What is the extent of thickness measurement at the special surveys of a chemical tanker?

The ladder, and the one place it is not the oil tanker's

Set the chemical measurement table beside the double-hull oil tanker's and they read the same: nothing but suspect areas at five years; each deck plate in the cargo area and one section at ten; two sections and the full band inside the cargo area at fifteen; three sections, every bottom plate and the band full length after that.

One thing is worth saying out loud because it surprises people who work bulk carriers: each deck plate arrives at the second survey, not the third. On a bulk carrier the deck goes complete at fifteen years. Here it is complete at ten. UR Z10.3 R21

Two societies again put work into the first column where the baseline puts none. LR asks for a full-beam strip of deck plating inside 0,5 L amidships, sited over a ballast tank where the ship has one — the same first-survey item it applies to oil tankers of both hull forms. DNV splits the difference by hull: it asks for a full-beam deck strip on a single-hull chemical tanker and leaves the column entirely blank on a double-hull one. DNV renewal 1, both hulls

That DNV split is the useful observation. The baseline writes one measurement table for chemical tankers of either hull form; DNV writes two, and they differ at exactly the survey where a double skin makes the most difference to what can corrode.

Two age bands that do not meet, on facing pages

This requirement has two boundary faults, and they are not interpretation questions — they are what the column headings say.

A ship of exactly five years falls in no column of the measurement table. The first column is headed age < 5 and the second 5 < age ≤ 10. Five itself is named by neither. UR Z10.3 Table II

A ship of exactly ten years is inside the measurement column and outside both close-up columns. The two close-up tables head their second column 5 < age < 10; the measurement table heads its matching column 5 < age ≤ 10. One document, one survey, two different treatments of the same birthday — and the tables sit on facing pages. UR Z10.3 Tables I.1, I.2 · II

Neither fault changes what a competent surveyor asks for — the survey number, not the arithmetic, decides the column in practice. Both are worth knowing before the argument, because they are the kind of thing that gets discovered at the worst moment and then costs an afternoon.

Two close-up tables, chosen by hull form

Unlike the measurement table, the close-up side is split: one table for a single-skin chemical tanker and one for a double. Picking the wrong one produces the wrong member list from the first survey onward, and nothing downstream will catch it.

On the single-skin table the first survey takes one complete transverse web frame ring from a ballast wing tank, one deck transverse from a cargo tank — or the deck structure in its place — and a bulkhead lower part. At the second survey the ballast wing tank gives up every ring, and both of its transverse bulkheads; each remaining ballast tank gives a deck transverse. UR Z10.3 Table I.1

A note under both tables handles the ship with no centre cargo tanks — the arrangement you get with a centre longitudinal bulkhead. There, the transverse bulkheads in the wing tanks are surveyed instead. The same note also defines what a ballast double hull tank means, and it means more than the double bottom: double bottom plus double side taken together. UR Z10.3 Table notes I, II

Which section has to catch a ballast tank — and which does not

The placement footnote works the same way as on a single-hull oil tanker: a ballast tank lying within 0.5 L amidships has to fall inside at least one of the sections, and the body text repeats it for the two-section and the three-section cases alike.

It does not reach the single section at the second survey. The footnote attaches only to the later entries, so the one section a ten-year chemical tanker owes carries no amidships condition and no ballast-tank condition in the baseline. What still governs it is the general rule on choosing sections — take them where the largest reductions are suspected, or where the deck readings have already shown them. UR Z10.3 fn (1) · §2.4.5

ABS and DNV both close that gap in their own way — ABS by requiring at least one of the two sections at fifteen years to lie in way of a ballast tank, DNV by requiring at least one inside 0.5 L amidships. On either society the single section still stands free.

The intermediate survey, and the year it can be spread over

Below ten years the intermediate survey sets no gauging line at all. What it requires is an overall survey of representative ballast tanks chosen by the surveyor — and where that reveals no visible structural defect, it goes no further. Anyone reading across from the oil tanker requirements will look for a corrosion-level measurement item here and not find one. UR Z10.3 §4.2.2

Above ten years it becomes the previous special survey in full, and only tank pressure testing is lifted out — narrower even than the oil tanker relief, which also drops the hull girder strength evaluation. The measurement extent is not reduced at all.

Two scheduling provisions sit with it and both are worth having on a job sheet:

• The survey may be spread. It can be begun at the second annual survey and carried through the following year to be completed at the third — so the work does not have to land in one attendance. §4.2.3.2
• The dry dock is optional below fifteen and compulsory above. Under fifteen an under-water survey may be considered in place of the dock; over fifteen that option is gone, and the lower portions of the cargo and ballast tanks are close-up surveyed and gauged in the dock. §4.2.3.3

The twenty per cent deck rule — chemical tankers only, ABS only

This is the one rule in the whole chapter that turns a local finding into a structural one at a stated threshold, and it exists for this ship type alone.

On a chemical carrier under enhanced survey, where substantial corrosion covers more than a fifth of the deck surface of a tank, the whole deck structure above that tank — longitudinals and web frames included — is gauged to the same patterns. No IACS requirement carries anything equivalent, and no other society in this set does either. ABS 7-3-2/7.5.2(f)

Worth pricing before the survey rather than during it: on an ABS chemical tanker the difference between nineteen and twenty-one per cent of one tank's deck is the difference between a local escalation and gauging an entire deck structure.

Escalation, and no relief at all

The widened grid is four sheets — bottom, inner bottom and hopper on the first, deck structure on the second, the remaining member groups across the other two. Each row sets an extent and a pattern rather than one of the two.

The sheets carry no age band and no survey number. The same four apply from the first special survey to the last, which is a small mercy: unlike the survey columns, there is nothing to select and nothing to get wrong.

And there is no CSR alternative here. The coat-and-check route that a Common Structural Rules bulk carrier gets on an identified substantial-corrosion area has no counterpart in this requirement, which does not mention the Common Structural Rules anywhere. A chemical tanker in that band goes to the widened grid and finishes it before the survey is credited. UR Z10.3, CSR absent

One last document-location point. KR keeps the chemical tanker's widened grid out of the Rules entirely — the Rules clause points at a table in the Guidance volume. A ship carrying the Rules alone has the trigger and not the pattern. KR Ch 3, 404. item 4(2)

Sources

1. UR Z10.3 Rev.21IACS — Chemical tankers on enhanced surveyofficial text ↗
2. A.1049(27)IMO Res. A.1049(27) — 2011 ESP Codeofficial text ↗
3. ABS Part 7ABS — Rules for Survey After Construction, Jul 2026official text ↗
4. DNV Pt.7 Ch.1 Sec.4DNV — Rules for Classification: Ships, survey requirements, Jul 2022official text ↗
5. LR TM GuidanceLloyd's Register — Thickness measurement and close-up survey guidance, Nov 2016official text ↗
6. KR Part 1Korean Register — Rules and Guidance for the Classification of Steel Ships, Part 1, 2026official text ↗

Class-society and IMO documents are indexed and linked, never re-hosted. Where a class rule is stricter than the IACS baseline, the class rule governs.

Related questions

• Extent of TM at the special surveys of a double-hull oil tankerK6.10
• Extent of TM at the special surveys of a single-hull oil tankerK6.11
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• How many gauging points per plate?K6.2