The correlation between tribological and functional aspects on casing and tubing joints.

Grigoras, Stefan ; Stirbu, Cristel ; Hanganu, Lucian Constantin 等

1. INTRODUCTION

The paper presents different constructive aspects of the joints for tubing and casing (fig. 1). The solution with the largest and modern application is the b) variant.

Any of the mentioned constructive versions presents the cylindrical thread (two steps with different diameters) which aim is to make equal the distribution of the thrust loads along the joint. This thread does not the seal. This is the reason for which, special sealing zones are provided, inside, in the middle or even outside, the joint. These sealing need considerable thrust loads, which cause elastic deformations. The outer and the inner diameter of the joint are limited by the well and by the drift, respectively, for any pipe diameter. The most of the components of the joint are loaded by: the column length; the make up torque; the inner and the outer pressure; the bending of the column. The joints of that type are realized under an original patent (Gafitanu et al., 1994). The joints are used in the extractive oil and gas industry.

[FIGURE 1 OMITTED]

2. THEORETICAL APPROACH

The theoretical analysis of an integral joint for tubing and casing raises some few problems (Gafitanu et al., 1996). The inner seal (IS in fig. 1)--is a Hertzian contact; its aspect is presented by figure 2. The two straight lines materialize the sphere of the pin ([R.sub.H] radius).

The normal load [F.sub.H] (keeping the contact stresses under a limit imposed by the material) produces an axial load [F.sub.aH]:

[F.sub.aH] = 5.72 x [pi] x [[sigma].sup.2.sub.Hp] x [D.sub.c] x [R.sub.H]/E x sin [beta] (1)

where, [[sigma].sub.Hp] is the stress limit; [D.sub.c] is the contact diameter.

This force generates a component of the make up torque (the friction torque in the contact point H, [M.sub.a1]). The contact pressure limits the minimum value of the contact stresses. The seal with the plastic material gasket G is described in figure 3.

The axial load [F.sub.aG] is introduced by the make up torque. This load is placed between the limits imposed by the sealing functioning and the gasket resistance. The friction moment between the end of pin and the gasket is noted with [M.sub. a2].

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

The outer seal (OS in fig. 1) is of metal/metal type; its aspect, before deformation of the components is presented by figure 4. Thrust load [F.sub.aO] (between the limits imposed by the box resistance and the sealing) generates a friction moment [M.sub.a3] as a component of the make up torque. The central shoulder (CS in figure 1) is the surface of the make up torque limitation. The contact surface and the pressure on this surface are two very important parameters, because of the sizes and the clearances between the pin and the box. The aspect of the central shoulder contact is presented by figure 5, where [F.sub.aCS] is normal load on contact:

[F.sub.aCS] = [sigma].sub.aC] x [pi]/4 ([D.sup.2.sub.oP] - [D.sup.2.sub.iB] (2)

with: [[sigma].sub.aC]--the limit contact stress. The friction moment is [M.sub.a4]. The limits are the minimum pressure on the shoulder and (as maximum value) about 50% of yield material limit.

The thread, defined on two middle diameters, [D.sub.m1,2], for each step, is loaded, in the moment of mounting by the axial force:

[F.sub.aT] = [F.sub.aH] + [F.sub.aG] + [F.sub.aO] + [F.sub.aCS] (3)

which produce the moment:

[M.sub.T] = [F.sub.aT][D.sub.mT]/2 x tg ([alpha] + [rho]) (4)

where, [D.sub.mT], [alpha] and [rho] are specific for the selected thread.

The make up torque, imposed by the designer of well is:

T = [SIGMA] [M.sub.ai] + [M.sub.T] (5)

and its determination finishes the joint calculus.

3. RESULTS

The efficiency of the joint is determined like the ratio between the critical cross section of the joint and of the pipe. The wall thickness of the pipe is the final factor of the efficiency. For the variant a) of fig. 1, the efficiency is very low (about 40%) and for the variant c)--the same figure- the efficiency is the greatest (120%) (Grigoras & Stirbu, 2002).

The evolution of the make up torque according to the pipe material resistance is presented in (Grigoras et al., 2008).

The outer pressure produces only the loading of the pipe body. The joint has a superior resistance, because the greatest wall thickness. The figure 6 presents the evolution of the pipe resistance (for collapse pressure), according to its material. The sealing functioning and the joint resistance were tested with inner pressure applied by liquid nitrogen. The pressure brought about traction stresses in the minimum cross section of the joint as the yield limit of pipe material (Grigoras & Stirbu, 2001).

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

The maximum thrust load (maximum parting load) supported by joint (at breaking limit), according to pressures and weight of pipes, for the proposed materials are presented in fig. 7, for the same pipe dimensions. The length of the extractive column, for each material has the distribution in fig. 8, for 5.5" pipe.

[FIGURE 8 OMITTED]

4. CONCLUSIONS

The new original joint was realized under a personal patent for a large dimensional scale of pipes. The patent has as object the b) variant of figure 1. The collapse and inner pressure represent critical parameters only for pipe bodies (the joint is more resistant). The length of the column and the maximum thrust load are taken over by the joint. Its efficiency is the same important factor. The make up torque evolution, according to the material, is the ascendant, for quality increasing of steels. The difference between the maximum and minimum value is larger for high resistant materials. The accuracy of mounting is necessary to increased for H40.... C90 materials. The selection of a material on all its parameters by designer must be that which is able to preserve the functioning requirements.

5. REFERENCES

Gafitanu, M., Grigoras, St. & Stirbu, C. (1994), Tubing and Casing Joint, Patent no. 106902 C1, OSIM, Bucharest

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

Grigoras, St. & [section]tirbu C. (2002), Theoretical Aspects on "Petrotub" Threaded Joints Functioning, BALKANTRIB'2002, Kayseri

Grigoras, St., Hadar, A., Marin, C., Stirbu, C. & Stoica, G. (2008), Theoretical Researches and Experimental Tests for a New Version of Tubing and Casing Thread Connection, DAAAM International Wienna, ISSN 1726-9679, ISBN 978-3-901509-68-1

Grigoras, St. & [section]tirbu C. (2001), Functional aspect of a threaded joint for tubing and casing, First National Conference on Recent Advances in Mechanical Engineering, Patras