Section 2 – Definitions

ECDA Terminology

The following are definitions of some key terms used:

Active

A state in which a metal is corroding without significant influence or shielding by the reaction product.

Aerobic

Oxygen-containing.

Anaerobic

Free of air or uncombined oxygen.

Anomaly

Any deviation from nominal conditions in the external wall of a pipe, its coating, or the electromagnetic conditions around the pipe.

Anode

The electrode of an electrochemical cell at which oxidation occurs. Electrons flow away from the anode in the external circuit. Corrosion usually occurs and metal ions enter the solution at the anode.

Aspect Ratio

The ratio of crack length to crack depth.

Asphalt Coating

Asphalt-based anti-corrosion coating.

B31G

A method of calculating the pressure carrying capacity of a corroded pipe (see references ASME B31.G).

Categorization

The process of estimating the need for repair of each indirect inspection indication based on current corrosion activity plus the extent and severity of prior corrosion.

Cathode

The electrode of an electrochemical cell at which reduction is the principal reaction. Electrons flow toward the cathode in the external circuit.

Cathodic disbondment

The destruction of adhesion between a coating and the coated surface caused by the products of a cathodic reaction.

Cathodic protection (CP)

A technique to reduce the corrosion of a metal surface by making that surface the cathode of an electrochemical cell.

Class Location

A geographical area, as classified according to its population density and other characteristics that are considered when a pipeline is designed, and pressure tested.

Classical SCC

A form of SCC on underground pipelines that is intergranular and typically branched and is associated with an alkaline electrolyte (pH about 9.3). Also referred to as high-pH SCC.

Classification

The process of estimating the likelihood of corrosion activity at an indirect inspection indication under typical year-round conditions.

Close interval survey (CIS)

A method of measuring the potential between the pipe and earth at regular intervals along the pipeline.

Cluster

A grouping of stress corrosion cracks (colony). Typically stress corrosion cracks occur in groups consisting of hundreds or thousands of cracks within a relatively confined area.

Coal Tar Coating

Coal tar-based anti-corrosion coating.

Colony

A grouping of stress corrosion cracks (cluster). Typically stress corrosion cracks occur in groups consisting of hundreds or thousands of cracks within a relatively confined area. See Cluster.

Collinear

Lying along the same line (coaxial). A term used to describe spatial relationship of adjacent cracks.

Corrosion

The deterioration of a material, usually a metal, that results from a reaction with its environment.

Corrosion activity

A state in which corrosion is active and ongoing at a rate that is sufficient to reduce the pressure carrying capacity of a pipe.

Corrosion Assessment Techniques:

Level 1 assessments shall be limited to components with one-sided widespread pitting damage designed to a recognized code or standard using an equation that specifically relates pressure to a required wall thickness. The only load considered is internal pressure.

Level 2 assessments can be used to evaluate components that do not meet Level 1 assessment criteria. The Level 2 Assessment procedures are used to evaluate all four categories of pitting: widespread pitting, localized pitting, pitting within a locally thin area, and a locally thin area in a region of widespread pitting.

The Level 2 Assessment rules provide a better estimate of the structural integrity of a component because a measure of the actual damage parameter, the pit-couple, is directly used in the assessment. The Level 2 assessment should be used when the pitting damage occurs on both sides of the component. Level 3 assessments can be used to evaluate components that are not covered by, or do not pass a Level 1 or Level 2 Assessment.

Level 3 Assessment procedures are intended to evaluate more complex regions of pitting, loading conditions, and/or components with details where only limited design rules are provided in the original construction.

Covered Segment

A segment of gas transmission pipeline located in a high consequence area. The terms gas and transmission line are defined in § 192.3.

Crack Coalescence

Joining of cracks that are in close proximity to form one larger crack.

Critical Flaw Size

The dimensions (length and depth) of a flaw that would fail at a given level of pressure or stress.

Defect

A pipe wall anomaly that reduces the pressure carrying capacity of the pipe.

Direct-current voltage gradient (DCVG)

A method of measuring the change in electrical voltage gradient in the soil along and around a pipeline to locate coating holidays and characterize corrosion activity.

Direct examination

Inspections and measurements made on the pipe surface at excavations as part of ECDA.

Disbonded coating

Any loss of adhesion between the protective coating and a pipe surface as a result of adhesive failure, chemical attack, mechanical damage, hydrogen concentrations, etc. Disbonded coating may or may not be associated with a coating holiday. See also cathodic disbondment.

ECDA Region

A section or sections of a pipeline that has similar physical characteristics and operating history and in which the same indirect inspection tools are used.

Electrolyte

A chemical substance containing ions that migrate an electric field. For the purposes of this recommended practice, electrolyte refers to the soil or liquid adjacent to and in contact with a buried or submerged metallic piping system, including the moisture and other chemicals contained therein.

Electromagnetic inspection technique

An above-ground survey technique used to locate coating defects on buried pipelines by measuring changes in the magnetic field that are caused by the defects.

External Corrosion Direct Assessment (ECDA)

A four-step process that combines pre-assessment, indirect inspections, direct examinations, and post-assessment to evaluate the impact of external corrosion on the integrity of a pipeline.

Fatigue

The phenomenon leading to fracture of a material under repeated or fluctuating stresses having a maximum value less than the tensile strength of the material.

Ferrous material

A metal that consists mainly of iron. In this Recommended Practice, ferrous materials include steel, cast iron, or wrought iron.

Fracture Toughness

A measure of a material’s resistance to static or dynamic crack extension. A material’s property used in the calculation of critical flaw size for crack-like defects.

Girth Weld

The circumferential weld that joins two sections of pipe.

Gouge

A surface imperfection caused by mechanical damage that reduces the wall thickness of a pipe or component.

GTI

Gas Technology Institute

High-pH SCC

A form of SCC on underground pipelines that is intergranular and typically branched and is associated with an alkaline electrolyte (pH about 9.3). Also referred to as classical SCC.

Holiday

A discontinuity in a protective coating that exposes unprotected surface to the environment. This includes disbonded areas and laminations.

Hoop Stress

Circumferential stress in a pipe or pressure vessel that results from the internal pressure.

Hydrostatic Testing

Pressure testing of sections of a pipeline by filling it with water and pressurizing it until the nominal hoop stresses in the pipe reach a specified value.

Immediate indication

An indication that requires remediation or repair in a relatively short time span.

Instruction

A detailed sequence of steps to be followed to apply an NDE method of inspection.

Interaction Rules

According to ASME B31.4[1] there are two type of Interaction Rules:

Type I Interaction – If the circumferential separation distance, C, is greater than or equal to 6 times the wall thickness required for design, the areas A1 and A2 should be evaluated as separate anomalies. If the circumferential separation distance is less than six times the wall thickness, the composite area (A1 + A2 − A3) and the overall length, L, should be used. See figure 1 below.

Type II Interaction – If the axial separation distance, L3, is greater than or equal to1 in. (25.4 mm), the areas A1 and A2 should be evaluated as separate anomalies. If the axial separation distance is less than 1 in. (25.4 mm), area A1 plus A2 should be used and the length, L, should be taken as L1 + L2 + L3. See figure 2 below.

Indication

Any deviation from the norm as measured by an indirect inspection tool.

Indirect inspection

Equipment and practices used to take measurements at ground surface above or near a pipeline to locate or characterize corrosion activity, coating holidays, or other anomalies.

In-Line Inspection (ILI)

The inspection of a pipeline from the interior of the pipe using an ILI tool. These tools are known as pigs or smart pigs also called intelligent or smart pigging.

ILI Tool

An instrumented device or vehicle that uses a nondestructive testing technique to inspect the pipeline from the inside or that uses sensors and other equipment to measure one or more characteristics of the pipeline. Also known as intelligent or smart pig.

Intergranular Cracking

Cracking in which the crack path is between the grains in a metal (typically associated with high-pH SCC).

Instant “OFF” potential

A measurement of a pipe to electrolyte potential made without perceptible delay following the interruption of cathodic protection.

Investigative Dig

An inspection of a pipeline at a discrete location exposed for examination.

Leak

Product loss through a small hole or crack in the pipeline.

Low-pH SCC

A form of SCC on underground pipelines that is transgranular and is associated with a near neutral-pH electrolyte. Typically, this form of cracking has limited branching and is associated with some corrosion of the pipe surface. Also referred to as near-neutral-pH or non-classical SCC.

Magnetic Particle Inspection (MPI)

A nondestructive inspection technique for locating surface cracks in a steel using fine magnetic particles and a magnetic field. See also ASTM E 709.10.

Maximum Allowable Operating Pressure (MAOP)

The maximum internal pressure permitted during the operation of a pipeline.

Mechanical Damage

A type of anomaly in a pipe cause by the application of an external force. Mechanical damage can include denting, coating removal, metal removal, metal movement, cold working of the underlying metal, and residual stresses, any one of which can be detrimental.

Metallography

The study of the structure and constitution of a metal as revealed by a microscope.

Microbiologically induced or influenced corrosion (MIC)

A form of accelerated corrosion that results from certain microbes and nutrients in the soil.

Monitored indication

An indication that is less significant than scheduled and which does not need to be addressed or require remediation or repair before the next scheduled re-assessment of a pipeline segment.

Near-Neutral-pH SCC

A form of SCC on underground pipelines that is transgranular and is associated with a near-neutral-pH electrolyte. Typically, this form of cracking has limited branching and is associated with some corrosion of the crack walls and sometimes of the pipe surface. Also referred to as low-pH or nonclassical SCC.

Operator Qualification (OQ)

A regulated covered task on a pipeline facility identified by a pipeline operator that is performed on that facility, is an operation or maintenance task, is a regulatory requirement and affects the operation or integrity of the pipeline.

Pipe to electrolyte potential

The potential difference between the pipe metallic surface and the electrolyte that is measured with reference to an electrode in contact with the electrolyte. This measurement is commonly termed pipe-to-soil.

Polarization

The change from the open-circuit potential as a result of current across the electrode/electrolyte interface.

Potential Impact Circle

A circle of radius equal to the potential impact radius (PIR).

Potential Impact Radius (PIR)

The radius of a circle within which the potential failure of a pipeline could have significant impact on people or property. PIR is determined by the formula r = 0.69 * (square root of (p*d2)), where r = is the radius of a circular area in feet surrounding the point of failure, p is the maximum allowable operating pressure (MAOP) in the pipeline segment in pounds per square inch and d is the nominal diameter of the pipeline in inches.

Note: 0.69 is the factor for natural gas. This number will vary for other gases depending upon their heat of combustion. An operator transporting gas other than natural gas must use Section 3.2 of ASME/ANSI B31.8S to calculate the impact radius formula.

Remediation

Repair or mitigation activity an operator takes on a covered segment to limit or reduce the probability of an undesired event occurring or the expected consequences from the event.

Residual Stress

The locked-in stress present in an object that results from the manufacturing process, heat treatment, or mechanical working of the material.

RSTRENG

An industry approved program designed to calculate the safe pressure carrying capacity of corroded pipe (see references PHMSA, PRCI, ASME).

Rupture

A failure of a pipeline that results from fracture propagation and causes an uncontrolled release of the contained product.

Scheduled indication

An indication that is less significant than immediate, but which is to be addressed before the next scheduled re-assessment of a pipeline segment.

Segment

A portion of a pipeline that is (to be) assessed.

Shielding

Preventing or diverting the cathodic protection current from its intended path to the structure to be protected.

SCC

An SCC cluster is assessed was defined to be to be “significant” by the Canadian Energy Pipeline Association (CEPA) (3) in 1997 provided that the deepest crack, in a series of interacting cracks, is greater than 10% of the wall thickness and the total interacting length of the cracks is equal to or greater than 75% of the critical length of a 50% through-wall flaw that would fail at a stress level of 110% of SMYS. CEPA also defines the interaction criteria. The presence of extensive and “significant” SCC typically triggers an SCC mitigation program (See discussion under Post-Assessment Step), but a crack that is labeled “significant” is not necessarily an immediate threat to the integrity of the pipeline.

Sound engineering practice

Reasoning exhibited or based on thorough knowledge and experience, logically valid and having technically correct premises that demonstrate good judgment or sense in the application of science.

Specified Minimum Yield Strength (SMYS)

The minimum yield strength of a material prescribed by the specification or standard to which the material is manufactured.

Stray current

Current through paths other than the intended circuit.

Stress Corrosion Cracking (SCC)

Cracking of a material produced by the combined action of corrosion and tensile stress (residual or applied).

Stress Corrosion Cracking Direct Assessment SCCDA

The stress corrosion cracking direct assessment process.

Subcritical Crack

A crack that is not large enough to cause spontaneous failure at a specific pressure or stress.

Structure-to-Electrolyte Potential

The potential difference between the surface of a buried or submerged metallic structure and the electrolyte that is measured with reference to an electrode in contact with the electrolyte.

Subcritical Crack

A crack that is not large enough to cause spontaneous failure at a specific pressure or stress.

Tensile Stress

Stress that tends to elongate the material.

Tenting

A tent-shaped void formed along the seam weld of a pipeline where the external tape coating bridges from the top of the weld to the pipe.

Terrain Conditions

The collective term used to describe soil type, drainage, and topography. Often used as input in the generation of SCC predictive models.

Transgranular Cracking

Cracking in which the crack path is through the grains of a metal (typically associated with near-neutral-pH SCC).

Voltage

An electromotive force or a difference in electrode potentials, commonly expressed in volts.

Yield Strength

The stress at which a material exhibits a specified deviation from the proportionality of stress to strain. The deviation is expressed in terms of strain by either the offset method (usually at a strain of 0.2%) or the total-extension-under-load-method (usually at a strain of 0.5%).