METHOD DETERMINES LEAK RESISTANCE OF 8-ROUND THREAD

Nandakumar K. Mallenhalli
XL Systems
Beaumont, Tex.

Determining the internal pressure that a tubular joint can resist without leakage may reveal the reason why some API 8-round threads leak.

The calculation presented here puts the concept in the right perspective and will aid an engineer in making a decision that can help avoid costly leakage in tubular strings. The method is based on recent technical papers published by ASME.

The main concept taken from the ASME papers1-3 (specifically, References 1 and 2) is that no leakage will occur if the interface bearing pressure is equal to or greater than the pressure differential at all times. Also these pressures should be within the elastic range.

What this means is that:

The internal pressure that the joint is subjected to should not exceed the yield.
The interface bearing pressure that is created when the joints are being made up should not exceed the yield at any time during makeup.
The internal pressure should be less than or equal to the interface bearing pressure at all times.

API has recognized these ASME papers and has incorporated changes in Section 3.2 of Bulletin 5C3, July 1989 (Bulletin on Formulas and Calculations for Casing, Tubing, Drill Pipe, and Line Pipe Properties).

In Specification 5CT (Specification for Casing and Tubing), December 1990, Section 4, API recommends hydrostatic test pressures based on the lowest of the following:

A percentage of yield stress, Equation 8.2.1.1, Bulletin 5C3.
Internal leak resistance pressure based on interface bearing pressure between pipe and coupling resulting from makeup and internal pressure, Equation 8.2.2.1, Bulletin 5C3.

These specifications lead to superfluous data on the pressures capable of being held by a joint. Therefore to obtain a usable pressure the following four steps are recommended:

Calculate minimum yield of the coupling based on Lame's or Barlow's formula at both hand-tight and power-tight plane (Fig. 1).
Calculate internal pressure leak resistance (interface bearing pressure) as per Section 3.2.1 of API Bulletin 5C3, July 1989, both at hand-tight and power-tight plane.
Tabulate the calculated values in a table for different numbers of turns. For selecting the number of turns, go from less than hand-tight turns to hand-tight turns to slightly above hand-tight turns, staying in the vicinity of hand-tight plane. Similarly do this for the power-tight plane.
Check to see if the interface bearing pressure tabulated for different number of turns from Step 2 exceeds the yield pressure calculated in Step 1 before the hand-tight plane or before the power-tight plane.
If it exceeds, then makeup the joint with less turns. Select this as the interface bearing pressure.
Check API specifications for the hydrostatic test pressure. This information can be referenced from either Specification 5CT or Bulletin 5C2 (Performance Properties of Casing, Tubing, and Drill Pipe).

If the value obtained from this step for internal pressure exceeds the value of either Step 1 or the selected value in Step 3, we know that the joint will leak.

EXAMPLE

The following case is for 7-in., 38 lb/ft, Grade P-110, LTC, 8-round thread.

STEP 1

The minimum yield pressure of the coupling from Barlow's formula at the hand-tight plane is:

Pyield = (1.75 x Y x t)/D

where:

Y = 110, 000 psi

D = Nominal OD of coupling=7.656 in. (from Bulletin 5CT or 5C2)

t = Wall thickness at weakest section = (D - E1)/2 + 0.3763

E1 = Pitch diameter at hand-tight plane = 6.90337 in. (from Standard 5B)

Pyield = 9,462 psi at hand-tight plane.

From Lame's formula:

Pyield =

(D2 - [D - 2(0.875 x t)]2

(D2 + [D - 2(0.875 x t)]2

The values of D, t, and Y are the same as shown previously and Pyield = 9,867 psi at hand-tight plane. Assume that the yield of the coupling takes place at 9,462 psi.

STEP 2

The maximum interface bearing pressure, P, due to makeup and internal pressure, at the weakest section (hand-tight plane) in the coupling is:4

ETNp(W2-E12

P =

2 E1W2

where:

E = Modulus of elasticity = 30 X 106 psi

T = Thread taper, in./in. = 0.0625

E1 = Pitch diameter at hand-tight plane = 6.90337 in.

W = Coupling OD = 7.656 in.

N = Number of thread turns makeup

p = 0.125 in. (pitch of 8-round thread)

Using the above formula, Table 1 is generated.

STEP 3

From Table 1, we can conclude that when the makeup involves three turns (nominal makeup or hand-tight-plane makeup), the coupling may yield because the interface bearing pressure (9,520 psi) exceeds the yield pressure (9,462 psi) for the coupling. Once the coupling yields during makeup, there is no way of knowing the internal pressure that the coupling can resist.

STEP 4

The API specification for 7-in., 38 lb/ft casing, Grade P-110 is that internal pressure resistance equals 9,520 psi.5

From Table 1, it can be concluded that the interface bearing pressure in the joint having exceeded the yield pressure of the coupling at hand-tight makeup will not be able to withhold this internal test pressure.

On repeated makeup, the coupling might develop a much lesser interface bearing pressure due to yield, and consequently withhold much less internal pressure.

Also if we have less than hand-tight makeup, say two turns, then the interface bearing pressure developed is 6,347 psi. If this is subjected to an internal test pressure of 9,520 psi as per API requirement, then the joint will certainly leak.

This analysis should also be carried out at the power-tight plane.

The engineer can be certain to have no leak if the interface bearing pressure remains higher than or equal to the internal pressure at all times, and if he or she contains the interface bearing pressure within the elastic limit during all makeups and checks to see if API test pressure would satisfy the above.

It should also be remembered that the above analysis holds good for quenched and tempered material only. If it is not, then the elastic limit is different in both the longitudinal and hoop direction. This will have to be taken into consideration for calculating the interface bearing pressure.

REFERENCES

Blose, T.L., and Vingoe, R.L., New Developments in High Pressure Oil Well Tubular Products, ASME, 1972.
Blose, T.L., Leak Resistance Limit-Tubular Product ASME, 1970.
Eichberger, L.C., and Miller, T.V., Internal Pressure Design of Casing and Tubing Strings, ASME, 1972.
API Bulletin 5C3, Section 3.2, 1989.
API Bulletin 5C2, Table 1, March 1982, pp. 8-9.