PL Risk Based Assessment Theory

Take note

This overview explains the key concepts and methodology of Risk Based Assessment (RBA) for Pipelines.

Risk Based Assessment is a methodology used to evaluate the integrity of Pipelines and determine appropriate Inspection Intervals and the Next Inspection Date (NID). A key concept in RBA is the Remaining Life or Remnant Life of a Pipeline.

Remnant Life

Remnant Life refers to the period between the Last Inspection Date (LID) of a Pipeline and the date at which it is no longer fit for purpose to operate. The RL can be calculated using the following factors:

• Expected corrosion rates:

  • Future Corrosion Rate: The anticipated rate of corrosion going forward.

  • Past Corrosion Rate: The corrosion rate observed between the last assessment and now.

    Note: The Past and Future Corrosion Rates are specified by a corrosion engineer.

• Corrosion Tolerance: The amount of wall thickness that can be lost without compromising the pipeline's integrity. This is based on:

  • Any wall loss from historically measured defects.

  • The minimum allowable wall thickness (MAT).

The Remaining Life date is calculated using the formula:

Where:

The Remaining Corrosion Tolerance (RCT) is the Corrosion Tolerance at the Assessment date and the Future CR is specified by the user i.e. Corrosion Engineer.

The Remaining Corrosion Tolerance is calculated using different formulas depending on the results of In-Line Inspections:

• If Defects were detected during the ILI.

• If no Defects were detected or no defects were detected in a section.

• If no ILI was conducted.

Note: In the Corrosion Tolerance calculations the in-accuracy (e.g. 10% of wall thickness) of the instrument is taken into account for the Remnant Life calculation.

Traffic Light Status

The RBA methodology uses a traffic light system to categorize the Pipeline's Status based on its Remnant Life and the end of Required Life:

• Red: RL < 1 year

• Orange: 1 year < RL < 3 years

• Yellow: 3 years < RL < End of Required Life

• Green: End of Required Life < RL
  Note: For the Shell Group’s full definition of a pipeline traffic light, refer to AMS MEC RP 4-04.8.

Remaining Corrosion Tolerance

To calculate the RL you need to know the RCT. Corrosion tolerance is the Wall Thickness a Pipeline can afford to lose without chances of failure. It is calculated differently depending on the availability of inspection data:

Pipelines with In-Line Inspection data and detected Defects

• The past wall loss is known from the inspection data.

• Defects are assumed to grow at a constant L/D ratio.

• The lowest Corrosion Tolerance calculated for any section is the limiting factor for the entire Pipeline.

RCT for Pipelines with ILI data and detected Defects

The formula to calculate the RCT for Pipelines with ILI data and detected Defects is:

The CT data comes from the ILI run and the WallLoss is determined from the ILI run date to the Assessment date based on the Past Corrosion Rate.

On the Failure Assessment diagram, the CT of a Pipeline with Defects is the distance from the defect point to where the assessment curve and a line drawn from the origin (0,0) through the defect point intersect, measured along the x-axis. The shape of the assessment curve depends on the specific method used (B31G, RSTRENG, or DNV RP-F101) and the pipeline's maximum allowable operating pressure (MAOP) and design data. If there is interaction between multiple defects, this interaction should be considered when determining the overall corrosion tolerance of the pipeline segment.

Pipelines with ILI run but no detected Defects and Pipelines Without Inspection

• The starting point is the Nominal Wall Thickness.

• The applicable L/D ratio is unknown.

• Therefore, the RCT will be defined based on the expected Corrosion Type and an associated, default L/D ratio.

RCT for Pipelines with ILI run but no detected Defects and Pipelines with no ILI

The formula to calculate the RCT for Pipelines with ILI run but no detected Defects and Pipelines with no ILI is:

For the calculation of the RCT for Pipelines with ILI Inspection and no defects reported the Limiting State is based on the expected corrosion morphology (general, grooving, or pitting). On the L/D diagram in the image below, the 3 LimStates are represented as a point on the Maximal Allowable Size Metal Loss Curve. The General corrosion LimState is determined by finding the point on the length axes that represents 1000 times the wall thickness. A line is drawn from this point to the Maximum allowable size metal loss curve. The intersection of this line with the curve represents the General corrosion LimState. The Grooving Corrosion LimState is a point on the same curve corresponding to 80 times the wall thickness. The Pitting Corrosion LimState is a point on the same curve corresponding to 20 times the wall thickness. To determine the Characteristic Thickness (CT), a line is drawn from each LimState point to the depth axis (y-axis). The intersection of this line with the depth axis represents the CT. The distance from the origin (0,0) to this intersection point is the CT value.

The WallLoss refers to the Wall Loss that occurred between the ILI run date and the Assessment date based on PastCR.

For pipelines/flowlines with no ILI run, the wall loss calculation is different for internal and external corrosion:

• For Internal Corrosion, the Wall Loss is calculated from the date the Pipeline / Flowline went into service to the Assessment date. This is based on the Past Corrosion Rate (PastCR):

• For External Corrosion, the Wall Loss is calculated from the Installation date of the Pipeline / Flowline to the Assessment date. This is also based on the Past Corrosion Rate (PastCR):

Interval Factor and Next Inspection Date

After calculating the RCT and RL, you need to calculate the Next Inspection Date (NID). To determine the NID it is first necessary to calculate the Interval Factor (IF). The NID is calculated as:

TheIF is influenced by:

• Assessment strategy (Design Intent, Extended Life, or Fit for Purpose)

• Confidence rating (Very Low, Low, Medium, High, Very High)

• Criticality or consequence of failure

A higher confidence rating, which reflects lower uncertainty in the assessment, generally allows for longer Inspection Intervals.

Assessment Strategy

To calculate the IF, we need to determine the Assessment Strategy. The Assessment Strategy is based on the pipeline's Remaining Life and is classified into three categories:

1. Design Intent:

   • The pipeline's RL is less than or equal to the user-specified End of Design life.

2. Extended Life:

   • The pipeline's RL is between the End of the Design life and the End of the Required life.

3. Fit for Purpose:

   • The pipeline's RL is based on its Fitness for Purpose, which may extend beyond the End of Required life.

The calculation of the IF varies depending on the selected Assessment Strategy for the Pipeline. There are two main approaches:

• When the Assessment Strategy is Design Intent or Extended Life, the IF calculation includes Probability of Failure (PFail)

• When the Assessment Strategy is Fit for Purpose.

IF calculation when Assessment Strategy Design Intent or Extended Life

When the Assessment Strategy is Design Intent or Extended Life the Interval Factor is determined based on the Criticality and Confidence Rating, with a higher Confidence Rating leading to a longer Interval Factor. Confidence Rating can be very low, low, medium, high, and very high. It reflects uncertainty in the:

• Stability / predictability of the Degradation rate.

• Number and quality of previous Inspections.

• Process stability.

Confidence rating is performed for the entire Pipeline, not for a section. The Interval Factor is a function of the Criticality and Confidence Rating:

Criticality is a function of the Susceptibility to Failure (StF) and Consequence:

The StF is calculated as the ratio of the Assessed Corrosion Rate to the Design Corrosion Rate:

IF calculation when Assessment Strategy Fit for Purpose

When the Assessment Strategy is Fit for Purpose the Interval Factor is determined based on the Consequence and Confidence Rating, without considering the Criticality. In this case, the Consequence can be specified per section of the Pipeline. The Interval Factor is based on the Consequence of Failure and Confidence Rating: