Safer deck operations on anchorhandlers

17 February 2006

Lassoing a buoy

Anchorhandling operations have traditionally been risky for the crews of offshore vessels working on deck. Both the offshore oil and gas industry and the shipowners are putting a high premium on reducing this risk as far as practicable.

At the heart of the system is a pair of cranes that travel the full length of the working deck on the cargo rails. The cranes are fitted with two jibs each, deploying a range of special manipulators which act as powerful hands. They work in combination with equipment mounted in the working deck itself - centring devices, towing pins and shark jaws - to carry out operations with wires, chains and synthetic fibre ropes under tension that previously involved potentially risky manual intervention by the Able Seamen (AB) on deck. Now, the AB can carry out the various operations from selected safe vantage points, operating the equipment by wireless remote control. In addition to this type of work, the cranes have other functions. They can move items of equipment from and to any point on the deck, extend their jibs beyond the stern of the vessel to capture anchor buoys and bring them on deck using a lasso and catch PCP wires. Finally, they are a key part of the pennant wire spooling system.

In developing this equipment, Rolls-Royce has drawn on wide experience from its Rauma Brattvaag deck machinery. Large winch systems form the heart of anchor handling tug supply vessels, providing towing capability as well as being the primary tools in laying out moorings for floating rigs and platforms. Winches are already complemented by stern rollers that ease the passage of wires and anchors over the stern of the vessel. Working loads in the 500 tonne region are common, and the new equipment fits in with these existing deck systems. Rauma Brattvaag winches, chain jacks, fairleads and other equipment are also widely specified for installation on semi-submersible rigs, drillships and FPSOs, giving Rolls-Royce unrivalled insight into offshore mooring and towing systems

Crane system
Two hydraulic boom cranes are mounted on tracks running the full length of the cargo rails that shelter the working deck of the vessel, one to port, the other to starboard. The basic crane units, which are rated for three tonnes at 12,8 metres and five tonnes at nine metres, are equipped with twin jibs and manipulators and have to be adapted for each vessel design. A power pack in each crane supplies hydraulic power to move the cranes and work the various tools. A trolley system allows the cranes to be operated anywhere with full load along the deck, and they can also reach out beyond the stern of the vessel.

When lifting, either crane can reach beyond the centreline, so that loads can be moved to and from any point on the deck. The lifting jib can be used to push wires and chains sideways across the deck to position them between the towing pins or in the shark jaw, if great weight, the tugger winch can be used. This jib is also fitted with a hook that can be opened and closed hydraulically. Its main function is to capture the PCP wire hanging from the rig crane and bring it on board, a job which formerly involved a crew member standing riskily right at the stern with a boathook.

The cranes work together for many operations. For example, to bring a buoy on board a lasso is first laid out on deck, each crane then picks up one side of the loop and together they take the lasso out beyond the stern and drop it over the buoy. The winch then pulls the lasso up over the stern roller, bringing the buoy on to the deck.

Manipulators
Attached to the working jibs of both cranes are hydraulically operated manipulators that can be used like powerful hands to carry out many jobs involving wires and chains. They are controlled from a safe distance by an AB using a neck-slung control unit with wireless connection. Chain claws engage individual chain links and hold them while chain shackles are connected. Two cranes work in partnership. The chain claw enables heavy chain links and shackles to be picked up from the deck.

A crane can use a special wire gripper to remove accumulated twist from wires. This is an important function in safer deck operations. Wire have an inbuilt tendency to twist as the tension in them changes. Relieving tension causes the wire to rotate rapidly, and a free end can lash about the deck, especially if some chain links are attached, with risk to life and limb. The soft lined jaws of the wire twister can either grip the wire tightly, or allow controlled rotation to release twist without danger. The complete jaw mechanism can rotate 360 degrees in one plane and also tilt partially in another. While this crane is holding the wire in the right location and orientation, the other offers up the shackle. The only manual intervention needed is securing the shackle pin. For many of these tasks the cranes will be working in conjunction with the towing pins and shark jaws.

Towing pins, shark jaw and centring system
One or two sets of equipment are mounted in the aft end of the working deck to control wires, ropes and chains during anchorhandling and towing operations. When not in use, everything retracts to leave a flush deck. The new Rolls-Royce towing pins incorporate arms which stow within the cylindrical pin and fold out to form an arch over the rope or wire. When fully extended the pins are high enough to capture lines from secondary winches installed above the main winch. The arms prevent wires escaping and by partially retacting the pins into the deck the arch forces the wire down to an optimum working height above deck. By this means, wires and chains can be brought down into the shark jaw which carries the loads during coupling and uncoupling operations. Centring devices used in conjunction with these items of equipment comprise a pair of steel sectors which rotate up from their stowed position flush with the deck. They are used when a towline lies at angle to centreline of the vessel as it runs from winch to stern roller. It is picked up by the relevant sector which forces it into line wth the gap between the towing pins so that it can be captured by the shark jaw.

Towingpins, shark jaw and centering systems are normally operated from the wheelhouse, where the operator has a excellent view over the working deck, and propulsion and winch controls to hand. But if required they can also be wireless operated by the crew on deck.

Pennant coiler
Pennant wires are normally taken on board in large coils. When they are uncoiled and paid out over the stern the wire can easily lash around and cause injuries. To reduce this risk, Rolls-Royce has developed a coiling machine that enables operations to be carried out under full control. Pennant wire coils can be fitted to the coiling device using the travelling cranes. The pennant winder is driven by a Low pressure hydraulic motor which is located under working deck level. A spooling sheave to be fitted on the shark jaw is a part of the system if the pennant wire want to be spooled on/off the AHT-winch. The spooling sheave can fit in on existing vessels without rebuilding, and can carry the pennant coil on top.

Training and operation planning
The crew can be trained in the use of these Safer Deck Operations systems using a simulator. For example Rolls-Royce is a partner in the full scale offshore simulator at the university college in Ålesund in Norway where ship operations and anchorhandling activities can be realistically simulated. An advantage is that complex operations can be run through beforehand, including “what if” scenarios, without hazarding vessel or crew, ensuring that the real life procedure goes smoothly and safely.

A UT 712 L AHTS currently under construction for Olympic Shipping will be the first to go into service with a complete outfit of the new deck equipment.