The term drillstring is used to describe the tubulars and accessories on which the drillbit is run to the bottom of the borehole. The drillstring consists of drillpipe, drillcollars, the kelly and various other pieces of equipment such as stabilisers and reamers, which are included in the drillstring just above the drillbit (Figure 1). All of these components will be described in detail below. The drillcollars and the other equipment which is made up just above the bit are collectively called the Bottom Hole Assembly (BHA). The dimensions of a typical 10,000 ft drillstring would be :
The functions of the drillstring are:
• To suspend the bit
• To transmit rotary torque from the kelly to the bit
• To provide a conduit for circulating drilling fluid to the bit
It must be remembered that in deep wells the drillstring may be 5-6 miles long.