Bit & BHA Whirl: Basics & Corrective Actions

Whirling is one of undesired motions which can be faced during drilling operations. It is an eccentric rotation about a point other than the geometric center. It is categorized in three types: 

- Forward whirl where the center of the motion rotates in the same direction as the drill string 
- Backward whirl where the center of the motion rotates in the opposite direction of the drill string 
- Chaotic whirl where the motions is a combination of backward and forward rotation 

If the whirl rate of one of the BHA components (drill collar, heavy weight ...) is the same as the rotation of that equipment then the whirl is called synchronous whirl. In synchronous whirl the same face of the drill string or BHA is permanently against to the borehole wall and this can lead to localized wear which can be recognized at surface when pulling out of the hole. The localized wear causes the reduction of the mechanical properties of that part of the BHA. The backward whirl can cause cracks which can grow from the outside to the inside of the pipe body leading to a failure and result in washout or a twist-off. During the chaotic or intermittent whirl, the BHA may repeatedly impact the borehole wall, this behavior can generate high lateral shocks and damage down hole tools such as MWD. The BHA whirl can be started when the stabilizers pass through ledges or formation change. 

Fig 1 - Forward and Backward Whirl

Bit whirl is the off center rotation of the drilling bit and also can be forward, backward or intermittent. It can be faced with any types of drilling bits (PDC, tri-cone or core …). It can cause over-gauge holes, slower rate of penetration and high wear rates on bits. Backward bit whirl can cause loss of bit inserts resulting in bit failure. The bit whirl starts usually at the connections or formation change and can continue till the next connection. 

The whirling has many features, it can be recognized at surface as lateral vibration (side to side motion), also high level of torque. After pulling out the hole, many signs can be considered as results of whirling problem such as cutter damage generally on the shoulder and gauge area, localized wear on tool joints and damage on the stabilizers blades. 

Available corrective actions are limited to this type of problems, but there are some hints to be mentioned: 

- Bit whirl is a dynamically stable so starting drilling in good situation can help to avoid this problem. 
- If whirling is severe, the drill string should be pulled off bottom and start drilling again without initiating whirl and apply reduced RPM and increased weight on bit 
- Changing drilling parameters can contribute to destroy the whirl pattern
- Using roller reamers rather than stabilizers can reduce the impact of whirling and improve the drilling performance.

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