Submerged Turret Loading

The Submerged Turret Loading (STL™) system represents state-of-the-art technology within offshore loading. STL technology offers a flexible, safe, and cost-effective solution for oil loading tankers (OLT) and floating storage and offloading (FSO) units. STL technology has a high availability in harsh environments and is well proven since 1993.

The STL, mooring, and riser can be completed independently from the OLT or FSO. The system is installed on-site before the FSO or OLT arrival at the field. Upon arrival, the vessel hooks up to the STL on its own. The disconnect feature of the buoy simplifies unplanned repair, upgrading, inspections, and replacement of the FSO as well as abandonment of the field. The system accounts for connecting in sea states between 5 and 7 m Hs (significant wave height). 

STL basics

A buoy moored to the seabed is pulled in and secured in a mating cone in the vessel. The buoy incorporates a turret connected to the mooring and the riser(s). All STLs are based on standardized mating cone geometry in the vessel.

Permanent or disconnect mooring

The STL system can be designed for disconnect service (OLT or shuttles) or for permanent mooring throughout the field life (FSO). Disconnection takes only minutes and can be performed in any weather condition. The buoy drops clear of the vessel and floats in an equilibrium position approximately 30 to 50 m below sea level.

STL integration

With bearing structures integrated in the STL buoy as opposed to bearings integrated directly in the hull, the turret integration in the vessel is simplified. Vessel modifications are limited to standard steelwork. The STL compartment is a cylinder extending from the bottom to upper deck, with the receiving cone as the lower part. With a diameter of 10 to 12 m, the impact on the hull is minimal.

STL system references

STL systems in service cover a wide range of applications, from the harsh North Sea environment to cyclone-prone areas such as China or Australia. The mooring caters to seas as high as 19 m Hs (significant wave height), water depth up to 2500 m, vessel sizes up to ultra-large crude carriers, throughput up to 600,000 BOPD, etc.

STL for high-pressure LNG regas

The STL is widely used to moor LNG carriers—either floating storage regasification units or regas vessels.

STL in ice

Icebergs and ice ridges in Arctic environments are better addressed with the quick disconnect features provided by the STL system. STL systems are verified in ice environments, and model tests have documented the survival of a vessel moored in 1.6 m-level ice with a drift speed of 0.75 m/s.