What is the extent of thickness measurement at the special surveys of a liquefied gas carrier?
The gas carrier is the one ship type in this chapter whose intermediate survey schedules no gauging at all — not a reduced list, none, at any age, in every document held. Its close-up table also stops at three columns where the measurement table has four, so after ten years the two halves of the survey grow apart.
The short answer
- First survey: suspect areas, and one full-beam strip of deck plating inside 0,5 L amidships.
- Second: the deck inside the cargo area, and one transverse section — cut through a ballast tank where there is one amidships.
- Third: two sections, and the wind-and-water band inside the cargo area.
- Fourth and after: three sections, every bottom plate, the band full length — and the duct keel.
- Intermediate survey: no gauging is scheduled, at any age.
Not an ESP ship, and it shows in the drafting
Bulk carriers, oil tankers and chemical tankers are surveyed under the enhanced survey programme. A liquefied gas carrier is not. It is covered by a requirement in the general family, and the whole shape of this page follows from that one fact.
Three consequences run through everything below. The tables are shorter. The escalation pattern is offered as guidance rather than set as a provision. And suspect areas are printed as the last row of every column — beneath the systematic items and beneath the close-up measurements — where the ESP requirements print them first. The obligation is identical; the position on the page is not, and a survey scoped from the top of the column downward can reach the bottom with the budget already spent. UR Z7.2 Table II
The ladder
It climbs the way the tanker ladder does, with one addition at the top that no other baseline in this chapter carries.
| Survey | Systematic extent |
|---|---|
| No.1 | One strip of deck plating across the full beam, inside 0,5 L amidships, cutting a ballast tank where the ship has one there. |
| No.2 | Each deck plate in the cargo area, and one transverse section — same amidships and ballast-tank condition. Selected strakes outside the cargo area. |
| No.3 | The deck, two sections, and all wind-and-water strakes inside the cargo area. |
| No.4 and after | The deck, three sections, each bottom plate, the band full length — and duct keel plating with its internals. |
Note the first two columns. The amidships condition and the ballast-tank condition are attached from the first survey here, not from the third as on a bulk carrier — so a gas carrier's first section is more tightly specified than a twenty-year bulk carrier's. UR Z7.2 R8
Two societies differ at the first survey in opposite directions, as they do on tankers. ABS carries suspect areas and nothing else; DNV carries the deck strip and omits the suspect line. BV reproduces the baseline word for word.
The close-up table stops at three columns. The measurement table has four
This is the structural fact worth taking away from the page, and it is easy to miss because the two tables sit near each other and look alike.
The close-up table has three columns, and its last one is headed age > 10. The measurement table has four, and splits ten-to-fifteen from over-fifteen. So a gas carrier's close-up scope reaches its maximum at ten years and then never grows again, while the measurement scope keeps climbing through two more steps. UR Z7.2 Table I · Table II
The close-up ladder itself is short:
- No.1 — one web frame in a representative ballast tank, topside, hopper side or double hull side; and one transverse bulkhead in a ballast tank, lower part only.
- No.2 — every web frame in one ballast tank, and that tank has to be a double hull side or a topside one unless the ship has neither; one web frame in each remaining ballast tank. The bulkhead is now taken complete.
- No.3 and everything after — every web frame and every transverse bulkhead, complete, in every ballast tank.
The bulkhead is the item to watch: lower part at five years, complete from ten. That is a change of member, not a change of count, and it is set by which numbered item of the table applies rather than by any wording in the column itself.
The intermediate survey schedules no gauging at all
On every other ship type in this chapter the intermediate survey above ten years becomes the previous special survey in all but name. On a gas carrier it does not, and the point is not that the requirement is lighter — it is that there is no measurement line to be light about.
Four documents were read on this and all four agree:
- The baseline — the intermediate section carries a ballast tank overall survey and a close-up table, and nothing else, at any of its age bands.
- BV — its intermediate table has no measurement column at any of its three bands, and the clause beside it adds none.
- ABS — adds instrumentation, gas detection and emergency shutdown testing, and close-up survey. No gauging extent is named.
- LR — its intermediate table carries no gauging line in any column.
That does not mean no readings are taken. Suspect areas and previously identified substantial corrosion still carry their standing obligations, and the surveyor can call for measurement where the close-up survey gives cause. It means the plannable part of a gas carrier's intermediate survey contains no gauging — which is a genuinely different commercial shape from every other ship type on this page. UR Z7.2 · BV · ABS · LR
The annual survey is the same story and one step further: LR does not print an annual table for this ship type at all — a gas carrier falls into the all-remaining-ships table, which sets no calendar-driven gauging.
The duct keel, and the layer ABS adds on top
The duct keel is in the baseline here. On bulk carriers it appears only in the ABS column and nowhere else in the set; on a gas carrier the requirement itself puts duct keel plating and internals into the fourth survey. It is the one item where this ship type asks for more than the ESP requirements do.
Above that, ABS and DNV add the same non-cargo layer they add everywhere at the fourth survey: exposed deck plating over the full length, representative first-tier superstructure decks, keel plating full length, and bottom plating at cofferdams, the machinery space and the after end of tanks. DNV brings the fore and aft peak ballast tank internals in one survey earlier, at the third. ABS SPS 4 · DNV renewal 3, 4
ABS also carries its whichever is longer construction here: main deck plating over the greater of the amidships 0.5 L and the cargo tank section. On most gas carriers the two are close; where they are not, ABS asks for the greater.
Two statements of the same escalation, and they differ
The requirement says the same thing twice about substantial corrosion, in two places, and the two versions are not identical.
The clause says the extent is increased until the substantially corroded area is bounded, and offers a named table as the pattern for the additional readings.
A note under the measurement table says the extent is to be increased to the satisfaction of the Surveyor — and does not name the table at all.
One version gives you a grid to work to; the other gives you a person to satisfy. Both are in force. In practice the grid is the sensible starting point and the note is the reason a surveyor can ask for more than it — but a programme that cites only the clause has quoted half the rule. UR Z7.2 clause · Table II note 3
Substantial corrosion is defined here as it is throughout: wastage past three quarters of the allowable margin while the member is still inside acceptable limits. And the pattern table for the widened readings is two lines — plating, and stiffeners — with no row for a girder, a floor, a bracket or a web frame. On a gas carrier those members are covered by the surveyor's judgement and nothing else.
Where the ceiling actually is
The same table note carries a second open-ended clause: the Surveyor may extend the measurements as deemed necessary. Put beside the escalation sentence, it means the ceiling of a gas carrier survey is not in the table. Two of the four corners of the requirement are discretionary.
One more asymmetry, recorded because it costs money if discovered late: the special survey clause carries no completion deadline for the widened readings, while the annual survey clauses require the extra work before the annual survey is credited. The deadline is real — it is simply not written in every clause that needs it. UR Z7.2 §2.4.2
The practical position for a gauging contractor: on a gas carrier, quote the systematic ladder firmly, quote the intermediate survey as a rate rather than a figure, and expect the escalation to be argued rather than looked up.
Sources
- UR Z7.2 Rev.8IACS — Liquefied gas carriers, hull surveysofficial 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 ↗
- BV NR467 Part ABureau Veritas — Rules for the Classification of Steel Ships, Part A, Jul 2026official text ↗
- 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.