Spiral Drill Collar
Product Description
Drill collar is a device used at the lowest part of a drill string, an important part of the bottom hole assembly (BHA). It is always a very thick-walled tube to apply weight to the drill bit. The spiral cut is added to a drill collar to prevent the likelihood of differential wall-sticking by reducing wall contact between the drill collar and the wall of the hole. The spiralling starts *2 from the bottom of the slip recess or *9 from the box end and ends *0 from the pin end. Normally, a drill collar is threaded at both ends, male on one and female on the other. Except for thickness as a special characteristic, a drill collar owns other unique features, gravity, rigidity, etc.
Drill Collar Classification:
Drill collars can be simply put into three categories according to their structure and material, namely the slick drill collar, the spiral grooved drill collar and the non-magnetic drill collar.
1.The slick drill collar is normally a smooth thicked-wall tube, with a thickness in the range of *8 to *3, about 4 to 8 times of that of the drill pipes. It is machined into a uniform cylindrical shape and made up of plain alloy steel.
2.The spiral drill collar is also machined from plain alloy steel, but it is normally lighter by 4 to 6 percent than the slicker drill collar. This kind of drill collar is widely used nowadays because the special spiral design largely hold back of the possibility of sticking accidents by reducing the area of contact between the wall and the collar.
3.The non-magnetic drill collar is manufactured from the non-magnetic alloy steel. As the magnetic measuring instruments can sense the geomagnetic field when measuring the direction of the borehole, the instruments must be kept working in a non-magnetic environment. However, in drilling process, the magnetic rigs always affect the accuracy of the measuring instrument. Therefore, the utilization of a non-magnetic drill collar can ensure a more reliable measuring result.
Drill Collars in Drilling Application:
1.Drill collars play a vital role in drilling process. Due to the structural features, drill collars can provide weight for drilling purpose, which is drill collars one and foremost specialty. Gravity works on a number of drill collars to offer enough downward force needed to break rocks with the drill bit.  
2.Apart from providing weight on the bit (WOB), drill collars also work as a shock absorber to dampen the vibration and shaking when drilling takes place. In this way, the drilling process can be carried out in a much steadier state.
3.Thirdly, the presence of the drill collars allows the pipelines to stay in the tensile state, while guarantees the rigidity of the drill string under great compression. Thus, the drill efficiency and quality are greatly improved.