Before selecting a downhole tool for a particular Bottomhole Assembly (BHA) or a drill string, knowledge and fundamental theory involved in the tool is essential so that the tool can be used effectively at optimum operating conditions. Whether a simple tool or a complex tool, ultimate aim is to derive full potential out of the tool. Also some of these tools are energy conversion tool which needs meticulous planning and usage of the operating variables and needs analysis considering the overall drilling system.

The Drillstring is the major component of the whole drilling system. The Drillstring is designed to connect the bit to the surface tool and equipments and provide a conduit for circulation of the drilling fluid. The drillstring consists of bit, bit sub, bottom hoe assembly, and drillpipes. Typical bottomhole assembly consists of drillcollars, different downhole tools, and heavy weight drillpipe. Various downhole tools such as stabilizers, reamers, positive displacement motors..etc. are added for several purposes. These tools are added to achieve best bit performance so that a usable and less tortuous hole is drilled. The Bit is rotated using either a rotary table and Kelly bushing or a downhole motor. Depending on the mode of operation, the drillstring and the downhole tools are exposed to stresses and torque. Improper design of the drillstring can result in the failure of the drillstring components.