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8/10/2019 Datp Process

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External Corrosion Direct Assessment Summary

ECDA is a four-step process. Each step is mandatory for an effective ECDA program. ECD

requires integration of information:

1) Pre-assessment - combining a pipelines physical characteristics, operating history a

prior inspections;

2) Indirect inspection - conducting complementary above the ground eld voltage & curre

examinations);3) Direct examination - exposing pipe for surface measurements including remaining w

thickness and

4) Post-assessment integrating all as a comprehensive integrity assessment includi

validating the process and the estimating the time interval until the next integrity evaluati

is required.

The ECDA Standard - NACE RP 0502

The NACE ECDA Standard is intended to address external corrosion for onshore ferro

pipe. ECDA is a continuous improvement process. Through successive ECDA application

a pipeline operator will identify and address locations where corrosion activity has occurre

is occurring, or may occur. As such, ECDA provides an advantage over alternative integr

assessment methodologies such as in line inspection (ILI) and pressure testing by locati


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Pre-assessment:

Pre-assessment requires the integration of historical, construction, operations and maintenance

records. This can be a simply meeting of company experts, using paper records pulled from

different departmental ling cabinets. Eventually the integration of electronic records from

these diverse departments can be used with rules devised by the experts (i.e. the integrity or

with consequences, risk assessment modeling) to presort the huge amounts of information

and present the summaries on a simple map. Maps have been found useful to bring together

abstract integrity evaluations as a magnitude at a location. Once the volumes of data have been

organized then decisions can be made. These decisions are the beginning of any integrity or

risk management program.

The typical records which are available in various departments should be: purchasing and

construction les, up to date alignment sheets, operating history, foreign crossing history, thirdparty adjacent construction and one call notices, land use and typical vegetation descriptions,

topography and elevation diagrams, leak and rupture locations and reports, maintenance

history with forensic reasons for the pipe replacements and other repairs, current and prior

above ground inspection reports, previous ILI reports, coating condition and other observations

of electrolytes and soils in excavation reports, corrosion survey reports for test lead, CIS, DCVG

or other above ground inspections, and there could be more. Data integration is neither trivial

nor inexpensive.

The minimum set of data to be collected is organized into ve sets of elements; pipe related,

construction related, soils and environment related, corrosion protection, and pipeline operations

considerations. These ve are further subdivided into individual data items as detailed in the

NACE standard. Each element has been provided with comments which help the operator

judge how each element may impact the tool(s) selection, the denition of the ECDA region(s),

and the interpretation of the above ground inspections to determine the integrity of the pipe line.

The NACE standard requires more data than in the prescriptive requirements of B31.8S.


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Denitions are provided to reduce the uncertainty especially when current jargon has one

department collecting the same observation but with a different label and unfortunately a different

set of units. Consistency in collecting data over the years is essential. As a hint databases can

be designed to have the jargon on the screen but use the standard denitions and units in the

background for each of the elds. The data is needed to correlate the voltage/current readings

with the observed wall loss in those sites excavated. The operator needs to prove that the

integrity assessment algorithms and assumption can link similar sites with similar soils, pipe

and coating materials, and therefore similar patterns in voltage/current readings will suggest the

similar distributions and depths of corrosion pits and general wall loss.

The outcome from Pre-assessment provides the operator with three essential conclusions:

1. ECDA is a viable process to measure integrity2. Two complimentary tools chosen to inspect the corrosion protection performance

3. The start and nish locations of each ECDA region.

The NACE Standard provides guidance for the selection of appropriate complementary above

ground inspection techniques. Not all the locations along the pipeline are suitable for ECDA

inspections. A convenient concept used to select the appropriate inspection technology from

a list of possibilities is to complete the current path. The wire from the multimeter to the pipe

is the easy trace, but the path back starts across the coating or damage, travels through the

soil to the ground bed and back up the wire lead. This return path can sometimes become

interrupted. Rock backll, paved roads, lack of soil moisture (high resistivity), coating shielding

liquid pockets, and other local situations can interrupt this return path and make the inspection

invalid. With greater experience the ECDA Tool Selection Matrix will be able to give probability

of detection and the probability of identication (POD & POI), the chance of nding and the

chance of detecting severe corrosion rather than a yes /no indication.

The region is dened by two complementary tools and although the complete; start to nish,

distance from A to D maybe suitable for ECDA a short section may have to use an alternate

inspection tool say to measure what is happening under a paved parking lot were an

electromagnetic approach is more convenient. This suggest there now will be segments A to B

for the rst two tools selected, B to C under the pavement with different inspection techniques

and C to D for the rst set of tools again.


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Direct Examination:

The object of the direct examination step is to dig at least those indications wh

most severe. Excavations are also needed to collect information that will help pred

corrosion rates. The following activities are to be completed in this the third ECDA s

1. Categorization of indications found in step two into minor, moderate, severe

severe; the latter two classications are to be are immediately excavated;

2. Excavation and data collection is done in areas were corrosion activity is mos

operator must have procedures for what data is to be collected before durin

excavation.

3. Measurement of the coating damage and corrosion defects underneath. T

is to extend the bell hole either way until no severe coating damage or nocorrosion is found for at least ve feet.

4. Evaluations of the remaining strength of the defects can be done using th

cluster length and maximum depth, and the methodology of B31G, RST

similar engineering analyses such as API RP 579,

5. Root cause analysis will help determine the appropriate mitigation activit

future corrosion is mitigated.

6. In-Process evaluation is an interim check on the assumptions. If smaller tha

defects are found then the criteria for severe, moderate or minor can be

worse then the operator needs to perform additional excavations or ree

appropriateness of using ECDA for the integrity evaluation. A small sample si

leads to erroneous assumptions.

Essentially a prudent operator follows a well proven procedure. The pipe is excavat

normal safety rules, the coating and ditch inspected, while samples of liquids and oth

related deposits are taken as the coating is removed and the pipe cleaned to barecorrosion wall loss; the depth, axial and circumferential extent of the defect photog

measured. Next operators should use some form of crack detection technology suc

or MPI to inspect for cracks particularly if mechanical damage is suspect. All immed

and most of the scheduled defects uncovered at each site will be repaired while the p

exposed for examination. The corrosion defect geometry can be put into the

calculation to provide the immediate or scheduled classication. The NACE RP 050

provides additional guidance on classication and repair.

In-process evaluation is an essential real time component of the analysis. The EC

helps nd representative corrosion but might not nd all the corrosion defects in an EC

If corrosion defects are found that exceed allowable limits then it should be assume

similar defects might be in the same ECDA Region. These feed back loops are in

quality processes as shown below.

The most difcult determination is the number of bell holes an operator is required t

If the above ground inspections nd locations were the criteria are not met the

must occur. The operator should immediately excavate multiple severe indication

moderate or severe indications in regions of moderate prior corrosion, and moderate

in regions of prior severe corrosion. The remaining severe indications, the remainin

indications in regions of moderate prior corrosion and minor indications in region

prior corrosion need to be scheduled for later action. The in process evaluation and

Indirect Inspection:

The objective of the indirect inspection step is to estimate areas of active corrosion. Different

survey crews probably conduct individual inspections in different years. Each set of results will

need to be superimposed for comparison. It is important to maintain a meta data set or an

estimate of quality for these survey results. The survey monuments for the alignment sheet, the

air photography, the prior year inspections could all have location opinions that differ from the

current survey crew. The CIS crew might use GPS with a +/- half a foot accuracy while the original

alignment sheet survey could be +/- 60 feet. Eventually someone will have the responsibility to

align these records onto the map prior to making integrity decisions. It is preferable to keep the

accuracy always with the original record since this essential quality term allows the interpreter

to control how far drawings can slide around over each other on the light table to see if theinspections or other ECDA indications fall roughly on top of each other. The meta data wrapper is

needed for constructing error bars, these guide the analyst in determining how far the image can

be safely manipulated. Today this electronic manipulation will probably occur in a information

system software package which is designed for geographical measurements and allows rubber

banding of known reference locations such as that provided by Baseline Technologies.

Once the indications from the above ground inspections are aligned with the Pre-assessment

and historical knowledge, the excavation sites can be located and prioritized for the examination

step. These would be classied as severe; those with the highest likelihood of corrosion activity,

moderate; possible or questionable corrosion activity, or minor; lowest likelihood of corrosion

activity. NACE provides a table to guide operators when deciding how to place locations in

the severity classication. The two inspections should provide consistent results both between

inspections and with the Pre-assessment hypotheses.

Discrepancies need to be resolved to see if the ECDA Regions were correctly chosen.Examinations of the severe category can begin while the discrepancies are still being

investigated. This clarication of the discrepancies may require reevaluating the ECDA Region

and possibly conducting a third complementary inspection.


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analysis will help the operator decide to excavate and recoat or x the impressed CP system. A

larger number of excavations can be expected if severe indications are indicated.

If the inspections return no indications or moderate indications then at least two excavations are

required at likely sites of corrosion for the rst application of ECDA. If the root cause investigation

or in process evaluation steps at these two sites suggest the assumptions require modication,

then the operator must conduct more excavations. If severe corrosion is found then the operator

must return to the Pre-assessment step and consider if ECDA is appropriate, are the inspection

techniques correct, and are there new start and nish points for the ECDA Regions?

Post-Assessment:

This stage prioritizes the mitigation and repair schedule, validates the process, and determines

the period of time until the next integrity evaluation for each of the ECDA Regions. Priority and

time require all the information gathered in the previous three steps. The standard assumes that

all of the immediate defects will have all been addressed (CP xed, coating repaired or pipe cut

out) and the scheduled defects prioritized so the worst will be xed rst during the interval until

the next integrity evaluation. Any of the moderate and probably most of the minor indications

will be expected to have been mitigated by either upgrading the cathodic protection system or

excavating to evaluated the corrosion then recoating before burial.

The distribution of defects remaining in the system will be a correlation of the Pre-assessment,

Inspection and Excavation data to determine the classication by inspection categories; versus

the number and size of the corrosion defects found in the associated bell holes. It is assumed

that the severe indications will have a corrosion distribution skewed towards greater coating

problems, and maybe wall loss, while the minor indications are locations of limited coating

damage with little or insignicant wall loss. The largest defect in these distributions of defect

size with the remaining moderate, and minor indications (the severe having been mitigated) andthe corresponding corrosion rate for each indication can be used in the performance regulations

to predict the length of time until the defect could corrode sufciently to leak. The data to support

both the estimate the size of the largest remaining defect and the corresponding corrosion rate

may not be available early in the program.

The prescriptive calculation for period between integrity evaluations is based on a conservative

half life estimate in the ECDA standard. Essentially the remaining half life of a ECDA Region is

determined by the size of the thinnest defect not repaired or replaced. The half life is determined

by dividing the remaining wall thickness by the corrosion rate and multiplying the answer by

0.43, a conservative constant.

Once the direct examinations are initiated, expected correlations will be conrmed. When this

information is correlated with soil analysis and other pre-assessment data, additional digs may

still be required. The ndings during direct examination will improve the correlation between

the CIS and DCGV voltage signals and the coating and pipe condition (general or pitting wall

loss). The correlation would determine the further extent of additional excavations. Using soilmoistures and other analyses when combined with other pre-assessment data, may allow

corrosion rates to be calculated. An estimate of the distribution of the defect sizes remaining

behind after mitigation and the corrosion rate at each site will set the re-inspection intervals

determined in step four, Post-assessment.


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Performance Measurement:

Performance or ECDA Effectiveness of the immediate activity is obtained by digging two bell

holes at selected locations. One location was to be categorized as scheduled (monitored if

no Scheduled exist) and one were there will be no inspection indications found. If defects are

found that are larger than expected and they would result in shorter intervals between integrity

evaluations then the whole ECDA process shall be re-evaluated or repeated or an alternative

integrity method such as ILI or hydro testing used.

Long-term improvement in ECDA performance can be tracked from a decreasing number of

excavations, the increase in total number of miles of pipe inspected, and/or the increase in

inspection mileage by each of the tools employed. There are a variety of suggestions to show the

integrity of the pipeline is improving. Monitoring change is the heart of continuous improvement.The ECDA process requires annual re evaluation and management of change. Assumptions

are continually evaluated and discrepancies force review of these assumptions. The ow chart

requires annual updating and reevaluation of the decisions made in prior iterations.

The collection of interrelated records and data needs to be organized for internal and external

auditing as well as to ensure continuous improvement. The historical data once veried remains

stable but the new decisions made, the current integrity inspections and excavations and post

assessment decisions need to be recorded in a consistent fashion. Section 7 of the NACE

ECDA Standard provides guidance and each operator is encouraged to document their records

in a clear concise, workable method that are pertinent to the four ECDA steps; pre-assessment,

inspection, examination, and post-assessment.

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