One of the most challenging design efforts associated with deepwater subsea completions is the surface-controlled subsurface safety valve (SCSSV) system. The potential for hydrates to form in the wellbore of deepwater wells forces the SCSSV to be set at increasingly greater depths as the depth of water increases, For conventional wellbore-pressure-sensitive SCSSVS, increasedsetting depths normally yield higher operating pressures, Additionally, dynamic effects inherent in the control system tend to drive operating pressure higher still. These higher operating pressures adversely affect practically the entire system, and must be recognized and accommodated early in the design process to avoid critical limitations in the subsystems which support the SCSSVS (i.e., control system, umbilical, and tree and wellhead system). Likewise, reliability and operability of the SCSSV system must be addressed early in the design phase. The design parameters affecting the SCSSV system for Mustique and Shasta are typical of those encountered in deepwater subsea completions. This paper reviews how these challenges were addressed by the design team for the Mustique and Shasta projects.
The Shasta and Mustique subsea satellite wells are located in the central Gulfof Mexico, The two Shasta wells are located in 860 and 1,000 ft of water respectively in Green Canyon Block 136, These gas wells have shut-in tubing pressures of approximately 3,700 psi, are completed with 5,000-psi horizontal trees, and are tied-back 9.5 and 11 miles respectively to Texaco's GC-6 platform (Figure 1). The single Mustique gas well is located in 832 R of water in Garden Banks Block 240, has a shut-in tubing pressure of approximately 7,400 psi, and is completed with a conventional 3-1 /16-in. X I-13/16-in. 10,000-psi subsea tree. The Mustique well is tied-back to Chevron's production platform in GB-236 (Figure 2). Both systems utilize an electro/hydraulic control system with a retrievable control pod mounted on each subsea tree. The coordination of the design, manufacture, testing, and installation of the two systems was performed by a team of contractors' representatives seconded to a team working for Hardy Oil and Gas USA Inc., the operator of the projects.
Early in the project it was recognized that an operable and reliable SCSSV system was critical for project success. For that reason, considerable research and analysis went into the selection of the SCSSV system configuration and components, and as a result of that research, a system utilizing tandem non-equalizing, flapper-type SCSSVS with selective capability and in-line hydraulic control line filters was selected.
The extent to which the SCSSV control system pressure would have to be elevated above shut-in pressure was not tiny appreciated initially. Due to project time constraints, the control umbilicals had to be ordered before final hydraulic analyses could be performed.