Previous-generation subsea wellhead equipment was conceived as an extension of well-known surface wellhead equipment. Contemporary performance criteria for subsea wellhead equipment require new technology from the designer/manufacturer. This paper describes the role of a single design concept, independent load support, in addressing these criteria and illustrates the resulting configuration of a severe-service subsea wellhead system.
Traditional subsea wellhead design, dating back to the early 1960's, uses a high-pressure wellhead body containing one or more casing hangers and their annular seal assemblies or packoffs. The wellhead body includes a single internal annular shoulder, available in the difference between the sealing diameter [blowout preventer (BOP) size] and the minimum ID (bit size). The casing preventer (BOP) size] and the minimum ID (bit size). The casing hangers in the wellhead body land on this shoulder and on one another in sequence. The total downward axial load from casing weight and from the wellbore pressure end load thus is carried on this single shoulder,
The size of this shoulder, and hence its bearing-support area, is fixed by the BOP and bit sizes for the first internal casing string (Fig. 1). These sizes are independent of the aggregate load that the shoulder must support. The load capacity of this shoulder eventually is overwhelmed by higher pressure, deeper and heavier casing strings, and hardness restrictions for sour-gas service. Even mechanical augmentation of that shoulder area is of limited benefit.
The most appropriate design approach to the load limitations of conventional wellhead design is the use of independent load support. Put simply, this involves the use of multiple load paths into the wellhead bore, instead of a single path through the lower annular load shoulder.
The use of multiple load paths is not new. Designers of "unitized" surface wellheads previously provided individual annular load shoulders along the length of a multiple hanger head to support individual casing hangers. These shoulders were designed by taking the available annular area (from the BOP size and bit clearance) and dividing it into multiple. smaller shoulders.
The drawback of this old surface wellhead design is that it offers no added load-bearing support area. Furthermore, the adjacent wellhead wall, upon which the packoff must act, has a different diameter at each hanger level. Consequently, the packoffs cannot be interchanged between the hangers. packoffs cannot be interchanged between the hangers. The limitations of traditional subsea wellhead design can be appreciated through consideration of the contemporary design criteria for subsea wellhead. In the discussion that follows, use of 18 3/4-in., 15,000-psi BOP's is assumed to be the most severe case for design of contemporary wellheads.