Ballast Tank Wastage at Intermediate Survey
A 1998-built, 18,000 dwt general cargo ship is at an intermediate survey in Hong Kong. In No.2 port ballast tank the original side-shell plating is 12.0 mm and the allowable minimum is 9.6 mm after the 20% diminution. Gaugings over a 25 m² area give 8.9, 9.0, 9.1, 9.3 and 9.4 mm; pitting up to 1.5 mm is present and the coating is broken down over about 30% of the area. The adjacent side longitudinals gauge 7.8–8.2 mm against an allowable 7.0 mm. The owner's superintendent proposes to wire-brush, apply a surface-tolerant coating and postpone steel renewal for 12 months, citing a fixed cargo and the cost of dry-docking. The vessel is classed and Hong Kong-flagged; the class surveyor is authorized to issue statutory certificates. The master has not been consulted. The next scheduled dry-docking is in 10 months. The tank is currently empty and gas-free.
1. Decide what the class surveyor should do about the owner's proposal and the tank's compliance.
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The gauged 8.9 mm is below the 9.6 mm allowable minimum, so the tank no longer meets class requirements. Wire-brushing and coating over sub-minimum steel does not restore strength and would hide continued corrosion; the owner's proposal should not be accepted. The lowest readings govern, not the average of the readings; the fact that some points are 9.3 or 9.4 mm does not make the 8.9 mm point acceptable.
The surveyor should issue a condition of class requiring permanent repair by a defined date. If the owner needs one voyage before repair, a temporary repair may be considered only with class approval and with restrictions such as limited ballast head, no heavy ballast and reinspection before sailing. The adjacent side longitudinals are above their allowable minimum, but the coating breakdown over 30% of the area suggests the wastage is not isolated. The surveyor should require a wider gauge pattern and check the frames and brackets before accepting any repair scope.
2. Define the repair scope and the verification needed after repair.
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The repair should crop out all areas below 9.6 mm plus a margin, typically renewing to about 10.0–10.5 mm. Fit insert plates using an approved welding procedure and qualified welders. After welding, carry out MPI or UT on the welds as required by class, re-gauge the repaired area and check the adjacent structure. Test the tank for tightness and apply a suitable coating system.
A doubler on the side shell in a ballast tank should generally be avoided unless class specifically approves it for a limited period; doublers create a corrosion trap and can initiate fatigue cracking. All gaugings, repair locations, NDT results and coating details should be recorded in the class survey report and the ship's maintenance records. If the repair is temporary, the conditions and the date for permanent repair must be recorded in writing.
3. Explain the statutory, flag and ISM consequences if the repair is not completed.
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Class must notify the Hong Kong flag administration of the condition of class. If the repair is not completed by the due date, class may suspend the vessel's class, which in turn invalidates the statutory certificates issued through class. The vessel should not load cargo or sail until class confirms that the repair and any conditions are cleared.
The owner should raise an ISM non-conformity and review the tank inspection and coating maintenance regime, since the breakdown and wastage indicate a systemic maintenance issue. Port State Control would be likely to detain the ship for a hull deficiency affecting seaworthiness. The master should be informed of the condition and the operational restrictions; the charterer's cargo fixture is not a justification for sailing with sub-minimum structure.