Its
been a long day since i've posted my last journal. Today I'm gonna share a bit
of my internet research about Virtual Anchor length. This term is usually used
in the offshore project especially in pipeline project. lately I am concern
about this field of study, Pipeline, because now I'm working on the Pipeline
division in one of the offshore consultant company.
So
lets begin with the article. Because i realize that I'm just a newbie in the
"blog world", so today i will just repost the article that i have
found in the internet about this topic, please be advice :). next time I will
be more interactive in posting some good knowledge.
A pipeline restrained by fixed anchors will experience a series of
stresses including longitudinal, bending, and axial. Virtual anchor lengths are
taken as the distance required for the frictional force provided by the soil
surrounding the pipe to equal the forces applied by thermal/ pressure expansion
and the soil’s resisting friction per unit length of pipe.
Anchors are placed strategically along the pipeline to help
prevent movement. Unrestrained pipeline movement can cause damage to the
connecting piping and equipment. The length of piping required to form the
virtual anchor is known as the active length. It must be noted that the force
required to fully restrain a pipe is not a function of its length. Factors that
influence the required force to restrain pipe include temperature, pressure and
percentage strain within the pipe.
Soil provides a constant frictional force along a buried steel
pipeline. The magnitude of the frictional force depends on the burial depth,
pipe weight, soil density, and coefficient of friction between the soil and
steel. Therefore, starting from a free end, the total restraint exerted by the
soil on the pipe gradually increases until it reaches the fully restrained load
at the virtual anchor. At this point, the naturally occurring forces are
balanced with a restraint point. Similarly, moving along the pipe away from the
virtual anchor, the pipe expansion becomes gradually minimized until a point of
zero expansion is reached indicating the pipe is fully held in place.
Axial expansion is calculated by taking the average of the full
axial restraint at one end of the pipe and zero restraint at the opposite end
of the pipe. When compared to above ground piping, the total axial expansion at
the free end of a buried pipeline is half of the calculated value for similar
scenario involving above ground pipe.
In reality most pipes do not have a totally free end but have
some resistance due to soil restraint as the pipe exits the ground and from the
connection to above ground piping. This acts to reduce the expansion at the
‘free’ end. Soils with lower friction resistance or pipes with less depth of
cover have longer active lengths and thus have greater expansion at the free
end.
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| Figure. Virtual anchor length - Visual representation |
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