RSTRENG Tips

Owned by Joseph Pikas
Dec 13, 2022
3 min read

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.

Definitions

Pipe OD

Outside diameter of pipe (Pipe Description Section will be Added)

Wall Thickness

Wall thickness of the pipe. (Pipe Description Section will be Added)

SMYS

Specified Minimum Yield Strength (SMYS) of the pipe (Pipe Description Section will be Added)

UTS

Ultimate Tensile Stress (UTS) used in Non-PRCI Calculations

Factor – Design Factor

Design factor of the pipe (Pipe Description Section will be Added)

Measurement Method

Corrosion depth or the remaining wall thickness.

Corrosion Depth

Corrosion depth (pit depth) data in mils or millimeters

Remaining Wall Thickness

Remaining wall thickness data in mils or millimeters

Modified SMYS

A calculated value that is given by SMYS * F – Design Factor.

P Design (Barlow)

P – Design/Design Pressure/Barlow’s Pressure is calculated using (2*SMYS* thickness*F) / OD. 

Established MAOP

Maximum Allowable Operating Pressure (MAOP) is established by the pipeline operator

Maximum Safe Pressure

The max safe pressure is calculated from the established MAOP.  However, the pipeline operator may want to uprate the pipeline pressure to P Design..  Therefore, consideration should be given to run the assessment for both values.

 Factor of Safety

The safety factor is based on Class location.  Example:  Reciprocal of a Class 1 pipe 0.72 is 1.3888888 or 1.39 or for a Class 3 pipe 2.0.  Failure pressure is calculated first, then multiplied by the appropriate safety factor to determine a safe operating pressure.

Color Code of Results

Yellow is equal or greater than Established MAOP.  Red is less than Established MAOP.  P Design and Established MAOP can be the same psi especially with liquid pipeline operators. 

Class Location

Class location is defaulted Class 1, Div II or 0.72 Design factor.  If Class 1, Div I is required, it must be manually inputted.  A special permit is required for this class.

Corrosion Growth Rate

User-defined or default 0.016 inches/year (0.4064 mm/year) used for remaining life and re-assessment interval calculations. 

Field Repairs

Repair Criteria (Direct Examinations)

  • Multi-step Screening Process

  1. ASME B31G Level 1 (Parabolic Shape)

  2. Modified B31G Level 1 (Arbitrary Shape

  3. 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

  1. Reduce Pressure

  2. Repair

  3. Replace Cylinder of Pipe

  4. 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.

  1. Zero out all metal loss areas that failed the first effective length.

  2. Run RSTRENG a second time using the manual method  or the automated method  until the remaining metal loss feature(s) pass.

  3. Use results to place sleeves as required.