Experience with Dry Gas Internal Corrosion Direct Assessment Methodology for a Gas Pipeline Segment Located in Southern China

Dry gas (DG) internal corrosion direct assessment (ICDA) methodology was applied to a gas pipeline segment located between Zhong County and Enshi, a most west portion of the Zhong-Wu transmission pipeline in Southern China.This line was constructed in 2003 and put in operation in 2004.  Internal corrosion was suspected because of possible water intrusion into the line during upsets and due to water condensation.

ICDA was used because both the gas flow and pressure were low, which made the line unpiggable.  ICDA is a four-step process based on the principle that for dry gas pipelines liquid collects at local low points in the pipeline, and that it is at these locations that internal corrosion is most likely to occur. The four-step process described in the NACE DG-ICDA Standard SP 0206-2006 was implemented, while improvement was made in several areas over the standard requirements. ICDA was determined feasible after a thorough review of the historical data of the line. In determining critical angle, we first processed the daily operating data for the past two years including temperature, pressure, flow and water content. We then performed a calculation of daily critical angles before the overall critical angle for the pipeline segment was evaluated and determined.  In determining inclination angles, we not only performed calculations using data of both pipe elevation and depth-of-cover, but also we conducted a field visit to verify or revise the inclination angles. In the Direct Examination Step, GWUT was employed. The result showed that the improved method was effect and high efficiency.This experience can advance the validation and application on the implement of ICDA method for gas transmission pipeline systems.