When we do a mooring campaign of an FPSO, the water depth and prevailing weather and current conditions will dictate the capacity, size and number of anchors and mooring legs to be installed.
The number of legs usually varies from 6 up to 20.
The FPSO either has a spread moor or a turret system.
In a spread moor the FPSO has 4 mooring porches onto which the mooring legs are connected, 1 porch on each corner of the vessel: port side fwd, port side aft, starboard fwd and starboard aft. The FPSO is moored in a predetermined orientation which is compatible with local weather and currents.
A turret mooring system entails the mooring legs getting connected to a built-in turret. This creates the effect of a geostationary static Turret but allowing the FPSO hull to rotate around the turret on a huge bearing system. A Turret is usually selected for areas where weather patterns and directions vary.
The numbers are staggering, but on an installation in ultra deep water, each mooring leg might typically have a Minimum Breaking Load of up to 5 000 Tons.
We moored an FPSO with a Turret system in 1900m water depth in Angola last year. It had 9 moorings legs in total, arranged into 3 clusters. Using the biggest commercially available chain on the market with chain diameter of 165mm and each link weighing 248kg.