NC-26 Thread Root Rolling

Drill string manufacturers do not cold roll slimhole (below 4 in. OD) connectors, this is due to the dimensional restrictions of the smaller inner diameter [4.23, 4.24]. However, as seen in chapter 3, these connectors contain significant SCFs, which are an obvious crack initiation site under fatigue loading. Therefore, it was apparent that in order to perform a full series of fatigue tests that included an investigation into the effects of residual stress, it would be necessary to design and fabricate a compact cold rolling device.

4.7.3.1 Box Rolling System Design

The major factor in designing a “miniaturised” cold rolling device was that there was very little space in which to apply a very large force. The clearance diameter at the LET of the box is only 50 mm, and for effective thread rolling the applied forces must be large enough to produce plastic deformation of the material. For this reason, hydraulic power was selected, as it offers high power to weight and power to bulk ratios, along with efficient transfer of energy from one point to another.

The first design was very much a prototype, which incorporated a single acting push cylinder connected to a hydraulic hand pump. The device was designed for the tool post of a large centre lathe. A small cylinder was brazed into a supporting body, a car brake pipe was brazed to the rear of the cylinder, which acted as the transmission conduit for the hydraulic fluid. This was then coupled to a flexible hydraulic hose and connected to a hand pump. A small piston and roller was fabricated to locate into the cylinder and was sealed using a car brake cylinder seal.

This “Mark I” box rolling equipment. Fig. 4.29, produced some notable thread root deformation. However, repeated operation of the equipment exposed some weaknesses in this prototype, and a more substantial design was manufactured. The “Mark II” device was machined from a solid hexagonal bar, with the hydraulic cylinder fitted and brazed into it, along with the car brake pipe supply line. Fig. 4.30. However, the braze and the car brake pipe were unable to withstand repeated high pressurisation of the system, resulting in considerable leakage. Therefore, a “Mark III” design was manufactured which removed these fabrication weak spots. The cylinder and hydraulic supply line were machined out of the solid hexagonal bar. The pump supply hose was attached directly to the solid body, as seen in Fig 4.31.

4.7.3.2 Pin Rolling System Design

Without the internal dimensional constraints, the external thread rolling device was considerably easier to design. Having identified the weak spots of the hydraulic system from the box rolling device, the piston cylinder and supply line were machined

from solid bar. In order to maintain consistency and interchangeability between the female and male thread rolling equipment, the piston bore was machined to receive the piston from the box rolling device. Figure 4.32 presents the pin rolling equipment in operation.

4.7.3.3 The Roller

The two primary roller design criteria were hardness and size. The roller needed to be hard enough, to deform the parent material, without itself becoming damaged, and small enough, to fit into the tight geometry of the box connection. However, the roller can also be used to modify the thread root geometry, as with the B.S.R.A. investigation [4.20], in which the edge radius of the roller was larger than the root radius of the thread form. The current oilfield practise is to cold roll with a roller of the same edge radius and thread root radius [4.23, 4.24]. A larger root radius has the benefit of a reduced SCF, and so improved fatigue performance as shown in Figure 4.33 [4.25].

Modification of a standard thread root radius to improve fatigue strength has been reported by Yukushev [4.26], in which the root radius of an MIO was increased by 35%, as shown in Fig. 4.34. This resulted in a doubling of the fatigue strength over a standard metric thread form. However, it is stated that due to difficulties in production, such a thread form is not recommended for industrial use. Yet one drillstring manufacturer has successfully managed to modify an existing oilfield connection thread form and take it to full scale production. The Super Strength Thread (SST) thread form is a modified NC thread with an enlarged root radius. Fig. 4.35. The radius is machined into the thread root, enlarging the radius from the standard 0.038 in. (0.9652 mm) to 0.057 in. (1.4478 mm). The new root radius reportedly reduces the maximum SCF by 45% and greatly improves fatigue performance [4.27].

The benefit of an enlarged the root radius has been demonstrated. However, enlarging the root radius by plastic deformation, as in the B.S.R.A. investigation.

rather than by material removal, has the advantage of a lower SCF, combined with a layer of compressive stress in the thread root. Therefore, the NC-26 roller was manufactured with a 0.048 in. (1.219 mm) edge radius, 0.010 in. (0.254 mm) larger than the root radius of the NC thread form. The dimensions, mechanical properties and heat treatment details of the roller are listed in Table 4.6. The complete piston and roller assembly can be seen in Figure 4.36.

4.7.3.4 Rolling Forces & Thread Deformation

In order to determine a suitable force for rolling some preliminary tests were performed on small sections of the thread. Visual evidence of thread root deformation was found using a hydraulic pressure of between 2000 and 4000 psi (135 and 275 bar). From basic hydraulic theory, the force a hydraulic cylinder can generate is equal to the hydraulic pressure multiplied by the cross-sectional area of the cylinder. However, this does not account for any losses in the system. It was therefore necessary to calibrate the forces of the system empirically. The piston force and pump pressures were calibrated with the use of a digital pressure transducer and a 50kN load cell, as shown in Figure 4.37.

The rotary shouldered connection is tapered from between 2 and 3 inches per foot. If the thread is cold rolled (from the LET to the free end) with equipment that runs parallel to the thread form, the system would encounter a pressure drop as the roller moved down the taper. It was found that rolling the length of the NC-26 connector produced a 35% drop in roller force, due to moving down the taper. To overcome this pressure / force loss, a hydro-pneumatic accumulator was fitted to the system. This energy storage device was charged to the operating pressure of the system, thus enabling the working pressure to be maintained along the length of the thread. Figure 4.38 illustrates the significance of working with an accumulator fitted to the system.

Effective cold rolling requires permanent thread root deformation. The amount of deformation was measured using a deep throat micrometer fitted with anvil tips, which allowed access to the thread root. The measurement of thread root deformation

is the only practical non-destructive method of assessing the effectiveness or quality of rolling. Current industry targets for thread root deformation range from 0.001 to 0.010 in. (0.0254 to 0.254 mm) [4.29, 4.30]. This range may suggest a lack of consistency in the rolling procedure within the industry.

Due to demands on time, oilfield thread manufacturers and service yards only roll over the threaded section once [4.23, 4.24]. Yet, from the earliest reports of thread root rolling from the Wohler Institute [4.19], multi-pass rolling has been mentioned. It is reported that thread roots were rolled a number of times, with one investigator going so far as to roll the thread root ten times. However, the reported results fail to mention the optimum number of passes required for maximum fatigue performance. The effect of multi-pass rolling was mentioned by the B.S.R.A. investigators [4.20, 4.21], who double pass root rolled, on the basis that multi-passing increased deformation and improved surface finish. Indeed, the Engineering Science and Data Unit (ESDU) investigation into the fatigue strength of large diameter bolts [4.28] recommends a minimum of 4 roller passes, and states that additional passes produce insignificant benefit to the fatigue strength of the component. Figure 4,39 shows the level of deformation produced in an NC-26 box thread for increasing hydraulic pressure and by repeated roller passes. The level of thread root deformation was confirmed by sectioning the connection, and measuring the thread form on an optical projector. Figure 4.40 shows the level of thread root deformation for a box thread rolled 3 times at 4000 psi.