Offshore production platforms have closely spaced wellheads thereon. Such spacing may be 71/2 to 10 foot center-to-center. The danger of a fire on such a platform is serious and the seals between the wellhead and the casing hanger have not generally been capable of withstanding the temperatures generated by an adjacent fire for a period of time sufficient to ensure that all of the subsurface safety valves have been closed to prevent release of fluids from the wellhead which would further fuel the fire. Additionally, in many applications the stem seal on a valve fails when close to a fire and allows fluids to escape which may further feed the fire.
Materials, such as Grafoil (a registered trademark of Union Carbide Corporation), have been developed which can provide a seal when exposed to high temperatures for a reasonable period of time. One form of such material is shown in U.S. Pat. No. 4,256,317. A casing hanger seal assembly, which is actuated by a piston, is suggested by U.S. Pat. No. 3,561,527. Spring loading of a seal assembly is shown in U.S. Pat. No. 4,160,551, which patent also discloses the use of alternate layers of graphite and spring metal as the sealing elements of a seal assembly. U.S. Pat. No. 1,324,775 discloses a piston actuated packing gland for sealing around a piston rod with springs positioned between the piston and the packing rings. The packing rings are hollow and wedge-shaped and have hollow, oppositely positioned wedge-shaped metal rings between each packing ring. The metal rings support the packing rings and the interengaging wedge shapes urge the packing rings inward against the piston rod.
None of the known prior art provides a seal assembly which, when subjected to an external fire, will provide an effective seal for sufficient time to ensure that the fluids contained by the seal assembly are not released to fuel the fire.