Tank Quantitative RBI (API RP 581)

Tank Quantitative RBI (API RP 581 Part 5)

Quantitative risk-based inspection of an atmospheric storage tank to API RP 581 Parts 1 to 5: damage factors and probability of failure for the bottom, the bottom edge and each shell course, the financial consequence of a leak or rupture, risk in US$ per year, and the target date and inspection effectiveness needed to stay within your risk target over the plan period

New here? Read the User Manual — the equations, the tables used, what is and is not calculated, and how the plan is set.

◉ API RP 581 4TH ED. (JANUARY 2025) · PARTS 1–5, TANK METHOD PART 5 SECTION 4 · API 653 5TH ED. ADD. 4 · NOT A REGULATORY SUBMISSION TOOL

⚠ Edition and scope. Built on API RP 581, 4th edition (January 2025), the current edition. It computes the Part 5 tank method in full for the bottom and bottom edge (thinning damage factor from Table 4.3, financial consequence from Sections 4.19–4.26) and for the shell courses (Part 2 thinning damage factor; environmental, component-damage and business-interruption costs from Sections 4.5–4.17), and the Part 4 inspection plan. Not computed: the Part 3 Level 1/2 flammable, toxic and injury consequence areas for the courses (enter their costs from your own study if you have them), and damage mechanisms other than thinning (SCC, external damage, brittle fracture: Part 2 Sections 5–21). The default costs are RP 581’s examples in 2001 US$; your risk targets and cost data are your company’s (Part 4 6.2). API 653 6.4.2.2.2 asks for a qualified team and a validated method.

Identification

Company

Tank ID

Location

Record Type

Units

RBI date (Part 4 6.4 Step 1)

Summary

Describe the tank, the bottom and the shell courses.

1. Tank, Product and Plan Period (Part 5 4.1, 4.6; Part 4 6.4)

Tank type (Part 2 Table 3.1)

Diameter m

Maximum fill height m

Course height m

Product (Part 5 Table 4.5)

Liquid density kg/m³

Dynamic viscosity Pa·s

Release prevention barrier

Foundation under the bottom

Management systems score, % (Part 2 3.5)

Plan period, years (Part 4 Step 1)

Financial risk target, US$/yr

Maximum DF target (optional)

Minimum DF target (optional)

Maximum inspection interval, years (optional)

A management systems score of 72 % gives F_MS = 1.0 (Part 2 Eq 2.8). Targets are the owner’s (Part 4 6.2) and apply to each component; leave a target blank to skip it. The thickness target is the minimum thickness of each component.

2. Financial Consequence Inputs (Part 5 4.17, 4.26; Part 3 4.12)

Soil under the tank (Table 4.7)

Depth to groundwater m

Environmental sensitivity (Table 4.6)

Fluid leaving the dike, %

Of that, staying on-site, %

Of the rest, staying off-site on land, %

Lost production, US$/day

Outage multiplier (Eq 3.85)

Bottom material (Table 4.16)

Shell material (Table 4.16)

Cost factor on the 2001 hole costs

Eq 5.38 limit (see the manual)

Course failure: damage to surrounding equipment, US$ (Part 3 Level 1/2, optional)

Course failure: injury cost, US$ (Part 3 Level 1/2, optional)

The percentages follow Eqs 5.20–5.23: the share leaving the dike, then the share of that staying on-site, then the share of the rest staying on off-site land; what is left reaches surface water. A sand or soil pad sits on the native soil chosen here; choose concrete-asphalt only where the bottom sits on a slab. Area and injury consequences for the courses need the Part 3 Level 1 or 2 analysis, which this tool does not run.

3. Tank Bottom Damage Factor and Probability of Failure (Part 5 4.2, 4.3)

Last bottom thickness measurement (or in service since)

Thickness then, t_rdi mm

Minimum thickness t_min mm

Corrosion allowance in Eq 5.2 mm

Effectiveness of the last inspection (Annex 2.F Table 2.F.5.3)

Bottom construction (F_WD)

Maintained to API 653 (F_AM)

Settlement (F_SM)

Bottom corrosion rate

Corrosion rate mm/yr

Internal lining (Table 4.1)

Lining expected life, years

Lining applied

Lining condition at the last inspection (Table 4.2)

Annex 2.B.14 bottom corrosion rate

Soil-side base rate mm/yr

Native soil resistivity (Table 2.B.14.4)

Pad material (Table 2.B.14.5)

Drainage (Table 2.B.14.6)

Cathodic protection (Table 2.B.14.7)

Highest operating temperature °C

Product-side base rate mm/yr

Product-side condition (Table 2.B.14.11)

Steam coil heater (Table 2.B.14.13)

Water drawn weekly or after every receipt (Table 2.B.14.14)

Product-side corrosion (2.B.14.3.3)

Model the bottom edge (TANKBOTEDGE, the 600–750 mm next to the shell) separately (Part 5 4.2)

Edge thickness, t_rdi mm

Annular ring / critical zone t_min (API 653) mm

Edge corrosion rate mm/yr

Edge: last inspection effectiveness

4. Shell Courses: Thinning DF and Consequence (Part 2 4.5.7; Part 5 4.5 – 4.18)

Last shell thickness survey

Shell yield strength MPa

Shell tensile strength MPa

Joint efficiency E

Corrosion rate confidence (Part 2 Table 4.5)

Shell inspections since the survey: A

Course 1 is the bottom course. Leave t_min blank to use API 653 4.3.3.1 with the product’s specific gravity and the Table 4.1 allowable stress; that stress is also used in the strength ratio (Part 2 Eq 2.14). Count the A–D inspections done since the survey date (Part 2 Step 8).

Course	t_rdi mm	Rate mm/yr	t_min mm	Liquid above	t_min used	A_rt	DF now	COF, US$	Risk now, US$/yr

5. Risk and Inspection Plan (Part 1 4.3; Part 4 6.4)

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