In recent years the number of
corrosion failures in onshore and offshore pipelines has steadily decreased due
to the introduction of corrosion management strategies by pipeline operators. A
vital component in effective corrosion management is an accurate determination
of corrosion growth rates.
In-line inspection using
high-resolution pigs to detect and size corrosion can become the basis for
defining a future safe operating strategy. A methodology has been developed
which includes the direct comparison of magnetic flux leakage or ultrasonic data
between two inspection runs. Consequently, any new sites of corrosion or sites
at which corrosion growth has occurred can be identified and the extent of the
growth quantified.
Utilizing the variation in corrosion
depth, corrosion growth rate and material properties, and reliability
methodologies the probability of failure is established as a function of time.
This approach has been used for internal corrosion management, and can also be
applied for management of excavation for external corrosion. Attention is given
to the approaches used to estimate corrosion growth rates and the reliability
methodologies which allow the probability of failure to be determined. Case
studies are presented of the successful use of the above methods.
At the start of the 1990's there were
concerns over the increasing threat of corrosion to pipeline integrity. For
example:
· -Corrosion
was the major cause of reportable incidents in North America
· -Corrosion
was the major cause of pipeline failure in the Gulf of Mexico
· -Corrosion
in a North American onshore oil pipeline had required over $1 billion in
repairs
· Internal
corrosion along the complete length of pipelines had resulted in significant replacement programs e.g.
However, world-wide, the number of
pipeline incidents caused by corrosion (internal and external) has remained at
around 25% and in North America, the number of corrosion leaks repaired each
year by pipeline operators has steadily decreased in the 1990's.
The reason for these trends is the
increasing use of corrosion management technologies to reduce corrosion risks.
Indeed, it is now expected that pipeline operators utilise appropriate
maintenance to prevent corrosion failures. For example, a North American operator
has recently been fined a record $30 million because "corrosion caused
most of the (300 oil) spills and they could have been prevented with proper operations
and maintenance".
Operators can now combine the benefits
of high resolution inspection, detailed corrosion growth analysis and
reliability methods in the development of their corrosion management
strategies.
ESTIMATION OF CORROSION GROWTH
RATES
The ability to accurately determine
corrosion growth rates is an essential input parameter into any effective
corrosion management strategy as it allows operators to better define and plan
future rehabilitation and operating regimes. Historically corrosion growth
rates have been estimated by comparing the sizes of a small sample of defects
in successive inspection runs or by using equations such as de Waard and
Milliams (10) for estimating rates in 'sweet' oil and gas pipelines. However,
run comparison software has now been developed (11) which provides a direct
comparison of the inspection signals between 2 or more inspection runs (Figure
1) and allows:
(i) the identification of internal
and/or external corrosion features which have grown between the inspection
operations,
(ii) the location of new sites of
internal and/or external corrosion to be identified, and
(iii) represent
Source
George Gilbert Mattew
Student ID. 155 12 061
Course: KL4220 Subsea Pipeline
Prof. Ir. Ricky Lukman Tawekal, MSE, Ph. D./ Eko Charnius Ilman, ST, MT
Ocean Engineering Program, Institut Teknologi Bandung
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