Schlumberger

Industry Article: Ultrasonic Advancements

Publication: Oilfield Technology
Publication Date: 11/01/2016

David Simpson, Cameron, a Schlumberger company, explains how flowmeter accuracy helps eliminate underdosing and the need to overdose in subsea chemical injection.

Finding the right balance in chemical injection is not always about determining the correct dose rate of inhibitor; many times it is about the means of injection. Accuracy of the subsea chemical injection flowmeter is the critical element that can help eliminate the occurrence of underdosing and overdosing. On deepwater projects with complex architecture and long step-outs, operators face high cost for not getting the dosage right. These chemical injection operations involve many uncertainties, including that of traditional chemical injection metering valve flowmeters that create even more uncertainty and cause operators to inadvertently underdose, or in some instances, purposefully overdose the production system to be sure full inhibition is achieved.

Accuracy of the flow as measured within the chemical injection metering valve (CIMV) can mean millions of dollars in cost savings and can make the difference between successfully managing the flow assurance challenges and having to make a best guess resulting in under or overdosing. Operators are pumping as much as 10 to 20% excess chemicals in a year just to protect from meter inaccuracy. Accurate and reliable flow measurement of chemical inhibitor is paramount to increasing operational efficiency and can reduce operational expenditure by tens of millions of dollars over the life of the field.

Sources of measurement inaccuracy

 Finding the right balance in chemical injection is not always about determining the correct dose rate of inhibitor; many times it is about the means of injection.
Figure 1: Subsea installation of a PULSE LF CIMV

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. Blockage occurs as a result of particulate contamination of chemicals being injected through the CIMV and failure of moving parts in some CIMV designs. Also, not knowing what chemical will actually be metered by the CIMV during flowmeter production can have serious repercussions on some flowmeter designs. Not knowing what chemical will actually be put into the CIMV system results in a best-guess on the part of the CIMV designer (viscosity can be critical to calibrating some traditional CIMV flowmeters), and ultimately, potential system under performance. And then there are the operational decisions made to change out the injection chemicals used in the field, which means the CIMV is no longer even remotely tailored for the chemicals being used today. Accuracy of CIMVs drops off, and failure can occur due to contamination or blockage, costing the operator millions of dollars in remediating flow assurance issues and in intervention costs.

Best guess costs money

Under and overdosing to keep production flowing incurs considerable additional cost. The under injection of treatment chemicals can result in scale or paraffin buildup in well production strings or pipelines, reducing the production rate. If the scale or paraffin exists for an extended period of time, the well may have to be shut in to undergo a batch treatment, resulting in deferred production and intervention costs. In the case of corrosion inhibitor treatment programmes, SURF (subsea umbilicals, risers and flowline) facilities can, in severe cases, be rendered unavailable for production until failed components are replaced.

In the case of overdosing, chemical excess costs are significant (potentially, a company could spend more than US$2 million annually to over-inject just one well), and the additional tankage ties up valuable deck space on the platform. The chemicals are expensive and involve associated transportation, storage, and handling costs.

Focus on CIMV design

Subsea operators have been in search of higher accuracy, reliable chemical injection flowmeters that have low native pressure drop requirements, are particulate tolerant (thus needing less stringent chemical cleanliness requirements), and having ever higher pressure ratings. They are also looking to identify Capex savings in umbilical and installation costs, Opex saving on chemical usage over the life of the field, space and weight savings on the host facility, and the ability to control chemical injection in deepwater/sub-ambient wells.

Previous generations of CIMV technology, using mechanical flowmeters along with other designs that use pressure differential measurements across small orifices and tightly fitting moving parts, did not address these needs. Injection chemical cleanliness is critical; strict cleanliness limits are specified for the chemicals and subsea filters required in the CIMVs. Flow measurement accuracy can also be heavily influenced by the properties of the injected chemicals, requiring project-specific calibration with specific chemicals during manufacture.

Finding the right balance in chemical injection is not always about determining the correct dose rate of inhibitor; many times it is about the means of injection.
Figure 2: Microbore non-intrusive line-of sight ultrasonic flowmeter developed for the PULSE LF CIMV

Now, use of a non-intrusive line-of-sight ultrasonic flowmeter is replacing these previous flowmeter designs, introducing a step change in CIMV technology. The non-intrusive nature of these ultrasonic flowmeters, which measure flow rate by time-of-flight measurement, means that the flowmeter is inherently debris tolerant with no moving parts, is chemical independent with a very low native pressure drop, and does not require any subsea filtration. These ultrasonic flowmeters deliver better than 3% of reading accuracy with an extremely large turn-down ratio that allows a greater injection range from a single device. Cameron PULSE* ultrasonic chemical injection metering valves have been developed for low-, medium-, and high-flow applications to address the complete chemical injection portfolio from low dose inhibitors injected at less than 0.25 l/hr to thermodynamic hydrate inhibitors (such as mono-ethylene glycol [MEG] or methanol injected up to 26 500 l/hr). These ROV retrievable devices are self-regulating chemical injection metering valves, requiring only one user-defined input (flow rate set point).

The PULSE LF* low-flow ultrasonic chemical injection metering valve (injection range 0.25 - 600 l/hr) is designed for low-dose inhibitor (LDI) injection to combat corrosion or scale buildup in the production system. For example, where particulate blockage is a recognised cause of CIMV failure due to the very low injection rates required. The PULSE MF* medium-flow ultrasonic chemical injection metering valve (injection range 80 - 11 000 l/hr) and PULSE HF* high-flow ultrasonic chemical injection metering valve (injection range 160 to 26 500 l/hr) are designed for hydrate management in large-bore subsea gas production projects, where methanol or regenerated MEG are injected at startup and shutdown or continuously during production to prevent hydrate formation and plugging. The safe prevention and/or removal of hydrate plugs represents 70% of deepwater flow assurance challenges; the remaining 30% deal with waxes, scale, corrosion, and asphaltene remediation. Large volumes of thermodynamic inhibitors such as MEG or methanol are injected in a very accurate and controlled manner, preventing the formation of hydrate plugs that would otherwise very quickly form and block the subsea production system, causing a shutdown.

Finding the right balance in chemical injection is not always about determining the correct dose rate of inhibitor; many times it is about the means of injection.
Figure 3: PULSE CIMV system architecture

Due to the high cost of MEG, it is often stripped out of the produced gas in a MEG regeneration process and recycled in the system. This leads to a contamination buildup in the lean MEG from the regeneration process and also from contamination in the distribution system. Any chemical injection metering valve injecting this lean MEG stream must be robust to counter such high levels of contamination.

The ability offered by the medium- and high-flow CIMVs to precisely control the injection rates of MEG from a common distribution network to individual wells allows operators to better manage their MEG budget, allowing higher allocation of MEG to where it is needed most. The smarter allocation of MEG maximises production on each well across the field, and reduces the over injection volume, enabling a reduction in the required size and cost of the regeneration plant on the host facility.

How the technology works

At the core of the PULSE LF CIMV is a microbore non-intrusive line-of-sight ultrasonic flowmeter. The recently launched ultrasonic flowmeter has no moving parts, and unlike a Venturi-type flowmeter, is pressure and chemical independent with a very low native pressure drop. Flow rate is determined by time-of-flight measurement of an ultrasonic pulse emitted and received by a pair of transducers travelling with and against the chemical flow in a small-bore tube. The onboard ultrasonic control module precisely determines the difference in time of flight of the two ultrasonic pulses travelling in the metering path, and it is the Delta T measurement that is used to calculate flow rate. This flowmeter development addresses the key limitation of present low flow chemical injection technology, which is sensitivity to blockage, by having capability to accurately and reliably meter chemical inhibitors without the need for filtration. The ultrasonic flowmeter features an extremely high turn-down ratio, allowing one meter to be used across a wide range of injection applications, is particulate tolerant (contaminated fluid can easily pass through the unrestricted flowmeter tube), provides consistent high accuracy of reading independent of changes in chemical properties such as viscosity, and reliably measures chemical inhibitor flow rate.

The new PULSE LF CIMV shares the same system architecture as the field proven PULSE MF & PULSE HF CIMV which were launched in 2010, whereby the ultrasonic flowmeter is combined in a closed-loop control with an electrically actuated throttling valve. Real time feedback for the flowmeter is used to control the throttling valve, maintaining a user-defined injection rate set point indefinitely, irrespective of up- and downstream system disturbances. Process status indications and diagnostic data is streamed back to give operators a clear view and control of what is happening at the subsea injection point. The PULSE LF CIMV utilises a micro-needle and seat throttling valve to precisely control the low-dose inhibitor injection rate between 0.25 and 600 l/hr. The PULSE MF and PULSE HF CIMVs use a multiple orifice valve (MOV) choke trim technology to give erosion-resistant precise control over the injection of thermodynamic inhibitors ranging from 80 to 26 500 l/hr.

Packaged as an ROV retrievable device with on-board diagnostics, the PULSE CIMV’s streams back performance data, enabling full inhibition without the need for overdosing. They also provide highly accurate and reliable injection control, ensuring the chemicals are consistently injected at the correct location and rate.

Highly accurate and reliable ultrasonic flowmeter technology delivers significant chemical Opex savings as a result of increased measurement accuracy alone. Used with a hydrate management system, significant potential exists to reduce not only the size, weight, and Capex of the required MEG regeneration plant, but also its running cost. Further savings can be realised in the cost of the MEG or methanol distribution system, especially for long step-out tiebacks or satellite wells, due to the low pressure drop nature of the ultrasonic flow measurement technique compared to traditional Venturi-type flow measurement.



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