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Publication Date: 2015-Aug-04
Document File: 9 page(s) / 1M

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The Prior Art Database


The tool electronics of a directional drilling tool needs a geostationary reference in order to make accurate measurements downhole and hence it resides inside the stationary housing of the directional drilling tool. In order to keep this portion of the drilling tool stationary, an anti-rotation device is mounted on the exterior of the stationary housing. This anti-rotation device has a springing mechanism through which it grabs onto the formation and prevents the tool from rotation. The anti-rotation device comprises of parts that are press-fitted onto each other. Since it is mounted on the exterior of the drilling tool, it is subjected to the harsh drilling environment such as drilling mud with cuttings, high temperatures, unintended external forces etc. Such a harsh environment causes the traditional press fitted part to wear and tear rendering the tool to be ineffective due to loss of interference between the mating parts. This paper describes a method of preparing the press fitted joint using shape memory composite to overcome the disadvantages of using traditional press fitted joints in such downhole harsh environments.

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Press fit and shrink fit are forms of interference fit widely used for holding parts together. Press fitting is similar to shrink fitting except that parts are not heated and higher pressures are necessary to assemble. Press fitting requires less energy to assemble but the bond is weaker compared to a shrink fit.

Press fit is one of the widely used techniques to assemble parts in drilling tools. Often parts in drilling tools either require press fitted joints for torque transmission or for holding them together in the presence of external loads. It is preferred to press fit over shrink fits since these tools often have to be dismantled during repair and maintenance. Decoupling a shrink fitted joint reliably is not feasible due to variation in coefficient of thermal expansion of the material.

In downhole drilling tools, press fitted parts are often assembled using a hammer or mallet. Since the parts associated are machined to close geometric dimensioning and tolerancing, they have to be hammered carefully to its mating part. Hammering is usually subjected to an HSE risk of hand/finger injury. When such weakly bonded press fitted joints are exposed to harsh environment during drilling, the joint is worn out resulting into rattling/breaking of parts and tool failure thereof.

This paper describes a simple, reliable, effective, and HSE friendly method to shrink fit parts by using shape memory composite materials thereby reducing a lot of labor time and effort in making and breaking these tools.

Shape memory alloys, their use in couplings and fittings.

Shape Memory Alloys hereon termed as SMAs can absorb and dissipate mechanical energy wherein a shape change is induced by a change in temperature. With a change in temperature, there is a shift in atomic shear lattice structure resulting in the associated shape change commonly termed as phase transition.

SMAs exist in two phases: high temperature austenite and low temperature martensite comprising of two variants twinned and detwinned form, each with their unique material properties. Shape memory alloys could be one way or two way depending on the number of shape memory they exhibit and the memory training it undergoes. Figure 1. below shows a schematic of the shape memory effect microscopically.

Figure 1: Microscopic Schematic of Shape Memory Effect [Ref 1]

The most widely used shape memory alloy is “Nitinol” named after (Nickel-Titanium Naval Ordnance Laboratory). Traditional Nitinol comes with following mechanical properties.

Nitinol Mechanical Properties

Young's Modulus


approx. 83 GPa (12E6 psi)


approx. 28 to 41 GPa (4E6 to 6E6 psi)

Yield Strength


195 to 690 MPa (28 to 100 ksi)


70 to 140 MPa (10 to 20 ksi)

Ultimate Tensile Strength

fully annealed

895 MPa (130 ksi)

work hardened

1900 MPa (275 ksi)

Poisson's Ratio


Elongation at Failure

fully annealed

25 to 50%

work hardened

5 to 10%

Hot Workability


quite good