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Inspection planning and evaluation for unpiggable pipelines

The use of in-line intelligent pigs is usually the preferred method of inspection to determine the condition of subsea pipelines. A large proportion of installed pipelines are, however, such that pigging is not possible. Alternative means of inspection are therefore necessary for these lines. Identifying suitable inspection techniques and strategies is becoming an increasing concern as subsea pipelines age and, in many cases, are in operation beyond their originally anticipated design lifetimes.

Inspecting unpiggable lines relies on techniques that are externally applied. A range of externally applied techniques are available for subsea pipeline inspection. These include ultrasonic corrosion mapping, long range ultrasonic testing (LRUT), guided waves introduced by electromagnetic acoustical transducers (EMATs) and eddy current methods, e.g. SEF.

Given the typical lengths of subsea pipelines it is generally only possible to achieve very low levels of coverage in an externally applied inspection, even when using screening type methods such as LRUT. Coverages achieved will typically be less than 1% of the total line length and, in many instances, substantially less than this. This is in contrast to inspection using intelligent pigs where close to 100% coverage is usually achieved. Strategies for inspection of unpiggable lines subsea therefore need to very carefully consider the performance of the inspection tools.

Sonomatic assists clients with planning and evaluation of unpiggable pipeline inspections, providing the following services.

  • Bayesian statistical analysis for planning and evaluation of compliance inspections for dry gas lines where the purpose of the inspection is to validate absence of corrosion.
  • Sampling inspection scheme development and evaluation of results for lines where the conditions are sufficiently homogeneous to allow a sufficient quantification of degradation characteristics based on small coverage.
  • Statistical simulation to assess the effectiveness of sampling options and to evaluate the reliability of the results. This includes consideration of both quantified sampling and screening type approaches.
  • Planning and evaluation based on Direct Assessment methods, e.g. ICDA or ECDA.

Sonomatic has worked with many of the world’s leading oil industry clients in developing and providing practical solutions for unpiggable pipelines inspection. Some examples are covered in the case studies below.

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