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Oilfield Technology
August
2015
ease of testing etc., and material sampling, i.e. retaining the
as-received surface condition and microstructure, sampling from
within the wall of the product, relaxation of residual stress, relevance
to service etc. These factors influence the degree of confidence in the
results and the extrapolation to consideration of FFP for the intended
service duty.
Examples of factors to consider include:
Ì
Influence of specimen geometry and extraction upon relevance
of the test method to service.
h
Uniaxial tensile specimens must be selected from within
the body of the material (to allow the machining of threads)
and are fully machined. Therefore, these cannot sample
the surface microstructure and condition. However, this is a
reproducible geometry which provides confidence that there
will be little variation between specimens influencing test
performance.
h
Double cantilever beam (DCB) is a specimen geometry used
for the crack arrest method to derive a threshold stress
intensity factor in sour service applications. This approach is
frequently adopted for downhole tubular, but not suited to
testing weldments.
Ì
Weldments need special consideration.
h
The extraction of material from weldments result in a
relaxation of the residual stresses which can contribute to
environmentally assisted cracking. This is the reason the
specimens need to have high strain applied in testing, per
ISO15156. Due to the thermal cycle (development of the
heat affected zone [HAZ]), dilution of the weld metal in the
root and presence of heat tint oxide, the microstructure
in weldments differs from the parent material. All of these
factors need to be taken into consideration in testing
performance; thus the usual recommendation is root-intact
4-point bend (4pb) testing. Retention of a significant portion
of the wall thickness is also necessary to mitigate the normal
misalignment from acceptable hi-lo parameters for line pipe,
although this is much more tightly constrained in weldments
for risers subjected to fatigue.
Ì
Installation may alter material properties.
h
Material subjected to plastic strain to reproduce the
installation method, e.g. reeling, must be sampled carefully
to ensure the specimens are correctly located. Consequently,
this results in only a small portion of a girth weld being
tested.
Ì
Damage modes must be reproduced by the test method(s)
selected.
h
It is also necessary to ensure all appropriate damage
modes are assessed in materials selection, including testing
when required. In the latter case, ensure the test method
is suitable, e.g. stress oriented hydrogen induced cracking
(SOHIC) for which Exova has developed a test method (as
there is no standard technique, although full ring testing
can induce this damage in susceptible material). The
company is intending to run a joint industry project (JIP)
for round-robin assessment for potential adoption as a
Standard.
Techniques which do not require extraction of specimens offer
potential advantages:
Ì
No relaxation of residual stresses.
Ì
Retention of the as-received material condition.
Ì
Depending on the technique adopted, testing of entire girth
weld.
Testing of clad components requires the isolation or removal
of the carbon steel. In long-term cyclic loaded testing for corrosion
fatigue endurance studies in particular, isolation can be difficult to
guarantee via coatings. Removal of the substrate means that the
material cannot be loaded to the degree appropriate for the service.
Standards
It is imperative that the industry has appropriate standards to
assure commonality of materials, processes, and testing protocols.
However, it is essential that users and specifiers understand the
limitations of techniques, whether this relates to the impact of
sampling upon the material condition tested, or the method’s
ability to reproduce the service conditions, particularly for elevated
temperature and pressure. Additionally, as the offshore sector
places increasing demands on materials to handle more aggressive
conditions (with new field developments in ultra-deepwater and/or
HPHT produced fluids) test techniques need to be reviewed, modified
or developed. This allows laboratory testing to provide confidence in
materials performance and qualification.
Exova supports international standards development and
maintenance via leadership and representation on committees
such as NACE MR0175/ISO15156, NACE TM0177 and TM0284
(SSC and HIC test methods respectively). The company’s
participation extends to the support of the European Federation
of Corrosion Working Party 13 and the two sour service guidance
documents (EFC16 and EF17), and support to NACE International and
NACE International Institute, amongst others.
Figure 2.
Slowstrain rate test (SSRT)/ripple strain rate test (RSRT)
equipment withautoclaves for CRA testing.
Figure 3.
Autoclaves allowelevated temperature andpressure testing of
CRAs for qualification tests ofmaterials andweldments.
