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The V window opening used for rotary drilling permits the
drawing pipe from the pipe rack into the derrick. The minimum
height of the window opening measured in the clear parallel to the
centerline of the derrick side from the top of the derrick floor joists
Rotary drilling derricks have a clear opening on the
drawworks side. For the 94-ft, 122-ft and 136-ft derricks this is a
minimum of 10 ft to each side of the centerline of the derrick and 7
1/2 ft above the top of the floor joists. For 140-ft, the 147-ft and
189-ft derricks no actual minimum is specified except that the area
any cross bracing or interfering members. Hence, the design of the
derricks utilizes outside bracing on the drawworks side, called
wing bracing.
Practically every derrick manufacturer makes some derricks
which are outside the API standard dimensions.

Supporting Capacity of Conventional Derricks

Conventional derrick capacities run from 86,000 Ib to
1,392,000 Ib. The API safe working load capacity (not failure) is
the maximum dead load, the derrick is designed to support with a
factor of safety of approximately 1.94. This value, given on the
nameplate attached to the derrick, is the allowable column load
capacity of a single derrick leg multiplied by 4. A factor of safety
of approximately 1.94 is provided in the API column formula to
provide for the added effect of wind loads, drill pipe setback in the
derrick, dead weight of the derrick, dynamic loads and the effect of
anchoring the deadline in different positions.
The safe working capacity changes when the deadline is
anchored in different positions. These are as follows:
1. Anchored in corner opposite drawworks side – reduces
capacity 20 %.
2. Anchored at calf wheel opposite drawworks – reduces
capacity 7 %.
3. Anchored at corner on same side as drawworks – reduces

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