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.
