Traditional flow measurement technologies used in subsea chemical injection metering valves for LDIs typically use either Venturi-type flow measurement (whereby a pressure drop is measured across a precision orifice) or a positive displacement flow measurement technique which employs a rotating or stroking piston to measure volumetric flow rate. Accuracy with these techniques can be heavily influenced by the properties of the injected chemicals, requiring project-specific chemical calibration during manufacture of these CIMVs. Inaccuracies in flow measurement can also stem from particulate contamination and blockage in the CIMV and from the fact that CIMVs can be engineered years in advance of being put into service (oftentimes with limited knowledge of the chemicals to be injected). A little understood fact is that CIMV system designers are often not provided with critical chemical data, while the CIMV is in the design stage. And then there are operational decisions made to change out injection chemicals used in the field during its life.
These events can render the CIMV as being not properly tailored for the chemicals being used, and ultimately, potential system under performance occurs. Blockage happens because of particulate contamination of chemicals being injected through the CIMV, blocking onboard filters or tightly fitting moving parts or orifices used for flow measurement.
Effects of incorrect inhibitor dosage
Delivering the optimum amount of inhibitor is key. Should this not happen, problematic and costly repercussions can occur, especially in deepwater. Under- or overdosing are often tied to chemical injection flowmeter accuracy, which can be heavily influenced by the flowmeter design and properties of the injected chemicals. Under injection can result in scale or wax buildup in production strings or pipelines, for example, lowering the production rate. Should the scale or wax exist in the line for an extended period, the well may have to be shut in to undergo a batch treatment, incurring deferred production and intervention costs. In the case of corrosion inhibitors, SURF (subsea, umbilicals, risers, and flowline) facilities may have to be taken offline until failed components are replaced.
Should systematic overdosing occur, significant chemical excess costs can result; not only in the chemical costs themselves, but also in the additional chemical tankage which takes up valuable deck space on the platform. For instance, based on the selection of the CIMV design alone, a company could inadvertently spend more than USD 2 million overinjecting just one well over the life of the field. Also, excess levels of LDIs in the export crude may affect its value at the refinery.
Ultrasonic flow measurement
The microbore, nonintrusive PULSE LF low-flow ultrasonic chemical injection metering valve, for injection rate control of LDIs, offers a highly reliable, debris-tolerant CIMV with best-in-class injection rate accuracy. This single, retrievable unit offers an injection range from 0.25 to 600 L/h (achieving a turndown ratio of 2,400:1) with an injection rate accuracy of better than 2% of reading above 2 L/h compared to the industry standard Venturi-type flowmeter that may only deliver accuracy of 5 to 10 percent full scale.
The PULSE LF CIMV was developed following success of the PULSE MF medium-flow ultrasonic chemical injection metering valve and PULSE HF high-flow ultrasonic chemical injection metering valve (used to inject high volumes of MEG or methanol as part of a hydrate mitigation system). The microbore ultrasonic flowmeter at the heart of the PULSE LF CIMV delivers nonintrusive, debris-tolerant flow measurement (based on the “delta T” time-of-flight measurement technique) with no moving parts, is chemical independent with a very low native pressure drop, and does not require any subsea filtration (Figure 2).