Page 86 - Reliability and Maintainability of In service Pipelines

Pipeline Inspection and Maintenance 75

This form of maintenance allows for the budget to only be used as it is neces-
sary rather than systematically at regular intervals. Cost-effectiveness as well as
knowledge are two major benefits of corrective action. When allowing a pipeline
to run its full course whilst being monitored, it can be documented how long the
raw pipeline will last under the impact of the substance flowing through it, loads
applied to it, as well as external factors which may cause the pipeline to deterio-
rate. This information can allow engineers to decide whether to redesign the sec-
tion of pipeline which will be replaced, once failure has occurred, or to replace it
with the exact same material if the term served by the pipeline is sufficient. This
will also help to predict the life span of the future pipelines designed using same
material.

Symbols

a depth of the equivalent rectangular stress block, (mm)
A the acid-consuming capability of the wall material
2
A s area of tension reinforcement in length b, (mm /m)
b unit length of pipe, 1000 mm
B 1 crack control coefficient for effect of spacing and number of layers of reinforcement
c the average rate of corrosion (mm/year)
crack control coefficient for type of reinforcement
C 1
d distance from compression face to centroid of tension reinforcement, (mm)
diameter of rebar in inner cage, mm
d b
[DS] dissolved sulfide concentration (mg/L)
0
f design compressive strength of concrete, (MPa)
c
f y design yield strength of reinforcement, (MPa)
F crack width control factor
F c factor for effect of curvature on diagonal tension (shear) strength in curved components
F d factor for crack depth effect resulting in increase in diagonal tension (shear) strength
with decreasing d
F N coefficient for effect of thrust on shear strength
h overall thickness of member (wall thickness), (mm)
i coefficient for effect of axial force at service load stress
k acid reaction factor
J is pH-dependent factor for proportion of H 2 S
w the width of the stream surface
P’ perimeter of the exposed wall
M s service load bending moment acting on length b, (Nmm/m)
M u factored moment acting on length b, (Nmm/m)
N s axial thrust acting on length b, service load condition (1 when compressive, 2 when
tensile), (N/m)
N u factored axial thrust acting on length b, (1 when compressive, 2 when tensile), (N/m)
s is the slope of the pipeline