The Southern African Institute of Mining and Metallurgy
8th International Corrosion Conference
N Webb
COMBINED EXTERNAL COATING AND CATHODIC
PROTECTION ASSESSMENT TECHNIQUES – A CASE STUDY FOR A
PIPELINE IN WEST AFRICA
Neil WEBBc , Brian WYATTa, Marcel ROCHEb, John THIRKETTLEa,
a
Corrosion Control Associates Ltd, 3 Ivy Court, Action Trussell, Staffs, ST17 0SN, UK
b
Total SA, Tour Coupole 22D60, 92078 Paris-La Defense Cedex, France
c
Corrolec cc, 9 Kareeboom Ave, Fourways Gardens, 2191, RSA
SUMMARY
Recent developments in survey techniques and measuring equipment now allow for an
accurate assessment of external coating condition and cathodic protection performance. These
developments build on 20 years of experience of using Direct Current Voltage Gradient
(DCVG) and Close Interval Potential Survey (CIPS) methods. The techniques use the latest
advances in measurement systems coupled to a bespoke data processing system. The paper
describes tests and trial surveys carried out on a hydrocarbon transmission pipeline in West
Africa during 2005. The survey works described and conclusions drawn are from the data
collected. Of particular interest are the apparent capacitance effects which are thought to
occur under disbonded coating and the characteristics that these may show on the integrated
CIPS and recorded DCVG profiles. This paper was originally presented at the Cefracor
conference in Aix, France, in June 2006
The pipeline was originally installed on a phased basis between 1989 and 1994. The external
coatings were three layer polyethylene factory coating and shrink sleeve field joint coating.
An impressed current cathodic protection system was installed. The pipeline has subsequently
been affected by disbondment of the coatings. It was a primary objective of the trials to
attempt to identify such affected areas. In addition to a technically detailed account of the
survey works undertaken and the results obtained the authors will consider the practical and
commercial aspects of these surveys with input that provides a perspective from the specifier,
pipeline surveyor, pipeline operations personnel and the pipeline owner.
It is intended that the data presented will provide greater awareness of the benefits and
limitations of the techniques used and their role for both stand-alone interpretation and the
integration of the survey information with other inspection methods used for external
corrosion assessment.
The application of the described techniques is well developed and proven to locate and
characterise through–coating defects and to provide a full audit of cathodic protection status.
However, the possibility of the techniques being able to indicate disbonded coating is
unproven and further tests and trials are planned which may allow a greater understanding of
the electrical characteristics which are believed to be associated with disbonded coatings
which may or may not be filled with an electrolyte. Further work is currently planned in West
Africa, South Africa and Scotland in order to improve the understanding of this technique and
to determine if the reported indications do actually represent a method of locating disbonded
coatings.
Keywords: External Coating, Cathodic Protection, DCVG, CIPS, corrosion assessment.
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The Southern African Institute of Mining and Metallurgy
8th International Corrosion Conference
N Webb
INTRODUCTION
It is widely accepted that no single method or technique exists that will locate where external
coating systems have become disbonded from steel pipelines. Indeed, recent international
technical seminars have addressed this issue in an attempt to create impetus for the conception
of such a technique using innovative instrumentation.
Recent advances in techniques for accurate cathodic protection performance assessment and
external coating condition assessment/defect location have promoted the possibility that these
systems, if used jointly, may provide sufficient data that can subsequently be analysed to
show trademark indications where the coating may have become disbonded. At the
commencement of this study it was considered possible that the characteristics of the data
profiles at these locations could indicate through–coating defects that were associated with
disbonded coating areas significantly larger than the through–coating defects that were known
to be locatable. During the work certain unexpected electrical characteristics were recorded
that may provide indications of the presence of disbonded coating.
The techniques used are well developed and have been previously described [1]. The
possibility that these data might locate disbonded coatings is in its infancy but has now been
observed on pipelines both in West Africa and Scotland. Similar effects have been seen when
surveying plastic sheathed power cables in South Africa. This Paper focuses on surveys and
analysis of data collected from a pipeline located in Gabon, West Africa.
The pipeline was constructed between 1989 and 1994 in phases to transport hydrocarbons
from Production Centres to a process terminal some 234 km distant. The route of the pipeline
is through heavily vegetated tropical rainforest to open scrublands. Numerous water courses
and open saline water crossings are transited whilst crossings include vehicle access tracks
and surfaced roadways.
Pipeline Details
Terminal
Cathodic Protection
Installations.
Transformer Rectifier Stations
Section 3 – 29Kms
Station
Section 2 – 100Kms
Solar Station
Total Length
234Kms
Station
Section 1 – 105Kms
Line constructed in sections as shown above.
Section 1 – Constructed 1989
Section 2 – Constructed 1989
Section 3 – Constructed 1994
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Production
Centre
Section operating at
temperature higher than
55°C
(13 Kms)
The Southern African Institute of Mining and Metallurgy
8th International Corrosion Conference
N Webb
Technical details of the pipelines are shown hereunder:
•
•
•
•
•
Constructed to 100 bar MAOP
Inlet pressure to transmission pipeline – 40 bar at Production centre
Pipeline inlet temp – 60° C (to prevent wax deposition in Production Centre)
General electrolyte conditions – wet compacted sand
pH 5.4 (measured at a location where corrosion underneath disbonded coating was
detected)
• External Coating:
o Factory applied coating - 3 Layer Low Density Polyethylene (LDPE) applied in
accordance with internationally recognised corporate specification including FBE
thickness of >70 microns.
o Field joints - heat shrink sleeves with hot melt adhesive applied to fast curing liquid
epoxy on brush-cleaned surface.
o Coating application fully inspected during installation in both factory and field.
• Pipeline wall thickness – 7.67 mm
• Corrosion allowance on wall thickness – zero
Cathodic Protection was installed at the time of construction and was of impressed current
design. Conventional cathodic protection stations, powered by transformer-rectifier units,
were installed at the end of each pipeline section between the Production Centre, intermediate
Stations and the Terminal where ac power was available and two additional solar powered
stations were installed at intermediate points within Section 1.
Several years after the commissioning of the cathodic protection systems, excessively
negative pipe/soil potentials were recorded in Section 1 for a short period of time due to the
malfunction of one of the solar powered systems. The solar powered system controllers were
vulnerable to lightning strikes in the area and this resulted in uncontrolled excessive current
output; these were subsequently decommissioned. This occurrence in 1991 led to the
readjustment of cathodic protection levels throughout Section 1 in accordance with
international standards.
At that time the cathodic protection criteria for minimum (least negative) pipe/soil potential
levels was set at –950mV and maximum (most negative) potential levels –1150mV with
reference to a saturated copper/copper sulphate reference electrode. Generally, potential levels
have been maintained in accordance with these criteria but have normally been recorded as
energised (ON) potentials recorded with the cathodic protection flowing. Recent survey
potential measurements recorded with current interrupters fitted to the cathodic protection
power sources, to allow the more accurate measurement of Instant OFF (polarised near IR
drop error free) potentials have indicated that operating levels have actually been less negative
than –950mV with respect to Cu/Cu SO4.
An Intelligent Pigging / In Line Inspection [Magnetic Flux Leakage (MFL)] survey carried
out in 2004 indicated the presence of external corrosion on the transmission pipeline. A total
of 13600 defects were reported and these were categorised as follows:
12494 № Defects (92%) –
metal loss at less than 20% wall thickness
1168 № Defects (approx 8%) –
metal loss between 20% and 40% wall thickness
11 №
Defects (approx 0.08%) – metal loss greater than 40% wall thickness
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The Southern African Institute of Mining and Metallurgy
8th International Corrosion Conference
N Webb
The maximum metal loss defect recorded by the MFL intelligent pig was pitting to 57% of
wall thickness (4.38mm) recorded some 2.2Kms from the Production centre. An analysis of
the defects recorded was carried out in accordance with ASME B31G specification and this
concluded that the mechanical integrity of the pipeline, under the service conditions, had not
been compromised. The areas of greatest corrosion damage have been repaired.
Following receipt of this information, three excavations were carried out along the pipeline
route close to locations where defects had been recorded and these endorsed the findings of
the intelligent pig survey. External corrosion was confirmed and pit depth measurements were
in accordance with the expectations from the intelligent pig survey. The cause of the external
corrosion was established as disbonded coating such that cathodic protection currents were
prevented from flowing to the steel substrate surface. Pipe/soil potential measured in a
conventional manner (ON) with the reference electrode positioned close to the pipeline at the
location indicated a level of –1012mV with respect to Cu/CuSO4. Investigations carried out at
the time concluded that the disbonded coating was confined to the heat shrink field joints. It
was later established that the coating disbondment had also occurred to the factory applied 3
Layer LDPE [2].
A survey technique was therefore required that would indicate the presence of disbonded
coating together with locations where corrosion may not have occurred to the steel substrate
but the risk prevailed. Ideally the characteristics of disbonded coatings under the conditions
where the void was either filled with a conductive electrolyte or not was required. As no
single proprietary method was available to determine this condition, trials were carried out
using traditional, well proven methods. These included Close Interval Potential Survey
(CIPS) techniques to closely examine the profile of applied cathodic protection values
together with a recorded Direct Current Voltage Gradient (DCVG) to examine the
characteristics of coating defects and the capacitance effects of voltage gradient and
attenuation. The intent was to determine, as far as possible using highly accurate
measurements, that the cathodic protection potential at the steel interface was operating at a
level that complied with international standards for minimum polarised values and that
voltage gradients were acceptable and operating within conventional limits.
A trial survey programme was set up and carried out during April and May 2005 under
different circumstances to replicate the prevailing conditions and to characterise the pipeline
in an attempt to determine the most likely locations of disbonded coating.
SURVEY DETAILS
In order that the trials would provide meaningful data a specification was prepared to ensure
that the testing carried out was to the highest possible accuracy. Consultants were therefore
engaged to prepare a specification for the proposed trials. The specification detailed
requirements for the characterisation of the pipeline in respect of cathodic protection polarised
potentials, cathodic protection currents flowing, other DC earth currents flowing along the
pipeline and AC earth currents flowing along the pipeline. This was required in order to
provide base indices against which the close interval potential and direct current voltage
gradient surveys could be calibrated. Specified and rigorous requirements were also detailed
for the close interval potential and direct current voltage gradient survey equipment, the
accuracy of synchronisation between current interrupter units and data loggers, the accuracy
of measured voltage values, the regular calibration and demonstration of all these
characteristics in the field, and the competence of operating personnel. This was to ensure that
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The Southern African Institute of Mining and Metallurgy
8th International Corrosion Conference
N Webb
precision measurements were recorded by personnel experienced in the measurement and
interpretation of the data that was collected. In total nine tests were carried out on selected
sections of pipe where coating defects including disbondment had been previously noted and
on sections which were thought to be free of coating defects and disbondment. Seven of these
tests were recorded CIPS and DCVG, where data sets were recorded and profiled for analysis.
A recorded Pearson survey was also used as a comparative technique in an attempt to seek
synergy. The Pearson survey provided information regarding through–coating defects, where
the steel substrate had become exposed to the electrolyte but provided no information which
was indicative of coating disbondment. Following a two trial surveys this technique was
abandoned.
For the purposes of clarity the following information is provided:
Close Interval Potential Survey (CIPS)
The technique entailed the measurement of the structure/electrolyte/reference electrode
potential (ON and Instant OFF) at close intervals along the route of the selected pipeline
section. Typically, pipe/soil potential measurements were recorded at between 1 and 2 metre
intervals along the pipeline route. The potential measurements were recorded by a specifically
designed cathodic protection measurement data logger from which a graphical profile of the
pipe/soil potential data could be produced following the completion of the data collection
survey. The accuracy of potential measurements recorded was determined, in part, by the
location of the reference electrode relative to the protected structure. This was as close to the
crown of the pipe as reasonably possible but was restricted in the field by the practical
placement of the reference electrode with respect to the pipeline. It was not always possible to
place the electrode directly over the crown of the pipeline at all locations due to vegetation
growth and other obstacles.
Right of way showing Jungle encroachment
CIPS surveyor with DCVG trailing cable.
In order that the Instant OFF potential measurements were as accurate as possible, minimising
the IR drop error caused by the cathodic protection current flow, the applied cathodic
protection current was interrupted for a brief time period. Typically all cathodic protection
current sources to the section of pipeline being protected were interrupted simultaneously and
the Instant OFF potential measured, before depolarisation of the pipe but after switching
spikes and pipeline capacitive effects had decayed. This wait time for switching spikes and
reactive effects was normally within 100 – 200 ms of the switch OFF time. It was necessary
that the data logger measurement period occurred immediately after this wait period and that
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The Southern African Institute of Mining and Metallurgy
8th International Corrosion Conference
N Webb
all switching devices and the data logger were sufficiently synchronised. The 10ms specified
accuracy of the system ensured this, and that all switching devices were switched open circuit
for the entire OFF measurement period. In order to minimise depolarisation of the pipe during
the survey a current interruption cycle ratio of 4:1 ON: OFF was typically used.
Where fluctuating stray earth currents were suspected to be present and affecting the pipe/soil
potential with time, a second (static location) data logger was deployed to collect pipe/soil
potential data simultaneously on the same synchronised time base as the (mobile) data logger
which was used to survey along the pipeline. The data was used for the purpose of
identification of interaction effects which could cause local variances in pipe/soil potential
measurement at specific times/locations.
Direct Current Voltage Gradient Survey (DCVG)
The technique measured voltage gradients caused by the cathodic protection current flow
through the soil to the pipeline (without any additional or increased impressed current which
is sometimes employed in analogue DCVG measurements). Greater current density occurs at
coating defects that expose bare steel with resulting increased voltage gradients in the soil.
The locations of defects in the external coating are therefore identified by peaks in the soil
voltage gradients.
In conventional, non-recorded analogue DCVG surveys, the measurements recorded are not
absolute but show changes in the voltage gradient by indicating a fluctuating value on a centre
zero analogue instrument.
Transformer
Rectifier
Precision
Timer /
Switching
Device
Optional
Static
Data
Collector
Defect
Defect
Defect
Stray
Current
Soil Variance
0
Metallic
Contact
Static Data
on another
graph
-200
DCVG
+200
0
-200
Precise Time Linkage Mobile , DCGV and Static
Combined Enhanced CIPS and Recorded DCVG to Indicate Defect Size
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The Southern African Institute of Mining and Metallurgy
8th International Corrosion Conference
N Webb
In the combined CIPS and DCVG technique used in the survey work (see previous
illustration) the actual ON/OFF values were measured and recorded on the data logger at the
same time and at the same location as the ON/OFF potential measurements. A detailed
resumé of the technique may be found in reference [1]. The data collections were effectively a
combination of the CIPS, DCVG and Intensive methods defined in EN 13509: 2003 and DIN
50925: 1992.
The simultaneous recording of CIPS and DCVG data allows the calculation of defect size in
terms of the formulae developed by Baeckman and Schwenk [3]
ANALYSIS OF RESULTS
During the surveys, it was noted from the daily data download and review that there appeared
to be a large number of soil voltage gradient indications, and that these indications were
frequently not of the classical sine wave shape that would be expected of individual defects.
Given that the pipeline field joints were a known point of concern, the survey team was
concerned that the standard spacing between the front and back poles for DCVG
measurement (10m) was possibly too close to the pipe joint length of 12m to allow adequate
resolution. Surveys with lateral electrode DCVG measurements and trials with both 9m and
18m spacing in the front – back trailing electrode DCVG measurements were undertaken, and
a final electrode spacing of 9m front – back trailing electrodes was selected for all further
work.
During the survey, it was further noted that there was a significant variation in soil potential
gradient measurements depending on the exact position of the poles. This variation was of a
similar magnitude to the actual gradients being measured. Further, it was at times impossible
to survey over the crown of the pipe due to encroachment of vegetation. The accuracy of the
individual reference electrodes was checked on several occasions prior to, during and after
individual surveys. These tests comprised both comparison against a standard electrode and
measurement of differential between electrodes at a common point during the survey. The
electrode potentials were found to be within the specified limits at all times. The survey
conditions were ideal, with no significant stray current, and wet conditions ensuring good
electrical contact between the electrodes and the ground. The “scatter” was therefore related
to the positioning of the electrodes, resulting from very small potential gradients in the soil.
As the linear resolution of the survey was one metre, several readings were obtained per metre
due to the slow survey speed. In order to extract significant data from the results, the raw data
was processed to provide a single “average” data point per metre. This exercise has clarified
the graphs and facilitated the identification of very small gradients along the route.
The figure below illustrates the resulting traces of CIPS, DCVG and calculated defect size
from a typical section of the survey.
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The Southern African Institute of Mining and Metallurgy
8th International Corrosion Conference
N Webb
Defect Size
Equivalent Defect
radius (m)
0.2
0.1
0
-0.1
-0.2
0
100
200
300
400
500
300
400
500
DCVG
-100
Gradient (mV)
Chainage (m )
-50
0
50
100
0
100
200
CIPS
-1500
Defect marker
-700
Access path
Detour
-900
Marker post
201.6
-1100
Defect marker
-1300
TP 90
Potential (mV)
Chainage (m )
-500
0
100
200
Chainage (m )
300
400
500
Test 2 - 22/04/2005 - CIPS and Recorded DCVG from TP 90 towards Rabi
The extensive, albeit small, gradient pattern exhibited along the pipeline sections surveyed
was found to be related to the pipe joint length. There were many instances where the gradient
peaks were exactly 12m apart. A typical example is shown in the figure below.
Recorded DCVG channel indicating 12m signature
Gradient (mV)
-50
-25
0
25
50
650
662
674
686
698
710
722
734
746
Chainage (m )
758
770
782
794
806
818
830
Extract of the “gradient” channel plotted on 12m grid
However, most of these did not have through–coating defects. This is indicated by these
locations showing the same soil potential gradient profile with the CP current on and off;
through–coating defects would have shown significant variation between the ON and OFF
soil potential gradient. Those that did exhibit a differential between “ON” and “OFF”
gradients are the most likely candidates for corrosion underneath the disbonded sleeves as
they would appear to have through–coating defects that will allow electrolyte to access the
steel surface under the disbonded coating.
Examination of the survey results revealed that those indications with a differential between
“ON” and “OFF” soil potential gradients were accompanied by a characteristic “valley” in the
CIPS traces. This is further evidence that these locations are typical of through–coating
defects: they may be with or without disbonded coating.
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The Southern African Institute of Mining and Metallurgy
8th International Corrosion Conference
N Webb
The soil potential gradient traces which show no difference between “ON” and “OFF”
(standing gradients) cannot be related to through–coating defects as there is no current flow.
Similar characteristics have been observed on other surveys where disbonded coatings were
known to exist, and where the electrical characteristics of the outer insulation layer had been
compromised. The exact mechanism of this phenomenon is unknown at present, but is
postulated to be related to capacitance effects between the pipe and the soil.
Indications are that the combined CIPS and Recorded DCVG survey has apparently provided
a means of locating disbonded sleeves and differentiating between those where corrosion
beneath the sleeve is likely to occur as a result of capillary water ingress.
The “standing” and “differential” gradients revealed by the survey are apparently associated
with joint sleeves, with the differential gradient locations displaying typical coating defect
characteristics in the CIPS results. Defect sizes are apparently small, which is commensurate
with capillary defects under sleeves.
It is intended that these indications will be verified by excavation and visual examination of
pertinent locations.
A second aspect of the survey, which is related to the conventional use of CIPS to establish
polarised CP levels, was the relatively high IR factor identified in the ON readings for the
pipeline. The pipeline CP system had historically been operated based on the ON readings at
test posts. Using a relatively conservative IR factor, extrapolation of historical test post
potential data revealed that the pipeline had possibly not been fully protected. This may have
contributed to the extent of corrosion at the disbonded sleeves. The survey also confirmed that
the test post data at 3km intervals was not fully representative of conditions throughout the
pipeline.
The following figure indicates a recent set of test post ON data with an extrapolated OFF
data-set based on IR factors observed during the survey.
ON and extrapolated OFF potential data from 3km spaced test points.
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The Southern African Institute of Mining and Metallurgy
8th International Corrosion Conference
N Webb
CLIENT PERSPECTIVE
It is too early to conclude whether this new technique (without over-impressed DC) gives
significantly more detailed and valuable information, compared with the association of
“conventional” ON/OFF CIPS and DCVG (unrecorded, generally performed using additional
current source to accentuate the voltage gradients at the coating defects, carried out
separately). The first analysis of data has given a good enough impression to justify further
study of the “Integrated method”. This will be done, first through a complete and detailed
comparison of the results obtained on the sections surveyed with the conventional methods
and this new one, together with the results of In-line Inspection. Depending on the results of
this comparative study, a programme of new surveys and measurements will be formulated
and carried out on the field.
The final objectives are to be able to develop survey methods that will ensure:
- Detection of all corrosion locations underneath disbonded coatings
- Identification and location of disbonded coating areas
- Appreciation of corrosion risks underneath disbonded coatings
(presence or not of electrolyte and renewal thereof).
In order to achieve these ambitious objectives (if they are achievable), a lot of effort and
further field testing will be required in the near future.
Collaboration by all - end users and service specialised companies - is necessary to expedite
this process.
***
REFERENCES
[1] Brian Wyatt, “Advanced Systems Of Overline Assessment Of Coatings And Cathodic
Protection”, UMIST Cathodic Protection Conference, UMIST, Manchester10-11th February
2003.
[2] Marcel Roche, Denis Mélot, Gildas Paugam, “Recent experience with pipeline coating
failures”, 16th International Conference on Pipeline Protection, Paphos, Cyprus, 2-4 Nov.
2005, BHR Group
[3] Von Baeckmann, W. Schwenk, W. & Prinz, W., eds., “Handbook of Cathodic Corrosion
Protection”, 3rd Edition, Gulf Publishing Co, Houston, Texas, 1997
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