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Fitness-for-service project on Middle Eastern pipeline

Internal blistering was discovered in one of Kuwait Oil Company's 20" diameter cross-country pipelines. A fitness for service assessment based on the damage present was essential and the client needed advice on future pipeline operation and integrity management.

KOC despatched a 200mm long specimen of the longitudinally welded pipe to TWI for analysis, along with an un-damaged sample of spirally welded pipe from the same pipeline for comparison purposes.

TWI's approach involved several of its in-house disciplines. Both destructive and non destructive tests were going to be required to confirm the material's properties, identify the damage mechanisms and to characterise the extent of the damage. From these tests, and the results of in-line inspection (pigging) carried out by KOC, the team would be able to draw conclusions about the fitness for purpose of the pipeline as a whole and make recommendations for future operation, inspection and integrity management.

The work began with a detailed review of the pipeline operating history, and the available in-line inspection data. This was followed by non destructive tests on the supplied materials using ultrasonic testing and magnetic particle inspection. Lastly destructive testing including tensile, hardness, and toughness tests, together with chemical analysis and metallography, were carried out in order to characterise the materials and the damage.

The operating history and in-line inspection reports revealed that the line had suffered a failure in 1976, with 'laminations and blistering' reported at the failure site. The pipeline had also been briefly co-opted to transport sea water for fire-fighting in 1991, following the Gulf War, before being returned to high pressure gas transport service. Other than this brief service with sea water, the known history of the pipeline only involved the transport of clean, dry gas.

An inspection report was available from 1981, recommending approximately 6km of repairs, although there were no records indicating whether this report had been actioned. The in-line inspection showed that although the line was predominantly long-seam welded, there was approximately 7km of spiral welded pipe, predominantly in 12m lengths, distributed throughout the pipeline. Internal corrosion damage was almost completely absent from the spiral welded pipe, but was widespread in the long-seam welded pipe.

Fig. 1. Magnetic particle inspection of the pipeline sample examined by TWI, showing internal blistering (A) and hydrogen induced cracking (B)
Fig. 1. Magnetic particle inspection of the pipeline sample examined by TWI, showing internal blistering (A) and hydrogen induced cracking (B)

Based on the in-line inspection results, and the tests on the supplied pipe, TWI reached a number of conclusions about the origin, mechanism and fitness for service of the pipeline:

  • The sample of long-seam welded pipe examined by TWI contained significant hydrogen induced cracking (HIC) and blistering, as well as evidence of internal corrosion.
  • However the sample of spiral welded pipe contained no cracking or internal corrosion, and had a microstructure which is much less susceptible to HIC than the long-seam welded pipe.
  • The internal corrosion mechanism could only be related to the transport of an aggressive product. Therefore, the absence of internal corrosion in the spiral welded pipe indicated that it was probably installed as part of a 1981repair programme.
  • The internal blistering observed in the long-seam welded sample originated before the 1976 failure, as a result of the transport of a sour product. This is likely to be the same product that is responsible for the internal corrosion. The blistering is therefore unlikely to have been actively growing by a hydrogen charging mechanism since before the 1981 repairs.
  • The transport of sea water in 1991 had little, if any, effect on the condition of the pipeline.
  • The observed cracking in the pipe sample inspected by TWI had a severity which dictated that it could not be accepted as fit-for-service according to API 579-1/ASME FFS-1, even though it had almost certainly been operating for more than twenty years without incident.

TWI recommended to KOC that the FFS methodology was not appropriate for re-validating the 40km pipeline, due to the severity of the blistering in the tested sample, and the lack of knowledge about the extent and severity of damage on the remainder of the pipeline. However, KOC advised that the future operating pressure of the pipeline was very low, and the required revalidation period was only 5 years. On this basis, TWI was able to recommend an alternative revalidation methodology, based on a hydrostatic proof test as described in ASME B31.8S.

KOC implemented TWI's advice, and successfully completed a hydrostatic test at 1.7 times the operating pressure, prior to recommissioning the pipeline. TWI's expertise in inspection, testing, fitness for service assessment, and pipeline integrity management were combined to enable KOC to make significant savings on inspection and repair works, while demonstrating the safety and integrity of the pipeline.

KOC implemented TWI's advice, and successfully completed a hydrostatic test at 1.7 times the operating pressure, prior to recommissioning the pipeline. TWI's expertise in inspection, testing, fitness for service assessment, and pipeline integrity management were combined to enable KOC to make significant savings on inspection and repair works, while demonstrating the safety and integrity of the pipeline.

For further information read about TWI services to the Oil and Gas industry or email contactus@twi.co.uk.

Fig. 2. Hydrogen induced cracking viewed under optical microscopy. The image shows a microstructure containing a significant number of laminar inclusions which act as nucleation sites for HIC. Distortion of the microstructure caused by the cracking can be seen, along with step-wise crack growth, which is characteristic of HIC
Fig. 2. Hydrogen induced cracking viewed under optical microscopy. The image shows a microstructure containing a significant number of laminar inclusions which act as nucleation sites for HIC. Distortion of the microstructure caused by the cracking can be seen, along with step-wise crack growth, which is characteristic of HIC

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contactus@twi.co.uk