Yards Moving Forward - GL Group

of pipelines?
in order to identify those parameters critical to the risk of the
whole system. Semi-quantitative methods have been found to
be adequate for such an analysis, enabling the most costeffective
remedial measures to be taken for any identified
risks. Using a stepwise, semi-quantitative method, the riskdriving
parameters and the quantification of the risk for a
pipeline network can be calculated.
GL Methodology
The GL methodology splits the pipeline system to be
assessed into different sections, called TAGs. Each TAG is
deemed to have a relatively constant risk associated with it
because the physical characteristics of the section and its
environment remain unchanged. However, each change in
the pipe’s wall thickness, or a known damage, for example, or
a change in water depth for an offshore pipeline, would require
a split into different TAGs. Consequence-specific factors
will also impact how a pipeline system is split up into TAGs.
Such factors include pipeline location, volumes transported
and the product being transported. For land pipeline the
location factor would take into account aspects such as population
density, road crossings and the proximity of buildings
for a given area along the route. In addition, the risk assessment
must accommodate any readjustments, such as the
construction of new installations in the vicinity of the pipeline,
that may take place over time.
The GL methodology also utilizes an index-based
approach to risk evaluation. Risk is presented by using a risk
matrix to provide a clear overview of the relative contribution
of the probability of failure (PoF) and consequence of failure
(CoF) factors. Considerable effort is required to estimate the
PoF factor and criteria such as design, operation, third party
impact and corrosion are weighed up in the analysis.
Consequences considered in the determination of the CoF
factor include human safety, environmental impact, economic
repercussions and reputational/political fallout.´The
PoF and CoF factors are incorporated in the GL Index which is
used to indicate the relative severity of the risk. The highest
risk is when both the PoF and CoF are serious. Risk acceptance
criteria are the limits above which the operator will not
tolerate risk to the installation.
Optimising Pipeline Inspections
Risk acceptance criteria are also used to help determine
the most appropriate type of inspection and the optimum
timing for inspections of relevant parts of the pipeline system.
SAFETY PRECAUTION
Obviously, it is important that an inspection is carried out
prior to the acceptance limit being breached. This could be
likened to preventative maintenance. Timely inspections will
enable not only a reassessment of the risk level based upon
the gathering of firsthand information onsite but also the
detailed evaluation of any damage. Such an evaluation could
necessitate the timely repair or replacement of the degraded
component, i.e. corrective maintenance. The main reason for
implementing a risk-based approach to inspection planning
is to focus inspection efforts on items where the safety, economic,
environmental or reputational risks are identified as
being high. At the same time, inspections of those parts of the
pipeline network identified as low-risk systems can be scaled
down in an appropriate manner.
Software Package
Germanischer Lloyd has incorporated its newly developed
procedure into an RBI software package called GALIOM
for Pipelines. The software not only enables application of the
GL methodology to all types of onshore and offshore pipeline
systems but also facilitates the time-consuming iterative processes
for determining the probability and consequences of
failure for each TAG section in a system. This risk-based
evaluation and inspection methodology provides a powerful
tool for improving pipeline inspection strategies. To determine
the risk it is important to consider the time-dependent and
time-independent factors, as both are required for a sufficiently
accurate risk assessment.
Germanischer Lloyd believes that simple qualitative
methods should be replaced with more accurate semi-quantitative
risk evaluation whenever possible. Nevertheless, the
qualitative approach is still required for many technical
aspects because it provides an efficient process for determining
the major contributors to risk. Based on semi-quantitative
methods, the GALIOM for Pipelines risk evaluation and
inspection program enables an accurate assessment of the
probability of failure due to factors such as design, loads, fatigue
and corrosion as well as the consequences. The project
has confirmed that for most pipeline systems corrosion represents
a significant contribution to the overall risk. The GL
methodology enables an estimation of the probability of corrosion,
the size of flaws and their annual growth rate, taking
into account different boundary conditions as well as inspection
data, if available. The probability index is calculated
using a limit state function for corrosion flaws to provide a
relatively fine graduation of the probability of failure. ■ MC
For further information: Ulrich Adriany, Pipelines and Infrastructure, Deputy Head of Dept.,
Phone: +49 40 36149-7423, ulrich.adriany@gl-group.com
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