For casing pipe, most popular threading type is LTC (Long thread connector), STC (Round threading connector) and BTC (Buttress threading).Threads are used as mechanical means to hold the neighboring joints together during axial tension or compression. For all casing sizes, the threads are not intended to be leak resistant when made up.
The design of those threading types you can find them in API 5CT or API 5B.
Connection strength and tightness are two main coupling technical indicators,both of those two type of threading are not suitable for:• Heavy oil thermal recovery• Ultra-deep well• Heavy corrosion environment
STC coupling can withstand strength of the tubular 60~80% of the pipe tensile, BTC coupling can holdmore tensile but it’s sealing performance is poor under circumstance like high internal pressure.
Both two type couplings in generally use lead, zinc, copper, graphite or silicone oil composition to improve seals, but those seals only work under temperature of 60~95 Celsius. Also for STC coupling, if under huge external pressure and axial tensile stress, when heavy impact or bent happens, the threading will have more chance to slippage.
API standard threading coupling primarily seal the pipe by thread grease, metal plating and thread engagement of interface. STC thread root to tip clearance is 0.152mm, BTC threading gap is the whole height of tooth. Bottom to tip is 0.178 for pipe under 193.7mm, and 0.229mm for pipe above 219.1mm.
The internal yield pressure is the pressure that initiates yield at the root of the coupling thread.
PCIY = coupling internal yield pressure, psi,
Yc = minimum yield strength of coupling, psi,
W = nominal outside diameter of coupling, in.,
d1 = diameter at the root of the coupling thread in the power tight position, in.
This dimension is based on data given in API Spec. 5B, Threading, Gauging, and Thread Inspection of Casing, Tubing, and Line Pipe Threads, and other thread geometry data. The coupling internal yield pressure is typically greater than the pipe body internal yield pressure. The internal pressure leak resistance is based on the interface pressure between the pipe and coupling threads because of makeup.
PILR = coupling internal pressure leak resistance, psi,
E = modulus of elasticity, (3.0 × 107 psi for steel)
T = thread taper, in.,
N = a function of the number of thread turns from hand-tight to power-tight position, as given in API Spec. 5B, Threading, Gauging, and Thread Inspection of Casing, Tubing, and Line Pipe Threads,
pt = thread pitch, in.,
Es = pitch diameter at plane of seal, in., as given in API Spec. 5B, Threading, Gauging, and Thread Inspection of Casing, Tubing, and Line Pipe Threads.
This equation accounts only for the contact pressure on the thread flanks as a sealing mechanism and ignores the long helical leak paths filled with thread compound that exist in all API connections.
In round threads, two small leak paths exist at the crest and root of each thread. Buttress threads have a much larger leak path along the stabbing flank and at the root of the coupling thread. API connections rely on thread compound to fill these gaps and provide leak resistance. The leak resistance provided by the thread compound is typically less than the API internal leak resistance value, particularly for buttress connections. The leak resistance can be improved by using API connections with smaller thread tolerances (and, hence, smaller gaps), but it typically will not exceed 5,000 psi with any long-term reliability. Applying tin or zinc plating to the coupling also results in smaller gaps and improves leak resistance.
The round-thread casing-joint strength is given as the lesser of the fracture strength of the pin and the jump-out strength. The fracture strength is given by
The jump-out strength is given by
Fj = minimum joint strength, lbf,
Ajp = cross-sectional area of the pipe wall under the last perfect thread, in.2,
= π/4[(D – 0.1425)2 – d2],
D = nominal outside diameter of pipe, in.,
d = nominal inside diameter of pipe, in.,
L = engaged thread length, in., as given in API Spec. 5B, Threading, Gauging, and Thread Inspection of Casing, Tubing, and Line Pipe Threads,2
Yp = minimum yield strength of pipe, psi,
Up = minimum ultimate tensile strength of pipe, psi.
These equations are based on tension tests to failure on 162 round-thread test specimens. Both are theoretically derived and adjusted using statistical methods to match the test data. For standard coupling dimensions, round threads are pin weak (i.e., the coupling is noncritical in determining joint strength).
The buttress thread casing joint strength is given as the lesser of the fracture strength of the pipe body (the pin) and the coupling (the box). Pipe thread strength is given by
Coupling thread strength is given by
Uc = minimum ultimate tensile strength of coupling, psi,
Ap = cross-sectional area of plain-end pipe, in.2 ,
Ac = cross-sectional area of coupling, in.,
= π/4(W 2 – d12).
These equations are based on tension tests to failure on 151 buttress-thread test specimens. They are theoretically derived and adjusted using statistical methods to match test data.
Extreme-line casing-joint strength is calculated as
Acr = critical section area of box, pin, or pipe, whichever is least, in.2.
When performing casing design, it is very important to note that the API joint-strength values are a function of the ultimate tensile strength. This is a different criterion from that used to define the axial strength of the pipe body, which is based on the yield strength. If care is not taken, this approach can lead to a design that inherently does not have the same level of safety for the connections as for the pipe body. This is not good practice, particularly in light of the fact that most casing failures occur at connections. This discrepancy can be countered by using a higher design factor when performing connection axial design with API connections.
The joint-strength equations for tubing given in API Bull. 5C3, Formulas and Calculations for Casing, Tubing, Drillpipe, and Line Pipe Properties, are very similar to those given for round-thread casing except they are based on yield strength. Hence, the ultimate tensile strength/yield strength (UTS/YS) discrepancy does not exist in tubing design.
If API casing connection joint strengths calculated with the previous formulae are the basis of a design, the designer should use higher axial design factors for the connection analysis. The logical basis for a higher axial design factor (DF) is to multiply the pipe body axial design factor by the ratio of the minimum ultimate tensile strength, Up, to the minimum yield strength, Yp.
Tensile property requirements for standard grades are given in API Spec. 5C2, Performance Properties of Casing, Tubing, and Drillpipe, and are shown in Table 3 for reference along with their ratio.
Table 3-Tensile Property Requirements
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