Monitoring Top of Line Corrosion with Eddy Current Technology Combined with Magnetic Flux Leakage Method

Corrosive processes at the inside of pipe lines can be a major risk factor for pipeline operations. To facilitate the monitoring of Shallow Internal Corrosion, SIC, an In-Line Inspection (ILI) Tool that is based on Eddy Current (EC) technology has been developed.

Several cases exist were the prevalent growth of internal metal loss corrosion is the major risk for pipeline operation. One important example is Top of the Line Corrosion (TLC) in wet gas lines due to condensation. Heavy wall lines, which are common in off-shore applications, are often used to allow a specified maximum  corrosion growth rate over the life time of the structure. A good approximation even of marginal corrosion growth rates to monitor the degradation process is thereby required. These estimations can be subsequently used to predict failure pressures and life times in the context of limit state design guidelines.

The presented EC technique gives a depth measurement of metal loss defects in terms of absolute figures that complements relative wall loss measurements of Magnetic Flux Leakage (MFL) tools. By inspection of heavy wall lines in offshore applications SIC tool measurement results can outperform MFL depth sizing of shallow internal corrosion, or likewise assist MFL defect identification or depth sizing e.g. by increased distinction of individual pits in dense clusters due to its high lateral resolution of defect surface measurement.

Following the combined inspection approach, both measurement methods complement one another and show synergetic effects, which can be used to increase the overall sizing performance for corrosion growth monitoring.

Results of ILI tool pull tests that were performed in lines with TLC incidents demonstrate the value of combined action and evaluation of MFL and SIC techniques.