When drilling wells for the extraction of hydrocarbons there are two established conventional methods of suspending casing and sealing the annulus:
by the use of a conventional wellhead stack with slips and elastomeric seals, which often involves removal of the BOP stack during slip installation, or
by the use of a unified casing head (multibowl) with mandrel type hangers and seal assemblies run through the BOP.
A recent joint development programme between a wellhead equipment supplier and an oil company seeks to provide the advantages of both systems by providing casing tensioning capability, positive casing hanger lock-down, and a high performance metal sealing system. The advantages of this system are particularly applicable to offshore platform installations in terms of both economics and safety.
An operating company is developing offshore fields with deep, hot, corrosive wells and is experiencing many problems with existing wellhead systems:
Unified wellhead with mandrel type hangers and
Conventional wellhead stack up with slips.
Consequently, this company and a wellhead equipment supplier have joined in the design of a new surface wellhead.
Up to now the operating company has mostly used unified wellheads for oil field developments on the coast of Africa and conventional wellhead stack ups for gas field developments in the North Sea. Problems have been experienced with both systems, particularly in Angola, where they have been the most serious.
For instance, since 1984 an oil field in Angolawas developed with uncemented length of deviated production casing of about 2400 meters. The temperature variation between shut-in and production modes was 50°C. This, over the uncemented casing, caused a 120 ton up thrust. On the first installed wells, with unified wellheads, the as-cemented weight of the casing was not sufficient to avoid buckling of the uncemented casing and vertical movement of the casing hanger. A combination of the temperature and fretting caused very rapid degradation of the elastomeric seals. Later wells are equipped with conventional wellhead stacks, with the casing tensioned on the slips. With this pre-tension there has not been buckling of the casing and only slight vertical movement of the wellheads. However, the system does not give completely satisfactory service:
The life of the elastomer seals is uncertain for this temperature range.
Critical situations have been experienced at the end of the cementation job, with instability of the annulus.
Since the installation of the slips involves the removal of the BOP stack, cutting casing, and the installation of new spools etc., safety was jeopardized, time was lost, and costs increased significantly.
Therefore, a new system had to be designed providing:
Tensioning capability for the casing
Simple installation of the hangers through the BOP and stacking them in one wellhead housing
Metal annulus isolation seals