The subsea SpoolTree system (also known as "Horizontal" or "Valveless" tree) features a spool with a wellhead connector below and a wellhead connector profile on top; the tubing hanger is hung off inside the spool; the valves are situated on the side of the spool and there are no valves in the vertical bore of the wellhead/tree system. This configuration requires the well completion to be run after the tree has been installed but it enables the completion to be pulled without the tree being removed. The running tools and accessories required for the SpoolTree are simpler than for a conventional system. A description of the concept as applied for the first time in the Gryphon Field is provided.
The novelty of the concept lies in the configuration of the equipment, not in the technology employed The system offered cost and delivery time advantages, but of prime concern was the safety implication of using this novel equipment, and a comprehensive safety evaluation was carried out.
The safety features of the Gryphon SpoolTree system are described and all 7 safety studies investigating the System from various angles are summarized.
The final section addresses future potential applications of the Concept.
When the decision on the Gryphon Field development configuration was taken, around the middle of 1992, it required that the wells would be completed subsea and linked to a FPSO moored nearby (Ref. 1). There would be four different types of well:
an aquifer water supply well;
a dual-purpose gas (injection/production) well.
At the time, the option of using pump-assisted dumpflood ('pumpflood') wells to replace the water supply well and the injectors (see Ref.2) was still under consideration. The consequence for the subsea trees would be that types (ii) and (iii) above would be replaced by a tree allowing full-bore access to a 7" completion, designed for facilitating wireline access for the running/retrieval of 7" nominal hydraulic downhole pumps.
At the time of initial enquiry, requirements common to all subsea trees were formulated as follows:
The project schedule allowed 9 months from committal date to deliver of the first trees.
Simplicity and proven technology should be employed where possible to safeguard the schedule requirements.
Safety levels should be at least as good as existing designs.
Cost reductions (compared to existing hardware) must be sought.
Pressure and temperature levels are not excessive, and standard 5000 psi equipment would be adequate.
18 3/4 " wellheads with standard casing configuration would be used, though certain casing strings may be omitted in certain wells.
Valves would be operated by direct hydraulic control, both by the Workover Control system as well as by the Production Control System.