RSTRENG (Effective Area) Calculations

Owned by Drew Lafleur (Deactivated)
Dec 13, 2017
2 min read

What Actions must be Taken to Address Integrity Issues?

According to DOT 192.933 a calculation of the remaining strength of the pipe shows a predicted failure pressure less than or equal to 1.1 times the maximum allowable operating pressure at the location of the anomaly. Suitable remaining strength calculation methods include, ASME/ANSI B31G; RSTRENG; or an alternative equivalent method of remaining strength calculation.  The objective is to evaluate structural integrity that starts with Barlow’s formula for internal pressure.

For U.S. gas and hazardous liquid pipelines, the formula for predicting the pressure that may cause an area of the pipe wall to yield is known as Barlow’s formula:

  • P=2t(SMYS)/D

This is important to understand when calculations like ASME B31.G, Modified B31.G or RSTRENG have specific limitations such as:

  • Mechanical Defects (3rd Party Damage)
  • Stress (Installation, Bending, etc.)
  • Welds
  • Fittings
  • Excessive Overburden
  • Combined with other defects
  • Greater than 45 degrees circumferentially
  • Crack Like Defects

A recent case was brought to Technical Toolboxes attention regarding circumferential corrosion greater than 50% around the pipe at an angle of 90 degrees axially to pipe.  The pitted area was approximately 1-inch length in the axial direction and totally around the circumference of the pipe.  This can occur from time to time from construction problems or at air soil interfaces on risers.   All remaining strength calculations have limitations, and this must be understood.  Then what are some of the methods to assess the integrity of the pipe.  Using ASME/API 579 Fitness for Service, finite element analysis (FEA), or going back to Barlow’s formula to determine if the remaining wall thickness meets the structural integrity for internal pressure for operating conditions of the pipeline.

Many pipeline operators have used Barlow’s formula as an assessment method when conditions such as these arise and combine it with an appropriate repair.  ASME/API 579 Fitness for Service is another way to address these issues.  Pipeline operators are beginning to understand the benefits as the refining and petrochemical industry has done to determine whether construction defects or in-service deterioration threaten the integrity of their assets.