Stabilisers consist of a length of pipe with blades on the external surface. These blades may be either straight or spiral and there are numerous designs of stabilisers (Figure 8). The blades can either be fixed on to the body of the pipe, or mounted on a rubber sleeve (sleeve stabiliser), which allows the drillstring to rotate within it.

The function of the stabiliser depends on the type of hole being drilled. In this section we are concerned only with drilling vertical holes. Drilling deviated holes will be dealt with later. In vertical holes the functions of stabilisers may be summarised as follows:
• Reduce buckling and bending stresses on drill collars
• Allow higher WOB since the string remains concentric even in compression.
• Increase bit life by reducing wobble (i.e. all three cones loaded equally).
• Help to prevent wall sticking.
• Act as a key seat wiper when placed at top of collars.

Generally, for a straight hole, the stabilisers are positioned as shown in Figure 9.Normally the stabilisers used will have 3 blades, each having a contact angle of 140 degrees (open design). When stabilisers begin to wear they become undergauge and are less efficient. Stabilisers are usually replaced if they become 1/2” undergauge (3/16” undergauge may be enough in some instances).

Drillstring Stabilisers

Institute of Petroleum Engineering, Heriot-Watt University

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