TWI Bulletin, November - December 2010
Case Study: Fitness for service assessment of pipelines
When internal blistering and multiple cracks were discovered in one of Kuwait Oil Company's 20" diameter pipelines, they sought expert help from TWI.
A fitness for service assessment based on the damage present was essential and the client also wanted advice on future pipeline integrity management. KOC despatched a 200mm long specimen of the longitudinally welded pipe to TWI for analysis, along with an un-damaged sample of replacement spirally welded pipe for comparison purposes.
TWI's approach involved several of its in-house disciplines. Both destructive and non destructive tests were required to confirm the material’s properties, identify the damage mechanisms and to characterise the extent of the damage. From this the team would be able to draw general conclusions about the fitness for purpose of the pipeline as a whole and make recommendations for future operation, inspection and integrity management.
The action plan began with non destructive tests on the supplied materials firstly using ultrasonic testing with both a zero degree and an angled probe.TWI then performed magnetic particle inspection, metallography and chemical analysis.
Lastly destructive testing and Vickers hardness measurements were carried out on tensile specimens and Charpy toughness specimens machined from the supplied material.
Based on the results of its inspection and tests on the supplied pipe TWI reached a number of conclusions about the origin, mechanism and fitness for service of the observed cracking:
The sample of long-seam welded pipe examined by TWI contained significant hydrogen induced cracking 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 sample of spirally welded pipe was almost certainly installed in response to the repair recommendations contained in a 1981 in-line inspection report. The cracking observed in the long-seam welded sample originated before 1976, and is unlikely to have been actively growing by a hydrogen charging mechanism since before 1981. The inspection identified only one defect in the location of the sample that was examined by TWI, however its investigation revealed ten individual blisters, and a significant area of hydrogen induced cracking. So the results of the inspection cannot be relied upon to quantify the extent or distribution of HIC damage in the pipeline.
The ultrasonic in-line inspection did not provide sufficiently detailed information to enable a fitness for service assessment to be completed on any identified HIC features. 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, unless all areas of hydrogen induced cracking were identified and repaired or replaced. The pipeline was able to be re-validated without the repair of all existing hydrogen induced cracking based on a hydrostatic proof test method, as described in ASME B31.8S.
Finally TWI was able to advise its client that burst testing was recommended to provide an estimate of the safe working and test pressures.
