Tensile Strain Capacity of Energy Pipelines

• Author / Creator

  Cakiroglu, Celal

• Pipeline structures can be subjected to excessive tensile strains due to a variety of environmental agents including but not limited to slope instabilities, seismic forces, freezing and thawing cycle of the permafrost in arctic regions. In addition to environmental and geotechnical effects, large strains can also occur during offshore pipe laying operations using reeling. Another occasion where the increase of the tensile strains can cause damage to pipelines and the environment is the accumulation of strains in the vicinity of cold bends. Experimental research showed that tension side fracture of cold bends is a probable mode of failure albeit being mostly overlooked in practice. This mode of failure due to tensile strain is highly dangerous because of the explosion associated with it in case of gas carrying pipelines. Therefore further study of that failure mode is necessary in order to identify the conditions leading to it. In the recent years a strain based design approach is increasingly adopted in the management of pipeline systems in order to guarantee a safe and economical pipeline operation. An integral part of the strain based design methodology is the prediction of the tensile strain capacity of pipeline structures under a combination of external forces, in the presence of defects in the pipe wall which can be caused due to corrosion or imperfections in the manually or automatically manufactured girth welds. The presence of pipe wall flaws due to corrosion is especially critical in case of vintage pipes which constitute a large portion of the Canadian pipeline network. On the other hand the tensile strain capacity prediction equations currently available in the literature are not applicable to vintage pipes with low yield strength. Therefore, there is a dire need for the study of vintage pipes in order to have a better understanding of their structural response under tensile strain. In the scope of this research program the tensile strain capacity of energy pipelines is investigated both experimentally and using finite element analysis. In the experimental part of the research program eight full scale tests are carried out with vintage pipes having X52 steel grade and flaws of different length and height in the pipe wall. Each full scale test specimen is prepared in order to have a unique combination of the internal pressure and the flaw dimensions. The specimens are subjected to axial tensile force up to the point of rupture at the flaw location. The test results showed that, the tensile strain capacity of the pipes greatly vary not only due to the changes in the flaw height and length but also due to the level of the internal pressure. This condition shows that the effect of the internal pressure should not be ignored despite not being included in the CSA code for the oil and gas pipeline systems as one of the parameters affecting the tensile strain capacity. The second part of the research program is focused on the finite element analysis and classification of failure modes of cold bends under large tensile strains. In order to have a better understanding of the tension side fracture of cold bends under excessive tensile strain, the effects of the internal pressure level and the pipe steel grade on the structural response of cold bends are investigated. Parametric studies are carried out which span a range of internal pressure values causing hoop stresses between 20% and 80% of the pipe yield strength. The parametric study of the internal pressure is repeated for X60, X65, X70 and X80 steel grades. The results of these finite element simulations showed that, two different types of structural response can be observed depending on the level of the internal pressure and the steel grade. In order to develop a rigorous decision making procedure for the mode of the structural behaviour, a linear classification algorithm is utilized based on the results of the finite element simulations.

• Subjects / Keywords

  • Pipelines
  • Classification
  • Vintage pipes
  • Tensile strain capacity
  • Strains

• Graduation date

  Fall 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3FX7436R

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.