Summary
A primary consideration with coiled tubing (CT) is that it is consumed by
fatigue loading during routine operations. Also, rugged oilfield conditions
routinely lead to corrosion and other mechanical surface damage. Since fatigue
is a surface phenomenon, the presence of a surface imperfection has a
significant influence on fatigue-damage mechanisms. This paper describes the
study of magnetic-flux-leakage (MFL) inspection signals caused by surface
defects in the form of milled circumferential grooves in steel CT. The focus of
the investigation is to identify and estimate the size of surface defects on
the basis of characteristic MFL signal features. It is demonstrated that this
effort is greatly enhanced by finite-element analysis (FEA). The ultimate
objective is to extract surface-flaw dimensions accurately from conventional
MFL signals. These dimensions are used in computer CT life-prediction
models.
An axisymmetric FEA model is developed and used to calculate leakage flux
density solutions for milled circular and rectangular shaped grooves in
1.75-in.0outside0diameter (OD), 0.156-in.-wall-thickness (WT), 90-ksi CT
samples. FEA results are compared to axial and radial MFL signals measured with
an experimental inspection unit. Favorable agreement is observed between
experimental and FEA data. Furthermore, signal features are correlated with the
known slot geometries to identify basic geometry-recognition patterns for
different circumferential grooves. Signal features reveal qualitative and
quantitative trends relative to surface-flaw dimensional characteristics.
The need persists to make the operator’s string-management decision-making
process more reliable and automatic with respect to determining fatigue life
expectancy. The obstacle here is that because of the inherent inaccuracies in
commonly used MFL inspection techniques, reliable real-time flaw-evaluation and
characterization capability is limited.
Introduction
End users of CT are keenly aware--and much has been published concerning
this matter--of the fact that bending fatigue is one of the primary threats to
in-service CT integrity and has tended to impede the progress of CT usage.
Fatigue problems are further intensified by rugged oilfield conditions that
routinely cause corrosion and other surface damage such as scratches, nicks,
gouges, dents, and impressions. Since fatigue is a surface phenomenon, the
presence of a surface imperfection can accelerate fatigue-damage mechanisms and
reduce the useful life of CT significantly.
Current CT integrity-assurance and string-management needs are being driven
by usage demands energized in part by reliable analytical fatigue
life-prediction models (Tipton et al. 2002), the more severe service conditions
accompanying the progression into deeper and higher-pressure wells (McCoy et
al. 2002; Stanley 2005), and the strong outlook for existing and future
conventional well applications for which CT intervention can provide unrivalled
solution (Adam 2003). Consequently, the number of CT strings in service and
user inventories is growing rapidly. The implication is that the major need of
end users is increased confidence and reliability of CT, whether new or
used.
The natural responses to the emergence of the CT industry have been the
development of technologies to identify problematic conditions and to make the
operator’s integrity-assessment process and string-management decisions more
efficient and automatic. The detection and characterization of metal loss
represents an essential part of a CT integrity-monitoring system (Stanley
1996). Inspection techniques have been adapted from the pipeline and
oilfield-tubulars sectors, where MFL has become the most popular means of
detecting flaws in CT. MFL has been selected over ultrasound because it is a
technique that is not affected seriously by the state of the pipe surface.
However, string-management decisions based on the indications from MFL
inspection still require manual proving to characterize the defect that caused
the MFL response (Stanley 2005; Moran et al. 2002; Stanley 2004a; Stanley
2004b).
Sources such as Moran et al. (2002), Stanley (2005), and others (Stanley and
Varner 1998; Stanley 1996; Stanley 2004b; Rosen 1997; Rosen 1998; Stanley 2001)
document and describe the challenges facing the development of CT-inspection
technology. To address these challenges, experimental MFL inspection of CT
specimens with flaws of known geometries permits systematic examination and
mapping of the signals to determine how much geometric information can be
obtained from magnetic nondestructive testing. At the University of Tulsa, this
process is performed by means of a simple and compact bench-top MFL inspection
unit with the benefit of repeatability.
This paper describes the measured MFL inspection signals obtained from the
bench-top inspection unit for circumferential flaws of known geometry.
Furthermore, a 2D axisymmetric FEA model is described that is used to
conveniently simulate this technique and reproduce the axial and radial MFL
signals. The FEA predictions are compared to the experimental data.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
14 February 2006
- Meeting paper published:
4 April 2006
- Manuscript approved:
24 August 2008
- Published online:
16 March 2009
- Version of record:
1 March 2009
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