What are the permissible diminution limits for oil tankers, class by class?

What each scheme is a limit of

The question "what does class X allow on the deck of a tanker" has an answer for every society on this page, and the answers cannot be lined up beside one another. Not because the societies disagree about corrosion, but because they are not all measuring the same thing. Before a single figure is useful you have to know what kind of number it is. There are five kinds on this page.

A percentage of the as-built or original thickness. ABS, LR, ClassNK, KR and INSB work this way. What varies is the word for the base. KR defines original thickness as the thickness on the drawing, but adds that where the Classification Technical Rules separately specify a required thickness for a member, that figure is followed instead — so on some members the base is not the drawing at all. KR Annex 1-5 Tbl 1 n.4

Four separate figures per member. BV and RINA do not publish one number for a plate. They publish four, and each is assessed on its own, in order: an isolated area, meaning a groove or a patch of severe local corrosion within one item; the item, meaning the average condition of that one plate or that one stiffener; a group, the average across all elements of the same kind, all deck plates together, all bottom longitudinals together; and a zone, the average across every longitudinal element in the deck zone, the neutral axis zone or the bottom zone. Lifting the second number out of that set and calling it "the BV limit" is the single most common way this table gets misread. BV Pt A Ch 2 App 3 [4.3.4]–[4.3.5]

And the BV percentage is not taken off the as-built. It is taken off the rule thickness, with a wear allowance subtracted afterwards: t_min = (1 − W/100) t_rule − t_wc, where t_wc is zero in general. Only where the rule thickness is not available may the as-built be used instead. On a ship where the builder put steel on above the rule requirement, the BV limit and a limit computed from the as-built are different numbers. BV Pt A Ch 2 App 3 [4.3.1]

A diminution coefficient. DNV publishes no percentage in its main table. It publishes a coefficient k and the minimum thickness is t_min = k · t_{gr_off}, where the gross offered thickness is the as-built less any documented owner's voluntary addition. A coefficient of 0.80 reads across to "20 % off" only if the voluntary addition is zero or the paperwork proving it is missing. Where the owner's addition is documented, the same coefficient gives a higher minimum thickness than a flat 20 % of the as-built would. DNV-CG-0182 Sec.2 [3.1.2]

A computed renewal thickness. CSR gives no percentage anywhere. The number a reading is compared against is t_ren = t_as-built − t_c − t_{vol_add}, and the corrosion addition t_c is built up from the compartments on either side of the member. The resulting percentage is an output, and it is different for every member. CSR Pt 1 Ch 13 Sec 2 [2.1.1]

A minimum renewed thickness. CCS, where it publishes a figure at all, states the percentage the other way round — the thickness that must remain, not the wastage that may be lost. Its 75 % for structural members in fore and aft peak ballast tanks of ships of 90 m and above is the complement of a 25 % wastage allowance, not a 75 % one. CCS Ch 3 §II.2 Tbl 3.1

Nothing on this page converts one of these into another. Where a section says one society is stricter than another, it is comparing two figures of the same kind or it says which kinds are being set side by side and what the comparison is worth.

Which column you are reading

Six of the eleven schemes on this page do not have a single column for an oil tanker. They have two or more, and something about the ship decides which one you are in. Get this wrong and every figure below it is wrong.

ABS splits on the bottom configuration and the contract date. Table 1A is headed for non-CSR tankers of 90 m and over built to ABS class, and it carries three columns: contract 2005 or later, double bottom tankers; contract between 1962 and 2004, double bottom tankers; and contract 1962 or later, single bottom tankers. The figures quoted on this page are the first of the three. Where the other columns differ, each section below says so, because the difference is real — on the side shell it is 20 % against 25 %. Two further conditions sit on the whole table: for a CSR ship the individual wastage allowance comes from Part 5A/B instead, and for a vessel built to another society's rules the ABS Technical Office that did the initial plan review has to be asked. ABS 7-A1-4/35.3 Tbl 1A

LR splits on ship category. A non-CSR oil tanker, chemical tanker or liquefied gas carrier of L ≥ 90 m is Category 1 and reads §1.5.1. Category 2 is every remaining ship type at L ≥ 90 m; Category 3 is any ship under 90 m, so a small tanker drops out of Category 1 on length alone and picks up the looser figures. The same section sets LR's substantial corrosion at wastage past 75 % of the permissible diminution, taken on the individual plate or the individual stiffener, which is why the category decision moves the substantial-corrosion trigger as well as the renewal line. LR TM Guidance §1.5

KR splits on class, and the split is not what the name suggests. Class I is a ship of 90 m and above, or a ship carrying liquid cargo, in either case classed at a Classification Survey during Construction after 1 July 1998. Class II is everything else that is not Class III, and Class III is high speed and light craft. So a small tanker is still Class I on the liquid-cargo limb, while a tanker classed after construction rather than during it falls outside the table altogether and takes separate requirements of the Society. KR Annex 1-5 Tbl 1 n.1–2

INSB splits on category on the same shape as LR but with a different list. Category 1 is an oil tanker, chemical tanker, dry bulk cargo ship, combination carrier or liquefied gas ship of L ≥ 90 m; Category 2 is the remaining types at L ≥ 90 m; Category 3 is anything under 90 m. Categories 2 and 3 share one column in the local table, and the difference between Category 1 and that shared column is 10 percentage points on most of the hull envelope. INSB Tbl C · Tbl D n.5

RINA splits on a contract date. Table 9 is for ships contracted for construction on or after 1 June 2000. Table 10 covers earlier ships and is not the same table with different numbers — it is a shorter table, and on several rows it prints two figures where Table 9 prints three. Every RINA figure on this page is Table 9; where Table 10 differs, the section says so. RINA Pt A Ch 2 App 2 Tbl 9/10

And the schemes do not agree on where on the ship they apply. This matters as much as the numbers. BV states its criteria are, as a rule, for the structure within the cargo area of ships over 90 m; outside the cargo area the percentages may be increased by 5, and for ships under 90 m the same increase applies except to the deck and bottom zone figures — but the two increases cannot be applied at the same time. DNV Sec.2 [3.1] is written for structure within 0,4 L amidships and hands structure outside it to [3.2] and a separate table. INSB's global table is keyed to position: one set of figures over 0,5 L amidships, a looser set at 0,075 L from the ends, linear interpolation between. ABS puts no cargo-area limit on the table itself, but note 1 requires that internals included in longitudinal strength be continuous or effectively developed at ends throughout amidships 0,4 L. The rows on this page are cargo-area structure; that is the common ground, not a statement that every scheme stops there. BV [4.3.6] · DNV Sec.2 [3.1] · INSB Tbl D · ABS Tbl 1A n.1

Two more scope conditions worth carrying before the figures start. DNV Sec.2 does not apply to a ship with the class notation CSR, to a container ship, to naval vessels, to high speed and light craft, yachts and aluminium vessels, or to a ship that has undergone a major conversion — each of those is sent elsewhere in the guideline. And CSR reaches an oil tanker only where it is a double hull oil tanker of L_LL ≥ 150 m contracted on or after 1 July 2015. DNV Sec.2 [1.1] · CSR Ch 1 Sec 1

Strength deck and sheer strake

Start where the technician starts, on the weather deck. On the strength deck plate of a non-CSR tanker the percentage schemes converge harder than anywhere else on the ship. ABS 20 % — and here uniquely all three ABS columns print the same figure, so the double bottom and contract-date question does not bite on this row. LR Category 1 20 %. KR Class I 20 %. INSB Category 1 20 %. DNV k = 0.80 on plating within 0,4 L amidships, which is 20 % of the gross offered thickness. BV and RINA both print 20 % in the item column. seven sources, deck plating row

Two societies sit outside that. ClassNK gives 20 % of original thickness + 1 mm — the same percentage with a millimetre added on top, so on a 18 mm deck plate the NK limit is 13.4 mm against 14.4 mm for a flat 20 %. It is the most permissive local figure on this row. The NK table does not split by ship type at all: the same band covers shell plates, strength deck plates, tight bulkheads in deep tanks and inner bottom plates. NK App 3.5 (1) PRS and CCS publish no figure of their own. PRS Part I §5.1.3.6 sets the acceptance criteria by reference to the PRS Rules Part II — Hull and to named IACS URs, and Part II is not held here; CCS Chapter Three §II.1 refers the local strength criteria to paragraph 2, Appendix 1, Chapter 5, Part One of the CCS Rules, also not held. Those cells are not blank because nobody looked. They are blank because the number lives in a document this handbook does not hold, and the clause that proves it is cited. PRS Pt I §5.1.3.6 · CCS Ch 3 §II.1

Now the part that gets missed. On BV and RINA the deck plate does not have one limit, it has three that apply to it plus a fourth that applies to everything around it: 25 / 20 / 10 for isolated area, item and group, sitting inside a deck zone criterion of 10 %. The group figure is the one that bites. A single deck plate may be 20 % down, but the average of every deck plate together may only be 10 % down. Compared honestly against the item column, BV and RINA are level with ABS and LR; compared at the group column, they are twice as strict as anything else published. Item 1 in both tables is not the deck plate alone — it collects deck plating, deck stringer, sheer strake and the longitudinal bulkhead upper strake, so all four are assessed as one group. BV Tbl 6 item 1 · RINA Tbl 9 item 1

The sheer strake is the row where the source structures diverge without the numbers diverging. ABS gives it its own line, 20 % in all three columns. BV and RINA fold it into item 1 with the deck. KR names it in the first row with the strength deck plating. ClassNK treats it as a shell plate and names the sheer strake only for the flat bar longitudinal beams on it. LR §1.5.1 carries no separate sheer strake line at all — the figure comes from Table 1.5.5, which works by frame number and strake position rather than by member name, and gives deck-and-sheer plating 20 % for Category 1 and 30 % for Categories 2 and 3. If your sheet records a sheer strake reading without the strake letter and the frame, the LR route to a limit is closed. LR Tbl 1.5.5 · ABS Tbl 1A · NK App 3.5

Side shell and longitudinal bulkhead: the neutral axis zone

Move down the side and the picture changes in a way that has a structural reason behind it. Steel near the neutral axis contributes little to the hull girder section modulus and a great deal to shear, so the schemes that model the hull girder explicitly loosen the zone figure here while holding the local figure. BV and RINA print 25 / 20 / 15 for side shell plating against 25 / 20 / 10 for the deck — same isolated area, same item, a looser group — inside a neutral axis zone criterion of 15 % against 10 % for the deck zone. BV Tbl 6 items 5/7 · RINA Tbl 9 items 5/7

The ABS column question does bite here. Side shell plates read 20 % for a tanker contracted 2005 or later with a double bottom, and 25 % in both the 1962–2004 double bottom column and the single bottom column. That is the opposite direction from the way people expect a rule to age: the newer ship gets the tighter allowance, not the older one. Report the contract date and the bottom configuration and the ABS reading follows; report neither and there is a five-point spread with no way to close it. ABS Tbl 1A side shell

LR holds side shell plating at 20 % for Category 1 and does not loosen it toward the neutral axis; a Category 3 tanker under 90 m takes 30 % instead. KR is at 20 % Class I and 20 % + 1 mm Class II, INSB at 20 % Category 1 with wind and water strakes included in the same row, ClassNK at 20 % + 1 mm, DNV at k = 0.80 from the "side and longitudinal bulkhead plating" line. LR §1.5.1 · KR · INSB · NK · DNV Tbl 1

KR adds something the other schemes do not put on this row at all: a second trigger that is stated in millimetres, not per cent. On an oil tanker or chemical tanker, a shear strength evaluation is required when the average corrosion of any strake in the side shell or longitudinal bulkhead exceeds 2,0 mm for Class I or 3,0 mm for Class II. On a 16 mm strake, 2,0 mm average is 12.5 % — the extra evaluation fires well before the 20 % renewal line does. This is a per-strake average, so it needs the strake identity on the sheet, not just the plate. KR Annex 1-5, shear strength

On the longitudinal bulkhead the divergence is about how the source cuts the structure up, not about severity. BV and RINA give the bulkhead itself 25 / 20 / 15 as item 7, but hand the upper strake to item 1 in the deck zone and the lower strake to item 9 in the bottom zone — so one bulkhead can be assessed under three different group criteria depending on height. ABS prints three separate rows and, in the contract 2005 or later column, gives all three 20 %; in the 1962–2004 column the same three rows read 20 / 25 / 25. ClassNK and KR split by bulkhead type instead of by orientation: a tight bulkhead of a deep tank takes 20 % + 1 mm on NK and 20 % Class I on KR, while a watertight bulkhead that is not a deep tank boundary takes 25 % on NK and 25 % Class I on KR. NK's table footnote defines a deep tank as a tank used for the carriage of water, oil or other liquids forming part of the hull in holds or tween decks. BV/RINA items 1·7·9 · ABS Tbl 1A · NK · KR

One condition on this row that is easy to lose and changes the whole reading on a double hull ship: BV and RINA do not give the inner skin its own allowance. Table 6 note (2) and Table 9 note (2) both state that on a double hull oil tanker the structural elements of the inner skin — plating, longitudinals, girders, bulkheads — are included in the corresponding elements of the outer skin. The inner skin longitudinal bulkhead is therefore assessed as part of the side shell group, not as a bulkhead of its own, and the inner bottom with the bottom plating. Neither document held gives a reason for the arrangement; what it does in practice is put the two skins into one average, so a sound outer skin can carry a wasted inner skin through the group criterion and a wasted outer skin can drag a sound inner skin down. BV Tbl 6 n.(2) · RINA Tbl 9 n.(2)

Bottom, keel and inner bottom

The bottom is the other hull girder flange and the schemes treat it as one. BV and RINA put bilge and bottom strakes, the longitudinal bulkhead lower strake and the keel plate together as item 9 at 25 / 20 / 10, inside a bottom zone criterion of 10 % — the deck figures again, mirrored. ABS bottom plates read 20 % for contract 2005 or later double bottom tankers, 25 % in the 1962–2004 double bottom column, and back to 20 % for single bottom tankers. That is not a typographical accident and it is worth pausing on: an old single-bottom tanker is held to a tighter bottom allowance than an old double-bottom one, because the single bottom plate is the only bottom there is. LR, KR Class I, INSB Category 1 and DNV are all at the equivalent of 20 %; ClassNK is at 20 % + 1 mm. ABS Tbl 1A · BV item 9 · RINA item 9

The keel is a special case in the one document that gives it a row. ABS Table 1A prints a Keel Plates line with no percentage in it and hands the reader to note 8. The trigger there is the neighbour: a keel plate comes out once it has fallen to whatever minimum thickness ABS allows on the adjacent bottom plating. So the keel has no allowance of its own — it inherits whichever bottom figure applies to the plate next to it, which in turn depends on the column. LR names bottom shell plating and gives no keel line; ClassNK and KR give none either; BV and RINA fold the keel plate into item 9. If your sheet has a keel reading and no adjacent bottom plate reading, the ABS route to a judgement is incomplete. ABS Tbl 1A n.8

On the inner bottom the ABS table splits into an outermost strake row and an other-plates row. In the contract 2005 or later column both read 20 %; in the 1962–2004 column they read 20 % outermost and 25 % other plates; and the single bottom column prints dashes, because a single bottom tanker has no inner bottom to gauge. Do not read a dash as "no limit". LR gives inner bottom plating 20 % and inner bottom longitudinals 25 %; ClassNK names inner bottom plates in its first band at 20 % + 1 mm; KR names it in the first row at 20 % Class I. ABS Tbl 1A · LR §1.5.1 · NK · KR

And a CSR ship reverses the whole picture on this member. The inner bottom plating forming the bottom of a cargo oil tank or slop tank carries the heaviest one-side corrosion addition in the tanker grid, t_c1 = 2,1 mm, because that is the face that sits under the cargo and the water that settles out of it. Add the ballast-side value beneath it, round up to the next half millimetre, add the 0,5 mm reserve, and the renewal thickness on that plate takes far more than 20 % off the as-built. On a non-CSR sister the same plate is judged at 20 %. Same member, same cargo, two entirely different numbers, and neither is wrong. CSR Ch 3 Sec 3 Tbl 1 · Ch 13 Sec 2 [2.1.1]

Longitudinals and stiffeners: where the schemes part company

This is the row where the eleven schemes are furthest apart, and the spread is not noise. Take a deck, side or bottom longitudinal in the cargo area of a non-CSR tanker. ABS gives one figure to everything internal — 20 % in the contract 2005 or later column, 25 % in the other two — and does not split web from flange at all; the row collects longitudinals, girders, transverses, struts, bulkhead webs and stringers and brackets in one line. LR gives 25 %. DNV gives k = 0.75 for stiffeners, which is 25 % of the gross offered thickness, while girders and stringers stay at 0.80. KR gives 25 % Class I and 30 % Class II. INSB gives 25 % — and this is one of three rows where INSB Categories 2 and 3 carry the same figure as Category 1. The other two are the stiffeners-and-corrugated-plating line of the watertight and oiltight transverse bulkheads, and the stiffeners line of the miscellaneous members. All three are stiffener lines, all three read 25 % in both columns, and on those three the category question does not move the figure. ClassNK is the most permissive at 30 %. ABS · LR · DNV Tbl 1 · KR · INSB · NK

Notice the direction. Almost every scheme allows more off a longitudinal than off the plate it is welded to — LR 25 against 20, DNV 0.75 against 0.80, KR 25 against 20, NK 30 against 20 % + 1 mm. None of the documents held states why. What can be said from the sources themselves is that ABS attaches a structural condition to the internals row rather than a looser number: internals included in longitudinal strength must be continuous or effectively developed at ends throughout amidships 0,4 L. A discontinuous longitudinal does not get the allowance at all. ABS Tbl 1A n.1

BV and RINA go the other way and split the member. Web 25 / 20, flange 20 / 15 — isolated area and item — with a group criterion of 10 % for deck and bottom longitudinals and 15 % for side shell longitudinals and stringers. The flange is held tighter than the web, and the group figures follow the zone the longitudinal belongs to, exactly as the plating does. That is the only place in this comparison where a document distinguishes the two parts of a stiffener by number, and it means a BV reading needs the web and the flange recorded separately or it cannot be assessed. On RINA the split is newer than the ship in many cases: Table 10, for ships contracted before 1 June 2000, prints 25 / 20 for web and flange together. BV Tbl 6 items 2/6/10 · RINA Tbl 9/10

Three exceptions on this row that a summary would flatten, and each one is the difference between a pass and a renewal:

• KR pulls deck and sheer strake longitudinals out of the 25 % row. Longitudinals attached to the strength deck plating and the sheer strake take 20 % Class I and 20 % + 1 mm Class II under the first row of the table, not the 25 / 30 % secondary-stiffener row. KR also defines the term: a secondary stiffener is a member supported by a primary supporting member which does not itself support another reinforcement member. KR Annex 1-5 Tbl 1 n.6
• ClassNK does the same thing for flat bars. Flat bar longitudinal beams on the sheer strake and strength deck take 20 % + 1 mm under the first band, not the 30 % that covers webs and faces of frames, longitudinal beams, stiffeners and brackets. And where high tensile steel is used in the bottom longitudinals of a single bottom tanker, the web limit drops to 25 %. NK App 3.5 (1)
• LR's Table 1.5.5 states the split as a rule, not a list. For Category 1 the longitudinal structural members split is plating 20 %, stiffeners 25 %. Note 6 to the same table then tells you what to compare against those figures: take a plate as an average over its whole area, and take a stiffener as an average over the span between its supports. A single low reading is not the number. INSB Table C note 3 carries the same instruction for its own table. LR Tbl 1.5.5 n.6 · INSB Tbl C n.3
• CSR does not use the stiffener at all to pick the number. Which corrosion addition applies to a longitudinal is set by the location where it lands on the attached plating, not by where the web sits. A deck longitudinal inside a ballast tank and one inside a cargo oil tank take different values although both are deck longitudinals. CSR Ch 3 Sec 3 [1.2.3]

Read the whole row and the honest summary is this: on a longitudinal, ABS in its newest column is the tightest published local figure at 20 %, the BV and RINA flange item figure of 15 % is tighter still on that one part of the member, and ClassNK at 30 % is the most permissive — a ten-point spread on the same steel. There is no engineering statement in any of the four documents that reconciles them. They simply differ, and the ship's class decides.

Transverse bulkheads, web frames and cross ties

These members carry no hull girder bending, and most schemes give them a little more room than the longitudinal structure. On transverse bulkhead plating, LR is at 25 % with stiffeners and horizontal stringer plating also at 25 %; ClassNK at 25 % for watertight bulkhead plates; KR at 25 % Class I and 30 % Class II; INSB at 25 % Category 1 and 30 % Categories 2 and 3, with stiffeners and corrugated bulkhead plating at 25 % in both columns. ABS is the outlier in the other direction: transverse bulkheads share a row with other plates of longitudinal bulkheads and the topside and hopper tank sloping plating, so they take 20 % in the contract 2005 or later column and 25 % in the other two. LR · NK · KR · INSB Tbl C · ABS Tbl 1A

BV and RINA give the transverse bulkhead its own item with four sub-lines: plating 25 / 20 / 15, stringer web 25 / 20, stringer flange 20 / 15, stiffener web 30 / 20, stiffener flange 25 / 15. The item's note carries a scope condition that is easy to lose: it includes swash bulkheads and the forward and aft peak bulkheads, which are outside the cargo area. That is one of the few places where a cargo-area scheme reaches beyond the cargo area by name. BV Tbl 6 item 18 n.(3) · RINA Tbl 9 item 18

DNV is the only scheme that changes the number because of how the bulkhead is built. A plane transverse bulkhead takes k = 0.75; a corrugated one takes k = 0.80 on both flange and web; and note 2 to the table adds that a bulkhead designed with two plate flanges connected by vertical webs — a double skin bulkhead — also takes 0.80. So the DNV allowance on a tanker's transverse bulkhead is 25 % if it is plane and 20 % if it is corrugated or double skin, off the gross offered thickness in each case. The construction type has to be on the sheet. DNV-CG-0182 Sec.2 Tbl 2 n.2

Web frames in cargo oil tanks and ballast tanks are where LR's tank-type vocabulary has to be read carefully. LR gives web frame plating and face plates 20 % and web frame secondary structure 25 %, under the heading SWBT and COT — where SWBT is any tank put to the use of salt water ballast, LR's footnote naming the peaks, the wing and the centre tanks, the double bottom, the side tanks and the deep tanks, and where COT is a cargo oil tank. That definition is broader than "ballast tank in the cargo area" and it is doing work: it is what puts a peak tank web frame on the same line as a cargo tank web frame. KR gives web and face of a primary supporting member, and the floors and girders of the double bottom, 20 % Class I and 25 % Class II; ClassNK gives primary members web and face 25 %; INSB gives transverse structural members in cargo oil and water ballast tanks 20 % Category 1 and 25 % Categories 2 and 3; DNV splits web 0.80 from flange 0.75; BV and RINA give deck, side shell, longitudinal bulkhead and bottom transverse web frames the same set — web 25 / 20, flange 20 / 15, brackets and stiffeners 25 / 20. LR §1.5.1 SWBT/COT · KR · NK · INSB · DNV · BV/RINA items 13–16

CCS has one thing to say on this row and it is worth carrying, because CCS is otherwise a column of "not published" on a tanker. The Guidelines note that the current CCS Rules define no specific corrosion limit for the interior members of peak ballast tanks, and step in with a figure of their own: a minimum renewed thickness of 75 % of as-built for structural members in fore and aft peak ballast tanks of ships of L ≥ 90 m, and 70 % below 90 m. Read it in the CCS direction: 75 % must remain, so 25 % may go. CCS Ch 3 §II.2 Tbl 3.1

The cross tie is the one member where two schemes that share nothing else move in the same direction. DNV gives the cross tie k = 0.85, the highest coefficient on any member row in its tables and therefore the tightest allowance in the DNV set for a steel member — 15 % of the gross offered thickness against 20 or 25 % for everything around it. One figure in the same table goes higher, and it is not a member row: the note below the table takes any strength member made of stainless steel or of aluminium to k = 0.90, which is 10 %, whatever the member is. BV and RINA drop the cross tie web item figure to 15 % where every other web frame in the table sits at 20 %, giving the set web 25 / 15, flange 20 / 15, brackets and stiffeners 20 / 15. Three of the eleven schemes do not see the cross tie at all: ABS names struts inside the internals row, ClassNK and KR fold it into primary members, and LR's Category 1 list carries no cross tie line. Neither DNV nor BV nor RINA states a reason for singling it out. What can be said is that the two independent schemes that do name it both make it stricter, and no scheme makes it looser. DNV Tbl 2 cross ties · BV/RINA item 17

The hull girder, and where the zone figure sits

Every local figure on this page is conditional on the hull girder. That is not a footnote — on ABS it is written as the condition on the whole table: the individual wastage allowances are acceptable provided the section modulus is not less than 90 % of the greater of the modulus required at new construction or by 3-2-1/3.7.1(b). A tanker can pass every plate on Table 1A and still fail. For tankers of 130 m and above and over 10 years of age, ABS requires the sectional area calculations to be carried out by an ABS Technical Office — the attending surveyor does not settle it. ABS Tbl 1A n.4–5

Now the part the brief for this page exists to answer. The BV and RINA zone figure is not a fourth stricter limit stacked on top of the other three. It is two things at once, and both are stated in the source. First, it is the escape hatch: a greater diminution than the group column may be accepted for one group of items if, taking the other groups in the same zone into account, the overall diminution of the zone does not exceed the zone figure. BV works the example itself — deck plating averaging 12 % against a group criterion of 10 % can be accepted where the deck longitudinals are at 4 %, because the zone average comes out under 10 %. Second, it is the hull girder proxy. Note 1 to §4.3.5 traces the zone column back to a general requirement sitting on the girder itself: across the midship 0,4 L, the section modulus the ship has today still has to reach 90 % of the rule modulus. Run the zone check, though, and BV works from the original modulus rather than the rule one. At the owner's request the Society may run a direct calculation on the ship's current thicknesses to accept a greater diminution than the zone figure allows. BV [4.3.5] n.1 · RINA Tbl 9 zone

So the four BV and RINA numbers are a sequence, not a set of alternatives: isolated area first, then item, then group, then zone, and each step can rescue the one before it. The zone figures for a tanker are deck zone 10 %, neutral axis zone 15 %, bottom zone 10 %, each evaluated separately. On RINA, Table 10 — ships contracted before 1 June 2000 — publishes only deck zone 10 % and bottom zone 10 % and leaves the neutral axis zone blank. On an older RINA tanker there is no neutral axis zone criterion to fall back on. RINA Tbl 9/10 zone column

The other schemes state the hull girder criterion directly and in four different shapes:

• DNV — the reduced section modulus shall not be less than 90 % of the required modulus based on design bending moments, and in any case not less than 90 % of the minimum rule modulus — two tests, not one. Outside 0,4 L amidships a separate table allows an average reduction of deck or bottom of 10 % over the first and last tenth of the length, interpolated over the next tenth. And an ultimate strength check of the vertical hull girder bending capacity is required when the average reduction of deck or bottom structures exceeds 5 % on a single deck vessel of L > 150 m — a trigger that fires at a quarter of the local allowance. DNV Sec.2 [3.1.3]–[3.1.4]
• ClassNK — 90 % of the rule-required hull section modulus for L_f ≥ 130 m under the current rule set, dropping to 80 % at L_f ≤ 60 m with interpolation between. The figure also depends on which rule version the ship was built to: for oil and chemical tankers built to the 1973–1986 versions it is 87 %, and ships built to the 1964–1972 versions are held to the full rule requirement with no reduction at all. The build year is an input to the hull girder criterion, not just to the local one. NK App 4-5 6(1) Tbl 4
• INSB — no modulus percentage in the main path. Instead a positional table: over 0,5 L amidships for a Category 1 ship, topside plating 10 %, topside longitudinals 15 %, bottom plating 10 % on single bottom construction and 15 % on double bottom, bottom longitudinals 15 %. At 0,075 L from the ends the same five read 20 / 25 / 20 / 20 / 25 %, with linear interpolation between — the allowance opens up toward the ends, where the hull girder bending moment is smaller. Note 4 lets a greater reduction be permitted over 0,5 L amidships provided the section modulus from the gauged thicknesses is not less than 90 % of the Rules' modulus for the new ship, with a reassessment of scantlings then required. Note 6 adds a survey consequence rather than a strength one: where the reduction exceeds 0,75 of these values, additional transverse sections are to be measured. INSB Tbl D n.1–7
• CSR — at special surveys once the ship is over 10 years of age, the deck and bottom zone must hold 90 % of the section modulus calculated with the gross offered thickness, and the neutral axis zone area must stay at or above the zone area at gross offered thickness less 0,5 t_c. A screening rule passes the section without the zone-area calculation where the actual reduction of all contributing items is under 10 % for the deck and bottom zones and under 0,5 t_c for the neutral axis zone. CSR Ch 13 Sec 2 [2.2.2]
• LR, KR, PRS and CCS publish no hull girder percentage in the documents held. LR gives a strength deck residual buckling thickness t_r in Table 1.5.6 and states at Table 1.5.5 note 2 that for Category 1 oil tankers that requirement is complied with as advised by the attending Surveyor — a hedge, and it is LR's hedge, not this page's. KR Pt 1 Ch 3 106. 1 (2) requires longitudinal strength to be evaluated on oil tankers of 130 m and upwards after the ship reaches 10 years of age, to criteria set by separate requirements of the Society which are not held; what KR does publish is the shear strength trigger at 2,0 mm and 3,0 mm average strake corrosion. PRS Pt I §5.1.3.6 and CCS Ch 3 §I.1 both point out of the document held. LR Tbl 1.5.5 n.2 · KR 106.1(2) · PRS · CCS §I.1

Five different arithmetics, one common number: 90 %. ABS, BV, RINA, DNV, ClassNK, INSB's note 4 and CSR all land on 90 % of a section modulus, and then differ on which modulus — required at new construction, rule, original, minimum rule, or calculated with the gross offered thickness. Those are not the same denominator, so the same ship can sit above 90 % on one and below on another. The convergence is on the fraction, not on the test.

The full grid

This page is a reading of the tanker dataset — twelve members against eleven schemes, walked in the order a technician gauges them, with the reasoning attached. It is not the whole grid. Every cell, with its own clause reference, its own PDF page and its own conditions, sits in handbook table T01 — Permissible diminution, class by class, which carries the same eleven schemes across three ship types.

Open T01 — permissible diminution, all 451 cells ↗

Use the two together. The page tells you what kind of number you are holding and which column you are in; the table tells you the number and where it came from. Where the two ever disagree, the table is the checked source and this page is the one to correct.

What this means when you fill the sheet

Read back over the eight sections above and every one of them turned on something that is not the reading. Not one of these schemes can be applied to a bare millimetre value. Here is what the sheet has to carry for any of them to work on a tanker.

• The as-built thickness, per point. Not per member, not per strake — per point, because the strake changes thickness along its length. This is the field whose absence makes the whole set unusable: ABS, LR, ClassNK, KR, INSB and CCS all compute from it, DNV computes from it less the voluntary addition, and CSR subtracts the corrosion addition from it. Without it there is no denominator and no scheme on this page produces an answer. Everything else below can be recovered from the ship's papers after the fact. The as-built at the point you gauged cannot.
• Whether an owner's voluntary addition exists, and whether it is documented. DNV's coefficient is applied to the as-built less the documented voluntary addition, and BV works from the rule thickness with the as-built only as a fallback. On a ship built with extra steel, an office that does not know about it will compute a limit that is too low and pass a plate that should have been called.
• The contract date, and the bottom configuration. ABS Table 1A columns turn on contract 2005 or later and on double bottom against single bottom; RINA turns on contract on or after 1 June 2000; KR Class I turns on classification during construction after 1 July 1998; CSR turns on contract on or after 1 July 2015. Four different dates, and none of them is the delivery date.
• Length, and which length. The 90 m threshold sorts LR and INSB categories and KR Class I; CSR needs freeboard length L_LL ≥ 150 m; ClassNK's hull girder criterion is keyed to freeboard length; DNV's ultimate strength trigger to L > 150 m; the ABS and KR technical office triggers to 130 m.
• Position along the ship, not just the frame. DNV changes table outside 0,4 L amidships; INSB interpolates between 0,5 L amidships and 0,075 L from the ends; BV adds five points outside the cargo area. A frame number without the ship's length and perpendiculars cannot be turned into a position.
• The compartment on both sides of the plate. Cargo oil tank, slop tank, ballast tank, void, sea — this sets the CSR corrosion addition on each face, and on LR it decides whether the member is inside the SWBT and COT band. On a non-CSR ship it is still what tells the office whether the member is a deep tank boundary, which moves the ClassNK and KR figure by five points.
• Web separately from flange. BV and RINA hold the flange tighter than the web on every stiffener and web frame; DNV splits web from flange on web frames and floors. A single "longitudinal: 8.2 mm" entry cannot be assessed on either scheme.
• The strake identity, and enough points to average. KR's shear trigger is an average per strake. LR Table 1.5.5 note 6 and INSB Table C note 3 both put the limit on an average: a plate over its whole area, a stiffener over the span between its supports. BV's group and zone criteria are averages of averages. One low reading is not a failure on any of those schemes, and one high reading is not a pass.

Two habits that follow from that list. Record the compartment on both sides even when the ship is not a CSR ship — it costs one column and it is the field most often missing when the office has to go back and ask. And when the plate you are on is a keel plate, gauge the adjacent bottom plating in the same visit, because on ABS the keel has no allowance of its own and inherits that plate's.

Last thing. None of these figures is a target. LR says plainly that each maximum it publishes already carries five more years of average corrosion inside it. LR then sets substantial corrosion at wastage past 75 % of the permissible diminution — so on an LR tanker the extra close-up and gauging work starts at three-quarters of the number on this page, not at the number itself. What counts as substantial corrosion in the other ten schemes is a separate question and has its own page. LR TM Guidance §1.5

Sources

1. ABS Pt 7 App 1 Sec 4ABS Rules for Survey After Construction, Part 7 — July 2026official text ↗
2. BV NR467 Pt A Ch 2 App 3Bureau Veritas NR467 Rules for the Classification of Steel Ships, Part A — R26, July 2026official text ↗
3. LR TM Guidance Pt 1 Ch 5Lloyd's Register Thickness Measurement and Close-up Survey Guidance, Part 1 — Version 7.3, November 2016 (older edition held)official text ↗
4. NK Guidance App 3.5ClassNK Guidance for Undergoing Class Maintenance Surveys — 2014 edition (older edition held)official text ↗
5. RINA Pt A Ch 2 App 2RINA Rules for the Classification of Ships, Part A Ch 2 App 2 — 1 July 2024 interactive set (older edition held)official text ↗
6. DNV-CG-0182DNV-CG-0182 Allowable thickness diminution for hull structure — Edition July 2022 (older edition held)official text ↗
7. KR Pt 1 Annex 1-5KR Rules and Guidance for the Classification of Steel Ships, Part 1 — 2026 editionofficial text ↗
8. PRS Pt I §5.1.3.6PRS Rules for the Classification and Construction of Sea-going Ships, Part I — January 2026official text ↗
9. INSB Pt I Ch 3 Sec 13INSB Rules and Regulations for the Classification and Construction of Steel Ships, Part I — Issue 04 Rev.03, December 2024official text ↗
10. CCS TM Guidelines Ch 3CCS Guidelines for Thickness Measurement of Hull — Ver. 6.6, March 2025official text ↗
11. CSR Pt 1 Ch 13IACS Common Structural Rules for Bulk Carriers and Oil Tankers — 1 January 2024 version, in force 1 July 2024official 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.

Related questions

• How do I read a thickness limit?K5.1
• What counts as substantial corrosion in each class?K5.2
• What are the permissible diminution limits for bulk carriers, class by class?K5.4
• What are the permissible diminution limits for general dry cargo ships?K5.5
• What are the pitting acceptance criteria in each class?K5.6
• How is CSR renewal thickness calculated?K5.7
• What thickness limits apply to ships under 90 m?K5.8