Fitness for Service Pipeline
Fitness-for-service assessment is a multi-disciplinary approach to
evaluate structural components to determine if they are fit for
continued service. Pipelines may contain flaws or other damage, or may
be subject to more severe operating conditions than the original design
anticipated. Quest Integrity Group’s LifeQuest pipeline assessment
solution uses API 579-1/ASME FFS-1 fitness-for-service methodology to
deliver an assessment of the pipeline for continued operation at defined
maximum allowable operating pressure. An evaluation of remaining life
and/or inspection intervals may also be part of such an assessment.
The FFS of any particular material is determined by performing a
fitness for service assessment. Performing accurate FFS evaluations is
an integral aspect of fixed equipment asset integrity management.
On the other hand, failing to perform evaluations can lead to equipment
failures which can further result in injury, loss of life, and severe
financial and economic consequences.
The reason these examinations are performed is because even if a
piece of equipment has a crack or other defect, this doesn’t necessarily
mean that it’s unfit for service. Most equipment can continue in
service despite small flaws, and to repair or replace equipment that can
still be used would be an unnecessary and costly expense. Not only
that, but unnecessary weld repairs can actually do more harm than good,
as the quality of the new weld can often be less than the original one.
There are several ways to see if a flaw can cause a piece of
equipment to be no longer fit for service. For cracks, fracture
mechanics provides the mathematical framework for the examination by
quantifying combinations of stress, flaw size, and fracture toughness.
While cracks tend to be the most dangerous, they’re not the only flaw
that might warrant evaluation. Volumetric flaws such as corrosion pits,
porosity, and slag may reduce the load-bearing capacity of a structure.
Likewise, structural integrity may also be compromised by locally
thinned areas which come grinding out cracks, thus FFS methodologies
have been developed to evaluate local thinning. In these cases,
acceptance criteria are based on limit load analyses rather than
fracture mechanics models. Some examples of these different FFS
methodologies are the BS 7910 method, API RP 579-1/ASME FFS-1 method, and the MPC/AP method.
It is important to note though that FFS evaluation can’t provide an
absolute delineation between safe and unsafe operating conditions.
Uncertainties in input parameters such as stress, flaw size, and
toughness often lead to a large uncertainty in the prediction of the
critical conditions for failure. In general there are two ways to
address this uncertainty. The more traditional approach has been to use
conservative input values in a deterministic analysis. The result of
such an analysis is a pessimistic prediction of critical flaw size or
remaining life.
An alternative approach, one which is becoming more common, entails
performing a probabilistic analysis that incorporates the uncertainties
in the input data. The latter type of analysis does not result in an
absolute yes/no answer as to whether or not a structure is safe for
continued operation. Rather, a probabilistic analysis estimates the
relative likelihood of failure, given all of the incorporated
uncertainties. Probabilistic FFS analysis can be an integral part of a risk-based inspection (RBI) protocol, where inspection is prioritized according to the risk of significant injury or economic loss.
Daftar Pustaka :
http://pipelinesinternational.com/news/fitness-for-service_assessment_of_unpiggable_pipelines/53611
https://inspectioneering.com/tag/fitness+for+service
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