What is the extent of thickness measurement at the special surveys of a single-hull oil tanker?

The first survey is not empty, and this is where it matters

On a double-hull tanker the five-year column holds suspect areas and nothing else. On a single hull it holds real work: deck plating taken right across the beam of the ship, one strip of it inside the cargo area, and the strip has to be sited over a ballast space — a ballast tank if the ship has one, and failing that a cargo tank whose service is mainly water ballast. UR Z10.1 R25

It is deck plating, not a transverse section. The requirement's own wording keeps those apart, and the difference is the whole cost of the item: a strip of deck across the beam is a morning, a transverse section is a cut through the girder from keel to sheerstrake. A programme that reads the word section here and prices a full one has overspent; one that reads it and takes deck plating only at the third survey has under-delivered.

Now put that next to the double-hull page. Lloyd's Register carries this same first-survey requirement onto double-hull ships, where the baseline asks for nothing at all. Once you have read the single-hull column, the LR position on a double hull stops looking like an outlier and starts looking like a society applying one rule to both hull forms. LR Special Survey I

ABS adds a placement condition the others leave open: its strip is cut inside the amidships 0.5 L as well as over a ballast space. DNV strips the item back to the deck alone and drops the siting condition. ABS SPS 1 · DNV renewal 1

The ladder from there

The middle column of that table is worth reading slowly. No strake inside the cargo area is called for until the third survey — at the second, the wind-and-water item is the selected strakes outside the cargo area and nothing else. The reflex is to assume the band starts amidships and works outward; here it starts at the ends.

ABS words the deck at the second survey differently from everyone: main deck plating in full over whichever runs the longer of the amidships 0.5 L and the cargo area. On most tankers the cargo area is longer and the two readings agree; on a short-cargo-area ship they do not, and ABS asks for the greater. ABS SPS 2

The section has to catch a ballast tank

On a bulk carrier the placement condition is geometric — one section in the amidship area. Here it is structural, and it is sharper.

A footnote to the measurement table attaches to the pair of sections: a ballast tank lying within 0.5 L amidships has to fall inside at least one of them. The condition is repeated in the body text for both the two-section and the three-section cases, so it is not a drafting accident. The section is not just a cut at a place — it is a cut through a particular kind of space, chosen because that is where the corrosion is. UR Z10.1 Table II fn (1)

ABS stacks three conditions on the same pair at the third survey: both inside the amidships 0.5 L, cut through two different cargo tanks, and at least one of them taking in a ballast tank. A pair of sections that satisfies the baseline can fail any of the three. ABS SPS 3

Close-up: rings, and three letters

The close-up table is written in letters, defined beneath it, and the letters carry the whole meaning:

• (A) — the complete transverse web frame ring, taken with the members beside it.
• (B) — the deck transverse, with the deck members beside it.
• (C) — the complete transverse bulkhead, girder system included. Where the table asks for less, it names the lower part instead.

A ring is not the double-hull web frame. On a single hull the ring is the whole transverse frame taken round the section — and where the ship has a ballast wing tank the ring comes from there, or else from a cargo wing tank whose service is mainly ballast. Same siting logic as the deck strip at the first survey, and for the same reason. UR Z10.1 Table I

The progression: one ring and one deck transverse at the first survey, with bulkhead lower parts. At the second, every ring in the ballast wing tank, deck transverses one apiece across the remaining tanks, and the first complete bulkhead — a whole (C) rather than a lower part, in the wing ballast tank, both bulkheads of it. At the third, every ring in every ballast tank, every ring in one cargo wing tank, and every transverse bulkhead in the ship complete.

There is a layout quirk worth knowing before you read the printed table. In one column three separate entries are all labelled (A), stacked one under the other, and the column only makes sense if they are taken as three different requirements rather than one repeated. UR Z10.1 Table I, col. 3

The thirty per cent rule, and the direction it rounds

From the third survey a percentage enters the close-up table — the only one in the tanker chapter. In each cargo wing tank left over after the fully surveyed one, at least 30 % of the rings. ABS and DNV add the same figure to the deck and bottom transverses in each cargo centre tank.

And a note under the table settles what most percentages leave open: the 30 % is rounded up to the next whole number. On a tank with seven rings that is three, not two. It is one word in a table note and it changes the count in every wing tank on the ship. UR Z10.1 Table I Note 1

At the fourth survey the close-up column is two lines: the third-survey extent repeated, and further transverses at the classification society's discretion. As on the double hull, the ladder stops climbing and the discretion takes over.

Above ten years the intermediate survey is the special survey

The same structure as the bulk carrier, and the exclusion list is even shorter. Above ten years the intermediate survey takes the whole extent of the previous special survey, and exactly two things drop out unless the attending surveyor wants them: tank pressure testing, and the longitudinal strength evaluation of the hull girder. Nothing about the measurement extent is reduced. UR Z10.1 §4.2

Because the two surveys then ask for the same work, the requirement carries a clause that stops one set of gaugings being credited twice. It exists for exactly this age band, and it is the clause to have to hand when a survey plan proposes to reuse readings across the two attendances. UR Z10.1 §4.1.3

Above fifteen the intermediate survey includes a dry-dock survey, and the lower portions of the tanks are taken through overall survey, close-up survey and gauging there if not already done. Lower portions are defined — the parts below the light ballast waterline — and that definition decides whether the gauging crew is booked into a dock or into a tank cleaned in service. It is the single most schedule-relevant sentence in this requirement.

One drafting inconsistency to note: the intermediate clause writes its age band with a hyphen — 5 to 10 years of age — where the special survey columns use inequality signs. A ship sitting exactly on ten years is caught by two different conventions in the same document.

Reliefs, and what they are not

There are two relief clauses, one for each table, and they are separate: one lets the close-up extent be specially considered, the other lets the measurement extent be. Both are written as may be specially considered — not as may be dispensed with. A reduction under either is a judgement made at the survey, not a discount claimed before it. UR Z10.1 §2.3.4 · §2.4.4

And there is no CSR relief here at all. The coat-and-check alternative that a Common Structural Rules bulk carrier gets on substantial corrosion has no counterpart in this requirement — the document does not mention the Common Structural Rules anywhere. A single-hull tanker in the substantial-corrosion band goes to the widened grid and stays there. UR Z10.1, CSR absent

That widened grid is four sheets — bottom structure on the first, then deck, side and bulkhead structure across the other three — and each row sets both an extent and a pattern rather than one or the other.

Sources

1. UR Z10.1 Rev.25IACS — Oil 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 ↗

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

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