Casing Thread Connections
Oilfield tubular may be equipped with plain ends (no threads), have API-specified threaded casing thread connections, or proprietary (non-API) threaded connections.
API Casing Connections
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Oilfield casing conforming to API standards may be obtained with plain ends, but ends
are usually threaded and furnished with couplings such as:
- short thread and coupling (STC)
- long thread and coupling (LTC)
- buttress thread and coupling (BTC)
- Extreme-Line thread (X-line) for casing * with 8 round threads per inch (8 RD)
With the exception of Extreme-Line, male (or pin) threads are machined on plain-end pipe and later made up with a coupling.
A reduced OD (special clearance) coupling is offered on some sizes and weights to allow for additional clearance between pipe and hole. While providing this additional clearance, special clearance couplings often reduce the rating of the connection, usually in tension or internal yield and test pressure.
API Short/Long Thread and Coupling
The API Short Thread and Coupling (STC) and API Long Thread and Coupling (LTC) are used in pipe sizes of 4-1/2″, 7” and 9-5/8”. Figure 4 shows the LTC design.
The STC design is the same except that the coupling and the threaded pins are shorter. This design is externally threaded on both ends of a non-upset pipe. The single lengths are joined with an internally threaded coupling.
The thread profile has rounded threads and roots with a 60° angle between the thread flanks as shown in the figure. The thread density is 8 threads per inch (8 RD) on a 0.0625 inch per inch taper. When the coupling is made up, small voids exist at the roots of each thread.
Thread compound must be used to fill these voids in order to obtain a seal. LTC is not made in casing sizes larger than 13-3/8” because of the possibility of joint pull-out that can occur with the heavier weight casing.
API Buttress Thread and Coupling
This is used in conductor casing and surface casing applications, also where a higher joint strength is required. Figure 5 shows the BTC design. This design is externally threaded on both ends of a non upset pipe (as in the STC and LTC). The single lengths are joined with an internally threaded coupling.
The thread profile has flat crests and roots parallel to the taper cone. The thread density is 5 threads per inch on a 0.0625 inch per inch taper for sizes 13-3/8″ and smaller, and 0.0833 inch per inch taper for sizes 16″ and larger.
The BTC thread has higher joint and bending strengths compared to LTC (or STC). As a result, this thread is used often in deeper wells where higher hook loads are experienced.
Thread compound must also be used in order to obtain a seal with BTC. BTC is also run in horizontal wells where doglegs can cause high bending loads on the larger size casings. But BTC leak resistance is lower that that of LTC and STC.
Proprietary / Premium Casing Connections
Proprietary connections are available which offer premium features not available on API connections.
Among the special features for proprietary connections are:
- clearance OD of coupling for slimhole completions
- metal-to-metal seals for improved high pressure seal integrity
- high bending strength for deviated holes
- multiple shoulders for high torque strength
- a streamlined connection OD for easy running in multiple completions.
- recess-free bores through the connection ID for improved flow characteristics
- higher tensile strength for deep holes
- an integral connection to reduce the number of potential leak paths
- resilient seal rings for continuous corrosion protection
- high compressive strength for compressive loading situations
VAM Connection – No More Exist
This connection has a metal to metal seal for superior leak resistance. An internally threaded coupling with internal shoulders provide positive make up torque and a non-turbulent bore (see Figure 6). Also, due to its superior joint and bending strength, it is used as the completion casing liner (One of Types Of Casing) for the horizontal wells.
It is stocked in 7″, 9-5/8″ and 13 3/8″ sizes. This connection is noteworthy for its gas leak tightness, low hoop stress, high joint strength (equivalent to API buttress thread), high collapse strength and easy stabbing design. Its two step pin nose which incorporate a primary and reserve torque shoulder and metal to metal seal make it a good candidate for the deep, high temperature, high pressure Gas service.
Vetco LS, RL-4S, and Dril-quip S60 Connections for Large Casing Sizes
The Vetco LS connection is a high-strength integral design that accommodates high internal operating pressures, bending moments, and tensile loads. The pin/box mating shoulder has a 30-degree taper (see Figure 8). This results in the open end of the box being captured by the tapered shoulder of the pin and prevents the box from ballooning at the pin/box interface during periods of high internal pressure and large bending moments.
The Vetco RL-4S connection features dual stabbing guides and a high stab angle for easy stabbing. Self locking, four start thread forms allow fast quarter-turn makeup.
The Drill-quip S-60 connection features easy stabbing, no cross-threading, fast makeup, low torque and high-pressure sealing. Both of these connections save rig time and are commonly used in 24” and larger casing sizes.