A subsea wellhead system mainly consists of wellhead housing, conductor housing, casing hangers, annulus seals, and guide base. The high pressure wellhead housing is the primary pressure containing body for a subsea well, which supports and seals the casing hangers. The high pressure wellhead housing also transfers external loads to the conductor housing and pipe, which finally are transferred to the ground.
Wellhead Housing
The wellhead housing is primary housing which supports both the intermediate and production casing strings. In the API 17D, very detailed profiles are introduced.
Two kinds of subsea analyses are necessary to consider in the wellhead housing design procedure: load stress analysis and thermal analysis. The hanger landing shoulder will sustain loads from the tubing hanger. Normally, the finite element analysis and riser fatigue analysis will be performed to verify the design capacities.
In addition, thermal analysis is performed to determine the temperature profiles through the system so that temperature derating can be accounted for as appropriate. A prototype wellhead shall be tested to the test pressure as well as loaded with simulated casing loads and BOP test pressure with hangers in place simulating the real production environment in total without experiencing any permanent deformation.
Intermediate Casing Hanger
The intermediate casing hanger for the system lands in the first hanger position in the lower portion of the wellhead. The casing hanger can nominally be for either a 16" or 13 5/8" casing. The casing hanger features an expanding load ring that lands into the wellhead seat segments to suspend the casing and BOP pressure end loads.
The analysis is performed on existing field-proven hangers of similar designs to compare stress levels. Reliability data also will be collected from similar equipment, and lessons learned shall be incorporated into the design. Finally, through testing, the hanger is proof tested to the design loads, pressures, and combined loads in the exact sequence they would be applied in the field, without experiencing any permanent deformation.
Production Casing Hanger
The production casing hanger for the system lands in the second hanger position. The casing hanger can nominally be for either 11-3/4" or 10-3/4" casing. The casing hanger features an expanding load ring that lands into the second set of wellhead seat segment to support the casing and BOP pressure end loads. Using the approach above, detailed stress analysis and classical calculations shall be performed in similar fashion to the analysis performed on the intermediate hanger.
Lockdown Bushing
The lockdown bushing is used to permanently hold the production casing hanger in place so that the annulus seal assembly locked to the hanger does not move and get damaged during start-up/shut-in operations.
The design approach for this piece of equipment was handled in the same fashion. Design calculations and finite element analysis are performed in conjunction with the gathering of reliability lessons learned and FMECA to confirm the integrity for the design upfront. Tests shall be performed to confirm that the lockdown bushing and tool can fully function as designed, and load testing is also performed to confirm the load capacity.
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