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MATERIALS SELECTION & DESIGN FIGURE 6
could be deduced per J dimension (7.49 mm) and pin length L4
(101.20 mm) of
the No. 2 connection, and the maximum axial distance (6.0 mm) of the 65-degree JM^MT I\ \PM ÅMTL MVL WN 6W
.QO]ZM If the distance between the pin end
and coupling end at the mill end was L, then Equation (1) applies:
L = 230.57/2 – 7.49 = 107.80 mm (1) The calculation showed that the crack
position was located at the thread engage- ment of the pin and box where the thread KWUXW]VL _I[ ÅTTML IVL KW]TL VW\ JM corroded before cracking. Thus, the crack took place before corrosion occurred.
Deducing the Time of Crack Penetration and Mechanics Analysis
A change in the internal pressure of
\]JQVO IVL KI[QVO ZMÆMK\ \PM [MIT NMI\]ZM of the tubular string. The internal pres- sure of the tubing and casing rose nor- mally from May 29 to June 10, and then decreased abruptly, indicating that the casing string was leaking. The gas leaked through the annulus to the wellhead, showing that the leakage was very severe, and the crack had already occurred.
Mechanics Analysis The forces acting on the coupling were
internal pressure, circumference stress formed by make up, and tension during running.4
Mechanics are as follows.
Internal Pressure Change of Casing and Tubing This well testing action increased the
internal pressure of the casing. Internal pressure and external pressure of the cas- ing string are the same at the same depth, and internal pressure and external pres- sure counteract each other, so the internal pressure of the casing is zero. Internal pressure became higher than external pressure after well testing.
NACE International, Vol. 51, No. 4
Resistance to Crack of Coupling Material
The investigation indicated that the
minimum toughness requirement in transverseness was 103 J for V150 grade (X = 1,034 MPa).5 The toughness of the cracked coupling
at 20 ºC was 38.5 J, which is lower than the requirement above, so the brittle fracture of the coupling was all related to the material toughness.
Corrosion Medium Source There were severe corrosion pits on the fracture and on all the threads at the
Sketch of engagement of pin and box at the mill end of connection.
Circumference Stress Formed by Make Up If the coupling was acted upon only by
axial tension, then the crack should originate and extend from the stress con- centrating position. A large tensile stress was borne on the
coupling due to high make up torque. It was proven that torque was high by short J dimension and bigger residual stress at the mill end of the coupling.
Load Analysis on Casing String During Running The maximum weight of the casing
string is near the wellhead. The crack occurred at a depth of 836.75 m, so it was not related to the casing string weight. Overloading during running could be eliminated.
mill end of the coupling, so there must be a corrosive medium. If the corrosion medium was from the
water outside the casing annulus, then water could only access the inside of the casing if the external pressure exceeded internal pressure. During well testing, the internal pressure was higher than the external pressure before the coupling crack and at the moment the crack began, [W WQT IVL OI[ _W]TL ÆW_ W]\
but there was only slight
rust so the corrosion medium was not from the output liquid and gas inside the casing.
Conclusions and Suggestions 1) The coupling failure was brittle
fracture. The coupling crack was due to low material toughness and over torque of make up.
April 2012 MATERIALS PERFORMANCE 61