pipeline free span monitoring study in the Gulf of Thailand.
The deployment period lasted six months, and involved
monitoring a number of potentially high-risk pipeline free
spans, which were longer than the code, defined allowable
limits.
The solution
The free span monitoring kit was deployed on a selection
of spans across a range of pipeline diameters. The data
recovered showed that the pipeline free spans were
not experiencing VIV, despite the local environmental
conditions being in the range predicted by the analysis
to excite the span. Using the data collected and further
analysis taking account of the identified conservatisms, the
span anomaly limits were modified accordingly.
The benefits
The project confirmed that the pipeline free spans were
not a threat in their current state and gave the operator
the data needed to assess the acceptability of any future
free spans identified. Future free span lengths can be
compared to the calculated maximum allowable length
for each section of pipeline. Without this in situ data,
the free span would have been marked for remediation.
Equipped with this knowledge, the operator was able to
focus on the free spans that presented a risk and avoid
unnecessary and costly remedial works on the other
pipeline sections.
Span assessments in lifetime extension
As well as forming a fundamental part of pipeline design
and integrity management, free span assessments are also
a staple feature of pipeline lifetime extension studies. Free
spans are normally identified as a threat to a pipeline’s
extended life during the risk assessment stage. Fatigue
damage has often accumulated throughout the pipeline’s
20 - 25 year life, and the pipeline strength may have been
compromised during the period of operation to date –
the wall thickness of the pipeline may have been
reduced due to corrosion for example.
Jee draws upon its expertise in pipeline design
and integrity management to compliment the free
span assessments performed as part of the lifetime
extension process. The company understands how
the design codes have changed over the decades
of pipeline operation, and know where extra
conservatism has been added or removed. They also
have an appreciation of the code’s limitations, so
its engineers are able to effectively challenge them
without adversely affecting safety, maximising the
allowable extension to pipeline operating life. As
part of this process, free span anomaly limits may
be updated based on the results of the analysis
performed.
Summary of span services
Jee’s engineers have conducted thousands of
free span assessments using a combination of
calculations and numerical methods, such as FE
analysis and computational fluid dynamics (CFD)
to investigate, predict and solve potential and
existing spanning issues. This experience covers the
assessment of free spans in single pipe and more
complex solutions, such as pipe-in-pipe systems and
bundles.
In addition to its analysis based approach,
Jee has developed a unique free span monitoring
system in order to bridge the gap between the
assessments performed in accordance with
the available recommended practices for span
assessment and the ‘real world’ behaviour of the
span. This has allowed it to justify the continued
operation of pipelines with significant free spans
without the need for costly remediation and
the associated future integrity concerns that
remediation may bring.
Figure 3.
Jee’s span monitoring system.
Figure 4.
Free span identified during survey.
54
World Pipelines
/
FEBRUARY 2016
