The procedure for installing a spool type wellhead system (Figure 6) can be outlined as follows:
(a) The conductor (30″) is run and cemented in place. It is then cut off just above the ground level or the wellhead deck (on a platform);
(b) The 26” hole is drilled and the 20″ casing is run through the conductor and cemented. Sometimes a landing base is welded onto the top of the 20” casing so that it can rest on the top of the 30” conductor, to transfer some weight to the 30″ casing.
(c) The 20″ casing is cut off just above the 30″ casing and a 20″ casing head housing (lowermost casing head) is threaded, or welded, onto the top of the casing. The internal profile of this housing has a landing surface on which the casing hanger of the subsequent casing string (13 3/8”) lands. The housing has two side outlets which provide access to the 20”x13 3/8” annulus. The upper flange of the housing is used as the lower part of the connection to the BOP stack used in drilling the next hole section. A ring gasket is used to seal off the connection between the housing and the BOP stack.
(d) The 171/2” hole is drilled and the 13 3/8″ casing is run with the hanger landing in the 20″ housing. The casing is cemented in place. The BOP stack is disconnected and a casing spool (13 5/8″) is flanged up on top of the 20″ housing. The BOPs are made up on top of the 13 5/8” spool and the 12 1/4″ hole is drilled.
The process continues, with a new spool being installed for each casing string. Eventually a tubing head spool is installed. This spool allows the completion tubing to be suspended from the wellhead. The minimum I.D. of a casing spool must be greater than the drift I.D. of the previous casing. A protective sleeve known as a wear bushing is installed in each spool when it is installed and before the drillstring is run. The wear bushing must be removed before the next casing string is run. Finally the Christmas tree is installed on top of the wellhead (Figure 7). A ring gasket, approved by the API, is used to seal off the space between the flanges on the spools. The gaskets have pressure energized seals and can be rated up to 15000 psi.
The disadvantages of this type of wellhead are:
1. a lot of time is spent flanging up the spools;
2. the large number of seals, increases the chance of a pressure leak;
3. BOPs must be removed to install the next casing spool;
4. a lot of headroom is required, which may not be available in the wellhead area of an offshore platform.
Institute of Petroleum Engineering, Heriot-Watt University