Section 2 Corrosion margins

2.1.1 The allowable diminutions will depend upon the design philosophy to which the ship was built. Guidance is given in the following paragraphs on the allowances to be used. Plan Approval advice should be sought if doubt on allowable diminutions exists.

2.1.2 Local diminution is defined as that which is limited to an area of approximately 3 m².

2.1.3 Nominal thickness is the thickness of the material fitted, as shown on the approved plan. It should be noted that nominal thickness may be greater than a thickness derived from the rule formulation, see Figure 11.1.1 Material thickness and Figure 11.1.2 Plan notation. However, the nominal thickness should always be considered as the minimum, subject to an appropriate rolling allowance, as it is this value upon which global strength calculations are based. It could also be possible for the plans to include an enhanced thickness allowance, see Fig.10.1.2.

2.1.4 In general, the following are based on a net scantling approach with a nominal thickness:

• Rules for Naval Ships.
• Rules for Special Service Craft.
• Other Naval design standards.

The following are based on a gross scantling with prescribed diminutions:

• Rules for Ships

2.1.5 Areas of plating that are found to have substantial corrosion as defined in Ch 11, 2.2 Naval Ship and Special Service Craft Rules, Ch 11, 2.3 Ship rules and Ch 11, 2.4 Other design standards, are to be raised as memoranda items and are subject to annual inspection and gauging to the satisfaction of the Surveyor.

2.2.1 For ships designed to the Rules for Naval Ships or the Rules and Regulations for the Classification of Special Service Craft, for all structure diminution may be allowed down to the nominal thickness (excluding an enhancement), less 0,5 mm. This includes an allowance for the negative rolling tolerance of the plate which is normally 0,3 mm, regardless of thickness, and 0,2 mm to allow for the accuracy of measurement equipment.

2.2.2 For practical purposes, local diminution up to 10% may be allowed in members that are not longitudinally effective. See Vol 1, Pt 6, Ch 4, 1.4 Calculation of hull section modulus of the Rules for Naval Ships. Diminution between 0,5 mm and 10% is considered substantial corrosion.

2.2.3 For members that are longitudinally effective, diminutions greater than those given in Ch 11, 2.2 Naval Ship and Special Service Craft Rules 2.2.1 may be permitted subject to a recalculation of the buckling capability of the corroded section. However, there is no guarantee that a recalculation will allow unrestricted operation to be achieved or extreme and residual strength notations to be maintained. Such diminution is also considered substantial corrosion.

2.2.4 At transverse sections, the total diminution of the topsides and bottom areas should be minimal, where it exceeds 2%, plan approval advice should be sought.

2.3.1 For ships designed to the Rules and Regulations for the Classification of Ships (hereinafter referred to as the Rules for Ships) a corrosion margin is included in the scantlings. Allowable thickness diminution should be determined using the publication Thickness Measurement and Close-up Survey of Ships for the appropriate ship category.

2.3.2 Substantial corrosion for these ships is defined as that which exceeds 75 per cent of the allowable diminution but is less than the diminution limits for the particular structure under consideration. Such structure is subject to additional inspection.

2.3.3 Plans will show thicknesses that include an allowance for corrosion.

2.4.1 For ships designed to a standard other than those mentioned in Ch 11, 2.1 General and Ch 11, 2.2 Naval Ship and Special Service Craft Rules the corrosion margin and rolling tolerance are to be determined from the designer's calculations, with reference to the appropriate plan approval office upon transfer into class. Suitable thickness diminutions can then be determined and a suitable memoranda item raised which details the allowable thickness diminution and what is considered substantial corrosion.

2.4.2 In some cases the approved plans may be marked by the Plan Approval Surveyor, with the minimum allowable thickness in addition to the fitted thickness, this will be based upon global and local strength calculations.