1 General

1.1 Application

1.1.1 Net thickness approach

The net thickness, t, of a structural element is required for structural strength in compliance with the design basis. The corrosion addition, t_c, for a structural element is derived independently from the net scantling requirements as shown in Figure 1. This approach clearly separates the net thickness from the thickness added to address the corrosion that is likely to occur during the ship-in-operation phase. This approach enables the status of the structure with respect to corrosion to be clearly ascertained throughout the life of the ship.

1.1.2 Local and global corrosion

The net thickness approach distinguishes between local and global corrosion. Local corrosion is defined as uniform corrosion of local structural elements, such as a single plate or stiffener. Global corrosion is defined as the average corrosion of larger areas, such as primary supporting members and the hull girder.

1.1.3 Exceptions in gross scantling

Items that are directly determined in terms of gross scantlings do not follow the net scantling approach, i.e. they already include additions for corrosion but without any owner’s extra margin. Gross scantling requirements are identified with the suffix "gr" and examples are:

• Scantlings of superstructures and deckhouses as given in Ch 11, Sec 1.
• Scantlings of massive pieces made of steel forgings and steel castings.

Figure 1 : Net scantling approach scheme

1.2 Gross and net scantling definitions

1.2.1 Gross required thickness

The gross required thickness, t_{gr_req}, is the thickness obtained by adding the corrosion addition as defined in Ch 3, Sec 3 to the net required thickness, as follows:

t_{gr_req} = t_req + t_c

1.2.2 Gross offered thickness

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

t_{gr_off} = t_{as_built} – t_{vol_add}

1.2.3 Net offered thickness

The net offered thickness, t_off, is obtained by subtracting the corrosion addition from the gross offered thickness, as follows:

t_off = t_{gr_off} – t_c = t_{as_built} – t_{vol_add} – t_c

1.3 Scantling compliance

1.3.1 The net required thickness, t_req, is obtained by rounding the net thickness calculated according to the Rules to the nearest half millimetre. For example:

• For 10.75 ≤ t < 11.25 mm, the Rule required net thickness is 11.0 mm.
• For 11.25 ≤ t < 11.75 mm, the Rule required net thickness is 11.5 mm.

1.3.2 Scantling compliance in relation to the Rules is as follow:

• The net offered thickness of plating is to be equal to or greater than the net required thickness of plating.
• The required net section modulus, moment of inertia and shear area properties of local supporting members are to be calculated using the net thickness of the attached plate, web and flange. The net sectional dimensions of local supporting members are defined in Figure 2. The required section modulus and web net thickness apply to areas clear of the end brackets.
• The offered net sectional properties of primary supporting members and the hull girder are to be equal to or greater than the required net sectional properties which are to be based on the gross offered scantling with a reduction of the applicable corrosion addition, as specified in Table 1, applied to all component structural members.
• The strength assessment methods prescribed are to be assessed by applying the corrosion reduction specified in Table 1 to the offered gross scantlings. Half of the applied corrosion addition specified in Table 1 is to be deducted from both sides of the structural members being considered.
• Corrosion additions are not to be taken less than those given in Ch 3, Sec 3, [1.2].

Any additional thickness specified by the owner or the builder is not to be included when considering the compliance with the Rules.

Figure 2 : Net sectional properties of local supporting members

Figure 3 : Net sectional properties of local supporting members (continued)

The net cross-sectional area, the moment of inertia about the y-axis and the assosiated neutral axis position are to be determined applying a corrosion magnitude of 0.5 t_c deducted from the surface of the profile cross-section.