The Repair Design Check tests a repair or alteration to an existing tank against API 653 Section 9 (5th edition, 2014, Addendum 4, July 2025) before the work starts. Choose one of six repair types:
Shell plate replacement or insert (9.2) and door sheets (9.2.4): thickness, size, corner radius and the Figure 9.1 weld spacing.
Lapped patches on the shell (9.3): which of the three uses it is, its thickness by design with the joint efficiency 9.3 sets, the thickness limits, size and location.
New or replacement shell penetrations (9.9): the required shell thickness for the reinforcement area (9.9.2), stress relief (9.9.3), insert plates (9.9.6).
Welded-on bottom patches (9.11.1): inside and outside the critical zone.
Hot taps (9.15): Table 9.1, the extra limits for shell of unknown toughness, spacing, and the nozzle pressure test.
Before the work, not instead of approval
Every repair is approved before work starts by an authorized inspector or a storage tank engineer (9.1.3), on the basis of API 650 equivalence (9.1.1). The tool checks the limits Section 9 prints so the approval goes faster; it is an indicative check, not the approval.
Quick start
Choose a sample beside Summary of checks and press Load sample: each one is a repair on TK-101 (36 m × 13.8 m, G 0.9) that passes, so you can change one value at a time and watch the check that governs it.
1
Identification: Tank ID and a repair item reference (the weld map number works well).
2
Section 1: the repair type, D, the maximum liquid level, G and the height of the repair above the bottom (lowest point; centreline for a nozzle or hot tap). Then the shell: construction, toughness, material group, the course and its nominal thickness.
3
Section 2 shows only the inputs for the repair you chose. Distances are toe to toe of the welds, as Section 9 measures them.
4
Read the summary: each check names its clause. Tick the work record items as they are confirmed.
5
Calculate & Save, then Print / PDF report. For a job with several items, save each under its own reference and print them together from the Tank Repair List.
SI or US customary: two rule sets
API 653 prints every limit twice, and the two are not exact conversions: 13 mm against 1/2 in. (12.7 mm), 300 mm against 12 in. (304.8 mm), 48 MPa against 7000 lbf/in.² (48.3 MPa). Like the Hydrostatic Test tool, the Repair Design Check uses the set you choose in Units and rule set, so the result matches the column of the standard you work to. Records are stored in SI either way.
Limit
SI
US customary
Thin / thick shell plate
13 mm
1/2 in.
Least replacement plate, A
300 mm
12 in.
Lapped patch: smallest, largest
5 mm, 13 mm
3/16 in., 1/2 in.
Critical zone; patch there at most
75 mm; 6 mm
3 in.; 1/4 in.
Hot tap liquid head; stress, unknown toughness
0.9 m; 48 MPa
3 ft; 7000 lbf/in.²
Replacement plates and door sheets (9.2)
The Figure 9.1 letters as the tool uses them.
Letter
t ≤ 13 mm (1/2 in.)
t > 13 mm (1/2 in.)
Sample, 22 mm
R, corner radius
150 mm (6 in.)
greater of 150 mm (6 in.) or 6t
150 mm
B, to the shell-to-bottom weld
150 mm (6 in.)
greater of 250 mm (10 in.) or 8t
250 mm
H, to horizontal welds
75 mm (3 in.)
greater of 250 mm (10 in.) or 8t
250 mm
V, to vertical welds
150 mm (6 in.)
greater of 250 mm (10 in.) or 8t
250 mm
A, least plate dimension
300 mm (12 in.)
greater of 300 mm (12 in.) or 12t
300 mm
C, new vertical weld to bottom plate welds
greater of 75 mm (3 in.) or 5t
same
110 mm
t is the replacement plate thickness. 9.2.3.3 keys the same split on the existing plate, so if either plate is over 13 mm (1/2 in.) the tool uses the heavier column.
The plate is at least as thick as the thickest adjoining plate in the course, unless that plate is a thickened insert (9.2.1). Enter the thickness the as-built standard needs (from Shell Design, with the present G and level) to check it too.
When the plate crosses a horizontal seam, it extends past it by at least the greater of R and H. When it runs down to the shell-to-bottom joint at about 90°, B no longer applies and C does.
Radiography (12.2.1.6): circular plates at least one film; rectangular plates and door sheets one in a vertical joint, one in a horizontal joint and one at each corner; over 25 mm (1 in.) full length. The tool states the count for the Shell Radiograph Count checklist.
Door sheets: never across vertical riveted or lap-welded seams (9.2.4.5); in a lap-welded or riveted shell the original section may not go back across a horizontal seam (9.2.4.4), so a two-plate assembly of new plate is used (9.2.4.8); in a butt-welded shell a vertical offset is at least V (9.2.4.7), and without an offset the new welds and the crossed seam get extra MT or PT (9.2.4.6).
Lapped patches on the shell (9.3)
Use
Joint efficiency
Thickness limits
Closing a hole: a removed opening or a cut-out corroded area (9.3.2)
0.70
at least 5 mm; at most the smaller of 13 mm and the adjacent nominal plate; overlap 25 mm to 8t; hole at least 50 mm
Reinforcing a deteriorated plate, incl. below retirement thickness (9.3.3)
0.35
at most the perimeter thickness + the lesser of a third and 3 mm, and 13 mm; the plate under it not counted
Small leaks or isolated pitting (9.3.4)
0.35
as 9.3.3, and at least 5 mm; the shell away from the pits meets tmin
The thickness by design is the API 653 4.3.3.1 b) equation with the joint efficiency of the use: t = 4.9 H D G / (S E) (2.6 H D G / (S E) in US units), H from the lowest point of the patch to the maximum liquid level, S the smaller of 0.80 Y or 0.429 T in the bottom two courses and 0.88 Y or 0.472 T above, with Y = 205 MPa and T = 380 MPa when the material is unknown.
Thickness by design for the sample tank, S = 188.8 MPa.
The sample reinforcing patch at 12.0 m needs 4.32 mm; the 8 mm patch on 6.2 mm plate passes. Move it to 9.4 m on 6.5 mm plate and it needs 10.6 mm, but may be at most 8.67 mm: the tool says a lapped patch does not work there and points you to a replacement plate (9.2).
Only on a course of 13 mm (1/2 in.) or less as built (9.3.1.2), only when the owner/operator specifies it (9.3.1), and never to replace a door sheet or shell plate.
Size: at most 1.2 m (48 in.) high and 1.8 m (72 in.) wide, at least 100 mm (4 in.); corners rounded to 50 mm (2 in.) except at the shell-to-bottom joint (9.3.1.4, 9.3.1.7).
It may cross butt seams ground flush, overlapping at least 150 mm (6 in.) past them; never lap, riveted or other patch seams, distorted areas or unrepaired cracks; no opening inside it; Figure 9.1 spacing to other welds and patches (9.3.1.5, 9.3.1.8, 9.3.1.10). Inside the shell, 150 mm (6 in.) clear of the shell-to-bottom weld (9.3.1.6).
New or replacement shell penetrations (9.9)
Step
Rule
Sample: NPS 16 at 0.75 m, 28 mm thickened insert
Required shell thickness (9.9.2)
4.3.3.1 b): 4.9 H D G / (S E), H from the centreline to the maximum level, S from API 650 Table 5.2 (138 MPa, 20,000 lbf/in.², if unknown), E 1.0
4.9 × 13.05 × 36 × 0.9 / 160 = 12.95 mm
Required area (API 650 5.7.2.1)
vertical hole dimension × that thickness
412 × 12.95 = 5,335 mm²
Stress relief (9.9.3, API 650 5.7.4)
Group I – IIIA: NPS 12 and over in plate over 25 mm; Group IV – VI: over NPS 2 in plate over 13 mm. Group not confirmed: the Group IV – VI rule
required: 28 mm Group I plate
Insert plate (9.9.6)
over NPS 4 in shell over 13 mm that does not meet the current design metal temperature: an insert at least the greater of 2 × the opening and the opening + 300 mm (12 in.)
1,050 mm against 824 mm
Major alteration (3.25 a)
over NPS 12 below the design liquid level
yes
Nozzle Reinforcement does the full API 650 5.7 design: tick “use t_req” there, enter the thickness from this tool, and bring the area provided back here.
A penetration on a shell joint needs Purchaser acceptance and the joint radiographed for three times the opening diameter (9.9.5, API 650 5.7.3.4).
Welded-on bottom patches (9.11.1)
Figure 9.13 rules as the tool applies them.
Where
Rules
Anywhere
Least dimension 300 mm (12 in.) over a seam or patch; smaller patches at least 150 mm (6 in.), not over seams or patches, and 50 mm (2 in.) past the corrosion. Corners rounded 50 mm. Edges parallel to seams or other lap welds 50 mm away. Over a three-plate lap: 300 mm along each lap weld and crossing at 45° or 90°; otherwise lap seams crossed at 45° or more, 100 mm (4 in.) from the next three-plate lap. Over a dent: span at most 300 mm, at least 6 mm (10 mm for Annex M) and the bottom thickness.
Within 150 mm (6 in.) of the shell
Tombstone shape, sides square to the shell-to-bottom joint (9.11.1.2 b).
In the critical zone, 75 mm (3 in.) from the shell (3.11)
At most 6 mm (1/4 in.) thick; not over a patch or butt-welded to one; at most 600 mm (24 in.) along the shell; patches apart by half the shorter one; unknown toughness: 75 mm or 5t from vertical shell welds; operating at most 93 °C (200 °F) carbon steel, 38 °C (100 °F) stainless; bottom under the weld at least 2.5 mm (0.10 in.).
A patch thinner than the bottom is noted, not failed: 9.11.1.1 b) asks for the 4.4.5.1 calculation and owner approval.
The remaining bottom at the toe of the inside shell-to-bottom weld must still meet 4.4.5 and 4.4.6 at the next inspection (9.11.1.2.2): a record item, checked in Inspection Interval or Annular Plate Ring.
Hot taps (9.15)
Check
Recognized toughness
Unknown toughness
Size and shell thickness
Table 9.1: NPS 6 and under 5 mm (3/16 in.); NPS 8 6 mm; 10 8 mm; 14 10 mm; 16 11 mm; 18 13 mm (1/2 in.). Between rows, the next larger size. Over NPS 18: outside the table.
at most NPS 4
Liquid above the tap
at least 0.9 m (3 ft) during the work (9.15.1.2)
and the hoop stress 4.9 H D G / t below 48 MPa (7000 lbf/in.²): for 8 mm shell on the sample tank, H at most 2.42 m
Reinforcing plate
not across seams or to the shell-to-bottom weld (9.15.1.7); 300 mm (12 in.) from vertical welds (Figure 9.14); API 650 Figure 5.6 to horizontal welds and the bottom
also at least the shell thickness and the cutout + 50 mm (2 in.) across
Spacing to other nozzles
√(R T) toe to toe (9.15.3.1)
same
Four shell thickness readings around the location (9.15.3.2): the tool uses the least.
Sample: NPS 8 at 1.2 m in 20.6 mm shell; √(R T) = √(18,000 × 20.6) = 609 mm; nozzle pressure test before the machine goes on, at least 1.5 × the hydrostatic head = 167 kPa (9.15.5.3).
Where API 650 5.7.4 would call for stress relief, a hot tap is still allowed without it, but only with a controlled-deposition procedure (11.3.2) qualified with water backing (9.15.1.1). Not on the roof or in the vapour space, not on laminated or pitted plate, not where welding heat can cause caustic or stress corrosion cracking.
Major repairs, radiographs and the hydrotest
Result
Where it goes next
Major repair or alteration (3.25)
A replacement plate or door sheet below the design liquid level longer than 300 mm (12 in.), or a penetration or hot tap over NPS 12 below it. The tool says yes or no; the Hydrostatic Test tool in its existing-tank mode decides the test or the 12.3.3 exemption.
Radiograph count
Construction Checklists: API 653 Shell Radiograph Count (12.2.1) for the repair welds, and API 650 Shell Radiograph Count (8.1.2, Figure 8.1) for new-to-new joints.
Examinations
Construction Checklists: API 653 Repair Examination Checklist (12.1, 12.4).
Tank repair list: all items in one report
A repair job on one tank usually has many items. Check and save each one with its own repair item reference (R-01, R-02 …), then press Load the repair list for this tank in the Tank Repair List card. The list shows the latest save of each reference; an earlier save of the same item is counted but left out, so a corrected check replaces the first one.
The list adds up
From
Items with failing checks, and checks still needing input
each item’s saved checks
Hydrostatic test: required unless exempt under 12.3.3 when any item is a major repair or alteration (3.25)
the major-repair result of plates, door sheets, penetrations and hot taps
What to enter in the API 653 Shell Radiograph Count: circular plates, rectangular plates and door sheets, insert plates, and which go full length (its Fill the plate quantities from the tank repair list button reads the same items)
plate shapes and thicknesses; penetrations in an insert
Hot tap nozzle test pressures (9.15.5.3)
each hot tap, in its own units
Work record items confirmed and open
each item’s ticks
Untick an item to leave it out of the totals and the report; Open loads it into the page to change and save again.
Fill in the work order, contractor and inspector, then Print tank repair report: a cover page with the register, what the job needs and a sign-off block for the contractor, the authorized inspector or storage tank engineer, and the owner; then one page per item with its checks, rows, sketch and work record.
Each item is re-checked from its saved inputs, in the rule set it was saved in, when the report is made; the item you are working on is put back afterwards.
Nothing extra is saved: the list is read from the saved items, so it is always current.
Filling from the other tools
Tool
What it gives
Shell Design
D, design liquid level, G; at the height of the repair: the course, its thickness as designed, the thickness required and its allowable stress Sd
Shell Evaluation
D, maximum liquid level, G, riveted construction; at the height of the repair: the course t1 for the patch perimeter and the shell away from the pits (check it at the patch)
Enter the height of the repair first: the course values depend on it.
The latest record of each tool for the same company and Tank ID is used; the table under the button shows what was filled.
9.9.2, 9.9.3 with API 650 5.7.4, 9.9.5, 9.9.6, 3.25 a
Bottom patch
9.11.1.1 a – e, i – l; 9.11.1.2 a – j; 9.11.1.2.3
Hot tap
9.15.1.1 a, b; 9.15.1.2; 9.15.1.4; 9.15.1.7; 9.15.3.1; Figure 9.14 with API 650 5.7.3; 9.15.5.3
All
Work record: 9.1.3 – 9.1.7 and the Section 9 and 12 items for the repair
How the numbers were checked
Figure 9.1 was read from the published table for both columns and both unit sets; the test suite checks R, B, H, V, A and C at 10, 22, 30 and 40 mm and at 7/8 in.
Each sample is recomputed by hand in the tests and in this manual’s build: the lapped patch thickness, the 9.9.2 thickness and area, √(R T), the test pressure and the unknown-toughness liquid limit.
Every check was made to fail in turn, in SI and in US units: 89 tests, plus 25 for the tank repair list (latest save per item, totals, the combined report, and the page put back afterwards).
Saving, reopening and reports
Inputs are kept on this device as you type and offered back if the page closes; Calculate & Save stores one record per repair item; Show saved checks lists them with their type and whether they are major.
Print / PDF report: the summary, the rows and sketch for the repair, the work record and the inputs.
Open the tool on a type with a link: repair-design-checker.html?type=htap (also plate, door, lap, pen, bpatch).
Limits and interpretations
Nonmetallic repairs (9.4, Annex J), shell height changes (9.6), fixed and floating roof repairs (9.12 – 9.14) and complete new bottoms (9.11.2) are not checked.
The lapped patch design uses the 4.3.3.1 b) hoop equation with the joint efficiency of 9.3. API 653 gives no equation for a patch over a hole carrying the pressure across it (9.3.4.2); for a large hole check plate bending too.
For a penetration the tool finds the required thickness and area; the reinforcement and API 650 5.7.3 weld spacing are designed in Nozzle Reinforcement.
Figure 9.1: the heavier column applies if either the replacement or the existing plate is over 13 mm (1/2 in.), following 9.2.3.3.
Table 9.1 between rows: the next larger connection size. Hot tap spacing to horizontal welds and the bottom uses API 650 Figure 5.6 for an as-welded opening, as Figure 9.14 refers to.
The tank repair list reads the saved items; an item not saved, or saved without a repair item reference, appears as its own line.
Critical-zone rules apply when the patch comes within 75 mm (3 in.) of the shell; the tombstone rule within 150 mm (6 in.).
Glossary
Recognized / unknown toughness
Section 3: plate shown to meet the toughness rules, or not.
Critical zone
The bottom or annular plate within 75 mm (3 in.) of the inside of the shell (3.11).
Door sheet
A plate cut from the shell for temporary access and put back or replaced (3.13).
Major repair / alteration
Work listed in 3.25 that triggers the hydrostatic test unless exempt (12.3).
Toe to toe
Distance between the edges of two welds, as Section 9 measures spacing.
√(R T)
Hot tap spacing: shell radius times shell thickness, square-rooted.