This invention relates to improved rigid, dual radially locked subsea wellhead housings used in offshore drilling.
Early subsea wellhead system designs remained essentially unchanged for many years, typically employing a single landing or support shoulder between the 183/4" high pressure wellhead housing (referred to below as "the HP housing") and the 30" conductor housing (referred to below as "the LP housing"). This type of load-transfer mechanism operated relatively well for many years, until field failures occurred in the early 1980's while operating in high current situations. Subsequent studies revealed that external loadings, such as those generated by drilling risers or platform tiebacks, caused bending stresses in the casing extension below the HP housing, ultimately resulting in a fatigue failure of the weld between the casing and the bottom of the HP housing body. Numerous studies indicated that the magnitudes of the bending stresses in the extension below the HP housing were affected by several factors, including: (1) the type of interface between the HP and LP housings; (2) the radial clearances between the HP and LP housings; (3) the cement level in the annulus between the conductor pipe and casing extension below the HP housing; (4) the size of the wellhead extension; and (5) the external load.
As a result of the field failures and subsequent studies, the interface between the HP housing and the LP housing underwent several design changes. The end goal of most of the design efforts has been to eliminate fatigue failure in the casing immediately below the HP housing by increasing the rigidity between the HP and LP housings so that external loads imposed on the HP housing are more directly and efficiently transferred to the LP housing. Most designs implemented dual, axially spaced, sockets and a beveled landing shoulder at the interface between the HP and LP housings. However, prior "dual socket" designs included relatively high degrees of taper on the opposing interface elements, and require significant axial (or downward) force to keep the tapered elements in proper relative position so that the radial component of the tapers is maintained during load transfer. In other words, if vertical motion is present in the dual socket designs, the tapered sections of the landing shoulders can become misaligned, providing an increased radial separation or clearance between the HP and LP housings, resulting in decreased radial rigidity and greater potential for fatigue-related failure.
Thus, in order to maintain radial rigidity in such designs, it is also necessary to maintain axial rigidity. Therefore, significant attention is directed in such designs to providing means for maintaining axial rigidity to eliminate vertical movement between the HP and LP housings, often resulting in the use of complicated wedges or unidirectional latches. Further, as a consequence of meeting the goals of increasing both radial and axial rigidity, recent designs have resulted in complicated installation and locking procedures, often requiring the use of significant amounts of preload and expensive and cumbersome tools and camming devices. Moreover, once installed and used in accordance with such prior designs, the housings were difficult to refurbish and reuse.
Accordingly, there is a need for significantly improved radial lock designs for subsea housings that are effective simple to install, maintain and refurbish.