- Title
- Numerical failure assessment of corroded steel pipes
- Creator
- Mokhtari, Mojtaba
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Pipeline failure can lead to the leakage of hazardous materials, which may be followed by disastrous explosions. Every year, these failures cause hundreds of fatalities and injuries as well as extensive damage to the environment and facilities, costing billions of dollars. External pitting corrosion is one of the main reasons for pipeline failure. Finite element methods (FEM) and burst capacity models are commonly used to assess the remaining strength of corroded oil and gas pipelines with localized defects (pits). These methods can be potentially excessively conservative or they may present considerably scattered predictions, in part because they idealize the actual shape of complex pitting by simple approximations. For instance, in FEM (and also in experimental studies), artificial pitting corrosion is typically modelled using semi-elliptical (semi-ellipsoidal) or rectangular (cuboidal) geometry. In order to improve the accuracy of failure predictions, a novel defect modelling technique, termed Virtual Cumulus Cloud (VCC) technique, has been developed herein. For the first time, this technique allows isolated artificial, complex-shaped pits to be developed in FEM and experimental testing. Using the VCC technique, the present study evaluates the effect of the pit geometry idealizations on the accuracy of numerical and experimental predictions. In addition, the performance of well-known burst capacity models has been assessed. In addition, idealized pit geometries have been shown herein to underestimate stress concentration factors (SCFs) significantly. By implication, this may lead to overestimates of fatigue life for pitting corroded pipes under sustained cyclic loading. It has been demonstrated that the underestimation of SCFs can occur only when the corroded pipeline undergoes little or no plastic deformation, such as under low operating pressures or when the pits are small in size. Therefore, there is a critical operating pressure below which certain types of idealized pit morphology leads to underestimates of SCFs. A protocol has been outlined and a semi-empirical equation given to allow the critical operating pressure to be estimated. Novel methods have been presented herein to correctly estimate SCFs for pipelines with defects of complex morphology. This may prevent the leakage/explosion of metallic pipelines caused by applying conventional methods for stress or fatigue analyses of pitting corroded pipelines. In addition to these developments, the finite element simulations with complex-shaped pits assisted in developing the first 3D burst capacity models, the next generation of analytical models for predicting very accurately the burst pressure of corroded steel pipelines with localized defects of complex morphology. These models bring simplicity and accuracy together through using a new parameter, volume of the defect, to measure the burst pressure of pitting corroded pipes. The past 50 years has seen the development of numerous 2D burst capacity models that focused only on the longitudinal-section area of the corrosion defect. These have become increasingly complicated and less practical in efforts to increase their relatively low accuracy and stability. Contrary to these earlier models, the 3D models presented herein do not require the definition of an exact defect profile, yet they are significantly more accurate and more stable than well-known conventional models. Thus, they can significantly reduce the lifecycle costs of pipelines by eliminating or much reducing the unnecessary repair or replacement of corroded pipelines. All the conclusions and the new analytical models mentioned above were derived from 33 finite element simulations of pitting corroded X65 steel pipes. These were validated against seven full-scale burst capacity tests on X65 steel pipes. Then, the conclusions and the developed analytical models were verified against 17 finite element models of pitting corroded X42 steel pipes, which themselves were validated against six full-scale burst capacity tests with both complex-shaped and rectangular defects on X42 pipes.
- Subject
- remaining strength; complex-shaped pit; carbon steel pipe; volume loss
- Identifier
- http://hdl.handle.net/1959.13/1401294
- Identifier
- uon:34888
- Rights
- Copyright 2019 Mojtaba Mokhtari
- Language
- eng
- Full Text
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| View Details Download | ATTACHMENT01 | Thesis | 23 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 453 KB | Adobe Acrobat PDF | View Details Download |