Shell-Risk Based Inspection (S-RBI) - Theory

Risk-Based Inspection (RBI) is a risk-based approach to the inspection of Pressure Equipment in the Oil and Gas industry. This type of inspection analyses the Susceptibility to (likelihood of) Failures (StF), and the Consequences of the same Failures (CoFs), in order to priorities inspections.

This structured and multidisciplinary process is used to determine and document, cost-effectively, the optimum inspection, testing, and PM (e.g condition monitoring) requirements, of Equipment in its operating context, and safeguards integrity.

The RBI methodology, implemented in IMS, is based on the best practices as developed within Shell and complies with the API RP 580 standard on Risk-Based Inspection. This Shell Risk-Based Inspection (S-RBI) is a semi-quantitative approach to risk-based inspection. As written in API 580, this type of approach “… is geared to obtain the major benefits of the … two approaches (e.g., speed of the qualitative and rigor of the quantitative).”

The Workflow explains how S-RBI fits into the Degradation Management work process as implemented in IMS. 

Criticality (Susceptibility to Failure (StF) and Consequence)

The S-RBI method calculates Criticality. This is the base risk, i.e., risk when the effects of maintenance and inspection are disregarded. It relates to the original design with the actual operating conditions. The Criticality assessment is carried out in a semi-quantitative way, for each DM that applies to each (pressure containing) Component.

Criticality is based on the Component’s Susceptibility to Failure (StF) for a DM and the Consequences caused by the loss of primary containment. S-RBI plots these two classes on a modified version of the Shell Corporate Risk Assessment Matrix (RAM) (see the figure below) to determine the Criticality. Note: The overall Consequence is the highest Consequence category from Asset, People, Environment, and Community.

Risk Assessment Matrix used in S-RBI. For the implementation of this in IMS see Step 7 Determine Susceptibility to Failure (StF), Step 8 Rate Consequence of Failure (CoF), and Step 9 Review Overall Criticality.

Confidence

In addition to Criticality, the Confidence is also assessed during the S-RBI assessment. The Confidence Rating reflects the confidence (or uncertainty) in the forecast of the degradation, i.e., assessment of StF and the RL (for AR DMs).

The Confidence assessment is based on three Key Questions:

• Stability and controllability of the DM.
• Number and reliability/quality of inspections.
• Reliability of process (IOWs) monitoring.

Questionnaires, varying per DM, ensure a consistent approach to determine the Confidence rating. In most cases, IMS scores each key question +0.1, 0, or -0.1, depending on the answer. The scores are summed to determine the total score, which is limited to a maximum of 0.2. IMS then uses the total score to determines the confidence rating, which can range from very high (VH) to very low (VL) (see the table below).

Confidence Rating total scores:

Confidence, together with Criticality, determines the inspection intervals or process monitoring intervals. The Interval Factor (IF) for AR degradation is increased and decreased with the confidence score. Thus, better Confidence means longer inspection intervals. For NAR DMs, a high Confidence reduces the monitoring requirements or the need to redesign.

For the implementation of this in IMS see Step 10 Assess Confidence.

Deliverables: MII, NID, IS, Required Inspection Effectiveness

The S-RBI deliverables are a Next Inspection Date (NID) and/or an Inspection Strategy (IS). Additionally, the S-RBI results can be used to determine the required Inspection Effectiveness. These 3 will determine when and how to do the inspection for the Component’s DM.

For AR DMs, a Maximum Inspection Interval (MII) is derived from the RL and Interval Factor (IF) The IF is determined from the Criticality and Confidence rating. The NID is then determined form the MII. For NAR DMs, RL is not applicable and the monitoring regime and MII are only based on Criticality and Confidence rating. (See also Next Inspection Date (NID) – Horserace.)

Age-Related DMs

For AR DMs, the MII is determined from RL and IF, which in turn depends on the Confidence and Criticality (see the figure below).

The AR S-RBI work process. For AR DMs, the MII is determined from RL and IF, which in turn depends on the Confidence and Criticality.

IMS combines the Confidence and Criticality in a table to determine the IF. The table, shown in the figure below, has the “half-life” and in parenthesis the “full life” values for the IF. Certain Equipment or DMs, such as heat-exchanger tubes, underground piping, CUI and tank floors, uses the “full life” table for the IF determination, while the rest use the “half-life” table. Note: The “full life” is not applicable for air cooler or fin fan tubes, as the impact of a leak will be a release directly to atmosphere.

The "half-life" (and "full-life") IF for AR DMs are based on Confidence and Criticality.

The IF then determines the fraction of RL at which to inspect, i.e., the MII (see the figure below).

The Maximum Inspection Interval depends on the RL and IF.

The Maximum Inspection Interval is determined from:

   MII = IF × RL

With:

MII   - Maximum Inspection Interval [years]

IF     - Interval Factor [-]

RL    - Remnant Life [years]Example of a MII calculation.

Then the NID is determined from the MII and the Last Inspection Date (LID). This is the ultimate date by which the Inspection Should be completed.

  NID = LID + MII

With:

NID    - Next Inspection Date [years]

LID     - Last Inspection Date [years]

MII      - Maximum Inspection Interval [years]

Non-Age-Related DMs

Here the RL approach does not apply. The effective strategy is to prevent (process) conditions under which NAR DMs can initiate. The Maximum Inspection Interval (MII) is based on Confidence (largely) and Criticality – see below.

The NAR S-RBI work process.

Instead of a straight-forward MII, the RBI result could also be a monitoring and Inspection Strategy (IS) (or both) based on Confidence and Criticality – see below.

General Monitoring and Inspection Strategies (IS) for NAR DMs.

The Inspection Strategy tables vary per DM. For example, there are three Inspection Strategy tables for cracking, based upon the expected rate of cracking. Refer to Guidance per DM for specific strategy matrices per DM. Also refer to Guidance per Special Emphasis. In Step 11 you will find a specific guidance summary table, with applicable references per DM / Specials Emphasis Component.

Strategy Based - CUI

For Corrosion Under Insulation (CUI), it is generally challenging to make an accurate estimate of the RL, because degradation occurs in a very localized manner. Other factors, such as the condition of the Barrier coating (covered by the lagging/insulation) and the condition of the lagging (requires a structured external inspection review), play an important role, but are also in general difficult to assess. For this reason, the Inspection Strategies (IS) for CUI are based only on Criticality. 

Required Inspection Effectiveness

In addition to a NID and/or IS, the Inspection scheduling should consider the required Inspection Effectiveness. This is an API-term for inspection confidence. The higher the effectiveness, the higher the confidence that most severe defect has been found. The required Inspection Effectiveness should be used to determine the appropriate inspection technique and coverage for the specific DM. The required Inspection Effectiveness can be determined from the morphology of the DM and the current condition of the Equipment.

Morphology of the DM: In this context, the morphology is described by the Interval Factor (IF), which depends on the Confidence and Criticality.

Current condition of Equipment: As the Equipment inspected is nearing the end of its technical life (i.e., the RL decreases) it becomes more critical that a life-determining defect is found, i.e., exceeding corrosion allowance. This implies, a higher Inspection Effectiveness is required to find this defect with sufficient confidence.

Interval Factor (IF) and Remaining Life (RL) can thus be combined to determine the Required Inspection Effectiveness (see below).

Required Inspection Effectiveness Category - Guideline for Vessels*:

*Not applicable for bundles, coils, RVs, underground piping and tanks. 

The required Inspection Effectiveness should be used to determine specific inspection techniques and coverage. The inspection techniques and coverage will depend on the specific DM / Special Emphasis Component. Refer to Guidance per DM and Guidance per Special Emphasis. In Step 11 you will find a specific guidance summary table, with applicable references per DM / Specials Emphasis Component.

Key S-RBI Implementation Requirements

S-RBI is relative risk, so assumptions are made, and good judgement is required. Therefore, for an S-RBI assessment to be effective, take note of the following:

• Form an RBI assessment team: Process Engineer, Operations, Corrosion Engineer, Plant Inspection, Maintenance (Reliability), Regulatory Authority or Specialist (on request).
• Compile an RBI Corrosion Control Premises at the start of an RBI project to document information required for the risk assessment. This information can be captured directly in IMS, at the unit level and further down in the CL descriptions (free text fields). This is really Step 1 and Step 2 of the Degradation Management Implementation work process.
• Integrate RBI in the overall inspection process and data analysis in IMS.
• Harmonize RBI with CCD, TAs, on-stream Inspections, and Maintenance process.
• Include knowledge of experienced Inspection and Corrosion personnel.
• Define the IOW for key process parameters and review regularly. This is also part of Step 2 of the Degradation Management Implementation work process.
• Keep the Consequence analysis simple and realistic.
• Make realistic conservative assumptions and document this when data is unavailable.
• Review regularly (keep evergreen) and don’t project too far ahead.
• Incorporate change procedures, e.g., Management Of Change (MOC).
• Nominate RBI process owners.
• Follow the Degradation Management Implementation workflow (see Degradation Management Implementation - Overview), as well as the guidance for Special Emphasis Components and guidance per DM. Also, take note of the Equipment that is not suitable for RBI.

The S-RBI Workflow

See S-RBI Workflow.

Starting an RBI Analysis in IMS

To start an RBI Analysis in IMS see Step 3 Start S-RBI Analysis and Assign DMs to the Component.