What does UR Z10.4 require for double-hull oil tankers, in plain language?
Z10.4 is the enhanced-survey rule for double hull oil tankers. It fixes five extent tables: close-up extent, thickness measurement extent, tank testing, the substantial-corrosion expansion, and the intermediate survey — all keyed to the Special Survey number, not to the calendar.
The short answer
- Who it covers: every self-propelled double hull oil tanker, for the hull and piping in way of cargo tanks, pump rooms, cofferdams, pipe tunnels, void spaces in the cargo area and all ballast tanks.
- Cycle: Special Survey every 5 years, annual within ±3 months of the anniversary, intermediate at or between the 2nd and 3rd annual.
- Close-up: Table I, by Special Survey number — one web frame at SS No.1, all web frames in all ballast tanks at SS No.3.
- Gauging: Table II — suspect areas only at SS No.1, then 1, 2 and 3 transverse sections at SS No.2, 3 and 4.
- Current text: Rev.18 applies to surveys commenced on or after 1 July 2024.
What counts as a double hull oil tanker
The definition clause is the one technicians argue about, so read it in full. A double hull oil tanker is a ship built primarily to carry oil in bulk, whose cargo tanks form an integral part of the hull and are protected by a double hull running the entire length of the cargo area, made up of double sides and double bottom spaces used for water ballast or left as voids. The words “forming an integral part of the ship's hull” were added in a later revision and are underlined in the Rev.18 text — an independent-tank ship is not what this rule is aimed at. UR Z10.4 §1.2.1
The footnote pulls in ships you might not expect: the rule also applies to existing double hull tankers that do not comply with MARPOL Regulation 13F but have a U-shaped midship section. The cargo meant is MARPOL Annex I cargo. A combined cargo/ballast tank — one carrying cargo or ballast as a routine part of the operation — is treated as a ballast tank, which drags it into the ballast-tank close-up and coating regime; a cargo tank that may take ballast only in the exceptional case of MARPOL I/18(3) stays a cargo tank. UR Z10.4 §1.2.2 bis
Substantial corrosion is wastage in excess of 75% of the allowable margins but still within acceptable limits. On a CSR ship the same words mean a measured thickness between tren + 0.5 mm and tren, the renewal thickness. Coating is graded GOOD (minor spot rusting only), FAIR, or POOR — general breakdown over 20% or more, or hard scale at 10% or more, of the area under consideration. GOOD is the grade that buys relief later in the rule. UR Z10.4 §1.2.10 / §1.2.12
The survey cycle
Special Surveys run on a 5-year interval. An extension of class of 3 months maximum beyond the fifth year is possible in exceptional circumstances, and then the next period of class still starts from the pre-extension expiry date. The Special Survey may be commenced at the 4th Annual Survey; if it is commenced earlier than that, the whole survey has to be finished within 15 months for the work to be credited. UR Z10.4 §2.1.1–2.1.4
You cannot credit the same gauging twice: concurrent crediting of a space to both the Intermediate and the Special Survey is not acceptable. The dry dock survey is part of the Special Survey, and the lower portions of cargo and ballast tanks — below the light ballast water line — get their overall survey, close-up survey and gauging there if not already done. UR Z10.4 §2.1.5 / §2.2.2.1
Annual Surveys fall within 3 months either side of the anniversary date. The Intermediate Survey is held at or between the 2nd and 3rd Annual Survey, and its extent depends on age: at 5–10 years it is an overall survey of representative ballast tanks plus the suspect areas from previous surveys; from 10 years on it is the same extent as the previous Special Survey, minus pressure testing of cargo and ballast tanks and minus the hull girder longitudinal strength evaluation, unless the attending surveyor wants them. Over 15 years a dry dock survey becomes part of the Intermediate Survey too. UR Z10.4 §4.2 + Tbl V
Close-up survey extent, by Special Survey number
An overall survey of all tanks and spaces is done at every Special Survey. The close-up work on top of it is the grid below. Read the age bands as what they are — the Special Survey number, not the ship's birthday.
| Survey | Ballast tanks | Cargo oil tanks | Transverse bulkheads |
|---|---|---|---|
| SS No.1 age ≤ 5 | One web frame, in one ballast tank | One deck transverse, in one cargo oil tank | One complete, in a ballast tank; one lower part in a cargo oil centre tank; one lower part in a cargo oil wing tank |
| SS No.2 5 < age ≤ 10 | All web frames in one ballast tank, plus the knuckle area and the upper part (about 5 m) of one web frame in every other ballast tank | One deck transverse, in each of two cargo oil tanks | One complete, in each ballast tank; one lower part in two cargo oil centre tanks; one lower part in a cargo oil wing tank |
| SS No.3 10 < age ≤ 15 | All web frames, in every ballast tank | All web frames — deck transverse and cross ties included — in one cargo oil tank; one web frame in each of the others | All of them, in all cargo oil tanks and all ballast tanks |
| SS No.4+ age > 15 | Everything required for the 10–15 year band, plus any additional transverse areas the Society considers necessary | ||
Source for the whole table: UR Z10.4 Tbl I Note (6) fixes what the knuckle area is. Take the web frame where the slope hopper plating connects to the inner hull bulkhead and to the inner bottom plating; the knuckle area is that part of the frame, running out no more than 2 m from the corners, measured on the bulkhead and on the double bottom alike. Where no centre cargo tanks are fitted, the wing-tank transverse bulkheads are surveyed instead (Note 2). Note 1 fixes what counts as one ballast tank for the table: apart from the fore and aft peak tanks, the ballast compartments on one side — hopper tank, side tank and double-deck tank — are taken together with the double bottom, which counts across the full breadth where the longitudinal centre girder is not watertight and side by side where it is. UR Z10.4 Tbl I Note 1
Two levers move the table. The surveyor may extend the close-up survey — in particular in tanks whose structural arrangements have given trouble on sister ships, and in tanks with scantlings reduced under an approved corrosion control system. Where the hard protective coating is in GOOD condition, the Table I extent may be specially considered. UR Z10.4 §2.3.3 / §2.3.4
Thickness measurement extent, by Special Survey number
| Survey | Inside the cargo area | Wind-and-water strakes |
|---|---|---|
| SS No.1 age ≤ 5 | Suspect areas only | — |
| SS No.2 5 < age ≤ 10 | Each deck plate; 1 transverse section | Selected strakes outside the cargo area |
| SS No.3 10 < age ≤ 15 | Each deck plate; 2 transverse sections; all wind-and-water strakes | Selected strakes outside the cargo area |
| SS No.4+ age > 15 | Each deck plate; 3 transverse sections; each bottom plate | All strakes, full length |
Source for the whole table: UR Z10.4 Tbl II Suspect areas are gauged at every one of these surveys. On top of the grid, from Special Survey No.2 onwards, the structural members subject to close-up survey under Table I are gauged for general assessment and recording of the corrosion pattern — that item is absent from the Special Survey No.1 column, which carries suspect areas alone. UR Z10.4 Tbl II item 4 Where two or three sections are called for, at least one must include a ballast tank within 0.5L amidships.
The sections are not yours to pick at random: they are chosen where the largest reductions are suspected, or where the deck plating readings point. For tankers of 130 m and upwards over 10 years of age, the sampling method for the hull girder longitudinal strength evaluation is in Annex III Appendix 3. As with close-up, GOOD coating lets the Society specially consider the Table II extent. UR Z10.4 §2.4.5 / §2.4.6
Substantial corrosion expands the job
Find substantial corrosion and Table IV takes over. It runs to five sheets covering bottom and hopper structure, deck structure, wing ballast tanks, longitudinal bulkheads in cargo tanks, and transverse watertight and swash bulkheads. The pattern is nearly always the same: five points on a defined area of plating, three across a web and one or three on the flange. A sample of the rows:
- Bottom, inner bottom and hopper plating: a minimum of three bays across the double bottom tank including the aft bay, plus measurements around and under all suction bell mouths — five-point pattern on each panel between longitudinals and floors.
- Bottom, inner bottom and hopper longitudinals: minimum three longitudinals in each bay where the bottom plating is measured — three readings in line across the flange, three on the vertical web.
- Deck plating: two transverse bands across the tank, minimum three readings per plate per band. Deck longitudinals: every third longitudinal in each of the two bands — three readings vertically on the web, two on the flange if fitted.
- Wing ballast tank side shell and longitudinal bulkhead plating: on the upper strake, and on the strakes that sit in way of the horizontal girders, read the plating between every pair of longitudinals across at least three bays; on all other strakes, every third pair over the same bays.
- Upper and lower stools: a transverse band within 25 mm of the welded connection to the inner bottom or deck plating, and a band within 25 mm of the connection to the shelf plate — five-point pattern between stiffeners over one metre length.
Source: UR Z10.4 Tbl IV/1–5
Timing matters on the report. The extended measurements are to be taken before the survey is credited as completed — not raised as an outstanding item. Suspect areas identified at previous surveys are re-examined, and areas of substantial corrosion found at previous surveys are re-gauged. On CSR ships those substantial-corrosion areas have to be examined, and further thickness measurements taken, at annual and intermediate surveys as well. UR Z10.4 §2.4.2
Who may gauge, and where the points are defined
The gauging is carried out by a firm certified by the Classification Society under the principles in Table VII: the firm submits its organisation, its experience, the careers of its technicians — operators qualified to a recognised industrial NDT standard — the equipment and its calibration procedures, an operators' guide, the training programme, and a record format matching Annex II; then an audit of the firm; then an onboard demonstration with satisfactory reporting. The certificate is renewed or endorsed at intervals not exceeding 3 years, and approval may be cancelled where measurements were improperly carried out or improperly reported. The surveyor witnesses the gauging unless the Society does it itself, the firm attends the survey planning meeting, and gauging in close-up areas is done at the same time as the close-up survey. UR Z10.4 §7.2 / Tbl VII
The number and location of points sits in §7.3 and its Table 1, with Figures 5 to 8 showing typical double hull arrangements. That clause applies to CSR-built ships. On a ship that was not built to the IACS Common Structural Rules, how many readings are taken and where they fall is set instead by the society's own rules, or by whichever IACS UR applies, and it turns on how old the ship is and which structural elements are in question. Table 1 is where the arguments end: “selected plates” means a single point on one out of three plates; deck plating, bottom plating and wind-and-water strakes take at least two points per plate, at the 1/4 extremities or in areas of average corrosion; a transverse section takes one point on each plate and both web and flange on every longitudinal. For tankers older than 10 years, within 0.1D of deck and bottom at each transverse section, every longitudinal and girder is measured on web and face plate and every plate gets one point between longitudinals. Transverse rings take at least two points per plate in a staggered pattern and two on the flange, minimum 4 points on the first plate below deck. UR Z10.4 §7.3 Tbl 1 The per-plate patterns are in K6.2.
The report gives location, measured thickness and original thickness, plus date, equipment type, names and qualifications of personnel, and the operator's signature, following the principles in Annex II. The surveyor reviews the final report and countersigns the cover page, and the substantial corrosion found goes into the Executive Hull Summary with its diminution and corrosion pattern. UR Z10.4 §7.4 / Tbl IX
Building the Z10.4 sheet before you board
Draft Operator lays out the transverse sections and the Table I close-up locations against the survey number, so the drawing arrives on the tablet already scoped — ⊞ Scope → SS No.3.
Open Draft OperatorSources
- UR Z10.4 Rev.18IACS — Hull surveys of double hull oil tankers, Feb 2023official text ↗
- UR Z7IACS — Hull classification surveys, the base rule Z10.4 sits on top of (Z10.4 §1.1.2)official text ↗
- IACS CSRIACS Common Structural Rules — renewal thickness and acceptance criteria referenced by Z10.4 §1.2.10 and §8official 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.