This paper discusses the design and results of extensive prototype testing of an all metal sealing, weight set, 18-3/4"-15,000 psi subsea wellhead system. This system (Figure 1) has been developed to meet industry requirements of higher pressure and higher temperature in critical applications.
Since the all metal-to-metal sealing packoff is critical to the reliability of the system, emphasis is given to its design and testing. Seal energization and retrieval forces are minimized by the unique seal design. Additionally, the development of an alternative elastomer packoff for use when all metal sealing is not required is discussed. The computer optimized hanger design addresses the need for increased system capacity.
Operator requirements for drilling subsea wells in high pressure formations, with the potential for elevated temperatures drove the development an all metal-to-metal sealing, 18-3/4"-15,000 psi system. Additionally, operator preferences for weight set operation were a significant factor which influenced the development effort.
A conceptual study was performed to investigate alternatives for metal sealing and casing suspension. Multiple concepts were then selected for further development. Each concept was analyzed and prototypes constructed as needed to evaluate its feasibility. At the end of this phase preferred concepts were selected for in depth development. A test program was undertaken to optimize the metal-to-metal seal. In parallel, concepts were developed for a single trip hanger/packoff running tool. Detailed analysis and testing was also performed to verify the integrity of the preferred casing suspension concept.
During the next phase of the project, concepts were developed for all other critical elements of the system. Operators and experienced field service personnel were contacted to refine and improve each design. Once this was completed, detail designs were produced and full scale prototypes built and tested. Components were first tested individually and later full system integration tests were performed. Every attempt was made to simulate field conditions and the equipment was evaluated assuming worst case Operational scenarios. Emergency backup equipment was also developed and tested as part of this overall effort. Critical tests were witnessed by third party inspectors.
The high pressure wellhead housing is designed to carry a combined load of 7.1 million pounds on its bottom landing seat. This includes 15,000 psi working pressure across the wellhead bore and 1.0 million pounds weight for each of three casing strings. This is achieved by using high strength load segments which form an annular seat for the bottom casing hanger (typically 18-3/4" × 13-3/8"). These load segments are held in place in a unique way by a snap ring. The housing has three undercut (0.015" per side) surfaces used for sealing metal-to-metal annulus packoffs. It was observed from field operations that scratches in the wellhead bore are typically shallower than 0.015". Thus, the undercut areas minimize the possibility of scratches at the metal sealing locations.
The casing hangers are designed with passive load shoulders; i.e., with no trigger mechanisms or expandable load rings.