On the Nonlinear Behavior of the Piezoelectric Coupling on Vibration-Based Energy Harvesters

Vibration-based energy harvesting with piezoelectric elements has an increasing importance nowadays being related to numerous potential applications. A wide range of nonlinear effects is observed in energy harvesting devices and the analysis of the power generated suggests that they have considerable influence on the results. Linear constitutive models for piezoelectric materials can provide inconsistencies on the prediction of the power output of the energy harvester, mainly close to resonant conditions. This paper investigates the effect of the nonlinear behavior of the piezoelectric coupling. A one-degree of freedom mechanical system is coupled to an electrical circuit by a piezoelectric element and different coupling models are investigated. Experimental tests available in the literature are employed as a reference establishing the best matches of the models. Subsequently, numerical simulations are carried out showing different responses of the system indicating that nonlinear piezoelectric couplings can strongly modify the system dynamics

Ratcheting of Corroded Pipes

Corroded pipes for oil transportation can eventually experience Ratcheting after some years of operation. The evaluation of the defects caused by corrosion in these pipes is important when deciding for the repair of the line or continuity in operation. Under normal operational conditions, these pipes are subject to constant internal pressure and cyclic load due to bending and/or tension. Under such loading conditions, the region in the pipes with thickness reduction due to corrosion could experience the phenomenon known as ratcheting. The objective of this paper is to present a revision of the available numerical models to treat the ratcheting phenomenon. Experimental tests were developed allowing the evaluation of occurrence of ratcheting in corroded pipes under typical operational load conditions as well as small-scale cyclic tests to obtain the material parameters. Numerical and experimental tests results are compared. Shakedown models are also investigated as a practical tool for ratcheting prediction.</jats:p

Analytical Assessment of the Remaining Strength of Corroded Pipelines and Comparison With Experimental Criteria

Recently published analytical solutions for the remaining strength of a pipeline with narrow axial and axisymmetric volumetric flaws are described in this paper, and their experimental and numerical validation are reviewed. Next, the domains of applicability of each solution are studied, some simplifications suitable to steel pipelines are introduced, and an analytical model for the remaining strength of corroded steel pipelines is presented. This analytical solution is compared with the standards most widely used in the industry for assessment of corroded pipelines: ASME B31G, modified ASME, and DNV RP-F101. The empirical and analytical solutions are compared with respect to their most relevant parameters: critical (or flow) stress, flaw geometry parameterization, and Folias or bulging factor formulation. Finally, two common pipeline steels, API 5L grades X42 and X100, are selected to compare the different corrosion assessment methodologies. Corrosion defects of 75%, 50%, and 25% thickness reduction are evaluated. None of the experimental equations take into account the strain-hardening behavior of the pipe material, and therefore, they cannot properly model materials with very dissimilar plastic behavior. The comparison indicates that the empirical methods underestimate the remaining strength of shallow defects, which might lead to unnecessary repair recommendations. Furthermore, it was found that the use of a parameter employed by some of the empirical equations to model the assumed flaw shape leads to excessively optimistic and nonconservative results of remaining strength for long and deep flaws. Finally, the flaw width is not considered in the experimental criteria, and the comparative results suggest that the empirical solutions are somewhat imprecise to model the burst of wide flaws.</jats:p

Shakedown Study of Pipes With External Corrosion Under Cyclic Bending and Internal Pressure

Pipelines and rigid risers are susceptible to corrosion. This is also a concern for pipes onshore and on process plants of floating platforms. Once detected the corrosion defect on pipes under cyclic loads, the analysis carried out to decide on keeping the pipe in operation or repair/replace should consider that the defect may experience cyclic plasticity caused by stress concentrations and thickness reductions. In this condition, ratcheting can cause rapid failure. This paper presents a study combining experiments and different analysis techniques to evaluate the occurrence of ratcheting in pipes subjected to cyclic bending and internal pressure. Experiments with different defect geometries were carried out. Numerical analysis using incremental plasticity and shakedown procedures are presented and compared with the experiments.</jats:p

Low Cycle Fatigue of Corroded Pipes Under Cyclic Bending and Internal Pressure

Corroded pipes for oil transportation can eventually experience low cycle fatigue failure after some years of operation. The evaluation of the defects caused by corrosion in these pipes is important when deciding for the repair of the line or continuity in operation. Under normal operational conditions, these pipes are subject to constant internal pressure and cyclic load due to bending and/or tension. Under such loading conditions, the region in the pipes with thickness reduction due to corrosion could experience the phenomenon known as ratcheting. The objective of this paper is to present a revision of the available numerical models to treat the ratcheting phenomenon. Experimental tests were developed allowing the evaluation of occurrence of ratcheting in corroded pipes under typical operational load conditions as well as small-scale cyclic tests to obtain the material parameters. Numerical and experimental tests results are compared.</jats:p

Assessment of Design Collapse Equations for OCTG Pipes Under Combined Loads

Abstract The objective of this paper is to evaluate the design collapse equations presented in Chap. 8 and Annex F of the current standard ISO TR 10400 for casings under external pressure and axial tension. A nonlinear numerical model has been developed to analyze the performance of these equations to predict casing collapse under combined loads. Experimental tests have been performed with different diameters, d/h ratio, and steel grade to calibrate the numerical model. The design collapse equations shown in the ISO TR 10400 are replicated from the API 5C3. Due to the various limitations identified since the first publication of the American Petroleum Institute (API) equations, the API Work Group has assessed different models to be used as design collapse equations and Klever–Tamano (KT) model has shown to be reliable and more accurate. However, the API Work Group included the KT model in the API 5C3 as informative. For this reason, KT model is presented in the Annex F of ISO TR 10400. The work done in this paper has confirmed the better performance of KT model for most of the cases analyzed. For combined loading, the API collapse equation results in a simple strength derating method, while the KT model has achieved similar behavior for low values of axial tension when comparing the experimental results. The axial tension for the casings into the well is likely to be lower than 40% of yield strength. Therefore, the KT model has shown to be more convenient to well design than API equations.</jats:p