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COMPLETING THE JIGSAW

GETTING TO KNOW YOUR STRESS CORROSION CRACKING

Andrew Wynne Integrity Engineer for ROSEN (UK) FIGURE 1 THREE FUNDAMENTAL ELEMENTS REQUIRED
and winner of IGEM’s YPPC 2019 FOR STRESS CORROSION CRACKING (SCC)

What is stress corrosion cracking? grade, stress level, etc. A pipeline was
Stress corrosion cracking (SCC) is selected for inspection based on its
a form of environmental cracking coating type (asphalt). The client had
found worldwide, on both liquid and experienced a previous SCC failure on a
gas pipelines. External SCC requires pipeline with an asphalt coating.
the presence of three fundamental The hoop stress within this selected
elements in order to occur: a tensile pipeline was below 60 per cent SMYS
stress, a corrosive environment and a and the pipeline was relatively young
susceptible material (Figure 1). (<25 years) and of high grade (X65).
SCC literature suggests that pipelines However, other than the coating
with higher hoop stress, i.e., stress which could disbond and lead to
due to internal pressure, are more external corrosion, the pipeline did
susceptible to SCC. A corrosive not tick all the boxes for an SCC FIGURE 2 A PIPELINE RUPTURE
environment often forms as a result susceptible pipeline. - A POTENTIAL OUTCOME OF NOT MANAGING SCC
of damaged coating that exposes the
pipeline surface. This, in combination In-line inspection
with a poor cathodic protection (CP) In 2017, ROSEN ran an electromagnetic
system, can lead to external corrosion acoustic transducer (EMAT) tool through
and an increased risk of SCC. All steel the pipeline to identify axially orientated
pipelines are technically susceptible cracks. In addition, a geometry
to SCC, but modern pipelines with inspection tool was run alongside the
higher grade steel are considered less EMAT tool to identify any deformations,
susceptible to SCC than older and lower such as dents and wrinkles.
grade pipelines. The EMAT inspection reported
a significant number of crack-like
SCC Management indications, i.e., potential SCC sites. The
SCC can be a difficult pipeline threat first piece of the jigsaw was in place,
to manage; it is difficult to detect potential crack locations had been
and estimates of the rate of growth identified. In order to verify the EMAT
vary widely from 0.1mm/year to over signals, in-field verification of some of the same properties of those four spools
1mm/year. If not managed properly, the crack indications was carried out. found to have SCC; the same coating
SCC can result in a rupture (Figure 2) type, coating condition, soil type, hoop
leading to loss of production and, more In-field investigation stress, etc., and were constructed on
importantly, a potential loss of life. I attended one particular excavation the same date. Intuitively, Spool 2
SCC has been found in all corners site, which consisted of five adjacent should be as susceptible to SCC as the
of the globe. To date, SCC has not spools (Figure 3) crossing the top of a other four spools.
been reported in the United Kingdom. hill, where the pipeline’s curvature was The four spools with SCC (Figure 3)
However, the potential for SCC in the visible. The pipeline was an overbend were subjected to cold field bending
UK gas transmission network certainly in order overcome the hill’s gradient. during construction in order to pass
exists, with an ageing network (over Four of the five spools excavated were the pipeline over the curvature of the
50 years old) and some pipelines found to have SCC. hill. This curvature was visible in-field
operating with hoop stress at or above All the EMAT-reported indications and was also reported during the ILI.
60 per cent specified minimum yield were confirmed to be SCC and additional Cold field bending introduced localised
strength (SMYS). shallow SCC indications, below the stress concentrations into the pipeline,
EMAT reporting threshold, were also raising stress levels such that SCC could
Case study identified in-field. Note that EMAT occur in the spools. However, Spool
A client conducted a SCC susceptibility reported no indication of SCC in Spool 2, 2 was never subjected to cold field
assessment for an entire gas a result which was confirmed in-field. bending and visibly remained a straight
transmission system. A number of The question raised by the client was: section of pipeline. Importantly, the
pipelines were selected for in-line why did spools 1, 3, 4 and 5 display SCC, deformations (wrinkling) introduced
inspection (ILI), with the intention whereas Spool 2 did not? Was there into spools 1, 3, 4 and 5 (Figure
of identifying SCC, based on various something unique about Spool 2 which 3) during construction were not
criteria: pipeline age, coating type, pipe resisted SCC? Spool 2 seemingly shared introduced into Spool 2.

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