The Calibration Method to Improve the Data Quality of Defect in Term of Size Deviation on Natural Gas Pipelines Inspected by MFL Pigging Method
Authors: Khajornkai Kitisiri , Navasin Homhual , Piman Nenkaew 
Published: February 2023

Severe corrosion phenomena is easily occurred on offshore pipelines especially in splash zone section. This zone needs to be closely monitored and inspected by Magnetic Flux Leakage (MFL) pigging method. Nevertheless, the MFL pig can run faster than the required speed to obtain the best inspected data or the ordometers of MFL pig lift off from the pipeline wall thickness during running in the vertical position. If there are any defects which can be directly inspected by Non Destructive Test (NDT) method, this result can be used to calibrate the MFL signal to minimize the errors from the speed excursion.

In general, MFL pigging technology is usually used to inspect the metal loss of natural gas pipelines. There are many external corrosions which scatter throughout the pipeline surface in each piggable pipelines. Most of the severe external corrosions are the significant and large defects which directly affect to the pipeline remaining strength. These defects are generally located on the pipeline section above the water surface. Some of them can be also locates underneath the composite wrap that are quite difficult to be verified. However, there is some defects which can be directly inspected by pit depth gauge. This valuable defect sizes are used for calibration of the other defect sizes in some hard spots of inspections.

After receiving the final report of MFL in 2021, the position of the new growth of corrosion defects underneath the original composite wrap and the new external corrosion feature in the surrounding area are located. The current corrosion defect found in the 2021 MFL report are compared with defects found in year 2014, 2007, 2004 and 1992. For the maximum depth of corrosion defect in 2021 MFL report, the remaining strength of this defect based on ASME B31G in longitudinal defect and Kastner in circumferential axis are assessed. The assessment of maximum allowable operating pressure (MAOP) remained lower than the current maximum allowable operating pressure. This new MAOP was directly afflected to the gas deliverable to customers and require PTT to verify the defects by NDT methods and repair it with the recommended repairing method according to Pipeline Research Council International (PRCI). This repair could support to maintain the current MAOP without any reduction on MAOP.

In the same way, PTT measured the external corrosion in the exposed area above the original composite wrap and revert the defect dimension to ILI vendor in order to recalibrate its dimension. Then, the ILI vendor reassessed the defect dimension depended on the actual dimension from field measurement. Mostly, the new defect depths was smaller than the first issue of final report in 2021. The ILI vendor submitted the new version of final report.

For the reparing history at riser section, this riser section was revamped by pipe wrap in 2003 because it contained localized corrosion with the depth of 3-5 mm, the width of 400 mm and the length of 200 mm. After composite wrap installation, the MFL inspection in 2004 and 2008 shown the large number of external corrosions between the former coating of splash zone and composite wrap. This section was replaced and repaired by the new composite wrap in 2021