Fatigue Crack and Wasted Deck Plating at Second Special Survey

The location is diagnostic: a bracket toe at a longitudinal-to-web-frame connection in a ballast tank is a classic fatigue detail, not a corrosion failure. Corrosion wastage is broadly distributed and shows as general or pitting loss; a single 180 mm crack initiated at a geometric stress concentration and running away from it is load-cycle driven, with the loss of coating acting as an accelerant (corrosion pits are crack initiators and reduce the effective section).

Consequences for the repair:

• Welding over the crack is not a repair. The crack tip must be removed - crop out the affected material and fit an insert plate with rounded corners, full-penetration welded, with the toe ground flush afterwards. Confirm removal by MPI/UT before the insert goes in.
• The detail, not just the crack, is the problem. Grind or re-profile the bracket toe, check the bracket thickness against the rule requirement, and consider a soft toe or a modified bracket shape, otherwise the crack returns inside the next survey interval.
• Look for siblings. Inspect every equivalent bracket in every ballast tank; fatigue at one detail is almost never unique, and finding them now is far cheaper than at sea.

The three numbers are different tests and must not be conflated:

• As built 18.0 mm, wastage 2.8 mm = 15.6% - inside the usual 75% renewal trigger in itself.
• Renewal thickness 15.5 mm: 15.2 mm is below it, so the plate has reached its renewal criterion at this survey.
• Class minimum 14.6 mm: 15.2 mm is above it, so the ship is not yet at the point where the scantlings are inadequate for strength.

That distinction is exactly the judgement the surveyor has to make: 'above minimum' does not mean 'acceptable to leave'. The default outcome under CSR is renewal of the affected strake at this survey. Any departure requires a documented engineering assessment - a thickened/allowable-thickness calculation with the local stresses, buckling and hull girder section modulus re-checked, plus a defined next-survey limit - accepted by the class, not agreed between owner and surveyor on the tank top.

Also check the extent: a 2 m x 3 m patch below renewal usually has neighbours close to it, so gauge on a grid and map the whole strake rather than renewing to the outline of one reading.

Reject the proposal as framed, and say why in engineering terms rather than rule terms:

1. Deferral is not the owner's to grant. Renewal is a class requirement; the only route is a formal allowable-thickness assessment with a defined re-inspection date, which may well be shorter than 2.5 years.
2. The coating proposal does not hold. 'Surface-tolerant' epoxies tolerate damp and marginally prepared steel, not hard scale and blistering. Applying over that condition traps corrosion cells and gives a false sense of protection; the surface must be brought to at least ST3, or abrasive-blasted for a 10-year-plus target. With coating 'poor' the tank should be treated as being in a corrosive environment for the next survey cycle, which means the corrosion allowance calculation changes.
3. The crack and the wastage interact. The tank that cracked is the tank with the failed coating; the two findings should be treated as one condition and the inspection scope widened accordingly.

A defensible package is: insert-plate repair with toe grinding and MPI, renewal of the deck strake to the mapped extent, coating to the manufacturer's specification at the correct surface preparation, plus close-up survey of all equivalent bracket details - with any deferral supported by a class-accepted strength assessment.