Inspectioneering Journal

Pioneering Approaches for Integrity Assessment of Multiphase - Difficult to Pig - Subsea Pipelines

By Vibha Zaman, P.E., Senior Consultant at Asset Optimization Consultants, and Peter Dsouza, Corrosion and Materials Engineer at LyondellBasell. This article appears in the May/June 2015 issue of Inspectioneering Journal.


Under growing public scrutiny, pipeline operators need to meet strict legislative and environmental requirements. They must demonstrate and document the safe performance of their pipeline assets by assessing risk factors, performing inspections, and implementing mitigation measures. According to the Pipeline and Hazardous Materials Safety Administration (PHMSA), it is estimated that 40% of the world's pipelines are difficult to pig or deemed "unpiggable," which means they cannot be inspected via inline inspection (ILI). Assessing the condition of these pipelines poses a special challenge for operators that will require a proactive approach to rethinking what safety means for business. Current U.S. Pipeline Safety Regulations require integrity assessments to be performed for certain onshore pipelines using any of the following methods:

  1. Inline Inspection

  2. Pressure Testing

  3. Direct Assessment

  4. Other technology that an operator demonstrates can provide an equivalent understanding of the condition of the pipe.

Regulations for offshore pipelines are somewhat less prescriptive and no requirements for integrity management currently exist. However, ensuring the integrity of offshore pipeline assets is no less important than ensuring the integrity of onshore pipeline assets. Since the implementation of the onshore pipeline integrity management regulations starting in 2002, new tools and technologies have rapidly been developed to make traditionally "unpiggable" pipelines piggable through the use of tethered pigs, robotic pigs, or specially designed pigs that can maneuver the tight bends which older generation pigs could not navigate. Despite these advances in technology, a large percentage of pipelines remain difficult to pig.

Carrying out pressure tests to verify pipeline integrity has its own disadvantages. Specifically, they do not identify the severity of flaws (other than critical flaws that result in failure) and they can cause existing flaws to increase in size. Additionally, pressure tests require pipelines to be taken out of service.

Therefore, direct assessment is often seen as the best option to verify pipeline integrity. Direct assessment of subsea pipelines is especially challenging due to the difficulty of pipeline access, as well as the limitations in available technology to perform subsea wall thickness inspections. Given that the US has over 34,000 miles of subsea pipelines, the longest offshore pipeline infrastructure in the world, with 10,377 miles of pipeline length over 30 years old and another 3,380 miles 20-30 years old, addressing these challenges requires action rather than reaction.

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