The failure of a pipeline used to transport wet C02 containing gas has highlighted the limitations that both intelligent surveys and ultrasonic testing (UT) inspection can have for the detection of internal grooving type corrosion. These limitations are presented, together with the inspection, testing and assessment programme that was subsequently introduced to evaluate the condition of other associated production facilities operating under similar conditions. Measures adopted to prevent the reoccurrence of such C02 induced corrosion damage are discussed. In addition, the use of a corrosion rate prediction tool enabled the risk ranking of the facilities under threat of C02 induced corrosion to be rapidly undertaken and therefore enable an inspection and assessment priority ranking to be made. A comparison is therefore made of the predicted levels of corrosion with those actually observed in practice.
On 22 March 94 a rupture occurred on 25.4 cm (inch) diameter buried pipeline used to transport approximately 1.5 MMsm3/d of wet gas (containing 0.8 mol% C02) from Yibal A station to Yibal D station. The pipeline, which had been installed in 1988, possessed a nominal wall thickness of 4.78 mm and was operating well within its agreed operating envelope (maximum allowable operating pressure of 9100 kPa) at a pressure of 5700 kPa. The failure was located 20 metres downstream of the pig launcher in the first joint after the bend entering the buried section. The force of the rupture was sufficient to propel a 2 metre section of the pipeline some 70 metres away and to cause significant damage to the pig launcher pipework. Figure 1 provides a schematic illustration of the location of the failure. Inspection of the failed section of the line revealed that the failure was caused by bottom of line C02 corrosion. It appeared that corrosion damage had initiated at a number of discreet locations in the 6 o?clock position which progressively developed into a series of channels or grooves running parallel with the length of the line for a significant distance. The channels, for the majority of their length, appeared to penetrate through approximately 80% of the full wall thickness (between 0.8 and 1 mm of remaining wall thickness was measured near the rupture initiation site). A ductile crack initiated at the centre of the failed section and propagated along the corrosion groove for a distance of approximately 1 metre in either direction. No other influencing factors were found (operating conditions, impact damage, mechanical defect, location of weld seam).
A prior intelligent survey conducted in October 1993 indicated the presence of some anomalies on the log which were interpreted in the flawlist as internal isolated pits (max. 40% wall loss) which did not interact nor affect the maximum allowable operating pressure of the line. The extent of corrosion, based on the intelligent survey data, was most severe near the pig launcher (where rupture subsequently occurred), while other affected sections were randomly distributed along the length of the pipeline. Based on the log, it was concluded that only 9 areas were affected, consisting of 1 to 2 joints in each case.
In January 1994 dig-ups were undertaken at the areas coinciding with the intelligent survey reported anomalies and subsequent manual ultrasonic testing (UT) inspection using a D-meter type thickness probe in these locations appeared to confirm the flawlist (i.e. 40wall loss). Both the intelligent survey and UT inspection techniques, however, were calibrated for pitting defects and not for grooves such as led to the failure. Consequently, the full severity of the defect was not acknowledged at the time. Ironically, the U
