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In this picture we see one of the semi-submersible oil rig's moorings. Most oil rigs are equipped with the shown anchor chain fairleads. This mainly to enable supply vessels to come alongside the rig, but also to give the moorings the best efficiency. The chaser ring is seen resting on the upper end of the anchor chain with the free end of the connected chaser wire stored on board the rig as shown. The setup shown in the picture is a typical oil rig mooring, and the supply vessel is seen approaching stern first, ready to receive the chaser wire.
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The supply vessel has now approached the rig and the wire from the tugger winch has been made ready on deck. The ship is equipped with two tugger winches, which are used for handling anchors on deck, retracting chaser wires as in this situation as well as a variety of other purposes. The tugger wire is laid out through the A-guides. The oil rig's crane carries the lose end of the chaser wire and lifts it downwards toward the supply vessel. A messenger wire is attached to the eye of the chaser wire.
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The crew of the supply vessel grab the messenger wire with a boat hook and connect it to the tugger wire. The tugger wire has a diameter of approximately 25-30 millimeters with a break load near 30 tons. The tugger winches are very powerful and can easily break the wire, so the winches are used very carefully when handling heavy loads.
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With the tugger wire connected to the messenger of the chaser wire, the crew start heaving in on the tugger wire. At the same time the oil rig crane pays out wire until the entire weight of the chaser ring and wire is carried by the tugger winch. Heaving in the tugger winch is continued until the eye of the chaser wire is past the shark jaws, at which point the jaws are closed. The tugger wire carrying the chaser wire is then payed out until the eye of the chaser wire is resting in the shark jaw. The tugger wire and the messenger wire can now be removed from the eye.
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With the tugger and messenger wire out of the way the ship's main working wire can now be connected to the eye of the chaser wire. The working wire is another name for the wire on the anchor handling winch, the supply vessel's main winch. The hookup is done manually by (usually) two crewmembers. The eye of the work wire and the eye of the chaser wire are connected with a shackle, and afterwards the work wire is heaved in until the weight of the chaser ring and wire are carried entirely be the anchor handling winch. At this point the shark jaws can again be opened.
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After opening the shark jaw the work wire with attached chaser wire is payed out according to the local water depth. Meanwhile the supply vessel starts moving ahead dragging the chaser ring along the anchor chain. This is done at low speed, 1 or 2 knots. The charterer has equipped the supply vessel with a navigational system showing the location of the mooring anchors and anchor chains on a screen. The ship's own position is also plotted on the screen, helping the officers on board the vessel to know when to expect being at the location of the anchor etc. This navigational equipment communicates with equal computer systems on board the oil rig so that also the rig can "see" where the supply vessels are in relation to the moorings.
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The ship has now pulled the chaser ring all the way out to the anchor. Exactly when the ring is at the anchor can be seen on board the supply vessel, since it will stop in it's slow forward movement. The throttle is eased a bit and heaving in the work wire is initiated. This will pull the vessel back until it is almost directly above the anchor. The continued heaving on the work and chaser wire will eventually lift the anchor out of the seabed. At this time, the oil rig will have payed out a bit on the chain, so that the tension on it doesn't overwhelm the equipment and the AHTS vessel.
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The supply vessel keeps heaving in the work wire whilst a little throttle ahead is maintained. This is to avoid the weight of the anchor chain pulling the vessel backwards. When the anchor reaches the same depth as the vessel's propellers the forward thrust is stopped so that the jet of water won't make the anchor spin and get entangled with the anchor chain. By now the oil rig will also start heaving in the anchor chain so the vessel will start moving slowly astern. This is a particularly dangerous part of the operation, since the low working deck is highly likely to get flooded when the vessel is moving astern! Depending on the water depth, the anchor chain might be dragging along the seabed while being winched in by the rig, or it might be hanging in a curve from the chain fairlead to the stern of the AHTS vessel.
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If the anchor has been deployed correctly when the rig moorings were laid out and if the anchor has been lifted off the seabed correctly it should face the correct way when reaching the vessel's stern roller. The A-guides are opened and the anchor and the connected chain are pulled onto the deck. When the anchor has passed the guides, these will be closed around the anchor chain. Also the the shark jaws will be closed around the chain and then the work wire can be slacked off and work on the anchor initiated.
If the anchor is not facing the right way on the stern roller it usually helps to lower it a bit and maybe give it a short burst of propeller thrust. If the anchor can't be brought to face the right way you can still just heave it on board, but working with the anchor (disconnecting chain and equipment etc.) will be a little more difficult. The best way of pulling the anchor on deck is with the anchor, as shown, laying on it's back with the anchor chain going atop of the anchor between the two legs.
Depending on what the assignment is the anchor can now be disconnected from its anchor chain and the supply vessel's work wire connected to the chain instead. Then the oil rig can be towed by the anchor chain. The anchor can also remain connected to the wire and the vessel will then sail back to the oil rig whilst the rig heaves in the anchor chain. When back at the rig the vessel lowers the anchor back out over the stern whereby the oil rig can pull the anchor onto its "bolster" where it is stowed when the rig is not moored. The chaser wire can then be disconnected from the work wire and the lose eye of the chaser wire lifted back onboard the oil rig. There are several other possibilties depending on the type of work being performed, and this guide is just an example of how a mooring anchor can be retreived from the seabed.
