Engineering critical assessments and their use
During the past 40 years or so, the industry has used the
engineering critical assessment (ECA), which is a fitness
for service (FFS) assessment, that has become increasingly
important in today’s harsher environments for mitigating
and preventing pipeline failures. ECAs are designed to
evaluate the significance of any flaws within a structure or
component with respect to structural integrity, achieving this
by determining the maximum flaw size that can be tolerated
with respect to fracture, fatigue growth or plastic collapse
under specified loading conditions within its intended design
life.
ECAs take into account the installation and operational
phases of a pipeline and can be divided into two categories:
)
)
‘Elastic’-based ECAs examine methods that do not result
in permanent deformation during the pipeline installation
process, for example landlines, S-lay and J-lay installation.
)
)
‘Plastic’-based ECAs examine methods using high
plastic strain, such as pipeline reeling. As a result, ECA
techniques can provide FFS assessments for projects with
a range of differing installation methods and operational
environments, including deep, intermediate and shallow
waters.
ECAs can be carried out for many purposes, including
the assessment of flaws within structures that have become
apparent as part of ongoing inspection. In this case, an ECA
can then be used as part of the process to justify ongoing
service based on FFS and to set inspection intervals, helping
to determine if and when the structure needs to be repaired
or replaced. Increasingly, ECAs are also used as part of the
manufacturing process.
With respect to pipeline welding, ECAs have been
used to replace arbitrary volumetric ‘good workmanship’
weld flaw acceptance criteria with FFS-based acceptance
criteria that consider the complete lifecycle of the pipeline.
Mechanised welding can result in low height flaws, but
with lengths that would normally be rejected by good
workmanship criteria. These low height flaws are less
significant with respect to FFS and the use of automatic
ultrasonic testing (AUT), where flaw height can be measured
accurately, has helped to drive the change from good
workmanship to an FFS acceptance criteria, thus reducing
unnecessary weld repairs.
In offshore pipeline welding, an ECA can in theory
relax the weld flaw acceptance criteria, although
unfortunately this is not always the case. As installation
methods become more complex and operational
conditions become more onerous, ECA-based FFS
acceptance criteria can sometimes be more restrictive
than good workmanship criteria. This follows the principle
of FFS, as in today’s industry, good workmanship-based
acceptance criteria may not be enough to satisfy the
arduous installation and operating conditions frequently
imposed on pipelines.
The importance of material characterisation
The two key material properties used as inputs for an ECA
are tensile strength and fracture toughness, with the quality
and repeatability of the tensile and fracture mechanics test
data being essential to the ECA process. The input data for
an ECA should be carefully selected to be both realistic and
conservative, which will provide confidence that the analysis
is robust in providing tolerable weld flaw sizes than can be
accepted without compromising the integrity of the pipeline
over its service lifetime.
Further to normal weld procedure qualification (WPQ)
testing, additional testing to characterise the parent and
weld material properties specifically for the ECA is part of
the process that Element can provide within an integrated
ECA testing and analysis service.
Specialised tensile samples are used to allow
characterisation of the tensile properties of the material
under various conditions of cyclic strain and temperature
that simulate the process of the pipeline installation and
operational conditions. Single-edge notched tensile (SENT)
geometry fracture toughness samples producing JR curves
are used to obtain the fracture toughness relevant to the
pipeline installation process, while single-edge notched
bending (SENB) and/or SENT geometry fracture toughness
samples in a strained and aged condition are used to
characterise the operational fracture toughness when the
pipeline installation has involved plastic strain.
An integrated ECA service
To successfully deliver an ECA, a multidisciplinary approach
must be taken at all times, involving materials engineering,
fracture and fatigue analysis and NDT expertise. Element’s
ECA services are fully integrated, with its advanced materials
testing facilities in Houston, Aberdeen and Breda assisting in
the production of essential materials input data, delivered to
internationally recognised codes and standards.
High quality data and analysis within an integrated
service ensures more reliable modelling of installation and
service conditions, thus optimising weld flaw acceptance
criteria and minimising costs and time spent on unnecessary
repairs, while ascertaining overall integrity of the pipeline.
Figure 2.
Pipeline installation has become considerably more
sophisticated and there is now increased demand to provide
the confidence that pipelines will function safely for longer
periods in deeper, colder and sourer deepwater environments.
32
World Pipelines
/
JUNE 2015
