Threaded casing connections are oft eii rat ed according to their joint efficiency and sealing characteristics. .Joint efficiency is defined as t h e tensile 5treiigtIi of t h e joint divided by the tensile strength of the pipe. Generally, failure of the j o i n t is recognized as jumpout. fracture. or thread shear.
Jumpout: I n a juiiipout. the pin arid hox separate with little o r 110 daiiiage to the thread eleiiieiit. Iri a conil~ressioiifailure. t lie pin progresses furt I i c
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into the ))ox..
Fracture: Fracturing occurs wlien tlie pin t Iireaded sect ioii separates from the pipe body or there is an axial splitting of the, coupling. C;enerally this occurs at t h e last engaged thread.
Thread Shear: Thread shear refers to tlie stripping off o f threads froin t l i v pin and/or box.
C;enerally speaking. shear failure of most threads under axial load does not occur. In most cases. failure of V-shape threads is caused by juiiipout or occasionally. hy fracture of the pipe in the last engaged threads. Square threads provide a liigli strength connection and failure is usually caused I)!. fracture in the pipe near the last engaged thread. Many proprietary connect ions iise a modified butt r w s thread and soiiie use a negative flank aiiglr to iiicrrlase tlie joint strrngtli. 111 addition
to its function of supporting trnsion and other loads. a joint iiiiist also prevent t h e leakage of the fluids or gases which the pip? iiiust contain or exclilde. Consequently, t h e interface pressure Iwtweeii tlir mat iiig threads i n a joint iiiust be sufficiently large to obtain proper mating and scaling. This is accornl)lislied by thread interference, metal to riietal seal. resilient ring or coiiihiiiat ion seals.
Thread Interference: Sealing I~etn.eeiit Iir threads is achieved h y Iiaviiig t l i r thread meinhers tapered and applyirig a iiiakeup torqiir suffic.ient to \vedgc, the pin and box together and cause interfrwwct, Ijetweeii t lie t Iirvail elements. Gaps between the roots and crests and I ~ t w e e nt h e , flanks of t l l c , mating surfaces. which are required t o allow for niacliining tolerance. arc’ plugged by a thread coinpound. The reliability of these joints is. therefore. related to the makeup torque and tlir gravity of t h e thread c o i i l p o ~ ~ i dEX. cessive makeup or insufficient rriakrup can hot 11 be har~iifiilt o the sraliiig properties of joints. The need for excessive makeup torque to generate liigli pressure ofteii causes yielding of the joint.
Metal-to-Metal Seal: There are two types of iiietal-to-nirtal seal: radial and shoulder. Radial is iisuall?. u s e d as tlie primary s ~ a and l the >boulder as tlic backup seal. .A radial seal gencrall!. occiirs I x t wreii flanks a n d lwtween t Ilr, crests and roots as a result of: 1)ressurc’ duv t o thread intrrfmwce created 1 ) ~