Détection et caractérisation de bouchons dans des pipelines à l aide d ondes guidées

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Détection et caractérisation de bouchons dans des pipelines à l aide d ondes guidées More info about this article: http://www.ndt.net/?

les conditions de température et de pression peuvent conduire à la formation de bouchons d hydrates dans les pipes pendant les périodes d arrêt de production.

1 Détection et caractérisation de bouchons dans des pipelines à l aide d ondes guidées More info about this article: RESUME Bastien Chapuis 1, Frédéric Jenson 1, Laurent Pomié 2 1 CEA-LIST, Gif-sur-Yvette, France 2 TECHNIP, France Lors de l exploitation de gisements d hydrocarbures à grandes profondeurs, les conditions de température et de pression peuvent conduire à la formation de bouchons d hydrates dans les pipes pendant les périodes d arrêt de production. Les exploitants cherchent à minimiser l utilisation de produits chimiques qui sont aujourd hui nécessaires pour éviter la formation de ces bouchons. Technip travaille à la mise au point d une technologie active de chauffage du pipe à l aide de câbles chauffants disposés autour du pipe et protégés par un second pipe qui contient le premier (design dit de «Pipe in Pipe») pour remédier au colmatage de ligne par bouchon d hydrates et/ou les dissoudre. Afin d éviter des conséquences catastrophiques en cas de fonte non maitrisée du bouchon (poche de gaz conduisant à l explosion du pipe ou détachement brusque du bouchon sous l effet de l importante pression amont) des études sont conduites dans le cadre d une collaboration entre Technip et le CEA. Des expériences de détection, localisation et caractérisation de bouchons de glace (supposés représentatifs de bouchons d hydrates au niveau acoustique dans un premier temps) ont été effectuées sur la ligne du CETIM à l aide d un système magnétostrictif émettant des ondes guidées à plusieurs fréquences. Il a ainsi été montré que la détection et la caractérisation (épaisseur de glace déposée sur la paroi du pipe) du bouchon est possible à grande distance. INTRODUCTION Hydrates can form at pressures and temperatures found in natural gas and oil pipelines, causing blockages. Several mitigation techniques and subsea architectures can be applied in order to avoid or to minimize the risk of forming hydrates. The most common technique today, during production and shutdown, is the use of thermodynamic inhibitors injected in the flowline. The required concentration of these inhibitors is very low and their performance quite good, but they are very toxic. Technip has developed an Electrically Trace Heating Pipe-in-Pipe (ETH-PIP, Figure 1) for the purpose of transporting gas and liquid production fluids. This technology can be used to keep the temperature high enough in the pipe to prevent hydrate formation during normal production or shutdown and thus protects against the risk of plugging lines without the use of toxic inhibitors.

In order to correctly operate such ETH-PIP and to avoid the risk of local pressure build-up or plug run-away (Figure 2), a good knowledge of the exact location and shape of the plug (in order to

The present paper presents the feasility and performances assessment of a non destructive and non intrusive technology based on guided waves propagation in the inner pipe to locate and to

In a first part, simulations to understand the mechanism of plug detection using guided waves and to identify the best configuration are presented.

2 In order to correctly operate such ETH-PIP and to avoid the risk of local pressure build-up or plug run-away (Figure 2), a good knowledge of the exact location and shape of the plug (in order to evaluate the volume of gas that will be released during plug melting) is required. Figure 1: Electrically traced Heating Pipe in Pipe. Figure 2: Hazardous event during hydrate plug melting. The present paper presents the feasility and performances assessment of a non destructive and non intrusive technology based on guided waves propagation in the inner pipe to locate and to characterize the hydrate plug. This work has been realized in the framework of a collaboration bewteen Technip and the CEA. In a first part, simulations to understand the mechanism of plug detection using guided waves and to identify the best configuration are presented. Then, experiments of ice plug detection at a real scale are described. Ice plugs are supposed, in a first approximation, to be representative of hydrate plugs from an acoustical point of view but are simpler to manipulate. 1. GUIDED WAVES SIMULATIONS a. Guided waves theory It is not the purpose of this part to introduce the theory of guided waves in pipes; however some key notions are necessary to understand the underlying choices made for the numerical simulations are briefly presented. The first notion is the concept of modes. In a pipe an infinite number of elastic guided wave modes can propagate in the pipe wall. These modes are grouped in three families described in Figure 3. Torsional and longitudinal modes have

method generally consists in selecting the most appropriate mode and the frequency for which it will be the most sensitive to the defect.

3 axisymmetric shapes, whereas flexural modes are not axisymmetric, and are rarely used in practice since are more complicated to exploit. Each mode has different properties: propagation velocity, displacement shape, attenuation, sensitivity and selectivity to the defect considered The first step in the definition of a guided wave NDT method generally consists in selecting the most appropriate mode and the frequency for which it will be the most sensitive to the defect. Figure 3: Families of guided waves modes propagating in a pipe. The second characteristic of guided waves is the dispersion phenomenon: the wave propagation velocity depends on frequency as described in Figure 4. This phenomenon can limit the propagation range and should be quantified by simulation when it cannot be avoided. Figure 4: Dispersion curves of a pipe (axisymmetric modes only): Blue curves for longitudinal modes, red curves for torsional modes. b. Simulation of guided wave mode propagation in OD 4 pipe with an ice plug Numerical simulations have been performed in order to determine the best operational GW conditions for hydrate plug detection, i.e. the mode and the operating frequency most sensitive to the plug (maximizing the reflection coefficient on the plug) and the most selective (not affected by other parameters like liquid content on the pipe, in order to limit false alarms). Simulations have been performed using the module dedicated to guided waves of the simulation software CIVA (CIVA GW v10 [1]). This module allows: The computation of the elastic modes that are likely to propagate in planar and tubular waveguides (dispersion curves);

The computation of the guided wave field generate by a probe (in order to optimize the generation and detection of a given guided mode by a probe); The simulation of the signal (Ascan) that would be

18 mm steel pipe (V L = 5900, V T = 3240 m/s, = 7.9 kg/m 3 ), with or without ice plug (V L = 3944 m/s, V T = 2029 m/s, = 0.92 kg/m 3 ).

4 The computation of the guided wave field generate by a probe (in order to optimize the generation and detection of a given guided mode by a probe); The simulation of the signal (Ascan) that would be obtained in a real inspection (simulation of the emission, propagation, interaction with a flaw, detection). The simulations have been performed for a OD 4 WT 3.18 mm steel pipe (V L = 5900, V T = 3240 m/s, = 7.9 kg/m 3 ), with or without ice plug (V L = 3944 m/s, V T = 2029 m/s, = 0.92 kg/m 3 ). Several simulations were performed to find the most suitable frequency and signal response analysis in reflection for different conditions of pipe clogging. The best candidate for long range detection is the first torsional mode T(0,1) mode due to its elastic characteristics (not dispersive, purely tangential displacement) and the availability of commercial systems for generating/detecting this mode. As explained in literature, the optimal frequency for detecting a hydrate plug using T(0,1) mode and reflection measurements is at the cut-off frequency of the second torsional mode [2] & [3]. This frequency depends on the thickness of hydrate deposit on the pipe. The optimal frequency decreases when the hydrate layer thickness increases. For the 4 OD pipe of the CETIM loop which will be used for experimental validation, presented in the next section, the working frequency is around 50 khz (Figure 5 and Figure 6). With T(0,1) mode at low frequency we could expect to determine the position of the plug with a precision of around 20 cm in the most favorable case. The plug thickness detection threshold will be very dependent on the acoustic impedance mismatch (thus on the mechanical properties of plug: density and wave velocities and plug front face shape) and the signal to noise ratio of the acquisition system. Figure 5: Dispersion curve for a full plug. The second torsional mode is indicated as T2.

5 Figure 6: Result for 50% clogging. 2. EXPERIMENTAL RESULTS a. Experimental set up Experimental guided wave detection of ice plugs have been performed on a pipe of the CETIM loop in Senlis, France (Figure 7). The pipe used for the experiments has the following characteristics: Outer diameter: 4 Wall thickness: 3.18 mm Length: 24 m (4 x 6 m sections welded together) Material: Steel (V T = 3240 m/s) No coating. Ice plug formation has been performed using the Qwik-Freezer system. The system is composed of a jacket fitted around the pipe. Liquid CO 2 contained in cylinders is injected in the jacket and forms dry ice around the pipe. The dry ice, whose temperature is -78 C, locally cools the water in the pipe and forms the ice plug. The plug is approximately 50 cm long.

jacket liquid CO 2 dry ice Figure 7: Experimental setup for ice plug creation of the CETIM loop.

Magnetostrictive sensors (MsS) from the Southwest Research Institute (SwRI) available at CETIM have been used and allowed to work on a large frequency range.

A typical pulse-echo signal using one sensor at the extremity of the flowline is presented in Figure 8.

6 jacket liquid CO 2 dry ice Figure 7: Experimental setup for ice plug creation of the CETIM loop. Five sensors have been installed the flowline at different locations to determine the maximum detection range. Magnetostrictive sensors (MsS) from the Southwest Research Institute (SwRI) available at CETIM have been used and allowed to work on a large frequency range. During our experiments, central frequencies from 32 khz to 180 khz have been tested to cover the range of frequencies identified during numerical simulations. A typical pulse-echo signal using one sensor at the extremity of the flowline is presented in Figure 8. Even in absence of the plug, several echoes allow determining geometrical discontinuities of the flowline (sensors, welds, pipe end). An endoscope has been introduced inside the pipe to verify the geometry of plug extremities for one ice plug. The water inside the pipe has been removed for this operation. It showed that the ice plugs extremities were steep (Figure 9) which is a favorable case for the detection. Figure 8: Typical pulse-echo signal using sensor C5 emitting in the left direction.

7 Figure 9: Photographs of the ice plug taken with the endoscope after removal of the water. b. Detection and location of the ice plug using guided waves Pulse-echo measurements have been used to detect the several ice plugs created during the experiments. The formation of the plug lasted about 2 hours, the signals measured at the different frequencies have been collected at a regular basis during all the experiment. The signals collected at 45 khz central frequency for a plug located 1.5 m from the sensor C1 is presented in Figure 10. At T0, just before the first injection of CO 2 in the jacket, echoes from the 3 welds of the flowline are visible as well as the pipe end, located 23 m away. 8 minutes after the first injection of CO2 a signal indicating the presence of a new element can be detected 1.5 m from sensor C1. 2 hours later the plug is completely formed, the signal is attenuated through the plug and the echoes from the welds and the pipe end, located after the plug, are no longer visible. A final signal is recorded after removal of the jacket and the dry ice around the pipe. The echo is therefore only due to the plug, which is correctly detected and located. Figure 10: Short range plug detection. The time (in minutes) after the first injection of CO 2 is indicated at the left of the signals. Red arrow: echo from the plug.

After 26 minutes an echo is detected at 20 m from sensor C5 and the pipe end, located after the plug, is no longer visible.

8 Another result is presented in Figure 11 for a plug located 20 m away from the sensor C5 which is used for pulse-echo measurements. As in the previous case, at T0 the echoes from the welds and the pipe end are visible. After 26 minutes an echo is detected at 20 m from sensor C5 and the pipe end, located after the plug, is no longer visible. A final signal recorded after removal of the jacket and the dry ice around the pipe demonstrates that the plug is correctly detected and located 20 m away from the sensor with a signal over noise ratio of at least 14 db. Due to the length of the flowline, 20 m is the maximal distance that has been tested but an higher detection range can be expected. Figure 11: Long range plug detection. The time (in minutes) after the first injection of CO 2 is indicated at the left of the signals. Red arrow: echo from the plug. c. Plug form characterization The discrimination between a full plug and a crust lying at the bottom of the pipe has been studied comparing the response of a full plug and the response of a partial plug, obtained in a pipe half filled with water. In that case the plug was located at 8.4 m from sensor C2. The signals at the end of the experiments (T116) for the frequency 45 khz is presented in Figure 12. The discrimination between a plug and a crust can therefore be based on the measurement of the echo amplitude.

Figure 12: Difference in the echo amplitudes between a plug and a crust. Red arrow: echo from the plug.

First, simulations using CIVA software have been used to determine the best configuration for such inspection.

fluid. The operating frequency has been optimized to maximize the detectability of the plug.

9 Figure 12: Difference in the echo amplitudes between a plug and a crust. Red arrow: echo from the plug. CONCLUSION The results of a feasibitlity program of plug detection, location and characterization using guided waves have been presented. First, simulations using CIVA software have been used to determine the best configuration for such inspection. The torsional mode T(0,1) has been selected due to its elastic characteristics (not dispersive, purely tangential displacement) allowing a long propagation range in a pipe filled with an internal fluid. The operating frequency has been optimized to maximize the detectability of the plug. In a second part, some experiments at real scale have been performed to demonstrate the capability of guided waves to detect and to locate an ice plug (supposed, in a first approximation, to be representative of hydrate plugs from an acoustical point of view) located several meters from the sensor. Guided waves are also sensitive to the shape of the plug and can discriminate a full plug against a crust lying at the bottom of the pipe. ACKNOWLEDGEMENT The authors would like to thank the CETIM M. Berthelot and M. Emmanuel for their valuable help during the experiments. REFERENCES [1] CIVA CEA LIST Extende website: [2] Ma, J., On-line measurements of contents inside pipes using guided ultrasonic waves, PhD thesis, Imperial College, 2007 [3] Ma, J.; Lowe, M. & Simonetti, F., Feasibility study of sludge and blockage detection inside pipes using guided torsional waves, Measurement Science and Technology, 2007, 18,

Hydrate plug localization and characterization using guided waves

11th European Conference on Non-Destructive Testing (ECNDT 2014), October 6-10, 2014, Prague, Czech Republic Hydrate plug localization and characterization using guided waves More Info at Open Access Database