Figure 1. The lower part (1) is made up in the direction of the bit. The unit will only be able to transfer torsion equal to the spring force before the main units makes up through the helical thread. (Images courtesy of Tomax AS)
The AST concept is based on controlling drilling torque by regulating the weight-on-bit down in the hole to assist the driller attempting to do the same from surface. Placed anywhere in the bottomhole assembly (BHA), the AST function will prevent stall-outs and several other unwanted effects.

From the outside the AST works as follows:
• When the bit speed drops and reactive torque builds fast, indicating the bit is about to stall, the AST tool will contract.
• The fast contraction will off-load the bit and allow it to regain its original speed.
• With the bit back at speed the unit will gradually release the accumulated torsion.
The process can be repeated an unlimited number of times.

Figure 1 describes the internal AST process. An excessive torsion force with magnitude to overcome the compressed spring (3) will make the lower telescopic unit (1) with external helical spline (2) rotate and enter into the upper telescopic section. As a result, the unit becomes shorter in overall length and the push on the drillbit is released. When the bit is back at speed and the accumulation of force in the spring (3) will be released. In the opposite end of the helical threaded interface converting excessive torsion in to a linear force, the AST has an effective shock absorber designed to eliminate the risk of the BHA entering into axial occillations.

Preventing damage
Through the function described above, the AST will reduce destructive forces in three directions: axial, lateral and torsion.

Axial shocks and vibrations typically occur from weight transfer problems associated with deviated wells and friction. The downhole components are often mechanically resistant to this kind of loading, but axial forces can place excess weight on the cutters and initiate motor-stall problems. The AST works directly to counteract axial overload by allowing for 0.15 m (1¼2 in.) of compression.

Lateral shocks and vibrations are very destructive to downhole electronics and difficult to absorb once they occur. A good strategy is to inspect the bit and BHA coming out of hole accurately and of course keep track of the vibration status by measurement while drilling where available. The task of keeping lateral vibrations under control is becoming increasingly difficult with polycrystalline diamond compact-type bits in alternating formations because of the fixed cutters’ tendency to enter into bi-centric motions. A dominant cause of such destabilization of the bit is the stick-slip phenomena. Stick-slip is a common phrase to describe repeating accumulation and discharge of torsion. The torsion builds up in the long tubular string because the face of the bit stalls in the cutting process. The torsion builds until the bit breaks free and the torsion is released from the tubing in an uncontrollable manner.

The most severe destructive forces are often lateral vibrations being secondary effects from stall and stick-slip. The AST works actively to limit such effects where torsion peaks indicating the start of a stall or stick scenario will compress the AST spring and cause an instant contraction. This contraction will off-load and free the bit before forces reach a level where damages can be made. The AST will also control the forces on the bit during the release of the string absorbed force.

It is essential that the AST is designed to react to forces at a level where they present a risk. In cases where the forces do not exceed such levels, the AST tool will not provide any motion or other action. A solid end stop allows for direct transfer of pull force thru the tool to allow for fishing and full force jarring.

Figure 2. Note: The stall frequency was not accurately recorded. The recording is based on a subjective count of stalls per hour.
Potential for speed
The advantage of avoiding damage and wear is obvious, but not so obvious is the great drilling speed achievable when the motor can be loaded towards its maximum without the risk of stalling. A good case study was produced in a recent drilling job where the deviation combined with a light 31¼2-in. BHA and significant rig heave made it very challenging to advance the bit and keep it drilling. Frequent stall-outs were encountered (Figure 2). For other reasons than the drilling itself, the well was cemented back and the same section had to be drilled again. The operator called for AST technology to be applied for the second pass. In hole, the AST delivered an immediate reduction in motor stalls and later almost totally eliminated them. This opened for loading the 3.5-in. motor by another 550 psi, with a boost in drilling speed.

When comparing the results over the same depth interval, penetration was more than doubled with the AST tool and the stall-out problem solved. The reason for the improvement in penetration was obvious. By loading the motor up to a differential pressure of 800 psi, torque doubled while motor rpm dropped only by 10%. Horsepower output was consequently up in the range of 40 hp, more than double the initial 20 hp from 220 psi without the AST.

Field record and applications
AST tools have to date done more than 50 jobs worldwide, mostly in coiled tubing (CT) applications. The technology has proven valuable for plug and scale drilling operations but also in drilling of new formation. The tool is easy to use and requires no particular consideration at surface other than good drilling practices.

An overall observation from the operations is the potential of the AST technology to help overcome the lack of rotary capacity in CT drilling while at the same time maintaining the advantages of superior well entry efficiency. The Anti Stall Technology is proven to have the same good effect on metal mills as on drill bits and the technology might in places move the window for CT to wells previously requiring a rotary capacity. In pursuit of such an achievement all AST operations have been closely monitored and the tool optimized for the best possible performance. A key factor in the process has been tool stroke, where it has been found that a long stroke and wide dynamic range is important to make the tool work efficiently without having to guess the downhole conditions every time the tool is going in to a new well.

The AST technology and the AST tool have over the last 2 years shown convincing results in both in controlled tests and a growing number of CT operations. This new addition to drilling technology is currently being used for close to all relevant operations involving a downhole motor on the Norwegian Continental Shelf. Together with a number of other improvements the tool has led to an increase in value to CT drilling and milling with a consequent rise in activity in the same area.