Contents of Materials Performance - APR 2012

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

Page 62 of 76

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 the pin of No. 1. The maximum hoop [\ZM[[ _I[
58I IVL \PM I^MZIOM _I[
438 MPa at the mill end of the connection UILM ]X _Q\P \PM XQV WN 6W axial residual stresses were all plus, and \PM I^MZIOM _I[ 58I I\ \PM KZIKS WZQOQVI\QWV XWQV\ 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 °+ _I[ 58I \PM
58I IVL \PM 2 QV \PM TWVOQ\]LQVIT
LQZMK\QWV IVL 2 QV \PM \ZIV[^MZ[IT LQZMK\QWV 2 QV \PM \ZIV[^MZ[IT LQZMK\QWV Metallography
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 KI[QVO JZMIS[ QV +PQVI I[ _MTT I[ QV W\PMZ KW]V- tries. V150 grade casing was listed in before ! > OZILM KI[QVO _I[ ZMUW^ML
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 ous difference in corrosion pit dimensions between those at the crack position and threads away from the crack, indicating there was no big difference in the corro- sion medium and time between them. There was no corrosion on the pin and \PM JW` I\ \PM ÅMTL MVL WN \PM KWVVMK\QWV because the threads were protected com- pletely by thread compound. Data indi- cate that the crack occurred before cor- ZW[QWV XQ\[ _MZM XZWL]KML IVL \PM [M^MZM corrosion on the fracture and threads at the mill end of the connection took place after the crack occurred. The distance between the crack origi-
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