Section 13 Steel renewal criteria

13.1.1 The plans to be supplied on board the ship unit, see Pt 10, Ch 1, 2.3 Plans and information to be supplied on board the unit, are to include both the as-built and renewal thicknesses. Any Owner’s extra thickness is also to be clearly indicated on the drawings. The as-built Midship Section plan provided by the Builder and carried on board the ship unit is to include a Table showing the minimum allowable hull girder sectional properties for the mid-tank transverse section in all cargo tanks.

13.1.2 Re-examination and additional thickness measurements are required annually where the coating condition is POOR, as defined in Pt 1, Ch 3, 1.5 Definitions 1.5.12.

13.1.3 Steel renewal is to be carried out when the measured thickness is less than:

t = mm, rounded up to the nearest 0,25 mm

For tank bottom plating:

t = 6 + mm, rounded up to the nearest 0,25 mm

where

and are defined in Pt 10, Ch 1, 12.3 Corrosion additions 12.3.2.

13.2.1 General corrosion is defined as areas where general uniform reduction of material thickness is found over an extensive area.

13.2.2 Pitting corrosion is defined as scattered corrosion spots/areas with local material reductions which are greater than the general corrosion in the surrounding area. The pitting intensity is defined in Pt 10, Ch 1, 13.3 Local structure 13.3.1.

13.2.3 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 Pt 10, Ch 1, 13.3 Local structure 13.3.3.

13.2.4 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 Pt 10, Ch 1, 13.3 Local structure 13.3.3.

13.3.1 For local structure when general corrosion has occurred, steel renewal is required where the measured thickness is less than the renewal thickness:

t = + + mm, rounded up to the nearest 0,25 mm

where

and are defined in Pt 10, Ch 1, 12.3 Corrosion additions 12.3.2.

For inspection intervals greater than one year, this criterion assumes that a localised reduction in the net thickness can be tolerated.

For all levels of pitting intensity, steel renewal is also required where the average thickness across any cross-section in the plating is less than the renewal criteria for general corrosion given in Pt 10, Ch 1, 13.3 Local structure 13.3.1.

13.3.3 Where the overall height or breadth of edge corrosion is less than 25 per cent of the stiffener height or flange breadth, or in Pt 10, Ch 1, 13.3 Local structure 13.3.3, steel renewal is required where the measured thickness is less than:

• 0,7 (as-built thickness – Owner’s extra); or
• 1 mm less than the renewal thickness.

Steel renewal is also required where the average thickness across the breadth or height of the stiffener is less than the renewal criteria for general corrosion given in Pt 10, Ch 1, 13.3 Local structure 13.3.1.

Where edge corrosion extends over more than 25 per cent of the height or breadth of the stiffener, local renewal criteria for general corrosion as defined in Pt 10, Ch 1, 13.3 Local structure 13.3.1 is to be used.

13.3.5 For grooved areas, steel renewal is required where the measured thickness is less than:

• 0,75 (as-built thickness – Owner’s extra);
• 0,5 mm less than the renewal thickness; or less than 6 mm.

Members with areas of grooving greater than 15 per cent of the web height or 30 mm are to be assessed based on the criteria for general corrosion as defined in Pt 10, Ch 1, 13.3 Local structure 13.3.1, using the average measured thickness across the plating/stiffener.

13.4.1 The following actual hull girder sectional properties are required to be verified:

1. vertical hull girder moment of inertia, about the horizontal axis;
2. hull girder section modulus about the horizontal axis – at deck-at-side;
3. hull girder section modulus about the horizontal axis – at keel;
4. hull girder section modulus about the vertical axis – at side;
5. hull girder vertical shear area.

13.4.2 The minimum allowable hull girder section properties are to be calculated with every member at a thickness equal to its required net minimum thickness plus half the applicable corrosion addition given in Pt 10, Ch 1, 12.3 Corrosion additions.