STL, A New Approach to Field Development
Authors: Smedal A. , Syvertsen K. 
Published: October 2008

Submerged Turret and Loading (STL) is an efficient and safe system for offshore loading and storage. The loading system can effectively be adapted to specific field requirements and operational conditions, The STL is designed to meet the requirements for versatile and flexible field development, allowing loading operations to take place all year round in the harshest environments.

The STL concept has been selected for field development projects with different operational requirements i.e.Direct Shuttle Loading (DSL) which permits Loading of processed crude oil directly into shuttle tankers without the requirement for oil storage at the field. Two STL loading buoys located adjacent to the production installation allows continuos loading and efficient transport to the point of sale. A Direct Shuttle Loading system is installed at the HeidrunField (Conoco).STL as a Floating Storage Unit (FSU) application has beenselected for the Fulmar Field (Shell/Esso) and the Yme Field (Statoil). Both FSU?S are converted standard tankers (Aframax and Suezmax). and designed for operation in extreme North Sea environments. The STL system allows the FSU units to be disconnected for service and maintenance. A safe rapid disconnection can also be made if the environmental conditions exceed the design conditions.An STL buoy can be arranged as an effective offshoreLoading terminal. BP have selected STL for the Harding Field. The STL loading terminal will be used for export from a subsea field storage.Based on the STL technology Statoil have launched a new design, Multipurpose Shuttle Tanker (MST). This unit will be equipped with the STL ship systems allowing it to operate as an STL tanker for direct shuttle loading, as an STL loading terminal or as a storage unit. In addition the vessel is equipped for operation as a standard North Seashuttle tanker capable of loading from floating storage units and standard North Sea loading terminals via a loading hosein the conventional manner.

The MST tanker can also serve as a floating production and storage unit (FPSO). For operation as an FPSO the vessel is equipped with process facilities placed as modules on deck. The production risers are connected to a slightly modified STL buoy with an integral multi-path high pressure swivel unit (Rotating connector). In the production variant a specially designed STP buoy is required (Subsea Turret Production buoy). The connector allows the FPSO tanker to weathervane during production direct from subsea wells. The rotating connector is a new development satisfying all requirements for wellstream, gas and water injection, fluids and control umbilical functions.

A vessel of the MST type is in order, due for completion in late 1996.The production concept, developed from STL technology can effectively be used for conversion of standard tankers to FPSO or for new-built FPSO?S.

GENERAL

Offshore loading based on the use of shuttle tankers with dedicated loading equipment started in the North Sea in mid seventies. The offshore loading operations were found to be a safe and cost effective alternatives to export via pipelines for several North Sea Field developments.