Use of Failure Analysis Techniques to Improve Reliability by Material Selection in SAGD Applications
Schlumberger Artificial Lift has developed a new electrical submersible
pump (ESP) system for operation at 250ºC [482ºF]. In late 2009, the
system has gone through a benchmark test jointly launched by Schlumberger and
ConocoPhillips, successfully validating the temperature limit. During the
post-test evaluation, axial cracks were found in ceramic sleeves in the pump.
Although cracks did not translate into system failure, it was noted as a cause
of concern as it may affect the long-term system reliability.
In this work, defect investigation and failure analysis were performed,
combined with Finite Element Analysis (FEA) simulation and experimental
verification to reproduce and predict the failure mode. A numerical model was
developed on the basis of Theoretical Elasticity and showed that the
aforementioned failure mode is the result of axial thermal stress caused by
difference in the coefficient of thermal expansion (CTE) of components in the
shaft string coupled with the pump construction.
Experiments were conducted on a pump with the same construction type and
the failure mode was successfully replicated. By fitting the experimental
crack-temperature to the numerical simulation, the model is able to predict the
failure mode in ceramic sleeves. The validated numerical model was used to
screen suitable alloys for key components in the shaft string to reduce the
thermal stress. A special alloy was identified and experimentally verified to
increase the crack-temperature up to 280ºC [536ºF].