Abstract
In case severe corrosion is detected in a pipeline, repair or replacement can be very costly especially for offshore pipelines. The decision for the repair or replacement should be taken based on the accurate and validated inspection data. The fit for purpose analysis of oil and gas transport pipelines are based on the results of intelligent pigging. There are two types of pigs on the market used for this purpose: magnetic flux leakage (MFL) pigs and ultrasonic (UT) pigs. For the gas pipelines, MFL pigs are used as they do not require the presence of a liquid phase. However, visual inspections of some pipe sections done after some repairs carried out on the pipelines operated by the authors company, showed that MFL tools exaggerate significantly localised thickness losses for the wet gas lines subject to top of line corrosion (TLC). Several studies were initiated to compare the MFL, UT and visual &direct NDT inspection results. The study confirmed that MFL may exaggerate significantly (up 100 %) the thickness losses. Consequently the decision to repair or replace a line corroded by TLC can not be taken based on only MFL or UT results. It is always necessary to cross-check the inspection results by another technique. The paper presents different pipelines inspection cases, details of studies and weak and strong points of different pipeline inspection tools.
