Page 29 - Reliability and Maintainability of In service Pipelines

18 Reliability and Maintainability of In-Service Pipelines

Case 2: Ring stability—steel pipe exposed to external pressure, water table, and
vacuum pressure
Ring stability of a pipe is determined by analyzing the amount of deflection
required to cause soil slip with or without added vacuum pressure and water table.
In most cases, external pressure is present and is always considered along with
other given factors. In this case, the steel pipe is also exposed to vacuum pressure,
therefore it is important to analyze the amount of vacuum pressure required to
cause soil slip to prevent pipe failure.
However, ring stability of a pipe is not only limited to the above factors. Pipe
stiffness also plays a significant role in the ring stability of pipes. Factors that can
affect the ring stability of pipes are ring stresses, which, if exceeding pipe capac-
ity, ultimately lead to pipe failure by causing pipes to collapse.
Ring stiffness of a pipe is defined as the ability to resist deflection. The stiff-
ness of a pipe is measured via a ratio consisting of the applied load(s), F, over
the measured deflection, D, as follows:
F
Pipeline stiffness 5 ð1:4Þ
D
Ring stiffness however is determined via the equation below and in this case
per unit length of pipe:

EI
Ring stiffness 5 ð1:5Þ
r 3
w 3 D
As I 5 and r 5 then:
12 2
w 3
2
Ring stiffness 5 E ð1:6Þ
3 D
Following installation of the pipe, it is important to also pressurize the pipe.
This pressurization leads to the internal pressure inside the pipe decreasing defor-
mation of the ring caused by deflection.
However, deflection of pipe does not exceed beyond the vertical compression
of the surrounding soil. The vertical deflection, at which soil slip occurs, is
referred to as the critical deflection of soil. When this soil slip occurs, it causes
the pipe to collapse (if flexible).
To minimize this effect, cement mortar coatings and linings are added to
pipes, although deflection limits can be better controlled by ensuring proper selec-
tion and management of embedment material, and installation techniques, instead
of controlling pipe stiffness.
In order to control pipe stiffness, not only an increased number of factors
need to be considered, which could prove to be complicated, but in the case
of pipe failure it will be more difficult to pinpoint the exact cause of the