API RP 581 · 4th edition (2025)

Tank Quantitative RBI — User Manual

Oct 19, 2026@Shamit
YEARS → TARGET DATE
Contents

    About this tool

    The Tank Quantitative RBI tool applies the API RP 581 method to one atmospheric storage tank. For the bottom, the bottom edge and each shell course it computes a damage factor for thinning, the probability of failure, the financial consequence of a leak or rupture and the risk in US$ per year, then finds the date each component reaches your targets and the inspection effectiveness that keeps it within them to the end of the plan period.

    Edition

    Built on API RP 581, 4th edition (January 2025), the current edition: Part 1 (risk and categories), Part 2 (POF and the thinning damage factor), Part 3 (financial consequence terms), Part 4 (inspection planning) and Part 5 Section 4 (storage tanks). API 653 6.4.2.2.2 asks that the RBI team be qualified and the method validated; this tool is a calculation aid for that team.

    Qualitative or quantitative?

    The Tank RBI (API RP 580) tool records a team’s categories. This tool calculates them from RP 581’s equations. Use the RP 580 tool for the framework and the API 653 factor lists, and this one for the numbers.

    Quick start

    1. 1
      Load a sample, or enter the tank in Section 1: diameter, maximum fill height, course height, product, release prevention barrier (RPB), foundation, management systems score, plan period and your targets.
    2. 2
      Section 2: soil and depth to groundwater, environmental sensitivity, where a release would go, lost production per day and materials. Enter the course area and injury costs if you have a Part 3 study.
    3. 3
      Section 3: the last bottom thickness measurement, its date and the effectiveness of that inspection; the corrosion rate measured or from Annex 2.B.14; construction, maintenance and settlement; any lining. Tick the edge to model it separately.
    4. 4
      Section 4: the last shell survey date, material strengths, rate confidence and inspections since, then one row per course (course 1 at the bottom).
    5. 5
      Read Section 5: risk by component, the risk curves, the matrix and the target date and inspection for each component. Save and print.

    What is calculated

    ItemMethodComputed?
    Bottom and bottom edge DF (thinning)Part 5 4.3: Eq 5.2, Table 4.3, Eq 5.3Yes
    Shell course DF (thinning)Part 2 4.5.7 (structural reliability, Bayesian update)Yes
    Bottom corrosion ratePart 2 Annex 2.B.14 (Eqs 2.B.37, 2.B.38) or your measured rateYes
    POFPart 2 Eq 2.1 with Table 3.1 GFFs and Eq 2.8 FMSYes
    Bottom financial consequencePart 5 4.19 – 4.26 (Eqs 5.33 – 5.54)Yes
    Course financial consequence: environment, damage, productionPart 5 4.5 – 4.17; Part 3 4.12.2, 4.12.4Yes
    Course area, flammable, toxic and injury consequencePart 3 Level 1 / 2No: enter the costs from your study
    SCC, external damage, brittle fracture DFsPart 2 Sections 5 – 21No
    Risk, categories, matrixPart 1 4.3, Table 1.3Yes
    Target date and inspection planPart 4 6.4Yes (risk, maximum and minimum DF, thickness and interval targets)

    Bottom and edge damage factor

    The wall-loss fraction is (Eq 5.2):

    Art = max[ 1 − (trdi − Cr,bm · (agetk − agef,rc)) / (tmin + CA), 0 ]

    • trdi is the thickness at the last measurement (or the furnished thickness at the in-service date) and agetk the years since. The tool projects it at every half year of the plan.
    • tmin: 2.54 mm (0.1 in.) without an RPB, 1.27 mm (0.05 in.) with one (API 653); for the edge, the annular-ring or critical-zone minimum you enter. Art stays zero until the projected thickness falls below tmin, so the bottom DF sits at 0.1 and then rises steeply: in practice the thickness target usually comes first.
    • Lining: the remaining expected life at the measurement, divided by the condition factor (Table 4.2: good 1, average 2, poor 3), delays corrosion (Eq 5.1). The online-monitoring factor of Eq 5.1 is not applied (taken as 1): as printed it would shorten the credit for a monitored lining.
    • The base DF is read from Table 4.3 for the effectiveness of the last inspection (A – E, examples in Annex 2.F Table 2.F.5.3), interpolating between the 0.05 steps of Art.
    • Eq 5.3: DF = max(base × FWD × FAM × FSM, 0.1), with FWD 1 welded / 10 riveted, FAM 1 maintained to API 653 / 5 not, FSM 2 settlement beyond API 653, 1 within, 1.5 never evaluated, 1 concrete foundation.
    InspectionSoil side (Table 2.F.5.3 examples)
    Afloor scan over 90 % with UT follow-up, hand scan of the critical zone
    Bfloor scan over 50 % with UT follow-up, or EVA with scan follow-up
    Cfloor scan 5 – 10+ % plus near the shell, or circle-and-X pattern, helium/argon, hammer test, coupons
    Dspot UT, flood test
    Eineffective

    Bottom corrosion rate (Annex 2.B.14)

    SideEquation and factors
    Soil sideCRS = CRSB × FSR × FPA × FTD × FCP × FTB × FST; base 0.13 mm/yr (5 mpy). Resistivity 1.5 – 0.66 (1.0 with an RPB), pad 1.5 – 0.7, drainage 1 / 2 / 3, CP 1 / 0.66 / 0.33 (only with verified API 651 CP), RPB not to API 650 1.4, temperature 1.0 – 1.4.
    Product sideCRP = CRPB × FPC × FPT × FSC × FWD; base 0.05 mm/yr (2 mpy). Wet 2.5, temperature 1.0 – 1.4, steam coil 1.15, water drawn weekly or after each receipt 0.7.
    Combinedgeneral product-side corrosion: the sum; pitting or localised: the greater. Never below 2 mpy (0.05 mm/yr).

    A measured rate from inspection history is preferred (Part 2 4.5.2); the Annex is for when there is none.

    Shell course damage factor

    • Art = Cr · agetk / trdi (Part 2 Eq 2.12), from the last shell survey.
    • Strength ratio SRp = (S · E / FS) · tmin / trdi with flow stress FS = (YS + TS)/2 · E · 1.1 (Eqs 2.13, 2.14).
    • tmin is yours or, left blank, API 653 4.3.3.1: 4.9 D (H − 0.3) G / (S E), at least 2.5 mm, with H the liquid height above the course bottom, G from the product density and S from API 653 Table 4.1 (lower of 0.80 Y and 0.429 T for the bottom two courses, 0.88 Y and 0.472 T above). With that tmin, S cancels out of SRp.
    • Prior probabilities for the three damage states (1×, 2×, 4× the rate) come from your rate confidence (Table 4.5) and are updated by the A – D inspections since the survey (Table 4.6, Eqs 2.16, 2.17).
    • β for each state with COVΔt 0.20, COVSf 0.20, COVP 0.05 (Eq 2.18); DF = Σ Po · Φ(−β) / 1.56 × 10−4, at least 0.1 (Eqs 2.19, 2.20, with FIP, FDL, FOM = 1 for a tank shell).

    Bottom consequence

    1. 1
      Hydraulic conductivity of the soil for water, the mean of the Table 4.7 bounds (Eq 5.35), corrected to the product by density and viscosity (Eq 5.33). Seepage velocity = kh / porosity (Eq 5.34).
    2. 2
      Hole: 3.175 mm with an RPB, 12.7 mm without (Table 4.8); number of holes max(nint((D/30.5)²), 1) (Table 4.9). Head: the fill height, or 0.0762 m with an RPB.
    3. 3
      Leak rate by Eq 5.37 (Bernoulli, very permeable soil), 5.38 (Giroud, low permeability) or 5.39 (between).
    4. 4
      Detection time 7 days on a concrete or asphalt foundation, 30 days with an RPB, 360 days otherwise; the leak lasts until detected or the tank is empty (Eqs 5.44, 5.45).
    5. 5
      Time to reach groundwater = depth / seepage velocity (Eq 5.47). The leak is split between groundwater and subsoil in proportion (Eqs 5.48 – 5.50) and costed at the Table 4.6 rates (Eq 5.51).
    6. 6
      Rupture: the whole inventory, weighted by its GFF share, split between the dike, on-site soil, off-site soil and water by your percentages (Eqs 5.52, 5.26 – 5.30).
    7. 7
      Component damage (Eq 5.53: 5,000 / 120,000 US$ scaled by (D/30.5)² and the material factor), business interruption (5 / 50 days weighted, times lost production, Part 3 Eqs 3.85, 3.87). FCtotal is their sum (Eq 5.54).
    US$/bbl (Table 4.6)LowMediumHigh
    In the dike101010
    On-site surface soil505050
    Off-site surface soil100250500
    Subsoil5001,5003,000
    Groundwater1,0005,00010,000
    Surface water5001,5005,000
    The Eq 5.38 limit

    RP 581 switches from Eq 5.39 to Eq 5.38 when kh ≤ C37 · (d1.8 / (Cqo h0.4))1/0.74, with C37 = 1.408 × 10−8 and kh in m/day. Checked numerically, that constant marks the point where Eqs 5.38 and 5.39 meet only if kh is in m/s. Read with m/day, Eq 5.39 is used for fine sand, very fine sand and silt, and there it falls to near zero (up to 20 orders of magnitude below Eq 5.38 at the switch). The US customary C37 (6.995 × 10−5) is also inconsistent with the SI value by the factor 0.3048. The tool therefore applies C37 as 1.408 × 10−8 × 86,400 for kh in m/day (continuous), and offers the printed reading as an option in Section 2. All other constants were checked: the SI and US sets of Eqs 5.37 – 5.39 agree within 0.5 %.

    Course consequence

    • Holes of 3.175, 6.35 and 50.8 mm and a rupture (Table 4.4) at the bottom of the course; head = fill height − (i − 1) × course height (Eq 5.7); a course above the liquid has no consequence.
    • Leak rate 0.543 × 0.61 × A × √(2 g h) bbl/day (Eq 5.5, A in mm²); detection 7 days for the small hole, 1 day otherwise; volume limited to the liquid above (Eqs 5.9 – 5.15).
    • Environmental cost of the GFF-weighted leak and rupture volumes split by your percentages (Eqs 5.19 – 5.31).
    • Component damage 5,000 / 12,000 / 20,000 / 40,000 US$ and outage 2 / 3 / 3 / 14 days, GFF-weighted (Part 3 Eqs 3.83, 3.85), times the material and cost factors and lost production.
    • Damage to surrounding equipment and injury costs (FCaffa, FCinj) need the Part 3 consequence areas, which this tool does not compute. If you enter them from your own study they are added, and FCaffa adds its outage by Eq 3.86. Until then the summary carries a note.

    Risk, categories and the matrix

    • POF = GFF × DF × FMS (Eq 2.1). GFF: 7.22 × 10−4 per year for the bottom and edge, 1.0 × 10−4 for a course (Part 2 Table 3.1). FMS = 2.38 e−0.012 · score (1.0 at 72 %).
    • Risk = POF × FCtotal, US$ per year. The tank figure is the sum of its components.
    • Categories from Part 1 Table 1.3: probability by the DF range (≤ 1, 1 – 10, 10 – 100, 100 – 1000, over 1000), because Note 1 says the POF ranges do not apply to tank bottoms and courses; consequence A – E at US$ 10 k, 100 k, 1 M and 10 M.
    • The matrix shading is an example (the RP 580 Figure 7 pattern used across Tank Protocol); RP 581 leaves the ranking to the owner (Part 1 4.3.2).

    Target date and inspection plan (Part 4 6.4)

    1. 1
      The RBI date and plan date set the period; DF, POF, risk and thickness are computed every half year.
    2. 2
      For each target you set, the date it is reached is interpolated: financial risk target, maximum DF target, the thickness reaching tmin, and the maximum interval since the last measurement. The earliest is the target date.
    3. 3
      For a risk or DF target the tool tries a C, then B, then A inspection at the target date and keeps the first that holds the plan-date risk and DF within the targets. For the bottom an inspection moves the Table 4.3 column (a worse one does not); for a course it adds one inspection to the Bayesian update.
    4. 4
      Consequence driven: if the risk target is already exceeded at the lowest DF (0.1), or the DF is at or below your minimum DF target, inspection cannot reduce it; the tool says so and points to reducing the consequence (RPB, leak detection, inventory, containment) or reviewing the target.
    5. 5
      Thickness reaching tmin: inspect (C or better) and repair, or assess fitness for service, before that date. If even an A inspection is not enough, the plan says repair, replace or mitigate.

    Targets are the owner’s (Part 4 6.2) and apply to each component. Leave one blank to skip it.

    Saving and the report

    • Fill in Identification (company, tank or item ID, location) and the record type: Just for research stays private to you; Official is visible to your company.
    • Calculate & Save stores the inputs and results in the rbi581TankCalcs collection.
    • Print / PDF report opens a printable page of everything on screen: identification, the standard and edition, each section’s inputs and results, and any notes. Use Save as PDF in the print dialog. The report carries the edition and scope note, the bottom and course results, the risk table, the risk curves and matrix, the plan for each component, the checks and the inputs.

    Limits

    • RP 581 4th edition (January 2025), the current edition.
    • Thinning only. Add SCC, external corrosion and brittle fracture by Part 2 if they apply; use the API 571 tool to screen mechanisms.
    • Course area and injury consequences are not computed (Part 3 Level 1 / 2); the course consequence is environmental, damage and production unless you enter those costs.
    • Default costs are RP 581’s 2001 US$ examples: replace them with your own, or scale the hole costs with the cost factor.
    • Bottom Table 4.3 has one column per effectiveness (one inspection); the best inspection in the period is used. The Eq 5.1 online-monitoring factor for linings is not applied, and the Eq 5.38 limit uses the continuous reading by default (see above).
    • Use the Shell Evaluation, Bottom & Annular Plates, Settlement and Tank Shell FFS tools for the thicknesses, minimums and settlement you enter here.