During a loaded voyage of a bulk carrier it was discovered that water had entered into the double bottom tank No.4, amidships. After inspection it was found that the water was coming through the ship’s side via the access trunk from the top wing tank and then to the double bottom ballast tank. The ingress water was estimated to be approximately 300 cubic meters per hour. Fortunately, the leak was limited to the double bottom ballast tank and the ballast pumps were able to keep the tank empty until the crack could be dealt with.
A 5 m long vertical crack was found in the ship side below the waterline. The crack was repaired afloat by fitting an external cofferdam before inserting a new plate. The insert plate had to be divided into 3 smaller plates due to high shear forces in the area in order to reduce the risk of overloading the remaining ship side plate. The crack could have seriously affected the safety of the ship if not dealt with in time.
The probable cause of the incident is grooving corrosion along the welds, which for some reason had been left unattended over the years, see the photo at the beginning of the post. The reduced thickness in the groove caused a fatigue crack to initiate and propagate, see photo below.
Lessons to be learned
- Grooving corrosion is important to look for during routine inspections, also including access trunks between different ballast tanks, and suspect areas are to be descaled before inspection and evaluation of condition.
- Proper coating and maintenance of coating in such areas will prevent grooving corrosion and possible cracking as a consequence of that.
- The hull inspection and maintenance system on board should also include access trunks if such spaces are not already clearly identified for special attention during inspection.
When doing repairs of defects which are serious enough to affect the Class, the owner should involve Class as required by the Rules for retention of Class. Such involvement is particularly important when doing repairs of main structural elements such as in this case, for assessment of the damage, review of repair method/design improvement and repair location (afloat or in dock).
Source: DNV
Officer of the Watch 
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