GE Oil & Gas - PII Pipeline Solutions SPRING 2008

Crack Ranking

A new standard for ILI crack inspection
Our UltraScan™ CD in-line inspection (ILI) technology has logged more than 14 years of successful SCC and seam-weld crack inspections. During that time, pipeline operators have saved tens of millions of dollars by avoiding unnecessary line replacements and costly hydrostatic tests that provide only limited information on the future integrity of the pipeline.

Due to the unique morphology of cracks in steel pipelines, significant post-inspection work is often required in order to prioritize critical flaws by level of severity. This work may involve substantial excavations and NDE evaluations in order to decrease the conservatism built into ILI crack sizing. This process helps to ensure that the most severe cracks are given first priority.

In order to help operators with this important and costly challenge, GE Oil & Gas - PII Pipeline Solutions now includes our Crack Ranking service as part of our standard offering for all new crack inspections.

What is Crack Ranking?
PII's process provides operators with better resolution of individual defects, much like RSTRENG (LAPA) does for quantifying corrosion wall loss based on our MFL and ultrasonic measurement surveys. Our ILI crack inspection tools have historically identified crack length and the deepest point in an individual crack. By using depth profiling, we are able to assess the severity of a crack using a higher resolution crack profile, as shown in Figure A.

For SCC fields, individual cracks within the field are identified and isolated. An 'equivalent crack' is then derived using CEPA inter-linking rules to represent the field, as shown in Figure B. Once the depth profile for individual cracks and the 'equivalent crack' for SCC is complete, the crack severity is assessed using Linear Elastic Fracture Mechanics (LEFM).

PII's integrity engineers prefer the API 579 assessment methodology for reasons defined in the PICA article.

Once the cracks are assessed using Linear Elastic Fracture Mechanics, the crack is then ranked relative to the other cracks on the pipeline segment and is given an Estimated Severity Factor (ESF). The cracks and corresponding ESF rankings are then provided to the operator in the final report. The end result is a crack severity prioritization list that provides the operator with a huge head start in establishing a sound dig validation and repair program.

Finally, this new enhancement offers significant value to any crack assessment (see PICA article) performed on the pipeline inspection data. The accuracy and optimization of any crack assessment exercise is heavily dependent on the accuracy of the inspection data and subsequent NDE validation data. The new Crack Ranking service included in our crack inspection is another tool to ensure the most accurate data for your crack assessment algorithms.


UltraScan™ CD Inspection followed by Engineering Crack Assessment

The Moomba-to-Sydney Gas Pipeline, operated by Agility on behalf of its owner, The Australia Pipeline Trust (APA), transports natural gas from central Australia to Sydney on the east coast – more than 1,300 km.

The pipeline was commissioned in 1976, and a failure occurred in 1982 due to SCC cracking. The pipeline’s maximum allowable operating pressure (MAOP) was down-rated to 6 MPa from a design MAOP of 6.895MPa, and a length of approximately 30 km was replaced. The operating strategy was changed to mitigate the probability of SCC occurring or developing.

In 2004, during excavations following a GE Oil & Gas – PII Pipeline Solutions metal loss survey, further significant SCC colonies were discovered. The MAOP was further down-rated to 5.5 MPa – a significant reduction in throughput for such an important pipeline.

In 2004-05, Agility contracted with PII to perform an UltraScan™ CD survey to investigate the prevalence of SCC in the first section of the pipeline. Following a successful survey, a number of excavations confirmed the tool findings. Defect locations, lengths and depths had all been predicted accurately.

In addition to investigating a significant repair program based on the UltraScan™ CD results, Agility also contracted with PII’s Integrity Services group to perform detailed engineering critical assessments of the crack colonies. Particular attention was given to their immediate significance to the integrity of the pipeline; and to analyzing potential growth mechanisms to predict the number of significant features arising as a function of time. In addition, material testing was carried out on samples of the pipe.

As a result of this work, Agility was able to develop a strategy for the safe future operation of the pipeline with the aim of reinstating the original MAOP while assuring the integrity of the line.

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