Top of Line Corrosion in Multiphase Gas Line Operational Feedbacks

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The first case of top of line corrosion (TLC) in Tunu gas field was reported by Gunaltun et al in 19991. Inline
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.

Product Number: 51323-19486-SG

Author: Suryani, Rudi Rinaldi, Albertino Prabowo, Farih Mitraningsih, Aji Samiaji

Publication Date: 2023

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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.