Theoretical research and experimental tests for a new version of tubing and casing thread connection.

Grigoras, Stefan ; Hadar, Anton ; Marin, Cornel 等

1. INTRODUCTION

The extractive industry uses many types of joints for tubing and casing. The integral joint is obtained of the pipes bodies.

Figure 1 presents the aspect of an integral two steps cylindrical thread joint ([D.sub.OP]--the outer pipe diameter; [D.sub.IP]--the inner pipe diameter; CS--central shoulder; OS--outer seal; IS inner seal; plastic gasket). The thread is placed on two cylindrical steps. The maximum outer diameter [D.sub.OBOX]--and the minimum diameter [D.sub.IPIN] are limited by the bore and the drill diameter (the well technical conditions).

[FIGURE 1 OMITTED]

2. THEORETICAL APPROACH

The joint functioning is assured by the thread, by the sealing zones, by the minimum area of the cross section, the wall thickness, by the bending and combined resistance of the critical area and by the thread resistance and screwing capacity. The main functional parameter of any joint is the make-up torque. The cylindrical thread is placed on two steps. The two steps have pitch continuity, for manufacturing process assurance. The thread assures the strength end the montage. The metal/metal contacts represent the sealing zones, presented by figure 2. In the inner contact (fig.2a), the theoretical spherical end of the pin (radius R) is replaced by two tapered zones, A[C.sub.1] and [C.sub.1]B. The stiffness of the metal/metal linear contact, of the end of the pin, of the gasket and the elastic deformations of the contact, of the gasket and of the pin generates the axial load [F.sub.aI]. This load must be produced by the thread. The outer seal (fig.2b) is a metal/metal linear contact too. The stiffness of the contact, of the end of the box and of the pin is the main parameter of the axial load of this zone, [F.sub.aO]. This load is produced by the screwing. The maximum value of [F.sub.aO] is limited by the box resistance and the minimum value is shown by the sealing functioning.

[FIGURE 2 OMITTED]

The contact between the pin and the box, on the central shoulder (figure 3) is the contact which limits the value of the makeup torque. The contact pressure brings about to the axial load [F.sub.aCS] determination, by limiting the pressure values to the material yield limit. The make-up torque is the functional parameter which assures the load on the contact.

The friction forces in all the described contacts, like the friction in the thread, produce the friction global torque, which is the make-up torque:

T = [T.sub.I] + [T.sub.O] + [T.sub.TH] + [T.sub.CS] (1)

where: TI, TO, TTH and TCS are the friction torques in the inner (index I), outer (index O) contacts; in the thread (index TH) and on the contact shoulder, respectively. Another parameter of the joint functioning is his efficiency, defined by the ratio between the critical area of the joint, AcrJ (for traction) and the area of pipe section, AP, (American Petroleum Institute, 1993):

[epsilon] = [A.sub.crJ]/[A.sub.p] x 100 [%] (2)

efficiency-defined by (2)--is around 60 ... 70%, for this joint type. Other loads on the joint are the outer pressure, the inner pressure, the traction (column weight), the bending of the column and the thermal lengthen of the column (compression). The thread loads are the nonlinear distribution of traction load along the joint, the wear produced by the screwing and the bending and compression effects. An element of the wall of pipe or joint is represented in figure 4.

The components of equivalent stress in the wall are:

--the normal axial stress: [[sigma].sub.ax] = f([[sigma].sub.t], [[sigma].sub.b]);

--the normal radial stress: [[sigma].sub.r] = f([p.sub.i], [p.sub.o], [p.sub.c], [p.sub.s]);

--the normal tangential stress: [[sigma].sub.t] = f([p.sub.i], [p.sub.o], [p.sub.c], [p.sub.s]),

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

where: [[sigma].sub.t] is the traction stress, [sigma].sub.b]--the bending stress, [p.sub.i]--the inner pressure, [p.sub.o]--the outer pressure, [p.sub.c]--the equivalent compression pressure produced by the thread and [p.sub.s]--the pressure produced by the screwing, (Gafitanu et al., 1996). The equivalent stress, calculated using these components limits the load of the pipe and joint (Gafitanu et al., 2001, Stirbu, 2002).

3. EXPERIMENTAL TESTS

Experimental tests have been developed on different lots of joint, for many sizes and for different materials too. The makeup torque was calculated by our theoretical method (American Petroleum Institute, 1993). The aim was to determine the resistance of the joint and the sealing efficiency evaluation. We have developed tests under inner pure pressure. For the initial tests, water and oil were the active fluids. The pressure can be considered as the normal radial stresses in the pipe and joint wall. They stop at the yield limit of the pipes steels (maximum values were 1000 bar). Figure 5 presents the aspect of a joint, destroyed under inner pressure. The sealing functioning was perfect, if the tests keep the values of the makeup torque and of the manufacturing precision. The combined tests represent the experimental evaluation of the functional and strength parameters of the joints, simultaneously for minimum two loads: the inner pressure and the traction force. Table 1 presents the results of the tests, for 3 specimens of the joint. The sealing efficiency is remarkable (Ianus et al., 2007).

We have created the condition for the experimental tests developed under inner pressure, traction forces and bending of the column, taken over by the joints. The testing fluid was the liquid nitrogen. The angle of the column bending was maximum 100/20m column length, (American Petroleum Institute, 1993). In this case, the axial stresses in the joint wall increase with 120 %, and the metal/metal seals lessen the normal loads on the contact surfaces.

The minimum value of the make up torque assures the sealing efficiency, in perfect conditions and the resistance is perfect, too. Figure 5 shows a test under bending, inner pressure and traction on a column obtained of three joint pipes. The maximum inner pressure was 40 MPa.

[FIGURE 6 OMITTED]

4. RESULTS

Figure 6 presents the theoretical axial load repartition on the thread pitches, along a joint between two pipes. The 1 curve represents the load produced by screwing and the curve number 2 is the repartition of the column weight. The superposition of the two component leads to a uniform load distribution. The figure number 7 presents the evolution of make-up torque values for any used steel, according to (American Petroleum Institute, 1993). The experimental tests show that the joint assures the sealing and resistance parameters for well column in the oil or gas extractive industry.

[FIGURE 7 OMITTED]

5. CONCLUSIONS

The presented joint is an original construction, because of its thread, sealing zones and manufacturing process. The dimension of the joint are placed in the same domain like Hydril, Extrem Line, Vam and other companies. Joint performances are greater comparing with all the analyzed joints. The experimental tests on pipes and joint confirm the choice of a type of joint (the technical solution) and the original algorithm of joints design. The tests developed under pure inner pressure show the maximum efficiency of meta/metal seals, the role of anticorrosive barrier of the gasket and the resistance of the pipe and joint. The combined tests (inner pressure and traction forces) determine the diminishing of contact stresses in the metal/metal seals. If the central shoulder is rationally placed and all the length dimensions depend on its position, the optimum functioning of the joint is assured. The manufacturing accuracy has a very important role in the joint evolution. The main advantage of the new joint is the diminishing of thread stresses and the time of the screwing (reduced with 50%).

6. REFERENCES

American Petroleum Institute (1993), Bulletin on Formulas and Calculations for Casing, TUBING, Drill Pipe, and Line Pipe Properties, ISO 10400/1993.

Gafitanu, M.D, Grigoras, St. & Stirbu, C. (1996), Original Joint for Casing and Tubing. Theoretical Analysis, Proceedings of the 26--The Israel Conference on Mechanical Engineering, Haifa.

Gafitanu, M.D., Grigoras, S. & Stirbu C. (2001), Functional Aspects of a Thread Joint for Tubing and Casing, Second World Trybology Congress, Vienna.

Ianus, G., Grigoras, St., Hanganu, C.L. & Borzan, M. (2007), The Effects of the Damaged Structure of Grease's Soaps on Ball Bearings Vibrations, Mat. Plast., 44 (1), p. 47-50.

Stirbu, Cr. (2002), Tubing and Casing Joints. Resistance and Functional Aspects, Meridian Engineering, no.2, Edit. TEHNICA INFO, Chisinau, Moldova.

Tab. 1. Tests parameter for combined load.

 Inner Traction
 pressure force
Specimen [MPa] [N] Sealing efficiency

1 70...71 8 x [10.sup.5] Losses=0 /3.5 hours
2 71...73 8.5 x [10.sup.5] Losses=0 /72 hours
3 * 69 7.8 x [10.sup.5] Losses=0 /one hour
 * --test on joints without gasket.