AHT Tugs

The modern Anchor Handling Tug (AHT) has developed over the years from a purpose-built vessel for buoys and moorings - as used by the Harbour Authority and the Navigational Aid organisations, like Trinity House and the Northern Lights, who lift and maintain the buoyage systems around the UK and Ireland.

The Anchor handling vessels developed by the Royal Navy, to lay moorings for the Naval Fleet, changed over the years as the ship sizes increased. The largest Admiralty mooring required increasingly large vessels. As the sizes increased, so the vessel build costs increased and these vessels were then required to cover additional work like fuelling, storing, victualling, watering, etc.

The early anchor-handlers were pontoon barge type vessels with large bow rollers on A-frames and large crane facilities to lift anchors and chain onto the barge decks. It was soon found that these vessels (with small modifications) were very useful in the construction industry, but had poor seagoing qualities.

With the new requirement for towage of oil rigs and anchoring of other large structures - in ever increasing water depths and in areas further and further from the ports and safe havens - new designs had to follow.

The anchor handling tug today is required to have the following attributes -

• Sea going (100A1 notation)
• Powerful (50 ton bollard pull upwards)
• Multifunctional

See below for a video of an Anchor Handling Tug (AHT) in action. Note - the tug in the video is an Anchor Handling ASD - is both an AHT and an ASD tug.

AHT tugs - Seagoing

Able to steam and operate world-wide under an IACS Classification Society member (BV, GL, DNV etc).

AHT tugs - Powerful

Able to have adequate power to tow structure to their designated positions and then ensure they are able to maintain position to lay the structures, securing anchors. On completion, have adequate power within the winches in slow speed to break the anchors and chains from the sea bed. Have adequate craneage to lift anchors and chains from stowed positions.

AHT tugs - Multifunctional

To offer fire protection. Usually FiFi minimum of 2,400m3/hr jet and spray with accommodation cooling. Oil recovery into ships holding tanks; craneage; large open deck for storage and hauling anchors; stern roller in order to enable rolling of structures, chain and anchors onto the deck. Work or DOTI (rescue) boat service. Good/high forecastle structure. Low speed multifunctional waterfall winch to enable controlled winch movements.

The AHT is generally a powerful twin-screw fixed or Azumuthing thrusters with bow and sometimes stern thrusting units, able to work offshore.

Inshore works are still covered by 'Shoalbuster' vessels (multi role tugs with AHT characteristics) and 'Multicat' self-propelled pontoons.

See below for videos of a Shoalbuster (top) and a Multicat (bottom) in action.