STL - The Basic Concept

Basis of the STL system is the buoy moored to the seabed. The buoy is pulled into and secured in a mating cone in the bottom of the vessel and thus connecting the mooring system. Internal in the buoy is the turret with connection to the mooring and riser systems. The outer buoy hull can rotate freely with the vessel around the turret by means of internal turret bearings. Oil, gas or other media are transferred through a swivel to the piping system of the vessel. Disconnected, the buoy will float in an equilibrium position with a pickup line to the surface, ready for new connection.

Fast operation
With a suitably designed mooring system, the STL system can be safely operated more or less independent of weather conditions and sea states. Should the tanker for any reason, have to disconnect from the buoy, a safe release can be performed in any weather. This can be carried out either as a normal controlled operation or as an emergency disconnecting. Connection to and disconnection from the STL Buoy is a straightforward and fast operation. Under average conditions connection can be completed in about one hour from the time the tanker arrives at location. Normal controlled disconnection can be done in less than half an hour. An emergency disconnection can be undertaken within a few minutes.

An integrated solution
The basic concept of the STL system is an anchored submerged buoy with an integrated turret. The mooring lines and the riser for the fluid transfer are connected to the turret within the STL Buoy. The tanker is connected to the system by pulling the STL Buoy into a mating recess in the bottom of the ship. When the STL Buoy is locked to the tanker the vessel can freely weather wane without the aid of propulsion. When not connected the STL Buoy is floating in an idle submerged position at a designed water depth. This depth may be between 25 and 40 meters.

Geometrical standardization
All STL/ STP systems today are based on the same mating cone geometry in the vessel leading to standardization of the specialized steelwork in the vessel as well as an excellent basis for utilization and reuse of vessels. Per December 2002, 25 vessels are or are being equipped with the STL/ STP mating cone.

Flexible mooring system
The STL installations operative today are installed in water depths from 85 to 350m and with design significant wave heights of up to 16.4m. Systems in construction will have design significant wave heights up to 18.4m. Verification of mooring systems through ocean basin tests have verified the feasibility of the STL/STP mooring systems for water depths ranging from 40 to 900m. Mooring system analyses have demonstrated the suitability of the STL/ STP mooring system for water depths ranging from 20 to more than 2000m.

Mooring system verification
Correspondence between analyzed and model tested data versus real life full scale mooring line tension measurements on Heidrun have demonstrated very good accuracy. In normal operation excursion from system centre will be monitored using either HPR or DGPS systems.

STL References