This paper presents the design of a metal-to-metal seal suitable for use in subsea wellheads along with some of its more salient performance characteristics. A Packoff Qualification Test Series being used to qualify the seal for subsea drilling service is also presented. The test series includes Defects Testing wherein mechanical defects are purposely manufactured into the metalsealing packoff and related sealing areas to represent operating field conditions. The sizes of these defects were determined from a survey of used wellheads that have been in service.
The environments in which sealing systems on subsea wellhead equipment must operate are necessarily harsh. The primary sealing surface of the wellhead housing has to suffer the rigors of the drilling operation, and the sealing system itself has to be transported through the BOP stack and riser system (in current water depths, this could mean a distance of 7500 tt.l2300 m). Seal misalignment, contaminants, and well fluids can also contribute to the hostile environment. This environment must be properly defined and better understood if effective seal systems are to be developed, since we cannot assume that seal systems which operate satisfactorily in laboratory testing will also perform well in actual field service.
The metal seal shown in Figure 1 exhibits several desirable characteristics to meet the rigors of offshore drilling. The seal consists of inner and outer "leg" sections of metal which are spread apart by an energizing ring which is forced downward. As the metal sections move apart, "wickers" (a wicker is a leadless threadform) which are machined into the OD of the hanger and the ID of the subsea wellhead bite into the metal seal sections. This biting action has been shown through extensive testing to generate a reliable metal-to-metal seal. Although not addressed in this paper, a significant amount of axial motion lockdown is also generated in addition to the sealing action.
Figure 2 is a typical plot of force exerted on the energizing ring versus its downward displacement. Figure 3 is a typical plot of the average pressure across the area of the wickers vs. their depth of bite obtained during model testing. Note that the average contact pressure across the wicker sealing area is approximately 70,000 psi (4.8s million Pa). Although- this contact pressure on a full size seal is estimated to be somewhat less (because hoop yielding occurs at the same time that biting action into the wickers is. taking place), it will significantly exceed the design working pressure of 15,000 psi (1 million Pa), thereby preloading the seal against the hanger OD and the housing ID.
Any claimed metal-to-metal seal for subsea wellheads should undergo a series of stringent tests to qualify its performance during any development or seal evaluation phase. Testing must be conducted at least to the extremes of expected operating conditions such as temperature extremes. Table 1 shows eight such tests along with six test condition variables.
