Drillpipe is the major component of the drillstring It generally constitutes 90-95% of the entire length of the drillstring. Drillpipe is a seamless pipe with threaded connections, known as tooljoints (Figure 2). At one end of the pipe there is the box, which has the female end of the connection. At the other end of each length of drillpipe is the male end of the connection known as the pin. The wall thickness and therefore the outer diameter of the tooljoint must be larger than the wall thickness of the main body of the drillpipe in order to accommodate the threads of the connection. Hence the tool joints are clearly visible in the drillstring.
Each length of drillpipe is known as a joint or a single. The standard dimensions for drillpipe are specified by the American Petroleum Institute. Singles are available in three API length “ranges” (see Table 1) with range 2 being the most common. The exact length of each single must be measured on the rigsite since the process used to manufacture the drillpipe means that singles are not of uniform length. Since the only way in which the driller knows the depth of the drillbit is by knowing the length of the drillstring the length of each length of drillpipe (and all other drillstring components) made up into the drillstring must be measured and recorded on a rillpipe tally. The drillpipe is also manufactured in a variety of outside diameters, and weights (Table 2) which assuming a specific gravity for steel of 490 lb/cuft, is a reflection of the wall thickness of the drillpipe. The drillpipe is also manufactured in a variety of material grades (Table 3). The specification for a particular string of drillpipe could therefore appear as:
5” 19.5 lb/ft Grade S Range 2
All of these specifications will influence the burst, collapse, tensile and torsional strength of the drillpipe and this allows the drilling engineer to select the pipe which will meet the specific requirements of the particular drilling operation.
Care must be taken when using the specifications given in Table 2 since although these are these are the normally quoted specifications for drillpipe, the weights and dimensions are ‘nominal’ values and do not reflect the true weight of the drillpipe or the minimum internal diameter of the pipe.
The weight per foot of the pipe is a function of the connection type and grade of the drillpipe and the weight per foot that should be used when calculating the true weight of a string of pipe is given in Table 13.
The weight of the pipe calculated in the manner described above will reflect the weight of the drillpipe when suspended in air (“Weight in air”). When the pipe is suspended in the borehole it will be immersed in drilling fluid of a particular density and will therefore be subjected to a buoyant force. This buoyant force will be directly proportional to the density of the drilling fluid. The weight of drillpipe when suspended in a fluid (“Wet Weight”) can be calculated from the following:
Buoyant Weight (“Wet Weight”) of Drillpipe = Weight of pipe in Air x Buoyancy Factor
The buoyancy factor for a particular density of drilling fluid can be found from Table 15.
Institute of Petroleum Engineering, Heriot-Watt University