Manual for Determining the Remaining Strength of Corroded Pipelines

Scope

This document is intended solely for the purpose of providing guidance in the evaluation of metal loss unpressurized pipelines and piping systems. It is applicable to all pipelines and piping systems within the scope of the transportation pipeline codes that are part of ASME B31 Code for Pressure Piping, namely:ASME B31.4, Pipeline Transportation Systems for LiquidHydrocarbons and Other Liquids; ASME B31.8, GasTransmission and Distribution Piping Systems; ASMEB31.11, Slurry Transportation Piping Systems; anadems B31.12, Hydrogen Piping and Pipelines, Part PL. Where the term pipeline is used, it may also be read to apply to piping or pipe conforming to the acceptable applications and within the technical limitations discussed below.

Nomenclature

A = local area of metal loss in the longitudinal plane

AC = cross-sectional area of Charpy impactspecimen

A0 = local original metal area = p Lt

CV = Charpy V-notched impact absorbed energy

D = specified outside diameter of the pipe

d = depth of the metal loss

E = elastic modulus of steel

L = length of the metal loss

Le = effective length

M = bulging stress magnification factor

MAOP = maximum allowable operating pressure

MOP = maximum operating pressure

PF = estimated failure pressure = 2SFt/D

PO = operating pressure, may equal MAOP or MOP

PS = safe operating pressure = PF/SF

SF = estimated failure stress level

Flow = flow stress, defined in para. 1.7(b)

SO = hoop stress at the operating pressure, calculated as POD/2t

SUT = specified ultimate tensile strength at temperature, may equal SMTS

SYT = specified yield strength at temperature, may equal SMYS

SF = safety factor

SMTS = specified minimum tensile strength at ambient conditions

SMYS = specified minimum yield strength at ambient conditions

t pipe wall thickness

z = L2/Dt

ze = Le2/Dt

Analysis Level

The user may choose to conduct a Level 0, Level 1,Level 2, or Level 3 analysis, depending on the quantity and quality of data available with which to perform an evaluation, and on the desired degree of refinement of the analysis.

a) A Level 0 evaluation is one that relies on the tables of allowable defect length and depth found in section 3.These tables are carried over without change from earlier editions of ASME B31G and have been supplemented by the addition of tables in metric units. It is intended that a Level 0 evaluation be conducted in the field without the need for performing detailed calculations.

(b) A Level 1 evaluation is a simple calculation that relies on single measurements of the maximum depth and axial extent of metal loss. It is intended that a Level 1evaluation be conducted in the field by an engineer, corrosion technician, coating inspector, or other individual having appropriate training. A Level 1 evaluation is also suitable for use in prioritizing metal-loss anomalies identified by inline inspection.

(c) A Level 2 evaluation is one that incorporates greeter level of detail than a Level 1 evaluation in order to produce a more accurate estimate of the failure pressure. It typically relies on detailed measurements of the corroded surface profile, accounting for the actual distribution of metal loss, and involves repetitive computations that may be facilitated by the use of computer software or spreadsheets. It is intended that a Level 2evaluation be conducted by an engineer or technician having appropriate training. A Level 2 evaluation maybe suitable for use in prioritizing metal-loss anomalies identified by high-resolution inline inspection.

(d) A Level 3 evaluation is a detailed analysis of specific flaw in accordance with a user-defined methodology, with full justification for loadings, boundary conditions, material properties, and failure criteria. It is intended that a Level 3 evaluation be conducted by technical specialist having appropriate expertise in the subject of fitness-for-service assessment.

Material Properties and Other Data

(a) Specified minimum material properties shall bemused when conducting Level 0, Level 1, or Level 2 evaluations for the purpose of determining the need for are pair. Actual material properties from mill test reports(MTRs) or laboratory testing, if known with sufficient confidence to warrant their usage, may be used with Level 3 evaluations. Statistical representations of material properties may be used with Levels 1, 2, or 3 for purpose of establishing a probability of failure; however, the details of such analyses are outside the scope of this document.

(b) S Flow stress is a concept relevant to fracture mechanics and is used in the Level 1, Level 2, and Level 3evaluations. It is not a property specified in a material grade or finished product standard. Research indicates that it may be defined variously as given below.

(1) Flow for plain carbon steel operating at temperatures below 250°F (120°C) may be defined by S flow = 1.1 SMYS. S flow shall not exceed SMTS.

(2) S flow for plain carbon and low-alloy steel having SMYS not in excess of 70 ski (483MPa) and operating at temperatures below 250°F (120°C) may be defined by

S flow = SMYS + 10 ksi (69 MPa). S flow shall not exceed SMTS.

(3) S flow for plain carbon and low-alloy steel having SMYS not in excess of 80 ksi (551 MPa) may be defined by S flow = (SYT + SUT)/2, where SYT and SUT are specified at the operating temperature in accordance with the ASME Boiler and Pressure Vessel Code, Section II, Part D; applicable pipe product specification; or room temperature strength multiplied by the temperature derating factor specified by the applicable construction code. Linear interpolation of strength values is allowed between listed temperatures.

(c) This document does not prescribe which definition for flow stress should be used where more than undefinition applies. Where more than one definition applies, the various definitions produce acceptable though not necessarily identical results when used with any given evaluation method. It is noted that S flow was defined as 1.1 SMYS in previous editions of B31G. This definition remains an inherent element of the Level 0 assessment and is recommended with the Level 1 assessment performed in accordance with para. 2.2(a).

(d) Only the specified nominal wall thickness shall be used for the uncorroded wall thickness when conducting a Level 0 evaluation. If known with confidence, the actual uncorroded wall thickness may be used with a Level 1, Level 2, or Level 3 evaluation, with a suitable adjustment of the hoop stress due to internal pressure.

(e) Pipe body material may be considered to have adequate ductile fracture initiation properties for purposes of this Standard if the material operates at a temperature no colder than 100°F (55°C) below the temperature at which 85% shear appearance is observe din a Charpy V-notched impact test.

(f) Electric resistance welded (ERW) seams that have been subjected to a normalizing heat treatment, singled double submerged arc welded seams, and girth welds made using the shielded metal arc, gas metal arc, flux cored arc, and submerged arc processes (manual or3automated, and in any combination) are considered to have adequate ductile fracture initiation properties for purposes of this Standard. Other seam and weld types shall be investigated to establish fracture properties before applying methods described herein to metal loss affecting such welds. Consideration shall be given to the disposition of workmanship flaws or manufacturing flaws within a weld or seam that could interact with metal loss due to corrosion.

(g) Some operating conditions, such as low temperature service, or long-term exposure to sour environments or to very high temperatures, could adversely affect the ductility and fracture toughness properties of some materials. It is the user’s responsibility to consider such conditions where necessary before applying methods described herein.

EVALUATION METHODS

Level 0 Evaluation

Tables of allowable length of corrosion are found in section 3. The tables are carried over without change from previous editions of B31G and have been supplemented by the addition of tables in metric units. They were calculated from the equations for a Level 1 evaluation in accordance with the original B31G methodology. They provide a ready reference of maximum corrosion lengths for a range of pipe sizes and depths of corrosion. The tables may be used to determine the maximum allowable longitudinal extent of a contiguous area of corrosion or an interacting cluster of metal loss areas. Evaluations shall be carried out consistent with the procedure described in the following steps:

Step 1. Determine pipe diameter and nominal wall thickness from appropriate records or direct measurement of the pipe.

Step 2. Determine applicable pipe material properties from appropriate records.

Step 3. Clean the corroded pipe surface to bare metal.Care should be taken when cleaning corroded areas of a pressurized pipe.

Step 4. Measure the maximum depth of the corroded area, d, and longitudinal extent of the corroded area, L

Step 5. Locate the table corresponding to the size of the pipe, D.

Step 6. In the table, locate the row showing a depth equal to the measured maximum depth of the corroded area. If the exact measured value is not listed, choose the row showing the next greater depth.

Step 7. Read across to the column showing the wall thickness of the pipe. If the nominal wall thickness is not listed, use the column for the next thinner wall. The value, L, found at the intersection of the wall thickness column and the depth row is the maximum allowable longitudinal extent of such a corroded area.

Step 8. The metal loss area on the pipe is acceptable if its measured length, L, does not exceed the value of L given in the table.

The tables produce results that may be more conservative than those obtained by performing a Level 1, Level 2,or Level 3 analysis, particularly for operating hoop stress levels less than 72% of SMYS, and also for very long corroded areas. Therefore, the tables may show that a given corroded area is unsuitable for the current operating pressure, while the use of equations given below may show that it is acceptable.

The tables were designed to provide a minimum factor of safety of 1.39 for pipelines operating with a hoop stress of 72% of SMYS. Application of the tables to the evaluation of corrosion in pipelines operating at hoop stress levels greater than 72% of SMYS will result in a factor of safety that is proportionately less.

Level 1 Evaluation

Level 1 evaluations shall be carried out consistent with the procedure described in the following steps:

Step 1. Determine pipe diameter and nominal wall thickness from appropriate records or direct measurement of the pipe.

Step 2. Clean the corroded pipe surface to bare metal. Care should be taken when cleaning corroded areas of a pressurized pipe.

Step 3. Measure the maximum depth of the corroded area, d, and longitudinal extent of the corroded area, L

Step 4. Determine applicable pipe material properties from appropriate records.

Step 5. Select an evaluation method and calculate the estimated failure stress, SF.

Step 6. Define an acceptable safety factor, SF

Step 7. Compare SF to SF * SO

Step 8. The flaw is acceptable where SF is equal to nongreater than SF * SO, or where PF is equal to or greater than SF * PO.

If the flaw is unacceptable based on Step 8 above, the pressure can be reduced such that it is less than PF/SF

(a) Original B31G

Note that previous editions of B31G incorporated a definition for flow stress of S flow p 1.1 SMYS. For consistency in comparison to results obtained from evaluations performed to an earlier edition, use of the same definition for flow stress is recommended.

(b) Modified B31G. For z ≤ 50,

[References: (1) Keener, J. F., and Viet, P. H., “ProjectPR3-805: A Modified Criterion for Evaluating the Remaining Strength of Corroded Pipe,” AGA Catalog No. L51609, Dec. 22, 1989; (2) Keener, J. F., and Viet, P. H., “New Method Corrects Criterion for Evaluating Corroded Pipe,” Oil & Gas Journal, Aug. 6 and Aug. 20, 1990.]

(c) API 579 Level 1. The “API 579 Level 1” assessment, when reduced to its simplest form, is of a similar format to the other Level 1 methods presented herein, and therefore qualifies as a Level 1 assessment for purposes of meeting the requirements of this document.

Level 2 Evaluation

Level 2 evaluations are performed using what is known as the Effective Area Method. Level 2 evaluations shall be carried out using a procedure similar to the ten steps described for Level 1, except that the Effective Area Method generally requires several measurements of the depth of corrosion or remaining wall thickness throughout the corroded area. The Effective Area Method is expressed as follows:

The Effective Area Method evaluates, by iteration, all possible combinations of local metal loss, A, with respect to original material, A0. It requires for input a detailed longitudinal distribution or profile of metal loss. The detailed profile is established by obtaining several measurements of metal loss or remaining wall thickness throughout the metal loss area. Such measurements may be arranged in a grid pattern, or may follow a “river bottom” path through the deepest areas of metal loss. Increments of measurement need not be uniform, subject to limitations of application software. If using a grid pattern, the analysis must be repeated along each meridian to establish the governing solution. For a corroded profile defined by n measurements of depth of corrosion including the end points at nominally full wall thickness, n!/2(n - 2)! iterations are required to examine all possible combinations of local metal loss with respect to surrounding remaining material. The local solution resulting in the lowest calculated failure stress shall govern.

Owing to its iterative nature, it is a practical necessity to use a computer program or other algorithmic approach (e.g., a spreadsheet) in order to carry out an evaluation using the Effective Area Method. The “API 579 Level 2” assessment, when reduced to its simplest form, is equivalent to the Effective Area Method presented herein, and therefore qualifies as a Level 2 assessment for purposes of meeting the requirements of this document. Refer to API 579-1/ASME FFS-1 for detailed instructions