Mooring systems are designed to keep rigs on location by exerting a restoring force on the rig when metocean conditions push the rig away from its station over the well. Anchors in the seabed are attached to the rig using large chains or lines. Mooring system configurations are often described based on the ratio of the water depth to anchor radius as follows: catenary mooring (typically 1:2 or greater); semitaut (typically 1:1.4); and taut (typically 1:1). The mooring system components vary depending on the system configuration. DW catenary and semitaut mooring systems are often comprised of steel wire and chain segments. Taut mooring systems will usually incorporate synthetic rope segments.
Source: API RP 96, Deepwater Well Design and Construction, First Edition, March 2013. Global Standards