– NET SCANTLING APPROACH

Symbols

tas_built : As-built Thickness: the actual thickness, in mm, provided at the newbuilding stage, including tvoluntary_addition, if any.

tC : Corrosion Addition Thickness: as defined in Ch 3, Sec 3, in mm.

tgross_offered : Gross Thickness Offered: the actual gross (full) thickness, in mm, provided at the newbuilding stage, excluding tvoluntary_addition, the owner’s extra margin for corrosion wastage, if any.

tgross_required : Gross Thickness Required: the gross (full) thickness, in mm, obtained by adding tC to the Net Thickness Required.

tnet_offered : Net Thickness Offered: the net thickness, in mm, obtained by subtracting tC from the Gross Thickness Offered

tnet_required : Net Thickness Required: the net thickness, in mm, as required by the Rules that satisfy all the structural strength requirements, rounded to the closest half millimetre.

tvoluntary_addition : Thickness for Voluntary Addition: the thickness, in mm, voluntarily added as the owner’s extra margin for corrosion wastage in addition to tC.

1. General philosophy 1.1

1.1.1

Net Scantling Approach is to clearly specify the “net scantling” that is to be maintained right from the newbuilding stage throughout the ship’s design life to satisfy the structural strength requirements. This approach clearly separates the net thickness from the thickness added for corrosion that is likely to occur during the ship-in-operation phase.

2. Application criteria 2.1 General

2.1.1

The scantlings obtained by applying the criteria specified in this Rule are net scantlings as specified in [3.1] to [3.3]; i.e. those which provide the strength characteristics required to sustain the loads, excluding any addition for corrosion and voluntarily added thickness such as the owner’s extra margin, if any. The following gross offered scantlings are exceptions; i.e. they already include additions for corrosion but without voluntarily added values such as the owner’s extra margin:

• scantlings of superstructures and deckhouses, according to Ch 9, Sec 4

• scantlings of rudder structures, according to Ch 10, Sec 1

• scantlings of massive pieces made of steel forgings, steel castings.

2.1.2

The required strength characteristics are:

• thickness, for plating including that which constitutes primary supporting members

• section modulus, shear area, moments of inertia and local thickness for ordinary stiffeners and, as the case may be, primary supporting members

• section modulus, moments of inertia and first moment for the hull girder.

2.1.3

The ship is to be built at least with the gross scantlings obtained by adding the corrosion additions, specified in Ch 3, Sec 3, to the net scantlings. The thickness for voluntary addition is to be added as an extra.

3. Net scantling approach 3.1 Net scantling definition

3.1.1 Required thickness

The gross thickness required, tgross_required, is not less than the gross thickness which is obtained by adding the corrosion addition tC as defined in Ch 3, Sec 3 to net thickness required, as follows:

C

The gross thickness offered, tgross_offered, is the gross thickness provided at the newbuilding stage, which is obtained by deducting the thickness for voluntary addition from the as-built thickness, as follows:

addition

Net thickness offered, tnet_offered, is obtained by subtracting tC from the gross thickness offered, as follows:

C

3.1.4 Net section modulus for stiffener

The net transverse section scantling is to be obtained by deducting tC from the gross thickness offered of the elements which constitute the stiffener profile as shown in Fig 1.

For bulb profiles, an equivalent angle profile, as specified in Ch 3, Sec 6 [4.1.1], may be considered.

The net strength characteristics are to be calculated for the net transverse section.

In assessing the net strength characteristics of stiffeners reflecting the hull girder stress and stress due to local bending of the local structure such as double bottom structure, the section modulus of hull girder or rigidity of structure is obtained by deducting 0.5tC from the gross thickness offered of the related elements.

Shadow area is corrosion addition.

For attached plate, the half of the considered corrosion addition specified in 3.2 is deducted from both sides of the attached plate.

Figure 1: Net scantling of stiffener

3.2 Considered net scantling

3.2.1 Yielding check of the hull girder

The net thickness of structural members to be considered for the yielding check of the hull girder, according to Ch 5, Sec 1, is to be obtained by deducting 0.5tC from the gross thickness offered.

3.2.2 Global stress such as stress due to hull girder bending moment and shear force

The net thickness of structural members to be considered for stress due to hull girder bending moment and shear force according to Ch 5, Sec1, is to be obtained by deducting 0.5tC from the gross thickness offered.

3.2.3 Buckling check of the hull girder

The net thickness of structural members to be considered for the buckling check, according to Ch 6, Sec 3, is to be obtained by deducting tC from the gross thickness offered.

3.2.4 Ultimate strength check of the hull girder

The net thickness of structural members to be considered for the ultimate strength check of the hull girder, according to Ch 5, Sec 2, is to be obtained by deducting 0.5tC from the gross thickness offered.

tf

hw

tw

bp

tp

bf

3.2.5 Direct strength analysis

The net thickness of plating which constitutes primary supporting members to be checked stresses according to Ch 7 is to be obtained by deducting 0.5tC from the gross thickness offered.

The net thickness of plating members to be considered for the buckling check according to Ch 6, Sec 3, using the stresses obtained from direct strength analysis, is to be obtained by deducting tC from the gross thickness offered.

3.2.6 Fatigue check

The net thickness of structural members to be checked for fatigue according to Ch 8 is to be obtained by deducting 0.5tC from the gross thickness offered.

3.2.7 Check of primary supporting members for ships less than 150 m in length L

The net thickness of plating which constitutes primary supporting members for ships less than 150 m in length L, to be checked according to Ch.6, Sec.4, [2], is to be obtained by deducting tc from the gross thickness.

3.3 Available information on structural drawings

3.3.1

The structural drawings are to indicate for each structural element the gross scantling and the renewal thickness as specified in Ch 13, Sec 2.

If thickness for voluntary addition is included in the as-built thicknesses, this is to be clearly mentioned and identified on the drawings.