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.
Off take of oil from the Heidrun field is achieved through a Direct Shuttle Loading (DSL) system. This approach eliminates the need for an intermediate storage facility, allowing continuous production and transfer of oil directly from the Heidrun TLP to shuttle tankers. Purpose-built or appropriately converted tankers with an integral bow turret locate and connect to a Submerged Turret Loading (STL) buoy which functions both as a tanker mooring point and a termination for the flexible offloading line. The system is designed to permit the tankers to remain connected during loading and to disengage km the STL buy on completion of loading in all weather conditions up to and including the 100 year storm. This paper describe a implementation of the Heidrun DSL system from conception to first oil. It gives the background for choosing the DSL system and information on the data generated to support the selection process. Design, fabrication and installation of various components are explained to give an sight into the challenges that had to be overcome for realization of this 'first-of-its-kind' system in a record time of about one year Installation of the complete DSL system in the summer of 1994, approximately one year ahead of the original plans, mabled full scale in situ testing of the system with a purpose modified shuttle tanker. The two-month program provided the equivalent of one year of operational experience with the system before first oil. The paper addresses data obtained during the full scale testing, and comparison with analytical results. The operation of the Heidrun DSL system is also described, These data together with the experience gained during realization of this bold concept will give key information on how such a concept can be effectively applied to any major or marginal field development scenario either as an off take system or in conjunction with an FPSO/FSO. Introduction The DSL represented a method of offloading oil which required a bold decision for its implementation for a major development like Heidrun. When in summer of 1992 Conoco decided to use this concept for Heidrun, the STL technology was still in it a infancy and had not advanced beyond the preliminary design stage. Nevertheless, the idea was promising and offered potentially substantial cost savings when compared with other alternatives. However, the 'first-of-its-kind' and uninvolved nature of the STL technology, combined with the extremely tight schedule for its implementation, presented the Conoco Project Team with a formidable challenge. An STL system was used for the first time in October 1993, albeit in a permanent connection mode, for the UK Fulmar field. Some Fulmar experience was useful, including the marine operation aspects, in service data, etc. which led Conoca to adopt a modified approach to marine operations and very rigorous testing and trial programs for implementation of the Heidrun DSL system.
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