Through the Looking Glass

SwRI-developed device helps engineers get a closer look at hydrates

By André Barajas

André Barajas was a senior research engineer in the Mechanical and Materials Engineering Division. A mechanical engineer with professional interests in the areas of heat transfer, fluid dynamics and multiphase flow, Barajas recently led a team that designed and built a novel high-pressure natural gas multiphase flow facility in which he has conducted research on the formation and agglomeration of gas hydrates.

As the oil and gas industry gathers hydrocarbons from offshore reservoirs located beneath progressively deeper waters, it is increasing its use of multiphase flow lines to lower costs by reducing the number of flow lines required. In this low-temperature, high-pressure flow line environment, hydrates - slushy solids composed of water and a gas (methane or carbon dioxide, for example) - can form, agglomerate and block the flow. Because of this, the industry has identified hydrate remediation and flow assurance as top priorities in developing technology for production of deepwater leases.

The industry is also striving to achieve higher "field realism" in research and development. This is especially important in the areas of multiphase flow and natural gas, where similar methods cannot be used to scale laboratory data directly to field conditions because of the changing ratio of the gas density to the liquid density in the field. Laboratory experiments that realistically simulate field conditions decrease the risk involved in applying laboratory data to full-scale systems. Although industry wants laboratories to use more rigorous, field-like test conditions, such as higher flow rates and elevated pressures, there are limited techniques for understanding fluid flow phenomena under these conditions.

Eyeing the problem

Perhaps the most effective technique for understanding fluid flow behavior in multiphase flow applications is visual inspection. A visual account of flow phenomena for both laboratory and field studies can reveal information that would be difficult, if not impossible, to obtain using only instrumentation.

Visualization and optical access methods have been used extensively for research in applications where the test conditions were less challenging. These methods include sight glasses, transparent piping and camera probes. However, work remains to advance these methods to operate reliably in more challenging, elevated-pressure environments.

Optical access device

Using internal research funding, Southwest Research Institute (SwRI) engineers developed a probe and methodology to observe activity inside high-pressure vessels. This optical access probe comprises off-the-shelf components and attaches to a valve on a pipe that has at least a five-eighths-inch opening for pipeline operating conditions that include pressures up to 3,600 psig and temperatures from 15 degrees to 150 degrees Fahrenheit. The probe consists of a pressure-containing housing with a sapphire window into which a borescope can be inserted. Because it was designed for use in multiphase flow lines, it includes a window-cleaning mechanism. A manual crank allows the operator to lower the optical probe assembly (which includes a light source and a lens) to the desired position in the pipe. Still photography and video both can be obtained using the probe.

SwRI researchers are using this device on several projects. For one study, engineers replicated conditions that would cause hydrates to form and photographed the results, which showed how hydrates tend to form in long flow lines. The hydrates were deposited along the upper two-thirds of the pipe (approximately between the 8 o'clock and 4 o'clock positions), while gas continued to flow through the center of the pipe and a hydrate slurry continued to flow along the bottom one-third of the pipe. This experiment confirmed how hydrates adhere to pipe walls under certain conditions.

Hydrate formation within a high-pressure flow line can be directly observed and photographed using an optical access probe attached to a pipe valve. Hydrates frequently occur in offshore multiphase flow lines located in deep, cold water and can cause problems for the free flow of natural gas.

For another project, researchers used the probe to diagnose operation problems in a cyclone separator, which is used to remove excess liquid from a gas stream as it exits a high-pressure separator. Because liquid was being measured and injected into the gas flow farther downstream, removing liquid from the gas was critical to the success of the experiment. For this reason, the probe was located downstream of the cyclone separator to detect the presence of liquids. Whenever a liquid was detected, the separator level was modified to operate without liquid carry-over and to collect the required data. Video data were provided to the client to prove that carry-over was not an issue during the testing.

In both cases, deciphering the flow conditions inside the pipe would have been very difficult had not the optical probe allowed the flow regimes to be effectively determined.

Future applications

The SwRI optical access device is portable and easily can be installed on
any pipeline. Future laboratory and field troubleshooting applications include
the following:

• separator technology development and testing
• flow assurance research, including hydrate and paraffin deposition monitoring
• modeling, including computational fluid dynamics codes, transient pipeline simulators and prediction verification for flow regimes under various pipeline conditions
• wet-gas and multiphase flow meter calibration
• wet-gas sampling
• field applications.

Flow visualization by optical access is valuable in understanding fluid flow phenomena within enclosed, pressurized systems. Visualization, along with numerical, analytical and experimental methods, leads to greater understanding of fluid flow phenomena. As technology advances, the need to understand complex fluid flow phenomena at elevated pressures has increased. In particular, the energy industry has become increasingly interested in understanding and solving complex flow problems at elevated pressures.

Acknowledgment
The author wishes to thank current and former SwRI employees Robert Beeson, Russell Burkey, Mark Jones, Chris Kuhl, and Steve Petullo for their efforts in making this a successful project.

Published in the Spring 2003 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.

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