DRAFT last reviewed 2026-08-25 reviewer rev 1 6 sources

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

A double-hull tanker's systematic extent climbs in a straight line and is easy to hold in your head. The work is in the close-up column, where web frame means one member in a ballast tank and a different one in a cargo tank — and where a five-metre figure is written as three to five by one society alone.

The short answer

First survey: suspect areas, and nothing else on the IACS baseline.
Second: the deck inside the cargo area, plus one transverse section.
Third: two sections, and the wind-and-water band inside the cargo area.
Fourth and after: three sections, every bottom plate, and the band over the full length.
Close-up all the way through — web frames, deck transverses and transverse bulkheads, gauged as they are surveyed.

The systematic ladder, in one table

Unlike the bulk carrier chapter, where each survey changes shape, the tanker columns climb by adding one thing at a time. It is worth seeing all four at once.

Survey	What the column adds
No.1 age ≤ 5	Suspect areas. That is the whole of it.
No.2 5 < age ≤ 10	Each deck plate inside the cargo area, and one transverse section. Selected wind-and-water strakes outside the cargo area.
No.3 10 < age ≤ 15	The deck, two sections, and all wind-and-water strakes inside the cargo area. A selection still outside it.
No.4 and after age > 15	The deck, three sections, each bottom plate, and the band over the full length.

Two things to notice. The bottom arrives only at the fourth survey — the same late entry as on a bulk carrier. And the wind-and-water band goes complete inside the cargo area one survey before it goes full length, so between fifteen and twenty years the same band is measured to two different rules depending on where you are standing. UR Z10.4 R18

Two societies break the first rung, in opposite directions

The first survey is the one place in this table where the documents genuinely disagree about whether there is any scheduled work at all.

Lloyd's Register asks for a section at five years. Where the baseline column holds suspect areas and nothing else, LR requires one section of deck plating right across the beam, inside 0,5 L amidships — and it puts a condition on where the cut falls: through a ballast tank, or, if the ship has none there, through a cargo tank given mainly to water ballast. That is a full section of work at an age where three other documents ask for none. LR Special Survey I

DNV leaves every row blank. Its measurement table has no entry at all in this column — not a reduced list, an empty one. Read beside the bulk carrier chapter, where DNV also left a column blank, a pattern appears: DNV states nothing where the requirement is nothing, rather than printing the standing suspect-area line the others repeat. DNV Table 22

The practical reading: on an LR ship the five-year survey carries a real gauging job that a programme copied from the UR will not contain, and on a DNV ship the blank column is not a gap to be filled by analogy.

Where the sections may be cut

As on a bulk carrier, the count is agreed and the placement is not. Three positions:

The baseline names a number of sections inside the cargo area and stops there.
DNV asks for complete sections and, from the third survey, requires at least one inside 0.5 L amidships — the same construction it uses on bulk carriers.
ABS confines them all: one section inside the amidships 0.5 L at the second survey, two inside it at the third — and adds a condition no other document carries, that the two be cut through two different cargo tanks. ABS SPS 2 · SPS 3

The ABS condition is the one to plan around. Two sections inside the middle half of a tanker can easily fall either side of one bulkhead and sit in the same tank — which satisfies the count everywhere else and fails here.

Close-up: the same two words mean two different members

This is the part of a tanker survey that has no bulk carrier equivalent, and the part where a plan goes wrong quietly.

The close-up table uses web frame in two entries, and defines it separately for each in its own notes. In a ballast tank it means the vertical web in a side tank, the hopper web in a hopper tank, the floor in a double bottom tank, and the deck transverse in a double deck tank where one is fitted. In a cargo oil tank it means something else entirely: the deck transverse, the longitudinal bulkhead structural elements, and the cross ties where fitted. UR Z10.4 Table I notes

A technician who counts web frames the same way in both tank types has the wrong member list on one side of the ship's cross-section, and the error is invisible in the total.

The ladder itself runs like this:

No.1 — in a ballast tank, a single web frame and a single transverse bulkhead; in a cargo tank, a single deck transverse; and in one cargo centre tank and one cargo wing tank, the lower part of a transverse bulkhead.
No.2 — all web frames in one ballast tank, and in every remaining ballast tank the knuckle area plus the upper part of one web frame. A deck transverse in two cargo tanks. A transverse bulkhead in every ballast tank.
No.3 — every web frame in every ballast tank; every web frame in one cargo tank and one in each remaining cargo tank; and every transverse bulkhead, cargo and ballast alike.
No.4 — the third survey again, plus additional transverse areas as judged necessary.

Where no centre cargo tanks are fitted, the port and starboard cargo wing tank bulkheads come into scope instead — a relief carried as a table note by the baseline and written into the column by ABS. ABS Table 11 note 6

One structural point that changes the count on a DNV ship: DNV limits the ballast tank web frame entry to tanks within the cargo area at the first two surveys. The baseline sets no such limit, and its own definition of a ballast tank reaches the fore and aft peaks. DNV renewal 1 · 2

The knuckle, and the five metres that one society writes as three

At the second survey the sampled ballast tanks are not surveyed whole. Two bounded pieces of one web frame are taken instead, and both are defined rather than left to judgement.

The knuckle area is the part of the web frame surrounding the place where the slope hopper plating meets the inner hull bulkhead and the inner bottom plating, reaching two metres out from it. It is a small, precisely bounded region and it is there because that corner is where the coating goes first. UR Z10.4 Table I note (6)

The upper part is where the documents part company over a number. The baseline, the statutory annex, ABS and RINA all write it as approximately five metres. DNV writes approximately three to five metres. On a tall web frame that lower bound is a materially smaller piece of steel, and it is the only figure in this column that differs between documents. DNV renewal 2

What the fourth survey actually says, and who says it

The close-up column at the fourth survey is not written out in most of these documents. It reads back to the third and adds a line. That construction is fine until two things go wrong with it, and both do.

First, the read-back is written oddly. The column is headed for ships over fifteen years and its cell reads back to the requirement for ships of ten to fifteen. Nothing is wrong with the meaning, but a reader checking one cell in isolation sees a fifteen-plus column pointing at a ten-to-fifteen rule and reasonably wonders whether the page is right. It is.

Second, who decides the addition changes with the document. The added line is additional transverse areas as deemed necessary, and the body named is not the same one throughout: the class requirement says the Society, the statutory annex says the Administration, ABS says the Surveyor. On an ESP-notation tanker all three texts are live, and they name three different decision-makers for the same discretion. UR Z10.4 · A.1049(27) · ABS

ABS and DNV avoid the whole construction by reprinting the full list in the fourth column instead of reading back. It costs them a page and removes the ambiguity entirely — and it is the reason those two are the easier documents to quote from on board.

Traps worth carrying

BV prints two close-up tables on facing pages — one for single hull, one for double. It also prints two extended-measurement tables. Taking the single-hull one onto a double-hull ship puts the widened extent on bottom structure only, which is a large and quiet under-scope. The clause that says which table goes with which hull is the one to read first. BV Tab 1/2 · Tab 4/5
ABS sets no measurement line at the first survey. Its close-up table carries the survey but not a paired gauging item, so on the ABS grid the first special survey has a close-up examination with nothing scheduled to be measured on it. ABS Table 10
A footnote marker with no footnote. Two column headings of the DNV close-up table carry marker 8), and the notes printed beneath the table run 1 to 7. There is no note 8 on the page. Whatever it was meant to say is not in the document held. DNV Table 21
BV attaches a pitting rule to the escalation. Where bottom plating pitting reaches twenty per cent intensity or more, the measurements extend to establish both the thickness clear of the pits and the pit depth. It is a second trigger alongside substantial corrosion, and it is keyed to a percentage of area rather than a loss of thickness. BV §6.5.3

The escalation itself behaves as it does everywhere: a reading in the substantial-corrosion band takes the extent into the widened grid — five sheets for this hull form, covering bottom and inner bottom, deck, double side and inner side, transverse bulkheads, and the rest — and the extra work is finished before the survey is credited.

Sources

UR Z10.4 Rev.18IACS — Double hull oil tankers, hull surveysofficial text ↗
A.1049(27)IMO Res. A.1049(27) — 2011 ESP Codeofficial text ↗
ABS Part 7ABS — Rules for Survey After Construction, Jul 2026official text ↗
DNV Pt.7 Ch.1 Sec.4DNV — Rules for Classification: Ships, survey requirements, Jul 2022official text ↗
LR TM GuidanceLloyd's Register — Thickness measurement and close-up survey guidance, Nov 2016official text ↗
BV NR467 Part ABureau Veritas — Rules for the Classification of Steel Ships, Part A, Jul 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.