Measuring and Judging One Inch of Differential Settlement on a Ringwall Tank
One inch is not a verdict. API 653 Annex B splits measured settlement into uniform settlement, planar tilt, out-of-plane shell settlement, edge settlement and localized bottom depressions, and each has its own limit. On a 100 ft by 40 ft tank, permissible out-of-plane deflection is about two inches, while permissible edge settlement over a narrow settled band can be under one.
Annex B is normative, and it is a decomposition procedure before it is an acceptance criterion. Elevations around the shell are reduced to a uniform component, a rigid tilt plane fitted as a cosine curve, and the residual out-of-plane deflection that actually stresses the shell and the shell-to-bottom joint. Only the residual is compared against B.3.2.1, and the residual at a point is that point's deviation from the average of its two neighbors, not its raw depth below datum. Edge settlement and localized depressions are judged separately, against charts and against a 0.37R rule, with no arithmetic in common with the shell criterion. An inch of differential settlement can therefore be comfortably inside the shell limit while an inch of edge settlement over a narrow band around the periphery is already a repair. The tank in question needs its survey decomposed before anyone argues about repair.
Source: Verified against API Standard 653, Tank Inspection, Repair, Alteration, and Reconstruction, 5th edition, November 2014, with Addendum 1 (April 2018), Addendum 2 (May 2020) and Addendum 3 (November 2023) — clauses 4.5, 6.3.1, 6.4.1, 10.5.6, 12.5.1, 12.5.2, 12.5.3 and Annex B (normative), B.1 through B.4, including Figures B.1, B.2, B.3, B.6, B.7, B.10, B.11 and B.12. Edition confirmed against API's ICP Publications Effectivity Sheet for the March, July and November 2026 API 653 exam administrations. The B.3.2.1 equation is attributed by API to Marr, Ramos and Lambe, Criteria for Settlement of Tanks, ASCE Journal of Geotechnical Engineering, Vol. 108, August 1982; the B.3.2.2 equation to Andreani and Carr, Final Report on the Study of Out-of-Plane Tank Settlement, report to API SCAST, May 2007.
| Tank diameter (ft) | Shell height (ft) | Points N per 12.5.2 | Arc length L (ft) | Smax at Y = 30,000 psi (in.) | Smax at Y = 36,000 psi (in.) |
|---|---|---|---|---|---|
| 40 | 32 | 8 | 15.7 | 0.53 | 0.63 |
| 60 | 40 | 8 | 23.6 | 0.95 | 1.14 |
| 80 | 32 | 8 | 31.4 | 2.10 | 2.52 |
| 100 | 40 | 10 | 31.4 | 1.68 | 2.02 |
| 120 | 48 | 12 | 31.4 | 1.40 | 1.68 |
| 160 | 56 | 16 | 31.4 | 1.20 | 1.44 |
| 200 | 64 | 20 | 31.4 | 1.05 | 1.26 |
One inch of what? The five components Annex B separates
Annex B of API 653 is normative and covers evaluation of tank bottom settlement. Its first move is not to judge anything. Clause B.2.2 divides shell settlement into three components: uniform settlement, rigid body tilt described as planar tilt, and out-of-plane settlement. Clause B.2.3 handles edge settlement, where the shell settles sharply around the periphery and deforms the bottom plate near the shell-to-bottom corner. Clause B.2.5 handles localized bottom settlement remote from the shell, meaning depressions and bulges occurring in a random manner.
Each of the five has a different consequence and a different limit. Uniform settlement induces no stress in the tank structure, and Annex B directs attention instead to piping, nozzles and attachments. Planar tilt raises the liquid level on the low side, increasing hoop stress in the shell, and can bind the peripheral seal on a floating roof and inhibit roof travel. Out-of-plane settlement is the component that produces internal stresses in the structure and drives out-of-roundness at the top of the shell.
This is why an owner reporting roughly one inch of differential settlement across two inspection cycles cannot be answered from that number. One inch distributed as planar tilt across a 120 ft tank is a hydraulic and roof-clearance question. One inch of out-of-plane residual on a short, wide tank is inside the criterion. One inch of edge settlement across a narrow band is a repair conversation. The tank inspection interval framework sets when you look; Annex B sets what the looking means.
The survey has to be valid before the number means anything
Clause 12.5.2 defines the point count: N = D/10 with D in feet, rounded up to the next higher even whole number, never fewer than eight, uniformly distributed around the circumference, with maximum spacing of 10 m (32 ft). Figure B.1 repeats those constraints for external shell measurements and requires at least four equally spaced diametrical measurement lines. Figure B.2 governs internal bottom measurements with the tank out of service and adds a maximum spacing of 3 m (10 ft) across the diameter.
Clause B.2.1 adds requirements that decide whether two cycles of data are comparable at all. Measurements are to be performed by personnel experienced in the procedures, using equipment capable of sufficient accuracy to distinguish settlement differences, and settlement measurement locations should be re-used in any future settlement surveys and evaluations. A trend built from points that moved between surveys is not a trend.
B.2.1 also names the systematic error that corrupts shell surveys. Where the bottom is distorted or corroded beyond the shell, measurements taken near bottom lap welds can produce significant errors in measured elevation, and repaired plates, replaced plates or slotted-in bottoms may not sit parallel to the original bottom. The annex suggests surveying the elevation of the weld between the first and second shell courses instead. For bottom and edge measurements, readings taken while the bottom is not in contact with the soil can overestimate or underestimate; where the result is near the allowable, the annex asks for the measurement to be repeated with the bottom forced down onto the soil.
Subtract the two components that do not stress the shell
The construction in Figure B.3 works in a fixed order. The actual settlement is plotted with points around the circumference as the abscissa. The vertical distance from the abscissa to the lowest point on the curve is the minimum settlement, and it is called the uniform settlement component; a line through that point parallel to the abscissa becomes the datum for adjusted settlements. What remains after that subtraction is everything except the uniform component.
The plane of rigid tilt is then represented by the optimum cosine curve fitted through the adjusted data. Vertical distances between the irregular measured curve and that cosine curve are the out-of-plane settlements, written Ui at data point i. Uniform settlement and tilt have been removed by construction, not by judgment, and only what is left is compared against the structural criterion.
That ordering explains why raw survey spreads mislead. A tank with an eight-inch spread between its highest and lowest elevation points can carry almost all of that as uniform settlement and tilt and be structurally comfortable, while a tank with a two-inch spread concentrated at two adjacent points can fail B.3.2.1. Anyone reading a settlement report should look for the fitted cosine curve before reading any headline number, which is a useful first check for any reader of a settlement report.
The cosine fit and the R-squared gate
Clause B.2.2.4 e) states the accepted method: solve for constants a, b and c to find the optimum cosine curve of the form Elevpred = a + b cos(theta + c), starting from a least-squares fit chosen to minimize the sum of the squares of the differences between measured and predicted elevations. The curve is only considered valid, meaning it accurately fits the measured data, where R squared is greater than or equal to 0.9, with R squared equal to (Syy minus SSE) divided by Syy.
The annex is explicit about what a low R squared means. Where out-of-plane settlement concentrates in one or two areas, the least-squares fit under-predicts the local settlement and is not conservative, and R squared will typically fall below 0.9. In that situation the annex directs the engineer to more rigorous curve-fitting procedures, or to the alternative procedure in B.2.2.5 where the settlement does not indicate a well-defined rigid tilt plane at all.
This is a gate, not a formality. A report that presents a cosine curve without stating R squared has not demonstrated that the tilt plane it subtracted was real, and every out-of-plane number downstream of that fit inherits the doubt. Obtaining a statistically valid curve may require more measurement points than the Figure B.1 minimum, which is a survey planning decision, not a desk exercise after the fact.
S is not the settlement you measured
The single most misread step in Annex B is the difference between Ui and Si. Ui is the out-of-plane settlement at point i, the vertical distance from the measured curve to the fitted cosine curve. Si is the out-of-plane deflection at that point, and Figure B.3 defines it as Si = Ui minus the average of its two neighbors: for point 11, S11 = U11 minus one half of U10 plus one half of U12.
Si is what gets compared against the permissible value. A uniform sag spread evenly around several adjacent points produces large Ui values and small Si values, because each point's neighbors have moved with it. A single sharp local dip produces a large Si. The criterion is about local curvature of the shell, and reporting Ui as though it were Si will fail tanks that are fine.
The note to B.2.2.4 e) closes a second trap. Adding measurement points improves the cosine fit, but feeding all of them into the B.3.2.1 equation shortens the arc length L and shrinks the allowable. The annex permits all points to be used for the curve fit while a subset spaced no further than 32 ft, with eight as the minimum, is used to calculate Si and Smax, provided the subset includes the points furthest from the fitted curve. With 16 points taken at 15 ft spacing where 8 are required, the equation becomes Si = Ui minus one half of Ui-2 plus one half of Ui+2.
The out-of-plane limit, and why height beats diameter
Clause B.3.2.1 gives an equation for the permissible out-of-plane deflection where the cosine-curve approach was used. In words: the allowable grows with the square of the arc length between survey points and with the yield strength of the shell, and falls as tank height and Young's modulus rise. Use the equation and units exactly as printed in your licensed copy of API 653. API attributes the equation to Marr, Ramos and Lambe, published by ASCE in 1982.
Combine that with the point-count rule and a non-obvious result falls out. Since N = D/10, the arc length L is pi times D divided by N, which reduces to about 31.4 ft for every tank of 80 ft diameter or larger. Diameter cancels. For any large tank the permissible out-of-plane deflection is a function of shell height and material yield alone, which is why the table above shows a 200 ft by 64 ft tank with a tighter limit than an 80 ft by 32 ft tank. Below 80 ft diameter the eight-point minimum takes over, L shrinks with diameter, and small tanks end up with the tightest limits of all.
Run it once for the tank in question. A 100 ft diameter, 40 ft tall tank in A36 material gives L = 31.4 ft, and Smax = 11 times 986 times 36,000 divided by (2 times 29,000,000 times 40), which is 0.168 ft, or 2.02 in. In A283 Grade C at 30,000 psi the same tank permits 1.68 in. An inch of local out-of-plane deflection on that tank is inside the criterion with margin, and the conversation moves to the edge and the bottom.
When there is no tilt plane: B.2.2.5 and the K equation
Clause B.2.2.5 applies where a well-defined rigid tilt plane cannot be determined, or where the B.3.2.1 limit is exceeded and the owner wants an alternative to rigorous analysis or repair. Instead of a fitted curve, B.2.2.5.1 works from the plotted profile: an initial settlement arc length Sarc and maximum settlement are read from the points where the settlement slope changes direction, and additional measurement points halfway between those points refine both the arc length and the location and magnitude of the maximum. The refinement step may be repeated.
Clause B.2.2.5.3 adds a rule for one specific pattern: where the plot indicates a fold pattern about a diameter of the tank, the maximum out-of-plane settlement should be determined using a settlement arc length of 50 percent of the tank circumference.
Clause B.3.2.2 then supplies an acceptance equation that combines a tabulated factor K with the settlement arc length, tank diameter and height, and the shell yield strength and modulus, subject to a fixed upper ceiling. K is tabulated in API 653 by diameter band and roof type (open-top and fixed-roof tanks are treated differently, and the table does not cover every diameter). Read the factor and ceiling from your licensed copy rather than from secondary sources. Clause B.3.2 also excludes several geometries from both criteria, including abrupt ridges, discontinuities such as low nozzles in the settled region, fold patterns in cone roof tanks where the fold line runs adjacent to or through a line of roof columns, and combined shell and edge settlement patterns.
Edge settlement is where an inch usually fails
Clause B.2.3.3 splits the allowable in two. Bew is the allowable where the settled area contains a bottom lap weld essentially parallel to the shell, within plus or minus 20 degrees, and it is read from Figure B.11. Be is the allowable for areas with no bottom welds, with butt welds, or with lap welds essentially perpendicular to the shell within plus or minus 20 degrees, read from Figure B.12. Both charts plot allowable settlement in inches against R, the radius of the settled area, with curve families for tank diameters from 20 ft up to 160 ft and larger. Since Bew is the more conservative of the two, the annex suggests evaluating every settled area against Bew first and only splitting them out if that fails.
Clause B.3.4.4 handles welds at an arbitrary angle by interpolation: B equals Be minus (Be minus Bew) times sine of the angle of the weld to a tank centerline. Clauses B.3.4.1 and B.3.4.3 set an inspection trigger well below the allowable. Where measured settlement B exceeds 75 percent of the allowable, all shell-to-bottom welds and bottom welds should be examined visually and with magnetic particle or liquid penetrant methods, and all indications repaired or evaluated for risk of brittle fracture and fatigue before the tank returns to service. Clause B.3.4.2 extends visual examination to any welds within 300 mm (12 in.) of either side of the breakover area.
Clause B.3.4.6 states the basis and the limits of these charts: they were developed for typical 6 mm (1/4 in.) thick bottoms with minimal corrosion, can be applied with reasonable accuracy to 8 mm and 10 mm bottoms, and can be applied to generally corroded bottoms provided areas near all welds remain thicker than 5 mm (3/16 in.). For locally corroded bottoms, thin areas below 5 mm within the settled zone must be smaller than 300 mm (12 in.) across and must not include a weld. A corroded lap-welded floor around the periphery puts the tank outside the chart basis, and that is an engineering evaluation, not a chart lookup.
Localized depressions and bulges: the 0.37R rule
Clause B.3.3 covers depressions and bulges remote from the shell with a single equation: BB = 0.37R, where BB is the maximum height of the bulge or depth of the local depression in inches and R is the radius of the inscribed circle in the bulged or depressed area in feet. In SI the same relationship is written BB = 0.031R with both terms in millimeters. Figure B.10 presents it graphically, with the caption stating that repairs or a rigorous assessment should be conducted where the observed depth or height sits above the line.
The geometry input is the inscribed circle radius, not the width of whatever the crew measured across. A shallow dish 20 ft across permits 0.37 times 10, or 3.7 in. of depth. The same depth over a 3 ft inscribed radius permits only 1.1 in. and fails. Clause B.2.5.2 notes that acceptability depends on localized stresses in the bottom plate, the design and quality of the lap welds as single-pass or multi-pass, and voids below the bottom plate, and that the limits apply to bottoms with single-pass lap-welded joints.
Figure B.10 carries two curve families, one for a depression or bulge anywhere in the bottom and one for a partial ring-type depression or bulge at the edge only. Reading the wrong family is a common error in reports, and it is one of the items worth catching during independent review of an inspection report rather than during the next turnaround.
What triggers repair, and the trap in a two-cycle trend
Clause B.3.1 frames the whole annex honestly: the methods are not mandatory and approximate the maximum permissible settlement, and experience has shown that where settlements exceed them, further assessment or repair is required. Clause B.3.2.4 preserves the alternative, allowing a more rigorous evaluation by an engineer experienced in tank settlement analysis in place of repair. Clause B.4.2 describes what repair means when it is required: plate exceeding acceptable strains, typically 2 to 3 percent, should be replaced, re-leveling will not remove the plastic strain, welds in high-strain areas should be removed and replaced or subjected to a fitness-for-service evaluation, and the condition that caused the settlement should be corrected. Jacking and leveling are done in conjunction with plate and weld replacement, not instead of it.
Now the part that decides the reader's case. Clause B.3.4.5 states the usual practice of comparing measured edge settlement against the allowable without adding an allowance for further settlement, on the presumption that most settlement occurred in the first few years. It then names the exception: where significant additional settlement is expected, an engineer experienced in tank settlement evaluation should evaluate the settlement expected at the next inspection against the B.3.4 limits, and the annex calls this analogous to a corrosion allowance. An inch accumulated across two inspection cycles is evidence of active settlement, which puts the tank squarely in that exception. The comparison has to be made against the projected value at the next due date, not against today's reading. Clause B.3.4.5 also flags erosion of the pad adjacent to the tank as a cause that will continue until the pad is repaired.
One number keeps this in proportion. Clause 10.5.6.2 sets the construction tolerance for a foundation true to a horizontal plane: where concrete ringwalls are provided, the top of the ringwall shall be level within plus or minus 3 mm (1/8 in.) in any 9 m (30 ft) of circumference and within plus or minus 6 mm (1/4 in.) over the total circumference measured from the average elevation. An inch is four times the total construction tolerance and a fraction of the in-service acceptance limit, which is exactly why it alarms operators and why the decomposition, not the headline number, settles the question. Where the projection crosses a limit, the route forward is an engineering evaluation (which may include an API 579-1 fitness-for-service assessment, which Atlantis NDT does not perform) or a defined repair, and the survey interval belongs in the plan alongside the external inspection clocks. Atlantis NDT can help collect and organise settlement survey data; contact us about data collection; the evaluation itself belongs to the authorized inspector and, where needed, a tank engineer.
How many elevation points does an API 653 settlement survey need?
Clause 12.5.2 sets the minimum number of points as N = D/10 with D in feet, rounded up to the next higher even whole number and never fewer than eight, with maximum spacing between points of 10 m (32 ft). Figure B.1 repeats the eight-point minimum and the 32 ft spacing for shell surveys. Figure B.2 governs internal bottom surveys with a maximum spacing of 3 m (10 ft) across the diameter and at least four equally spaced diametrical measurement lines.
Is one inch of differential settlement acceptable under API 653?
The question has no answer until the inch is classified. One inch of out-of-plane shell deflection on a 100 ft diameter, 40 ft tall tank in 36,000 psi material sits inside a permissible value near two inches. One inch of edge settlement across a settled band a foot or two wide, in an area containing a bottom lap weld parallel to the shell, can exceed the Figure B.11 allowable and require repair or detailed analysis.
What is the R-squared test in the cosine curve fit?
Clause B.2.2.4 e) fits an optimum cosine curve of the form Elevpred = a + b cos(theta + c) by least squares, then tests the fit with R squared equal to (Syy minus SSE) divided by Syy, where Syy is the sum of squares of differences between average measured elevation and measured elevations and SSE is the sum of squares of differences between measured and predicted elevations. The curve is valid only where R squared is at least 0.9.
How is edge settlement measured correctly?
Clause B.2.3.2 directs that the breakover point is found by laying a straight edge on the unsettled floor and observing where the floor separates from it, with B measured as shown in Figure B.6. Where the tank floor is cone up or cone down, Figure B.7 requires B to be measured from a projection of the unsettled floor rather than from level. Measurements taken while the bottom is not in contact with the foundation can overestimate or underestimate significantly.
When does API 653 actually require a settlement survey?
Clause 12.5.1 makes one mandatory: a settlement survey shall be conducted for all existing tanks that undergo a hydrostatic test, except tanks with a documented service history of acceptable settlement values where no settlement is anticipated during the hydrotest. Everything else is scheduled by the owner. Annex B.1.1 directs that measurements be taken at a planned frequency based on an assessment of soil settlement predictions, and on prior service history where no baseline data exists.
What does exceeding an Annex B limit actually require?
Clause B.3.1 states that the methods are not mandatory and approximate the maximum permissible settlement, and that experience has shown further assessment or repair is required where settlements exceed them. Clause B.3.2.4 permits a more rigorous evaluation by an engineer experienced in tank settlement analysis in place of repair. Clause B.4.2 requires plate exceeding acceptable strains, typically 2 to 3 percent, to be replaced, and notes that re-leveling does not remove plastic strain.
Running the settlement programme: frequency, field survey practice and the NDE that follows
Short answer: API 653 does not give one calendar frequency for settlement surveys. Annex B says surveys during operation should follow a planned frequency, based on predicted soil settlement or, for older tanks with no baseline, on service history. A full settlement survey is mandatory around a hydrostatic test, unless the tank's history shows settlement is acceptable and none is expected. Inch-scale movement across two cycles counts as active settlement. That calls for a shorter, trend-based survey interval set by the owner and the authorized inspector, plus an engineer's projection to the next inspection date.
The sections above explain how to break a survey into its Annex B components and judge each one. This section covers the practical side: survey frequency, repeatable data, ringwall behaviour, the NDE that follows, and who decides. It paraphrases API Std 653. Check limits and clause numbers against your licensed copy of the current edition.
Check the edition first: Addendum 4 (July 2025)
The source note near the top of this article lists API 653, 5th edition, with Addenda 1 through 3. API has since issued Addendum 4 (July 2025), which revised the text on tank-bottom thickness measurement. The Annex B settlement method is still the framework, but before you sign an evaluation, confirm the edition and addendum your inspection programme follows, and check whether your jurisdiction or contract fixes a specific edition. For example, federal pipeline rules incorporate a dated edition by reference. API's own standards pages are the place to confirm what is current. Do not rely on an older edition copied online. Those copies are often decades out of date.
How often to survey: build the frequency, don't look it up
Annex B treats survey frequency as engineering planning, not a fixed interval. Predicted settlement depends on soil type, fill history, foundation design and tank age, and the owner is expected to set the frequency from those factors. The 2003 edition, which is publicly available because federal regulation incorporates it, already said this. It also said that tanks with no baseline data should be monitored on the basis of their prior service history. Later editions kept that approach.
In practice, most owners combine planned surveys with surveys triggered by events. A defensible plan usually includes the following triggers, each with a reason recorded in the tank file:
| Trigger | Survey normally taken | Why it matters |
|---|---|---|
| Hydrostatic test after repair, alteration or reconstruction | Shell elevation survey before filling, during filling and at full test level | API 653 requires settlement measurement during a hydrotest unless a documented history shows acceptable settlement with none expected |
| Out-of-service internal inspection | Internal bottom elevation survey plus edge breakover measurements | This is the only time bottom depressions, bulges and edge settlement can be measured directly |
| Routine in-service or external inspection finds signs of movement | Shell elevation survey at the established points | The visual signs are shell-to-foundation gaps, ringwall cracking, water ponding at the chime, or binding on a floating-roof seal |
| Measured rate is accelerating between surveys | Shorter survey interval set by the owner, with a projection to the next due date | Annex B allows edge settlement to be compared against the limit as measured only when little further movement is expected |
| Site events | Survey after the event | Flooding, pad erosion, nearby excavation or piling, a major leak that saturates the subgrade, or seismic activity |
| Service change to heavier product or higher fill height | Baseline survey before the change and a follow-up after the first full cycles | A higher foundation load changes the settlement prediction that the original frequency was based on |
| Foundation repair or re-leveling | New baseline survey | The earlier trend no longer describes the current geometry |
The reader's case, roughly one inch of differential settlement over two inspection cycles on a ringwall, falls on the "accelerating" row. The useful figure is the rate at each survey point, not the total. Divide each point's change by the time between surveys. Then ask the engineer to project the uniform, tilt, out-of-plane and edge components to the next planned inspection, and set the next survey date so that a limit cannot be crossed unseen. Inch-per-cycle movement is often surveyed annually until two consecutive surveys show the rate is falling. That is an owner decision and should be recorded as one. It is not a code number.
Ringwall foundations: what an inch means on concrete
A reinforced concrete ringwall carries the shell on a stiff, narrow footing, while the bottom plate rests on compacted fill inside it. The two parts can settle differently, and the pattern of movement shows which part is moving.
- The ringwall itself moves. If soil beneath the ringwall varies around the circumference, the shell elevations show tilt and out-of-plane settlement. This is the shell survey covered earlier in this article. Look for cracked or rotated ringwall segments at the low points, and for a gap opening between the bottom plate and the top of the ringwall.
- The fill inside the ringwall consolidates. The bottom drops relative to the shell, and the breakover forms just inside the ringwall. Annex B assesses this as edge settlement, and the shell survey can show almost nothing while the floor is already near its limit. This is the most common reason an "acceptable" shell survey and a failing internal inspection both turn up on the same tank.
- Water gets in at the chime. When the seal between the bottom plate projection and the ringwall cracks, or the ringwall top rotates away from the plate, rainwater can run under the bottom. That drives soil-side corrosion of the floor near the shell, where the code's bottom thickness limits are tightest and where edge settlement already concentrates strain. Settlement and underside corrosion often show up together on ringwall tanks, so they should be evaluated together.
The ringwall levelness tolerance quoted earlier applies to new or reconstructed foundations. It is not an in-service acceptance limit.
Collecting survey data that can be compared cycle to cycle
A trend is only meaningful if each survey is repeatable. Annex B asks for experienced personnel, equipment accurate enough to resolve the differences being measured, and the same points reused in later surveys. In field terms, that means:
- A stable benchmark away from the tank and its foundation. A benchmark on the ringwall or a nearby support moves with the tank and hides the uniform component.
- Permanent, marked survey points. Use points that can be found again, such as paint-marked or stamped locations on the shell, or the course-one-to-course-two weld where the bottom projection is distorted. Number them in the same direction every time.
- A recorded product level and temperature. Settlement under load is partly elastic. Comparing a survey at a high fill level with one at a low fill level adds a load-dependent difference that is not settlement. Survey at a consistent fill level, or at least record the level so the engineer can account for it.
- Closure of the level loop. Close the survey back onto the benchmark and record the closure error. If the closure error is close to the out-of-plane value being judged, the survey cannot support the evaluation.
- The method and instrument recorded. Digital levels, total stations and 3D laser scanning all produce elevation data, each with its own precision. Use the same method for the Annex B points every cycle.
- Repeatable internal bottom grids. When the tank is out of service, survey along fixed diametrical lines and mark the edges of depressions.
For shell geometry beyond elevation, such as roundness, verticality and local shell distortion near low points, see our tank 3D scanning page, which explains what a scan captures that a level loop does not.
Hydrotest settlement monitoring
API 653 makes a settlement survey mandatory for an existing tank that undergoes a hydrostatic test. The only exception is a tank with documented service history showing acceptable settlement, where none is expected during the test. Settlement is measured during filling and when the water reaches the full test level, using the same point count and spacing rules discussed earlier in this article.
Owners usually set hold points before filling, with limits fixed in advance by the engineer. If movement is unexpected, the owner's engineer and the authorized inspector decide whether to continue, and the survey team reports readings promptly. Keep the hydrotest survey as the new baseline.
The NDE a settlement finding should trigger
Annex B links settlement directly to weld examination. Where edge settlement passes a fraction of its allowable, shell-to-bottom and nearby bottom welds are to be examined visually and by magnetic particle or liquid penetrant methods, and any indications repaired or evaluated, as described earlier in this article. Beyond that code-driven step, a settled tank usually needs a wider inspection scope. The table below lists the methods most often used and what each one can and cannot show.
| Concern | NDE method | What it shows | Limits |
|---|---|---|---|
| Cracking at shell-to-bottom weld and lap welds in the breakover zone | VT, then MT or PT | Surface-breaking cracks from strain at the bend | Needs cleaned weld surfaces, and shows only surface-breaking flaws |
| Soil-side floor corrosion under low areas and at the chime | Floor scanning by magnetic flux leakage with UT prove-up | Screens the floor area for underside metal loss, and UT then sizes the remaining thickness | Coverage drops near the shell, at lap welds and on heavily distorted plates. Read the scanner's dead zones off the report. |
| Annular or critical-zone plate thickness | UT thickness and scanning | Remaining thickness where the code's limits are tightest | Needs access inside the tank. Readings near the shell weld need careful probe placement. |
| Shell distortion near tilt or out-of-plane low points | Shell geometry survey or 3D scan, plus UT thickness | Out-of-roundness and local distortion, along with the thickness the stresses act on | Geometry data supports an engineering evaluation but does not make the acceptance decision |
| Nozzle and piping strain from uniform settlement or tilt | VT of nozzle reinforcement, supports and piping flexibility, with MT or PT of nozzle welds if distress is seen | Signs of overstress where the tank connects to fixed piping | Overstress before cracking is not visible, so the piping engineer may need a stress check |
| Bottom welds after plate replacement or re-leveling | Vacuum box testing of new welds, as API 653 requires for bottom repairs | Leak paths in new or repaired bottom welds | Proves tightness only. It is not a volumetric weld examination. |
The comparison between MFL and UT floor scanning, including probability of detection and dead zones, is covered in MFL versus UT floor scanning. Remaining-thickness rules for pitted floors are covered in tank floor pitting and minimum remaining thickness.
Who decides what
Settlement work involves several parties, and many disputes after the fact come from unclear roles. API 653 puts the owner/operator in charge of the programme, and gives the authorized inspector the inspection judgements. Settlement evaluations beyond the simple Annex B checks belong to an engineer experienced in tank settlement.
| Party | Responsibility |
|---|---|
| Owner/operator | Sets the survey frequency and the programme, commissions engineering evaluations, decides on repair, re-leveling or a change of operating limits, and keeps the records |
| API 653 authorized inspector | Inspects the tank, records signs of settlement, reviews the survey and NDE results, and sets or recommends inspection intervals under the code |
| Tank or geotechnical engineer | Breaks down the survey, runs the Annex B checks or a more rigorous analysis, projects settlement to the next inspection, and designs any foundation repair |
| Surveyor | Provides repeatable elevation data, benchmark control and closure records |
| NDE contractor | Performs the weld, floor and shell examinations the scope calls for, under qualified procedures, and reports results to the authorized inspector |
Atlantis works on the NDE and data side of this table. Where earlier text on this page suggests we evaluate settlement, read it within that limit: we collect and report the data, and the inspector of record and the engineer evaluate it.
The regulatory overlay in the United States and Canada
- Pipeline breakout tanks (PHMSA). Under 49 CFR 195.432, operators inspect each in-service breakout tank at least once each calendar year, at intervals of no more than 15 months. Atmospheric and low-pressure steel aboveground breakout tanks are inspected under API Std 653, but the regulation does not allow API 653's risk-based interval provisions to be used to set inspection intervals. The edition incorporated by reference is the one that applies, so check 49 CFR 195.3.
- SPCC facilities (EPA). 40 CFR 112.8(c)(6) requires each aboveground container to be tested or inspected for integrity on a regular schedule and whenever material repairs are made, following industry standards. Field-erected tanks are commonly managed under API 653. Shop-fabricated tanks are commonly managed under STI SP001. A settlement finding is part of the integrity record the plan must keep.
- States and provinces. Some states add their own tank rules, and in Canada, aboveground storage tank requirements come from provincial fire codes and environmental regulations. Confirm with the authority having jurisdiction whether API 653 is referenced and which edition applies.
For how settlement surveys fit into the wider set of inspection intervals, see API 653 tank inspection intervals. For a field checklist, see the API 653 tank inspection checklist.
Common mistakes in settlement programmes
- Doing only a shell survey on a ringwall tank where the fill inside is what is settling.
- Comparing edge settlement with its limit as measured, with no projection, on a tank that is still moving.
- Reporting MFL floor coverage without its dead zones, so the strip next to the shell was never scanned.
- Evaluating settlement and floor corrosion separately when both act on the same plate.
How Atlantis supports this
Atlantis provides NDE for aboveground storage tanks under the owner's API 653 programme. Our tank inspection services include floor MFL scanning with UT prove-up, shell and annular UT thickness, and VT, MT and PT of shell-to-bottom and bottom welds in settled zones. ASNT-certified technicians do the work under ASNT Level III oversight. Elevation and geometry data can be collected alongside the NDE, using conventional survey or 3D scanning. Results go to your API 653 authorized inspector, who remains the inspector of record, and to the engineer evaluating settlement. We do not perform the Annex B acceptance evaluation, fitness-for-service work or foundation design. For inspector-side support on tank work, see API 653 tank inspection support.
Ask for a scope for a tank showing active settlement. We'll send a quote.
Settlement survey questions owners ask
How does API 653 want me to handle settlement measurement frequency for a ringwall tank with about one inch of differential settlement over two cycles?
API 653 does not set a fixed frequency. It expects a planned frequency based on predicted settlement and service history. Movement of about an inch over two cycles counts as active settlement, so the owner should shorten the survey interval, track the rate at each point, and have an engineer project each settlement component to the next inspection date before comparing it with the Annex B limits.
Is a settlement survey required at every API 653 inspection?
No. A survey is required with a hydrostatic test, unless the tank's history shows acceptable settlement and none is expected. Otherwise the owner schedules surveys, and inspections should record signs of settlement for follow-up.
What instrument should be used for a tank settlement survey?
Any method accurate enough to resolve the differences being judged and repeatable from one cycle to the next. Digital levels and total stations are common, and 3D laser scanning adds shell geometry. Use the same method, benchmark and points each cycle.
What NDE is needed after a settlement finding?
Annex B calls for visual and MT or PT examination of shell-to-bottom and bottom welds once edge settlement passes a fraction of its allowable. Floor MFL with UT prove-up, and UT of the annular zone, are commonly added to check for underside corrosion in the settled area.
Does re-leveling a tank fix settlement damage?
No. Re-leveling corrects geometry but does not remove plastic strain. Overstrained plate and welds still need replacement or engineering evaluation, and the cause of the settlement needs correcting.
Do PHMSA breakout tank rules allow risk-based settlement or inspection intervals?
For breakout tanks under 49 CFR 195.432, the regulation applies API 653 but excludes its risk-based interval provisions for setting inspection intervals. Check the edition incorporated by reference.
Who signs off a settlement evaluation?
The engineer who carried out the evaluation, under the owner's programme, with the API 653 authorized inspector reviewing it for the inspection record. Surveyors and NDE contractors supply data but do not accept the tank.
Need repeatable settlement and NDE data for your authorized inspector? Request a quote.
Need the NDE behind your API 653 programme?
Atlantis carries out tank floor, shell and settlement NDE and reviews the data; your Authorized Inspector stays inspector of record.