RE:RE:RE:From the other CGX BBOk phantom, I'll post my friend's response to your question to me here since you copied my message to you here: On land or on a platform offshore, the wellhead or starter head is installed after the surface casing is run. Subsequent casing strings that extend back to surface and that are not liner strings, have a wellhead section and a seal assembly installed on them. The seal assembly isolates previous casing strings from fluids and pressures encountered while drilling the current hole section. If it is the intent of the operator to produce the well, a tubing hanger is installed on top of the last wellhead section. The completion with the tubing is then run through the hanger. A Christmas tree, which a set of valves is then installed on the tree, which the well can produce through.
Subsea is different. Any casing strings run are hung off inside the wellhead section which is run with the 20 inch or 22 inch surface casing. Once the operator decides he is going to produce the well, the completion and tubing string are run with the subsea tree. The tree latches up to the 18-3/4" connector on the wellhead that the BOP had been latched up.