Performance Characteristics for an Interstitially Insulated Coaxial Pipe Using Wire-Screen Mesh for Deepwater Applications
- Egidio Marotta (Texas A&M University) | Dong Keun Kim (Texas A&M University) | Leroy S. Fletcher (Texas A&M University)
- Document ID
- Society of Petroleum Engineers
- SPE Projects, Facilities & Construction
- Publication Date
- September 2008
- Document Type
- Journal Paper
- 1 - 7
- 2008. Society of Petroleum Engineers
- 4.3.1 Hydrates, 4.3.4 Scale, 4.2 Pipelines, Flowlines and Risers, 4.3 Flow Assurance
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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.
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.
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