published last reviewed 2026-08-04 reviewer — rev 2 5 sources

What changes for thickness measurement on CSR ships?

On a CSR ship the number your reading is measured against is t_ren = as-built minus the corrosion addition minus any voluntary addition. The corrosion addition is a built-up figure from a rule table, not a percentage of the as-built, so the same reading can pass on a pre-CSR sister and fail here.

The short answer

Which ships: bulk carriers of freeboard length 90 m and above, double hull oil tankers of 150 m and above, self-propelled, unrestricted navigation, contracted for construction on or after 1 July 2015.
The renewal number: t_ren = t_as-built − t_c − t_{vol_add}. Renewal is required once the measured thickness falls below it.
The corrosion addition t_c: the two one-side values from the rule table added together, rounded up to the next half millimetre, plus a reserve thickness of 0.5 mm. Never less than 2 mm in total, or 1.5 mm for stiffener webs and face plates and for internals of dry spaces.
The band: a reading in t_ren ≤ t_m < t_ren + 0.5 mm is not a renewal but it is not clear either — it is the range in which alternative solutions may be adopted under the survey URs.
Hull girder: over 10 years of age the measured thicknesses also have to keep the section modulus at deck and bottom at or above 90% of the value calculated with the gross offered thickness.

Read this before you quote a number

Everything below is the CSR 1 January 2024 version, which entered into force on 1 July 2024. That is the text that was read for this page and every clause is cited from it.

The edition currently in force is the Consolidated 1 July 2025 edition, which is this same text with Corrigenda 1 applied. Those corrigenda were not available when this page was written and are not reflected here.

Before this page is published, a reviewer has to open the consolidated edition and confirm each cited clause and each table value against it. Cite these numbers as "CSR 1 Jan 2024 version" with the clause, never as "Consolidated 2025".

Which ships are CSR ships

Three conditions have to be met together. The ship is a bulk carrier or a double hull oil tanker; it is self-propelled and has unrestricted navigation; and it was contracted for construction on or after 1 July 2015. The contract date is the date the building contract was signed between the prospective owner and the builder, and IACS Procedural Requirement No.29 settles the edge cases. It is the contract date that decides, not the keel date and not the delivery date, so a ship delivered in 2017 can still fall outside. CSR Pt 1 Ch 1 Sec 1 [1.1.1]

Then the size limit, and it is different for the two ship types. Bulk carriers, single side skin and double side skin alike, are caught at a freeboard length L_LL of 90 m and above. Double hull oil tankers are caught at 150 m and above, an oil tanker being a ship that has to comply with Annex I of MARPOL 73/78. CSR Pt 1 Ch 1 Sec 1 [1.2.1]/[1.3.1]

Five bulk carrier types are named as being outside the rules even when they meet the length: ore carriers, combination carriers, woodchip carriers, ships whose inner bottom is adapted for self-unloading, and cement, fly ash and sugar carriers where loading and unloading is not done by grabs heavier than 10 tons, power shovels or anything else that can damage the hold structure. Read the ship's particulars and the class notation before you assume. If the ship is not a CSR ship, the renewal figures on this page do not apply to it. CSR Pt 1 Ch 1 Sec 1 [1.2.1]

The net thickness approach, and where the corrosion addition comes from

This is the thing that makes a CSR ship different at the gauge. The strength calculation is done on the net thickness — the steel that has to be there for the structure to work. The corrosion addition is worked out separately and added on top, and the sum is the gross thickness that gets built. The two are kept apart deliberately so that the corrosion state of the ship can be read off at any point in its life. No credit is given anywhere for coatings. CSR Ch 1 Sec 2 [4.3.4]

Four thicknesses are defined, and the office will use all four. t_{as_built} is what was actually fitted at the newbuilding stage. t_{vol_add} is any extra the owner or the builder chose to put on as their own margin against wastage, over and above the rule corrosion addition. The gross offered thickness is t_{gr_off} = t_{as_built} − t_{vol_add}, and the net offered thickness is t_off = t_{as_built} − t_{vol_add} − t_c. Voluntary addition never counts towards rule compliance. CSR Ch 3 Sec 2 [1.2]

The corrosion addition itself is built up, not picked off a single line. Each side of a plate or a stiffener gets its own one-side value, t_c1 and t_c2, according to what is on that side. The two are added, rounded up to the next half millimetre, and a reserve thickness t_res of 0.5 mm is added on top. For a member sitting wholly inside one compartment the one-side value is simply doubled and the same half-millimetre reserve added. Whole-member minimum is 2 mm, dropping to 1.5 mm for stiffener webs and face plates and for internals of dry spaces. Stainless steel and aluminium members carry the reserve alone, t_c = 0.5 mm, and the minimums do not apply to them. CSR Ch 3 Sec 3 [1.2.1]–[1.2.2]

One-side corrosion addition t_c1 or t_c2, in mm — the members a technician meets most often
Where the surface faces	Member	Oil tanker	BC-A / BC-B, L_LL ≥ 150 m	Other BC
Ballast water tank, bilge tank, drain storage tank, chain locker	Face plate of PSM, within 3 m below top of tank	2.0
	Face plate of PSM, elsewhere	1.5
	Other members, within 3 m below top of tank	1.7
	Other members, elsewhere	1.2
Cargo oil tank, slop tank	Inner-bottom plating / bottom of tank	2.1	not applicable
	Face plate of PSM, within 3 m below top / elsewhere	1.7 / 1.4	not applicable
	Other members, within 3 m below top / elsewhere	1.7 / 1.0	not applicable
Dry bulk cargo hold	Transverse bulkhead, upper part	n/a	2.4	1.0
	Lower stool — sloping plate, vertical plate, top plate	n/a	5.2	2.6
	Transverse bulkhead, other parts	n/a	3.0	1.5
	Hopper tank sloped plating, inner bottom plating	n/a	3.7	2.4
	Other members, upper part; webs and flanges of upper end brackets of single-side-skin side frames	n/a	1.8	1.0
	Webs and flanges of lower brackets of single-side-skin side frames	n/a	2.2	1.2
	Other members, other parts	n/a	2.0	1.2
Exposed to atmosphere	Weather deck plating	1.7
Exposed to atmosphere	Every other member	1.0
Exposed to seawater	Shell plating between minimum design ballast draught and scantling draught waterlines	1.5
Exposed to seawater	Shell plating elsewhere	1.0
Fuel and lube oil tank	One value for the whole compartment; the rule breaks out no members here.	0.7
Fresh water tank	One value for the whole compartment; the rule breaks out no members here.	0.7
Void spaces ⁽⁸⁾	Spaces that nobody normally enters: reached only through bolted manholes and the like, pipe tunnels, and the inner face of a stool space that bounds neither a dry bulk cargo hold nor a ballast cargo hold.	0.7
Dry spaces	Internals of machinery space, pump room, store rooms, steering gear space	0.5

Source for the whole table: CSR Ch 3 Sec 3 Tbl 1 These are one-side values; two of them make a member. Fuel and lube oil tank, fresh water tank and void spaces are three separate rows of the source table that happen to print the same 0.7; only the void-space row carries the "not normally accessed" description, so a fuel oil tank or a fresh water tank takes 0.7 whether or not anyone goes into it. Note (8) hangs off that same void-space row and it bites on the shell: when you work out the corrosion addition for the outer shell plating, treat a pipe tunnel as a water ballast tank, so the shell plate that bounds a pipe tunnel does not take the void-space 0.7 on its inboard face. Ch 3 Sec 3 Tbl 1 note (8) The full grid, including the eight footnotes that add 0.3 to 1.0 mm for chain locker, heated cargo and heated fuel oil boundaries, and the definitions of "upper part" and of the lower-stool zone where no stool is fitted, belongs in handbook table T10 — CSR renewal thresholds. Two rules that catch people out: the corrosion addition of a stiffener follows the location where it lands on the attached plating, and where one plate is touched by more than one value, the heaviest value is applied to the whole strake — with one exception, for vertical corrugations arranged by vertical seams in oil tankers, where the actual corrosion additions above and below the line 3 m below top of tank may be used for the parts above and below that line respectively. Ch 3 Sec 3 [1.2.3]–[1.2.4]

Renewal thickness: the number your reading is compared against

For local structural elements — that is local supporting members and primary supporting members — steel renewal is required when the measured thickness t_m is less than the renewal thickness. CSR Ch 13 Sec 2 [2.1.1]

t_ren    = t_as-built − t_c − t_vol_add     renew below this
t_repair = t_as-built − t_vol_add           insert plate, not less than this
band     = t_ren ≤ t_m < t_ren + 0.5 mm     alternative solutions may be adopted

Two things follow. First, t_ren is a derived figure: it is nowhere a fixed percentage of the as-built thickness, so you cannot work it out from the reading alone. Second, when a wasted area is renewed, the insert goes back at the as-built thickness less any voluntary addition, in steel of the same or better grade and yield stress. The insert is not put back at the renewal thickness. CSR Ch 13 Sec 2 [2.1.2]

Worked through, on a side shell plate of a double hull tanker with a ballast tank inboard and the sea outboard, in the strake below the ballast draught waterline: inboard face 1.2 (ballast tank, other members, elsewhere), outboard face 1.0 (shell plating elsewhere). Sum 2.2, rounded up to 2.5, plus reserve 0.5 gives t_c = 3.0 mm. On a plate of 16.0 mm as-built with no voluntary addition, t_ren = 13.0 mm and the band runs 13.0 to 13.5 mm. That is 19% off the as-built before renewal bites — on the hopper sloped plating of a large BC-A bulk carrier the same arithmetic gives a much bigger allowance, because the one-side value there is 3.7. The percentage is an output, never an input. worked from Ch 3 Sec 3 Tbl 1 + Ch 13 Sec 2

The half-millimetre band, and the hull girder check

On a pre-CSR ship the trigger for extra attention is wastage past 75% of the allowable margin — a proportion of a diminution allowance. On a CSR ship it is a fixed band instead: the reading sits at or above the renewal thickness but less than the renewal thickness plus the reserve thickness, and the reserve thickness is 0.5 mm flat, whatever the member and whatever the as-built. That is the range in which alternative solutions may be adopted under the survey rules the society applies. Same wastage on the same plate, one ship pre-CSR and one CSR, and the two are judged by different arithmetic. CSR Ch 13 Sec 2 [2.1.3]

Local corrosion is only half of it. CSR splits corrosion into local — uniform wastage of a single plate or stiffener — and global, the average wastage of large areas such as primary supporting members and the hull girder. Both have to be satisfied; steel renewal is required if either allowance is exceeded. CSR Ch 13 Sec 1 [1.2]

The global check runs at special surveys once the ship is over 10 years of age, using the measured, renewed and reinforced thicknesses. The current hull girder section modulus at deck and at bottom must stay at or above 90% of the value calculated with the gross offered thickness; alternatively the summed measured item areas of the deck zone and of the bottom zone must each stay at or above 90% of the same zone calculated gross. For the neutral axis zone the measured plating area must stay at or above the gross figure less 0.5 t_c. There is a screening rule: if the actual reduction over all contributing items of a transverse section is under 10% for the deck and bottom zones and under 0.5 t_c for the neutral axis zone, that section passes and the zone-area calculations are not needed. Which plates and stiffeners belong to which zone is fixed by the rule, and side shell plating appears in all three depending on height. CSR Ch 13 Sec 2 [2.2] / [1.2]

Pitting, edge corrosion, grooving: where they are not

This needs saying plainly, because the opposite is widely assumed. The 1 January 2024 CSR text carries no acceptance criteria for pitting, edge corrosion or grooving. The whole of the renewal chapter is two sections: principles and survey requirements, then acceptance criteria, and the acceptance criteria are the two given above — local corrosion against t_ren, and global corrosion against the hull girder percentages. Nothing on pit depth, nothing on pitting intensity, nothing on a groove or a wasted free edge. CSR Ch 13 Sec 2, whole section

CSR is explicit about where those requirements do live. The renewal chapter hands the whole hull survey job back to whichever of a society's own rules or documents brings the Z10 series in: UR Z10.2 if the ship is a single side skin bulk carrier, UR Z10.4 if it is a double hull oil tanker, UR Z10.5 if it is a double side skin bulk carrier. That hand-back covers the survey extent, the measurement patterns and the local corrosion acceptance criteria that go with them. CSR gives the strength arithmetic; the Z10 series gives the survey. On a CSR ship you work to both at once. CSR Ch 13 Sec 1 [1.1.4] / [2.1.1]

What this means when you fill the sheet

A reading of 13.2 on a 16.0 plate is a routine entry on a pre-CSR ship and sits inside the reserve band on the CSR sister worked above. The reading has not changed. What changed is the number underneath it. So the as-built column on your sheet stops being paperwork and starts being the input to the judgement — a wrong as-built value on a CSR ship moves t_ren by the same amount and can turn a renewal into a pass.

The values are on board, and CSR requires them to be. One copy of the midship sections, construction profiles, shell expansion, transverse bulkheads, aft and fore part structures and machinery space structures has to be supplied on board indicating both the newbuilding and the renewal thickness for each structural item, and any thickness for voluntary addition has to be clearly marked on the plans as well. The onboard midship section also carries the minimum required hull girder sectional properties for the representative transverse sections of all cargo holds, which is what the office needs for the global check. Superstructures, deckhouses and casing carry the newbuilding thickness only. CSR Ch 1 Sec 3 [2.2.3] · Ch 13 Sec 1 [1.3]

In practice the drawing you are handed on board does not always carry the renewal column, and then the office has to rebuild it: as-built minus the corrosion addition for that member in that compartment minus any voluntary addition. That is three lookups per member, not one, and it depends on which side of the plate faces what. Two habits pay for themselves on a CSR ship:

Record the compartment on both sides of every plate you gauge, not just the location. Ballast one side and sea the other is a different corrosion addition from ballast one side and cargo hold the other.
Note whether the point is within 3 m below the top of the tank, because that band carries its own higher value in ballast tanks, cargo oil tanks and slop tanks with the weather deck as tank top.
On stiffeners, note where the stiffener lands on the plating — that is what sets its corrosion addition, not where the web sits.
Where a reading falls in the 0.5 mm band, say so on the sheet. It is not a failure and it is not clean, and the surveyor needs it flagged rather than buried in a column.

The full value grid — every compartment against every member category, with the footnote additions and the "upper part" and lower-stool definitions — belongs in handbook table T10, CSR renewal thresholds, not on this page.

Sources

CSR Pt 1 Ch 13IACS CSR BC & OT, 1 Jan 2024 version (in force 1 Jul 2024) — Ship in Operation: Renewal Criteriaofficial text ↗
CSR Pt 1 Ch 3 Sec 2IACS CSR BC & OT, 1 Jan 2024 version — Net Scantling Approachofficial text ↗
CSR Pt 1 Ch 3 Sec 3IACS CSR BC & OT, 1 Jan 2024 version — Corrosion Additionsofficial text ↗
CSR Pt 1 Ch 1 Sec 1IACS CSR BC & OT, 1 Jan 2024 version — Application and scopeofficial text ↗
CSR Pt 1 Ch 1 Sec 3IACS CSR BC & OT, 1 Jan 2024 version — Documentation, plans supplied onboardofficial 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.