acoustic reflectometry for blockage detection in … · 2017. 10. 20. · acoustic reflectometry...

ACOUSTIC REFLECTOMETRY FOR BLOCKAGE DETECTION IN PIPELINE

JOSÉ L. A,VIDAL

Petrobrás Research Center - CENPES/PDEP/TOOL

Av.Horácio de Macedo 950- Cidade Universitária – 21941-915 -Rio de Janeiro-RJ

E-mail:[email protected]

LUCIANA L,SILVA

Ocean Engineering Department, Federal University of Rio de Janeiro, COPPE/UFRJ

Cidade Universitária – Bloco C - Ilha do Fundão— 21945-970- Rio de Janeiro-RJ

E-mail: [email protected]

Abstract-- Flow assurance is an important aspect of offshore, particularly deepwater pipeline design and operation, since one of

the critical issues is the eventual initiation and growth of hydrate or paraffin blockages under certain conditions. Ideally, operators

would benefit from online information regarding position and extent of an eventual blockage in a pipeline.The aim of this work is

to apply acoustic technology to design and make a prototype that can be used in a pipe to efficiently identify and measure

blockages. The technique uses a short duration sound pulse that is injected into the pipe. When the acoustic pulse encounters an

impedance discontinuity, a portion is reflected back towards the acoustic source and microphones or hydrophones. Analysis of the

measured signal reflections can provide valuable data related to location and size of the blockages. The experiments were

numerically reproduced with good correlation proving the potential of the technique.

Keywords—Blockage Detection, Pipeline, Flow Assurance, Hydrate, Acoustic Pulse, Reflectometry, Acoustic Technic

1 Introduction

Hydrate, paraffin wax and incrustation deposition in

oil and gas transport pipelines are an important challenge

for the development of deepwater environments. These

types of deposition always occurs along the inner wall of

the pipeline, which reduces the internal diameter and

eventually results in the blockage (N.X. Thanh,1999;

K.A. Papadopoulou,2008). During the service life,

pipelines should not fail because such failures could lead

to environmental, human, and economic costs. In this

context, flow assurance is critical for deepwater projects

in order to avoid pipeline blockages. Therefore detection

and localization of the formation of blockages at its early

stage is very important for remedial actions before

catastrophic effects occur. In recent years, detection of

blockage in pipelines has been an attractive area of

research and several methods have been proposed (K.A.

Papadopoulou,2008; O.A. Olugboji,2011). The

conventional methods for blockage detection include

flow pressure monitoring detection, thermographic,

radiographic methods, and others (K.A.

Papadopoulou,2008; O.A. Olugboji,2011; X. Chen,2007;

M.S. Ghidaoui,2005). Flow pressure monitoring

detection uses quick-acting valve to generate a water

hammer and the the reflections produced by any

blockage can be recorded. This method

has its disadvantages, like the high transient pressure

may cause considerable damage to the pipeline.

Thermographic and radiographic methods typically

require the use of a Remotely Operated Vehicle (ROV)

and needs to be near the location and these methods are

expensive and time-consuming operations.

The acoustic pulse reflectometry, a non-invasive

measurement technique and remotely operated, has been

shown to be an appropriate tool to solve this problem.

The technique is operated by injecting a small acoustic

signal into the fluid within the pipeline to remotely

detect blockages, and several components, such as

valves, flanges and T-sections located at long distances,

based on the analysis of reflected acoustic signals, as

these reflections carry sufficient information to identify

blockages and components (K.A. Papadopoulou,2008;

B.J. Forbes, 2003; A. Tolstoy, 2009; G.O. Albors,2007).

When the acoustic pulse encounters an impedance

discontinuity, a portion of the pulse is transmitted past

the discontinuity and a portion is reflected back towards

the acoustic source. Acoustic reflections are then

measured and analyzed to estimate the location of

impedance discontinuities within the systems under

investigation, also a sound wave is travelling inside a

pipe and the wavelength is large compared with the

diameter of the tube, wave propagation is very nearly

one-dimensional. That is, waves inside the pipe can be

considered to be plane waves. These waves can travel for

large distances with low attenuation (J.M. Hovem,2007).

Anais do XX Congresso Brasileiro de Automática Belo Horizonte, MG, 20 a 24 de Setembro de 2014

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This paper provides details of the results that have been

obtained from experiments tests and numerical model

that have been designed to determine the capability of

using the acoustic technique to detect blockages in

pipeline systems.

2 Acoustic Technique Background

The three dimensional acoustic wave equation

expressed in cylindrical coordinates is (P.M. Morse,1968;

N.R. Harris,1997):

∇2𝑃 =1

𝑟

𝜕

𝜕𝑟(𝑟

𝜕𝑃

𝜕𝑟) +

1

𝑟2(𝜕2𝑃

𝜕𝜃2) +

𝜕2𝑃

𝜕𝑧2=

1

𝑐2𝜕2𝑃

𝜕𝑡2 (1)

where P is acoustic pressure, t is time, c is the speed of

sound and r, and z are respectively radial, angular and

longitudinal variables. Assuming a variable separable

solution, P(r, , z, t)=R(r)()Z(z)T(t). The acoustic wave

equation for cylindrical symmetry can be solved to give a

general solution:

P(𝑟, 𝑧, 𝑡) = 𝐴 J (𝑘 𝑟)e

( ) (2)

where J (𝑘𝑟) is a Bessel function of zero order, kb is a

constant and 𝑘 is the exponential is a propagation

constant (wave number). For a pipe of radius a, a

boundary condition can be applied at r=a, In the case of

rigid wall the condition to be applied is that the particle

velocity normal to the wall is null, (𝜕𝑃 𝜕𝑟) = 0⁄ . When

analysing sound propagation in pipes it can often be

assumed that the wave front is uniform throughout the

cross-section. This assumption is limited to those cases

where the frequency f is below the cut-on frequency for

the first non-plane mode. For circular cross-sections this

limits given by the first zero of J1′ , the derivative of the

Bessel function of the first kind, as given by

𝑓 <1.84𝑐

2𝜋𝑟 (3)

For a pipe with internal diameter of 4'' filled the air, the

cut-off frequency is 1.96kHz.

3 Methodology

To test the acoustic system to locate and measure

blockages, tests were carried out in a small-scale pipeline

located in the Subsea Technology Laboratory at the

Federal University of Rio de Janeiro. An experimental

setup with a pipe of 4” internal diameter and length of

approximately 95m was constructed and composed of

several spools sections, each 500mm long, located in

different positions of pipeline to insert blockages, see

Figures 1 and 2. The tests can be performed in air and/or

water using different sizes of blockages and in different

positions. Paraffin blockages were made to simulate

different stages of obstruction, which covered

approximately 10, 25, 50 and 75% of the pipe's area, as

showed in Figure 3. For each were made three different

lengths 100mm 250mm and 500mm. The signal

generated by an acoustic pulse generator was passed

through an amplifier to drive a loudspeaker located at

one end of the pipeline, which transmitted the signal into

the fluid in the pipe. One microphone (BSWA model

MPA416) was mounted at a distance of 6.4 m from the

acoustic emitter. This microphone captures the incident

pulses and reflections returning from the blockages

under test, and the measurements were recorded using a

fit-for-purpose data acquisition system with sampling

rate of 100kS/s per channel. The ideal frequency content

of the acoustic signal injected into the pipe is the subject

of current research. However, in this work a short period

square pulse, with a width of 1ms, has been used.

Figure 1: Pipeline Layout (pipe of 4” internal diameter and

length of 100m). Above is schematic view and below is frontal

view with dimensions.

Figure 2: Zoom in the region of spool for insertion of

blockages.


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Figure 3: Different sizes of paraffin blockages with 10, 25, 50

and 75% of the pipe's area. The percentage in this figure refers

to the percentage of the pipeline area that will be blocked.

In parallel with the experimental program, finite

element model using ABAQUS (version 6.9-1) were

developed using three-dimensional finite element model

that simulate acoustic wave propagation. The internal

fluid was discretized with 3-D acoustic elements

(AC3D4), see Figure 4. This element is defined by four

nodes with one degree of freedom at each node: pressure.

The reference pressure is used to calculate the sound

pressure level. The acoustic pressure in the fluid is

determined by the wave equation

1

𝑐2𝜕2𝑃

𝜕𝑡2− ∇2𝑃 = 0 (4)

P is the sound pressure, t is the time and c the sound

speed. There are some assumptions: the fluid is

compressible, the fluid is inviscid (no viscous

dissipation) and the density is uniform throughout the

fluid. The appropriate size of the elements is chosen in a

manner so that the propagating waves are spatially

resolved. In (F. Mosera,1999 ), it is recommended that

more than 10 nodes per wavelength be used, can be

expressed as

𝑙𝑒𝑙𝑒𝑚 =𝜆𝑚 𝑛

10 (5)

where 𝑙𝑒𝑙𝑒𝑚 is the element length and 𝜆𝑚 𝑛 is the shortest

wavelength of interest. The Figure 4 shows, for better

visualization, a zoom of the fluid mesh. The blockages

were simulated using a reduction of the cross-

sectional area of the pipe. We assume only plane waves

propagate through the fluid and in the case the outer

boundary of the internal fluid is represented by a non-

reflecting acoustic impedance. The acoustic pressure–

time input (from an acoustic emitter) used in the

numerical analysis was obtained from experimental tests.

Figure 4: Finite element mesh with zoom in the region of the

blockage.

4 Experimental Results

The laboratory results are presented for pipeline filled

with air. A typical normalized intensity plot with distance

for a pipe with no blockage is shown in Figure 5 (unit of

pressure Pa). It shows a significant reflected intensity

caused by the end of at 88 m from microphone, with no

significant reflections along the rest of the pipe. The

Figure 6 shows the results for the same pipe but with a

blockage of 75% of area and length of 500mm that is

placed in different positions on the pipe, 12.5m, 31.5m,

41.0m, 54.5m, 83.0m from the microphone. In all cases,

it is possible to locate the blockage and the farther the

blockage, the lower the intensity of the reflected signal

due to attenuation factor, which quantifies the rate of

exponential decay that occurs with the propagation. It is

also possible to see the multiple reflections from tests

performed on blockages closer to the microphone.

10% 25% 50% 75%


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Figure 5: Response with the pipe without blockages.

Figure 6: Response of the pipe with blockage of 75% of

area and length of 500mm placed in different positions

on the pipe, 12.5m, 31.5m, 41.0m, 54.5m, and 83.0m

from the microphone.

Tests were performed with all the blockages placed in the

same position in the pipe 12.5m from microphone.

Figure 7(a) shows the normalized pressure results due to

reflection from blockages. All obstructions have the

same length of 500mm but simulate different stages of

blockage (10, 25, 50 and 75% of the area’s pipe). When

comparing results of different blockages, while the effect

of each obstruction is visible, the differences between

obstructions are only very slightly visible. Figure 7(b)

shows the results in the frequency domain after

performing the FFT of the signals. The differentiation

between them is more pronounced when analyzes the

signals in the frequency domain. The frequency contents

of all is approximately the same between 50Hz to 750

Hz, it shows that as the size of the blockage increases,

the amplitude of the reflected signal increases. This is to

be expected as the length of the blockage will also be

increasing, as can be seen in Figure 8. The blockages

have the same area (10%) and different lengths of

100mm, 250mm and 500mm.

Figure 7: Difference signal for varying blockages area. (a)

Response in time space and (b) frequency analysis of

blockage signals.


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Figure 8: Difference signal for varying blockages length. (a)

Response in time space and (b) frequency analysis of blockage

signals.

The laboratory experiments show that the acoustic

technique is a promising method for detecting pipe

different stages of blockages. The waves propagate at the

local speed of sound and the difference in arrival time

between transmitted waves and reflected waves is used

to calculate the distance from the microphone to the

blockage. The localization errors in all experiments were

between 0.15 and 1.05%. The uncertainty in sound of

speed measurement of the pulse signal can cause the

uncertainty of the location of blockage.

5 Numerical Results

Figure 9 illustrates a sequence of acoustic wave

propagation for a pipe with blockage (75% and 500mm).

The Figure 9(a) shows the behavior of acoustic wave

propagation before it reaches the blockage and the Figure

9(b) shows the result of the wave after passing through

the blockage. It is possible to see that a portion of the

incident wave is transmitted and part is reflected.

Figure 9: Sequence of acoustic wave propagation for a pipe

with blockage (75% and 500mm): (a) acoustic wave before it

reaches the blockage and (b) transmitted and reflected wave

after passing through the blockage.

Therefore, a comparison of numerical results obtained

with experimental results was conducted. Figure 10

shows some results of the comparison between the

numerical and experimental results for the cases where

the blockage is located 12.5m from the microphone and

the sizes of obstructions vary as: (a) 10% and 250mm,

(b) 25% and 500mm, (c) 50% and 250mm and (d) 75%

and 100mm. Results show that the numerical model can

be used to adequately capture the experimental observed

behavior. The numerical model can be used to make

predictions of the behavior of acoustic propagation

different pipelines systems as well as different types of

fluid.

(a)

(b)


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Figure 10: Comparison between the numerical and

experimental results for the cases where the blockage is located

12.5m from the microphone and the sizes of obstructions vary

as: (a) 10% and 250mm, (b) 25% and 500mm, (c) 50% and

250mm and (d) 75% and 100mm.

6 Prototype for Field Testing

After carried out the study to verify the feasibility of

the acoustic technique for detecting blockages, a

prototype was developed for oil and gas industry. High

power acoustic emitters can be mounted as shown in the

Figure 12. This technology will normally be required to

survey long lengths of pipeline and therefore low

frequency signals will be required. The specific signal

that is used should be selected according to the pipe

configuration with a compromise on survey length and

accuracy.

Figure 12: Acoustic emitters mounting arrangement.

The Subsea Technology Laboratory - Federal

University of Rio de Janeiro and Petrobrás jointly

conducted a field test using the acoustic monitoring

system. The tested water-filled pipeline

of 12” internal diameter and length of approximately 5

km. The tests were performed with flow in the pipeline

and at pressurized condition (150 psig). Figure 1 shows a

map showing pipeline system in red color.

Figure 12: Tested pipeline system.


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During these tests, the gas gun was located

approximately 240m of water collection station. Figure

13 shows how the acoustic system was connected to the

pipeline.

Figure 12: Acoustic system connection.

The Figure 13 shows the recorded data assuming a

speed of sound of approximately 1,450 m/s. The incident

signal has 25 ms of duration (highlighted area 1). In the

range between 25 ms and 67 ms it is possible to identify

multiple reflections from the interface between the

mechanical assembly of acoustic system to the pipe

(highlighted area 2). Between 67 ms and 336 ms there is

no significant reflections, just the background noise from

flow (highlighted area 3). After 336 ms it is possible to

see a significant difference in the reflected signals

indicating the existence of any obstacle. The distance

between the test point and the water collector station is

the same seen by the acoustic system, highlighted signal

showed in area 4 (Figure 13).

Figure 13: Field test response.

7 Conclusions

This paper deals with the analysis of acoustic

technique that can be used in a pipeline to efficiently

identify and measure blockages during operation on line

(no shutdown). Experimental tests and numerical

simulations are carried out considering acoustic

propagation behaviors in pipeline showing their

influence on system dynamics. The laboratory and field

results show that the technique can be used for gas pipes

and water pipes with large diameter. Results show that

the system enables remote and continuous monitoring of

pipeline and provides an indication of location and the

relative size of blockage. Tests indicate that the proposed

approach is capable of identifying blockages as small as

10% of the pipe area. It is possible that smaller

blockages can be identified, but tests were not performed

with blockages less than 10% of the area's pipe, but the

numerical model shows that it is possible to identify

obstructions with 1% of pipe area. The prototype for oil

and gas industry showed good results for water-filled

pipeline and it would appear to be a suitable to perform

detections.

Acknowledgments

The authors would like to thank the financial support

from CENPES/PETROBRAS.

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acoustic reflectometry for blockage detection in … · 2017. 10. 20. · acoustic reflectometry...

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