Бурение грунтовых зондов, установка энергетических колодцев

In general, casing couplings are specified by the types of threads cut on the pipe ends and coupling. The principal design features of threads are: form, taper, height, lead and pitch diameter (Fig. 1.7).

Form: Design of thread form is the most obvious way to improve the load bearing capacity of a casing connection. The two most common thread

d2 = di + toper

(b)

di d2

Fig. 1.7 : Basic elements of a thread. The thread taper is the change in diameter per unit distance moved along the thread axis. Thus, the change in diameter.

^2 — <A, per unit distance moved along the thread axis, is equal to the taper per

unit on diameter. Refer to Figs. 9 and 10 for further clarification.

forms are: squared and V-shape. The API uses round and buttress threads which are special forms of squared and V-shape threads.

Taper: Taper is defined as the change in diameter of a thread expressed in inches per foot of thread length. A steep taper with a short connection

provides for rapid makeup. The steeper the taper, however, the more likely

it is to have a jumpout failure, and the shorter the thread length, the more likely it is to experience thread shear failure.

Height: Thread height is defined as the distance between the crest and the root of a thread measured normal to the axis of the thread. As the thread height of a particular thread shape increases, the likelihood of jumpout failure decreases; however, the critical material thickness under the last engaged thread decreases.

Lead: Lead is defined as the distance from one point on the thread to the corresponding point on the adjacent thread and is measured parallel to the thread axis.

Pitch Diameter: Pitch diameter is defined as the diameter of an imaginary cone that bisects each thread midwav between its crest and root.

Threaded casing connections are often rated according to their joint efficiency and sealing characteristics. Joint efficiency is defined as the tensile strength of the joint divided by the tensile strength of the pipe. Generally, failure of the joint is recognized as jumpout, fracture, or thread shear.

Jumpout: In a jumpout. the pin and box separate with little or no damage to the thread element. In a compression failure, the pin progresses further into the box.

Fracture: Fracturing occurs when the pin threaded section separates from the pipe body or there is an axial splitting of the coupling. Generally this occurs at the last engaged thread.

Thread Shear: Thread shear refers to the stripping off of threads from the pin and/or box.

Generally speaking, shear failure of most threads under axial load does not occur. In most cases, failure of V-shape threads is caused by jumpout or occasionally, by fracture of the pipe in the last engaged threads. Square threads provide a high strength connection and failure is usually caused by fracture in the pipe near the last engaged thread. Many proprietary connections use a modified buttress thread and some use a negative flank angle to increase the joint strength.

In addition to its function of supporting tension and other loads, a joint must also prevent the leakage of the fluids or gases which the pipe must contain or exclude. Consequently, the interface pressure between the mating threads in a joint must be sufficiently large to obtain proper mating and sealing. This is accomplished by thread interference, metal to metal seal, resilient ring or combination seals.

Thread Interference: Sealing between the threads is achieved by having the thread members tapered and applying a makeup torque sufficient to wedge the pin and box together and cause interference between the thread ele­ments. Gaps between the roots and crests and between the flanks of the mating surfaces, which are required to allow for machining tolerance, are plugged by a thread compound. The reliability of these joints is. therefore, related to the makeup torque and the gravity of the thread compound. Ex­cessive makeup or insufficient makeup can both be harmful to the sealing properties of joints. The need for excessive makeup torque to generate high pressure often causes yielding of the joint.

Metal-to-Metal Seal: There are two types of metal-to-metal seal: radial and shoulder. Radial is usually used as the primary seal and the shoulder as the backup seal. A radial seal generally occurs between flanks and between the crests and roots as a result of: pressure’ due to thread interference created by
makeup torque, pressure due to the radial component of the stress created by internal pressure and pressure due to the torque created by the negative flank angle (Fig. 1.8). Shoulder sealing occurs as a result of pressure from thread interference, which is directly related to the torque imparted during the joint makeup. Low makeup torque may provide insufficient bearing pressure, whereas high makeup torque can plastically deform the sealing surface (Fig. 1.8(c)).

Resilient Rings: Resilient rings are used to provide additional means of plug­ging the gaps between the roots and crests. Use of these rings can upgrade the standard connections by providing sealing above the safe rating that could be applied to connections without the rings. Their use, however, reduces the strength of the joint and increases the hoop (circumferential) stress.

Combination Seal: A combination of two or more techniques can be used to increase the sealing reliability. The interdependence of these seals, however, can result in a less effective overall seal. For example, the high thread interference needed to seal high pressure will decrease the bearing pressure of the metal-to-metal seal. Similarly, the galling effect resulting from the use of a resilient ring may make the metal-to-metal seal ineffective (Fig. 1.9).

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