First Top of Line Corrosion (TLC) case was reported in 1999 in Tunu field, and since then, there were several other pipelines from the neighboring fields, installed from 2007 to 2015 that were experiencing TLC and was confirmed using in-line inspections (ILI). The TLC rates were found to be 0.5-1 mm/year in average with the maximum rate of 2-3 mm/yr for gas containing CO2 up to 8.5% and around 80-90 barg operating pressure with 80-90°C inlet temperature. Interestingly, on several pipelines, the maximum corrosion rates happened at about 2-5 kilometers downstream the inlet risers. As part of mitigation, some of the pipelines were designed with 10 mm corrosion allowance and when required, spray pigging and recently, volatile corrosion inhibitor, are employed during operation at some of the pipelines. Thanks to proper mitigations, despite high TLC rates, no adverse consequences were experienced on the pipelines. This paper will discuss on the correlation on the field data and inspection results of the pipelines, compares the predicted TLC rate with reality, and discusses the efficiency of various mitigation methods. Lesson learned are also provided to illustrate the evolution of TLC mitigation strategies to ensure the integrity of the pipelines.
The first case of top of line corrosion (TLC) in Tunu gas field was reported by Gunaltun et al in 19991. In-line inspection (ILI) of two carbon steel pipelines distributing multiphase effluent, showed up to 50% metal loss at 11.00 - 01.00 o’clock at several sections. Visual examination of a cut section showed that the top of line was covered with iron carbonate layer with deep pits, and severe metal loss occurred on large surfaces at these area. No corrosion was visible at the bottom side of the examined section. The pipes were buried under river delta bed except at the sections where TLC happened (dogleg and upheaval buckling). In this section, cooling effect increases the water condensation rate beyond internal company limiting threshold, and it was confirmed from ILI result a significant corrosion rate on this section.
