Effects of incorrect chemical dosage
Problematic and costly repercussions can occur should the inhibitor dosage be over or under the optimum application rate. Reasons for under- or overdosing are often tied to chemical injection flowmeter accuracy, which can be heavily influenced by the properties of the injected chemicals. Under injection of treatment chemicals can result in scale or paraffin buildup in well production strings or pipelines, lowering the production rate. If the scale or paraffin exist in the line for an extended time period, the well may have to be shut in to undergo a batch treatment, incurring deferred production and intervention costs. Where corrosion inhibitors are being injected, subsea umbilicals, risers, and flowline (SURF) facilities can, in severe cases, be taken offline until failed components are replaced.
In the case of overdosing, chemical excess costs can be significant and the additional chemical tanks take up valuable deck space on the platform. For instance, a company could spend more than US$1 million to overinject just one well over the life of the field. Furthermore, excess levels of LDIs in the export crude may affect its value at the refinery. Over the life of a field, accurate flow measurement and control of LDI injection could reduce operational expenditure by tens of millions.
The challenges of metering LDIs
With LDI injection, particulate block-age is a recognized cause of CIMV failure; this blockage is partly caused by the extremely low required injection rates, sometimes less than 0.5 L/h [3.17 gal/d] and the fact that particulate contamination can be introduced into the chemicals during transport, storage, and subsea distribution. These particulates can block moving parts inherent to many flowmeter and CIMV designs.
Traditional flow measurement technologies used in subsea chemical injection metering valves typically use Venturi-type flow measurement. Inaccuracies in flow measurement can stem from particulate contamination and blockage in the CIMV and from the fact that CIMVs are engineered years in advance of being put into service, often with limited knowledge of the chemicals to be injected. Such events render the CIMV as being not properly tailored for the chemicals being used, and ultimately, potential system under performance occurs.
A new dawn in CIMV flowmeter design
The latest low-flow CIMV flowmeter design delivers accuracy better than ±2% of reading for LDI injection compared to the industry standard Venturi-type flowmeter that may only deliver accuracy of 5-10% full scale. Launched at OTC 2016, the new Cameron PULSE LF low-flow ultrasonic chemical injection metering valve features a microbore nonintrusive, line-of-sight ultrasonic flowmeter. Featuring no moving parts, the flowmeter delivers debris-tolerant flow measurement, is chemical independent with a very low native pressure drop, and does not require subsea filtration. Developed to address the complete LDI chemical injection portfolio from 0.25 L/h to 600 L/h [1.6 to 3800 gal/d], this flowmeter is combined in closed loop control with a throttling valve, providing a self-regulating device requiring only one user-defined input—low rate.
The PULSE LF CIMV's flowmeter addresses the key limitation of present LDI chemical injection technologies—sensitivity to blockage. The flowmeter is particulate tolerant, meaning that contaminated fluid can easily pass through the unrestricted flowmeter tube. It also provides consistent high accuracy of reading independent of changes in chemical properties such as viscosity, and reliably measures chemical inhibitor flow rate. Real-time feedback from the flowmeter enables autonomous control of the throttling valve, maintaining a user defined injection rate set point indefinitely regardless of up- or downstream system disturbances.
Packaged as an ROV-retrievable device with onboard diagnostics, the PULSE LF CIMV enables full inhibition without the risk of under- or overdosing. Operators now have the option to reliably deliver LDIs via cost efficient subsea distributed chemical injection systems with precision regardless of chemical properties or contamination, giving them the option to make chemical decisions independent of the installed hardware.