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MATERIALS SELECTION & DESIGN Analysis on Causes of a Well Casing Coupling Crack FIGURE 4 FIGURE 5
Corrosion morphology on the threads near crack origination. Corrosion pits at fracture origination.
Residual Stress Measurement The measurement locations were 90 degrees apart along the circumference, at I LQ[\IVKM WN
UU NZWU \PM ÅMTL MVL WN
the No. 1 coupling. The same measure- ments were taken 38 mm from the mill MVL IVL \PM ÅMTL MVL NWZ \PM 6W KW]- pling and 93 mm from the mill end.
Result of Residual Stress Measurement
The hoop residual stresses were plus
IVL UQV][ J]\ VW\ PQOP I\ \PM ÅMTL MVL of the connection not made up with a pin WN 6W
58I IVL \PM I^MZIOM _I[
438 MPa at the mill end of the connection UILM ]X _Q\P \PM XQV WN 6W
Material Tests Chemical Analysis
The chemical components were in
IKKWZLIVKM _Q\P \PM [XMKQÅKI\QWV WN \PM mill according to the test result.
60 MATERIALS PERFORMANCE April 2012
Mechanical Property Test
58I IVL \PM
LQZMK\QWV IVL 2 QV \PM \ZIV[^MZ[IT LQZMK\QWV
The corrosion pits near the crack were \W UU LMMX IVL \W
mm in diameter. The corrosion pits away NZWU \PM KZIKS _MZM
\W UU LMMX IVL \W UU QV LQIUM\MZ
Micro-Fractography There were corrosion pits on the frac-
ture (Figure 5). Micro-fractography indi- cated that the fracture was brittle. Energy chart analysis showed that S was 0.03 to _\
IVL +T _I[ \W _\
Test Result Analysis Rupture tends to take place easily for
V150 grade casing during use and trans- port because the technology for this OZILM KI[QVO Q[ VW\ aM\ ^MZa UI\]ZM
NZWU )81 ;8-+ +< QV fracture problems.
! L]M \W
Occurrence Sequence of Crack and Corrosion Pits There were corrosion pits on the frac-
ture and threads near the crack and away NZWU \PM KZIKS
nation and the end of the coupling was ! UU 2 LQUMV[QWV I\ \PM UQTT MVL WN \PM connection could not be measured for No. 1, but the engaged position (L) of pin and box at the mill end of the connection
NACE International, Vol. 51, No. 4