Page 136 - Failure Analysis Case Studies II

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Fig. 6. View of longitudinals, centre girder and bottom shell from port side of vessel

The dock had been pumped dry to within about 20 in. (- 0.5 m) of water. A fair amount of thick
bunker oil was still present on the sides of the ruptured tanks, and this oil was running over the
fracture face at the bottom of the shell, and was dripping down from the upper parts of the
longitudinal bulkheads. The fracture face of the side shells and gunwales was inspected from a skip
suspended from a crane. Unfortunately, due to the tortuous nature of the deck plate fracture, it was
impossible to inspect this area in great detail. Similarly, it was impossible to view the longitudinal
bulkheads from close at hand. However, it was possible to assess the amount of deformation which
had been experienced by these members. Visual examination of the bottom shell fracture faces,
longitudinals and the bilge keel of the aft section of the ship as a whole was made by walking along
the bottom of the dock. Much of the fracture face of the sheer strake, side shell, bottom shell and
flat plate keel were covered in oil, and had to be cleaned to allow close inspection of the fracture
surface. The fracture occurred forward of the wash bulkheads in No. 3 tank (Fig. 2).
As can be seen from the general view in Fig. 5, a clean break of the bottom shell plate occurred
with little or no deformation (see also Fig. 6). Deformation did occur to the ship’s plate on both
sides in the region 20-30 ft (69 m) below the deck plate. Severe deformation also occurred to the
deck plates, and to the upper sections of the longitudinal bulkheads (Fig. 7). Apart from these
regions, the failure for the most part was accompanied by little or no ductility, being macroscopically
brittle.
From a detailed examination of the fracture surfaces, three possible initiation sites were detected.
All three sites were clear of oil, and had experienced some corrosion of the fracture faces. The site
which appeared to cause the most significant damage, at least during the initial incident described
by the crew, appeared to be located in the port bilge keel (Figs 8 and 9). The chevron or herring-