Assessment of Crack in Corrosion Defects in Natural Gas Transmission Pipelines
Hosseini, Seyed Aliakbar
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Pipelines are one of the safest forms of transportation for oil and gas. However, pipelines may experience some defects, such as cracks, corrosion and cracks in corrosion, during service period. In this thesis, the current defect assessment methods for crack, corrosion and crack in corrosion defects are reviewed. The aim of this study was to evaluate the effect of the crack in corrosion defects on the failure pressure of natural gas transmission pipelines. Consequently, a series of burst tests with varying defect depths were undertaken on end-capped, seam-welded API 5L Grade X60 (433 MPa yield stress) pipeline steel of external diameter 508 mm (20 inch), 5.7 mm wall thickness. Defects were created by pre-fatiguing the pipe to create a crack. The number of cycles required to create a fatigue crack were varied between 75000 to 150000 cycles based on the desired final defect depth. For the (CIC) defects, the pipe was pre-fatigued to create a sharp crack, and the artificial corrosion defect was simulated by machining a rectangular groove over the fatigue crack. The rupture tests were conducted by pressurizing the pipe until failure occurred. Results were analyzed using various assessment methods. For the artificial corrosion defects, the predicted failure pressures based on RSTRENG were more reliable than those based on Modified B31G. This study revealed that CorLAS provided the least conservative prediction for crack defects, whereas the other methods provided more conservative estimates of failure pressure. Moreover, the predicted failure pressure of the level 3 FAD for API 579 cylinder equations had better agreement with experimental results in comparison with the other methods, i.e. BS7910 and NG-18. The failure pressure for CIC defects for pipes tested fell between corrosion defects (lower bound) and crack defects (upper bound). The transition to crack defect behavior only occurs when the crack defect depth is significant or vice versa. It should be noted that the crack to corrosion ratio is not the only parameter to evaluate a CIC defect. There are other parameters such as total defect depth and defect profile, which affect the failure behavior of a CIC defect.