Free Span Analysis of Sub Sea Pipelines

7/31/2019 Free Span Analysis of Sub Sea Pipelines

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By,

D.N.ADITHYAR15021006

M.Tech- PLE( II sem)


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Free spanning is usually caused by acombination of seabed movement, waveaction, and current effects.

When a fluid flows across a pipeline, theflow separates, vortices are shed, and aperiodic wake is formed. Each time avortex is shed it alters the local pressuredistribution, and the pipeline experiences atime-varying force at the frequency ofvortex shedding


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Under resonant conditions, sustained

oscillations can be excited and the pipeline

will oscillate at a frequency. This

oscillation will fatigue the pipeline and can

eventually lead to catastrophic failure.


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In-Line Oscillations:o These oscillations are excited at flow

velocities lower than the critical velocities

for crossflow motion

To prevent this in-line response at either

mode of vortex shedding excitation, it is

suggested that the stability parameter (Ks)be larger than 1.8


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Dynamic Stresseso The presence of bottom currents can

cause significant dynamic stresses, if fluid

structure interaction (vortex shedding) inthese free-span areas causes the pipeline

to oscillate.

o

These oscillations can result in fatigue ofthe pipeline welds


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The frequency of vortex shedding is a

function of the pipe diameter, current

velocity.

Pipeline failure due to vortex excited

motions can be prevented if the vortex-

shedding frequency is sufficiently far from

the natural frequency of the pipe spansuch that dynamic oscillations of the pipe

are minimized


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Vortex-Shedding Frequency:o Frequency is the frequency at which pairs

of vortices are shed from the pipeline

fs =SUc/Dwhere

fs = vortex-shedding frequencyS = Strouhal NumberUc = design current velocity (Strouhal Number is the dimensionless frequency of the vortex shedding and is afunction of the Reynolds Number)


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where

fn =pipe span natural frequency

Ls= span length

Me = effective massCe = end condition constant


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Reduced Velocity:o The reduced velocity, Ur , is the velocity at

which vortex shedding induced oscillations

may occur.


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Critical Span Length:o The critical span length or the unsupported

pipeline length at which oscillations of the

pipeline occur for a specific current is

based on the relationship between the

natural frequency of the pipe free span

and the reduced velocity


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The critical span length for crossflow

motion is expressed as:


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General Considerationso No location along the pipeline route does

the unsupported pipeline span length

exceed the critical span length

o The selection of the allowable span length

is an important risk assessment type

solution


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Current Velocity Selectiono The calculated reduced velocity, stability

parameter, Reynolds Number, and critical

span length should all be based on a

current velocity that is perpendicular to the

pipeline.


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End Condition Selection:o Pinned-Pinned: Used for spans where

each end is allowed to rotate about the

pipe axis.

o Pinned-Fixed: Used for the majority of

spans, any span that does not fit the othertwo categories


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Design Parameterso Conservative design should be based on

the avoidance of in-line motion for the

design bottom current

o The allowable pipeline span length should

always be designed such that cross-flow

motion will never occur


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Trenching The sea bed:


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Rock dumping:


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The free span which is a night mare for

design engineers was studied and the

main cause of failure of pipeline in free

span region was found to by Vibrationsinduced in pipeline due to formation of

vortices.

The various methods for preventing Freespan were also studied.


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Documents

Free Span Analysis of Sub Sea Pipelines