Page 70 - Reliability and Maintainability of In service Pipelines

Pipeline Inspection and Maintenance 59

and the stress intensity factor for a crack pit in a pipe under axial stress:
!
3
p ffiffiffiffiffiffi X a 2c R a 2c R
K I2a 5 πa σ i f i ; ; 1 σ bg f bg ; ; ð2:21Þ
d a d d a d
i50
For internal and/or external crack pits, the difference in formulations of stress
intensity factor (Eqs. 2.20 and 2.21) lies in geometry functions (i.e., f i and f bg ),
which have been presented in different tables by Laham (1999). Due to the propa-
gation of corrosion, crack depth (i.e., a) and crack length (i.e., 2c) change with
time so the stress intensity factors are time variant.

2.3 Inspection Methods

The majority of current pipeline inspection methods involve the use of manual
labor and human evaluation. The main concern is that it requires large amounts of
time to complete the assessment of defects. In addition, human error is expected
in the evaluation which in some cases result in inaccurate assessment. This has
encouraged engineers and researchers to innovate and develop potential solutions
for inspection of pipeline networks. Pipes run for hundreds of kilometers and in
many cases they are underground. Therefore the innovative methods for assess-
ment can be automatic inspection systems. The merit of those systems is that they
greatly reduce the time taken to properly inspect pipes over long distances and
also if applied correctly, can eliminate the human errors.
In this section, conventional methods and some of the most recent advanced
methods for pipeline inspection are presented.

2.3.1 CCTV METHOD

Currently, the most common method used to investigate the internal condition of
pipes and perform defect assessment is known as closed circuit television
(CCTV). There are a number of leading worldwide companies who produce a
diversity of CCTV modules to accommodate a range of pipe sizes and scales of
work.
CCTV is relatively cheap and identifies easily physically damaged pipeline
walls or pipelines under threat of corrosion, crack, or deformations. The technol-
ogy allows engineers to decide whether to act further on the pipeline structure,
conduct more testing to assess the wellness of the pipeline structure, relocate the
pipeline system, or replace it entirely.
In a CCTV device system, a remote controlled car is placed inside the pipe-
line and is used as a visual inspection tool to view the inside of the pipe