Pipelines
like other slender strcutures with compressive forces, can buckle globally if
the axial compresssion goes beyond a certain level. Buried pipeline normally
tend to buckle in upheaval direction (upheaval buckling) and exposed pipelines
normally tend to buckle laterally (lateral buckling).
In
most cases, evaluations relevant to the global buckling threat will already
start taking place in e.g. feasibility studies carried out during the concept
phase. With regard to global buckling, the system risk review and strategy
development activity should be initiated by participating in such early studies
[1].
The
most relevant failure modes of global buckling are as follows [1]:
Local
buckling, which is normally the governing failure mode resulting from excessive
utilization. Local buckling appears as wrinkling or as a local buckle on the
compressive side of the cross section. Local buckling can lead to excessive
ovalisation and reduced cross-section area. This means reduced production, or
even full production stop if e.g. a pig should get stuck. A locally buckled
pipeline cannot stand an increased bending moment in the pipeline. This could
lead to pipeline collapse and full production stop.
Loss
of containment, as a result of:
Fracture,
is failure on the tensile side of the cross section also resulting from
excessive utilization. Fracture leads to leakage or full bore rupture, meaning
reduced production, or even full production stop.
Low
cycle fatigue, which can occur for limited load cycles in case each cycle gives
strains in the plastic region; i.e. the utilization is excessive in periods.
Low cycle fatigue may lead to leakage or rupture, meaning reduced production,
or full production stop.
Hydrogen
induces stress cracking (HISC), can occur in martensitic steels (“13%Cr) and
ferritic-austenitic steels (duplex and super-duplex). Blisters of free hydrogen
can create cracks in steel or weld at a CP/anode location when the steel is
exposed to seawater and stresses from the buckle. The pipeline utilization does
not have to necessarily be excessive. HISC leads to leakage or full bore
rupture, meaning reduced production, or full production stop.
True Stories
In
January 2000, a 17km 16-Inch pipeline in Guanabara Bay, Brazil, suddenly
buckled 4m laterally and ruptured, leading to a damaging release of about
10,000 barrels of oil, and to great embarrassment to the operator. Field
observation showed that as a result of temperature increase, the pipeline
displaced laterally, when failure took place. Operating pressure and
temperature of the pipeline were 400bar and 95°C, respectively. The soil
beneath the pipeline was very soft clay with about 2kPa undrained shear
strength at seabed [2].
In
December 2003, side-scan sonar survey of a 10km pipeline transported wet gas in
South East Asia, identified six lateral buckles along the pipeline length. The
original pipeline design did not consider lateral buckling as a design issue;
consequently, the effect of lateral buckling on the pipeline integrity was not
clear. Results of a detailed lateral buckling study showed that the pipeline
should be replaced within few years. Design Methodologies
Pipeline
design against lateral buckling involves three main Levels:
LEVEL 1: SCREENING
In
this level, an analytical approach (e.g. Hobbs [4]) will be used to check if
the pipeline is susceptible to lateral buckling. The results of this level
answer to the following questions:
Is
pipeline susceptible to lateral buckling?
Which
areas of the pipeline are susceptible to lateral buckling?
Can
we avoid lateral buckling by changing the concrete coating thickness of the
pipeline?
LEVEL 2: LATERAL BUCKLING ANALYSIS OF THE
PIPELINE
In
this level, a detailed finite element analysis will be performed on the areas
of the pipeline, which found to be susceptible to lateral buckling in Level 1
analysis. The results of this level answer to a main question: are limit state
conditions acceptable in areas of the pipeline with unplanned buckle?
LEVEL 3: MITIGATION
If
the answer to Level 2 question is no, this level will be commenced. In this
level, a mitigation measure will be selected based on project and client
requirements. The most well known mitigation methods are as follows:
Increasing
the concrete coating thickness in selected regions of the pipeline. One example
of this approach is Reshadat 16-Inch oil pipeline in Persian Gulf, the concrete
weight coating thickness of the first 5km of the pipeline was increased from
45mm to 65mm.
Laying
of the pipeline in zig-zag shape (snake lay). This method was successfully
utilized in South Pars Phase 6 and 7, Jade pipeline in North Sea, Penguins flow
line in North Sea
Laying
of the pipeline on pre-installed sleepers (vertical upset method). This method
was successfully utilized in PC4-B11 pipeline in Malaysia, King flow line in
gulf of Mexico.
Other
less popular methods such as adding either expansion spool or buoyancy modules
at selected intervals, and rock dumping also may be used.
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