Ship type: Tanker for Oil Size (grt): 140,000-150,000 Year of build: 1992

Crack at termination of bilge keel amidship

Course of events
During a loaded voyage, a single hull VLCC experienced an oil leakage to sea, apparently coming from the starboard side cargo tank, around amidships.

Extent of damage
Limited to the crack in the shell plate on starboard side.

Probable cause
The vessel was dry-docked after cargo discharge, and a crack was found in the hull plating at the termination of the bilge keel amidships, in cargo side tank No. 3 starboard, see photo.

The bilge keel had been constructed in sections along the ship's length for production reasons (terminated at each section joint, thus splitting the bilge keel in 5 parts on each side), which is a quite common solution today. The crack had initiated just at the bilge keel pad plate and propagated further into the hull plating, with the resultant oil leakage.

The most critical bilge keel endings are those in the midship area, where the hull girder longitudinal stresses are at a maximum. This, together with the unfavourable design of the bilge keel ending, lead to high notch stresses and finally a crack in the hull plating around the bilge keel toe.

Contributing factors to the damage are:

• The tapering of bilge keel ending was relatively sudden (1:3) and a smaller angle would have given a larger area for transferring longitudinal stresses in the bilge keel into the hull structure.
• The pad plate was only 25 mm longer than the bilge keel itself, thus creating a sudden ending of the bilge keel toe and hence a hard point where stresses were transferred directly into the hull plating.
• The pad plate was not continuous along the ship's length. A continuous pad plate would reduce the notch stress at the intermediate bilge keel toes.

Several similar cases have been reported previously.

Lessons to be learned

• Non-continuous bilge keels are not a favourable solution on larger ships, as fatigue cracking is a known problem. Such bilge keel end terminations should be thoroughly considered during approval and construction phase in order to keep the notch stresses low.
• Non-continuous bilge keels end terminations should be subject to thorough inspection for possible signs of cracks when vessel is in dry-dock.

Date: 2003-03-04