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Failure along spiral welds in a gas pipeline

TWI has worked with the Kuwait Oil Corporation (KOC) to find the root cause of failure in a critical high-pressure wet sour service gas pipeline in West Kuwait.

The pipeline had suffered several leaks, with the most recent due to a crack along a spiral weld. During repairs the pipeline was found to have severe internal corrosion, which led to the pipeline being decommissioned. After inconclusive in-house investigation and testing, KOC contacted TWI to conduct a failure investigation, which included extensive examination and testing of the failed pipe.

TWI experts noted that the pipe material had been correctly selected for wet sour service and externally the pipeline was in good condition, indicating an effective coating and cathodic protection system during service.

However, visual inspection of pipe samples showed severe internal pitting in the bottom of the pipe in the 4-8 o’clock position, with a spiral weld failure in the same area. Phased array UT confirmed that the crack was longer on the inside than on the outside, revealing that the cracking originated internally.

TWI found that the pipeline had suffered from extensive internal corrosion caused by the presence of water, CO2 and H2S gases, suggesting that the dehydration plant, which was the main source of internal corrosion mitigation in this pipeline, was not functioning adequately. 

The team also found that a small weld misalignment had resulted in a slight roof-topping of the spiral weld. This will have increased in-service stresses in these regions, and a misalignment of the spiral weld would cause higher local tensile stress when under pressure. In addition, regular pipeline shutdowns would have added a small amount of cyclic loading.

Following the investigation, TWI concluded that none of these factors was particularly severe on its own, however, when combined, the effect could be significant. The probable cause of cracking was a combination of increased localised stress at the spiral weld, in combination with corrosion due to a severe sour service environment. Eventually the local stress increased to such an extent at the spiral weld that cracking occurred.

Following the results of this investigation KOC replaced the pipeline with a seamless sour service pipe rated to the current level of H2S and water cut.

For further information, please email contactus@twi.co.uk. 

Avatar Chi-Ming Lee Senior Corrosion Engineer

Dr Chi-Ming Lee is a Senior Corrosion Engineer, working in the Material Performance and Corrosion Department at TWI Ltd, based in Cambridge, UK. His responsibilities include management of consultancy related projects, from a corrosion and asset integrity prospective, for a range of clients from the oil and gas industry and other sectors.

Prior to joining TWI Ltd, Dr Chi-Ming Lee worked in the field of corrosion and pipeline cathodic protection at several companies, including Clariant, PIM and Cathodic Protection Service Ltd. Notable is his 3 year period at Clariant, where his role was a member of the corrosion awareness campaign team within BP, tasked to reduce the cost of corrosion, across all business units.

Dr Chi-Ming Lee received a degree in Metallurgy and Material Science, and PhD on the topic of pitting corrosion from University of Nottingham, UK. He is also a Chartered Engineer and a Member of The Institute of Materials, Minerals and Mining (IOM3).

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