FAQ

Will the AST address lateral vibrations?

Yes, indirectly. Strong lateral vibrations primarily from whirl effects are produced when severe stick-slip makes the angular velocity of the BHA continuously traverse the critical RPM regions for the BHA.

Will the AST address high frequency torsional vibrations?

Due to the internal absorber, the AST is not capable of addressing frequencies in the 50 Hz range directly (self-excited drill-collar harmonics). The tool will however work on the rock cutting interface in a way that will keep the energy dissipated through such oscillations (i.e. the amplitude) at a minimum while drilling.

Will the AST perform better closer to the bit?

The effect of the AST is practically the same anywhere in the BHA.

The force and displacements required for the AST to affect the cutters move at the speed of sound in steel. At this speed, a difference of say 120’ or 40m has little impact. The advantage of moving the AST 120’ or 40m closer to the bit is 1.2% and 2.4% improvements in response at 120 and 240 RPM respectively.

Placing the AST closer to the bit in a low angle well also puts increased weight on the tool. This potentially increases the normal internal forces and friction losses. This must be weighed against the advantage mentioned above.

Can the AST have an effect on MWD decoding?

Even at full stroke, the AST will not displace enough mud to create a detectable pulse or frequency. Consequently the tool has no attenuating or disturbing effect on MWD signals.

In some challenging environments, such as in offshore drilling, the AST will make it easier to go on bottom without disturbing the MWD survey transmission.

Can the AST have an effect on directional drilling?

The AST tool is generally placed in a position where it does not affect directional performance.In cases where stick-slip is a problem, the effect of the AST will be to improve the orientation ability of 3D systems.

When used with a motor, the AST will make tool-face orientation more effective. In fact, it becomes so effective that care needs to be taken not to “over-steer”.

In some challenging environments, such as in offshore drilling, the AST will make it easier to go on bottom without disturbing the MWD survey transmission.

Does the AST create tension in the BHA – can it pull the bit off bottom?

The AST manipulates the compression in the lower BHA – and does not produce tension, i.e. the AST cannot pull the bit off-bottom.

A significant torque is required for the AST to contract. This means that the cutters need to have a firm grip in the rock for the AST to shorten. Once the AST has reduced the weight enough for the bit to rebuild speed, the AST contraction has already stopped.

Why is it recommended to space out the AST when drilling with an under-reamer?

The telescopic portion of the AST will otherwise come close to the under-reamer and potential side-forces will create friction that makes the AST less effective. This is also the reason why the AST should NOT be positioned right on top of a stab or roller-reamer. Minimum spacing is about 10’ (3m).

Is there a limitation on dropping balls through the AST?

None other than the given ID. 1/16” (1 ½ mm) clearance is recommended.

Can the AST create its own axial vibrations?

There is a built-in dampener in the tool to prevent the internal spring from entering its harmonic region. The contraction of the tool itself is relatively slow (governed by the RPM) and will only create minor axial accelerations – up to about 2g if the tool is working very hard.

I have an AST in the BHA and still see stick-slip?

You can be assured that the observed stick-slip is not from the cutters and presents minimal risk from shocks.

To confirm that the severity of the vibrations is low – use the MWD shock data for reference. The shock data gives a direct indication of risk.

Low energy oscillations are typically a result of friction-induced effects combined with self-excited harmonic effects. Note that the initiating friction can be from cuttings accumulated in the hole.

What happens at the Threshold lines shown un the XY-plot?

The lower threshold is the pre-tension of the tool. Torsional peaks need to exceed this level to be counteracted by the AST. The threshold is set at a level that presents no risk of harm to the BHA and bit and serves to avoid movement in the tool while tripping.

The upper threshold or operational limit means the tool bottoms out and becomes a collar.

What types of failures are seen, what are their consequences and frequency?

The dynamic function of the AST makes the seal subject to continuous wear. Current tools have three-fold seal redundancy plus a “soft-leak” system that keeps speeds low and prevents erosion/wash-out and tripping in a leak situation. The MTBF for leaks is in the range 8500 hours, i.e very high.

The AST is otherwise a mechanical tool and the most recent X-Series AST is designed with a high bending capacity with superior resistance to fatigue. The tools have consequently been free from mechanical failures for 30 000 rotary hours.

Historical failures and stories: Two back-off / twist off’s happened in the process of fielding the technology both as a result of inferior solutions and operational issues outside anyone’s control. The mechanical issues causing the failures where closed out in 2008 and two mechanical issues since have had root causes outside the tool design.

The X-Series AST’s mechanical reliability is greater than 30 000 rotary hours.

Does the AST have a job-specific setup?

There is no upper RPM limit.

When drilling vertical wells, the bit RPM should be 120 or higher to ensure the contraction of the AST is faster than the elongation of the string when reacting to torque spikes.

As the friction above the AST increases with increased deviation, the requirement for a minimum RPM goes away.

Is there an RPM limit on the AST?

No, as long as the tool is right for the main hole parameters (see AST comparison), the long stroke length accommodates a wide variety of parameters.

What is the temperature limit on the AST tools?

The standard limit is 200 °C (390F)

The 6 ¾” tools can be set up to 220 °C (425F)

What differentiates the AST from a shock-absorber?

The AST will actively reduce the WOB to keep the torque steady. This means the tool will regulate the input force to the bit in order to balance the drilling parameters.

In other terms, the AST is a regulator not an absorber.

Can I measure the AST action?

Yes, by running a instrumented AST.

Note that measuring shocks above and below the tool yields no meaning as the function is not to absorb shocks but to prevent their occurrence at the bit.

For measuring the AST contribution, the best way is to compare the same BHA with and without an AST.

Is there any effect of the AST when running a rock bit?

The AST will be effective on the roller-cone bit if torque or bit bounce is a problem.

This can often be the case with modern designs that have high offset angles in order to produce faster ROP using the “drag” or scraping effect.

Is there any benefit to running the AST on a turbine BHA?

The depth of cut and reactive torque is very low on a turbine. The benefit of running the AST will be small unless the challenge is to ensure a constant WOB.

Is there any benefit to running the AST on a motor assembly?

Yes! When running an AST with a motor, one can run the motor at higher differentials as the AST will reduce the risk of stalling. Steering is also greatly enhanced.

Is there any benefit to running the AST on a coring BHA?

If coring mixed formations, the AST will help to reduce the chance of jamming the barrel from reactive shocks when crossing formation heterogeneities. Note that if an AST is run for coring, the driller needs to pay more attention to parameters as the torque trace will not be as effective for determining if the core is jammed.

Is there any benefit to running the AST in milling operations?

Yes, when milling, the chance of stalling is high, particularly when establishing the cutting face.

What is the maximum distance from a bit?

To be inside the safe/best operational window based on our statistical data, we recommend having the tool within 60 metres of the bit.

Can the AST work in both vertical and horizontal sections?

Yes, it is suitable for both vertical and horizontal sections. The configuration for horizontal sections is different from lower deviations. See AST comparison.

Can the AST be combined with shock and bumper subs?

No, these devices will counteract the contraction and eliminate the AST effect.

What pressure drop do I put in for the AST?

The AST has a straight-thru ID and can be treated like a collar with the same OD and ID in the hydraulic calculations.

Can I use the same AST from one section to another?

If the configuration is right and the hours low, the only risk is mud drying out inside the tool – so this is the only issue to take into account.

If you find dry, hardened mud on the BHA in the set-back, do not re-run.

What happens if we need to activate the jar with an AST in the string?

Jarring down: The AST will compress when applying WOB to fire the jar. This compression will also add torque to the lower BHA that can help to make it come free.

Jarring up: No difference.

Why not put the AST below the under-reamer where my problem is?

The key is to use the AST to make the under-reamer run smoothly and remove any spikes from the bit at the same time. The worst shocks in an under-reamer BHA are shot downwards when the reamer is released from a stall with full torsional load from above.

Is it possible to service the AST on the rig?

No, all tools need to be shipped to a Tomax workshop for maintenance and repair.

Will the AST help if we have issues?

Yes, the AST will both absorb axial shocks and counteract any remaining reactive torque spikes from the same.

Heave and rough seas constitute the primary reason for use of AST in some applications.

What is the maximum service hours recommendation

200 rotating hours but there is more on this subject in the User Manual.

When within the 200 hours range, the standard surface-check should be performed to verify the status of and wear on the tool.

An additional tip to judge downhole loads comes from the fact that the tool is under constant dynamic load in the same way that the Rotary Steerable System is; so if you have the RSS changed, the AST should be replaced as well.

What is the main cost element in the AST?

The AST is a dynamic tool with both rotation and axial stroke. The seal system’s complexity is about three times that of a Rotary Steering Head and requires both seals and seal surfaces to be frequently serviced.

Has the tool been tested and proven by scientific method?

Yes, operators and independent research institutes have conducted several controlled comparisons.

Can I get extra value from the AST by attending training classes?

The most important factor for value is the planning. Tomax has experienced personnel that will support this process.

In the field, the key is to stick to the plan and have access to the User Manual and the anti-stall.com site for instructional videos and documentation.