Multiphase wet gas flowlines of Tunu field cross the Mahakam River at several locations. Even though the corrosion monitoring systems have shown negligible internal corrosion rates, inspection by intelligent pigging, after six years of service, of two of these flowlines indicated severe internal top of line corrosion (TLC). Corrosion occurred in three locations in one line and at two locations in the other one. The lengths of the corroded pipe sections vary between 10 and 100meters. Further investigations showed that corrosion took place at locations where the pipes were in direct contact with river water. These were unburied pipe sections (doglegs or pipe sections where upheaval buckling occurred) being subject to heavy cooling and consequently heavy water condensation. A pipe portion was removed. Visual inspection and laboratory examination confirmed the proposed explanation. This paper describes in details the field operating conditions, presents the results of inspection and laboratory studies and describes remedial actions undertaken to solve the problem.
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