Pipelines installed in marine environments for
transportation of oil and gas from the offshore platforms has become
“lifelines” of the oil industry. The water depths in which these pipelines are
laid may vary from tens of meters to hundreds of meters. These offshore
pipelines can be laid on the sea bed surface, they can be buried, or they can
be trenched. When the pipelines are laid in the sea, they are exposed to direct
flow of sea water .
Scour Caused by Span
Source : https://edarikahiki.files.wordpress.com/2013/02/118-15.jpg
This causes scour around the pipeline, which leads to
suspended free spans of the pipe line. The pipeline along the length of the
suspended span may or may not sag in the generated scour hole. In case of a
sagging pipeline, the pipeline may reach the bottom of the scour hole, which is
followed by backfilling which eventually lead s to self – burial of the
pipeline.
If free-spans are long, the vortex induced vibrations (VIV)
can cause the pipeline to undergo fatigue damage and severely reduce the
pipeline design life.
Underwater pipeline protection is of major concern for the
Oil and Gas operating companies. They carry high pressure crude oil, gas, and
products and hence their health is of major concern. Given the amount of threat
present in the underwater pipeline, the following protection :
Rock and Gravel Dumping
Rock and gravel dumping provides a protective layer of rip-
rap around the pipeline. Various installation techniques have been employed
from the surface vessels, namely:
From a side- dumping barge or vessel with individual stones falling to the sea
bed, From a split- hopper barge as one big mass, From a barge through
a pipe to reduce the fall velocity of the rock and improve placement accuracy.
Rock and Gravel Dumping
Source :
The type of material req uired and the amount of material
required is determined by the site location and hence the method of deployment
depends on it. The material used to form the protective layer must offer
sufficient resistance to withstand the flow induced forces (enhanced sh ear
stress, vortex action). The stability of the local sea bed material can be
calculated based upon knowledge of the local flow field around the pipeline.
Mattresses
Prefabricated mattresses have been used in bed protection or
preparation schemes. These mattresses can be installed in a controlled manner as compared to the rock
dumping method. Mattresses are often used to provide the much needed protection
to pipelines but it can also be adopted for other sea bed structures. One of
the major advantages of using mattresses is that they are flexible and can be
laid to suite the local bed contours.
Mattresses
Source : http://www.formshore.com/images/pipeline-pyramid-img1.jpg
The various types of protective mattress used for pipeline protection
are:
• Fascine mattress: it synthetic filter fabric strengthened with synthetic or
natural fascines,
usually overlain by rock dump material
• Block mattress – it continuous array of concrete blocks held together by
cables and laid on the sea bed or individual blocks held in it pattern on the
sea bed by synthetic nails
• Cell mattress — mesh baskets filled with sand or gravel, large rocks in large
wire mesh also called gabion baskets
• Concrete mattress – the mesh baskets of the cell mattress are filled with
underwater concrete instead of ballast
• Stone asphalt mattress – a synthetic filter fabric ballasted with it stone
and asphalt mixture
• Ballast mattress — a heavy synthetic fibres woven mattress is double folded
at both sides and tilled with sand or gravel.
Whilst the rock rip- rap is held in place due to its own
weight and resistance between the rock and the underlying layers. The
mattresses are often held in place by the use of soil pins or anc hors.
The resistance to pull out presented by the soil fixings, or
the tensile strength of the material joining the mattress to the anchor, is
designed to resist the uplift and drag due to hydrodynamic forces. Poorly
designed fixings have historically been the most common cause of failure of
these types of protection devices. Steps are often taken to fill any unevenness
in the bed beneath the structure and this provides additional protection
from scour.
Trenching (Pipelines) Or Increasing Structure
Embedment Trenching of pipeline or increasing the structure embedment in
the sea bed provides the much needed sheltering from wave and current forces
which cause scour. This increases the pipeline stability and also the
margin of safety against the undermining by scour as a result of greater
soil-structure interaction.The trenching of
pipeline into the sea bed not only provides protection from the scour
activity but it also reduces the hydrodynamic load on the pipeline
thereby adding stability to it. The increased embedment of the pipeline
results in reduced flow fields around it when compared to pipeline
resting on initial bed level. When trenching or increased embedment of
pipelines are not feasible in harsh environmental conditions it may
be necessary to stabilize them with anchors.
In a region of active sediment movement or sand waves, sea
bed needs to be ploughed flat prior to the installation. This causes the sand
waves to reform and migrate over the pipeline resulting in changes to the
pipeline cover over time, thereby providing cheap and effective way of
protecting and stabilizing a pipeline. The embedment process can be enhanced
with the use of a spoiler placed along the top of the pipe.
Sumber :
https://anggraenieka.wordpress.com/2014/01/30/pipeline-free-span-mitigation/
(www.ijert.org%2Fbrowse%2Fmay-2012-edition%3Fdownload%3D45%253Ascour-mechanism-detection-and-mitigation-for-subsea-pipeline-integrity%26start%3D10&ei=QiANUbuKGsytrAeXxYHgCQ&usg=AFQjCNEiFAjXiCF7mwSAik3AcPbyY_EhXg&bvm=bv.41867550,d.bmk&cad=rja)
Dega Damara Aditramulyadi
Student ID : 15512046
Course : KL4220 Subsea Pipeline
Lecturer : Prof. Ir. Ricky Lukman Tawekal, MSE, Ph. D.
Eko Charnius Ilman, ST, MT
Ocean Engineering Program, Institut Teknologi Bandung
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