of stress concentration factors is evaluated in view of the residual fatigue life enhancement of damaged pipes, beside repair procedure costs. Based on results of the parametric study, recommendations about the repair procedure using laminates of epoxy matrix reinforced with glass fibers will be proposed, comprising indications of the more suitable repair thickness, as a function of pipe and damage dimensions, in view of fatigue performance and cost concerns.

Fatigue is a major cause of failures concerning metal structures, being capable of causing catastrophic damage to the environment and considerable financial loss. Steel pipelines used in oil and gas industry for hydrocarbon transportation, for instance, are submitted to the action of cyclic loads, being susceptible to undergo fatigue failures. The phenomenon of metal fatigue is a complex process comprising different successive mechanisms. In general, four stages can be identified, representing microcrack initiation (nucleation), microcracking, macrocrack propagation, and final fracture. Fatigue damage prior to nucleation of microcracks is primarily related to localized plastic strain development at or near material surface during cycling. The microhardness of the material shows its ability to resist microplastic deformation caused by indentation or penetration, and is closely related to the material plastic slip capacity. Therefore, the study of changes in material surface microhardness during the different stages of fatigue process can estimate the evolution of the material resistance to microplastic deformations and, consequently, provide relevant information about the cumulated fatigue damage on the surface. The present work is part of a research study being carried out with the aim of proposing a new method based on microstructural changes, represented by a fatigue damage indicator, to predict fatigue life of steel structures submitted to cyclic loads, before macroscopic cracking. In a previous work, the X-ray diffraction technique was used to evaluate these changes. This technique presents several advantages, since it is non-destructive and concerns the surface and subsurface of the material, where major microstructural changes take place during fatigue. The most important parameter obtained by this technique is the full width at half maximum (FWHM) of the diffraction peak, which can provide information about the dislocation network density and estimate microdeformations. It was found that the evolution of this parameter with cycling presents three different stages, associated to the mechanisms of microcrack initiation, microcracking, macrocrack propagation, respectively. Here, the fatigue damage of pipeline steels is evaluated through microhardness testing. Different stages of changes in microhardness are also found and they are correlated to those observed with the X-ray technique and also with transmission electron microscopic (TEM) images from experimental tests performed with a similar material. This correlation can help to corroborate the X-ray diffraction results previously obtained and recommend then this non-destructive technique as the base of the method for predicting fatigue life of steel structures proposed here.

The present work aims to evaluate the microstructural mechanisms associated with the initiation of fatigue damage of steels used in the petroleum industry. Microdeformations and residual stresses (macrostresses) are evaluated by X-ray diffraction in real time during alternating bending fatigue tests performed on samples taken from an API 5L X60 grade steel pipe. Samples under two different conditions are considered: as-machined and annealed. Microdeformations and residual stresses are estimated from measurements of the full width at half maximum (FWHM) and displacement of the diffraction peak, respectively. The evolution of microdeformations shows three distinct stages (Stages 1–3). Increasing stress amplitude accentuates variations in FWHM and reduces the duration of each stage. Similar variations are observed for the residual stresses. The results from annealed samples allow the comprehension of the role of the initial structure. Changes in the density and distribution of dislocations are observed by transmission electron microscopy using the technique of focused ion beam. Cyclic uniaxial tests are carried out in order to evaluate the material behavior, in both as-machined and annealed conditions, under cyclic loadings. The cyclic material behavior is correlated to the evolution of microdeformations observed during the fatigue bending tests.

Rigid risers are widely employed in the offshore industry nowadays, assuming the vertical (riser tower) or catenary (steel catenary riser, SCR) configurations. During operation, these structures undergo dynamic loads generated by the action of currents and waves. Rigid risers can also be subjected to collision from neighboring vessels or impact of heavy objects launched from them, resulting in the introduction of defects. The possibility of a fatigue failure must be addressed since these defects induce high localized stresses in the damaged section. The aim of this work is to evaluate the stress concentration induced by plain dents on rigid risers under combined dynamic bending and constant tension loads. A finite element model is developed to reproduce denting and spring back processes and estimate the stress concentration on the dented riser under combined bending and tension loads. The model is used in a parametric study to evaluate stress concentration factors (SCFs) for varying dimensions of dents and risers. Analytical formulae are developed considering the results from the parametric study to estimate SCFs of dented risers, which can be used in a theoretical fatigue life study, modifying standard S-N curves, and help to forewarn a fatigue failure.

FPSO (floating production, storage and offloading) units can be subjected to mechanical damage in their side panels caused by collision with supply vessels. Even if the ultimate strength of the panel is not significantly affected by small damage, the stress concentration in the collided region may lead to the initiation of fatigue cracks, considering the long period of operation undergone by these vessels. The aim of this work is to evaluate stress concentration factors (SCFs) in damaged FPSO side panels and estimate their effect on the fatigue life through a theoretical study. A finite element model is developed to reproduce a supply vessel collision and evaluate resulting SCFs under in-plane compression load. A parametric study is carried out considering different damage magnitudes and the results obtained are used to develop an analytical expression to provide SCFs as a function of dimensions of damage and panel. SCFs provided by this expression could be used in a theoretical fatigue life study that can estimate the residual fatigue life of collided FPSO side panels and help to forewarn a fatigue failure under the event of an accidental collision.

Steel pipes used for oil and gas exploitation undergo the action of cyclic loads that can cause their failure by fatigue. A consistent evaluation of the fatigue behavior should take into account the micromechanisms of fatigue damage initiation, which precede macroscopic cracking and macrocrack propagation. In this work, microstructural changes in terms of variations in microdeformations and residual stresses (macrostresses) are evaluated by X-ray diffraction in real time during alternating bending fatigue tests performed on samples taken from an API 5L X60 grade steel pipe. Three stages of microstructural changes are detected. It is found that their amplitudes and durations are proportional to the level of alternating stress applied. Changes in density and distribution of dislocations are observed by transmission electron microscopy combined with the technique of focused ion beam. To understand the role of the initial dislocation structure, fatigue tests on annealed samples are performed under the same test conditions. Again, three stages of changes are observed, but with an increase in microdeformations during the first stage instead of a decrease as found for as-machined samples, suggesting the influence of the initial state of the dislocation network. The results obtained are very encouraging for the consideration of microstructural evolutions in the development of an indicator of fatigue damage initiation in steel pipes.

The present work aims to evaluate the microstructural mechanisms associated with the initiation of fatigue damage in steels used in the petroleum industry. Microdeformations and residual stresses (macrostresses) are evaluated by X-ray diffraction in real time during alternating bending fatigue tests performed on samples taken from an API 5L X60 grade steel pipe. Microdeformations are evaluated from measurements of the full width at half maximum (FWHM) of the diffraction peak and residual stresses are estimated from the peak displacement. The evolution of microdeformations shows three regular successive stages of changes. The amplitude of variation of each stage is intensified with increasing stress amplitude, while the duration is reduced. A similar evolution is found for residual stresses, whose stages of changes have nearly the same durations as those of microdeformations. Changes in density and distribution of dislocations are observed by transmission electron microscopy combined with the technique of focused ion beam. To understand the role of the initial structure, fatigue tests on annealed samples are performed under the same test conditions. Again, three stages of changes are observed, but with an increase in microdeformations instead of a decrease during the first stage due to the initial state of the dislocation network. The results are very encouraging for the consideration of microstructural changes measured by X-ray diffraction in the development of a future indicator of fatigue damage initiation in API 5L X60 grade steel pipes.

Steel pipe structures used in oil and gas industry, such as drill pipes, rigid risers and pipelines, undergo the action of cyclic loadings that can cause their failure by fatigue. These structures are made of high strength steels, such as API 5L grades X for instance. A consistent evaluation of the fatigue behavior should fundamentally be based on a local approach, in the dislocation scale, and take into account the micromechanisms of fatigue damage initiation, including microdeformations and microstructural changes, which precede the macrocrack propagation leading to final failure. In this work, the microstructural mechanisms of fatigue damage initiation in API 5L X60 grade steel pipes are investigated. Material properties of API 5L X60 steel are estimated according to chemical composition analyses, microscopic analyses, uniaxial tensile tests and Vickers micro-hardness tests. Samples are submitted to fatigue tests with reversed stress bending loadings. Microdeformations and residual stresses are measured with the aid of the X-ray diffraction method in real-time during fatigue tests. A numerical model is developed to reproduce the fatigue test loadings. The aim of the work is to provide ground for the development of a microstructural criterion for fatigue damage initiation in API 5L X60 grade steel pipes from the obtained experimental results. This criterion could allow a good prediction of the residual life of steel pipes previously submitted to fatigue loadings, before macroscopic cracking, and help to increase the reliability of oil and gas pipes.