US Strategic Petroleum Reserve relies on magnetic flow meters for accurate brine disposal measurement

With a capacity of 727 million barrels, the US Strategic Petroleum Reserve (SPR) is the world’s largest stockpile of government-owned emergency crude oil. Established after the 1973-74 oil embargo, the SPR provides the USA with a powerful response option should a disruption in oil supplies threaten the US economy. It is also the critical component for the United States to meet its International Energy Agency obligation to maintain emergency oil stocks and it provides a national defence fuel reserve.

The oil is stored in man-made salt domes created by injecting fresh water into salt deposits, which dissolves the salt, leaving an open cavern for storage. To retrieve oil from the salt caverns they pump water in the bottom, making the oil rise to the surface so it can be transferred to interstate pipelines or loaded into ships. When oil is added into storage they pump the brine water out for disposal. DM Petroleum Operations Company, Inc manages and operates the SPR under contract to the US Department of Energy (DOE) and manages four storage sites: two in Louisiana and two in Texas.

The US Environmental Protection Agency (EPA) requires periodic integrity tests on each brine disposal line at the SPR’s site for leak detection. The flow rate is measured simultaneously at a point onshore and at a second point that is miles out into the Gulf of Mexico near the dispersion nozzles. The meters are transported by barge offshore, inserted for testing purposes, then removed from the line and stored until the next tests are conducted. The onshore and offshore flows measured by the flow meters are compared to determine if the pipeline has a leak. The criterion for a successful test is a difference of 4% or less between the two measurements.

Once each year, usually between May and June when the weather tends to be calm and before the hurricane season starts, the two lines in Texas are tested. The Big Hill Brine Line is a 22.5 km long, 48-inch diameter pipe and is one of the DOE’s toughest flow metering applications. The second site is a 24-inch pipeline located approximately 9.5 km offshore. In the past a pitot tube-type meter was used to profile the pipe and measure the flow at the two points. The valuable time required by divers to accomplish flow profiling on the gulf sea floor and less-than-satisfactory results led to a search for a better means of measurement. The FPI Mag flow meters replaced the problematic pitot tube meters on 29 June, 1996. The recorded flows from the FPI Mag flow meters matched to within 0.21% and proved them as an excellent choice for the demanding task.

A lift boat - essentially a barge with legs - is used to transport the meters, crew and divers out to the offshore site. Once at the site, the legs are lowered to lift the barge off the water, providing a stable working platform. Divers locate the opening in the pipe, which is at a depth of about 12 m for the Big Hill Brine Line, and insert the FPI meter sensor. Once inserted, brine is flowed through the pipe and the flow data is collected and recorded at one-minute intervals for a minimum two-hour test period.

The same type of test is conducted onshore simultaneously. The onshore crew connects McCrometer’s converter directly to a laptop to convert the electrical signal from the sensor to live flow rates. The integrated system then compares the offshore and onshore measurements. A difference of 4% or less is considered a successful test. The tests are submitted to the EPA, and it is with these results that the EPA determines whether to renew SPR’s discharge permit.

The original FPI Mag meters and subsequent generations of the product have been in continuous use for periodic testing service since their original adoption for this application. The latest test, again conducted with FPI Mag meters, confirms their performance results.

“For years we have counted on this flow meter to perform testing; to date we have always performed the tests successfully,” said James Nguyen, Senior Pipeline Engineer with DM.

“Accuracy is extremely important to the testing process because of time and money”, said Nguyen. “We can’t afford to have equipment that is not working properly. The annual test cost for the contracting alone is $200,000 to cover the lift boat, a five-man diver team and equipment set-up. Plus we have our own offshore and onshore crews.”

The FPI Mag meter combines the ease of hot-tap installation with an accurate measurement across the full flow profile. The FPI Mag meter is accurate to ± 0.5% of reading from 0.3 to 10 m/s, and up to ± 1% of reading from 0.1 to 0.3 m/s.

Cost effective for new or retrofit applications because of its compact insertion design, the FPI Mag meter fits easily into limited-access confined spaces. It also can be removed from pipes under pressure for easy inspection, cleaning or calibration verification without an expensive shutdown and restart sequence, helping cut plant maintenance and instrument ownership costs.

The FPI Mag flow meter’s sensor features multiple electrodes across the entire pipe diameter. Electromagnetic coils installed inside the sensor produce magnetic fields. Stainless steel electrode pairs installed on the outside of the sensor collect the induced voltage cause by the flowing water. The total of each voltage signal is transmitted to the converter electronics, where it is converted to an average flow velocity. The converter then multiplies the average flow velocity by the pipe’s cross-sectional area to create a volumetric flow rate.

The FPI Mag is packaged with a heavy-duty 316 stainless steel sensor body for maximum structural integrity and, for this application, the optional Hastelloy electrodes compatible with the corrosive seawater environment. The sensor is coated with a NSF-certified 3M fusion-bonded epoxy coating for operational longevity. With no moving parts and a single-piece design, the FPI Mag flow meter contains nothing to wear or break, and is generally immune to clogging by grit or other debris.

“Over the years the product has been upgraded from the original fibreglass construction to steel. The more rugged the product is, the better for this tough environment. We need to rely on the equipment to perform accurately and properly so no retesting is needed. I’m not sure if any other meter could have been chosen to do the job with the given requirements,” said Nguyen.