Dogleg Produced by a Steerable System
When oriented drilling, the theoretical geometric dogleg severity or TGDS produced by the system is defined by three points on a drilled arc (Figure 12). The three points required to establish the arc are:
• The Bit
• The PDM stabilizer or Upper Bearing Housing Stabilizer.
• The first stabilizer above the mud motor (upper stabiliser).
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The radius of the arc is further determined by the tilt of the navigation sub, as seen in the Figure 12. The following basic relationship is produced by mathematical derivation.
TGDS(Degrees / 100ft) = 200xTiltAngle
L1 + L2
where:
Tilt angle = Bit tilt in degrees L = length between bit and upper stabilizer (L1 + L2)
L1 = length between UBHS and the upper Stabilizer
L2 = length between UBHS and the bit
This dogleg rate or TGDS is that created when the steerable system drills in the oriented mode. Furthermore, the theory considers that the system has full gauge stabilizers.