Martno1fan,
Sorry to disappoint you and your favourite theory but the answers to your questions are No, No and Yes. Bigger rigs than this have been anchored up by far smaller and much more basically equipped vessels than the B Dolphin. 50 tons bollard pull and 2,000 bhp was once considered adequate and chain handling involved lassoos and pelican hooks. It was an extremely hairy business.
A basic primer on anchor-handling:-
A rig carries its chain in lockers located in the corner columns. (Wire-moored units have storage reels on deck.) Moorings can vary from two to three on each column. The chain runs upwards from the lockers, over a powered winch on the top of the column, down the outside of the column via fairleads to a point near the column base and then out to the location of the anchor.
In shallow water (most of the North Sea) when mooring up, a crane on the rig passes the anchor via a pennant wire to the AHT which may carry it slung over the stern roller or may take it on deck. (The latter is usually the case if the mooring pattern includes seabed pipelines.) The AHT then moves out to the required location with the rig anchor winch paying out the chain under power until the pre-determined point for the anchor where it is released. The anchor location is often marked by a buoy.
In deep water such as this time, it is not possible for the rig to control the weight of the chain on its winch. Bear in mind that the rig chain was (I believe) 4". That is not chain with links 4" long but chain made from 4" diameter steel bar bent to shape and weighing about 150 kg/metre. It would be normal for the AHT to remove the rig anchor on its deck from the chain and feed the latter into its onboard chain storage lockers provided for this purpose. The BD had four with a capacity of 540 cub.m.. The AHT would then move away and the rig pay out more chain until at a predetermined point the rig would stop paying out and apply its winch brakes. The AHT would continue moving away while paying out rig chain from its lockers until the anchor position was reached. At this point, it would reattach the rig anchor to the rig chain (while the latter was secured in the Shark Jaws) before allowing it to go over the stern roller and then be lowered to the seabed. For long, deep-water moorings, (or mooring patterns crossing pipelines) it would be normal for another AHT to assist by sharing some of the weight of the chain while it was being deployed. This would be via a J-hook lowered over the stern roller until it caught on the chain. . Thus the rig anchor chain would be supported by the rig, the AHT laying the mooring and the second AHT somewhere between the two.
It would appear that this time the second AHT was having trouble staying connected by its J-hook to the chain and thus considerable fluctuations of load were being imposed on the B Dolphin. While these loads were over the stern roller, this would not endanger the vessel although it would be causing some anxiety. What does appear to have happened was that somehow the chain was able to move to the side and girt the vessel. Why this happened is the thrust of the inquiry. Remember " Give me a fulcrum and I'll move the world." It wouldn't have mattered if the B Dolphin was twice or three times her size, once girted with that weight of chain and with her winch emergency release u/s, she had no chance.
By the by, crudely speaking, towing pins are like twin posts that are recessed into the deck when not in use, ahead of the stern roller. Hydraulics raise them above deck and allow wire/chain to be coralled between them and stop it moving to the side. They may be independently raised or lowered.
Shark Jaws are also raised or lowered from flush with the deck and may also be closed or opened to grip wire or chain and prevent it moving in any direction. B Dolphin had two sets.
The towing pins would have been in use to keep the chain over the stern roller as the AHT moved away from the rig. The Shark Jaws would have been used to keep the chain steady while the rig anchor was being reconnected.
Barry M
