RSTRENG Tips

Technical Toolboxes receives many RSTRENG questions throughout the year; however, most of them are not related to operating the software. These questions are related to procedures, definitions, and field repairs.

Introduction

When metal loss features in pipelines are detected by in-line inspection (ILI), a decision to replace, repair, and assess remaining features. This decision is based on the prediction of the failure pressure of the corroded pipeline and must adhere to the US Code of Federal Regulations (CFR) Title 49, Parts 192 and 195, that stipulates that ASME B31G or RSTRENG be used to assess the remaining strength of corroded pipelines.

Procedures

RSTRENG procedures are designed for line pipe and should not be used to evaluate the remaining strength of pipe in corroded girth or longitudinal welds, or related heat affected zones, defects caused by mechanical damage, such as gouges and grooves, and defects introduced during pipe or plate manufacture, such as seams, laps, rolled ends, scabs, or slivers.” Other limitations include:

• Pipe vibration, fittings, wrinkle/ripple bends, etc.
• Crack-like defects.
• Combined corrosion and crack-like defects.
• Combined corrosion and mechanical damage.
• Metal loss defects due to mechanical damage (e.g., gouges).
• Metal loss in indentations and buckles, or metal loss that coincides with other damage.
• Metal loss in fittings.
• Pipelines that operate at temperatures outside their original design envelope or operating at temperatures in the creep range.

Field Repairs

Repair Criteria (Direct Examinations)

• Multi-step Screening Process
  • ASME B31G Level 1 (Parabolic Shape)
  • Modified B31G Level 1 (Arbitrary Shape
  • RSTRENG  Level 2 (Effective Area Method -EAM)

• The calculated failure pressure is then multiplied by the appropriate safety factor to determine a safe operating pressure.
• Then the decision is made to repair it or not.
• Repair or Mitigation
  • Reduce Pressure
  • Repair
  • Replace Cylinder of Pipe
  • Recoat

• Zero Out Method is option for long length metal loss (Greater than 25 inches) to reduce the number of composite or steel sleeves needed.
  • Zero out all metal loss areas that failed the first effective length.
  • Run RSTRENG a second time using the manual method  or the automated method  until the remaining metal loss feature(s) pass.
  • Use results to place sleeves as required.

Definitions

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