RELIABILITY ANALYSIS OF INTACT PIPE

CONCEPTS

BARLOW'S FORMULA

Barlow’s Formula relates the internal pressure that a pipe can withstand to its dimensions and the strength of its materials.

$P=\frac{2St}{D}$

$P =$ Internal Pressure (MPa)

$S =$ Specified Minimum Yield Strength (MPa)

$t =$ Wall Thickness (mm)

$D =$ Outside Diameter (mm)

Rearranging the formula:

$\sigma=\frac{PD}{2t}$

$\sigma =$ Yield Strength Required (MPa)

$P =$ Internal Pressure (MPa)

$t =$ Wall Thickness (mm)

$D =$ Outside Diameter (mm)

DESIGN FORMULA ACCORDING TO CSA Z662:19

For more details, limitations and particular cases: CSA Z662:19 Oil and gas pipeline systems

For straight pipe, the design pressure for a given design wall thickness or the design wall thickness for a given design pressure shall be determined by the following design formula:

$P=\frac{2St}{D} \cdot F \cdot L \cdot J \cdot T$

Where:

$P = design\ pressure\ (MPa).$
$S= specified\ minimum\ yield\ strength (MPa).$

*As specified in the applicable pipe standard or specification. For pipe of unknown origin.

$t = design \ wall \ thickness\ (mm).$
$D = outside\ diameter\ of\ pipe\ ( mm).$
$F = design\ factor.$
$L = location\ factor.$
$J = joint\ factor.$
$T = Temperature\ factor.$

Design Factor (F):

The design factor (F) to be used in the design formula shall be 0.800.

Location Factor (L):

The location factor (L) to be used in the design formula shall not exceed the applicable value given in the next table.

Table N° 1. Location Factor.

Joint Factor (J):

The joint factor (J) to be used in the design formula shall not exceed the applicable value given in the next table:

Table N° 2. Joint Factor.

Temperature factor (T):

The temperature factor (T) to be used in the design formula shall be as given in the next table:

Table N° 3. Temperature Factor.

Note: For intermediate temperatures, the temperature factor shall be determined by interpolation.

Dimensions for the pipe, Outside Diameter and Thickness:

For existing pipe sections: ASME B36.10M-2015

The nominal wall thickness for steel carrier pipe shall be not less than the applicable value given in the next table:

Table N° 4. Nominal minimum thickness for different diameters.

STEEL GRADES:

For existing steel grades applied to pipelines: API SPEC 5L: Line Pipe

MARGIN OF SAFETY

We define a margin of safety ( $M$ ):

$M=R-Q$

Resistance: R

Load: Q

Resistance ( $R$ ):

Yield Strength of the pipe $\rightarrow$ S

Load ( $Q$ ):

Yield Strength Required $\rightarrow$ $\sigma = \frac{P \cdot D}{2 \cdot t \cdot F \cdot L \cdot J \cdot T}$

Pipe yield strength, internal pressure (MOP in this case), outside diameters, and thickness will be treated as distributions. According to the Table N° 5 we assign what type of distribution follows each variable and what is its coefficient of variation (COV).

Table N° 5. Statistical parameters.

[1] T.J.E. Zimmerman, P. Hopkins, N. Sanderson, Can Limit States Design Be Used to Design a Pipeline above 80% SMYS?, 17th International Conference on Offshore Mechanics and Arctic Engineering, Portugal, Lisbon, 1998.

[2] Jiao, G., T. Sotberg, R. Bruschi, and R. Igland. 1997. “The SUPERB project: Linepipe statistical properties and implications in design of offshore pipelines.” In Proc., 16th Int. Conf. on Offshore Mechanics and Arctic Engineering, 45–56. New York: ASME.

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