Page 98 - Air and gas Drilling Field Guide 3rd Edition

4.6 Drill String Design 89

FIGURE 4-24. Typical flapper type float valve for direct circulation operations (courtesy of
Baker Oil Tools).

4.5.2 Kelly Sub Valves
At the top of the drill string (just above the kelly) is a kelly cock sub, which is fitted
with a ball valve (see Figure 4-1). In the event of a subsurface blowout, the kelly
cock’s ball valve can be closed and the sub left made up to the top of the kelly. With
the ball valve closed, a pressure gauge can be made up to the top of the sub. Using
this pressure gauge, the ball valve can be opened and vital pressure information
obtained for the pressure inside the drill string (together with casing head annulus
pressure). This information is needed to design the well control procedure.

4.6 DRILL STRING DESIGN
One of the initial planning steps for a rotary drilling operation is the design of the
drill string. The drill string must have the strength to drill to the intended target
depth and be light enough so that the hoisting system can extract the string from
the well when the target depth has been reached. This section describes the API
procedure for drill string design [1, 7].