If a situation requires upgrading a rig's mooring system for a desired water depth or as special location conditions dictate, it may be cost-effective to pre-install an enhanced mooring system for the rig to use at that drilling location.
Anchors, chain, and wire can be installed on the location prior to rig arrival and off the critical path time of rig operations. Some of the rig's standard mooring components may need to be removed before arriving at the preset location.
Once the rig arrives at the drilling location, its remaining mooring equipment is connected to the pre-installed system with one or two anchor handling vessels (AHVs).
A single AHV can install each leg of the preset mooring system. The mooring leg components are transported to location and deployed at the designated coordinates. Surface buoys are commonly attached to each predeployed leg for quick recovery when the rig is connected. Once the rig arrives on location, the AHVs connect the rig to the pre-installed mooring lines.
The disconnection of the rig is accomplished in a similar manner with either the rig's mooring components being reinstalled or the rig being moved to another preset mooring system.
A preset system should allow the rig connection to be completed in less time than is required to run the rig's own mooring system. However, this is influenced by whether the rig's own mooring components have to be removed or reinstalled on the rig and various other factors that may affect the mooring procedure.
Water depth, length of well drilling schedule, as well as vessel and rig day rates, all play a significant role in determining the economics of a preset scenario. These factors should be studied to determine the most economical means of mooring the rig. Generally, it is most economical to use the rig's own moorings.
The preset system becomes economical when it is necessary to supplement the rig's moorings to a significant degree, beyond the simple extension wire addition, or if seabed features or pipeline interference make conventional anchor deployment impractical.