Internal Mooring Systems

The Internal Turret Mooring system is integrated into the forward end of a vessel and is supported on a large roller arrangement, located either inside a moonpool towards the keel of the vessel, or at deck level.

Breakthroughs in Internal Mooring Systems

In 1994, SBM Offshore was awarded the first turnkey FPSO project for a North Sea location. The Anasuria FPSO consisted of a newly built turret-moored monohull vessel with a storage capacity of 850,000 barrels, capable of a continuous operation for the design service life of twenty years in the unforgiving North Sea environment. The special tailor-made turret mooring system was designed to moor the FPSO vessel in all weather conditions up to and including the survival sea state of 12.8m significant wave height.

An internal turret consists of a chaintable structure anchored to the seabed via an array of mooring anchor lines. The chaintable structure is suspended from a heavy-duty roller bearing, which is located several meters above the keel of the FPSO. A radial assembly is used at the vessel main deck level to absorb the radial loads caused by the accelerations of the upper turret structural mass. The turret cylinder houses the riser i-tubes and supports the turret main deck, the manifold platform, the swivel stack, riser winches, and all of the fixed portions of the rigid piping. The Anasuria-type turret was designed to accommodate up to 14 risers of various diameters.

Another breakthrough in turret technology came in the late 1990s when SBM Offshore was called to moor two large, deepwater FPSOs for Petrobas, Brazil. The challenge was to accommodate 35 and 47 risers respectively in two weather vaning FPSOs for various functions. The swivel stacks used were among the largest in the industry, with up to 12 individual passes for hydrocarbons. This same technology was later applied to the Espadarte FPSO (285,000 DWT). The Espadarte‘s turret system accommodates 45 risers, servicing up to 15 production and 9 water injection wells.