Bianca Pinheiro

Fatigue analysis of aluminum drill pipes

An experimental program was performed to investigate the fundamental fatigue mechanisms of aluminum drill pipes. Initially, the fatigue properties were determined through small-scale tests performed in an optic-mechanical fatigue apparatus. Additionally, full-scale fatigue tests were carried out with three aluminum drill pipe specimens under combined loading of cyclic bending and constant axial tension. Finally, a finite element model was developed to simulate the stress field along the aluminum drill pipe during the fatigue tests and to estimate the stress concentration factors inside the tool joints. By this way, it was possible to estimate the stress values in regions not monitored during the fatigue tests.

Stress Concentration Factors of Dented Pipelines

A nonlinear finite element model was developed to assess stress concentration factors induced by plain dents on steel pipelines subjected to cyclic internal pressure. The numerical model comprised small strain plasticity and large rotations. Six small-scale experimental tests were carried out to determine the strain behavior of steel pipe models during denting simulation followed by the application of cyclic internal pressure. The finite element model developed was validated through a correlation between numerical and experimental results. A parametric study was accomplished, with the aid of the numerical model, to evaluate stress concentration factors as function of the pipe and dent geometries. Finally, an analytical formulation to estimate stress concentration factors of dented pipelines under internal pressure was proposed. These stress concentration factors can be used in a high cycle fatigue evaluation through S-N curves.Copyright

COMPARATIVE STRUCTURAL ANALYSES BETWEEN SANDWICH AND STEEL PIPELINES FOR ULTRA-DEEP WATER

Pipe-in-pipe systems are usually composed of two concentric metal pipes with or without an insulation material in the annulus region. Design requirements for ultra-deep water pipelines motivated the development of a new pipe-in-pipe conception in which the annulus is filled with materials that combine low cost, adequate thermal insulation properties and good mechanical resistance. The aim of this ongoing research project is to evaluate the structural performance of sandwich pipes with two different options of core material. Because of their wide availability and relatively low costs, the materials considered in this study were cement and polypropylene for the annulus, with pipes made of API X-60 grade steel. In this paper, a three-dimensional finite element model considering material and geometric nonlinear behavior was developed. This numerical model was used to perform a parametric study to determine the collapse envelopes of different pipe-in-pipe configurations under combined bending and external pressure. The collapse envelopes were compared with others obtained for steel pipelines of equivalent collapse pressure. The study showed that the pipe-in-pipe systems with either cement or polypropylene cores are feasible options to ultra-deep water pipelines fulfilling concomitantly both the requirements of structural resistance and thermal insulation.Copyright

High Cycle Fatigue of Pipelines With Plain Dents: Simulations, Experiments and Assessment

The objective of this work is to propose a methodology for assessing the fatigue life of dented pipelines according to the current high cycle fatigue theory. The proposed methodology employs S-N curves obtained from tensile test material properties and includes an expression to estimate stress concentration factors for spherical dents. Finite element analyses are carried out to determine stress concentration factors for different pipe and dent geometries. Using the numerical results, an expression to estimate stress concentration factors of dented pipelines is developed. Additionally, fatigue tests are conducted with the application of cyclic internal pressure on small-scale dented steel pipe models. Different pressure levels are employed, resulting in failures ranging from around 6000 to more than 106 cycles, enabling the determination of the endurance limit and of the finite life behavior of dented pipes. Furthermore, the Goodman and Gerber criteria to account for the mean stress are evaluated in view of the experimental results. The fatigue test results are used to validate the proposed assessment methodology for the analyzed conditions.Copyright

Toward a Fatigue Life Assessment of Steel Pipes Based on X-Ray Diffraction Measurements

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.Copyright

Pipeline Plain Dent Fatigue: A Comparison of Assessment Methodologies

A collection of results of fatigue tests undertaken on full scale pipeline specimens containing unconstrained plain dents is presented. A total of 61 test results are reported, comprising dents of different shapes and depths, introduced in pipe sections with nominal diameters of 12″ and 24″, D/t ratios varying from 18.6 to 77.2 and made of API 5L X42, X46, X52, X60, X70 and X80 steels. The fatigue test results are used to compare five plain dent fatigue assessment methodologies. The assessment methodologies most widely used in the industry, namely, API 1156 and EPRG, are evaluated; each one of these has two different versions, evaluated independently. A dent assessment methodology previously proposed by the authors is also is included in the study. The classic Palmgren-Miner’s cumulative fatigue damage rule is employed in order to correlate the results, since all tests were carried out with two different pressure cycles. Only two of the evaluated methodologies presented a correlation with the fatigue tests that justifies recommending their application in the industry.Copyright

Stress Concentration Factors of Damaged FPSO Side Panels Under Cyclic Loads

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.Copyright

STRESS CONCENTRATION FACTORS OF LONGITUDINAL AND TRANSVERSE PLAIN DENTS ON STEEL PIPELINES

The objective of this work is to evaluate the stress concentration induced by longitudinal and transverse plain dents on steel pipelines under cyclic internal pressure. This work is within a study to propose a new methodology to assess the fatigue life of dented steel pipelines based on the current high cycle fatigue theory. This methodology employs stress concentration factors induced by plain dents, which are used to modify material S-N curves of metallic structures under high cycle fatigue loadings. The proposed assessment methodology was validated according to small-scale fatigue test results of steel pipe models with spherical dents under cyclic internal pressure. Here, stress concentration factors induced by longitudinal and transverse plain dents on steel pipes under internal pressure are obtained from a previously developed finite element model. Several finite element analyses are carried out in a parametric study. Analytical expressions are developed to estimate stress concentration factors for these two different dent geometries as function of pipe and dent geometric parameters. With the inclusion of these expressions, the proposed assessment methodology is improved and is now able to deal with three different plain dent geometries: spherical, longitudinal and transverse dents.

High Cycle Fatigue Damage Evaluation of Steel Pipelines Based on Microhardness Changes During Cyclic Loads

The aim of this work is to investigate changes in material surface microhardness during the different stages of fatigue life. Samples of API 5LX65 grade steel were submitted to high cycle fatigue tests in which different stages of microhardness changes are observed and then correlated to those observed by TEM images.

Stress Concentration Factors of Dented Rigid Risers

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.Copyright