Sérgio B. Cunha

Comparison and Analysis of Pipeline Failure Statistics

Failure statistics for onshore pipelines transporting oil, refined products and natural gas from the USA, Canada and Europe are compared. Failure data from Brazilian pipelines are presented and included in the comparison. Failure rates for internal and external corrosion, human action and natural forces are analyzed and the expected failure rate for each failure mechanism is indicated. The effects of relevant construction and environmental factors on the failure rates are studied and mean trends are obtained. Furthermore, the sizes of the holes observed in each type of failure are also compared. Finally, the probability of ignition after a failure in gas and liquid pipelines is evaluated. This study may serve as basis for the estimation of failure frequency, hole size and probability of ignition for hazard liquids and gas pipelines.Copyright

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

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

Stress, Strain and Plastic Instability of Internally Pressurized Pipes With Axis-Symmetric and Narrow Volumetric Flaws: Analysis and Experiments

The mechanical behavior of internally pressurized pipes with volumetric flaws is analyzed. The two possible straining modes are identified and each is studied by means of an idealized flaw: the bending strain is associated with axis-symmetric flaws and the membrane strain to narrow axially oriented flaws. Linear elastic shell solutions for stress and strain are proposed. The plastic behavior is studied and equations for predicting the pressure at which the pipe fails by plastic instability are developed. The analytical solutions were validated by comparison with finite element simulations and burst tests on 3” diameter pipes. Two materials with very dissimilar plastic behavior, low carbon steel and austenitic stainless steel, were used on the experiments. Twenty-nine burst testes were carried out with pipe sections with axis-symmetric flaws and twelve were performed on pipe sections with narrow flaws. The analytical models for stress and strain agreed well with the numerical model, and both the analytical and the numerical predictions showed good correlation with the experimentally observed burst pressures.© 2006 ASME

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

Pipeline VIV: Analytical Solution, Experiments and Parameter Identification

The study presents a closed-form solution for the vibration of a simply-supported beam due to vortex shedding, assuming linear elasticity and considering fluid damping. The in-line and cross-flow fluid forces are coupled to the beam equation as harmonic nonhomogeneous terms. Experimental results of 2 DOF VIV of a flexible small scale pipe in a uniform stream are presented for perpendicular an oblique (at 60 degrees of the translation direction) pipe. The range of relative velocity is from 1 to 10. The performance of two fluid damping models (Venugopal, 1996; Blevins – modified, 1990) is evaluated by comparing their predictions to the measurements of the in-line and cross-flow oscillations. Finally, ranges for in-line and cross-flow force coefficients are proposed and compared to the literature.Copyright

An Analytical Interaction Rule for Corrosion Flaws

Corrosion is a degradation mechanism that is very common in pipelines and frequently the corrosion flaws develop close one to another. Since a defect causes a perturbation in its neighboring stress field, it is a common procedure to treat close by flaws as a single one, a cluster whose dimensions envelop the whole group of defects, and some criterion must be employed to determine if two flaws are close enough to justify its clustering. The defect interaction rules available at present have practical or experimental foundations and, sometimes, one is substantially different from another. The objective of this article is to present an interaction rule based on analytical solutions for stress and strain around a volumetric flaw in a pressurized thin wall cylinder. The two possible straining modes are identified and how strain and stress propagates from a flaw into the undamaged region of the pipe is solved in the linear elastic domain. Based in this solution, a defect interaction rule is developed. This analytical interaction rule is compared to selected interaction rules commonly employed by the industry nowadays.Copyright

Forecasting the Consequences of Pipeline Failures

The consequences of the accidental release of petroleum based liquids or natural gas from onshore pipelines are studied. Damage to property, environment and society are considered. Property damage and environmental reparation costs are evaluated directly from publicly available data. Straight forward regression models are proposed to quantify these types of consequence, considering the released fluid and the characteristics of the environment. Societal impact, taken as the number of casualties, is evaluated by combining approximated fire models, heat vs. mortality correlations, population density and the statistical value of life. For gas, a jet fire model is employed; the heat flux is parameterized by the pressure and the failure size. For liquid releases, either pool or jet fire model might be employed, according to the size of the hole. The heat flux of the pool fire model depends on the size of the pool, which is determined by a correlation between released volume and affected area. On the jet fire model the heat flux is parameterized by the release rate and the heat of combustion. This study may serve as basis for the estimation of the consequences of failure in the evaluation of the risk of operating hazard liquids and natural gas pipelines.Copyright

FATIGUE LIFE ANALYSIS OF STEEL PIPELINES WITH PLAIN DENTS UNDER CYCLIC INTERNAL PRESSURE

This work is within an ongoing study, which aims to propose a new methodology for fatigue life analysis of steel pipelines with plain dents under cyclic internal pressure. This methodology follows the current high cycle fatigue theory and employs stress concentration factors induced by plain dents to modify standard S-N curves. A previously developed and validated finite element model is extended to generate stress concentration factors for longitudinal and transverse dents, in addition to spherical dents. Several finite element analyses are carried out in a parametric study to evaluate stress concentration factors induced by the three dent types studied: spherical, longitudinal and transverse dents. Analytical expressions are developed to estimate stress concentration factors for these three dent types as function of pipe and dent geometric parameters. Small-scale fatigue tests are conducted to evaluate the finite life behavior of dented steel pipes under cyclic internal pressure. The methodology is validated in view of the fatigue tests results. Including expressions to estimate stress concentration factors for three different dent types (spherical, longitudinal and transverse dents), the proposed methodology can then be used for fatigue life analysis of dented steel pipelines under cyclic internal pressure.Copyright