Section 2 Repair of pitting

2.1.1 The welding of internally pitted bottom shell plating may be carried out afloat provided the following additional requirements are complied with:

1. That the pitting is adequately prepared in respect of shape, cleanliness and dryness.

2. Such repairs are not permitted in the following circumstances:

   1. Where pitting is closely spaced.

   2. On grade E or EH quality steels, or above.

   3. Where the thickness of the material left at the bottom of the pit is less than 6 mm.

3. Welding consumables are to be selected from the List of Approved Welding Consumables for use in Ship Construction.

4. The pits are to be welded in a downhand position, and the selection of correct consumables is to ensure that the appropriate toughness will be achieved under the conditions of accelerated cooling provided by the water on the outside of the shell.

5. The use of submerged arc welding in this application has not always provided optimal results. Where it is proposed as a repair, guidance should be sought from an appropriate welding specialist.

6. Whatever process and consumable is used, the heat input must not be less than 1,5 KJ/mm. If, for example, the welding is carried out with a covered electrode as small as 4 mm diameter, then it must be deposited at a controlled rate such that the length of weld run does not exceed the electrode length consumed.

7. Taking into account the rapid cooling due to the presence of water outside, a random inspection of the first run of the welding, followed by a random inspection of the finished weld, is to be carried out by the Surveyor. These random inspections should be supplemented by NDE methods for the detection or discovery of any cracks.

2.1.2 An alternative to Ch 12, 2.1 General 2.1.1 is to use an external cofferdam fixed by a qualified diving contractor to isolate the pitting from the water.

2.1.3 Full details of the exact nature, location and extent of repairs should be reported and a suitable note should be made in the Memoranda in the case of repairs involving the use of epoxy compound.

2.1.4 The permanent repair of bottom shell pitting by fitting large numbers of welded doubling plates is not acceptable.

2.1.5 Where the pitting occurs in the bottom of a tank forming part of a tank top, the intensity of the pitting should be examined and the Surveyor should consult with the Owner/Naval Authority as to the best method of repair to be carried out in the tank.