Fatigue Crack at a Side-Frame Bracket on an Ageing Bulk Carrier
At a special survey a 180 m, 45,000 dwt bulk carrier built in 2006 is found with a 140 mm long crack at the toe of the lower side-frame bracket, frame 62 port, No.4 hold. The bracket is 12 mm AH36; the adjacent side shell is 16 mm. The ship trades with alternate hold loading and is at anchor in ballast, nominated for a 45,000 t iron-ore cargo. The owner's superintendent proposes to “weld over the crack, grind smooth and load”, arguing that the crack is in a bracket, not the shell, and that hull girder strength is unaffected. The class surveyor has not yet attended. The next dry-docking is 14 months away and the charterer is pressing for the vessel to sail. Ultrasonic gauging of the surrounding side shell shows 15.2–15.6 mm against an allowable 12.8 mm. No crack indication has been found on the outside of the shell.
1. State the immediate disposition of the vessel and assess the owner's proposed “weld over and load” repair.
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The vessel should not load 45,000 t on the superintendent's instruction. The crack is 140 mm long, about 12 times the 12 mm bracket thickness, at a known hot spot. Under alternate hold loading the side frame and bracket experience out-of-plane bending, so a toe crack can grow rapidly and may migrate into the side shell. Welding over an unexcavated crack leaves crack faces, contaminants and a hard, crack-prone weld; it also prevents reliable NDT. It is not an acceptable permanent repair and should not be accepted even as a temporary repair without class approval.
Immediate steps: stop cargo operations; MPI the crack and at least 500 mm either side; UT or gauge the adjacent side shell and frame; establish whether the crack has entered the shell or the bracket has fractured. If the crack is confined to the bracket, a class-approved temporary repair could be considered: drill 8–10 mm stop holes at both tips, grind the toes smooth, MPI, and impose operating restrictions such as reduced cargo, no alternate hold loading and favourable weather. That is a voyage-specific measure, not a repair. If the crack reaches the side shell or the bracket is fractured, the vessel should remain at anchor until permanent repair.
2. Set out the permanent repair and the fatigue assessment needed to prevent recurrence.
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Permanent repair should begin with excavation of the crack to sound metal and MPI confirmation that no crack remains. The bracket should preferably be replaced with a redesigned detail: increased thickness at the toe, a softer toe geometry, and improved weld profile with toe grinding and, where appropriate, shot peening. If the side shell is affected, fit an insert plate rather than a doubler. Use an approved welding procedure and consumables, then carry out MPI and UT on completion. Check adjacent frames, brackets and hold frames, because the same detail may be cracking elsewhere.
The fatigue assessment should use the measured or calculated stress range at the detail, the class S-N curves and the actual loading pattern, including alternate hold loading. If the calculated remaining life is shorter than the intended service life, the detail must be improved rather than merely replaced in kind. The class may require repair of similar details in other holds and may impose a survey programme. Restore coating and drainage so that corrosion does not compound the fatigue problem.
3. Describe the class, flag and ISM actions the surveyor and owner must take before the ship sails.
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The surveyor should attend, verify the extent of the crack by NDT, and issue a condition of class with a defined repair deadline and any interim operating restrictions. The owner must not treat the charterer's pressure as a reason to sail with an unrepaired crack. Class must be informed immediately if the crack is found to affect the side shell or if temporary repairs are proposed. Because class is authorised to issue statutory certificates for the Hong Kong flag, a condition of class that affects the vessel's compliance must be notified to the flag administration.
The owner should raise an ISM non-conformity and a corrective action covering the inspection, repair and root cause. Records of the defect, MPI/UT results, repair procedure, welder qualification and post-repair NDT should go into the class file and the ship's planned maintenance system. If class approves a temporary repair, the conditions and due date must be recorded and communicated to the master in writing.