The wellhead is simply a crossover between the various casings and the Christmas tree or -temporarily - the BOP.
Unitized wellheads
The evolution from spool type wellheads to compact, or uniheads has occurred over a number of years. This evolution has generated a wellhead which is technically superior, offers enhanced safety and rig time saving, without a direct cost penalty. Experience has also shown, that there need be little or no distinction between the designs for exploration and development applications, provided that the unihead is maintainable.
Standardisation
The objective is not a standardization of equipment geometry and/or vendor but rather one of approach to the design.
Sealing
Endless and axi-symmetric seals are a must.
Seal interface geometry must be designed with tight tolerances.
Rule of thumb: Gap times Pressure rating equals a constant (0.005" ´ 1 5Ksi).
Corollary: Higher pressure ratings require tighter machining tolerances.
BOP connections
API connections are sub-optimal.
For new installations, state of the art connections - available from all vendors - should be considered.
On existing API connections, fasteners should be upgraded.
Housing
Matched strength connections are a must.
Coupling-like connections and/or butt-weld 'HOTHED' are cost effective.
Slip-on heads - fillet or socket welded - are not recommended.
Casing suspension
Matched strength connections are a must.
Coupling-like connections are cost effective.
Dated segmented slips are not encouraged.
Mandrel type hangers, emergency slips and seal assemblies must be run through and tested under BOP protection.
Internal geometry
45° seat angles are technically superior and cost effective.
Seat areas should be in line with the cross-sectional area of the supporting system: (surface casings) or landing plates for special cases.
Centralisation and/or nesting of items is paramount for sealing. A gun barrel approach is supported, items to have the same internal diameter drift. Outlets should be minimised and their sealing capabilities optimised.