A unique conjunction of events and circumstances led to the sinking of two Maersk offshore supply vessels in December, 2016, according to a Danish Marine Accident Investigation (DMAIB) investigation.
The 4,013gt Danish ships Maersk Searcher and Maersk Shipper capsized on the night of Dec 21-22 and sank in the Bay of Biscay some 65 miles off the French coast while being towed by another offshore supply ship, the Maersk Battler, 4,363gt, to Aliaga, in Turkey, to be broken up.
The DMAIB report said the two ships were prepared for scrapping in 2016. In the summer, a Maersk superintendent began drafting plans to tow them from Denmark to the scrapyard using an AHTS, the Maersk Chancellor, 2,887gt.
She lacked the winch arrangements for a double tow, so the superintendent began to plan for a side-by-side tow on one wire. “The superintendent was laid off in a round of dismissals before the tow got underway, and he was not given time to hand over his work to his colleagues,” said the DMAIB report.
“A trainee took over the preparations for the tow in December under guidance of a new operations manager. The team assumed that the draft plan for the towing rig was finalised and approved, and that all was ready to go. However, the initial draft was not a finalised towing plan.”
The plan was not designed for the towing vessel that Maersk Supply now decided to deploy, the Maersk Battler, which was equipped for a double tow and did not need the special side-by-side arrangement developed earlier.
With the Maersk Searcher and Maersk Shipper rigged side-by-side, the Maersk Battler left Fredericia on the morning of Dec 12. She headed south through the North Sea then on Dec 20, in the English Channel, the swell increased and the crew paid out the tow wire to about 2,000ft. At this time, they noticed that the fenders between the two PSVs had disappeared, and that the their hulls were making contact.
The crew did not believe that this posed a problem, as the vessels were due to be scrapped, and the captain planned to inspect the ships on reaching the Mediterranean. Inspecting them in choppy conditions in the Channel or the Bay of Biscay was considered too hazardous.
The weather worsened on Dec 21, with westerly swells reaching 15ft. In the morning, it was clear that both vessels had suffered damage to their superstructures, and that they were listing towards each other. Again, the crew did not perceive that this posed any serious risk, as the damage appeared to be above the waterline. The vessels continued to collide into the night.
At about 2330, the watch AB observed that the Maersk Searcher was riding deeper in the water and listing heavily. He alerted the Master, who realised that the she was about to capsize. She went over about 10 minutes later, and threatened to drag the Maersk Shipper down with her by the bridle.
The Maersk Searcher went down at 0022, and the Maersk Shipper capsized a few minutes later. The second vessel stayed afloat until 0600, when she sank.