Most semisubmersible drilling rigs are fitted with conventional chain or combination chain and wire catenary mooring systems. These mooring systems can employ high-holding power anchors with 3" or 3 1/4" anchor chain and mooring wire of at least 3" in diameter.
These types of systems are typically "self-contained" onboard the drilling rig. The limiting factor for conventional catenary mooring systems is their length, weight, and ability to hold the rig within the desired minimum watch circle in greater water depths.
During normal operations, the installation or recovery of these moorings is accomplished with either one or two anchor handling vessels (AHVs). Each AHV will work opposing mooring legs of the rig simultaneously while a tug is used for station-keeping assistance. Size and horsepower of the AHVs are key elements of the straightforward mooring operation.
To anchor the rig on location, the AHVs take the permanent chain chaser (PCC) from the rig and deploy the anchor to the desired location with the mooring line attached. The vessel lowers the anchor to the seabed, and the rig tensions the mooring line to set the anchor. The vessel then pulls the chaser back along the installed mooring leg until it reaches the rig and repeats the process until all mooring legs are installed and tensioned to the desired loading.
To recover the mooring legs, the process is reversed. The AHV recovers the PCC from the rig and pays out work wire while pulling the chaser to the anchor. A coordination of work wire deployed and bollard pull of the vessel unseats the anchor.
The rig then retrieves the anchor line while the AHV recovers the work wire and anchor. The PCC is passed to the rig where it is secured at the rig column once the anchor is bolstered. When all mooring legs are recovered, the rig is free to move.
Water depth, weather conditions, selection of anchor handling vessels, and planning efficiency can influence the time it takes to set or recover the mooring legs.
Using the rig's mooring system is normally the most economical means of mooring the rig, provided the location is within the optimum capabilities of the drilling rig.
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