Saturday, January 05, 2013

Notes Relative to Shell and Deck Plate Repairs

1. Recommendations for renewals should be decided on a reasonable and practical basis; that is only what is necessary to place the vessel in serviceable condition fit lo proceed. Appearance “as original” is not essential.

2. To avoid combining the shrinkage stresses of several welds in close proximity in restrained surrounding structures, it is best to avoid too-small insert plates, these are prone to fracture. if possible, renew full width, but in any case not less than 450 mm width, and at least three frame spaces in length (the transversely-framed vessels), or for a length at least as great as the strake width.

3. Always note in the survey report accurately on whether existing butts are utilized or new butts established. This becomes very important in assessing future gauging.

4. Avoid use of a replacement insert plate more than about two millimeters or 10% “t” under or over original thickness in order to minimize stress concentration effect of abrupt change of section.

5. Avoid fitting plates transversely: that is make sure direction of rolling is fore and aft.

6. Whenever plates are cut out for renewal, always check the four edges of surrounding plating to make sure there is sufficient remaining thickness for proper attachment.

7. Small damage areas, say under 450 mm diameter. may satisfactorily be repaired by use of a circular insert plate, but since a small closed circle of weld is prone to cracking, the weld should be made alternating from one side lo the other (back-step) and closely checked for cracks.

8. Adhere to the general rules of plate renewals such as to release seams beyond new butts before welding the butts, weld internals to renewal plate before welding plate joints, weld plate butt before the seams, weld seams toward free end, and not welding seams across an unwelded butt, etc.

9. Small, single or relatively straight cracks in plating may frequently be satisfactorily welded, but first carefully locate end of crack and vee-out as required. Do not attempt to weld up multiple branching or “star-shaped” fractures indicative of fatigue. Renew the detective area

10. On steel repair Jobs it is particularly important to check fit-up and back chipping. UT or Radiograph finished welding if practicable.

11. Where extensive plating renewals are made, extreme care must be taken with the welding sequence to prevent compressive buckling of the existing plating on the opposite side of the hull girder, and to minimize hull distortion (and consequent effect on machinery alignment).

12. Where extensive renewals are made lo bottom shell plating in way of machinery, steps must be taken to insure the maintenance of machinery and shafting alignment. Afloat alignment checks and gear tooth contact records should be made before and after repairs

13. Doublers or strength straps should normally not be thicker than the plating on which they are fitted. Local doublers on side shell, deck or bottom shell are allowed as a temporary repair only, except as original compensation for openings.  Any such doublers over damage or wastage must be removed before crediting the Special Survey.

14. Longitudinal reinforcement straps should have width tapered down al ends to minimize the possibility of fracturing at the toe of the end-fillet welding, a possible result where a fillet weld is perpendicular to the direction of fatigue loading. They should also be edge-welded and plug welded on about 300 mm centers if over 600 mm width, and where possible, in-line with internal longitudinal girders or bulkheads to obtain the maximum effect. The welding size should be the minimum required for strength, especially the plug-welds. Welding sequence should be from the middle towards the free ends.

15, Make certain that replacement plating material and welding electrodes used are the correct grade or type. If the proper Class Society steel is not available, clear identification against the mill sheet and possibly confirmatory tests may be required for equivalent substitutes certified by another society. Change of steel grade or substitution of welded joints for riveting must have prior to your Office approval.

16. No attempt should be made to heat and fair sharp indents or creases, part crop and renew.

17. Smooth shallow indents are better left alone unless the attached internals are significantly affected. If extensive in a transverse direction however (say, more than two strake widths), they may require repair or reinforcement in order to preserve longitudinal or local structural strength.

18. The primary factors to be considered in assessing the harmful effect of indents are as follows:
  • Distance from neutral axis (which is usually about 4D from bottom).
  • Probable sectional area of bottom already lost due lo wastage.
  • Transverse extent of indent; i.e., percent of width of hull girder affected.
  • Whether internal structural members in way.
  • Whether internals in way are significantly affected i.e. tripped or adrift from bottom plating.
  • Sharpness of indent, i.e., susceptibility to fracture or “hinging”.
  • The type and size of vessel relative lo expected stress levels and direction of stress (hogging or   sagging) in area affected. In vessels with machinery amidships for instance, critical bottom stresses may be compressive whereas in machinery-aft vessels such as tankers, ore carriers and bulkers, critical bottom stresses may be in tension.
  • Depth of indents is less important except for appearance and only rarely for docking blocking, scattered smooth indents up to 75 mm (3 in.) depth usually do not require action unless internals in way are damaged.