Page 12 - Offshore Well Construction

CHAPTER 8
Drill String Design
Petroleum Extension-The University of Texas at Austin

DRILL STRING COMPONENTS
The principal components of the drill string are as follows.

Kelly or Top Drive System (TDS)
It is not exactly part of the drill string but transmits and absorbs torque to or from the drill string while carrying all
the tensile load of the drill string.

Drill Pipe (DP)
Transmits power by rotating motion from the rig floor to the bit and allows mud circulation.

Drill pipe is subjected to complex stresses and loads as is the rest of the drill string. Drill pipe should never be run in
compression or used for bit weight except in high angle and horizontal holes where stability of the string and absence
of buckling must be confirmed by using modelling software.

Heavy Weight Drill Pipe (HWDP)
HWDP makes the transition between drill pipe and drill collars, thus avoiding an abrupt change in cross-sectional area.
It is also used with drill collars to provide weight on the bit, especially in 6" or 8H" holes where the buckling effect of
the HWDP due to compression is minimal.
HWDP reduces the stiffness of the BHA. HWDP is also easier or faster to handle than DC and, more important, reduces
the possibility of differential sticking.

Drill Collars (DC)
Provide weight on the bit, keeping the drill pipe section in tension during drilling. The neutral point should be located
at the top of the drill collars section: 75 to 85% (maximum) of the drill collars section should be available to be put under
compression (available weight on bit).

Other Downhole Tools
Include: stabilizers, crossover, jars, MWD, underreamer, etc.

They all have different functions, but have two major common points: their placement is crucial when designing the drill
string and they introduce ‘irregularity’ in the drill string; i.e., different ID/OD and different mechanical characteristics
(torsion/flexion, etc.), which must be taken into account when designing the drill string.

Drill Bit
See Chapter 6, Drill Bits.

DRILL STRING CONSIDERATIONS
Drill Pipe
The main factors involved in the design of a drill pipe string are
• collapse and burst resistance.
• tensile strength (tension).

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