Summary
The petroleum industry has endeavored to meet the high demand for crude-oil
products with exploration and production into ever deeper ocean waters. Under
these subsea environments, pipeline insulation is essential to prevent pipeline
blockage that results from the solidification of paraffin waxes that are
present in the crude oil. To maintain proper crude-oil temperatures above the
paraffin-solidification point, new insulation techniques with enhanced
thermal-insulation performances are essential in minimizing pipeline heat loss.
A new insulation concept that involves an interstitial wire-screen mesh has
been developed and with its thermal performance has been investigated, which
was initially quantified with test coupons under simulated environmental
conditions. These results correlated very well with comparable values from a
test of a subsequent prototype-pipe section. Furthermore, an analytical model
has been developed that compared very favorably with the experimental results
for the coupon-test runs. Ultimately, this study has confirmed the feasibility
and performance characteristics of the insulation concept, and it has also
demonstrated the thermal competitiveness of the interstitially insulated
coaxial technology.
Introduction
To meet the high demand for oil, industries have commenced the exploration
and production of offshore sites, whose locations are in ever deeper ocean
waters. Pipe insulation is mandatory in preventing blockage within the pipe
caused by paraffin and hydrate buildup that can occur because the water
temperature at seabed could be as low as 0 to 2°C (32 to 35°F). Crude oil often
contains a type of wax that begins to form solid-paraffin deposits on the inner
surface of the pipe when the oil temperature reaches the paraffin cloud point
(68°C, or 155°F); therefore, blockage can and does occur. When paraffin waxes
block the inside of the pipe, an additional process is needed to remove it,
which then leads to reduced production efficiency. To maintain the inner-wall
temperature above the paraffin-formation point, the heat loss from the pipe
wall must be minimized. Several insulation techniques have been developed to
overcome this thermal issue by the addition of low-conductivity materials and
coatings on the external pipe surface. These materials could obtain thermal
conductivity at a range of 0.08 to 0.15 W/(m·K), as shown in Table 1. However,
these techniques have often had severe limitations, such as damage caused by
large hydrostatic-pressure differentials and installation concerns (Watkins and
Hershey 2004; Choqueuse et al. 2002; Hallot et al. 2002). With the insertion of
one or more layers of a wire screen, as an interstitial material, within the
annulus of a coaxial pipe, a reduction in the heat-transfer rate, and thus
retardation in paraffin buildup, can be achieved without the limitations just
stated. Moreover, the manufacturer and installation process for subsea piping
will be greatly simplified (Marotta and Fletcher 2005).
Within the interstitially insulated coaxial pipe (IICP), heat loss is
reduced significantly through an enhanced thermal resistance that exists
between the two pipe walls. The annular gaps are of such length that even if an
interstitial fluid, such as air or argon, is present, the effects caused by
natural convection are negligible. Therefore, the dominant heat-transfer
mechanism for this system is conduction through nonconforming contacts between
the wall and wire screen and within the annular gaps. In addition, conforming
microcontacts within the wire screen and pipe wall itself provide an additional
heat-flow-resistant path between contacting interfaces (Kim et al. 2007). This
concept, where a wire screen acts as the interstitial insulation, dramatically
increased the thermal resistance when compared with a bare pipe. As a result,
the rate of heat loss from the inner hot wall to the outer cold wall decreased
by more than two orders of magnitude. The effective thermal conductivity ranged
from 0.018 to 0.080 W/(m·K) for this configuration.
In this present investigation, as an intermediate stage toward the
fabrication and testing of a full-scale pipe, an experimental investigation
with a prototype pipe-insulation system, which contains just two layers of
wire-screen mesh and liner placed between them, was conducted for the
measurement of the effective thermal conductivity and thermal diffusivity.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
29 May 2007
- Revised manuscript received:
15 November 2007
- Manuscript approved:
28 January 2008
- Version of record:
15 September 2008
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