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Section 12 SHIP IN OPERATION RENEWAL CRITERIA

1. Allowable Thickness Diminution for Hull Structure

1.1 General

1.1.1 Applicability

1.1.1.1 The purpose of this Section is to provide criteria for the allowable thickness diminution of the ships hull

structure.

1.1.1.2 The criteria apply only to ships in operation that are designed and built in accordance with this part.

1.1.1.3 Thickness measurements are to be used to assess the ships structure against the specified renewal criteria.

1.1.2 Wastage allowance concept

1.1.2.1 Wastage allowance is comprised of two aspects; local wastage allowance and overall hull girder wastage

allowance. Local wastage allowance is defined in 1.4and the overall hull girder wastage allowance is defined in 1.5.

1.1.2.2. Assessment against both local and overall hull girder wastage criteria is required during the operational life of

the vessel.

1.1.2.3 Steel renewal is required if either the local or overall hull girder wastage allowance is exceeded.

1.1.2.4 The newbuilding requirements within this part incorporates corrosion additions, see Section 6/3,and consider

all relevant loads and failure modes (e.g. yielding, buckling, and fatigue). No further assessment of the scantlings

against the requirements within this part is required during the operational life of the ship provided that the thickness

of any structural member remains greater than the renewal thickness specified herein.

1.1.3 Requirements for documentation

1.1.3.1 The plans to be supplied onboard the ship, see Section3/2.2.3,are to include both the as-built and renewal

thickness as defined in 1.4.2. Any owners extra thickness is also to be clearly indicated on the drawings.

1.1.3.2 The as-built Midship Section plan provided by the builder and carried on board the ship is to include a

table showing the minimum allowable hull girder sectional properties, as defined in 1.5, for the mid-tank transverse

section in all cargo tanks.

1.2 Assessment of Thickness Measurements

1.2.1 General

1.2.1.1 The minimum survey requirements for the maintenance of class of double hull oil tankers are defined in Part

Bof the Rules.

1.2.1.2 Thickness measurements are to be conducted in accordance with 5.2.6, Part Bof the Rules.

1.2.2 Assessment of local wastage

1.2.2.1 Thickness measurements are to be taken to confirm that the measured thickness is not less than the renewalthickness for general corrosion and local pitting/edge corrosion as defined in1.4.2 and 1.6respectively.See also 1.3.

1.2.2.2 When a survey identifies that steel renewal is required or structural defects are present which, in the opinion

of the Surveyor, will impair the ships fitness for continued service, remedial measures are to be implemented before

the ship continues in service.

1.2.2.3 Re-examination and additional thickness measurements at Annual and Intermediate Surveys are required

where the measured thickness, tm, is less than the allowable thickness at annual survey,tannual, defined as:

wasownbuiltasannual tttt = (mm)

Where:

tas-.built : as built thickness, in mm

twas : wastage allowance, as defined in 1.4.2.2town : owner/builder specified additional wastage allowance, if applicable,

in mm

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1.2.2.4 Where re-examination and additional thickness measurements are required by 1.2.2.3 then additional

measurements are to be carried out in accordance with Table 12.1.1 to determine the full extent of the corrosion

pattern.

Table 12.1.1 Additional Thickness Measurement in way of Structure Identified with tm< tannual

Structural

member

Extent of measurement Pattern of measurement

Plating Suspect areas and adjacent plates 5pointpattern over 1m2

Stiffeners Suspect areas 3 measurements in line across web

3 measurements in line across flange

1.2.2.5 At each Special Survey, thickness measurements are to be taken in way of critical areas, as considered

necessary by the Surveyor. Critical areas are to include locations throughout the ship with corrosion levels that are

likely to contravene1.2.2.3and/or are considered prone to rapid wastage.

1.2.3 Assessment of overall hull girder wastage

1.2.3.1 The hull girder sectional properties of the ship are to be calculated for the cross-sections as specified in Part

Bof the Rules, based on the thicknesses given by the thickness measurements, to confirm that the resulting hull

girder sectional properties are not less than the minimum allowable defined in 1.5.2. The actual sectional properties

calculated based on measured thicknesses and in accordance with Part Bof the Rules, are to be submitted to the

Society.

1.3. Categories of Corrosion

1.3.1 General corrosion

1.3.1.1 General corrosion is defined as areas where general uniform reduction of material thickness is found over an

extensive area.

1.3.1.2 Renewal criteria for general corrosion are given in 1.4.

1.3.2 Pitting corrosion

1.3.2.1 Pitting corrosion is defined as scattered corrosion spots/areas with local material reductions which are greater

than the general corrosion in the surrounding area.

1.3.2.2 The pitting intensity is defined in Fig. 12.1.1.

1.3.2.3 Renewal criteria for pitting corrosion are given in1.6.2.

1.3.3 Edge corrosion

1.3.3.1 Edge corrosion is defined as local corrosion at the free edges of plates, stiffeners, primary support members

and around openings. An example of edge corrosion is shown in Fig.12.1.2.

1.3.3.2 Renewal criteria for edge corrosion are given in 1.6.3.

1.3.4 Groove corrosion

1.3.4.1 Groove corrosion is typically local material loss adjacent to weld joints along abutting stiffeners and at

stiffener or plate butts or seams. An example of groove corrosion is shown in Fig. 12.1.3.

1.3.4.2 Renewal criteria for groove corrosion are given in 1.6.4.

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Fig. 12.1.1 Pitting Intensity Diagrams

5% Scattered

10% Scattered

20% Scattered

30% Scattered

50% Scattered

Fig. 12.1.2 Edge Corrosion

hstf

Flatbarstiffener

Attached plating

0.25hstf

Inverted angle orbuilt-up stiffener

bstf

0.25bstf

Attached plating

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Fig. 12.1.3 Groove Corrosion

Flatbar stiffener

Groove breadth Groove breadth

Attached plating

Groovebreadth

Groovebreadth

1.4 Renewal Criteria of Local Structure for General Corrosion

1.4.1 Application

1.4.1.1 The renewal criteria in 1.4.2generally apply to areas of structural members with general corrosion.

1.4.2 Renewal criteria

1.4.2.1 Steel renewal is required if the measured thickness, ttm, is less than the renewal thickness, tren, defined as:

5.2 = corrownwasbuiltasren ttttt (mm)

Where:

tas-.built : as built thickness, in mm

twas : wastage allowance, as defined in1.4.2.2

town : owner/builder specified additional wastage allowance, if applicable,

in mm

tcorr-2.5 : 0.5mm, wastage allowance in reserve for corrosion occurring in the

two and a halfyearsbetween Intermediate and Special surveys

1.4.2.2 The wastage allowance, twas, is given by:

21 += waswaswas ttt (mm) and rounded up to the nearest 0.5mm

Where:

twas : total wastage allowance of the considered structural member, in mm

twas-1 : wastage allowance for side one of the structural member

considering the contents of the compartment to which it is exposed, in

mm, as given Table 12.1.2

twas-2 : wastage allowance for side two of the structural member

considering the contents of the compartment to which it is exposed, in

mm, as givenTable 12.1.2

1.4.2.3 In no case is the wastage allowance, twas, to be less than 1.5mm, except in way of internals of dry spaces and

pump room where 1.0mmis applicable.

1.4.2.4 Wastage allowances for compartments not listed in Table12.1.2 will be subject to special consideration.

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1.4.2.5 Areas which need to be renewed based on the renewal criteria in 1.4.2.1are, in general, to be

repaired with inserted material which is to have the same or greater grade/strength as the original and to

have a thickness, trepair, not less than:

ownbuiltasrepair ttt = (mm)

Where:

tas-built : as built thickness, in mmtown : owner/builder specified additional wastage allowance, if applicable,

in mm

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Table 12.1.2 Local Wastage Allowance for One Side of Structural Elements

Compartment Type Structural Member

Ship in Operation

Component Wastage

Allowance, twas-1 ortwas-2

(mm)

Within 3mbelow top

of tank (1)2.0

Face plate of PSM

Elsewhere 1.5

Within 3mbelow top

of tank (1)1.7

Ballast water tank and chain

locker

Other members

Elsewhere 1.2

Within 3mbelow top

of tank (1)1.7

Face plate of PSM

Elsewhere 1.4

Inner-bottom plating/bottom of tank 2.1

Within 3mbelow top

of tank

(1)

1.7

Cargo oil tank (3)

Other membersElsewhere 1.0

Weather deck plating 1.7Exposed to atmosphere

Other members 1.0

Exposed to sea water Shell plating(2) 1.0

Top of tank and attached internal stiffeners 1.0Fuel and lube oil

tank (3), Elsewhere 0.7

Top of tank and attached internal stiffeners 1.0Fresh water tank

Elsewhere 0.7

Void spaces

Spaces not normally accessed, e.g. access

only via bolted manhole openings, pipe

tunnels, etc.

0.7

Dry spaces

Internals of deckhouses, machinery spaces,

pump room, store rooms, steering gear

space, etc.

0.5

Notes :

Only applicable to cargo and ballast tanks with weather deck as the tank top.

0.5mmto be added for side plating in the quay contact region as defined in Section 8/Fig. 8.2.2.

0.3mmto be added for tanks arranged with heating coils

1.5 Renewal Criteria of Hull Girder Sectional Properties for General Corrosion

1.5.1 General

1.5.1.1 The following actual hull girder sectional properties are required to be verified, see1.5.2-3:

(a) vertical hull girder moment of inertia, about the horizontal axis,Iv

(b) hull girder section modulus about the horizontal axis - at deck-at-side,Zv- dk

(c) hull girder section modulus about the horizontal axis - at keel,Zv- kl

(d) hull girder section modulus about the vertical axis - at side,Zh- side

(e) hull girder vertical shear area,Av- shr

1.5.2 Renewal criteria

1.5.2.1 Steel renewal is required if the actual hull girder sectional properties,Iv-tm, Zv-tm-dk,

Zv-tm-kl,Zh-tm-side,Av-tm-shr, calculated using the actual thickness measurements are less than the minimum allowable hull

girder sectional properties defined in accordance with1.5.3.

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1.5.2.2 The actual hull girder sectional properties listed in 1.5.2.1are to be calculated in accordance with Section

4/2.6, using the measured thicknesses.

1.5.2.3 If steel renewal is required due to reduced hull girder sectional properties this is to be done by replacing local

corroded structural elements. Any combination of structural elements may be replaced provided that the resulting hull

girder sectional properties satisfy 1.5.2.1. Local structural elements being renewed are to be replaced in accordance

with the requirements of 1.4.2.3.

1.5.3 Calculation of the minimum allowable hull girder sectional properties

1.5.3.1 The minimum allowable hull girder sectional properties listed in 1.5.1.1are to be calculated in accordance

with Section 4/2.6, using the thicknesses defined in 1.5.3.2.

1.5.3.2 The minimum allowable hull girder sectional properties in the corroded condition are calculated using the

same corrosion thickness reductions that are used during the newbuilding stage, thus linking the newbuilding and

ship in operation criteria. Therefore the calculation of the minimum allowable hull girder sectional properties is to

be based on a member thickness, t, given by:

owncorrbuiltas tttt = 5.0 (mm)

Where:

tas-built : as built thickness, in mmtcorr : corrosion addition, as defined in Section6/3.2


in mm

1.6 Allowable Material Diminution for Pitting, Grooving and Edge Corrosion

1.6.1 General

1.6.1.1 Steel renewal for pitting, grooving and edge corrosion is required if the measured thickness is less than the

criteria defined in 1.6.2, 1.6.3 and1.6.4respectively.

1.6.2 Pitting

1.6.2.1 For plates with pitting intensity less than 20%, see Fig.12.1.1, the measured thickness, ttm, of any individual

measurement is to meet the lesser of the following criteria:

( )ownbuiltastm ttt 7.0 (mm)

ttmtren1 (mm)

Where:

tas-built : as built thickness of the member, in mm


in mm

tren : renewal criteria for general corrosion as defined in 1.4.2.1

1.6.2.2 The average thickness across any cross section in the plating is not to be less than the renewal criteria for

general corrosion given in 1.4.2.1.

1.6.3 Edge corrosion

1.6.3.1 Provided that the overall corroded height of the edge corrosion of the flange, or web in the case of flat bar

stiffeners, is less than 25%, see Fig.12.1.2, of the stiffener flange breadth or web height, as applicable, the measured

thickness, ttm, is to meet the lesser of the following criteria:


ttm tren1 (mm)

Where:



in mm

tren : renewal criteria for general corrosion as defined in1.4.2.1

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1.6.3.2 The average measured thickness across the breadth or height of the stiffener is not to be less than that defined

in1.4.2.

1.6.3.3 Plate edges at openings for manholes, lightening holes etc. may be below the minimum thickness given in

1.4.2provided that:

(a) the maximum extent of the reduced plate thickness, below the minimum given in1.4.2, from the opening

edge is not more than 20% of the smallest dimension of the opening and does not exceed 100mm

(b) rough or uneven edges may be cropped-back provided that the maximum dimension of the opening is notincreased by more than 10%.

1.6.4 Grooving

1.6.4.1 Where the groove breadth is a maximum of 15% of the web height, but not more than 30mm, see Fig. 12.1.3,

the measured thickness, ttm, in the grooved area is to meet the lesser of the following criteria:


ttmtren0.5 (mm)

but is not to be less than

ttm= 6 (mm)

Where:



in mm

tren : renewal criteria for general corrosion as defined in 1.4.2.1

1.6.4.2 Members with areas of grooving greater than those in 1.6.4.1 are to be assessed based on the criteria for

general corrosion as defined in 1.4.2 using the average measured thickness across the plating/stiffener.

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