Summary
This study shows how a subsea pipeline leak can be modeled in a mechanistic
way. Special attention is paid to the implication of fixed-pressure boundary
conditions at both upstream and downstream locations, which is relevant to the
way the majority of field operations are actually performed. The use of
fixed-pressure boundary conditions leaves the change in inlet total flow rate
(Δqt in) and the change in outlet total flow rate
(Δqt out) as two possible leak-detection indicators that can
be monitored on a real-time basis. The two-phase flow of gas and oil mixtures
in subsea pipelines is analyzed by using Beggs and Brill's correlations. The
effect of different parameters on the mechanistic leak-detection modeling is
also investigated, accounting for gas compressibility, backpressure of the
system, pressure drop across the system, and gas/oil fraction at the leak.
Also presented in this study is a new method to predict the change in inlet
or outlet total flow rates (Δqt in or Δqt
out) in a form of contours with dimensionless leak opening size (dleak/D) and dimensionless leak position (xleak/L) in x and y axes. This new style of reporting
leak-detection indicators is believed to provide a convenient means to improve
data interpretation in actual field practice and laboratory tests.
Introduction
A safe and proper operation of complex deepwater infrastructure has been a
serious issue for the oil industry (Teal 2003; Tubb 2006; Carlsen and Mjaaland
2006; French et al. 2006). Even small oil-spill incidents may create a
considerable technical and financial adversity, facing a delay in oil and gas
production, a loss of hydrocarbon products, an additional expense for repair
and remediation, and a negative impact on the operating company’s reputation
(Don 2004). In fact, pipeline leaks have already become a frequent problem. It
is not only the environment, but also companies, including producers and
transporters, that suffer from leak accidents. Leaks from subsea pipelines are
just as common as those from inland pipelines. Earlier experience shows that
small leaks leading to spills of 1–5 gallons per hour is very difficult to
detect by using existing leak-detection methods, compared to leaks with
relatively large opening sizes (Mastandrea et al. 1990; Cranswick 2001; Teal
2003).
Pipeline operators use various types of detection approaches including both
hardware- and software-based methods (Jolly et al. 1992; Barlas 2001; Theakston
and Larnaes 2002; Bloom 2004; Liu et al. 2005). Scott and Barrufet (2003)
provide a thorough summary about recent developments in this area. Acoustics,
fiber optics, ultrasonics, and cable sensors are examples of hardware-based
methods, while mass balance, transient modeling, and pressure analysis are
examples of software-based methods. A software-based leak-detection method
identifies a pipeline leak which occasionally causes several immediate
detectable effects in terms of fluctuations in the monitoring pressures and/or
flow rates (Mastandrea et al. 1990; Bonn 1998). Earlier studies usually focused
on the drastic change in flow conditions resulting from the blowdown and
rupture of subsea vessels or pipelines (Norris and Puls 1993; Norris and
Hissong 1994). It is clear that such a relatively large leak can predict the
failure of multiphase flowlines reasonably well.
Looking into the impact of relatively small-size leaks in subsea pipelines,
Dinis et al. (1999) developed the concept of leak-detection modeling for a
single-phase incompressible flow. They claimed that a leak which is larger in
its size and located further downstream can be more easily detected by
comparing their modeling results with laboratory flow experiments in
9,460-ft-long horizontal flow loops. Their analysis was based on the flow rate
measured only at the outlet, ignoring the pressure and flow rate at the inlet.
The same concept was tested with a single-phase compressible gas line (Scott
and Yi 1998) by comparing the responses before and after the leak. Scott et al.
(1999) and Smith and Griffin (2001) further extended these leak-detection
procedures by utilizing an empirical two-phase flow-friction factor.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
1 February 2008
- Meeting paper published:
5 May 2008
- Revised manuscript received:
21 July 2008
- Manuscript approved:
26 July 2008
- Version of record:
15 December 2008
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