00 Overview

The tool evaluates pipeline metal-loss and mechanical anomalies against their governing failure pressure (P_f), deriving safe working pressure (P_safe = P_f / SF) and Estimated Repair Factor (ERF = MAOP / P_safe). Corrosion is assessed independent of orientation (axial vs. circumferential) since the governing B31G/RSTRENG limit-load formulations are not orientation-specific; nine anomaly classifications route to five calculation engines:

1. Corrosion — B31G: ASME B31G Original and 0.85dL Modified, evaluated simultaneously from a single reported length/depth (no profile required).
2. Corrosion — RSTRENG: Effective-area method using the full River Bottom Profile (RBP); falls back to a Modified-B31G-equivalent estimate if no valid profile is supplied.
3. Axial Milling & Gouge: NG-18 Surface Flaw model (C = 1.0 rectangular profile).
4. Circumferential Milling & Gouge: Kastner Plastic Ring Collapse formulation, accounting for combined external axial tension and bending (σ_L,ext).
5. Mechanical Dents: ASME B31.8 / B31.4 strain and depth criteria for Plain Dents (≤ 6.0% OD), Dents on Welds (≤ 2.0% OD), and Dents with Gouge (0.0% OD / immediate repair).

01 API 1163 / POF Tool Sizing Tolerance Implementation

Per standard pipeline inspection practice (API 1163 / POF), ILI tool sizing tolerance is applied conservatively to reported anomaly depth (d_rep), while nominal pipe wall thickness (t_nom) is preserved for structural bulging and hoop burst containment:

This avoids artificial compounding penalties where wall thickness reduction would simultaneously decrease pipe burst pressure and inflate the Folias bulging parameter z = L² / (D · t). For caliper ILI tools, dent sizing tolerance is defined in terms of outside diameter percentage (% OD, default 1.0% OD).

02 ASME B31G Original (Corrected Piecewise Formulation)

ASME B31G mandates a piecewise formulation that strictly transitions at z = L² / (D · t_nom) = 20 (equivalent to Folias factor parameter A = 4.0):

03 ASME B31G Modified (0.85dL)

Universal flow stress calculation automatically switches for high-strength steels (SMYS ≥ 550 MPa or Grade X80+):

04 RSTRENG — Effective Area Method

Iterates through all possible contiguous sub-segments [i, j] (where j > i) of the River Bottom Profile (RBP) to establish the governing combination of localized depth and sub-length. Optimized with precalculated cumulative area integration. If any sub-segment exceeds critical instability (1 − (A/A₀) / M ≤ 0), rupture capacity drops immediately to 0.

05 NG-18 Surface Flaw (Screening)

Standard Battelle surface flaw rectangular cross-section model (C = 1.0) with Folias factor M = √(1 + 0.26·(2c/√(Rt))²) with R the external radius and 2c the flaw length (NG-18 two-term Folias factor, Folias 1975, as restated in Lyons et al., Eng. Failure Analysis 2020, Eqs. 3–4). Flow stress is (SMYS+UTS)/2 as in BS 7910; Lyons uses 1.15·SMYS for the historical NG-18 method. Empirical material stress derating penalties deducted from calculated hoop failure.

06 Kastner Plastic Collapse with Longitudinal Interaction

Assesses circumferential anomalies under plastic ring collapse, applying a deterministic depth penalty (+0.5 mm) to gouges, restricted by a code local width guard (W ≤ πD / 2), deducting external axial tension (σ_L,ext), and strictly capped by the pipe hoop stress burst limit (P_hoop = (S_f − Δσ_gouge) · 2t_nom / D).

07 Safe Working Pressure & ERF

Note on Mechanical Dents: ERF (Estimated Repair Factor) is defined specifically for pressure containment reduction in metal-loss / wall thinning anomalies (ASME B31G). Dents are geometric deformation anomalies governed by ASME B31.8 / B31.4 allowable depth limits (%OD); therefore, ERF is Not Applicable (N/A) for dents.

08 Reliability & Assessment Modes

Deterministic Assessment: Standard FFS evaluation applying code allowable limits and nominal safety factors (P_safe = P_f / SF).

Monte Carlo Reliability Analysis (Optional): Executes a 5,000-iteration Monte Carlo simulation accounting for ILI tool sizing tolerances (±tol, interpreted as an 80%-confidence bound: σ = tol / 1.28) and material strength variability (COV = 5%). Confirm the confidence level quoted in the ILI vendor specification. Determines quantitative Probability of Failure (PoF: P_f < MAOP) and 95% lower-bound burst capacity (P₀₅).

08b Flow Stress, Status Criteria & Interaction

Flow stress. Original B31G uses 1.1×SMYS; Modified B31G and RSTRENG use SMYS + 69 MPa (not above UTS) below 550 MPa; (SMYS + UTS)/2 is used at or above 550 MPa and for X80. ASME B31G-2023 para 1.7(b)(3) permits (SMYS + UTS)/2 up to 80 ksi (551 MPa); use above that is a tool extension. NG-18 uses (SMYS + UTS)/2 and Kastner 1.15×SMYS. Because definitions differ, the minimum over methods is not strictly like-for-like.

Status. Critical: depth ≥ 80% of nominal wall (ASME B31G-2023 §1.2(f), §3(b)), loss of containment, or P_f ≤ 1.10×MAOP (immediate response; confirm against operator procedure). Repair: ERF > 1.0. Investigate: 0.95 < ERF ≤ 1.0. Monitor: 0.80 ≤ ERF ≤ 0.95.

Interaction. Per ASME B31G-2023 §1.12, flaws within 3t of each other (axially and circumferentially) interact and should be evaluated as one combined flaw. The register screens for this and marks candidates with ⚠. With Interacting (3t) → Assess as one flaw enabled, each group of interacting anomalies of the same defect type is assessed as a single rectangular flaw: L = overall axial extent including the gaps, depth = deepest member, and the lowest t, SMYS and UTS (highest MAOP and SF) of the group; RSTRENG profiles are not merged. Each member shows the combined result (⧉Gn) unless its own stand-alone result is worse. Groups are the transitive closure of the 3t test, so a chain of close anomalies becomes one long flaw. Cluster IDs from the import are also treated as interacting. This is a simplification of B31G §1.12 and PR-218 Appendix A (which also discuss effective-area combination of unequal depths); confirm against the operator procedure.

Empirical values. Gouge stress derating, the +0.5 mm gouge depth penalty and default growth rates are user-adjustable engineering assumptions and are not code values. State them in every report.

09 Corrosion Growth Rate

Historical inspection depths are normalized against nominal wall thickness to calculate corrosion growth rate (mm/year) with sub-day calendar precision:

10 Engineering References

• API 1163 / POF: In-Line Inspection Systems Qualification & Anomaly Sizing.
• ASME B31G (2012): Manual for Determining Remaining Strength of Corroded Pipelines.
• PRCI / Kiefner & Vieth: RSTRENG Effective Area Method (Project PR-3-805).
• Battelle Memorial Institute: NG-18 Surface Flaw Model.
• Lyons, C. et al.: Validation of the NG-18 equations for thick walled pipelines, Engineering Failure Analysis (2020) — secondary source for the NG-18 equations as implemented.
• Kastner, W. et al.: Critical Local Plastic Collapse of Circumferentially Cracked Pipes.
• API 579-1 / ASME FFS-1: Fitness-For-Service.
• ASME B31.4 & ASME B31.8: Gas and Liquid Pipeline Systems.