Ship arrestor project

The two-year project is aimed at introducing a technique that will enable a tow line and sea anchor to be attached to an unmanned vessel with engine failure. The sea anchor will reduce the ship’s drift and create more time for it to be reached by rescue tugs before grounding.

It is designed to be deployed by a conventional search and rescue helicopter that will drop the tow line around the winch gear on the foredeck of the abandoned vessel. The helicopter then lays the line upwind and releases it attached to a sea anchor. Because the tow line must pass around deck equipment and over the ship’s side it is subject to considerable chafing and only chain can be used for the first few metres.

Unfortunately the weight of conventional steel chain was found to be too great for helicopter deployment and threatened the viability of the entire project. Titanium chain was rejected because it is brittle and too expensive but it transpired that one of the project partners had access to steel compositions developed in the former Soviet Union for space and defence purposes. This resulted in a stud-less 24mm chain weighing just 11kg per metre yet with a tested minimum breaking load of 1,250 kN. Far superior to a conventional 34mm stud link anchor chain with breaking load of 1,308kN that weighs 27kg per metre.

Further trials will require the new chain to be fitted into an inflatable deployment ring that hangs below the helicopter whilst in flight. When inflated, the ring has a circumference wide enough to encircle a ship’s winches and deck gear. When the ring is lowered around the deck gear the chain is released to make a connection with the ship. The anchor line is then released from the helicopter and the sea anchor is deployed.

The success of the ship arrestor project could have a major effect upon maritime and environmental safety. When a sea anchor is deployed either by helicopter or by the ship itself, its effect is almost immediate. Tests have shown that a 30m diameter nylon parachute sea anchor can quickly turn a 100,000 dwt tanker into the wind and reduce its drift speed by 50 percent. This dramatically improves its sea-keeping ability and could be a vital factor in the ship’s survival by allowing more time for the distressed vessel to be reached by rescue tugs.