Page 27 - Reliability and Maintainability of In service Pipelines

16 Reliability and Maintainability of In-Service Pipelines

cracking or other functional requirements (e.g., leakage, deformation beyond
allowable limits (for flexible pipes), and excessive movement at the joints).
The principle for the design of a pipe is to ensure that both serviceability and
ultimate limit states are not reached. This includes consideration of one or more
of the following conditions: strain, stress, bending moment, and normal force or
load-bearing capacity, in the ring or longitudinal direction as appropriate; and
water tightness.
The design of a buried pipe involves the selection of appropriate pipe strength
and a bedding combination which is able to sustain the most adverse permanent
and transient loads to which the pipeline will be subjected over its design life.
The design of buried pipelines depends on factors including the structural
properties of particular pipes, the internal and external pressures, loads, and the
condition of the surrounding soil. The general steps involved in the design of bur-
ied pipelines are outlined below:

Step 1: Determining the wall thickness of the pipe due to internal pressure.
The wall thickness depends on the diameter of the pipe, flow pressure, and
routing of the material inside the pipe.
Step 2: Checking minimum wall thickness of the pipe for handling.
Step 3: The external loading during construction is analyzed in the pipe soil
embedment system, which includes earth loads, live loads, the water
table conditions, and the pressure in the pipe.

One of the most crucial factors to consider in pipe design is ring theory, which
stems from external loadings. External loadings will be analyzed to highlight the
stability of the pipe ring and it is dependent on the properties of the surrounding
soil, as soil pipe interaction is known to have an effect on the structural behavior
of the pipe. However, before understanding ring theory it is important to gain an
understanding of the stresses in the supporting soil surrounding the pipe.
Furthermore, the stiffness factor of the pipe is also a factor to consider, especially
its sensitivity based on the time of loading and how it can be altered by tempera-
ture changes.
To illustrate pipe design methods of buried pipelines case scenarios are used
where pipes are subjected to different conditions. Steel pipes are considered as
examples to explain what factors need to be considered for pipe design.
Case 1: Determining wall thickness for internal pressure and handling
Thickness of steel pipe is determined by ensuring the maximum value for the
internal pressure that is being analyzed does not exceed the limiting hoop tensile
stress. Hoop tensile strength is determined using σ h .
It can be seen in Fig. 1.1 that the illustrated cross-section of a steel pipe is in
static equilibrium, with forces equating to: