Development of an Electromagnetic Inspection Technique for Lined-Cylinder Concrete Pressure Pipe

By Keith J. Morton

A thesis subrnitted to the Department of Physics in confonnity with the requirements for the degree of Master of Science (Engineering)

Queen' s University Kingston, Ontario, Canada

November, 1999

Copyright O Keith James Morton, 1999

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An electromagnetic inspection technique was developed for non-destructive

testing of lined-cylinder concrete pressure pipe. Lined-cylinder pipe is a composite

concrete and steel pipeline designed for small (typically 400-1200 mm) diameter,

concrete pressure pipe applications. Concrete pressure pipe is used internationally to

transport water under pressure. Applications include urban water supply, power station

cooling and wastewater force lines. Lined-cylinder pipe is well engineered and in general

has an excellent sewice record and long life. However there is currently an urgent need

to pinpoint damaged pipe sections for repair, in order to pre-empt bazardous and costîy

failures and prevent the unnecessary replacement of entire lines.

Lined-cylinder pipe consists of a 25-50 mm concrete core centrifùgally cast inside

a thin, 1.2 mm steel cylinder form. The concrete is then prestressed with a spiral wire

wtap of high strength steel. The 5 mm diameter wire is wound under tension at almost

80% of its yield strength of about 1.6 GPa Catastrophic failure can occur if compressive

p r e m s is lost when many neighbouring wires are corroded through.

Inspection of lined-cylinder concrete pressure pipe was demonstrated in test

sections using a combination of remote field eddy current and transformer coupling

effects. The technique is sensitive to single and multiple wire breaks anywhere around

the circumference of the pipe. Calibrations of the number of broken wires were made for

the different bell, middle and spigot regions and in one case the calibration was used to

estimate unknown wire break damage in an underground test line. Also, the response to

moisture content in the mortar coating was clearly distinguished fiom the wire break

response. This realization of a viable inspection technology for lined cylinder pipe will

motivate commerciaiization of the technique, although M e r development is still

necessary to meet the challenging requirements of in-line inspection.

Ackno wledgements

1 wouid sincerely like to thank Professor David Atherton for his constant encouragement, contagious enthusiasm and guidance throughout al1 my Ume at Queen's. His abilities as engineer, teacher, manager and mentor are reflected in the productivity and success of the Applied Magnetics Group.

1 would also like very much to acknowledge the support and encouragement of my loving wife and best fiiend, Amy.

This research was supported by the Natural Sciences and Engineering Research Council of Canada

Table of Contents

1 INTRODUCTION 1

- - --- - -- - - -

2.1 Remote Field Eddy Current Inspection 14 2.2 Transformer Coupling o.Y- ------------------.----- 17 2.3 Signal Andysis 19 2.4 Lined-Cylinder Pipe bspection 19

3 EXPERIMENTAL TECHNIQUES 22

4 RESULTS 26

5 FüTURE WORK 40

6 CONCLUSIONS 41

References ----- -0- --II----------------------------- 42 viu ---------HI-p-- . ~ ~ . ~ ~ ~ o ~ ~ ~ ~ ~ ~ ~ o ~ - ~ - ~ ~ ~ ~ ~ ~ ~ ~ o ~ - - - o o ~ ~ ~ ~ ~ ~ ~ - 44

List of Figures

Figure 1 : A single length of 610 mm diameter prestressed concrete pressure pipe. The inset shows the exposed prestressing wires that keep the inner concrete core under comp~ssion. ---~~---~------------------------o--oo-------~------n----- 2

Figure 2: A burst concrete pressure pipe showing the broken prestressing wires and ruptured steel liner after catastrophic failure. ------------------------ 3

Figure 3: Views of the bell (left) and spigot (right) ends of a single pipe section. Concrete pressure pipe is assembled with mating bell-spigot constniction. An installed elastomer gasket ring maintains the pressure rating at the joint.------- 7

Figure 4: Joint detail of the lined-cylinder concrete pressure pipe showing the bell and spigot construction. Note the thin steel liner and welded joint rings ont0 which the inner concrete core is cast. The large black circles represent the spirally wound pre-essing wires. 8

Figure 5: Bar-wrapped pipe is very similar in construction to the lined-cylinder type, however lower tensile strength steel is used at manufacture so a thicker gauge tensioning wire and steel liner are required. --------------- 9

Figure 6: Joint detail of the embedded-cylinder type concrete pressure pipe. Note the additional layer of concrete between the steel liner and the prestressing wires. -- 1 O

Figure 7: Depiction (below) of the indirect and direct energy paths coupling RFEC probe coils in ferromagnetic pipe inspection. The vector combination of the inside and outside fields within the pipe yields the three distinct regions show in the magnitude profiles (above). a) The direct field zone, where the interna1 exciter coil field is strongly attenuated by eddy current opposition b) the transition zone where the direct and indirect paths recombine, in this case desûuctively c) the detector coil is piaced in the remote field zone where the received signal is dominated by the double through wall transit of the indirect energy path.------O 16

Figure 8: Schematic of concrete pressure pipe inspection showing the steel liner and prestressing windings, which are show above the pipe for clarity. The through wall transmission characteristic of the remote field indirect energy path provides access to the extemal windings h m inside the pipe. Currents are induced in the windings, which become an additional strong inductive link between the exciter and detector coils. A break in the windings disrupts this 'transformer coupling' modifying the detected signal. 18

Figure 9: Complex plane representation of the combhed phase and amplitude response in a detector coil moving undemeath a region of wall thinning [ 1 61. In undarnaged pipe the trace would remain at the no corrosion or 'full wall' value. ------- 20

Figure 10: Schematic of the tool instrumentation. The hct ion generator and power amplifier control the amplitude and fiequency of the exciter current. The induced voltage in the detector coil is amplified and sarnpled by a PC controlled lock-in arnplifkr, referenced to the exciter coil. An odometer, interfaced with the PC provides &stance measuremeats. ------------------------ 23

Figure 1 1 : The inspection apparatus inside the 6 10 mm diarneter lined cylinder pipe laboratory -pie. 25

Figure 12: compares the phase response of four separate scans in the 9 1 5 mm B WP. The plots show superimposeci logs of the 'no defect' and single broken wire cases. The upper plot was made using the 1st version of the inspection tool, while the lower was made with the 2nd version. This plot also highlights scan to scan reproducibility of the inspection method and its sensitivity to subtle changes in the background pipe signal. -- --UHHHIIIHHH<)-U------------ 27

Figure 1 3: a) Amplitude response to 12 neighbouring breaks in the 9 1 5 mm B WP cornparrd to the no break case. b) Corresponding phase response. ------ 28

Figure 14: Cornparison of the complex plane signai response for increasing numben of neighbouring wire breaks. ----- -HIHIIU---UII---I-- 30

Figure 15: Plot of rneasured cumplex plane signal amplitudes for known numben of wires breaks in the middle of the 9 1 5 mm B WP section. The linear least squares fit and 90% confidence levels are also shown. ------------------------------- 3 1

Figure 16: Measured signal amplitudes for wire breaks in the bell and spigot ends of the 91 5 mm BWP. The respective linear least squares fit and 90% confidence levels

Figure 17: Plot showing just the calibration fits for the bell, middle and spigot regions of the 9 1 5 mm BWP on the same scale. -O----------* 34

Figure 19: Distance log of the five buried 610 mm LCP test sections. The joints, called defects regions and measured defects are labeled. --------------------- 36

Figure 20: Third pipe section of the 610 mm underground LCP line plotted individually. -,--- ~ - - _ _ U _ _ _ U _ _ _ U _ - _ _ U _ _ _ U _ _ _ U _ _ _ U _ _ _ U _ _ _ U _ - _ _ U _ _ _ U _ _ _ U _ _ _ U _ - - - _ _ U _ - _ _ U _ - 37

Figure 2 1 : Complex plane polar plot of a single section of 610 mm LCP showhg the sensitivity of the inspection technique to rnoisture changes in the protective mortar coating. The response to rnoisture is differentiated Grom the wire breaks by the trace angle in the polar plot. ----------------------------------- 39

Liïi of Tables

Table 1 : Estimated number of wire breaks for the measured defect regions in the underground 610 mm LCP test line. ---------------- 38

ac

AWWA

BWP

CPP

ECP

LCP

PC

RFEC

altemating current

American Water Works Association

bar-wrapped pipe

concrete pressure pipe

embedded-c y linder pipe

lined-cylinder pipe

Personal Cornputer

Remote Field Eddy Cunent

Transformer Coupling

Notation

Flux Density

Incident flux density

depth

fiequency

magnetic penneability

conductivity

phase lag

1 Introduction

1. I Motivation

The motivation for this research was to design and demonstrate a non-destructive

inspection system for lhed-cylinder concrete pressure pipe. Concrete pressure pipelines

are highly engineered structures used intemationally for pressurized water transportation.

Primary applications include drinking water supply to large urban centres, power station

cooling loops and waste water force lines [Il.

Concrete pressure pipe design combines inexpensive and rugged concrete with the

tende strength of steel in a composite pipeline construction. The result is a

comparatively lightweight, long-lived and low maintenance pipeline material that also

has excellent intemal and external load bearing qualities. The concrete core is put under

compression by a helical, high-strength steel wire wrap that is wound around the pipe

under high tension. At manufacture the concrete is "prestressed" and will remain under

compression up to the maximum design pressure. A single section of a 610 mm diameter

CPP is s h o w in Figure 1.

Catastrophic failure can occur if many neighbouring wires are corroded through

[2]. Tensile hoop stresses in the concrete from the internai water pressure are then

unbalaaced and the pipe ruptures. A ruptured lined-cylinder CPP is shown in Figure 2.

Pipe failure results in water loss (the commodity), disruption of service, dangerous local

conditions and expensive emergency repair costs. Therefore the goal of concrete

pressure pipe inspection is therefore to detect broken prestressing wires and to evaluate

the seventy and extent of these defect regions so that preemptive steps can be taken to

rehabilitate or replace damaged sections [3].

There is cunently an urgent need for a viable method to evaluate the integrity of

in-service CPP pipelines. For some years now the American Water Works Association,

which is a regulatory body of manufacturers and operaton has active1 y sought out precise

inspection techniques for CPP. Lined-cylinder pipe inspection is particularly important

because it makes up approximately 70% of ail in-service CPP. in North Amenca alone,

there are over 20 000 km of in-service CPP in use by nearly every major water utility.

Although the longevity of concrete pressure pipe has been proven since its introduction

alrnost 60 years ago, curent estimates indicate that without suitable inspection

technoiogy to identlfy distressed sections as much as half of in-service CPP wil1 require

complete replacement or full relining within the next twenty years. Current replacement

costs typicaliy nui more than $10 millionh. Non-destructive evaluation is essential to

pinpoint at-risk sections for remedial action and to avoid unnecessary replacement of

entire Iines.

1.2 Concrete Ptessure Ripe

There are two principal concrete pressure pipe designs, embedded-cylinder pipe

(ECP) and lined-cylinder pipe (LCP) [l]. ECP is configured for large diameter

applications while LCP is typically used in smaller diameter applications and is the focus

of this work. A third variety, bar-wrapped pipe (BWP), is very similar in construction to

LCP and is also included in this study.

These three distinct designs span a wide range of diameten as well as pressure

and extemal load requirements. Internai water pressures are approximately 1.4 MPa, and

surge pressures are typicdly 40% larger. LCP and BWP are usually manufactured in

diameters ranghg from 0.4 m to 1.2 m. The ECP design is now used for most diameters

larger than 1.2 metres. The very largest can reach 7 metres in diameter.

1.3 Manufacture of Concreie Prssure P@e

Lined-cylinder pipe is manufactured in 6-7 metre long sections. Thin, welded

steel cylinders are used as forms for the inner concrete core and spiral prestressing wire.

The minimum steel liner thickness is 1.2 mm. First, joint rings are welded to the ends of

the steel liner pipe. The rings facilitate joint self-centering during installation of the

nnished pipe sections. The liner is then mounted on a lathe and a 25-50 mm thick layer

of concrete is centrifûgally cast onto the inside. This action creates smooth, dense

concrete with only the minimum required water content. Mer initial curing, a

continuous spiral of prestressing wke is wound, under tension, around the liner. The

high-tensile steel wire is approximately 5 mm in diameter. The actuai wire gauge and

winding pitch are predetermined to keep the concrete under compression up to the

maximum design pressure, which is the surge capacity. The applied wire tension is set to

almost 80% of the 1.6 GPa wire yield strength. Concurrent with the winding process the

wire is coated with a corrosion inhibiting sluny of highly alkaline Portland cernent. The

wire is anchored at each end of the pipe section by a cinch clamp welded to the steel

liner. Finally a dense mortar coating is mechanically impacted ont0 the outside of the

pipe. The mortar protects against mechanical damage to the wires during installation and

once in the ground provides an additional alkaline barrier to check corrosion.

The standard joint is made using m a h g bell and spigot construction and a

watertight rubber gasket. The finished bel1 and spigot ends of a 610 mm diameter LCP

are show in Figure 3. A cross-section of the bell-spigot joint is given in Figure 4.

Figure 5 shows a similar bell and spigot joint, but for bar-wrapped pipe (B WP)

constnaction. Lower tensile strength steel is used in the pre-tensioning wires, requiriog a

larger gauge or 'bar'. in BWP, the cylinder is designed to provide part of the hoop

strength so it also requires a thicker gauge steel. The additional strength provided by the

increased liner thickness makes BWP a suitable choice in applications where high

extemal bending stresses are a consideration.

The third design, embedded cylinder pipe (ECP), is shown in Figure 6. In ECP

the steel liner is embedded in a second layer of 80-130 mm thick concrete before the

prestnssing wire is wound on. The concrete core and liner and wire guages are

nominally larger than in LCP. The spiral wire can be anchored by eiiher a clamp set in

the outer concrete layer, or by a clamp welded to the imer steel cylinder. in this design

the steel cylinder has minimal load bearing capacity and acts as a supporthg membrane

during manufacture and installation. Because of their large diameter, ECP are typically

made in 5-6 metre sections. Al1 of the CPP varieties corne with a host of additional

custom built finings, T-joints, valves and access manholes.

1.4 Concrete Pressure Pipe Failure

Despite their excellent in-service record, CPP can fail if many adjacent wires

break relaxing the compressive stress in the concrete core. Unfortunately the cntical

number of broken wires is not well known and diffea for each diameter and design type.

It is known that more wire breaks can typicaily be sustained in middle of the pipe than in

the bell and spigot. Detemiining the cntical values has k e n hindered because a

satisfactory way of counthg broken wires has not k e n available. Pipes that have already

failed do give some indication, but additional prestressing wire damage invariably occurs

during rupture. in fact the number of broken wires may not be a complete measure of

failure. Actual rupture occurs because of compressive prestress loss in the concrete,

which is dependent on more than simply the number of wire breaks. Pipe to pipe

differences in the concrete properties, manufactunng differences and the pitch and page

and overall pipe diarneter are al1 contributing parameters that effect the critical failure.

Although adequate failure analysis lags behind, estimating the number of broken wires is

still the most crucial factor to effective nsk management and safe operation of concrete

pressure pipe.

Current estirnates for the critical nurnber of broken wires in the mid-section of

LCP give it as approximately 80 wires. This value is almost certainly an

ovenirnpli fication because it generalizes for al1 LCP pipe diameters, manufactures and

design applications. The estimated values for the bell and spigot are 40 breaks each. An

inspection system does not therefore need to be sensitive to a partiaily corroded wire. In

fact the prestressing wires are under such high tension that any wire damage quickly

propagates causing a full break.

Corrosion and hydrogen embrittiement are cornrnon causes of wire breaks in

concrete pressure pipe [4]. Over tirne, acidic soil conditions penetrate and depassivate the

alkaline moxtar and slurry coatings forming reactive sites where the coatings are

degraded. The effects are accelerated in aggressive soil conditions and when the mottar

coating has been damaged or cracked.

1.5 Lùted£yinder Ikspection Requirements

The general requirements for linedcylinder inspection are to locate regions of

broken prestressing wires and estimate the severity of the dainage. In addition the

inspection system must be completely intemal to the pipe. The pipes are almost always

buried d h g out extemal evaluation. intemal inspection necessitates consideration of

insertion and retrieval of the probe, propulsion, power for on-board instrumentation and

whether the inspection is performed in watered or de-watered lines. The most important

consideration though, is the ability to iaspect the extemal prestress windings fiorn inside

the concrete and steel layen. Therefore before full probe desigr, for in-service lines is

warranted, proposed techniques (consistent with the above requirements) must be

validated in test lines.

Several successfid non-destructive techniques already exist for pipe and tube

inspection. Magnetic Flux leakage is the current method of choice for corrosion

monitoring of steel pipelines. In CPP, however, the unit relative permeability of the

concrete layer provides too great a gap in the magnetic circuit to be effective.

Conventiod eddy current inspection is also well developed, but is severely lirnited by

skin depth attenuation on the inner surface of the steel liner.

Previous inspection of ECP has been attempted, phari ly with de-watered pipe

walk-through's, using visual and acoustic methods to locate delamination of the concrete

layers due to loss of prestress. Visual inspection identifies cracking in the inner concrete

core, and therefore only very major distress. Soundings for delamination fiom sonic "tap

and listen" techniques are dificult to interpret and are tirne consuming because the entire

intemal area of the pipe must be tested. Acoustic-emission monitoring of snapping wires

is also under development using transducer arrays in watered pipes [5 ] . However, the

past history of wire breaks cannot be determined. With the exception of acoustic

emission none of these methods can be applied to LCP because the smaller diameters

restrict hurnan access altogether.

2 Theory

The method that was developed for in-line inspection of lined-cylinder CPP uses

low frequency electromagnetics to interrogate the prestressing windings. The method is

based on the non-destructive Rernote Field Eddy Current (RFEC) technique. Using the

RFEC methodology in CPP an additional inductive interaction with the prestressing *es

was observed called Transformer Coupling [6]. The combined result is an effective

inspection technique for CPP and is known as the Remote Field Eddy

Cumnt/Transformer Coupling effect to underline the importance of each aspect.

2.1 Remote Field Eddj, Current Imspection

Remote Field Eddy Current inspection was pioneered by T.R. Schmidt in 1958 for

the inspection of oil well casings [7]. More recently, hi&-resolution RFEC probes were

developed for commercial inspection of heat exchanger [SI and s tem tubes [9], cast iron

water distribution lines [IO] and nuclear reactor pressure vessels [ 1 11. Remote Field

inspection probes typically operate using solenoidal coils in a send-receive configuration.

In contrast to traditional eddy cunent probes which are limited to the inner surface of the

pipe wall, RFEC can inspect the entire wail thickness of conducting pipes. The unique

f w of the RFEC. technique is then complete through wall inspection using only

intemal coils [12].

The exciter coi1 sets up a low fiequency ac magnetic field inside the pipe. This

exciter field induces strong circumferential eddy currents on the inside surface of the pipe

wall. The eddy currents react to oppose the changing magnetic field. Resistive losses in

the wall mean that the exciter field is not completely counter balanced, allowing it to

diffuse into the pipe wall with attenuation and phase delay. If sufficiently low

fiequencies are used, through wall transmission of the magnetic field is achieved.

In the particular case of ferromagnetic pipes the magnetic field energy then tends

to be guided along the pipe axis away fiom the exciter plane. As it travels along the pipe,

the field rediffuses back inside undergoing additional attenuation and a further phase

delay. h i d e the pipe the direct exciter field is quickiy attenuated in the axial direction

by the induced eddy currents. Figure 7 is a schematic of an RFEC probe in a steel pipe

showing these two distinct energy paths and their relative field magnitudes. Beyond

approximately two pipe diameters, in the 'remote field' zone, the external field path

dominates the interna1 signal path. A detector coi1 placed in this region is sensitive to the

double through-wall transit of the indirect energy path.

The transmission of the field through the pipe wall can be approximated by the

one dimensional skin depth equation for a semi-idnite slab. The accuracy of this

approximation and of huther refinements to the mode1 have both been verified

experirnentally [13]. The attenuation and phase lag for a plane wave nomally incident

on a semi-infinite, linear, homogeneous, isotropic conductor is given as a function of

depth, d, by:

Where Bo is the magnetic flw density at the metal surface, B is the flux density inside the

pipe wall and @ is the phase lag given by:

Where a and p are the standard material parameters of conductivity and permeability and

fis the excitation fiequency.

DIRECT I I l

FIELD i I I I

ZONE i TRANSMON ZONE i REMOTE FIELD ZONE

I N D I R E C T ENERGY TRANSMISSION PATH

Figure 7. Depic tion (below) of the indirect and direct energy puths ccoupling RFEC probe coils in fèrromagnetic pipe inspection. The vrctor com binatiun of the inside and oiitside fields within the pipe y ields the three distinct regions shown in the magnitude profiles (above). a) The direcr field zone. ivhere the infernal exciter coi1 field is strongly ottenuared by eddy cirrrent opposition b) the transition zone where the direct and indirect pcrths recombine in this case destructively c) the detector coi1 is pluced in the remotejield zone where the received signal is dominated by the double through wall transit of the indirect energy path.

The phase lag is seen to be linearly dependent on penetration depth. Wall

thinning due to corrosion therefore acts to reduce the phase lag while also decreasing the

field attenuation of the detected signal. The RFEC eddy current technique shows nearly

equai sensitivity to intemal and extemal corrosion [Ml. The probe is also sensitive to

material anomalies in the indirect coupling path such as changes in conductivity or

relative penneability [12]. The signal will also show characteristic responses to extemal

metallic objects such as support plates, heat exchanger tube fins and welds or, in the case

of concrete pressure pipe, the extemal prestressing wires. It is this last interaction that

makes the double through wall transmission of RFEC testing an integral part of CPP

inspection.

2.2 Transformer Covplng

The RFEC technique is used in CPP inspection to achieve interaction with the

prestressing wires fiom inside the steel cylinder. The send-receive coi1 arrangement, now

shown inside a schematic concrete pressure pipe, is given in Figure 8. The extemal

windings are shown above the pipe for clarity.

As in RFEC inspection the field near the exciter is sharpiy attenuated in the axial

direction by opposing eddy currents in the steel liner. The indirect energy path again

couples the two coils. The prestressing wires are usually anchored to the pipe ends

creating a closed electrical circuit. Currents are then induced in the spiral wire by the

extemal magnetic field. When both the exciter and detector are underneath the wires as

shown in Figure Bb, the induced currents in the windings provide an additional inductive

link between the coils.

Figure 8. Schemutic* uf concrete pressrrre pipe inspection shmei~g the steel liner und prcstressing windings. T ' latter are shown rrbnve the pipe jhr clr~rity. The tlirough wu11 iru~i.sn~i.s.sion churucteristic qf the remote field indirect energy puth provides uccess to the externul windingsfiom inside the pipe. Cirrren~.~ urr inrluced in the windings, which become an addltiond strong inductive /in& betwen the exciter rindrlrtector mils whrn thry ure both under u section of closed windings. A break in the windi~gs disrupts this 'trun.~/ormer cotrpling ' muLiifLing the dctecte Jsignal.

There are then three coupling paths between the two coils, the direct field fiom

the exciter, the remote field path and the 'transformer coupling' (TC) path. The TC path

dominates the RFEC effect. The strength of the transformer coupling is used, in some

cases, to reduce the overall inspection tool length. The resultant field at the detector coil

is the complicated vector interaction of these t h e mechanisms. When prestressing wires

are broken the transformer coupling effect is diminished changing the amplitude and

phase of the measured fields.

2.3 Signal Analysis

In RFEC signal analysis the phase and amplitude measurements are kequently

combined into a single complex plane diagram or polar plot. It is sirnilar to the voltage

plane representation of transmit-receive coils in eddy curent testing [15]. Changes in the

recorded phase and amplitude can then be viewed simultaneously. The signai amplitude

is the distance fIom the plotted value to the origh and the signal phase is given by the

angular position of the point in the plane. Figure 9 shows the RFEC signal trace for a

detector coil passing undemeath a region of wall thinning [16]. This monitor display has

been adapted for signal analysis in CPP inspection.

2.4 Lined-Cylinder Pipe Inspection

Commercial inspection of ECP is now offered by the Pressure Pipe Inspection

Company using the recently developed RFECfïC method. The premise of the current

research was to apply RFEC/îC to LCP inspection. Success in lined-cylinder pipe is not,

however, obvious. From the point of view of transformer coupling, LCP diffen

1 4 t y---.

Detector directly undemeath corrosion

No Corrosion - -

Inphase (microVolts)

Figure 9. Complex plane representation of the corn bined phare and amplitude response in a detector coi1 moving underneath a region of wall thinning [16]. In undamaged pipe the trace would remain at the no corrosion or 'full wall ' value.

significantly fiom ECP. In embedded cylinder pipe the prestressing wire is

wound around the second concrete layer and foms a closed circuit, providing the

essential inductive coupling. In LCP the prestnssing wires are wrapped directly

ont0 the steel liner.

3 Experimentrl Techniques

3.1 Test Sections

The two Canadian CPP manufacturers provided sample pipe sections. Lafarge

(Concrete Pressure Pipe Division), supplied a single section of 610 mm diameter LCP to

test for mid-section wire breaks. The section was 7.3 metres long and could be

conveniently tested in the lab. Defects in the bel1 and spigot ends could not be tested in

this sarnple because of end effects. Bell and spigot breaks were tested using three

sections of 915 mm BWP supplied by Hyprescon. in an unusual cooperation between

cornpetitors the Hyprescon pipes were stored at Lafarge's CPP manufachiring plant in

Stouffiille, ON. As previously noted the LCP and BWP are very sirnilar in construction

and responses in BWP pardel those in LCP.

3.2 Appuratus

The tool design for lined cylinder concrete pressure pipe inspection was based on

optimizing the RFEC/TC response to broken prestressing wires. A compact design was

mandatory in keeping with the fiiture access requirernents of a commercial tool. A

schematic of the essential instrumentation is shown in Figure 10.

The function generator (Chung Electronics 555 oscillator) was set to a convenient

sub-mains signal fiequency. The output voltage was boosted by an audio amplifier (Sony

XM-2501) that powered the exciter coil. The current in the coil was monitored with a

series amrneter (Fluke 73110. The detector signal was first pre-amplified (Ithaco 398 1 A)

and then passed to the lock-in amplifier PC board (Ithaco 39818). A lock-in amplifier

measures the amplitude and relative phase of small ac signals with respect to a reference

EXCITER COI L

n 1 1 AMMETER GENERATOR Y

/,-\ BATTERY POWER IUPPLY

-1- PC CONTBOL AND DATA ACQUISITION

DETECTOR COI L

Figure IO. Schematic of the tool instrumentation. The funcfion generafor und power omplijfer contml the amplitude and jieguency of the exciter currenf. The induced voltage in the defector coil is ampiified d s a m p I e d by a PC contrded iock- in amplijie,: rejerenced tu the exciter coil. An odometez interfuced with the PCprovides distance measurements.

signal. The reference input was the cumnt in the exciter coils. Measurements were

therefore made relative to the magnetic field at the exciter coil. The lock-in acts at as a

very narrow active band-pass filter centered at the reference frequency. An on-board

persona1 computer and custom software controlled the lock-in amplifier, data acquisition

and odometer interface. Storage batteries powered both the computer and audio

amplifier.

3.3 Erperimental

The assembled inspection tool was pulled slowly though the pipe by hand to test

the response to wire breaks. Breaks were made in the prestressing wires by cutting them.

While this is of course destructive and renden the pipe unfit for future service, broken

wires could be reconnected electrically to restore the no defect condition. The tool is

show being pulled into the 61 0 mm LCP in Figure 1 1. Al1 the scans were repeated to

check the reproducibility of the results.

'Ihree test lines were used to evaluate the inspection system. Up to 15 wire breaks

were put in the mid-section of both the 61 0 mm diameter LCP and the 9 15 mm diameter

B W already mentioned. Ten breaks at the bel1 end and nine at the spigot were also

introduced into the BWP. An additional 610 mm LCP underground test line, five pipe

sections long, was used to ver@ the performance of the tool for middle break detection.

The number of wire breaks was not known in advance.

In a final expriment a metre long section of the 6 10 mm LCP mortar coating was

wetted dom. The mortar was kept continually soaked in this area to simulate possible

variations in pipeline bedding moisture and its effect on the response to broken wires.

The mortar was kept wet by wicking action using a towel dipped in a water trough.

4 Results

4.1 Sensirivity to Broken Prestresshg Wires

The fmt application of the RFECiTC technique to lined-cylinder type concrete pressure

pipe was very promising. The first tests were conducted in the 9 15 mm BWP with a

single break in the middle of the centre section. Figure 12 shows phase logs using the

first generation LCP tool and the current version. Although there is a marked difference

between the two versions, the most important result is common to both: sensitivity to

broken prestressing wires.

This encouraging resuit motivated tool design changes to optimize wire break

sensitivity. The first change was to reconfigure the tool components for use in 610 mm

diameter pipe also. This included a move to instrumentation modules to allow

components to be interchanged for different scanning arrangements. Also, the

mechanical design and fabrication, steering adjustments, electrical cabling and storage

batteries were ail improved. These provided a noticeable four-fold improvement in break

resolution.

Two key improvements in inspection quality are underlined in Figure 12. The

fm is the obvious increase in signal strength for a single broken wire. The second is the

increased resolution of the subtle background fluctuations dong the pipe axis. The

overlaid break and no break signals also highlight the scan to scan repeatability. It should

also be noted that the inspection system was s h o w to be sensitive to wire breaks

an ywhere around the circurnference .

The amplitude and phase logs for 12 adjacent wire breaks in the rnid section of

the 915 mm bar wrapped pipe are given in Figure 13. There are three pipe sections in the

Curent version of LCP tool

1st version of LCP tool

O 2 4 6 8 10 12 14 16 18 Distaxe (m)

Figure 12. Phase response oîjour separate scans in the 9915 mm BWP The superimposed logs of the 'no defiet' and single broken wire cases show the technique 's sensitiviîy to a single broken wire. The lower plot was made using the 1" version ofthe inspection tool, while the upper was made with the T' version. This plot also highlights scan to scan reproducibility of the inspection method and ifs sensitivity to subtle changes in the backgroundpipe signal.

fi,

\ 12 Wire Breaks

Figure I 3.a) Amplitude response to 12 neighbouring breaks in the 91 5 mm B WP compared to the no break case. b) Corresponding phase response.

line and the two joints also give characteristic responses. The logs from the break and no

break cases are again superimposed for cornparison.

The amplitude and phase logs for every scan were combined in the complex plane

for analysis. Four such representations are given in Figure 14 to illustrate the response to

different numbers of adjacent broken wires. The defects were measured using proprietary

software developed by the Pressure Pipe Inspection Company.

4.2 Cafibratio~ of 915 mm Bar Wrupped me

Measurements for the Mddle breaks in the 9 1 5 mm BWP are combined in Figure

15. The linear dependence of complex plane signal amplitude on the number of

neighbouring wire breaks is clear. The measurement scatter is fiom scan to scan

variations in the signal peaks and measurement uncertainties. The total uncertainty in the

measurement is unknown so the least squares fit calculation is used as an estimate.

Assurning a normal distribution of measured values about the mean fit, confidence levels

can be quoted using the standard deviation of the fit. The 90% confidence levels are

shown as dotted lines above and below the solid least squares fit. The caiibration curve

cm then be used to estimate the number of broken wires in inspected pipe sections. Ln

this case, for middle defects regions in the 91 5 mm pipe, the number of multiple broken

wires could be estimated to within 5 breaks.

Calibration c w e s were also made for broken wires in the bel1 and spigot ends of

the 915 mm BWP are shown in Figure 16. The 90% confidence levels are also shown

with the least squares fit. Multiple spigot end defects could be estimated to within 3 wire

breaks.

No wire breaks 1 wire break

5 wire breaks 13 wire breaks

Figure 14. Cornparison of cornplex plane signal response for increasing numbers of neigh bouring w ire breaks. Axes are the quadrature component (vertical) versus the in-phase component (horizontal). No joint signal are shown.

in bell end, on the other hand, there was poor defect resolution. A single defect

and 10 defects are cmntly indistinguishable. Figure 17 compares the best fit lines for

the bell, middle and spigot regions together on the same scale.

4.3 Calibratiom of 610 mm Lined-Cylinder Pipe

A calibration curve for middle defects in the 610 mm LCP test section is given in

Figure 18. This pipe also showed a linear dependence between voltage plane signai

amplitude and the number of wire breaks. The scatter is again a result of scan to scan

variations in the length of the defect signal response and measurement uncertainties.

Here the confidence intervals allow a precision of 8 broken wires. It should be noted that

the response to a single broken wire is large compared to additional single increments.

The inspectioh tool was also tested on a five-section line of buried 610 mm LCP.

The pipe was fiom the sarne manufacturer as the 610 mm lab sample. The location,

number and magnitude of break regions were not known. The amplitude and phase logs

are plotted in Figure 19. Called defects are indicated in the phase log of the figure. The

3rd pipe section was found to have the largest number of distinct defect regions. it is

plotted individually in Figure 20. Defects in the bell (lefk) and spigot (right) ends were

not estimated because a calibration sample was not available for these regions. The mid-

section defects were estimated as follows:

Figure 19. Distance log of the jive buried 61 0 mm LCP test sections. The joints, called defecrs regions and rneasured defects are labeied

Figure 20. Third pipe section of the underground 6 10 mm LCP line ploned individual ly

Table 1: Estimated number of wire breaks for the rneasured defect regions in the underground 6 10 mm LCP test line.

Defect Region A

4.4 Sensitiviîy to Coathg MoiFiure

Estirnated Number of Wire Breaks 1-5

The final experiment was to test the effect of moisture content in the mortar

coating. This is of significant interest because concrete pressure pipes are almost always

buried and therefore can be constantly or periodically wet. Figure 21 shows that the

system is clearly sensitive to moisture in the mortar layer, however the moisture response

is differentiated fiorn-the wire break response by the angle in the complex plane. This is

critically important because moisture changes will then not conhse analysis of the defect

trace.

Wire Break Defect Signal

/' Signal Trace

In-Phase

From Wet Wrap

Figure 2 1 Cornplex plane polar p h of a single section of 61 0 mm LCP showing fhe sensitiviîy of the inspection technique to rnoisture changes in the protective rnorfar coating. The respome to moisture is differentiatedfi the wire breaks by the Race angle in the polar plot.

5 Future Work

The development of an inspection technique for LCP is just the first step in a

larger research and development process that promises to be challenging. Now that the

RFECiTC method has k e n proven to be an effective, quantitative solution to lined-

cylinder pipe inspection, a multi-disciplinary approach must be taken to complete hl1

commercialization.

For example, continued research is particularly needed in the area of caiibration

samples. Although the BWP parallels LCP behaviour, the calibration fits were different

and not transferable. The calibration was also shown to be different for different regions

of the pipe. ~ o a s t r u c t k and material differences between manufactures and differences

in pipe design such as diameter and wire pitch will add additional complication. It is

cornmon in non-destructive testing to calibrate against representative defects in product

samples before inspection, however, the large size and service lifetimes of LCP makes

on-site calibrations in identical pipe unredistic. Altemate calibration techniques are

needed so that a wide range of pipe diameters and types can be inspected.

Engineering development of a commercially viable inspection system must

address such critical factors as propulsion and steering through long pipes, the necessary

power and operating conditions such as water. vibration and the strict access

requirements.

A unified approach to these problems will M e r increase the development costs

because a wide cross-section of samples, built for different design conditions and fiom a

variety of manufactures exist.

6 Conclusions

A precision non-destructive inspection technique for lined-cylinder concrete

pressure pipe was developed. The thiough-wall transmission characteristic of remote

field eddy current testing was used to probe the prestressing windings fiom inside the

pipe. The complex combination of direct and indirect energy paths and the additionai

transformer coupling was then monitored to discem wire breaks. In above ground test

sections the method was shown to be sensitive to both single and multiple prestressed

wire breaks in the mid, bell and spigot regions of the pipe. The breaks could be detected

anywhere around the circurnference.

Calibrations for the number of wire breaks were demonstrated in the middle of the

610 mm LCP and in the middle. bel1 and spigot regions of the 91 5 mm BWP. The

calibrations were different for each region and pipe sample. The estimates for middle

wire breaks are accurate to 8 breaks in the 610 mm LCP and 5 breaks in the 91 5 mm

BWP. in the spigot end of the BWP pipe, the estimate is to within 3 breaks. The bell end

breaks cannot at this point be reliably estimated for the 91 5 mm BWP, but breaks are

clearly seen. The 610 mm LCP calibration was then used to evaluate the seventy of

middle defect regions in an underground test line with a previously unknown number of

wire breaks. The RFEC/TC technique was also shown to distinguish between changes in

the moisnire content of the concrete mortar coating and the response to broken wires.

The contribution of this research was to demonstrate a viable inspection method

for LCP inspection and to motivate cornrnercialization of the technique. The research has

also highlighted the cpntinuing challenges and dificulties toward commercial inspection

of LCP; such tools will be expensive, requiring significant investments of time and

resources.

[ I l American Water Works Association standard for "Prestressed Concrete Pressure Pipe, Steel Cylinder Type, for Water and Other Liquids".

[2]Dave Marshall, "TRWD Experience with Prestressed Concrete Pipe", Proc. ASCE Conf. On Pipelines in the Constmcted Environment, San Diego, CA, August 1998, pp. 557-565.

[3]D.L. Atherton, B.J. Mergelas, K.J. Morton, X. Kong, "Electrornagnetic Inspection of Prestressed Concrete Pipe, Proceedings of National Association of Corrosion Engineers, Ottawa, Canada, October 1999.

[4] R. E. Price, R.A. Lewis and B. Erlin, " Effects of Environment on the Durability of Prestressed Concrete Pressure Cylinder Pipe, Proc. ASCE Conf. On Pipelines in the Constructed Environment, San Diego, August 1998, pp. 584-593.

[5] W. Worthington, " An update on Acoustic Emission Testing of PCCP, Proc. ASCE Conf. On Pipelines in the Constmcted Environment, San Diego, August 1998, pp. 477- 484.

[6] B. J. Mergelas and D. L. Atherton, "In-line Electromagnetic Inspection of PCCP", Proc. ASCE Conf. On Pipelines in the Constructed Environment, San Diego, August 1998, pp 7 14-720.

[7] T.R. Schmidt, "History of the Remote Field Eddy Current Inspection Technique", Muterials Evuluu~ion, Vol. 47, No. 1, January 1989, pp. 14-22.

[8] D.I. Brown and Q.V. Le, "Application of the Remote-Field Eddy Current Inspection Technique to the In-service Inspection of Ferromagnetic Heat-Exchanger Tubing", Materials Evafuation, January 1 989, pp. 47-55.

[9] D.E. Russell, "Recent advances in RFEC Probe Designs," Paper presented at the Second International Conference on the Remote Field Technique, Kingston, Canada, August 199 1.

[l O] P.H. Ferguson, M.J. Heathcote, G. Moore, D.E. Russell;, "Cast lron Water Distribution inspection", Warer, April 1996 pp 6-8.

[I l ] D.L. Atherton, S. Sullivan and M. Daly, "A Remote-Field Eddy Current Tool for Inspecting Nuclear Reactor Pressure Tubes", British J. of NDT, Vol. 30. No. 1, January 1988, pp. 22-27.

[12] T.R. Schmidt and D.L. Atherton, "Understanding Remote Field Logs", ikfateials Evaluation, November 1 998, pp. 1 276- 1 362.

[13] D.D. MacKintosh, P.A. Puhach, and D.L. Atherton, " Through-Transmission Equations for Remote Field Eddy Cunent Inspection of Smdl-Bore Ferromagnetic Tubes", Materials Evaluation, June 1993,744-748

[14] T.R. Schmidt, "The Remote Field Eddy Cunent inspection Technique", Materials Evaluation, Vol. 42, No. 2, Febnaary 1984, pp. 225-230.

[15] V.S. Cecco, G. Van Drunen and F.L. Sharp, Eddy Currenf Manuai, AECL-7523, Rev. 1 , Atomic Energy of Canada Ltd., October 198 1 .

[16] D. Mackintosh, MSc Thesis " inspecting Small Bore Ferromagnetic Tubes Using the Remote Field Eddy Current Technique," Queen's University, November 1992.