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Successful inspection of critical stainless steel pipeline

Plant operators face an increasingly complex challenge when managing the integrity of assets – achieving operational excellence and maximum asset performance while minimising costs and maintaining the highest safety and environmental standards. To determine the correct approach to achieve this, pipeline owners and operators must integrate and assess critical data from different sources such as the pipeline route, materials, facilities, operations, inspections, repairs and modifications.

Through the application of detailed knowledge of materials degradation processes, inspection and structural integrity, TWI has a demonstrable track record in assisting members with such aims.

The project carried out by TWI on behalf of the client comprised:

  • planning and selection of appropriate inspection techniques
  • inspection activities
  • assessment of inspection results.

In order to correctly identify the appropriate inspection techniques, TWI first reviewed the historical operational data for the piping loop which had experienced high levels of thermal cycling. From this data TWI asset integrity engineers identified a number of possible damage mechanisms which may have affected the integrity performance of the pipe where it had undergone low temperature excursions.  These ranged from a possible reduction in toughness and external pitting to cracking resulting from fatigue or external stress corrosion cracking (SCC). TWI was able to identify the possible locations of such damage and recommend appropriate inspection methods for their detection 

Successful inspection of critical stainless steel pipeline using NDT techniques and Engineering Critical Assessment
Successful inspection of critical stainless steel pipeline using NDT techniques and Engineering Critical Assessment

For those NDT methods identified within the table, TWI carried out the following tasks: 

  • established outline NDT techniques based upon knowledge of flaw morphology and prior experience
  • developed appropriate calibration pieces to support the NDT techniques to be deployed
  • finalised the efficacy of the techniques via trials and modelling as appropriate, and modified the techniques as required
  • established written NDT procedures.

Following approval and demonstration of these procedures:

  • mobilised necessary tools and suitably qualified inspection personnel to carry out NDT on-site
  • issued comprehensive reports on NDT findings.

Our proposed scope of work to the client also included carrying out an Engineering Critical Assessment (ECA) to determine the critical tolerable flaw size, against which any crack-like flaws detected could be assessed.

ECA is based upon fracture mechanics principles to determine whether or not a flaw of a specific size will pose a risk of failure due to any combination of fatigue, brittle fracture and plastic collapse during the design life of a structure or component.

There are three essential elements of ECA: geometry and size of component and flaw, material properties (tensile and fracture toughness) and loading (cyclic and static stresses). The material properties required for the ECA are obtained from a tensile and fracture mechanics testing programme.

Possible damage mechanism Cause Effect Inspection method Locations to inspect
Austenite to martensite transformation Low temperature upsets in 2009 (down to -100°C) Reduction in ductility Eddy current or Ferritscope Compare weldments (weld and HAZ) which have experienced upset and those which have not. Focus at coldest locations
External chloride SCC Operating temperatures exceeding 60°C, in coastal environment, no protective coating External cracking Liquid penetrant inspection (PT), phased array ultrasonic testing (PAUT) Weldments (including attachments), bends, water trap areas
External pitting Operating in coastal environment, no protective coating Localised metal loss Visual inspection, PT Water trap areas
Fatigue Connected to rotating equipment and thermal cycling Fatigue cracking PT, PAUT Weldments (including attachments), where pipe is constrained, eg supports, etc
Thermal shock Fast thermal cycling Cracking PT, PAUT At and near injection points, weldments (including attachments), where pipe is constrained
Internal erosion Internal liquid flow, particulate contaminants Metal thinning Manual ultrasonic testing Low points (6 o'clock position), bends

For more information on TWI’s work in this area please email contactus@twi.co.uk

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