简介：管线所有的金属材料都是绝缘的，在海上恶劣的环境能防止流体流动发生问题。此技术采用的不是外部绝缘方法，比采用泡沫绝缘的方法更持久。

An all-metal insulated pipeline can guard against flow assurance problems in harsh subsea environments.

Cut-out of the IICP technology with only two layers (Marotta, E.E. and Fletcher, L.S., 2005).

Temperatures in deepsea environments can range from 32°F to 35°F (0°C to 2°C). As produced oil cools, paraffin wax, hydrates, and asphaltenes can be deposited on the interior of the pipe, which can build up and block the pipe. Current prevention measures include externally insulated pipe, pipe-in-pipe, electrical pipe heating, and chemical additives.

Liquefied natural gas (LNG) is primarily methane (CH4) that has been converted to liquid form for easier transportation and storage. Typically, LNG occupies only 1/600th of the volume of natural gas at stove burner-tip temperatures. The liquefaction process involves the removal of impurities such as dust, helium, water, and certain heavy hydrocarbons; then the gas is compressed. The natural gas is then condensed into a liquid at ~3.6 psi by cooling it to ~ -260° F (-163° C). Reduction in volume makes it easy to transport by pipelines or by cryogenic sea vessels.

Pipelines are convenient for transporting LNG but might not be considered flexible enough since there is a source-to-destination requirement. Moreover, once the pipeline diameter is chosen, the quantities of LNG that can be delivered are fixed by the pressures. LNG has been determined to be cheaper to transport compared to natural gas for offshore pipeline distances greater than 700 miles (1,126 km) or onshore pipeline distances greater than 2,200 miles (3,540 km).

The main challenge in transporting the LNG through pipes is to keep the natural gas in liquid state without significant boil-off and to prevent potentially destructive deposits (heavy hydrocarbons and hydrates) by effective insulation. The basic objective of interstitially insulated coaxial pipe insulation technology is to reduce the thermal conductance of the pipe. Therefore, pipe insulation is obligatory to minimize blockage in the pipe due to paraffin and hydrate build-up and to minimize boil off.

An all-metal approach

In view of these facts, the interstitially insulated coaxial pipe (IICP, patent pending) technology uses strategies that attempt to bring heat transfer rates below those of current technologies available to oil and natural gas.

The technology does not use external insulation, making it more durable than foam-insulated products. Additionally, the reduced footprint allows the pipe to be easily transported and installed without additional expenses at the installation site.

The two thermal properties of interest in designing insulation material are effective thermal conductivity and effective thermal diffusivity. It is desirable to have as low a value as possible for the effective thermal conductivity to minimize heat loss during steady-state operation. For LNG applications, it is the heat gain from the surrounding environment. Similarly, it is desirable to have as low an effective thermal diffusivity as possible so that heat transfer response to transients such as production start-up and shut-downs will be kept to a minimum. A two-layer representation of the IICP is shown in Figure 1.

The authors have measured the effective thermal conductivity of the IICP insulation technology (Marotta et al., 2008) in a laboratory-scale prototype. A comparison of the effective thermal conductivity for IICP insulation to air using a dimensionless thermal conductivity ratio, keff/kair, has been determined as a function of mean pipe wall temperature for several different volumetric flow rates. The effective thermal conductivity for the IICP had the highest values for volumetric flow rates equal to .1 and .15 gallons per minute (GPM, 275% and 150%, respectively). However, the effective thermal conductivity showed a large decrease as the mean pipe wall temperature was increased. The trend indicates that as the inlet flow rate was increased, the dimensionless thermal conductivity ratio decreased. As the inlet flow rates were decreased, there was a significant decrease in the rate of change of the dimensionless thermal conductivity ratio, while it remained nearly constant at the highest volumetric flow rate (3.5 to .40 GPM). The effective thermal conductivity of the IICP insulation technology (0.017 to 0.079 W/mk) compared to commercially available products.

Moreover, the authors have successfully measured the effective thermal diffusivity of the IICP insulation technology in the laboratory-scale test pipe. A dimensionless ratio of the thermal diffusivities, a/aair, for the IICP insulation technology with respect to air as a function of the mean pipe wall temperature was also used for comparisons.

These tests were conducted at different water inlet starting temperatures (t = 0) to determine the effect. At a low temperature of 35°F (2.5°C), the IICP insulation had a very low thermal diffusivity compared to air with a ratio equal to .002 (0.2% of thermal diffusivity of air) at the same temperature. As the temperature was increased, the ratio for the thermal diffusivities increased to .095, which represents 9.5% of the value for air. As the temperature was increased further, the differences among the four different inlet starting temperatures were observable but small enough to be considered within the uncertainty range. The thermal diffusivity for IICP insulation was compared with other conventional materials as shown in Table 2. The importance of the thermal diffusivity value occurs during production shut-down and start-up when the minimization of heat losses is of utmost importance for the prevention of heavy carbon buildup and hydrate formation. These cool-down or heat-up times provide important information to production crews for maintenance or repair work.

With these facts in mind, the development of an all-metal insulation that is interstitially insulated and can operate at extreme pressures and temperatures for LNG is of critical significance. These performance requirements can be met with a multilayer IICP insulation configuration with optimized parameters. The simplicity of the all-metal IICP insulation lends itself to ease of fabrication and increased durability and reliability, which go hand in hand with lower costs for installation and maintainance.

Results

The experimental results seem to indicate superior insulating characteristics for the IICP insulation technology when compared to current technologies (Marotta and Fletcher, 2006). Thus, the present technology shows promise for subsea oil and gas applications using an all-metal, wire-screen mesh as an insulating material.

However, to ensure the best performance in an actual pipe, optimization of the IICP insulation technology is required. Therefore, further experimental investigations may be needed to quantify the effects on thermal performance caused by pressure differences between the inner and outer walls and the effect of elevated temperatures that approach 930° to 1,111° F (500 to 600° C), and low pressures of 2-8 psi and temperatures of -239° to -293° F (-150° to -180° C). Due to the multi-layer all-metal construction, however, these extreme performance requirements can be met.