Docfoc.com 249591587 Class 10 Free Span Analysis.ppt

7/25/2019 Docfoc.com 249591587 Class 10 Free Span Analysis.ppt

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Pipeline Free Span AnalysisPipeline Free Span Analysis

Dr. Ir. Ahmad TaufikDr. Ir. Ahmad Taufik

(SUBSEA PIPELINE ENG.)(SUBSEA PIPELINE ENG.)


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Introduction

Pipeline spanning can occur when the contact between the pipeline and

seabed is lost over an appreciable distance on a rough seabed.

This will cause to static overload and thus overstress of pipeline section

due to bending stress and lead to the pipeline deformation or crack.

The condition, under right current speed, pipeline span length andweight will also causing vortex induce vibration VIV that may lead to

fatigue failure.

The discussion will focus will design how to avoid this possible

occurrence by calculating the allowable freespan in terms of static and

dynamic loading. The vortex shedding induced oscillations due to currents is the most

deepwater pipelines limiting factor for the allowable span length.


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Pipeline Integrity Management ystem

Offshore Pipeline and Risk

Associated with it

Free Span Lead to Bending


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Other Possible

Problems related

to Freespan


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Problem Description

Freespan!reespan can result in failure of pipelines due to excessive yielding and

fati"ue. It may also cause interference with human activities such as

fishing. !reespan can occur due to unsupported weight of the

pipeline section and dynamic loads from waves and currents.

ibration !

"# In Line ibration

$# %ross & Flow ibration


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tot

e

aDW

IC

L

=

2

Problem Description ' Static Anal(sis

#a $ %llowable static freespan

& $ end restrained constant

$ '.() for pinned*to*pinned conditionI $ Moment of inertia

e $ +"uivalent stress Von Misses-

$ /niformly distributed load per unit length

22

)( IDs FFW ++

!reespan can result in failure of pipelines due to excessive yielding andfati"ue. It may also cause interference with human activities such as

fishing. !or static loading the maximum allowable freespan sub0ect to

static loading


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Pipeline Integrity Management ystem

)a*imum Allowable Freespan Offshore Pipeline


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Problem Description & D(namic Anal(sis

!reespan can also vibrate due to interaction of unsupported weight ofthe pipeline section and dynamic loads from waves and currents.

ibration !

"#In Line ibration

$#%ross & Flow ibration


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Problem Description ' D(namic Anal(sis

A# In'Line Oscillations

The amplitude of the in*line motion is only "+, of those associated with

cross*flow motion. everal parameters are used in determining the

potential for vibration. These include the reduced -elocit(. Ur. and the

stabilit( parameter. Ks#

The first and second modes of in*line instability are associated with

symmetrical vortex shedding and have a peak response at reduced

velocities Ur- of 1.' and 2.3, respectively.

To prevent this in*line response at either mode of vortex shedding

excitation, stabilit( parameter /Ks0 1 "#2otton, 1''1-.

4nV also state that the resonant in*line vortex shedding induced

oscillation may occur when "#+ 3 Ur3 $#$, the shedding will be alternate.


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B# %ross'Flow Oscillations

+xcitation in the cross*flow direction is potentially more dangerous

than that in*line since amplitudes of response are much greater than

those associated with in*line motion. 5owever, these oscillation occur at much larger velocities than in*

line oscillations and are not normally governing. The limiting value

for cross*flow oscillations based on Dn is Ks3 "4#

Problem Description ' D(namic Anal(sis


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Design %onsideration

D(namic Stresses

The presence of bottom currents can cause significant dynamic

stresses, if fluid structure interaction vortex shedding- in these free*

span areas causes the pipeline to oscillate. These oscillations can result in fati"ue of the pipeline welds, which

can reduce pipeline life. The fre"uency of vortex shedding is a

function of the pipe diameter, current velocity, and trouhal 6umber.


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orte*'Shedding Fre5uenc(

The vortex*shedding fre"uency is the fre"uency at which pairs of vortices are shed from the pipeline and is

calculated based on the following7

where7

fs$ vortex*shedding fre"uency

S$ trouhal 6umberUc$ design current velocity

D$ pipe outside diameter

trouhal 6umber is the dimensionless fre"uency of the vortex shedding and its a function of the 8eynolds

6umber. 8eynolds 6umber Reis a dimesionless parameter representing the ratio of inertial force to viscous

force7

where vis kinematic viscosity of fluid 1.2 x 19*: ft2;sec for water at 39


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Pipeline 6atural Fre5uenc(

The natural fre"uency for vibration of the pipe span is given by the following formulas7

where

fn$ pipe span natural fre"uency

Ls$ span length

Me$ effective mass

Ce$ end condition constant

The end condition constant reflect the support conditions of the pipeline span.

Ce$ 1.99 =-2$ '.() pinned*pinned-

Ce$ 1.2: =-2$ 1:.: clamped*pinned-

Ce$ 1. :9 =-2$ 22.2 clamped* clamped-


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Pipeline 6atural Fre5uenc( The effective mass is the sum of total unit mass of the pipe, the unit mass of the pipe contents,

and the unit mass of the displaced water added mass-.

where

Mp$ unit mass of pipe including coating slug;ft or kg;m-

Mc$ unit mass of pipe of content slug;ft or kg;m-

Ma$ added unit mass slug;ft or kg;m-

The added mass is the mass of water displaced by the pipeline and is calculated based on the

following7

whereis mass density of fluid around the pipe seawater $ 2 slug;ft>or 192: kg;m>-.


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Reduced elocit(

The reduced velocity, Ur, is the velocity at which vortex shedding induced oscillations

may occur 7

!igure 1 presents the reduced velocity for cross*flow oscillations based on the

8eynolds 6umber 4nV, 1'(1-. !igure 2 presents the reduced velocity for in*line

oscillations based on the stability parameter Ks-.


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Figure "# Reduced -elocit( for cross'flow oscillations

based on the Re(nolds 6umber


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Figure $# Reduced -elocit( for in'line oscillations

based on the stabilit( parameter


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Stabilit( Parameter

% significant for defining vortex*induced motion is the stability parameter, Ks, defined

as7

where sis logarithmic decrement of structural damping $ 9.12:-.


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%ritical Span Length

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

fre"uency of the pipe free span and the reduced velocity.

The critical span length for cross'flow motion is7

The critical span length for in'line motion is 7


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Design %riteria

7eneral %onsiderations

!or preliminary design purposes, it is customary to design a pipeline

such that at no location along the pipeline route does the

unsupported pipeline length exceed the critical span length for which

in*line motion occurs due to vortex shedding, at any time during thedesign life of the pipeline.

5owever, in deep water, where traditional deployment of span

supports is not possible, this conservative design procedure can be

"uite costly 8h(9-

Thus, the selection of the allowable span length can become a risk

assessment type solution.


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Design %riteria

%urrent elocit( Selection

The calculated reduced velocity, stability parameter, 8eynolds

6umber, and critical span length should all be based on current

velocity that is perpendicular to the pipeline.

This design current should be based on the 199*year near bottom

current unless otherwise directed.


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Design %riteria

:nd %ondition Selection

The selection of the proper end conditions for the pipe free span has

a significant impact on the allowable span length selected.

The end condition selected can influence the calculated critical span

length by as much as :9 percent, thus making the selection of the

proper end conditions a critical step in selecting the proper

allowable span length.


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;#< Design %riteria

;#


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Design Steps

The following steps are based on the use of figure 1 and 2 to assist in

determining the allowable pipeline free span length.

Step "7 4etermine the design current 199*year near bottom

perpendicular to the pipeline-

Step $7 &alculate the effective unit mass of the pipeline

Step =7 &alculate 8eynolds 6umber

Step


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Design Steps

Step >7 ?ased on the terrain and conditions involved, determine the

type of free span end conditions and calculate the end condition

constant.

Step 27 &alculate the critical span length for both in*line and cross*flow

motionStep ?7 !or the ma0ority of pro0ects, the allowable span length is the

critical span length calculated for in*line motion. 5owever, when

economic factors warrant, the critical span length calculated for cross*

flow motion can be selected.

Step "+7 hen in*line motion is permitted, the fatigue life of the free

span should be calculated and evaluated for the pipeline.


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:*ample of Design %alculation

This example calculates the allowable span length to the cross*flow

oscillation based on the following information7

@utside diameter of pipe (D) $ 9.2):) m

Inside diameter of pipe (Di) $ 9.2:9' m4ensity of fluid in pipe (f) $ 19) Ag;m>

4ensity of pipe (p) $ 192B Ag;m>

Mass of pipe and coatings (Mp) $ )B Ag;m

Ainematic viscosity of external fluid (vk) $ 1.:3: x19*3 m2;sec

&urrent velocity (Uc) $ 9.>: m;s

&onstant for clamped*pinned ends (Ce) $ 1:.B


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Design %alculation

Step 1 ! :ffecti-e )ass


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Design %alculation

Step 2 ! Stabilit( Parameter

Step 3 ! Re(nolds 6umber

Step 4 ! Reduced elocities

/r$ 1.3 from !igure 2 for in*line motion

/r $ :.9 from !igure 1 for cross*flow motion


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Design %alculation

Step 5 ! %ritical Span Length for %ross'Flow )otion

Step 6 ! %ritical Span Length for In'Line )otion


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Fatigue Anal(sis 7uideline

The fatigue life e"uation presented in this section is based on the

Palmgren*Miner !atigue Model, which uses an *6 model based on

the %*C modified curve of the form7


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Fatigue Anal(sis 7uideline

here7

Lf $ fatigue life years-

Ls $ span length

Ds $ outside diameter of steel

fn $ pipe fre"uency 5D-

f /fn $ fre"uency ratio !igure >-

%;4 $ amplitude ratio !igure B-

Ti $ current duration hrs;day-.

This simplified fatigue life e"uation is expanded as follows7


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Figure =# %hart for determination of fre5uenc( ratio based on /V/ Dofn0#


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Figure


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Fatigue %alculation Procedure

The following steps should be followed when checking the fatigue life

of

free span length7

Step "! &alculate the pipe natural fre"uency

Step $7 4etermine the near bottom current velocity occurrence

distribution in histogram form using current duration blocks

Step =! !or each current segment determine the fre"uency ratio based

on /c;4fn- and !igure >.

Step


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Recommendation on Possible Pipeline Freespan

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Docfoc.com 249591587 Class 10 Free Span Analysis.ppt