SPIE Proceedings [SPIE Photonics East (ISAM, VVDC, IEMB) - Boston, MA (Sunday 1 November 1998)] Fiber Optic and Laser Sensors and Applications; Including Distributed and Multiplexed

Detection of Flowline Blockage Using Bragg Grating Sensors

John W. Berthold*McDermott Technology, Inc.

1 562 Beeson StreetAlliance, OH 44601

ABSTRACT

Real-time hoop strain measurement at multiple points along a flowline provides a way to monitor changes in the internalpressure gradient within the flowline. This information could be used to provide an early warning that a flow restriction isforming, locate the area of the blockage, and allow early intervention to control or eliminate deposition of material on thepipe wall. In the event of complete flow blockage, the method could be used to precisely locate the blockage.

In this project, we evaluated fiber Bragg grating (FBG) sensors for measurement ofhoop (circumferential) strain present inthe wall of pressure pipe. The tests were performed on a section of pipe to which multiple sensors were attached. Thepurpose of the testing was to characterize the hoop strain resolution, repeatability, and sensitivity and to determine themagnitude of error caused by the presence of bending strain and axial strain. The advantage of FBG sensors for thisapplication is that multiple sensors can be embedded in a single optical fiber which can then be attached to a pipeline. EachFBG sensor is wavelength encoded so that each sensors strain output signal corresponds to a known location along the lengthof the pipeline. Thus, it is possible to identify the location of a blockage in a pipeline by monitoring many locations alongthe pipe for small changes in hoop strain indicative of increased pressure drop.

The presentation will include a description of the tests and analysis of the performance characterization work. The results ofthe tests were positive. Analysis of the data for this application shows that it is possible to achieve submicrostrain resolutionand better than 2 microstrain repeatability with FBG sensors.

Keywords: pressure, hoop strain measurement, pipeline.

1.0 INTRODUCTION AND SUMMARY

The problems associated with wax and paraffm build-up in subsea flowlines are well documented. It is known that when theflowline temperature is significantly below the crude oil temperature, which is generally the case in deep water, paraffm mayprecipitate and deposit on the flowline walls. Various methods, such as chemical injection and mechanical augers may beused to remove the deposited material. The chemical methods can be costly and the mechanical methods are limited in thelength offlowline that can be cleaned. Sometimes flowline plugging occurs, and ifplug removal is not possible, it may benecessary to lift the plugged section from the water, cut it out and replace it with a new piece ofpipe. This latter approach isobviously very costly.

Presently, little effort has been devoted to the measurement of flowline conditions to improve the ability to predict when andwhere wax and paraffm build-up is likely to occur. It is generally known for example, that with particularly waxy crudes,deposition is most likely within a mile or so of the subsea well head. It is also known that the deposits may extend up to1000 feet or so down the length of the flowline. However, it is presently not possible to pinpoint with any degree of certainty(not even 1000 feet) the location of the build-up, or to track the rate ofbuild-up in real time. Both pieces of informationwould be very useful to operators who could use it to decide what types of intervention (chemical, mechanical, pigging, etc.)might be needed.

* Further author information --J.W.B. (correspondence): Email: [email protected]; Telephone: (330) 829-7272; Fax: (330) 829-7832.

Part of the SPIE Conference on Fiber ODtic and Laser Sensors

8 and Applications • Boston, Massachusetts • November 1998SPIE Vol. 3541 • 0277-786X/991$l0.OO

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/10/2013 Terms of Use: http://spiedl.org/terms

The purpose of the work described in this paper was to demonstrate the feasibility of fiber optic sensor technology to providethe important measurements needed to locate paraffm build-up in subsea flowlines. The method tested employscommercially available, multiple fiber optic Bragg grating (FBG) sensors embedded in a single optical fiber. The sensors areattached to the pipe and directly measure hoop strain at each attachment point. From a series of these measurements, apressure profile along the length of the flowlme can be obtained, and changes in this profile with time indicate deposit build-up on the inside surface ofthe pipe wall. The benefits ofthis approach include:

S Large number of gages per fiber with only one signal lead. No underwater power source required. No pipe through-wall penetrations required. Real-time monitor for early warning and to check effectiveness of control and remediation.

Tests were performed on a 1 6-foot section of 4-inch schedule 1 0 pipe to which three FBG sensors were attached. A fourthsensor was installed as a temperature reference. The purpose of the tests was to characterize the hoop strain resolution,repeatability, and sensitivity and to determine the magnitude of error caused by bending strain and axial strain.

2.0 APPROACH

In our test and evaluation effort, an optical fiber was spirally wrapped onto a pipe test section. In a practical application, anoptical fiber encapsulated in a metal sheath could be spirally wrapped around the outside of the flowline between thewellhead and platform as shown in Figure 2-1. The sheathed cable would be securely attached to the outside of the pipe atspecific measurement locations along the length where multiple FBG sensors embedded within the optical fiber detect thehoop strain in the pipe wall.

The diagram in Figure 2-2 shows the concept for interrogating a series ofFBG sensors to determine the strain in each. Lightfrom a broadband source is coupled into the fiber. The light passes sequentially through each of the multiple sensors alongthe cable, and a small amount of light from each sensor is reflected back toward the source. During manufacturing, eachFBG can be tuned differently so that each FBG reflects a different light wavelength. The individual FBG sensors cantherefore be identified and the strain can be related to the as-installed position of each sensor. Thus, when the fiber is locallystrained, the strain location is determined based on the wavelength, and the magnitude of the strain is determined based on asmall change from the nominal wavelength according to the gage factor 1.2 picometers per microstrain. (A microstrain isdefmed as 106 units of strain, or l06 inch/inch.)

9

Gulf of Mexico

Well head

/ Distributed sensor spirallywrapped around pipe

Figure 2-1 Distributed Fiber Optic System for Flowline


The measurement approach we evaluated in this project used FBG sensors spaced at 40 inch intervals along the length of asingle mode optical fiber with a buffer diameter of 0. 155mm. The optical fiber and sensors were produced by 3M BraggGrating Technologies. The sensors were proof tested to 2% strain after production. Each sensor was 1 5mm long and wasembedded in the fiber after stripping the polymide buffer coating. The sensors were produced by introducing a periodicmodulation of the refractive index into the core of the fiber[l]. It is straightforward to control the number of spatialmodulation periods and the spacing. Each grating reflects optical signals within the light guiding core at a wavelength whichis twice the optical spacing between the high-index and low-index regions (Bragg reflection condition). After the sensor wasembedded, the fiber was recoated with polymide. During testing, we interrogated the FBG sensors using a Fiber BraggGrating Interrogation System FBG-IS obtained from Micron Optics.

The purpose of the tests was to characterize the performance of fiber optic Bragg grating sensors for measuring themagnitude of circumferential hoop strain in the wall of pipes. Measurements were made on the reflected wavelength signalsfrom the fiber Bragg grating (FBG) sensors and these signals were converted to strain values. The measured strain valueswere analyzed and compared to reference measurements of hoop strain, axial strain, and bending strain obtained fromelectrical resistance strain gages. Data analysis was used to determine the FBG strain resolution and sensitivity andrepeatability to hoop strain changes. Cross-sensitivity of the FBG sensors to axial and bending strain along with FBGthermal sensitivity was also estimated from the data analysis.

A single optical fiber containing embedded Bragg grating sensors was positioned on the wall of a 4 inch schedule 10 pipewhich was 1 6 feet long. The configuration of the test section is shown in Figure 3-1. The optical fiber was positioned on thepipe in a modified spiral wrap so that the FBG sensor axis was oriented perpendicular to the pipe longitudinal axis. Thethree FBG sensors at locations A were bonded to the outside surface of the pipe with Micro-Measurements AE-lO adhesive.A fourth FBG sensor at location T was in thermal contact with the pipe but not bonded with adhesive. This FBG sensorprovided a signal sensitive to pipe temperature but not strain. Biaxial electrical resistance foil strain gages were attached tothe pipe wall at the locations indicated in Figure 3-1 and measured the strain field at the location ofthe bonded FBG sensorsat locations A1, A2 and A3.

A detailed drawing of the test section is shown in Figure 3-2 and a diagram of the instrumentation is shown in Figure 3-3.The test section was filled with water and could be pressurized to 700 psi maximum to introduce hoop strain. A jack screwlocated at the center of the test section could be used to introduce bending strain in the vertical plane. A hydraulic tensionassembly located at the free end provided the capability to introduce axial strain in the test section. The test section wasfitted with an internal plug located 76 inches from one end, which divided the test section into two segments, 1 and 2, which

10

SourceOpticalfiber

Figure 2-2 Distributed Fiber Optic Bragg Grating Sensors for Detecting Strain

3.0 TESTING


could be pressurized independently. The purpose of the plug was to simulate a flow blockage in the pipe. A pressuredifference could then be produced so that the pressure and resulting hoop strain on one side of the plug were different fromthe pressure and hoop strain on the other side.

'3,

End Viewshowing straingage locations

?Jnput

Fiber Optic Bragg Grating Sensor Detail• At each axial position A, four biaxial strain gages were located around the circumference of pipe at 12, 3, 6, 9 o'clock.

• Fiber optic Bragg grating sensors were all oriented so that at attachment locations A1, the fiber axis was perpendicular tothe pipe axis.

• Sensors A1, A2, A3 were located adjacent to biaxial strain gages at 12 o'clock.

• Sensor T provided a signal sensitive to pipe temperature but not strain.

Figure 3-1 Test Section DiagramBUTTRESS

HYDRAULIC TENSIONING DEVICE

Segment I _________(76")

II —

-Plug 14"End Support—__.

1 3.5" REF

• Approximate locations of FBG sensorsand strain gages spaced 3 feet apart

Figure 3-2 Detailed Scale Drawing of Test Section Mounting & Structural Loading

11

—14'

—11'

-8'-

Opticalfiber

Segment 2(116")

4" schedule 10 pipe

Not to scale

Fiber Layout for Measurement of Hoop StrainFiber optic Bragg grating sensors at locations A,

T

- Xgtransmitted'6'

BUTTRESS

Segment 2(116")

Center Jackr1L

BASE

SHIP CHANNEL 20 FEET LONG


Three sets of four biaxial strain gages (twenty-four output signals) provided reference measurements of hoop, bending, andaxial strain.

The output signals were collected by an Easy Data data acquisition system and recorded along with a time stamp on a floppydisk.

The wavelengths of the light signals reflected from the Bragg grating sensors were measured by a Micron Optics FBG-IS andconverted to strain readings which were stored in a data file in an IBM compatible 486 PC. Each data file was time stampedcoincident with the data from the strain gages.

Test Section

Figure 3-3 Instrumentation for Data Acquisition During Tests

At each test point the following data was recorded: Output from FBG sensor, output from 12 biaxial strain gages,thermocouple signals, pressure transducer signals for both segments 1 and 2 of the test section. Pressure was allowed tostabilize before readings were taken. Three data runs were conducted for sequences 2A, 2B, and 2C and data was recorded atthe following target pressure test points (see Figure 3-2 for location of test section segments 1 and 2):

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5.0 CONCLUSIONS

The spacing between the FBG sensors in the present test program was chosen to be 40 inches for convenience. Four FBGsensors with 40-inch spacing were readily installed on the 16-foot test section. It is clear that if desired, the sensor spacingcould be increased significantly to enable a single FBG sensor fiber to be installed along a pipe many tens of miles long.Figure 6-1 illustrates how the output of the FBG sensors can be used to locate a blockage in such a pipe. The plot is basedonmeasured data from Test 2C, and indicates how the FBG sensor signals can be converted to pressure readings to provide apressure profile along the pipe and to identify the location of a simulated blockage, which was welded into the pipe sectionbefore testing began.

"Blockage Locationi

Figure 5-1 Output From Three FBG Sensors (Converted to Pressure Units)Vs. Distance Down The Test Section Pipe

Note that when there is a 20 psi pressure difference between segments 1 and 2 (see Figure 3-2), the three-sensor array clearlyidentifies the blockage location to be between the 8-foot and 11-foot marks.

6.0 REFERENCES

1. G. Melts, W. W. Morey, and W. H. Glenn, Optics Lett., i4 (1989) 823.W. W. Morey, J. R. Dunphy, and G. Melts, Proc. SPIE, 1, (1991) 216.

2. W. W. Morey, G. Melts, and J. M. Weiss, "High Temperature Compatibilities and Limitations of Fiber GratingSensors," Proc. SPIE, 2360 (1994) 234.

17

BRAGG OUTPUT VS. PIPE LENGTHTest Series 2C

625 ___________ ______________________ _________________ ___________

620

615

61000)0)

605

600 ______ ______________________ _____________________ _____________________

5954 6 8 10 12

Pipe Length (ft)

. No Blockage 20 PSI Blockage


Documents

SPIE Proceedings [SPIE Photonics East (ISAM, VVDC, IEMB) - Boston, MA (Sunday 1 November 1998)] Fiber Optic and Laser Sensors and Applications; Including Distributed and Multiplexed