Converting gauging history into a steel renewal list before the dock date is fixed
Compute a short-term rate from the last two gaugings and a long-term rate from the as-built or CSR net scantling, take the more conservative, then project each plate forward to the dock date rather than to today. The output that matters is not a rate in millimetres per year but a renewal list in tonnes, produced before the yard prices the job.
Drydock is a fixed window bought at a day rate, and the scope has to be settled before it opens. Steel must be ordered and cut, tanks gas-freed and staged in a sequence, hot work permits and crews arranged, and the yard's contract priced as a lump sum with a per-tonne rate for additional steel discovered in dock. Every tonne discovered late is bought at that rate plus schedule. So the real question a corrosion rate module answers in marine is not what the wastage rate has been but what the diminution will be on the date the ship arrives, and how many structural items will have crossed a class renewal or substantial-corrosion threshold by then. That converts directly into plate tonnage, staging requirements and manhours. A system that reports historical rates and stops has produced a report; a system that produces the projected renewal list has produced a scope.
Source: Written against the IACS Unified Requirement Z10 series for hull surveys of bulk carriers, oil tankers and double-hull ships; the ESP Code adopted under SOLAS Chapter XI-1 Regulation 2 for enhanced survey programmes; IACS Recommendation No. 87 on guidelines for coating maintenance and repairs, and IACS guidance on thickness measurement scope and reporting format; the IACS Common Structural Rules net scantling approach and corrosion additions; IMO Resolution MSC.215(82), the Performance Standard for Protective Coatings for dedicated seawater ballast tanks and double-side skin spaces; the coating condition definitions GOOD, FAIR and POOR used in IACS survey requirements; individual class society rules (ABS, DNV, Lloyd's Register, BV, ClassNK) for allowable diminution and renewal criteria; ISO 8501-1 for surface preparation grades.
| Structural item | Last gauged | Projected at dock date | Class trigger | Call for the scope list |
|---|---|---|---|---|
| No.3 WBT upper deck longitudinal web, as-built 11.0 mm | 9.30 mm (May 2024) | 8.80 mm | Renewal at 20 % diminution = 8.80 mm | Renew — order plate now rather than discover it in the dock |
| No.3 WBT side shell strake C, as-built 16.0 mm | 14.10 mm | 13.70 mm | Substantial corrosion below 13.60 mm; renewal below 12.80 mm | 0.10 mm from a trigger that forces annual gauging — gauge early and know |
| No.2 COT deckhead, as-built 14.5 mm, pitted | deepest pit 4.9 mm | 5.6 mm projected pit depth | Pit depth and pitting intensity criteria, not the plate mean | Plate-mean rate is blind here; scope a dedicated pit survey and staging |
| Rudder blade plating, as-built 20.0 mm | 18.60 mm | 18.30 mm | Cavitation damage, not general wastage | Linear projection invalid — inspect at dock, hold a contingency allowance |
| No.5 WBT, recoated 2023, as-built 12.5 mm | 12.40 mm | 12.30 mm | Coating condition GOOD | Segment the rate at the coating event; the pre-2023 rate does not apply |
The dock window is fixed; the scope is what moves
A drydocking is bought as a slot. The date is agreed months ahead, the yard prices a lump sum against a defined scope, and everything discovered after the ship is on the blocks is charged at a per-tonne rate for extra steel plus whatever the schedule extension costs in day rate and in a lost fixture at the other end. The economic asymmetry is severe: a tonne of steel identified twelve weeks early is plate on a truck, while the same tonne identified in week two of the docking is plate, overtime, staging that was already struck, and a delay.
This is what makes the marine version of corrosion rate calculation a forecasting problem rather than a reporting one. The last gauging campaign happened at the intermediate survey, perhaps eighteen months before the dock opens. Nothing in that report tells you what the ship will be on arrival. What you need is every gauged item projected forward to the dock date, compared against the renewal criteria that apply to it, and the crossings counted and converted into tonnage and access.
It is also why the readiness question and the corrosion question are the same question. Which tanks must be gas-freed, staged and gauged before the dock, in what order, is determined by which items are close enough to a threshold that the answer changes the scope. Projecting first, then sequencing the pre-dock gauging around the projections, is a different and cheaper plan than gauging everything and sorting it out afterwards.
What a thickness measurement report actually contains
Hull gauging under an enhanced survey programme is performed by a thickness measurement firm approved by the class society, to a scope agreed in advance with the attending surveyor, and reported in the format the society requires — organised by structural member, strake, frame and tank rather than by anything resembling a corrosion monitoring location register. A single campaign on a large tanker or bulker produces thousands of readings.
Two things about that format cause persistent trouble downstream. First, the figure recorded against a plate is a summary of several readings taken on it, and the summarising convention varies: some firms report the lowest single reading, some report the mean, some report both, and the report does not always make the choice explicit. Second, the readings are located by structural reference, not by a permanent physical mark, so the next campaign's readings on the same plate are near the previous ones rather than at them.
The consequence is that the raw report is a snapshot of condition, which is what class requires of it, and not a repeatable measurement series, which is what a rate calculation needs. Building a defensible rate means importing the report with its basis attached to every figure, refusing to difference a lowest against a mean, and computing on the plate and strake population rather than pretending a spot-level series exists. A system that silently reduces each plate to one number and starts subtracting will produce confident rates that are mostly measurement variance.
Coating condition is the variable that segments everything
A ballast tank does not corrode at a constant rate. Under an intact coating applied and maintained to the IMO protective coating performance standard, wastage is close to nil for the coating's useful life. As the coating begins to break down — first at edges, welds and the tank top where condensation and oxygen concentrate — bare steel is exposed and wastage begins, and it accelerates as the exposed area grows. The curve is a long flat stretch followed by a knee.
Draw a straight long-term rate through a tank's whole history and you average across that knee, which gives an answer that is wrong in both directions at once. It over-states what will happen in the year after a recoat, encouraging renewal that is not needed. It badly under-states what happens in the years after coating failure, which is exactly the period in which the scope you are trying to predict gets written. This is why the coating condition assessment — GOOD, FAIR or POOR — belongs in the data model as a state variable per tank per survey, not as a comment in a survey report.
The practical rule is that rates are computed within a coating regime and never across a recoating or a documented change in condition. A tank recoated in 2023 starts a new series in 2023. Its pre-2023 history is retained, because it tells you what this tank does once the coating goes, and that historical post-breakdown rate is the right input for forecasting beyond the current coating's expected life. Two rates for one tank, used for two different horizons, is more honest than one rate used for both.
Pitting, grooving and cavitation do not obey a mean rate
General wastage is the mechanism a thickness-and-time model describes well. Several of the mechanisms that actually decide marine scope are not general wastage at all. Pitting in the inner bottom of cargo oil tanks, and deckhead pitting driven by acidic condensation in a sulphur-bearing crude with inert gas overhead, attacks a small fraction of the area very fast while the plate mean barely moves. Grooving corrosion runs as a narrow trench along the weld toe of side longitudinals, taking a critical section without appearing in any plate average. Cavitation erosion on and around the rudder and in the propeller wake is a hydrodynamic phenomenon whose rate has essentially no relationship to time in service.
Applying a mean-thickness rate to these is not conservative — it is blind. The plate mean on a pitted deckhead can look perfectly healthy while the deepest pits approach penetration, and class criteria for pitting are written in terms of pit depth and pitting intensity over an area precisely because the mean is uninformative. Grooving requires the groove depth and length, measured deliberately. Cavitation requires inspection, not projection.
So the model has to carry mechanism as an attribute of the location, and change its behaviour accordingly. General wastage locations get a projected thickness. Pitting locations get a projected deepest-pit depth and a flag that the plate mean is not the governing measure. Cavitation and other non-progressive damage gets excluded from projection entirely and put on the dock inspection list with a contingency allowance held in the budget instead. Being explicit that a location cannot be forecast is more useful than forecasting it badly.
Net scantlings, corrosion additions and the moving definition of nominal
For older ships the comparison is straightforward in principle: measured thickness against as-built thickness, with renewal when diminution exceeds the allowable percentage for that member. For ships built to the Common Structural Rules the arithmetic is different. Scantlings are specified as a net thickness required for strength plus a corrosion addition, and the assessment compares the measured thickness against the net scantling plus any remaining addition rather than against a percentage of as-built. Treating a CSR ship as if it were a percentage-of-nominal ship gives the wrong answer, usually in the unsafe direction.
There is a further complication that catches operators of second-hand tonnage. As-built thickness for a given plate may not be what the general arrangement says, because plate was substituted during construction, or because a previous renewal fitted a different thickness and the record of it did not travel with the ship at sale. Computing a diminution percentage against a nominal that was never installed produces either a phantom renewal or a phantom margin.
The data model therefore needs the governing basis stored per structural item: as-built with its source, or net scantling plus corrosion addition, together with any renewal history that changed the local thickness. Any renewal event resets that item's baseline to the as-fitted thickness of the new plate, with the prior series retained. That is the same discipline a good integrity system applies to a repaired pressure vessel, and for the same reason: after the repair you are measuring different metal.
From projected diminution to tonnes, staging and manhours
The projection only earns its keep if it converts into the things the yard prices. That requires geometry to be held alongside condition: for each structural item, its dimensions, thickness, material and location in the ship. An item flagged for renewal then resolves automatically to a plate area, a steel weight, an approximate weld length and a tank, which sums across the scope into a plate order, a rough manhour estimate by trade, and — critically — a list of which spaces need to be gas-freed, staged and access-cut.
That last item is often the binding constraint rather than the steel. Staging a deep tank, gas-freeing a cargo oil tank and cutting and reinstating an access opening consume dock days regardless of how many tonnes are renewed inside. Knowing at the planning stage that three tanks must be opened rather than one changes the dock duration, the labour curve and the price, and it is knowable from the projection months in advance.
It also changes the negotiating position. A yard quoting against an owner's defined and defensible scope prices differently from a yard quoting against a vague scope with a large discovery allowance, because in the second case the yard is pricing the risk. Arriving with a projected renewal list, its basis, and the gauging that supports it is worth real money before a single plate is cut.
Evaluating a system against your own fleet's gauging history
Give the vendor a real thickness measurement report from your last special survey, in the format your approved firm actually issues, together with the one before it. Ask them to import both and produce a projected renewal list for a dock date you nominate. Watch what happens at the seams. Does the import preserve whether each figure is a lowest reading or a mean? Does it recognise the same plate across two campaigns despite different operators and slightly different reference points? Does it refuse to project the rudder? Does it ask about coating condition, or silently assume it never changed?
Then check the outputs against the decisions you actually have to make. Does the renewal list carry tonnage and tank access, or only thicknesses? Does it separate items crossing a renewal limit from items crossing the substantial-corrosion threshold, since those have different consequences and different costs? Does it hold the renewal criteria for your class society and member types, and let you see which criterion was applied to each item? Can you re-run the whole projection against a different dock date in seconds when the fixture moves, which it will?
The Atlantis NDT inspection management ERP is built on Odoo 18, and the marine configuration holds structural items with their geometry, coating regime and governing basis so that a projection produces a scope rather than a spreadsheet. Corrosion rates are computed short-term and long-term separately, the conservative one governs, and rates are never drawn across a recoating or a renewal. It is affordable, accessible and fully customizable to your class society's criteria and your own gauging contractors' report formats. To see it run against your last two survey campaigns, or to request a scoped quote, contact info@atlantisndt.com.
Why project to the dock date instead of reporting the rate today?
Because the decision being made is a procurement decision with a lead time. Plate has to be ordered, cut and delivered, staging and gas-freeing sequenced, and the yard contract priced before the ship arrives. A thickness that is compliant today but crosses a renewal limit two months before the dock opens is a renewal, and finding that out in the dock means paying the yard's extra-steel rate plus schedule. The projection, not the historical rate, is the deliverable.
Why is point-to-point differencing unreliable on a ship's plating?
Because the same point is rarely measured twice. Gauging is performed by an approved firm whose operators change between surveys, on large plates, from staging or rope access, with the target point defined by frame and strake rather than by a permanent mark. Two readings nominally at the same location can be a metre apart on plate that wastes unevenly. Rates are therefore more defensible when computed on the plate or strake population than on individual spots, with single-spot jumps treated as suspect until re-measured.
How does coating condition change the corrosion rate calculation?
It segments it. A ballast tank with intact coating wastes at close to zero until breakdown begins, then accelerates as bare steel is exposed. A straight long-term rate drawn through a recoating event averages the two regimes and predicts neither: it over-predicts the year after a recoat and badly under-predicts the years after coating failure. Coating condition — GOOD, FAIR or POOR — should be held per tank per survey as a state variable, with rates computed within a coating regime and never across a recoat.
What does substantial corrosion mean and why does it matter to the dock plan?
Substantial corrosion is wastage in excess of 75 percent of the allowable margin while still within acceptable limits. Crossing it is consequential: it typically brings the item into an annual gauging regime and expands the survey scope, so it costs money and access even though no steel is renewed. Knowing which items will cross that line by the dock date lets you decide deliberately whether to renew early and reset the clock, or accept the surveillance burden.
How do you turn projected diminution into a steel weight the yard can price?
By carrying geometry with the measurement. Each structural item holds its plate dimensions, thickness and material, so an item flagged for renewal converts directly to area, tonnage and an approximate weld length. Summed across tanks that produces a plate order and a first-pass manhour estimate by trade. It will not match the final as-fitted figure, but it is close enough to order long-lead plate and to negotiate the lump sum from a position of knowing your own scope.
Is API 510, 570 or 653 inspector training part of this offer?
No. Those inspector certifications are administered by API and are not something Atlantis NDT issues or trains toward, and they are in any case not the qualification regime that governs hull survey work. What Atlantis provides is NDT training to ASNT SNT-TC-1A and ISO 9712 across Level I, II and III in UT, RT, MT, PT, ET, VT, PAUT and TOFD, ASNT Level III consulting, inspection management and reporting software, digital twins, 3D laser scanning, and independent report validation.
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