**White Paper on**

**Safety Factors for Assessing**

**Pipeline Anomalies**

**Prepared for the Joint Industry Project (JIP) Committee -**

**Alternate Design with Life Cycle Management**

**May 16, 2008**

Prepared by:

Integrity Management and Threat Response Team

**Joint Industry Project Committee**

**Alternate Design with Life Cycle Management**

**Steering Committee:**

Alliance Pipeline, Inc. Harold Kraft

Boardwalk Pipeline Partners Frank Maria

CenterPoint Energy Chris Bullock

El Paso Pipeline Group David Bowmaster

Panhandle Energy David Johnson

Spectra Energy Richard Kivela

TransCanada Joe Zhou

Williams Larry Borski

**Committee Support:**

Kiefner and Associates Michael Rosenfeld

Process Performance Improvement

Consultants, LLC John Zurcher

Mark Hereth

Keith Leewis

**Integrity Management and Threat Response Team:**

CenterPoint Chris Bullock

El Paso Pipeline Group Tim Keller

Panhandle Energy Jerry Rau

Spectra Energy Richard Kivela

Williams Robert Heine

KAI John Kiefner

P-PIC John Zurcher

Safety Factors for Assessing Pipeline Anomalies

John F. Kiefner, Ph.D., P.E.

May 8, 2008

![](media/image1.png){width="6.5in" height="0.7395833333333334in"}

> ![](media/image2.png){width="0.40625in"
> height="0.4479166666666667in"}Kiefner & Associates, Inc.\
> 585 Scherers Court (614) 888-8220\
> Worthington, Ohio 43085 www.kiefner.com

# Table of Contents {#table-of-contents .TOC-Title}

Executive Summary [3](#executive-summary)

Introduction [3](#introduction)

What Is Meant By "Predicted Failure Pressure"
[3](#what-is-meant-by-predicted-failure-pressure)

Basis for B31.8S Response Times [3](#basis-for-b31.8s-response-times)

Effects of Wall Thickness, Corrosion Rates, Integrity Assessment Levels,
and Operating Stress Levels on Response Times
[3](#effects-of-wall-thickness-corrosion-rates-integrity-assessment-levels-and-operating-stress-levels-on-response-times)

The Baseline ASME B31.8S Criteria
[3](#the-baseline-asme-b31.8s-criteria)

Effect of Corrosion Rate on Response Time
[3](#effect-of-corrosion-rate-on-response-time)

Effect of Wall Thickness on Response Time
[3](#effect-of-wall-thickness-on-response-time)

Effect of Integrity Assessment Level on Response Time
[3](#effect-of-integrity-assessment-level-on-response-time)

Effect of Operating Stress Level on Response Time
[3](#effect-of-operating-stress-level-on-response-time)

Implications of Notice of Proposed Rulemaking
[3](#implications-of-notice-of-proposed-rulemaking)

References [3](#references)

# List of Tables {#list-of-tables .TOC-Title .unnumbered}

Table 1. Maximum Allowable Corrosion Rates for Applicability of Figure 4
of B31.8S [3](#__RefHeading___Toc198020567)

Table 2. Response times for various assessment levels for design factor
of 0.67 [3](#__RefHeading___Toc198020568)

Table 3. Response times for various assessment levels for design factor
of 0.56 [3](#__RefHeading___Toc198020569)

# Table of Figures {#table-of-figures .TOC-Title}

Figure 1. How Modified B31G was used to calculate response times for
Figure 4 of ASME B31.8S [3](#__RefHeading___Toc198021381)

Figure 2. Base case response times required under ASME B31.8S
[3](#__RefHeading___Toc198021382)

Figure 3. Effect of growth rate on response time (wall thickness fixed
at 0.312-inch) [3](#__RefHeading___Toc198021383)

Figure 4. Effect of wall thickness on response time (corrosion rate
fixed at 7 mpy) [3](#__RefHeading___Toc198021384)

Figure 5. Effect of integrity assessment level on response time
[3](#__RefHeading___Toc198021385)

Figure 6. Effect of operating stress levels above 72% of SMYS on
response time [3](#__RefHeading___Toc198021386)

Figure 7. Basis for response times for design factor of 0.80
[3](#__RefHeading___Toc198021387)

Figure 8. Basis for response times for design factor of 0.67
[3](#__RefHeading___Toc198021388)

Figure 9. Basis for response times for design factor of 0.56
[3](#__RefHeading___Toc198021389)

Figure 10. Reasonable response times for design factor of 0.67
[3](#__RefHeading___Toc198021390)

Figure 11. Reasonable response times for a design factor of 0.56
[3](#__RefHeading___Toc198021391)

#  {#section .TOC-Title .unnumbered}

Safety Factors for Assessing Pipeline Anomalies

John F. Kiefner, Ph.D., P.E.

# Executive Summary

A current Notice of Proposed Rulemaking (NPRM) issued by the Pipeline
and Hazardous Materials Safety Administration (PHMSA) among other things
proposes response times for anomalies that are unreasonable when
compared with the industry's practices embodied in ASME B31.8S -- 2004,
"Managing System Integrity of Gas Pipelines" and with requirements
already embodied in Part 192 of federal pipeline safety regulations. The
purpose of this document is to demonstrate why the industry's current
practices represent a more reasonable approach to achieving and assuring
pipeline integrity than would be the case in the response time rules of
current NPRM are adopted.

The subject NPRM is primarily for the purpose of promulgating safety
regulations for pipelines intended to be operated at stress levels above
those that have historically been mandated by federal pipeline safety
regulations. Specifically, the proposed regulations would permit

a natural gas pipeline meeting very specific material and design
criteria to be operated with a maximum allowable operating pressure
(MAOP) based on a design factor of 0.80 in a Class 1 area, a design
factor of 0.67 in a Class 2 area, and a design factor of 0.56 in a Class
3 area. Previously, the design factors for these class locations (with a
few exceptions) have been limited by the federal regulations to 0.72,
0.6, and 0.5, respectively.

The NPRM also addresses response times for repairs on these pipelines
with higher design factors where such pipelines would be subject to the
special integrity management requirements associated with
"high-consequence areas". It is the differences of these response times
from the principles embodied in ASME B31.8S -- 2004 that are the subject
of this document.

Presented herein for consideration are alternative response times that
not only are more

reasonable than those embodied in the current NPRM, but also are
entirely consistent with

current gas industry pipeline integrity management practices as embodied
in ASME B31.8S. Whereas the NPRM proposes requirements which, in effect,
cause almost all anomalies found by the common integrity assessment
tools to be subject either to immediate repair or repair within one
year, the alternatives offered for consideration herein would allow
linearly decreasing response times based on the current predicted
failure pressures of the anomalies for each of the three newly proposed
design factors. The alternative response times in all cases default to
an immediate repair when the predicted failure pressure of any anomaly
has decayed to a level of 1.1 times the MAOP as is consistent with
current industry practice *and with current federal regulations*.
Alternative initial response times for anomalies with predicted failure
pressures ranging from 1.25 times the MAOP to1.8 times the MAOP are
suggested ranging for 5 to 20 years based on the very same principles
embodied in response-time requirements of B31.8S.

# Introduction

A current Notice of Proposed Rulemaking (NPRM) issued by the Pipeline
and Hazardous Materials Safety Administration (PHMSA) among other things
proposes response times for anomalies that are unreasonable when
compared with the industry's practices embodied in ASME B31.8S -- 2004,
"Managing System Integrity of Gas Pipelines" and with requirements
already embodied in Part 192 of federal pipeline safety regulations.
This document presents comparisons of the proposed requirements and the
industry's current practices in order to demonstrate that the
requirements proposed in the NPRM are indeed unreasonable and
unworkable.

As background the rationale for the safety factors to be applied to the
"predicted failure pressure" of an anomaly (i.e., a defect) in a
pipeline are discussed herein. The rationale is embodied in ASME B31.8S.
The latter provides a consensus-based methodology for pipeline integrity
management, and a key element of that methodology consists of
determining safe response times for repairing or replacing anomalies
based on their predicted failure pressures and

anomaly growth rates appropriate to the specific environment. To account
for uncertainties associated with analytical models, with determining
material properties, with determining

anomaly sizes, and with the assumed anomaly growth rates, factors of
safety are embodied in the B31.8S methodology. The safety factors are
based on achieving a minimum margin between the maximum operating
pressure (MOP) of the pipeline and the predicted failure pressure of the
worst-case unrepaired anomaly remaining in the pipeline. The "class
location" of a gas pipeline is not considered in establishing the
margin, nor should it be, because the methodology can be applied to both
natural gas and liquid petroleum pipelines. Instead, the response time
limit is based on the margin between predicted failure pressure and MOP.
The size of the margin varies with time, and the larger the margin, the
longer the response. Because the margin is in effect the safety factor,
safety factors are anticipated to decline linearly with the passage of
time but only to a preset minimum that is judged to be adequate. The
purpose of this document is to demonstrate why this is a more reasonable
approach to achieving and assuring pipeline integrity than would be the
case if the response time rules of the current NPRM are adopted.

# What Is Meant By "Predicted Failure Pressure"

The concept of predicted failure pressure (also known as "remaining
strength") can best be viewed in the context of the pressure-carrying
capacity of a pressurized pipeline. If the internal pressure in a piece
of defect-free pipe is increased without limit, the pipe will typically
burst when the hoop stress level reaches the ultimate tensile strength
of the material. The pressure level associated with this event is
typically called the "burst" pressure. A piece of pipe containing a
longitudinally oriented defect will fail at a pressure level below the
burst pressure because the defect takes away stress-carrying material.
The larger the defect, the lower will be the associated failure
pressure. Models of pipe material behavior exist that define a
relationship between defect size and failure pressure. Hence, such
models are used to "predict" failure pressure for a given-size defect.

Pipeline integrity assessment tools such as in-line inspection devices
are used to locate and characterized defects in pipelines so that they
can be repaired or removed before they become large enough to cause the
pipeline to fail at its maximum operating pressure. Pipeline operators
rely on well-validated models of pipe material behavior to assess the
potential effects of defects on failure pressure based on their sizes as
revealed by the in-line inspection (ILI) tools. The failure pressures
associated with each and every ILI anomaly are used to rank anomalies by
severity and to prioritize responses so that all anomalies that might
impair the integrity of the pipeline are repaired of removed.

Alternatively, a hydrostatic test utilizing a test pressure level well
above the maximum operating pressure may be applied to a pipeline to
demonstrate its pressure carrying capacity. In such cases pipeline
operators similarly rely on well-validated models to define the sizes of
defects that could potentially remain after a hydrostatic test to a
given pressure level. Those defect sizes are then compared to the
model-calculated sizes of defects that would cause the pipeline to fail
at its maximum operating pressure.

Since it is not possible to assure that pipelines are defect-free even
when they are newly installed, the pipeline industry relies on
techniques such as hydrostatic testing and ILI to find and eliminate
injurious defects, that is, those defects that could cause failure of
the pipeline at its maximum operating pressure. It is recognized that
defects which are too small to fail at the hydrostatic test pressure or
small enough to escape detection with ILI will remain. Moreover, if a
particular defect can become larger with the passage of time, the
defect, though initially non-injurious, could, at some time in the
future, become large enough to cause failure at the maximum operating
pressure. The well-validated models of pipeline material behavior allow
one to calculate a predicted failure pressure for a given defect on the
basis of its size and the properties of the pipe material. By comparing
the predicted failure pressure to the maximum operating pressure and
applying an upper-bound defect growth rate, one can estimate the margin
between the two pressures and use that margin to decide if and when
remedial action is necessary. In making such assessments, one must
recognize that the sizes of defects are predictable only to within
tolerances of the inspection technique employed, that material
properties may vary from the values assumed, that applied growth rates
may not always be as high as the worst-case growth rate, and that even
well-validated models of pipe material behavior are not perfect.
Therefore, the response times for remedial action must embody reasonable
factors of safety that are sufficient to compensate for the
uncertainties associated with measurements, assumptions, and
calculations.

The bases for the ASME B31.8S guidelines are described in detail in
Reference 1. The important point is that the industry's experience using
these guidelines shows that they are appropriate. To date operators who
have followed the guidelines in responding to anomalies detected by ILI
or conducting hydrostatic tests to the levels required to meet the
guidelines have experienced no failures from anomalies that were
evaluated and left unrepaired on the basis of their predicted failure
pressures.

# Basis for B31.8S Response Times 

The B31.8S response times are based on a well-validated pipe
material/defect behavior model known as Modified B31G^(2)^. The Modified
B31G model predicts failure pressures for longitudinally oriented
defects resulting from corrosion-caused metal loss (internal or
external). For a 36-inch-OD, 0.312-inch-wall, Grade X70 material, the
Modified B31G predicted failure pressures are shown in Figure 1.

![](media/image3.wmf){width="6.482638888888889in" height="4.4125in"}

[]{#__RefHeading___Toc198021381 .anchor}Figure 1. How Modified B31G was
used to calculate response times for Figure 4 of ASME B31.8S

Failure pressure is plotted on the y-axis versus the total length (L) of
the metal loss on the x-axis. Defect depth-to-thickness (d/t) curves for
values of d/t ranging from 0.1 to 0.9 are shown on Figure 1. Every metal
loss defect is assumed to have a profile area of 85% of d times L.
Horizontal lines on Figure 1 represent levels of constant failure
pressure. Hence, the burst pressure for this pipe material (defect-free)
is represented by the horizontal line at a pressure of 1387 psig, and
the 100% of SMYS pressure is represented by the horizontal line at a
pressure of 1213 psig. If a pipeline comprised of this material were to
be hydrostatically tested to a pressure level of 1213 psig, the maximum
sizes of defects that could remain after the test would be those with
length and depth/thickness combinations that lie on the 1213 psig line.
For example, a 5-inch-long defect could remain if its depth is less than
33% of the wall thickness, a 10-inch-long defect could remain if its
depth is less than 22% of the wall thickness, and a 20-inch-long defect
could remain if its depth is less than 19% of the wall thickness.

Response time in the context of B31.8S, Figure 4 can be calculated from
a plot such as Figure 1 by assuming that the appropriate time is the
time it takes for the failure pressure corresponding to 100% of SMYS to
degrade to the point where the failure pressure reaches 1.1MOP. For the
purposes of the lines drawn on Figure 4 of the B31.8S document, three
MOP levels were chosen, 72% of SMYS, 50% of SMYS, and 30% of SMYS. The
arrows on Figure 1 extending from the 100% of SMYS line to the lines
corresponding to 1.1 times each of these three MOP levels at various
defect lengths are the basis for the lines on Figure 4 of the B31.8S
document. The other requirement imposed by the committee responsible for
ASME B31.8S was that the allowed degradation times from a failure
pressure of 100% of SMYS would be 10 years for a pipeline operated at
72% of SMYS, 15 years for a pipeline operated at 50% of SMYS, and 20
years for a pipeline operated at 30% of SMYS. As will be seen, these
specifications for Figure 4 of B31.8S apply to a specific value of wall
thickness in conjunction with a specific corrosion rate. The base line
case was for a wall thickness of 0.312 inch and a corrosion rate of 7
mills per year (mpy).

One can determine from Figure 1 that a 20-inch-long defect cannot have a
d/t ratio larger than 0.19 in order to survive an integrity assessment
to 100% of SMYS. When the conditions are imposed that this defect cannot
have a failure pressure below 1.1MOP in 10 years for a 72% of SMYS
operation, in 15 years for a 50% of SMYS operation, and in 20 years for
a 30% of SMYS operation, it can be inferred from the amounts of growth
in the d/t ratios indicated by the arrows in Figure 1 that the
corresponding maximum allowable corrosion rates are 7.2 mpy, 10.4 mpy,
and 11.1 mpy for 72 %, 50%, and 30 % of SMYS operating levels,
respectively.

Next one can determine from Figure 1 that a 10-inch-long defect cannot
have a d/t ratio larger than 0.22 in order to survive an integrity
assessment to 100% of SMYS. When the conditions are imposed that this
defect cannot have a failure pressure below 1.1MOP in 10 years for a 72%
of SMYS operation, in 15 years for a 50% of SMYS operation, and in 20
years for a 30% of SMYS operation, it can be inferred from the amounts
of growth in the d/t ratios indicated by the arrows in Figure 1 that the
corresponding maximum allowable corrosion rates are 8.7 mpy, 11.0 mpy,
and 10.9 mpy for 72%, 50%, and 30% of SMYS operating levels,
respectively.

Further, one can determine from Figure 1 that a 5-inch-long defect
cannot have a d/t ratio larger than 0.33 in order to survive an
integrity assessment to 100% of SMYS. When the conditions are imposed
that this defect cannot have a failure pressure below 1.1MOP in 10 years
for a 72% of SMYS operation, in 15 years for a 50% of SMYS operation,
and in 20 years for a 30% of SMYS operation, it can be inferred from the
amounts of growth in the d/t ratios indicated by the arrows in Figure 1
that the corresponding maximum allowable corrosion rates are 10.0 mpy,
11.4 mpy, and 9.7 mpy for 72 %, 50%, and 30 % of SMYS operating levels,
respectively.

Note that the results are relatively insensitive to defect length.
Therefore, if a conservative value of maximum allowable corrosion rate
is chosen, the criteria can be based solely on wall thickness and
corrosion rate. Based on these observations and the fact that pipeline
operators are allowed to interpolate between the lines of Figure 4 of
B31.8S for MOP levels between those represented by the lines, it is
reasonable to say that for a pipe material with a wall thickness of
0.312-inch being operated at a stress level of 72% of SMYS, one can
utilize Figure 4 of B31.8S for cases in which the corrosion rate is
known to be 7 mpy or less. The effects of other corrosion rates for this
wall thickness and for any wall thickness are discussed below. Before
that discussion, however, it is useful to note how the allowable
corrosion rate changes for different wall thicknesses and that it does
not change appreciable with diameter.

Representative variations in maximum allowable corrosion rates with
variations in wall thickness and diameter are shown in Table 1. For each
diameter and wall thickness combination the maximum allowable corrosion
rates were calculated by the process described above using Modified
B31G.

[]{#__RefHeading___Toc198020567 .anchor}Table 1. Maximum Allowable
Corrosion Rates for Applicability of Figure 4 of B31.8S

+-------------+------------------+------------------+------------------+
| **Pipe**    | **Maximum        | **Maximum        | **Maximum        |
|             | Allowable**      | Allowable**      | Allowable**      |
| *           |                  |                  |                  |
| *Material** | **Corrosion      | **Corrosion      | **Corrosion      |
|             | Rate**           | Rate**           | Rate**           |
|             |                  |                  |                  |
|             | **for 72% of     | **for 50% of     | **for 30% of     |
|             | SMYS**           | SMYS**           | SMYS**           |
+-------------+------------------+------------------+------------------+
| 36-inch-OD  | 7                | 10               | 10               |
|             |                  |                  |                  |
| 0.31        |                  |                  |                  |
| 2-inch-wall |                  |                  |                  |
+-------------+------------------+------------------+------------------+
| 36-inch-OD  | 4                | 6                | 6                |
|             |                  |                  |                  |
| 0.18        |                  |                  |                  |
| 8-inch-wall |                  |                  |                  |
+-------------+------------------+------------------+------------------+
| 36-inch-OD  | 15               | 20               | 20               |
|             |                  |                  |                  |
| 0.62        |                  |                  |                  |
| 5-inch-wall |                  |                  |                  |
+-------------+------------------+------------------+------------------+
| 12          | 7                | 10               | 10               |
| .75-inch-OD |                  |                  |                  |
|             |                  |                  |                  |
| 0.31        |                  |                  |                  |
| 2-inch-wall |                  |                  |                  |
+-------------+------------------+------------------+------------------+

These results show that the rate is roughly inversely proportional to
wall thickness and independent of diameter. Although not specifically
shown herein, the rate is independent of material yield strength as
well. One can anticipate the latter point from the fact that ASME
B31G^(3)^ is independent of yield strength. Modified B31G is practically
independent of the material strength as well because the failure
criterion is SMYS/(SMYS+10,000). For Grades X52 through X80, this number
varies only from 0.84 to 0.89 and the effect of grade is actually less
than these numbers would suggest.

# Effects of Wall Thickness, Corrosion Rates, Integrity Assessment Levels, and Operating Stress Levels on Response Times 

## The Baseline ASME B31.8S Criteria

The baseline response time criteria of ASME B31.8S are shown in Figure 2
below. The curves can be used "as is" for any wall thickness as long as
the limits of Table 1 are observed. In simple terms, for a pipeline
being operated at 72% of SMYS, the maximum allowable corrosion rate for
the figure to be applicable is approximately 22***t*** where ***t*** is
the wall thickness of the pipe. The 22***t*** limit is applicable to
cases where the operating stress level is 72% of SMYS. It gives
conservative limits for pipelines being operated at less than 72% of
SMYS, but to avoid mistakes, the user should follow the 22***t*** limit
for all stress levels.

![](media/image4.wmf){width="5.976388888888889in"
height="4.086111111111111in"}

[]{#__RefHeading___Toc198021382 .anchor}Figure 2. Base case response
times required under ASME B31.8S

As an example consider a 24-inch-OD, by 0.281-inch X52 pipeline that is
operated at an MOP corresponding to 72% of SMYS. One can calculate that
the MOP is 876 psig. If the corrosion rate does not exceed 22 times
0.281 or 6 mpy, one can assess the remaining life using Figure 2. Assume
that an integrity assessment is performed by ILI and that all anomalies
with calculated failure pressures (based on Modified B31G) below 1217
psig (100% of SMYS) have been examined and remediated. The factor of
safety against failure of any remaining anomaly is 1.39 of 100/72. It
should be obvious that an equivalent margin of safety would exist after
a hydrostatic test of the pipeline to a minimum pressure of 1217 psig
(100% of SMYS). If the corrosion of a remaining anomaly with a failure
pressure of 1217 psig continues at a rate of 6 mpy, the failure pressure
of the anomaly will degrade in accord with the line labeled "72% of
SMYS" in Figure 2. After ten years have elapsed, the failure pressure
will have decayed to 1.1 times MOP or 964 psig, the limiting value
established by the committee. The anomaly must be addressed within the
10-year period.

Figure 2 can be used for pipelines operating at stress levels less than
72% of SMYS. If an identical pipeline that is operated at an MOP of 608
psig (50% of SMYS) were to be assessed and remediated by repairing all
anomalies with failure pressures at or below 1217 psig (100% of SMYS),
and if the corrosion rate is 6 mpy, the failure pressure of a
just-surviving anomaly will degrade to 669 psig (1.1 times the MOP of
608 psig) in a period of 15 years. The anomaly must be addressed within
the 15-year period (the 50% of SMYS line on Figure 2). If a third
identical pipeline is operated at an MOP of 365 psig (30% of SMYS), if
it is assessed and remediated so that no anomalies remain with failure
pressures below 1217 psig (100% of SMYS), and if the corrosion rate is 6
mpy, the failure pressure of a just-surviving anomaly will degrade to
401 psig (1.1 times the MOP of 365 psig) in a period of 20 years (the
30% of SMYS line on Figure 2). The anomaly must be addressed within the
20-year period.

The user of Figure 2 is allowed to extrapolate between the three
fixed-stress-level lines for intermediate values of MOP.

Situations where the wall thickness -- corrosion rate combinations cause
the degradation to occur at rates faster or slower than those
represented by the lines on Figure 2 can be addressed by modified
versions of Figure 2. Similarly, if the level of remediation after an
ILI only addresses anomalies with failure pressures less than 100% of
SMYS or a hydrostatic test is carried out at a level of less than 100%
of SMYS, the situation will need to be addressed by a modified procedure
using Figure 2. Finally, the means for dealing with a situation in which
the operating stress level exceeds 72% of SMYS (e.g., 80% of SMYS) is
discussed. The procedures for addressing such situations are discussed
below.

## Effect of Corrosion Rate on Response Time

The effect of a faster or slower corrosion rate than the one
corresponding to 22t can be determined as illustrated in Figure 3. Only
the situation involving a pipeline operated at 72% of SMYS is
considered, but situations involving other operating stress levels can
be handled in a similar manner.

![](media/image5.wmf){width="6.482638888888889in" height="4.4125in"}

[]{#__RefHeading___Toc198021383 .anchor}Figure 3. Effect of growth rate
on response time (wall thickness fixed at 0.312-inch)

## Effect of Wall Thickness on Response Time

The effect of a thinner or thicker pipe than the one corresponding to
the base case thickness can be determined as illustrated in Figure 4.
Only the situation involving a pipeline operated at 72% of SMYS is
considered, but situations involving other operating stress levels can
be handled in a similar manner.

![](media/image6.wmf){width="6.459722222222222in"
height="4.688888888888889in"}

[]{#__RefHeading___Toc198021384 .anchor}Figure 4. Effect of wall
thickness on response time (corrosion rate fixed at 7 mpy)

## Effect of Integrity Assessment Level on Response Time

The effect of an integrity assessment level less than 100% of SMYS
compared to the 100%-of-SMYS-level assumed for the base case thickness
is illustrated in Figure 5.

![](media/image7.wmf){width="6.482638888888889in"
height="4.420138888888889in"}

[]{#__RefHeading___Toc198021385 .anchor}Figure 5. Effect of integrity
assessment level on response time

## Effect of Operating Stress Level on Response Time

The effect of operating stress level on response time is shown in Figure
2 for operating stress levels of 30% of SMYS, 50% of SMYS, and 72% of
SMYS. In Figure 6 below, the response times for these stress levels are
the same as those shown in Figure 2, but the response time for an
operating stress level of 80% of SMYS is added for comparison.

![](media/image8.wmf){width="6.482638888888889in" height="4.4125in"}

[]{#__RefHeading___Toc198021386 .anchor}Figure 6. Effect of operating
stress levels above 72% of SMYS on response time

# Implications of Notice of Proposed Rulemaking

The foregoing discussions of the industry's rationale for response times
following an integrity assessment as embodied in ASME B31.8S-2004 lead
to the following assessment of response times that have been proposed by
PHMSA in conjunction with a current Notice of Proposed Rulemaking
(NPRM):

Pipeline Safety: Standards for Increasing the

Maximum Allowable Operating Pressure

For Gas Transmission Pipelines

Docket ID PHMSA-2005-23447; Notice 2

Docket No. RIN 2137-AE25

The NPRM, among other things prescribes response times for anomalies
found in pipelines operated at stress levels above 72% of SMYS up to and
including 80% of SMYS that are not consistent with the B31.8S philosophy
as described above.

In particular the NPRM, clause 92.620(d)(11) Making Repairs, would
classify anomalies as requiring "immediate repair" based on the
following (not all are direct quotes from the NPRM and maximum allowable
operating pressure is abbreviated as MAOP):

(B) The defect meets the criteria for immediate repair in § 192.933(d).
    ("A calculation of the remaining strength of the pipe shows a
    predicted failure pressure less than or equal to 1.1 times the
    maximum allowable operating pressure at the location of the anomaly.
    Suitable remaining strength calculation methods include, ASME/ANSI
    B31G; RSTRENG; or an alternative equivalent method of remaining
    strength calculation. These documents are incorporated by reference
    and available at the addresses listed in appendix A to part
    192.")[^1]

(C) MAOP was based on a design factor of 0.67 and the (predicted)
    failure pressure \< 1.25 times MAOP.

(D) MAOP was based on a design factor of 0.56 and the (predicted)
    failure pressure \< 1.4 times MAOP.

The NPRM would classify anomalies as requiring repair "within one year"
based on the following (again, not all are direct quotes from the NPRM
and maximum allowable operating pressure is abbreviated as MAOP):

(B) MAOP was based on a design factor of 0.80 and the (predicted)
    failure pressure \< 1.25 times MAOP.

(C) MAOP was based on a design factor of 0.67 and the (predicted)
    failure pressure \< 1.50 times MAOP.

(D) MAOP was based on a design factor of 0.56 and the (predicted)
    failure pressure \< 1.80 times MAOP.

To show that these proposed requirements are inconsistent with the
industry's B31.8S practices, it is useful to revisit the technique used
in the previous discussions of the B31.8S rationale. First it is
important to note that the definition of immediate repair in ASME B31.8S
is an anomaly with a predicted failure pressure less than 1.1 times
MAOP. The same definition for an immediate repair is embodied in §
192.933(d). With that in mind consider the predicted failure pressures
for a 36-inch-OD, 0.312-inch-wall, X70 pipe with an MAOP of 970 psig
corresponding to 80% of SMYS (a Class 1 design factor of 0.8). For Class
2 areas assume that X70 grade pipe is used and that, therefore, the wall
thickness for a Class 2 area (design factor of 0.67) is 0.466 inch. For
Class 3 areas assume that X70 grade pipe is used and that, therefore,
the wall thickness for a Class 3 area (design factor of 0.56) is 0.557
inch.

First, the predicted failure pressures for the Class 1 areas using
Modified B31G will be as shown in Figure 7.

![](media/image9.wmf){width="6.482638888888889in" height="4.4125in"}

[]{#__RefHeading___Toc198021387 .anchor}Figure 7. Basis for response
times for design factor of 0.80

Corrosion defects that survive a 100%-of-SMYS integrity assessment (a
hydrostatic test to 1213 psig for example) will grow in depth after the
test at a particular rate as indicated by the arrows until the failure
pressures decay to the 1.1 times MOP line at 1068 psig. This level is
designated by both ASME B31.8S and § 192.933(d) as an immediate repair.
One can determine from the arrows shown on Figure 7 that the
20-inch-long defect grows from 19% of the wall thickness at 100% of SMYS
to 32% of the wall thickness (a depth change of 0.041 inch) at 1068 psig
(1.1 times MOP). As shown in Figure 6 previously, the implied time
interval allowed under B31.8S for this to occur would be 5 years. The
5-year response time interval would be acceptable for a corrosion rate
of 8 mpy or less. In contrast, the NPRM implies that after a
100%-of-SMYS integrity assessment is performed, all anomalies found by
ILI that have predicted failure pressures of 100% of SMYS would have to
be repaired within one year or that a 100%-of-SMYS test would have to be
conducted after one year. A more reasonable approach, one that would be
consistent with B31.8S, is to require that for an MAOP based on a design
factor of 0.80, the response time for a defect with a predicted failure
pressure of 1.25 times MOP should be repaired within five years and that
the repair must be done immediately if and when the predicted failure
pressure decays to 1.1 times MOP.

Next, consider the predicted failure pressures for the Class 2 areas
using Modified B31G as shown in Figure 8. The pipe material is
36-inch-OD, 0.466-inch-wall, X70. For predicted anomaly failure
pressures of 1812 psig (100% of SMYS or 1.67 times MOP), anomalies of
6-, 15-, and 26-inch lengths must grow from d/t values on the
100%-of-SMYS line to d/t values on the 1.1-times-MOP line on Figure 8.
For predicted anomaly failure pressures of 1700 psig (1.4 times MOP),
anomalies of 6-, 15-, and 26-inch lengths must grow from d/t values on
the line designated by thick, dark arrows to d/t values on the
1.1-times-MOP line on Figure 8. For predicted anomaly failure pressures
of 1516 psig (1.25 times MOP), anomalies of 6-, 15-, and 26-inch lengths
must grow from d/t values on the line designated by thick light arrows
to d/t values on the 1.1-times-MOP line on Figure 8. For each of these
scenarios the times to failure and the associated corrosion rates are
summarized in Table 2.

![](media/image10.wmf){width="6.482638888888889in" height="4.4125in"}

[]{#__RefHeading___Toc198021388 .anchor}Figure 8. Basis for response
times for design factor of 0.67

[]{#__RefHeading___Toc198020568 .anchor}Table 2. Response times for
various assessment levels for design factor of 0.67

+-------------+---------+-------+---------+-------+---------+-------+
| **Length    | **1.67  |       | **1.4   |       | **1.25  |       |
| of**        | times   |       | times   |       | times   |       |
|             | MOP**   |       | MOP**   |       | MOP**   |       |
| **Defect,   |         |       |         |       |         |       |
| inches**    |         |       |         |       |         |       |
+-------------+---------+-------+---------+-------+---------+-------+
|             | **Years | *     | **Years | *     | **Years | *     |
|             | to**    | *Rate | to**    | *Rate | to**    | *Rate |
|             |         | in**  |         | in**  |         | in**  |
|             | 1.  **  |       | **1.1   |       | **1.1   |       |
|             | times** | **    | times** | **    | times** | **    |
|             |         | mpy** |         | mpy** |         | mpy** |
|             | **MOP** |       | **MOP** |       | **MOP** |       |
+-------------+---------+-------+---------+-------+---------+-------+
| **26**      | 12      | 12    | 10      | 11    | 5       | 11    |
+-------------+---------+-------+---------+-------+---------+-------+
| **15**      | 12      | 13    | 10      | 11    | 5       | 10    |
+-------------+---------+-------+---------+-------+---------+-------+
| **6**       | 12      | 14    | 10      | 12    | 5       | 10    |
+-------------+---------+-------+---------+-------+---------+-------+

Recall that both B31.8S and § 192.933(d) require immediate repairs for
defects with predicted failure pressures of 1.1 times MOP. What these
calculations show is that when the initial predicted failure pressure is
1.67 times MOP, a 12-year response time for the 0.466-inch-wall pipe
would be consistent with a corrosion growth rate as high as 12 mpy, that
when the initial predicted failure pressure is 1.4 times MOP, a 10-year
response time would be consistent with a corrosion rate as high as 11
mpy, and that when the initial predicted failure pressure is 1.25 times
MOP, a 5-year response time would be consistent with a corrosion rate as
high as 10 mpy. These are clearly reasonable and conservative criteria
for response times. In contrast, the NPRM would require defects with
predicted failure pressures of 1.25 times MOP to be repaired
immediately, and defects with predicted failure pressures of 1.4 times
MOP to be repaired within one year. The response times suggested by the
calculations in Table 2 are consistent with current industry practices
as embodied in B31.8S, and they are clearly more practical than those
embodied in the NPRM.

Lastly, consider the predicted failure pressures for the Class 3 areas
using Modified B31G as shown in Figure 9. The pipe material is
36-inch-OD, 0.557-inch-wall, X70. For predicted anomaly failure
pressures of 2166 psig (100% of SMYS or 1.79 times MOP), anomalies of
9-, 17-, and 28-inch lengths must grow from d/t values on the
100%-of-SMYS line to d/t values on the 1.1-times-MOP line on Figure 9.
For predicted anomaly failure pressures of 2183 psig (1.8 times MOP),
anomalies of 9-, 17-, and 28-inch lengths must grow from d/t values on
the line designated by thick, dark arrows to d/t values on the
1.1-times-MOP line on Figure 9. For predicted anomaly failure pressures
of 1698 psig (1.4 times MOP), anomalies of 9-, 17-, and 28-inch lengths
must grow from d/t values on the line designated by thick light arrows
to d/t values on the 1.1-times-MOP line on Figure 9. For each of these
scenarios the times to failure and the associated corrosion rates are
summarized in Table 3.

![](media/image11.wmf){width="6.482638888888889in" height="4.4125in"}

[]{#__RefHeading___Toc198021389 .anchor}Figure 9. Basis for response
times for design factor of 0.56

[]{#__RefHeading___Toc198020569 .anchor}Table 3. Response times for
various assessment levels for design factor of 0.56

+-------------+---------+-------+---------+-------+---------+-------+
| **Length    | **1.79  |       | **1.8   |       | **1.4   |       |
| of**        | times   |       | times   |       | times   |       |
|             | MOP**   |       | MOP**   |       | MOP**   |       |
| **Defect,   |         |       |         |       |         |       |
| inches**    |         |       |         |       |         |       |
+-------------+---------+-------+---------+-------+---------+-------+
|             | **Years | *     | **Years | *     | **Years | *     |
|             | to**    | *Rate | to**    | *Rate | to**    | *Rate |
|             |         | in**  |         | in**  |         | in**  |
|             | 1.  **  |       | **1.1   |       | **1.1   |       |
|             | times** | **    | times** | **    | times** | **    |
|             |         | mpy** |         | mpy** |         | mpy** |
|             | **MOP** |       | **MOP** |       | **MOP** |       |
+-------------+---------+-------+---------+-------+---------+-------+
| **28**      | 20      | 12    | 20      | 12    | 8       | 13    |
+-------------+---------+-------+---------+-------+---------+-------+
| **17**      | 20      | 13    | 20      | 13    | 8       | 13    |
+-------------+---------+-------+---------+-------+---------+-------+
| **9**       | 20      | 14    | 20      | 14    | 8       | 13    |
+-------------+---------+-------+---------+-------+---------+-------+

What these calculations show is that when the initial predicted failure
pressure is 1.79 times MOP, a 20-year response time for the
0.557-inch-wall pipe would be consistent with a corrosion growth rate as
high as 12 mpy, that when the initial predicted failure pressure is 1.8
times MOP, a 20-year response time would be consistent with a corrosion
rate as high as 12 mpy, and that when the initial predicted failure
pressure is 1.4 times MOP, an 8-year response time would be consistent
with a corrosion rate as high as 13 mpy. These are clearly reasonable
and conservative criteria for response times. In contrast, the NPRM
would require defects with predicted failure pressures of 1.4 times MOP
to be repaired immediately, and defects with predicted failure pressures
of 1.8 times MOP (higher than 100% of SMYS) to be repaired within one
year. The responses times suggested by the calculations in Table 3 are
consistent with current industry practices as embodied in B31.8S, and
they are clearly more practical than those embodied in the NPRM.

Comparisons of the response times embodied in the NPRM to reasonable
response times consistent with B31.8S and § 192.933(d) are shown in
Figures 10 and 11.

![](media/image12.wmf){width="6.459722222222222in"
height="4.695833333333334in"}

[]{#__RefHeading___Toc198021390 .anchor}Figure 10. Reasonable response
times for design factor of 0.67

Figure 10 shows response times for pipelines with an MAOP based on a
design factor of 0.67. The cases shown are valid for 0.312-inch-wall
pipe with corrosion rates ranging from 10 mpy to 12 mpy. Whereas the
NPRM would require immediate repair for a defect with a predicted
failure pressure of 1.25 times MAOP or less, the approach that is more
consistent with B31.8S would require immediate repair of a defect with a
predicted failure pressure of 1.1 times MAOP and would allow a linearly
decreasing repair interval from five years for defects with predicted
failure pressures of 1.25 times MAOP to immediate repair for defects
with predicted failure pressures of 1.1 times MAOP. Whereas the NPRM
would require repair within one year for a defect with a predicted
failure pressure of 1.4 times MAOP or less, the approach that is more
consistent with B31.8S would allow a linearly decreasing repair interval
from ten years for defects with predicted failure pressures of 1.4 times
MAOP to immediate repair for defects with predicted failure pressures of
1.1 times MAOP.

![](media/image13.wmf){width="6.4527777777777775in"
height="4.688888888888889in"}

[]{#__RefHeading___Toc198021391 .anchor}Figure 11. Reasonable response
times for a design factor of 0.56

Figure 11 shows response times for pipelines with an MAOP based on a
design factor of 0.56. The cases shown are valid for 0.312-inch-wall
pipe with corrosion rates ranging from 12 mpy to 14 mpy. Whereas the
NPRM would require immediate repair for a defect with a predicted
failure pressure of 1.4 times MAOP or less, the approach that is more
consistent with B31.8S would require immediate repair of a defect with a
predicted failure pressure of 1.1 times MAOP and would allow a linearly
decreasing repair interval from eight years for defects with predicted
failure pressures of 1.4 times MAOP to immediate repair for defects with
predicted failure pressures of 1.1 times MAOP. Whereas the NPRM would
require repair within one year for a defect with a predicted failure
pressure of 1.8 times MAOP or less, the approach that is more consistent
with B31.8S would allow a linearly decreasing repair interval from
twenty years for defects with predicted failure pressures of 1.8 times
MAOP to immediate repair for defects with predicted failure pressures of
1.1 times MAOP.

Lastly, for pipelines with an MAOP based on a design factor of 0.8, the
NPRM would require repair within one year for a defect with a predicted
failure pressure of 1.25 times MAOP or less. In effect that would mean
that a hydrostatic test to 100% of SMYS used as an integrity assessment
tool would have to be repeated annually. Alternatively, the remediation
of defects found by ILI that had predicted failure pressures of 100% of
SMYS would have to be addressed with one year. Neither requirement is
reasonable. In contrast the approach that is more consistent with B31.8S
would require immediate repair of a defect with a predicted failure
pressure of 1.1 times MAOP and would allow a linearly decreasing repair
interval from five years for defects with predicted failure pressures of
1.25 times MAOP to immediate repair for defects with predicted failure
pressures of 1.1 times MAOP. This scenario was illustrated previously in
Figure 6.

# References

1.  Kiefner, J., "*Schedule of Responses to Corrosion-Caused Metal Loss
    Revealed By Integrity-Assessment Results*", GRI-01/0085, August
    2001.

2.  Kiefner, J. F., Vieth, P. H., and Roytman, I. \"*Continued
    Validation of RSTRENG*\", PRC *I*nternational, Catalog No. L51749
    (1996).

3.  *Manual for Determining the Remaining Strength of Corroded
    Pipelines*, A Supplement to ASME B31 Code for Pressure Piping, ASME
    (1991).

[^1]: ASME/ANSI B31G is listed under REFERENCES as Reference 3.