What does UR Z7 add to hull surveys and thickness measurement?
Z7 is the baseline for the whole hull of every self-propelled vessel. The Z10 family, Z7.1 and Z7.2 sit on top of it and cover one part of one ship type; everything they do not name — engine room, superstructure decks, seachests, fuel and fresh water tanks — is gauged to Z7.
The short answer
- Z7 applies to all self-propelled vessels. The Z10 family, Z7.1 and Z7.2 are additions on top of it for one ship type and one part of the ship.
- Z7 special survey gauging: SS 1 suspect areas only; SS 2 adds one deck transverse section; SS 3 two sections, hatch covers and coamings, peak tank internals; SS 4 and after, three sections plus full-length deck, wind-and-water strakes and keel.
- No gauging table at annual or intermediate survey in Z7. Gauging there is triggered by suspect areas and by what the surveyor finds.
- Z7.1 covers general dry cargo ships of 500 gt and above; Z7.2 covers ships carrying liquefied gases in bulk.
- Z7 and Z7.1 carry a Corr.1 of May 2024; Z7.2 is still at Rev.8, May 2019.
Which document, which part of the ship
Z7 is the general hull survey requirement. It applies to all self-propelled vessels, and it tells you where to go for the extra requirements: tankers to Z10.1, bulk carriers to Z10.2, chemical tankers to Z10.3, double hull tankers to Z10.4, double side skin bulk carriers to Z10.5, general dry cargo ships to Z7.1, liquefied gas carriers to Z7.2. UR Z7 §1.1.3
Read the scope clause of the additional document and you see how narrow it is. Z7.1 reaches the hull structure and the piping over one belt of the ship: every ballast tank, plus the cargo holds, the cofferdams and pipe tunnels, and those void spaces and fuel oil tanks that sit inside the cargo area. Z7.2 draws the same belt on a gas carrier over a slightly different set of compartments — pump rooms and compressor rooms, cofferdams and pipe tunnels, void spaces and fuel oil tanks inside the cargo area, and again every ballast tank — and it leaves out piping that UR Z16 already covers. Both then say the same thing: what they contain is additional to the classification requirements for the remainder of the ship, and the remainder of the ship is Z7. UR Z7.1 §1.1.3 UR Z7.2 §1.1.2
In practice: on a bulk carrier you work the cargo holds and the ballast tanks to Z10.2, and you work the engine room structure, the exposed superstructure decks, the seachests, the fuel oil and fresh water tanks outside the cargo length area, the automatic air pipe heads and the anchor chain to Z7. Nobody hands you two survey plans — the class survey planning document merges them — but when the office asks why a poop deck plate is on the sheet at Special Survey No.4, the answer is Z7, not Z10.2.
Two more Z7 items a gauging technician meets. Chain cables are gauged at Special Survey No.2 and every subsequent special survey, and renewed where mean diameter is below the society's limit. UR Z7 §2.2.4 And the gauging firm must be approved under UR Z17, with two exceptions written into Z7 itself: non-ESP ships under 500 gross tonnage, and all fishing vessels. UR Z7 §5.3.1
Z7 thickness measurement extents, by survey and by age
Special survey gauging is set by one table, and the survey is not credited until any extension for substantial corrosion has been gauged as well. UR Z7 §2.2.11
| Survey / age | What must be gauged |
|---|---|
| SS 1 · age ≤ 5 | Suspect areas, anywhere in the ship. Nothing else is mandatory. |
| SS 2 · 5 < age ≤ 10 | Suspect areas. One transverse section of deck plating inside amidships 0.5L, in way of a cargo space where the ship has one. |
| SS 3 · 10 < age ≤ 15 | Suspect areas. Two transverse sections inside amidships 0.5L, in way of two cargo spaces where applicable. All cargo hold hatch covers and coamings, plating and stiffeners. Internals in the forepeak and afterpeak ballast tanks. |
| SS 4 and after · age > 15 | Everything in SS 3, but with a minimum of three transverse sections. Added on top: the exposed main deck, every plate of it, over the whole length of the ship; a representative amount of the exposed superstructure decks, meaning poop, bridge and forecastle; on every transverse bulkhead in a cargo space, the lowest strake and the strakes at 'tween-deck level, together with the internals behind them; every wind-and-water strake, both sides, stem to stern; every keel plate over the whole length, and bottom plates as well where they run past cofferdams, the machinery space and the aft end of tanks; seachest plating. How much shell plating around overboard discharges gets gauged is left to the attending surveyor. |
Source for the whole table: UR Z7 Table 1 Two notes under that table change the count. On ships under 100 m in length the sections may drop to one at SS 3 and to two at subsequent special surveys; on ships over 100 m, exposed deck plating inside amidships 0.5L may be called for at SS 3. Gauging of internals may be specially considered where the hard protective coating is GOOD.
At annual survey Z7 sets no extent table at all. What it requires is that suspect areas from previous surveys are examined, that the areas of substantial corrosion are gauged, and that the gauging is expanded until the extent of the substantial corrosion is known — all of it before the annual survey is credited. Ballast tanks are examined where the special or intermediate survey results made that necessary, and gauged where the surveyor considers it necessary or where corrosion is extensive. UR Z7 §3.2.4
At intermediate survey, likewise, Z7 gives no gauging table. It extends the second or third annual survey to internal examination: representative ballast tanks for ships between 5 and 10 years, all ballast tanks over 10 years, selected cargo holds on dry cargo ships over 15 years that are not covered by Z10.2, Z10.5 or Z7.1, and selected cargo spaces on ships over 10 years that are not dry-cargo-only and not covered by Z10.1, Z10.3, Z10.4 or Z7.2. Where the examination shows no visible defects it may stop at confirming the coating still works. Gauging follows from what that examination finds, not from a table. UR Z7 §4.2
When the extension for substantial corrosion is ordered, the pattern is the familiar one: plating gets a 5-point pattern over 1 square metre on the suspect plate and the adjacent plates; stiffeners get 3 readings in line across the web and 3 across the flange on the suspect area. Substantial corrosion in Z7 is wastage over 75% of the allowable margin, still inside acceptable limits. UR Z7 Table 2 · §1.2.8 The same table is Table III in Z7.1 and Table V in Z7.2, with the same numbers.
Z7.1 — general dry cargo ships
Z7.1 catches general dry cargo ships that are self-propelled, of 500 gt and upwards that carry solid cargo, and then cuts a long list back out again. Out go the ships already under Z10.2 or Z10.5; the dedicated trades, meaning container carriers, wood chip carriers and cement carriers; ro-ro cargo ships and refrigerated cargo ships; livestock carriers; deck cargo ships; and any general dry cargo ship built with a double side skin that runs the length of the cargo area and the full height of the hold up to the upper deck. Hybrid ships take Z7.1 only in way of the single-side-skin holds. UR Z7.1 §1.1.1
| Survey / age | Close-up (Table I) | Gauging (Table II) |
|---|---|---|
| SS 1 · age ≤ 5 | Selected shell frames in one forward and one aft hold with their 'tween decks; one selected hold transverse bulkhead; all hold hatch covers and coamings. | Suspect areas. |
| SS 2 · 5 < age ≤ 10 | Selected shell frames in every hold and 'tween deck; one transverse bulkhead in each hold; forward and aft bulkhead of one side ballast tank with stiffening; one transverse web with plating and framing in two representative ballast tanks of each type; all hatch covers and coamings; selected areas of deck plating and underdeck structure inside the hatch line; selected areas of inner bottom. | Suspect areas. Deck plating over one transverse section, taken in way of a cargo space and inside amidships 0.5L. Readings on the members under close-up survey, for general assessment and to record the corrosion pattern. |
| SS 3 · 10 < age ≤ 15 | All shell frames in the forward lower hold and 25% of frames in every remaining hold and 'tween deck, with end attachments and adjacent shell plating; all hold transverse bulkheads; all ballast tank transverse bulkheads with stiffening; all transverse webs in every ballast tank; all hatch covers and coamings; all deck plating and underdeck structure inside the hatch line; all inner bottom plating. | Suspect areas. Two transverse sections inside amidships 0.5L, in way of two different cargo spaces. Close-up members as above. Within the cargo length area, each deck plate outside the line of the hatch openings. All wind-and-water strakes within the cargo length area, and selected wind-and-water strakes outside it. |
| SS 4 and after · age > 15 | All shell frames in all holds and 'tween decks with end attachments and adjacent shell plating. Everything else as SS 3. | Suspect areas. Within the cargo length area: minimum three transverse sections inside amidships 0.5L; each deck plate outside the hatch line; each bottom plate including the lower turn of bilge; duct keel or pipe tunnel plating and internals. Close-up members as above. All wind-and-water strakes full length. |
Source for the whole table: UR Z7.1 Table I UR Z7.1 Table II Ships under 100 m: one section at SS 3, two at SS 4 and after. Hold bulkhead close-up is taken at set levels — immediately above the inner bottom, immediately above the 'tween decks, mid-height on holds without 'tween decks, and immediately below the main deck and 'tween deck plating.
The annual survey on these ships is not empty. Between 10 and 15 years it takes an overall survey of one forward and one after hold with their 'tween decks. Over 15 years it takes an overall survey of every hold, plus close-up of at least 25% of frames — the lower third of the side frame at the shell, the frame end attachment and the adjacent shell plating — in a forward lower hold and one other selected lower hold. Find something needing repair there and the close-up goes to all frames in those holds and is extended into the remaining holds. UR Z7.1 §3.2.5
The intermediate survey over 15 years is the one that surprises owners. It runs to the same extent as the previous special survey, with one carve-out: item 2c) of the special survey column for ships over 15 — each bottom plate including the lower turn of bilge — is not required. Tank testing, air pipe head survey, and internal examination of fuel oil, lube oil and fresh water tanks also drop out unless the attending surveyor wants them. It may be started at the second annual survey and finished at the third. UR Z7.1 §4.2.4
Z7.2 — liquefied gas carriers
Z7.2 is a hull document. It covers the ship's structure and the piping in the pump rooms, compressor rooms, cofferdams, pipe tunnels, void spaces and cargo-area fuel tanks, and all ballast tanks — but not the piping covered by UR Z16, and not the cargo installation, which is Z16's job. The cargo tanks themselves are not gauged under this document. UR Z7.2 §1.1.2
| Survey / age | Close-up (Table I) | Gauging (Table II) |
|---|---|---|
| SS 1 · age ≤ 5 | One complete web frame ring, adjacent members included, in a ballast tank chosen to represent the topside, hopper side and double-hull side types; the lower part of one transverse bulkhead in a ballast tank, with its girder system and adjacent members. | One section of deck plating over the full beam within 0.5L amidships, in way of a ballast tank where one exists. Readings on the close-up members for general assessment and corrosion pattern. Suspect areas. |
| SS 2 · 5 < age ≤ 10 | All web frames in one ballast tank — a double hull side tank or a topside tank, or another tank if neither is fitted; one web frame in each remaining ballast tank; one complete transverse bulkhead in each ballast tank. | Within the cargo area: each deck plate, and one transverse section within 0.5L amidships in way of a ballast tank where one exists. Selected wind-and-water strakes outside the cargo area. Close-up members. Suspect areas. |
| SS 3 · 10 < age ≤ 15 | All web frames in all ballast tanks; all transverse bulkheads in all ballast tanks. Same extent at every subsequent special survey. | Within the cargo area: each deck plate, two transverse sections, all wind-and-water strakes. Selected wind-and-water strakes outside the cargo area. Close-up members. Suspect areas. |
| SS 4 and after · age > 15 | As SS 3 — the close-up table stops increasing after 10 years. | Within the cargo area: each deck plate, three transverse sections, each bottom plate, duct keel plating and internals. All wind-and-water strakes full length. Close-up members. Suspect areas. |
Source for the whole table: UR Z7.2 Table I UR Z7.2 Table II Where two or three transverse sections are called for, at least one must take in a ballast tank inside 0.5L amidships, if the ship has one. On ships with independent type C tanks and a midship section like a general cargo ship, the gauging may be increased to include the tank top plating at the surveyor's discretion, and the close-up extent may be specially considered.
Gas carriers get their own intermediate close-up table, which Z7.1 does not have. Between 10 and 15 years: all web frames and both transverse bulkheads in one representative ballast tank, the upper part of one web frame in another representative ballast tank, and one transverse bulkhead in another representative ballast tank. Over 15 years: all web frames and both transverse bulkheads in 2 representative ballast tanks. Ballast tank overall survey at intermediate follows the same age split as Z7 — representative tanks from 5 to 10 years, all tanks over 10. UR Z7.2 Table III
Check the revision before you quote it. The current text of Z7.2 is Rev.8, May 2019, applied to surveys commenced on or after 1 July 2020. It carries no corrigendum — the May 2024 Corr.1 that landed on Z7 and Z7.1 did not touch it. UR Z7.2 Rev.8 note 9
Which document governs, by ship type
| Ship | Additional document | Rest of the ship |
|---|---|---|
| Oil tanker | Z10.1 | Z7 |
| Bulk carrier | Z10.2 | Z7 |
| Chemical tanker | Z10.3 | Z7 |
| Double hull oil tanker | Z10.4 | Z7 |
| Double side skin bulk carrier | Z10.5 | Z7 |
| General dry cargo ship ≥ 500 gt, single side skin | Z7.1 | Z7 |
| Liquefied gas carrier | Z7.2 (cargo installation: Z16) | Z7 |
| Container ship, ro-ro, reefer, wood chip, cement, livestock, deck cargo, double-side-skin general cargo | none | Z7 alone |
Source: UR Z7 §1.1 UR Z7.1 §1.1.1 The last row is the one that gets missed: those ships have no additional document, so the whole hull is gauged to the Z7 table. A single-hold ship in the excluded list still picks up the SOLAS II-1/25 water ingress detection items from Z7.1 §2.6 and §3.3.
What Corr.1 of May 2024 changed
Nothing in the extents. The corrigendum edits one clause in each document — the clause on acceptance criteria for thickness measurements, which points the reader at the society's own rules and at the specific URs that carry the diminution limits: UR S18 for corrugated transverse bulkheads, and UR S21 or UR S21A for cargo hatch covers and coamings on exposed decks. UR Z7 §1.5 UR Z7.1 §1.6
The change is in the footnotes to that clause, and it records what happened to S21A. Before: S21A applied to ships contracted for construction on or after 1 July 2012, and S21A Rev.1 to ships contracted on or after 1 July 2016. Corr.1 records that IACS withdrew S21A on 1 July 2024 and put UR S21 Rev.6 in its place, and adds a second footnote saying S21 Rev.6 applies to ships contracted for construction on or after 1 July 2024. The visible reference in the clause body becomes "S21 or S21A" instead of S21A alone. Z7 §1.5 fn.2–3
What that means on the sheet: for a hatch cover or coaming on a ship contracted before 1 July 2024 the acceptance criteria are unchanged — S21A still governs those hulls even though the UR itself is withdrawn for new contracts. For hulls contracted on or after that date, S21 Rev.6 is the reference. Neither document sets its own diminution numbers for hatch covers; both send you to the class rules and to S18 and S21/S21A. The implementation notes at the front of Z7 and Z7.1 stop at Rev.29 (surveys commenced on or after 1 July 2023) and Rev.15 (surveys commenced on or after 1 July 2020) respectively — the corrigendum carries no separate survey-date trigger of its own.
Sources
- UR Z7 Rev.29 Corr.1IACS — Hull classification surveys, May 2024official text ↗
- UR Z7.1 Rev.15 Corr.1IACS — Hull surveys for general dry cargo ships, May 2024official text ↗
- UR Z7.2 Rev.8IACS — Hull surveys for liquefied gas carriers, May 2019official 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.