## 4 Extensive Parametric Studies Using the Developed Empirical Equation  
Using the developed empirical equations, a parametric study was carried out to analyse the behaviour of API 5L X52 and X65 pipe grades for different combinations of geometric parameters. Generally, for API 5L X52 and X65 pipes, it was found that the normalised failure pressures overlap almost completely. However, the normalised failure pressures for X80 pipes are comparatively higher than that of the other two materials, as illustrated in Figures 8-11. Figure 8: Failure pressure predictions for API 5L X52, X65, and X80 pipe for multiple normalised effective defect lengths. Figure 9: Failure pressure predictions for API 5L X52, X65, and X80 pipe for multiple normalised defect spacings. For the effects of normalised effective defect lengths and defect spacing, it was found that as the normalised effective defect length increases from 0.00 to 1.80 for a defect spacing of 0.50, normalised effective defect depth of 0.4, and a normalised axial compressive stress of 0.30, the failure pressure decreases gradually up to a maximum of 28.17% and begins to plateau beyond a normalised effective defect length of 1.80, as illustrated in Figure 8. This trend is observed for all other conditions as well. Based on Figure 9, as the normalised defect spacing increases from 0.00 to 3.00 for a normalised effective defect length of 1.20, a normalised effective defect depth of 0.4, and a normalised axial compressive stress of 0.30, the failure pressure also increases (a maximum of 0.95%) due to the decrease in stress-strain overlapping regions. These results are consistent with the former literature [18; 32; 43]. Under all other geometric conditions, for a change in the parameters, the normalised failure pressures of API 5L X52 and X65 pipes are almost similar with a maximum percentage difference of 1.26%. However, the normalised failure pressure for high toughness pipe material is relatively larger, with a maximum percentage difference of 9.48%. As for the effects of defect depth, as the defect depth increases from 0.10 to 0.80, the failure pressure of the corroded pipe drops drastically with a maximum decrease of 71.88% for a normalised defect spacing of 0.50, a normalised effective defect length of 1.2, and a normalised axial compressive stress of 0.30 for a high toughness pipe (API 5L X80). It was observed that, beyond a normalised effective defect depth of 0.60, the normalised failure pressure of the three pipe grades begins to converge and overlap, as illustrated in Figure 10. Under all other conditions, the normalised failure pressures of the three materials exhibit the same failure pressure trend. Figure 11: Failure pressure predictions for API 5L X52, X65, and X80 pipes for multiple normalised axial compressive stress. Figure 10: Failure pressure predictions for API 5L X52, X65, and X80 pipes for multiple normalised effective defect depths. It was observed that when a normalised axial compressive stress of larger than 0.60 was imposed on a corroded pipe, the pipe buckles. This is also the case for pipes subjected to an axial compressive stress of 0.60, with normalised effective defect depths of greater than 0.60. The normalised failure pressures of the pipe remain constant for a normalised axial compressive stress values of 0.00 to 0.40. Beyond that, the normalised failure pressure begins to reduce gradually with a maximum pressure drop of 31.57% observed for the API 5L X52 pipe of 0.50 normalised defect spacing, 1.20 normalised effective defect length, and 0.40 normalised defect depth, as illustrated in Figure 11.  
In this study, the developed equation is accurate for API 5L X52, X65, and X80 pipes. However, the accuracy of the equation drops significantly (up to 30.00%) when applied to pipe grades that fall between the ranges of these materials, as the interval between the true ultimate tensile strength of high toughness and medium toughness pipe grades used in this study is large.