Investigation into the Sinking of the MV Derbyshire
As there was no evidence of structural failure being the cause of the sinking of the MV Derbyshire, the British Government did not immediately start an investigation.
However, numerous loss scenarios were being espoused by marine experts, and these have been whittled down to the two most likely causes of the sinking:
- Structural failure at frame 65.
- Collapse of the hatch covers.
The structural failure was to be initially investigated through money raised by the Derbyshire Family Association (DFA), a charity set up after the loss of the Derbyshire to clear the crew of being responsible through negligence for her sinking.
Structural Failure at Frame 65
The funds from the DFA were used to pay for investigations into cracks at frame 65 on some of the other Bridge Class OBO carriers.
This design was thought to promote cracking at this bulkhead. After the Kowloon Bridge, a sister ship, broke into three pieces after running aground, one of the breaks was at frame 65. Meanwhile other sister ships were reporting cracks appearing on the deck above frame 65, prompting an examination by the British Board of Trade (BOT).
This concluded that the transition from longitudinal girders to longitudinal bulkheads could have been responsible for the sinking of the Derbyshire. Here misalignment was also evident between the two components, adding to stresses and promoting cracks.
The DFA had by now raised enough money to fund an underwater survey of the Derbyshire wreck. This indeed confirmed that the Derbyshire was in fact in two pieces. The living quarters and engine room aft of the bridge were lying on the seabed a short distance from the rest of the hull.
A further more detailed investigation was once again carried out by the British BOT and confirmed that the wreck was in two pieces.
However, the stern section was found only 600m from the rest of the remains, and as she had descended through 4000m from the surface, it was therefore unlikely that she broke up on the surface, more likely breaking up on the way down. Marine experts in ships sinking therefore deemed that the design at frame 65 was not the cause of the sinking of the MV Derbyshire.
Collapse of the Hatch Covers
This is the most likely scenario cited by a subsequent BOT investigation; a summary is given below.
It was envisaged that as the MV Derbyshire was overtaken by Typhoon Orchid in the South China Seas, and she started to take heavy green water over the fo’c’sle head. These huge waves damaged /detached the boson’s foredeck stores ventilator covers along with the fore peak ballast tank ventilator covers.
This allowed these spaces to quickly fill, with every wave taking the ship down by the head. Subsequent waves dislodged the fore-cargo hold ventilator covers. With these being carried away, the ventilators were open to the sea.
As larger waves crashed over the starboard side of the bow, they dislodged the No. 1 cargo hatch cover and allowed copious amounts of seawater to enter the hold. This ruptured the internal bulkhead, allowing water into the No .2 hold, and this along with further pounding from subsequent waves caused the No. 2 hatch cover to disintegrate. A sketch of the layout and typical wave profile is shown below.
This had a domino effect on the rest of her nine holds' hatch covers, being acerbated as she sunk down by the head, her angle increasing downwards. Eventually as she slipped below the waves, the bridge, accommodation, and engine room broke away from the rest of the hull.
The total time estimated from No.1 hold hatch being breached to the sinking has been estimated as two minutes. This allowed no time for the crew to abandon ship.
It is a mystery, however, that no “Mayday" was picked up- if indeed one was issued. Even when I was at sea in the 1960's, a Mayday was transmitted simply and quickly by the pressing of a button, which also gave the current location of the ship in distress. I like to believe that the Radio Officer got the Mayday away, but atmospherics stopped it being received on mainland China, the many nearby islands, or other ships in the area.
After the non-appearance of the MV Derbyshire at her destination of Kawasaki, where the relief crew waited in vain, a formal search and rescue was carried out by the Japanese Marine Agency. Their ships and aircraft only found oil bubbling up and shining on the surface of the sea, but this was enough to confirm her final resting place, as shown in the sketch below.
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2. scitation.aip. Ultimate Limit Assessment of hull.
3. shipstructure. Overview of the MV Derbyshire and the Investigations.
4. IMO. Improvements to design of ore carriers.