ASSESSMENT OF CRACK IN CORROSION DEFECTS IN NATURAL GAS TRANSMISSION PIPELINES

Crack-like defects may occur coincident with corrosion defects and represent a new hybrid form of defect in gas and oil pipelines that is not directly addressed in the current codes or methods of assessment. There is a need to provide assessment and evaluate the integrity of the line as well as identify requirements for defect repair or line hydrotest. A numerical investigation was undertaken to evaluate the predicted collapse pressure of crack in corrosion (CIC) defects in typical fine pipe. Longitudinally oriented CIC defects were evaluated as long cracks occurring within long, corrosion grooves of uniform depth. This was a conservative representation of a finite length CIC defect. It was found that the collapse pressure for CIC defects varied between that of a long uniform depth crack and a long uniform depth corrosion defect. The transition to corrosion defect behaviour only occurred when the corrosion defect depth was significant (greater than 75% of the total defect depth). Finite-length CIC defects were then investigated using a numerical investigation to identify the effect of crack and corrosion length. The collapse pressure of a finite length crack within an infinitely long corrosion defect was found to be lower than a crack of equivalent total depth and length. This reduction in collapse pressure was attributed to increased local stresses in the vicinity of the crack due to the coincident corrosion. The predicted collapse pressure increased towards the crack-only value when the length of the corrosion defect was decreased to that of the crack. CIC defects were evaluated as cracks using the NG-18 approach and BS 7910 code (Level 2A FAD). The NG-18 approach conservatively predicted lower collapse pressures than the FE analysis, whereas the FAD approach was conservative for shallow defects and could be non-conservative for deeper defects. These results are attributed to the presence of the corrosion and the fact that no factor of safety was included in the analysis. Future studies will investigate experimental validation of the FE and FAD methods for this type of defect.