DERBYSHIRE - Loss of a Bulk Carrier

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MV DERBYSHIRE was a British Oil/Bulk/Ore, (OBO), carrier transporting ore from Canada to Japan when she was lost during the typhoon ORCHID on the 9th or 10th of September 1980. She went down with all 44 on board without any distress signal. DERBYSHIRE is the largest British bulk carrier ever lost and has been the object of several investigations and discussions regarding bulk carrier safety. This loss is just one in a long list of bulk carrier losses from the 1970s to the mid-1980s.

The Customer: 
To determine the maximum load a rule-designed hatch cover can carry, we set plastic collapse as the ultimate state. The T stiffener has a plastic shape factor, s, of 1.25. (The shape factor refers to how much the load can be exceeded from first yield to full yield of the cross section). Then the ratio of the full plastic collapse load and the design load is:
This means that we can increase the design pressure to a pressure of 3.125 (1.75 tonne/m2) = 5.47 tonne/m2 uniform pressure. This equals a water height of = 5.32 m over the hatch cover (Head = pressure/sea water density = 5.47 tonne/m2 / 1.028 tonne/m3). The collapse head shows the true pressure a rule-designed cover can carry before collapse. But did the cover truly fulfill the requirements? Finite element analysis of the hatch cover design shows that they would collapse under a static pressure of about 4.1 m (Ref. 1, pg 9) (hyperlink). This is above the ICLL requirement to resist a uniform pressure of 1.75 tonne/m2, but is still under the predicted collapse head of 5.3 m corresponding to the design pressure.
The Challenge: 
Required section modulus is thus 19.56 kNm / [0.40(235 MPa)] = 204 cm3. The actual section modulus of the T beam stiffener can be calculated or taken from a table (Z = Iz /ymax where Iz is the 2nd moment of inertia about the z-axis and ymax is the distance from the neutral axis to the furthest flange). We do not use the dimensions of the tapered end of the stiffener because the ends are approximated as simply supported because the loading is not symmetric, and because the side plate at the end of the hatch cover is flexible. The most critical part of the stiffener is the end condition at the girder, so stiffener height at this location must be used in the calculations. The stiffeners used fulfill the requirements, according to Ref. 1.
It can be questioned whether these calculations describe the structure's capacity correctly, since the transverse girders are not as effective as assumed. One reason is that the depth of the side girders are smaller than the depth of the stiffeners, so in fact, the actual moment capacity of the girders is about 0.2MP, where MP is the plastic moment capacity of the stiffeners. The narrow flange of the girders provides much less bending strength than necessary to restrain bending of the cover.