Following a crack detection ILI campaign, the next natural step is to perform an integrity assessment or a Fitness for Service assessment.
Understanding what the ILI data is telling you can be a daunting task. All crack-detection technologies report ‘crack-like’ calls; some, none or all of which may represent integrity threats. ILI crack detection technology performance is also highly sensitive to pipeline construction, the inspection conditions and the target cracking morphology.
These combined uncertainties, if not addressed, are transferred into the integrity assessment, subsequently resulting in uncertainty and therefore risk in the integrity management response. An over-conservative response remediating unnecessary features can be extremely costly to a Pipeline Operator; whilst an under-conservative response can lead to a more dangerous scenario – an under-estimation of the integrity threat.
Central to the current industry best-practice for crack management, API RP 1176, overlaying reported features with all available threat and tool performance knowledge is therefore critical to optimise the credibility of any subsequent integrity assessment.
Let’s look at an example:
A pipe body crack-like anomaly has been reported by an EMAT crack detection tool. The anomaly has been reported to be 2.5 mm deep with a length of 75 mm. Because SCC had previously been validated on the line, the anomaly was assumed to be SCC in the integrity assessment. No field verifications had been undertaken to validate this ILI signal or other similar ILI signal types and once assessed, the model predicted a burst pressure which is 25 bar lower than the current operating pressure. Yikes!!
The questions which will follow are likely to be:
- Do we need to shut down the pipeline immediately?
- How come the pipeline has not failed already?
- What level of conservatism is embedded in the assessment?
- Do we have a detailed understanding of the mechanism and are we modelling this correctly?
Upon review of the initial assessment and follow up verification dig, the following observations were made:
- The pipe material toughness was conservatively low in the absence of any material testing. However, recent tests have shown it to be significantly higher.
- The loads assumed in the fracture assessment assumed original MAOP. The pipeline was de-rated to a lower pressure which had not been considered in the current assessment.
- The crack assessment model employed in the assessment was not calibrated against a recent burst test and other models were considered more accurate.
- The integrity engineer assumed the flaw was SCC even though the anomaly was located in an area of the system where axial SCC susceptibility was identified as being ‘very low’ due to a high-performance coating system.
- The ILI signal data for this anomaly provided evidence that this was not consistent with other sites where SCC was confirmed but no evaluation of the likelihood of the ILI signal being SCC had been undertaken by the integrity engineer or the ILI vendor.
- The dig verification engineer confirmed the flaw was not SCC but a shallow gouge with dimensions less than those reported by the ILI.
The above example provides an overview of how layers of conservatism can create an unmanageable situation which, in a worst-case situation, can require hundreds of anomalies appearing to require an ‘immediate’ response. Clearly, more than just the ILI data is required to perform a credible crack assessment.
Frontline Integrity specialise in supporting Pipeline Operators to answer the following questions to reduce uncertainty and increase confidence in integrity assessment activities:
- Which of the ‘crack-like’ calls actually represent integrity threats that could fail the pipeline?
- How fast are these features growing?
- How and where has the ILI tool performed with respect to the integrity threat – could features be missing, incorrectly classified or sized inaccurately?
- Is the understanding of the pipeline materials and stress state at the anomaly locations sufficient to allow credible assessment of the failure pressures and failure mode of any cracks (leak vs rupture)?
In addition to the above, development of repair schedules and re-inspection intervals also require consideration of:
- A robust understanding of consequence of failure along the line.
- Regulatory requirements which may dictate maximum allowable intervals where a time-dependent cracking threat such as SCC has been validated (e.g. 49 CFR Parts 191 and 192 in the USA)
- Budgetary availability (e.g. how many repairs per year can be funded?)
- Operational windows (pressure reductions for repairs)
- Tool availability.
Once an ILI campaign is complete, it is important to review all aspects of the work and develop an overall management strategy for a system. For cracking threats, this is unlikely to be a standard ‘pig and dig’ approach, due to the number of areas of uncertainty that will inevitably exist surrounding the threat. Refining threat knowledge, material property data and ILI performance validation (in line with industry standard API 1163 for ILI performance qualification) should all be part of a continuous improvement cycle and is central to the current industry best practice for crack management, API RP 1176. Activities could include: destructive laboratory validation of cut-out features to identify evidence of growth; above ground surveys to identify cathodic protection issues or coating degradation to refine susceptibility models; fracture toughness testing; burst testing or even a pull test to improve confidence in ILI tool performance against the target threat.
How can we help Operators and what value do the Frontline Integrity team add?
Services Offered
Added Value
Collating all integrity threat knowledge to support a balanced ILI response in line with API 1176.
Confidence in assessment results and the resulting actions taken; avoiding unnecessary alarm.
Development or audit of a long term integrity management strategy to protect against failure and progressively reduce uncertainty and increase confidence surrounding the management of the threat.
Visibility of required work over the coming years to continuously improve threat knowledge and reduce uncertainty (e.g. field verifications).
Cost saving from long-term planning.
Independent audit of defect assessment and remaining life calculations for all anomaly types.
Increased confidence in using results to make integrity management decisions as well as identification of areas of uncertainty to feed into pipeline risk assessment.
Integration of management plans into annual risk assessment and RBI’s.
Embeds mitigations into comprehensive risk-management as part of PIMS to enable quantification and demonstration of their overall impact on pipeline risk.
Selection of validation techniques and expert review of validation results to feedback into interpretation of ILI data.
Understanding the reliability of field verification results to support the appraisal of ILI tool performance. Optimisation of techniques to appraise reliability.
Provision of tailored software solutions to support and trend field verification information to assist with the interpretation of integrity threats and ILI tool performance.
Consolidation of large datasets to expedite integrity management decision making.
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