Hydrate plug localization and characterization using guided waves

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1 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 Bastien CHAPUIS 1, Vahan BARONIAN 1, Frédéric JENSON 1, Laurent POMIE 2 1 CEA, LIST, Gif-sur-Yvette, France bastien.chapuis, vahan.baronian, 2 Technip, Rueil-Malmaison, France Abstract In some conditions of temperature and pressure, the exploitation of hydrocarbon deposits in deep waters can lead to the formation of hydrate plugs during shutdown operations. The operators try to decrease the use of chemical products necessary today to avoid the formation of such plugs. Technip is developing an active technology of pipe heating through the use of heating cables arranged around the pipe and protected by a second pipe around it (design called Pipe in Pipe ). These cables can be used to avoid the formation of the plug and, if necessary, to dissolve it. In order to avoid catastrophic failures in case of uncontrolled plug melting (pocket of gaz which could lead to pipe explosion or sudden plug dissociation of the plug from the pipe due to the important upstream pressure), a non-destructive and non-intrusive technology is necessary to locate and to characterize the hydrate plug. Studies are therefore performed to determine the capabilities of guided wave technology to provide such information. In this paper, we present a feasibility program that encompasses numerical simulations using CIVA software and experimental tests on ice plugs that are supposed, in a first approximation, to be representative of an hydrate plug from an acoustical point of view. Keywords: guided waves, plug detection, pipes, finite-element simulation, SAFE 1. Introduction In some conditions of temperature and pressure, the exploitation of hydrocarbon deposits in deep waters can lead to the formation of hydrate plugs during shutdown operations. The operators try to decrease the use of chemical products necessary today to avoid the formation of such plugs. Technip is developing an active technology of pipe heating through the use of heating cables arranged around the pipe and protected by a second pipe around it (design called Pipe in Pipe ). These cables can be used to avoid the formation of the plug and, if necessary, to dissolve it. Catastrophic failures can result of uncontrolled plug melting (pocket of gaz which could lead to pipe explosion or sudden plug dissociation of the plug from the pipe due to the important upstream pressure). It is therefore necessary to obtain information about the plug (location of the plug, quantity of hydrate, ). Some studies are performed by Technip and the CEA in order to determine the capabilities of guided wave technology to detect and to characterize an hydrate plug in a pipe. The feasibility program encompasses numerical simulations, using CIVA software and experimental tests on ice plugs that are supposed, in a first approximation, to be representative of an hydrate plug from an acoustical point of view.

In a first part, numerical simulations of guided wave interaction with a plug are presented.

In a second part, experimental tests of ice plugs detection using guided waves are presented. 2.

in the pipe [1-3]. It has been shown that a maximal reflection occurs at the cut-off frequency of T2 mode. T2 mode is the second torsional mode of the pipe with the solid layer (bi-layered waveguide).

2 In a first part, numerical simulations of guided wave interaction with a plug are presented. These simulations show the physical phenomenon involved in the plug detection and characterization and allow optimizing the experimental setup. In a second part, experimental tests of ice plugs detection using guided waves are presented. 2. Numerical simulations: interaction of guided waves with a plug Studies of Imperial College researchers have described the interaction of the first torsional mode of a pipe T(0,1) with a solid layer in the pipe [1-3]. It has been shown that a maximal reflection occurs at the cut-off frequency of T2 mode. T2 mode is the second torsional mode of the pipe with the solid layer (bi-layered waveguide). At this frequency, the incident T(0,1) mode (for which energy is totally located in the pipe wall) meets the bi-layered structure for which mode shapes present a strong impedance mismatch: all the energy is located in the solid layer. Some simulations have been performed at CEA in order to confirm this phenomenon and the interest of using the cut-off frequency of T2 mode for experiments. The model, implemented within CIVA GW 11 [4], couples a semi-analytical approach with a finite elements box around the perturbation of the waveguide thanks to a transparent boundaries formulation [5]. Only a limited zone around the transition between a mono-layered pipe and a bi-layered pipe must be meshed (Figure 1), the simulations are therefore very fast. Figure 1: Configuration and mesh for an infinite plug. Simulations have been performed for different plug thicknesses (Figure 2). Figure 2: Different plug thicknesses have been simulated. The plug thickness is expressed as a percentage of the inner radius of the pipe.

The results are presented in Figure 3. The reflection coefficient between 10 khz and 50 khz for the different plug thicknesses is plotted. Several sharp but strong reflections are identified.

3 The results are presented in Figure 3. The reflection coefficient between 10 khz and 50 khz for the different plug thicknesses is plotted. Several sharp but strong reflections are identified. As described in Imperial College results, these reflections occur at the cut-off frequencies of Tn modes, the modes of the bi-layered structure (see for instance the dispersion curves superimposed for the 75% thickness case on Figure 3). These results, combined with mode shapes at the cut-off frequencies of Tn modes, confirm the interaction mechanism of T(0,1) mode with a solid layer in a pipe identified by the Imperial College team. Figure 3: Reflection coefficient of a T(0,1) incident mode on an infinite ice plug of different thicknesses (6 OD, 7.11 mm WT pipe). The maximal reflection frequency therefore depends on the thickness of hydrate deposit on the pipe. The maximal reflection frequency decreases when the ice layer thickness increases. 3. Experimental demonstration at real scale 3.1. Experimental set up The experiments have been performed on a pipe of the CETIM loop in Senlis, France (Figure 4). 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 is performed using the Qwik-Freezer system. The system is composed of a jacket fitted around the pipe (Figure 4).

The plug is approximately 50 cm long and its formation takes approximately 2 hours with cycles of 8 minutes of injection of CO 2 and 10 minutes of waiting time, during which the guided wave

4 Material: Steel (V T = 3240 m/s) No coating. Ice plug formation is performed using the Qwik-Freezer system. The system is composed of a jacket fitted around the pipe (Figure 4). Liquid CO 2 contained in cylinders is injected in the jacket and forms dry ice around the pipe. The dry ice, at a temperature of -78 C, locally cools the water in the pipe and forms the ice plug. The plug is approximately 50 cm long and its formation takes approximately 2 hours with cycles of 8 minutes of injection of CO 2 and 10 minutes of waiting time, during which the guided wave acquisitions have been performed. Figure 4: Experimental setup for ice-plug formation. Magnetostrictive sensors (MsS) from the Southwest Research Institute (SwRI) have been used and allowed to work on a large frequency range. During these experiments, central frequencies from 32 khz to 180 khz have been tested to cover the range of frequencies identified during numerical simulations. The sensors have been used in pulse-echo operation mode (Figure 5). The time axis of the signal (Ascan) is transformed into distance thanks to the knowledge of the wave propagation velocity. Thus, the singularities that generate echoes can directly be located. Moreover, 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 6), which is a favourable case for the detection.

Figure 5: Typical pulse-echo signal using sensor C5 emitting on the negative (left) direction. The geometrical elements on the pipe (welds, other sensors, pipe end) are clearly visible on the signal.

Detection and location of the ice plug using guided waves Guided waves capabilities for the hydrate plug detection and characterization have been evaluated through different types of experiments on

5 Figure 5: Typical pulse-echo signal using sensor C5 emitting on the negative (left) direction. The geometrical elements on the pipe (welds, other sensors, pipe end) are clearly visible on the signal. Figure 6: Photographs of the ice plug taken with the endoscope after removal of the water Detection and location of the ice plug using guided waves Guided waves capabilities for the hydrate plug detection and characterization have been evaluated through different types of experiments on several plugs. During the experiments the signals at different frequencies have been acquired regularly during the formation of the plug. The position of the plug front face is determined from the time of flight of the echo. Figure 7 and Figure 8 give two examples of signals at low (1.5 m) and large (20 m) distance. The signal T0 is measured before the first injection of CO 2 in the jacket, T118 is a signal after the last injection (118 minutes after the beginning of the experiments), when the plug is totally formed. A last signal is presented: it is measured just after T118, after the removal of the jacket and the dry ice around the pipe. A few minutes after the first injection of CO 2, the echo coming from the plug is visible. It allows to detect the apparition (low thickness of ice deposited on the pipe wall) and the position of the plug. The comparison with the signal T118 and T118 without dry ice shows that the T(0,1) mode is also sensitive to the dry ice produced to form the plug. It was not obvious since the dry ice is not firmly bonded to the pipe wall. However, the pressure created inside the jacket is sufficient to have a noticeable effect. It can create difficulties in the interpretation of the

6 signals acquired during the formation of the plug, since the echo cannot, strictly speaking, be attributed to the plug only but also to the dry ice. 20 m is the maximal distance between plug and sensor achievable on CETIM loop. It is clear with the experiments presented here that the plug can be detected and located at such distance and probably at larger distances if necessary since the signal over ratio is higher than 14 db at 20 m. Figure 7: Short range detection (1.5 m) using sensor C1 emitting in the positive direction. The echo from plug is indicated with the red arrow. Time (in minutes) from the first injection of CO 2 is indicated on the left of each signal. Figure 8: Long range detection (20 m) using sensor C5 emitting in the negative direction at 45 khz. The echo from plug is indicated with the red arrow. Time (in minutes) from the first injection of CO 2 is indicated on the left of each signal.

7 4. Conclusion Results of a demonstration program of guided wave capabilities to detect hydrate plugs in a pipe have been presented. This study encompasses numerical simulations, using CIVA software (guided wave module) and experimental tests on ice plugs that are supposed, in a first approximation, to be representative of an hydrate plug from an acoustical point of view. It has been shown that guided mode T(0,1) can be used to detect and to locate the plug located several meters away from the sensors. This study is still ongoing. The influence of the presence of a pipe elbow on the plug detection capability is being tested, both experimentally and using simulation. The next step will be the detection of real hydrate plugs to demonstrate that the methodology developed with ice plugs can be transferred to hydrate plugs. Acknowledgments The authors would like to thank the CETIM F. Berthelot and K. Emmanuel for having given access to their experimental facilities and for their help during the experiments and M. El Bakkali (CEA) for his assistance on the experimental part of this study. References 1. Ma, J., On-line measurements of contents inside pipes using guided ultrasonic waves, PhD thesis, Imperial College, 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, Ma, J.; Simonetti, F. & Lowe, M., Scattering of the fundamental torsional mode by an axisymmetric layer inside a pipe, Journal of the Acoustical Society of America, 2006, 120, CIVA GW 11 CEA LIST Extende website, 5. Baronian, V.; Bonnet-Ben Dhia, A. S. & Lunéville, E., Transparent boundary conditions for the harmonic diffraction problem in an elastic waveguide, Journal of Computational and Applied Mathematics, 2010, 234,

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