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Electric intelligence

Published by , Editor
Oilfield Technology,

Simon Jagers, Samotics, the Netherlands, explains how measuring current and voltage topside can prolong downhole run life in electric submersible pumps.

Electric submersible pumps (ESPs) are a popular artificial lift method, particularly for deep, high-volume wells. Their modest surface footprint and ability to handle a wide range of flow rates and lift requirements make them versatile. They are generally low maintenance and cost-effective, compared with alternative assisted lift technologies. However, they do have weaknesses. The greatest of these is a short run life: downhole ESPs average just two years of service. By comparison, hydraulic submersible pumps routinely last for a decade or longer.

There are three reasons for this short ESP lifespan: inadequate design, improper installation and operational hazards. This article will focu on the last of these. Assuming you’ve chosen the right equipment and installed it correctly, what can you do to prolong its run life once it’s operational?

The answer hinges on the presence of high-quality data. Yet only 2% of all downhole ESPs have sensors installed. One reason for this meagre number is that a wellbore is an inhospitable place for sensitive transducers. A non-invasive monitoring technique called electrical signature analysis (ESA) can alleviate this problem. ESA systems install sensors topside rather than downhole, yet nonetheless provide asset health data of high enough quality to help resolve operational causes of ESP failure.

What is electrical signature analysis?

Almost all electrical and mechanical changes in an industrial drive train have an observable effect on the current and voltage drawn by the associated motor. Electrical signature analysis is a family of signal processing techniques that leverages this fact to provide early failure warning and real-time performance insights.

An ESA system installs non-contact current and voltage probes in the topside motor control cabinet housing the variable frequency drive (VFD) or power supply. After local signal processing, the data are encrypted and securely sent to the cloud. Machine learning algorithms then analyse this data to detect both developing damage and suboptimal operating conditions. Real-time performance reporting empowers operators to respond adequately to changing conditions; early fault alerts enable repairs to be planned weeks to months in advance.

Here are three ways ESA provides valuable information to boost an ESP’s lifespan.

ESA can track an ESP’s real-time performance

Because ESA systems capture both current and voltage, they can estimate a pump’s instantaneous head and flow using the affinity laws and the pump’s design specifications. This helps in two ways.

Figure 1. A sample real-time pump performance dashboard in an ESA system.

First, knowing that a pump is currently operating away from its best efficiency point (BEP) enables the operator to take timely action, before damage can occur. Off-BEP operation generates system stressors such as cavitation and recirculation, which directly weaken pump seals, impellers and bearings. If chronic, this behaviour will dramatically shorten pump run life. Knowing where an ESP is currently operating makes it possible to adjust system settings (such as the pump’s supply frequency) to stop this behaviour, preventing pump damage.

Second, inspecting the data over time can reveal trends that indicate changing wellbore parameters, requiring longer-term adjustments. For example, persistent operation to the far left of the curve paired with a chronic increase in the current spectrum’s noise floor is indicative of sustained cavitation, which in a downhole ESP can indicate increasing free gas levels. Comparing the ESA data with data from the system’s control valves can help distinguish between cavitation caused by gas entrainment and low flow arising from other causes.

Figure 2. Part of an ESA analysis to identify a chronically cavitating pump.

ESA can track power quality

By capturing the current and voltage signals feeding the pump’s motor, ESA systems have direct access to raw data on the quality of the power supply. ESA systems can directly signal grid-side issues such as voltage unbalance and harmonic distortion, even through a VFD. Catching these anomalies early is important, because every asymmetrical situation imposed on the grid side will cause machine inefficiencies and eventually lead to some kind of mechanical failure.

ESA provides two levels of defence against failure

By tracking pump performance and power quality in real-time, ESA systems help proactively prevent damage from occurring. Once damage has started to develop, ESA systems also serve as a second line of defence that catches many types of developing damage well in advance of failure. For example, over time sand will erode the pump’s impellers at their outer edge. This gradual decrease in impeller diameter will manifest as a long-term downward trend in active power despite no change in speed or well pressure. By tracking active power over time, ESA systems can identify impeller erosion well before the pump begins to show signs of failure.

The fault detection performance of today’s ESA systems is equal to or better than that of older, more familiar technologies such as vibration and temperature analysis. ESA will catch most electrical and mechanical faults in the motor, plus many (but not all) mechanical failures in the pump itself. Looking at pros and cons, ESA is the best monitoring technique to catch electrical faults early; in contrast, it performs poorly on faults arising in the pump’s far-end bearing.

Worth a closer look

While no single technique can eliminate all failures, ESA’s unique qualities offer a means to significantly improve ESP run life and operational performance. Hopefully this short overview will spur you to learn more about electrical signature analysis and its potential value in your downhole operations.

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