Anchor handling tug supply (AHTS) vessels are vessels which supply oil rigs, tow them to location, anchor them up and, in a few cases, serve as an Emergency Rescue and Recovery Vessel (ERRV).
AHTS vessels differ from Platform supply vessels (PSVs) in being fitted with winches for towing and anchor handling, having an open stern to allow the decking of anchors, and having more power to increase the bollard pull. The machinery is specifically designed for anchor handling operations. They also have arrangements for quick anchor release, which is operable from the bridge or other normally manned location in direct communication with the bridge. The reference load used in the design and testing of the towing winch is twice the static bollard pull. Source : Wikipedia