Anchor Handling Tug

Overview

An anchor handling tug supply vessel, usually shortened to AHTS, is the most powerful workboat in the offshore fleet. It tows drilling rigs and barges between locations, then runs out and recovers the heavy anchors that moor them on station. Both jobs demand enormous pulling force, so the vessel is built around two things: very high bollard pull from its propulsion, and a winch that can heave hundreds of tonnes of chain and wire over the stern. Between offshore jobs the same vessel often carries deck and tank cargo like a supply vessel.

The hull is a robust steel form with a low open stern, a full-width stern roller, and a heavily reinforced afterdeck. Propulsion is twin high-power diesels driving nozzled controllable-pitch propellers for maximum bollard pull. The defining system is the anchor handling winch, supported by stern handling gear that grips and guides the wire. A power system, a dual-station bridge, and offshore safety systems round out the vessel.

Hull and stern arrangement

The hull uses heavy plating and a closely spaced frame set to take the loads of towing and anchor work. The stern is cut low and open so that anchors, chain, and wire can be dragged over the stern roller, a large free-spinning barrel that lets heavy gear pass into and out of the sea without snagging. A deep stability skeg aft resists the sideways pull of a tow, and twin flap rudders give strong steering at the low speeds used for towing and station-keeping.

Propulsion and bollard pull

Bollard pull is the force the vessel can exert on a stationary line, and it is the headline number for an AHTS. To maximize it, each main diesel drives a controllable-pitch propeller inside a fixed Kort nozzle, which boosts thrust at low speed by accelerating the water through a duct. The pitch actuator sets blade pitch for full thrust without overloading the engine, and the gearbox includes a power take-off that taps engine power to drive the winch hydraulics or a shaft generator. A bow thruster and a stern thruster give the lateral control needed for dynamic positioning during anchor work.

Anchor handling winch system

The winch system is what sets an AHTS apart from a plain tug. The anchor handling winch carries separate drums for wire and chain, each driven by hydraulic motors through reduction gearing and held by heavy brakes, with line pulls measured in hundreds of tonnes. A separate towing winch handles the main tow wire, recovered chain stows in the chain locker below, and a spooling device lays the wire evenly on the drum. The whole system draws from a large hydraulic power unit fed off the engines.

Stern handling equipment

Working anchors at the stern in a seaway is dangerous, so the stern handling equipment controls the wire at every step. Towing pins rise through the deck to keep the wire centered, and the shark jaw grips chain or wire so it can be secured and worked without the load coming back on the winch. Tugger winches drag rigging across the deck and a stern crane handles anchors and buoys, all powered from the deck hydraulic power unit.

Power and bridge

The power system runs auxiliary diesel generators plus shaft generators that use main engine power for electrical loads, feeding the main switchboard with an emergency generator backing essentials. The bridge has a forward navigation station with radars and ECDIS and an aft control station where the operator works the winch control, thrusters, and DP console together while watching the working deck.

Accommodation and safety systems

The accommodation block in the forward house holds cabins, a galley, HVAC, and a fresh water plant. As an offshore support vessel it carries safety systems including external fire monitors fed by a FiFi pump, a fast rescue boat, and liferafts.

Anchor handling in practice

A mooring job shows how the vessel's systems work together. To set a rig anchor, the AHTS takes the anchor and its chain from the rig, runs it out to the chosen position while paying wire off the anchor handling winch over the stern roller, and lowers it to the seabed under control of the shark jaw and towing pins that keep the wire centered. Recovery reverses the sequence, with the winch heaving the anchor off the bottom against pulls that can reach hundreds of tonnes, which is why the winch system and the hull structure behind it are built so heavily. Throughout, the vessel holds station on dynamic positioning using its main propellers and thrusters, and the operator runs winch, thrusters, and DP together from the aft control station with a clear view of the working deck. Between mooring campaigns the same vessel tows the rig itself, where bollard pull is the limiting number, or carries deck and tank cargo to the field like a supply vessel. That versatility, high pull for towing, a powerful winch for anchors, and cargo capacity for resupply, is what keeps an AHTS in near-constant demand around an offshore field.