ACOUSTIC MEASUREMENTS DETECT SAND IN NORTH SEA FLOW LINES

Aug. 27, 1990
Trond Folkestad, Kanaga S. Mylvaganam Chr. Michelsen Institute Bergen, Norway Clamp-on acoustic emission (AE) transducers have been developed for detecting sand in flow lines on offshore oil producing platforms. Sand production is determined by using the integrated value of measured noise generated by collisions of sand particles on the pipe wall. During the field's life, sand production can do excessive damage and result in expensive repairs that might occur at any time and in any well.
Trond Folkestad, Kanaga S. Mylvaganam
Chr. Michelsen Institute
Bergen, Norway

Clamp-on acoustic emission (AE) transducers have been developed for detecting sand in flow lines on offshore oil producing platforms.

Sand production is determined by using the integrated value of measured noise generated by collisions of sand particles on the pipe wall.

During the field's life, sand production can do excessive damage and result in expensive repairs that might occur at any time and in any well.

Much effort has been put into developing techniques to reduce the risk of sand production and in designing monitoring systems to determine if sand is being produced. The intrinsically safe clamp-on AE system was developed by Chr. Michelsen Institute (CMI) in an extensive research and development project cosponsored by the Norwegian companies Statoil A/S and Fluenta A/S.

In a series of projects from 1984 to the present, CMI has made various feasibility studies on the principle of detecting particles in gas and liquid flows using clamp-on acoustic emission systems.1-6 CMI's work also includes the preliminary study,1 prepared for Norsk Hydro A/S, on sand monitoring for the TOGI (Troll-Oseberg gas injection) project.

The clamp-on AE system was tested in one of CMI's flow loops that is constructed with the same type of pipe sections as used offshore. These tests were performed under similar flow conditions as found in offshore production flow lines in the North Sea.

Additionally in 1989, the clamp-on AE system has undergone a long-term comparative test together with three other commercially available sand-monitoring systems at Veritec A/S (Veritas Offshore Technology & Services A/S of Det Norske Veritas).

Two units have also been tested in May 1990 on production flow lines on the Gullfaks B platform (Statoil A/S) in the Norwegian sector of the North Sea.

In the summer of 1990, two clamp-on AE sand-monitoring systems will be installed on British Petroleum's Ravenspurn A platform in the British sector of the North Sea.

SAND PROBLEMS

In many oil and gas producing areas, the influx of formation sand into the well bore presents a major obstacle to oil and gas production. Wells have been abandoned because problems associated with formation sand influx were severe and very costly to overcome.

Sand particles may accompany hydrocarbon flow from sandstone reservoirs. This sand can cause erosion of flow lines, valves, and other equipment. Partial or complete blockage of flow lines can occur if sand settles and forms sand beds.

In addition to blockage (called sand bridging) and the difficulty of removing formation sand, all production equipment is prone to erosion from sand particles in the flowing medium. Fig. 1 shows an eroded valve used on one of the platforms in the North Sea. In stainless steel pipe, sand particles can erode the inner protective layers and make the pipe susceptible to corrosion.

The formation itself may collapse from the instability caused by sand removal.

These problems make it important to monitor continuously formation sand in the flow stream. With monitoring, immediate action can be taken when excessive sand production is noticed.

Actions can range from changing choke size to complete shutdown. In some cases, the wells might need to be gravel packed.

MONITORING TECHNIQUES

On offshore production platforms, all existing techniques use intrusive mechanisms to monitor the sand in the flow stream. The most common techniques are:

  • Sand mechanically eroding specially designed hollow pipe containing high-pressure gas. After a hole erodes, the gas escapes and activates a pilot valve. These mechanical devices are massive and complicated, and they involve flanges, valves, etc.

  • Sand mechanically eroding a radioactive probe. Both radioactive material and associated radiation-monitoring devices are needed for this technique.

  • Sand-impacting probes based on the "tuning fork" principle. The impact generates high-frequency (600-800 khz) acoustic pulses that are monitored. One disadvantage to these devices is that ice plugs can completely destroy the tuning fork. Another disadvantage is that the tuning fork has to be placed across the pipe section fairly accurately. The probe end and the pipe wall have to lie within a certain interval. On Gullfaks A platform, the clearance has to be 2.5-6.5 mm.

  • Erodible resistance and an electrically similar nonerodible resistance coupled to a Wheatstone bridge. Both resistances are placed on an intrusive probe.

  • Optical transmission measurements giving particle concentrations and size.

This list is by no means complete and includes only the monitoring techniques most frequently used in the oil industry. However, the list does indicate the nature of the devices currently in use.

For reference, Table 1 lists the advantages and disadvantages of existing devices.

All of these methods are intrusive and thus cumbersome to maintain.

Most of these devices have to be replaced after establishing the presence of sand. On the other hand, a device that is not intrusive can be maintained easily and cheaply.

ACOUSTIC EMISSIONS

Suitably placed AE transducers are used for monitoring acoustic emissions created by sand particles colliding with the inner surface of the pipe.

A not too rigorous method can be used to illustrate the conversion of particle momentum during collision into an AE signal. Reference 7 contains a rigorous approach with experimental verifications of the theory.

CONVERSION METHOD

An AE transducer with a piezoelectric crystal will generate a voltage. This voltage is proportional to the displacement imposed on the crystal by the movement (due to sound waves) of the pipe's outer surface. Sand particles colliding with the pipe's inner surface will generate an impulse causing oscillations of the pipe's bulk.

This produces sound waves that are picked up by the AE transducers on the pipe.

Fig. 2 illustrates a single sand particle colliding with the pipe wall. This particle has a velocity, v, that forms an incident angle, a, with the pipe wall.

The change of the particle's momentum due to the collision and thus the impulse imparted onto the wall is given by mv (1 +e) sin a. The term e< l is the coefficient of restitution, and m is the mass of a single particle.

The corresponding AE signal of a single particle that can be measured on the outer surface of the pipe wall (np) can be approximated as:

[SEE FORMULA] (1)

where: Co is a factor representing the transfer function for the impulse imparted onto the inner wall, to the voltage from the AE transducer. The factor accounts for energy dispersion and damping, mainly due to transmission of acoustic energy from one material to another.

To determine the contribution of the AE signal from all the particles passing the section of pipe wall with the AE transducer in a given period of time, T, one has to consider the flow of particles and oil/gas/water in the pipe.

When passing a bend, the particles will hit the opposite wall of the bend due to inertia. The fraction of the particles hitting the wall, depending mainly on the kinematic viscosity, n, and on the average particle velocity,v, can be denoted by, k(-n,v) < 1.

The particles passing through the section of pipe per second will have a total mass, M (g/sec), and on average they will hit the wall with a perpendicular average velocity component, v sin a.

Thus, the total AE signal due to oil/gas/water flow and particle collisions, Np, obtained by a suitably placed AE transducer will be given by:

[SEE FORMULA] (2)

where: C1 = Co (1 + e) and Nf(V,.... ) is the noise due to oil/gas/water flow.

The technique of measuring Np relies on the increase in the integrated AE signal with increasing sand when compared to the stable value of the integrated AE signal without any sand in the oil/gas/water flow.

The principle is schematically shown in Fig. 3 for the periodic integration of the AE signal, with and without sand, over a period of time,T.

In Fig. 3, the AE signal without sand is given by Np (T,V,M = O,.... ) and is equal to Nf. To determine the amount of sand passing the cross-section of pipe per second, the inverted relationship of Equation 2

[SEE FORMULA] (3)

has to be determined empirically using statistical methods both during and after the measurements. This is not a topic covered in this article.

The AE signal without sand given in Fig. 3 by Np (,r,v, M = O,.... ), in theory, can be subtracted from the AE signal with sand, Np (r,V,MO.... ), to determine the amount of sand, M.

In the actual system, an empirical formula along the lines of Equation 3, has been developed. This can be illustrated by Fig. 4, where the empirical formula obtained from the Veritec tests is depicted graphically as a three-dimensional plot between the most important parameters. The three parameters are the amount of sand, M, the flow velocity, v, and the measured AE signal, Np.

It has been demonstrated that the empirical formula can be used to estimate the actual amount of sand with reasonable accuracy. This empirical formula has to be determined for each individual installation as part of the calibration of the clamp-on AE sand-monitoring system.

AE-SIGNAL ANALYSIS

The flow loop at CMI has an oil-flow velocity of 6 m/sec. This velocity is representative of normal offshore conditions. In the flow loop, the AE signal is obtained by a broad-band Bruel and Kjaer AE transducer, Type 8312 (Fig. 5). The AE signal without sand is shown in Fig. 5a.

The sand in the oil flow was increased in 2-g increments. This represents an increase of 0.64 g/sec in the sand flow.

The corresponding AE-signal plots are shown in Figs. 5b, c, and d. The AE signal with 1.92 g/sec of injected sand is shown in Fig. 5d.

In Fig. 5, the AE signals emitted due to sand particles lie within the frequency band of 300-800 khz. In these frequencies, the level of AE signals with 1.92 g/sec sand of the size distribution 75-150 u, increased by more than 6 db.

This level represents an easily detectable rise in the AE signal that can be processed to obtain the amount of sand in the oil flow. Integrating the AE signal over the frequency band will give a significantly larger increase than 6 db in the integrated value of the AE signal.

After establishing the range of frequencies in which the AE signal due to the collision of sand particles is predominant, the next step is to analyze how the AE signals in the frequency band of interest varies for varying sand concentrations. This has been done for the band of frequencies from 300-800 khz using two AE transducers specially developed by AV Technology Ltd. of Great Britain.

Their equipment for this purpose is certified as intrinsically safe by the British Approval Service for Electrical Equipment in Flammable Atmosphere (Baseefa), an organization in Great Britain undertaking certification and approval services.

Similar organizations are the Underwriters Laboratories Inc. (UL) in the U.S. and the Physikalisch Technische Bundesanstalt (PTB) of West Germany that follows closely the stipulations of the International Electrotechnical Commission (IEC) and European Committee for Electrotechnical Standardization (Cenelec).

Transducer response is seen in the AE-signal plots of Figs. 6a, b, c, and d. Figs. 6a, and c represent AE signals around the central frequency of each transducer, 450 and 650 khz respectively, without any sand in the oil flow. Whereas, Figs. 6b and d are AE signals with 1.92 g/sec of injected sand in the flow loop.

Fig. 6 shows that the AE-signal level is increased considerably due to the 1.92 g/sec of injected sand in the oil-filled flow loop. The concentration of injected sand is around 31 ppm by weight or 10 ppm by volume. The increase in AE-signal level as compared to the signal without sand is better than 10 db.

As mentioned earlier, integrating the AE signal, based on the concept shown in Fig. 3, over the frequency band 300-800 khz will give a significantly larger increase than 10 db in the integrated value of the AE signal for the same amount of sand.

The offshore user is interested to know whether the concentration of sand particles is above a certain level. The acceptable level of sand concentration specified by the various operators of offshore platforms in the North Sea varies considerably. However, the maximum concentration of fine sand particles used (size distribution 75-150 u) in the tests was about 31 ppm. This produced a 6-db rise in the AE signal.

Because the actual size of sand particles encountered in offshore operations may be considerably larger (confirmed by samples obtained from different wells in the North Sea), a corresponding increase in the level of the AE signal will certainly be observed.

CMI'S FLOW LOOP

The flow loop was constructed with the same type of pipe sections as used by Statoil A/S in its offshore installations. The flow loop consists of two horizontal sections and two vertical sections of pipe of the type API 5LX GR52 with 6-in. outer diameter and wall thickness of 21.9 mm.

The flow loop uses a centrifugal pump and has valves for injecting sand particles and for taking samples to analyze the sand concentration in the oil flow (Fig. 7). The oil used in the flow loop is N20 process oil that has a density at 15 C. of 0.868 kg/I., viscosity at 40 C. of 7.85 cSt, viscosity at 100 C. of 2.2 cSt, freezing point of -45 C., and a temperature of flammability of 150 C.

The pump has a volume flow of 4,380 I./min that corresponds to a flow velocity of 6 m/sec in the 6-in. pipe. The flow loop volume is 228 I.

AE-SYSTEM LAYOUT

The layout for the AE system is shown in Fig. 8. Preamplifiers are connected to the AE transducers by a short single-core superscreened cable of length less than 1 m. This length avoids unnecessary electromagnetic interference (EMI).

The AE transducers with their preamplifiers are classed "intrinsic safe" and therefore can be placed anywhere on an offshore platform without impairing safety.

The preamplifier is connected to the zener barrier using a single-core superscreened armored cable. The zener barrier is connected to the signal processor containing the integrating facilities that produce signals of the type shown in Fig. 3.

LONG-TERM TESTS

The tests done at Veritec A/S involved the AE system described in this article and three other commercially available devices developed for the detection of sand.

The test conditions were varied by changing the following parameters:

  • Sand concentration

  • Size of sand particles

  • Velocity of flowing medium

  • Composition of flowing medium. The composition was varied to have different gas/liquid ratios (GLR). Carbon dioxide (CO2) was the gas and water was the liquid.

  • Pressure and temperature.

Selecting CO2 effectively imposes the high-pressure effects of hydrocarbon gases produced on a flow loop because of the higher molecular weight Of CO2 compared to many of the hydrocarbon gases.

Sand sizes of 100, 250, and 500 u, were fed into the flow using a motor-controlled "screw feeder." By adjusting the motor speed stepwise, the amount of sand fed into the flow varied as a stepwise function with time.

Fig. 9 shows one of the many test series performed at Veritec A/S. The calculated amount of sand fed into the flow based on the speed setting of the motor activating the "screw feeder" is seen in Fig. 9a.

The corresponding real time outputs from the sand monitor are shown in Figs. 9b and c. These are for time averages of 45 and 5 sec and with the same integration time of T = 1 sec.

These measurements were made at a flow velocity of 6.9 m/sec, a particle size of 500 u, and with a GLR of 36. The immediate observation is that the AE signals as shown in Fig. 9b follow the step changes in the amount of sand fed into the flow.

For the first step (starting from the left), the calculated average sand feed is 0.05 g/sec. The second step corresponds to an average sand feed of 0.14 g/sec. The third step has an average sand feed of 0.24 g/sec.

The fluctuations seen in the measured AE signals with a short averaging time of 5 sec (Fig. 9c), for levels of sand feed that were assumed to be constant, were later found to be affected by the uneven feed of sand from the screw feeder.

In fact, the frequency observed in Fig. 9c is equal to the revolutions per second of the screw feeder. Feeding of the sand is rather intermittent with moments of no sand feed in between. Thus, we see an increase in frequency with increasing levels of sand fed into the flow.

The ability of the sand monitor to follow these very large and fast fluctuations in sand concentration is due to the automatic gain control and fast sampling done in the signal processor. The fast response of the clamp-on AE sand monitoring system makes it well suited for real time operations that are needed in detecting unusually high sand concentrations in short time intervals, in otherwise sand-free flow.

GULLFAKS B PLATFORM

Two clamp-on AE sand-monitoring systems have been specially mounted on the Gullfaks B platform by Statoil A/S in conjunction with the testing of another sand-monitoring device.

During the test, controlled amounts of formation sand from the Gullfaks B reservoir were injected into the production line. The clamp-on AE transducer system was mounted on the flow line.

The transducer system has the intrinsic safety classification and is connected to the signal-processing system. This includes an IBM compatible PC and a VME card system to gather, process, and display the necessary data. The signal-processing system can handle multiple clamp-on AE transducers and therefore has the possibility of simultaneous monitoring and displaying sand concentrations in up to 40 wells.

Two clamp-on AE transducer systems were mounted on the 6-in. production pipe of well No. 2 on the Gullfaks B platform. The units were tested using a controlled and deliberate injection of sand into the production line.

Fig. 10 illustrates the technique used in achieving controlled injection of sand. The injection of sand into the production line is performed as follows:

  • The sand in a gel is pressed into the production flow line just after target cap A.

  • The clamp-on AE transducers belonging to the sand-monitoring system are placed just after the next two target caps (after a 90 bend) B and C.

  • The signals from these sensors are processed to find the change in the AE signals due to various amounts of injected sand. The level of the integrated AE signal from the transducer system mounted after target cap B, is shown in Fig. 11 for an injected-sand concentration of 1.4 g/sec, a flow velocity of 2 m/sec, and a GLR of 68.

The level of integrated AE signal increases during the 700 sec injection period, starting at 200 and ending at 900 sec (Fig. 11). This corresponds to 1,000 g of sand injected into the production line. This AE signal is obtained with finer sand than the sand used in the long-term tests at Veritec or CMI.

The formation sand size from the Gullfaks B field lies between 25 and 150 u. The clamp-on AE sand-monitoring system registered the collisions of even these small particles. This was an indication of the good sensitivity of the system.

BP'S RAVENSPURN

Two units of the clamp-on AE sand-monitoring system have been ordered by BP for monitoring sand and proppant concentration in production flow lines on its Ravenspurn A platform in the southern North Sea.

The proppant consists of small ceramic balls, 200-1,200 u diameter, with a density of 3,080 kg/cu m. The balls are considerably denser than quartz sand, which has a density 2,600 kg/cu m.

Proppant is injected into the well to keep fractures in the formation open during production. The sand-monitoring units were planned to be installed in July and August 1990. The interesting point is that the Ravenspurn A platform is unmanned and all the monitoring and controlling is done using remote-control techniques from the central control room for the Ravenspurn field, located on the Cleeton platform.

The control room on the platform has a backup system on an onshore facility. The clamp-on AE sand-monitoring system will be sending data via modem and telecommunication radio link to the Cleeton platform.

The received data will be processed and displayed using an IBM-compatible PC, which acts as the operator interface. Because the distance between the Ravenspurn A platform and the Cleeton platform is only about 10 km, the usage of telecommunication radio link is natural. However, for long distances between an unmanned platform and the operator site, the operator might have to resort to satellite communications.

In case of an emergency caused by excessive sand production, the operator will try to prevent damage to the production lines of the platform by slowing or stopping the production completely, all via remote control.

In the era of unmanned operation (which is becoming more and more common) of platforms, various remote control techniques will be necessary. The clamp-on AE sand-monitoring technique surely will belong in this category.

SUBSEA APPLICATIONS

Placement of AE transducers on subsea flow lines will require considerable modifications to the AE transducers, the holder of the AE transducers, and the connections to topside processing units. In this context, the development of a subsea sand monitor for the TOGI project is worth mentioning.8-12 For the TOGI project, an acoustic clamp-on device has been developed after preliminary studies at CMI.

The sand monitor for the TOGI project gives a qualitative estimate of sand production and requires the placement of the AE transducers in a bend on the production flow line as described in Reference 9. The topside flow lines have target caps as shown in Fig. 10, and therefore do not have any smooth bends.

The sand monitor for the TOGI project has been primarily developed for detection of sand in subsea flow lines transporting gas. The monitor has a specially designed processing unit placed in a waterproof package which can be located to a depth of 400 m subsea.

The measurement sequence is initiated by a topside control computer, which can be the TOGI well controller or a dedicated topside computer. Because of the elaborate AE-transducer design and the processing methodology, the sand monitor for the TOGI project is a higher-cost topside alternative to the clamp-on AE sand-monitoring system described in this article.

ACKNOWLEDGMENT

The authors acknowledge the assistance of Atle Johannessen, Gunnar Wedvich, and Svein Vaerholm in developing the software and hardware needed in the clamp-on AE sand-monitoring system. We are grateful to Rune Norheim of Fluenta A/S for providing the site photos and some of the plots used in this article.

Further thanks are due to Robin Monk and Simon McWade of AV Technology Ltd. who assisted us in developing the AE sensors needed for our specific purpose.

Statoil A/S and Fluenta A/S financed the preliminary research and development activities and the prototype development.

REFERENCES

  1. Eknes, E., Heggstad, S., Wedvich, G., and Zachariassen, L.O., "Deteksjon av sand i gasstrom," Confidential report prepared for Norsk Hydro A/S, CMI No. 841217-1, Chr. Michelsen Institute, Bergen, Norway, 1985.

  2. Mylvaganam,K.S., "Development and Production of nonintrusive sensors for detection of sand in production pipes," Confidential proposal, Chr. Michelsen Institute, Bergen, Norway, February 1986.

  3. Folkestad, T., Hauge, R., Johannessen, A.A., and Mylvaganam, K.S., "Deteksjon av sand i olje. Vurdering av prinsipp for ikkeinntrengende sanddetektor," Confidential report prepared for Fluenta A/S, MI No. 861405-1, Chr. Michelsen Institute, Bergen, Norway, April 1987.

  4. Folkestad, T., Hauge, R., Johannessen, A.A., and Mylvaganam, K.S., "Ikkeinntrengende detektor for sand i olje-strAom," Confidential proposal to Fluenta A/S, CMI No. 861405-2, Chr. MIchelsen Institute, Bergen, Norway, May 1987.

  5. Folkestad, T., Hauge, R., Johannessen, A.A., and Mylvaganam, K.S., "Development of a Clamp-on Sensor for Detection of Sand in Flowing Oil," Confidential internal report, CMI No. 8614053, Chr. Michelsen Institute, Bergen, Norway, June 1987.

  6. Folkestad, T., "Development of a clamp-on sand indicator for offshore use," Confidential proposal to Fluenta A/S, CMI Proposal No. 4/88, Chr. Michelsen Institute, Bergen, Norway, February 1988.

  7. Illyes, A., "Die Energiewandlung bei ein und mehrmaligem Stoss einer Partikel gegen eine feste Wand," Berlin, Techn. Univ., Dissertation, 1986.

  8. Dahl, P. R., " Prototype Sanddetektor for TOGI." Offshore Hitech '89 Conference, Norsk Petroleum-steknisk Forening, June 7-8, 1989.

  9. Dahl, P. R., "Development of Prototype Sand detector for TOGI," Conference on Subsea Separation and Transport, Oslo, Norway, Mar. 2, 1989.

  10. Fossum, T. G., "Sand Detector for Subsea Installations," Conference Subsea 89, London, Dec. 56, 1989.

  11. Haugane, O., "Acoustic Sand Detection," Conference on Subsea Separation and Transport, Oslo, Norway, Oct. 6, 1987.

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