Good forecast for pipelines
David Hodges of Kiwa Quality Services looks at how ongoing research and data collation has been fundamental to identifying pipeline defects and predicting pipes' working lifetimes
Assets such as pipelines tend to have a life cycle: design, manufacture, installation,
service, failure, repair and eventually replacement. If the time and cost of
each stage could be identified, asset management would become a more scientific
discipline. With condition assessment and knowledge of a pipeline’s level of
deterioration, a forecast can be made of its service life. This can be of immense
value in predicting the reliability of pipelines over time, in assessing various
maintenance options including a no-maintenance programme and in determining
whether a pipeline can be upgraded. The final outcome is an optimal investment
policy for the life of the pipeline.
A knowledge of the properties of pipe materials combined with improved data
processing, enables relatively accurate forecasting of how and when things will
go wrong. There have been several examples of failures in pipelines which were
believed to have complied with all the standards and guidelines available at
the time of manufacturing. Static test methods, often given in standards, do
not necessarily reflect the nature of in-service failures, while research using
dynamic testing procedures has reproduced failures more closely resembling those
seen under operational conditions. Improved data quality and a better understanding
of failure processes has helped to reduce premature failures. This has led to
design improvements in products where weaknesses are known.
Inspections to assess the rate of physical change are part of the process for
quantifying how much longer a pipeline can be expected to operate safely and
productively. Determining a reliable value for the deterioration rate of the
pipeline and using this to predict the deterioration process in the future helps
determine the right course of remedial action.
Early research began by studying strengths and weaknesses of different pipeline
materials, applications and ages. This is particularly difficult as pipelines
are made in a very wide range of materials such as ductile iron, steel, GRP,
cement, PVC and polyethylene. Information was then obtained on the design characteristics,
manufacturing processes and test methods employed. In addition, information
had to be obtained on service history, including operating conditions, maintenance
procedures and records and any pipeline surveys.
Failure can be caused by a number of factors:
- external corrosion,
- internal corrosion,
- impact damage,
- erosion damage,
- incorrect operation conditions,
- loss of ground support,
Research increasingly focuses on the reliability of newer materials such as
PVC and polyethylene. This includes crack propagation, the long-term effects
of surface damage and the effects on the pipe and quality of the water it contains
when the pipes are laid in contaminated soil – all of which can affect the lifetime
of a pipeline.
Recent improvements in non-destructive testing technology, such as scanning
using magnetic and ultrasound sensors, can result in the precise identification
of defect size and location in a wide range of materials in varying operating
conditions. The application of such sensors is now the foundation for almost
all lifetime predictions.
The probability of failure increases as the pipeline ages. Curves of failure
probability versus time can be used as the basis for scheduling further inspections
and for planning rehabilitation or replacement by predicting the time of failure
if no remedial action is taken. The basic assessment methodology usually uses
techniques to determine the defect size that would be present in the pipeline
when it went into service and the defect sizes that can be tolerated at the
operating pressures. The fatigue life can then be calculated by determining
the number of pressure cycles required to enlarge the defect to a size which
would fail at the operating pressure. Using this approach it is possible to
determine the nature and distribution of defects that could be present in the
pipeline and the probability of failure. A study allows the operator to determine
which pipeline is at greatest risk from fatigue and schedule the inspection
of the pipelines accordingly.
Another approach is to study the existing defect population in the pipeline,
the likely future deterioration rates and the potential for reducing these rates
through, for example, reduction of pressure, the dosing of corrosion protection
additives and the use of lining techniques such as epoxy resin, spray lining
or polyethylene liners.
Assessment analysis techniques can also be used to quantify the benefits of
repairs, which has led to substantial improvements in the prediction of lifetimes.
There is still a long way to go in pipeline research before we can produce
a computer model showing the effects of service on the lifetime of pipelines.
Much progress has been made but we still do not fully understand what causes
a number of failures. There is a need to identify fully the mechanisms involved
in the ageing process particularly in plastics. We are unlikely to stop the
ageing process altogether but slowing it will delay failure and therefore lifetimes
will be extended. We do not have all the answers yet but we have already gone
a long way in identifying many of the factors involved, and we are now able
to model many of their effects. We may not yet have a pipeline able to last
several hundred years without maintenance but through continued research we
are getting closer to it.
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