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14 DERRICKS AND MASTS

Despite the fact that most of the steel associated with an oil
well is below ground, the oil derrick is the symbol by which an oil
well is known. To many people it is the trademark of the oil
industry — in fact they often give little thought to what is going on
below the ground, seeing only that which is above.
The oil derrick, however, has undergone an extensive period
of transition. In the majority of cases the permanent derrick is a
thing of the past. Conventional derricks are seldom left over the
hole. Many manufacturers have discontinued production of con-
ventional derricks. Also there are derrick construction contractors
with a large supply of conventional derricks which they rent for
drilling purposes.
Derricks in use today fall into three broad classifications:
conventional, portable (usually free standing), and portable mobile
types. All can be used for drilling and servicing but the first two
types are predominantly used for drilling and the last type for both
drilling and servicing.

Conventional Derricks

Considering the conventional derrick first, the reader is
referred to API Standard No. 4A API Specifications for Steel
Derricks (including standard rigs) and to Fig. 14.1 which shows
derrick nomenclature.
There are nine API sizes of conventional derricks. These
have the following dimensions (tabl. 14.1).
The height A of an API conventional derrick is the distance
measured along the neutral axis of the derrick leg from the top of
the derrick floor joists to the bottom of the water table beams or
bumpers.
The base square dimension B is the distance between the
neutral axis of adjacent legs at the top of the derrick floor joists.

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