Cutting-edge choke condition monitoring: What can we do to prevent choke failure?

GEIR INSTANES and KJETIL NYSATER, ClampOn 

Subsea production choke valves are engineered for efficiency and durability. However, unexpected challenges can arise, which are often kept confidential within the industry. For example, why did the choke wear out and need replacement so quickly, despite operating well below design pressure or flow capacity? 

The purpose of a production choke valve is to control the amount of fluid or gas being produced. This means that the choke valve will absorb or transfer a significant amount of energy, especially when the differential pressure across the valve is high and the well is producing either at a reduced rate or with a small choke opening. Choke valves generate broadband noise caused by pressure pulsations, similar to a powerful whistle or horn when the opening is small. When fully open, the flow passes through the choke without any pressure drop, thus not absorbing any energy. 

Common challenges faced by choke valves: 

1. Erosion and Corrosion: High fluid velocities, sand production and corrosive environments can cause significant wear and tear on valve components. 
2. Noise and Vibration: Pressure pulsations and high flowrates can generate excessive noise and vibration, potentially leading to structural damage. 
3. Poor Control: Achieving precise control can be difficult, especially under varying flow conditions and pressures. 
4. Trim and Body Wear: Continuous exposure to harsh conditions can lead to rapid degradation of the valve trim and body. 
5. Accessibility Issues: Maintenance and testing can be challenging, due to the valve’s location and the need for specialized equipment. 
6. Environmental Considerations: Managing water disposal and adhering to environmental regulations during testing and operation can be complex. 

For years, ClampOn has worked to improve the noise suppression algorithms in their sand detectors, to avoid interference from flow and choke noise. However, even with the latest generation acoustic sand monitors, choke noise can be so strong during low openings that it cannot be suppressed, especially in high-pressure gas conditions. This has been a nuisance for operators and ClampOn, as it results in false sand content readings during well startup. 

This raises the question: how much energy does the choke absorb/transfer when even the most advanced acoustic noise suppression cannot suppress it? The answer is likely “a lot.” So much so that if left at this narrow choke opening, the choke would probably fail long before the end of its design life. Therefore, should this information not be used to alert the operator, who can then increase the choke opening and move outside the unsafe operating window? 

Failures of subsea choke valves have become more frequent, with no single cause identified. Different failure modes are observed across various manufacturers and models. A common factor in these failures is that wells operate at higher pressures and flowrates, compared to older assets, leading to increased stress and higher risk to the choke valves. Failures in choke valves result in production stops, leading to economic losses and potential environmental dangers, if the failures lead to leaks into the sea, Fig 1. 

Fig. 1. Choke noise recorded by Acoustic Sand Detector – Raw Value.

Cavitation, acoustic cavity resonance and chatter are some of the known drivers of choke valve failures. These issues can also cause damage to components and pipework upstream and downstream of the choke. For example, pipeline welds and walls can crack, and prolonged pipework vibration driven by high-frequency noise from the choke can lead to weld failures. Intrusive instrumentation, such as pressure and temperature transmitters, can also fail. 

Acoustic cavity resonance occurs when sound waves resonate within a cavity, amplifying certain frequencies. This phenomenon happens when the frequency of the sound waves matches one of the natural frequencies of the cavity, causing the air inside to vibrate at that specific frequency. 

In the context of subsea choke valves, acoustic cavity resonance can occur when the flow of fluid through the valve creates pressure waves. These waves can resonate within any voids or cavities in the valve or surrounding pipework. The frequency of these pressure waves depends on the volume and shape of the cavity. When resonance occurs, it can lead to very high sound pressure levels, which can cause damage to the valve and surrounding components. 

MONITORING SOLUTION  

ClampOn specializes in permanent subsea monitoring systems, primarily for acoustic monitoring of sand production, corrosion and erosion monitoring, leak monitoring and physical sensing of pipework vibration—often combined in a single instrument. Nearly all subsea production trees have ClampOn’s Sand and Vibration monitor installed, although vibration monitoring is an optional functionality that is not always enabled. These instruments are typically installed on the flow control module, near the choke valve, to ensure accurate real-time monitoring of sand content.  

Vibration levels indicate whether pipework vibrations are below or above acceptable levels. However, flow-induced vibration is normally not a concern at this location, as the pipework here is short in length and rigid, which is the main reason why the functionality is not often enabled. 

The sand monitors are usually installed close to the choke, ideally downstream, because this is where the flow is highest. Any solids in the flow will generate high acoustic energy on impact, allowing the sand monitor to detect even the smallest amount of sand. This means that in existing installations, it is possible to enable the vibration monitoring part of the Acoustic Sand Detector, to pick up vibrations generated by the nearby choke valve and detect issues like chattering. Additionally, it is possible to widen the acoustic measurement area beyond just focusing on the acoustic energy from solid impacts, to capture more of the typically unwanted choke noise, such as cavitation or acoustic resonance. 

For new installations and retrofits, ClampOn’s latest development, the Choke Condition Monitor (CCM), will be installed directly onto the choke body. The CCM can measure vibration across a very broad range, compared to the standard Acoustic Sand and Vibration Detector (ASVD). It not only captures the frequency response from 0 to 20 kHz but also measures the acceleration/energy within this band. This capability allows for accurate measurement and calculation of the internal sound pressure level (SPL), Fig. 2. 

Fig. 2. ClampOn Choke Condition Monitor (CCM) installed on the choke body.

SPL is typically calculated using the pressure drop across the choke and the mass flowrate, as it has historically been difficult to measure directly. Existing guidelines have established safe operating limits for SPL, enabling instant alerts, if these limits are reached. This allows operators to act quickly, to avoid damage. 

The CCM has two operating modes. The standard running mode reports vibration RMS velocities with defined limits, similar to a traffic light system: green indicates no concern, yellow suggests that further diagnostics should be performed and red signals that immediate action is required. This system ensures that operators can safely manage the operating window without time-consuming calculations and analysis. 

The second mode is diagnostic mode, which allows for the retrieval of detailed information, such as broadband frequency response spectrum and raw data snapshots. These data can be used for further diagnostics or analysis, if needed. 

A choke valve is a complex mechanical assembly consisting of multiple parts, some of which are moving while others are static. These parts are made from a variety of materials, ranging from standard to more “exotic” materials designed to withstand erosion and cavitation. The design of choke valves varies by model, make and field, due to different flow conditions. Consequently, the various moving components have different natural frequencies, depending on their type and design. Direct measurement of these internal components is impossible, because they are inaccessible within the choke assembly. However, in diagnostic mode, it is possible to monitor the frequency content on the body of the choke valve and identify if any components are vibrating at levels higher than acceptable. Frequency measurements can also be used to determine the source of the vibration, Fig. 3. 

Fig. 3. Vibration sweep testing on a choke assembly, monitoring sleeve vibrations at various choke positions.

At times, it is necessary to operate the choke valve outside the safe operating window, particularly during start-up after the well has been shut in for some time. During this period, the wellhead pressure builds to a high level, resulting in significant differential pressure across the choke. The choke must be opened gradually during start-up, which places considerable stress on it during the initial ramp-up at a low opening. High transient sand production may also prompt the operator to reduce the choke opening during start-up or to maintain a low opening for longer durations, to minimize sand production from the reservoir. This is acceptable, as long as these periods are brief. 

To maintain control, it is important to have a monitoring system that continuously records acoustic and vibration levels from production start to the end of life. This information is stored in the topside data system and can be integrated into Likelihood of Failure (LOF) calculations. This makes it easy to determine if the choke is within the design life curve, or if maintenance plans should include a choke replacement and when. In simple terms, it can be used to identify and forecast the required maintenance. 

INTEGRATION AND QUALIFICATION 

By utilizing the existing ClampOn compact instrumentation platform and upgrading it with new MEMS sensors (micro-electromechanical systems), we have rapidly provided a qualified monitoring solution. No hardware changes to the instrument enclosure were necessary, ensuring that qualifications to applicable standards, such as API 17F, remain valid. We can confidently state a technology readiness level of 7, the highest in our industry. The signal interface adheres to the industry-standard CanOpen CiA 443 SIISL2, allowing this technology to be deployed in any new or existing subsea production systems. 

The instrument is securely fastened directly to the choke body, to ensure that vibration levels are accurately captured by the MEMS sensors within the instrument. This is achieved, using a bolted mounting plate for new installations or a mounting plate with built-in super magnets for retrofit cases. Both mounting options have high, known eigenfrequencies, ensuring that the fixture arrangement does not affect measurements. For existing assets without spare connections to the subsea control system, we can daisy chain the Choke Condition Monitor with our existing acoustic sand and vibration monitors or any other SIISL2 instruments on the module. 

Fig. 4. Example of a worn-out choke insert.

FIELD CASE 

ClampOn was recently contacted by a Norwegian operator, who experienced a choke collapse during the first hours of starting up a new well. They sought assistance in identifying the cause of the collapse and mitigating the risk of future failures. Early investigations suggested that strong vibrations were likely the cause of the collapse. Another potential cause could be the impact of solids or sand; however, the acoustic sand detector did not record any sand production, nor were there any indications of sand downstream of the production system, ruling this out. 

Material selection was evaluated, considering the combination of high differential pressure (DP) across the choke and potential high vibration, Fig. 4. 

The choke’s failure during the initial start-up procedure aligns with high vibration and/or high sound pressure levels at low choke openings, the latter being the most likely cause. Unfortunately, the acoustic sand detector located a few meters from the choke was not configured to use its onboard vibration monitoring capability, so no vibration data were available for evaluation. The acoustic raw data used for sand calculation showed very high levels prior to and during the failure, not only on the well in question but also on acoustic sand detectors on neighboring wells in the same template.  

Fig. 5. The ClampOn Choke Condition Monitor installed on the choke body and Acoustic Sand and Vibration Detector installed on the flow control module.

This indicates a very high sound pressure level at the time of failure. However, it is not possible to quantify the exact level, as the acoustic sand detectors use advanced filtration to remove as much flow noise as possible. Additionally, they are located several meters away from the source, and the noise is transmitted through the water and piping to the instrument. 

As of the time of writing, four choke failures have occurred, each with different choke trims and materials. One of the risk mitigations implemented was enabling the vibration monitoring capability on the existing acoustic sand detectors and installing the new Choke Condition Monitor directly on the choke body. This has now been done on one of the wells, which will be operational later this year. This will help the operator avoid producing with a choke at an opening that can cause high vibration or sound pressure levels, Fig 5. 

About the authors

GEIR INSTANES is the Vice President of ClampOn AS. He is one of the entrepreneurs of ClampOn and has been working with the development of ultrasonic sensors since the very beginning, in 1994. 

KJETIL NYSÆTER has more than 25 years of experience working in the oil and gas industry. He has worked at different departments in ClampOn, including manufacturing, field operations, RND and sales. He is now focusing on new product development and new applications.