October 18, 2001
ABS releases Castor report
The American Bureau of Shipping has now released the report of its investigation into the damage sustained by the Castor. (You can download it here).
The Castor experienced deck cracking between frames 72 and 73. The overall crack extends approximately 22 meters from the portside deck stringer strake to the starboard deck strake. There are five separate cracks which travel in series with some short sections of parallel crack growth.
The report notes that the Castor as constructed had scantlings in excess of the minimum requirement satisfying the hull girder section modulus criteria used at the time of construction. Calculations of Castors hull girder section modulus at the time of the incident determined that the vessel satisfied the present ABS Rules and IACS minimum standards. This explains why the vessel was able to survive for 39 days in heavy weather even after sustaining the deck cracking.
While the overall condition of the structure was sound, the survey after damage found that substantial corrosion did exist on the underside of the main deck plating and supporting longitudinals in way of No. 4 cargo/ballast tanks.
The underside of the plating and longitudinals had previously been coated. However, this coating had deteriorated over time exposing portions of bare steel to the corrosive atmosphere in the ullage space. While the area of corrosion was limited, the extent of the corrosion was excessive. In the most severe areas, steel originally 16 mm had wasted to 5.5 to 6.6 mm, or 60 to 65% wastage.
Steel in No. 4 tank which had been renewed with uncoated 14.5 mm plate in 1997 had wasted to 12 mm at the time of the incident. This represents corrosion rates of up to 0.71 mm/year or seven times the nominal 0.10 mm/year corrosion rate assumed in ABS Rules.
While the area of severe corrosion was limited, it did cover a large portion of the forward end of No. 4 tanks, particularly the center tank.
Survey and analysis of this structure, says the ABS report, leads to theconclusion that the wasted under deck longitudinals became detached during the heavy weather. This weakened the deck structure eventually leading to buckling and then to cracking of the plating.
The gaugings taken during the most recent special and intermediate surveys did not reflect the areas of excessive corrosion and therefore failed to adequately represent the condition of the vessels structure.
The survey reports for the most recent special and intermediate surveys did not make note of the areas of excessive corrosion in No. 4 tanks and therefore did not adequately represent the condition of the vessels structure.
The report makes these recommendations