There are two types of casing hanger in common use. Wellheads can be designed to accept both types of hanger.
Mandrel (boll weevil) Type Casing Hangers: This type of hanger (Figure 8) is screwed onto the top of the casing string so that it lands in the casing housing when the casing shoe reaches the required depth. Short lengths of casing, known as pup joints may have to be added to the string so that the casing shoe is at the correct depth when the hanger lands in the wellhead. The calculation which determines the length of pup joints required to achieve this positioning is known as spacing out the string. Although this is the most common type of hanger it cannot be used if there is a risk that the casing will not reach bottom and therefore that the hanger will not land in the wellhead.
Slip Type Casing Hangers: This type of hanger (Figure 9) is wrapped around the casing and then lowered until it sits inside the casing spool. The slips are automatically set when the casing is lowered (in a similar fashion to drillpipe slips) This type of hanger can be used if the casing stands up on a ledge and cannot reach its required setting depth. These types of hanger are also used when tension has to be applied in order to avoid casing buckling when the well is brought into production.
Institute of Petroleum Engineering, Heriot-Watt University