buried piping-C2UG.pdf

Chapter 11: Buri edPi pe Model i ng Thi schapter descri besan approach to model i ngconti nuous soi lsupportsusi nga col l ecti onof poi nt restrai ntsandthe CAESARI I processor that automatesthe con-

Contents C AESAR IIUnder gr ound Pi p e Model e r ----- 2 Usi ng th e Undergr ound Pi p e Model e r ----- 3 Note son t h e Soi lModel 9 Recommende d Pr oce dur es12 Ori gi na lUnburi ed Model1 3

CAESAR II Underground Pipe Modeler

CAESAR II - User Guide

CAESAR II Underground Pipe Modeler The CAESAR II undergroundpipe model eri sdesi gnedtosimpl i fyuseri nputofburied pi pe dat a.Toachi eve thi sobject ive the “M odeler”performst he fol lowingfuncti onsfor users: • Al lowsthe di rectinputofsoi lpropert ies.The “M odeler”cont ai nsthe equat ionsfor buri edpipe st i ffnessest hatare out li nedlateri nt hi schapt er .These equat ionsare used t ocalculate fi rstt he st iffnessesonaperl ength ofpi pe basis,andthengenerat et he rest raintsthatsimul at et he discrete buriedpi pe restrai nt. • Breaksdownstrai ghtandcurvedl engthsofpipe tol ocate soi lrest raints.CAESAR II usesazone concepttobreakdownst raightandcurvedsect i ons.W here transverse bearingisaconcern(nearbends,t ees,andent ry/ exitpoi nt s),soilrest raint sare l ocated i ncl ose proxi mit yandwhere axiall oaddomi nat es,soi lrestrai ntsare spacedfarapart . • Al lowsthe di rectinputofuser-defi nedsoi lsti ffnessesonaperl engt h ofpi pe basis. Inputparametersi ncl ude axi al ,t ransverse,upward,anddownwardsti ffnesses,aswell asul ti mat el oads.Userscanspecifyuser-defi nedst iffnessesseparat el y,orinconjunct ionwit h CAESAR II’saut omat icall ygenerat edsoi lsti ffnesses.

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Buried Pipe Modeling

CAESAR II - User Guide

Using the Underground Pipe Modeler

Using the Underground Pipe Modeler Users can start the Buried Pipe Modeler by selecting an existing job, and then choosing Input-Underground from the CAESAR II Main Menu. The Modeler is designed to read a standard CAESAR II input data file that describes the basic layout of the piping system as if it was not buried. From this basic input CAESAR II creates a second input data file that contains the buried pipe model. This second input file typically contains a much larger number of elements and restraints than the first job. The first job that serves as the “pattern” is termed the original job. The second file that contains the element mesh refinement and the buried pipe restraints is termed the buried job. CAESAR II names the buried job by appending a “B” to the name of the original job. Note

The original job must already exist and serves as the pattern for the buried pipe model building. The modeler removes any restraints in the buried section during the process of creating the buried model. Any additional restraints can be entered in the resulting buried model. The buried job, if it exists, is overwritten by the successful generation of a buried pipe model. It is the buried job that is eventually run to compute displacements and stresses.

When the Buried Pipe Modeler is initially started up, the following screen appears:

This spreadsheet is used to enter the buried element descriptions for the job. The buried element description spreadsheet serves several functions:

Buried Pipe Modeling

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Using the Underground Pipe Modeler

CAESAR II - User Guide

•

allows users to define which part of the piping system is buried.

•

allows users to define mesh spacing at specific element ends.

•

allows the input of user-defined soil stiffnesses

Typical buried pipe displacements are considerably different than similar above ground displacements. Buried pipe deforms laterally in areas immediately adjacent to changes in directions (i.e. bends and tees). In areas far removed from bends and tees the deformation is primarily axial. The optimal size of an element (i.e. the distance between a single FROM and a TO node) is very dependent on which of these deformation patterns is to be modeled Not having a continuous support model, CAESAR II or the user, must locate additional point supports along a line to simulate this continuous support. So for a given stiffness per unit length, either many, closely spaced, low stiffness supports are added or a few, distant and high stiffness supports are added. Where the deformation is “lateral”, smaller elements are needed to properly distribute the forces from the pipe to the soil. The length over which the pipe deflects laterally is termed the “lateral bearing length” and can be calculated by the equation: 75(π) [4EI/Ktr] 0.25 Lb = 0. Where: E = Pipe modulus of elasticity I = Pipe moment of inertia Ktr = Transverse soil stiffness on a per length basis, (defined later) CAESAR II places three elements in the vicinity of this bearing span to properly model the local load distribution. The bearing span lengths in a piping system are called the Zone 1 lengths. The axial displacement lengths in a piping system are called the Zone 3lengths, and the intermediate lengths in a piping system are called the Zone 2lengths. Zone 3element lengths (to properly transmit axial loads) are computed by 100*Do, where Do is the outside diameter of the piping. The Zone 2mesh is comprised of up to 4elements of increasing length;starting at 1.5times the length of a Zone 1 element at its Zone 1 end, and progressing in equal increments to the last which is 50*Do long at the Zone 3end. A typical piping system, and how CAESAR II views this “element breakdown” or “mesh distribution” is illustrated on the following page.

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Buried Pipe Modeling

CAESAR II - User Guide

Using the Underground Pipe Modeler

Zone Definitions

A critical part of the modeling of an underground piping system is the proper definition of Zone 1 (or lateral) bearing regions. These regions primarily occur: •

On either side of a change in direction

•

For all pipes framing into an intersection

•

At points where the pipe enters or leaves the soil

CAESAR II automatically puts a Zone 1 mesh gradient at each side of the pipe framing into an elbow. Note

Buried Pipe Modeling

It is the user’s responsibility to tell CAESAR II where the other Zone 1 areas are located in the piping system.

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Using the Underground Pipe Modeler

CAESAR II - User Guide

The left side of the Buried Element Description Spreadsheet displays below:

Buried Element Description Spreadsheet

There are 13 columns in this spreadsheet The eight not shown above carry the userdefined soil stiffnesses and ultimate loads. The first two columns contain element node numbers for each piping element included in the original system. The second three columns are discussed in detail below: Soil Model No. — This column is used to define which of the elements in the model are buried. A nonzero entry in this column implies that the associated element is buried. A 1 in this column implies that the user wishes to enter user defined stiffnesses, on a per length of pipe basis, at this point in the model. These stiffnesses must follow in column numbers 6through 13. Any number greater than 1 in the SOIL MODEL NO. column points to a CAESAR II soil restraint model generated using the equations outlined later under Soil Models from user entered soil data. From/To End Mesh Type— A check in either of these columns implies that a lateral loading mesh should be placed at the corresponding element end. For example: FROM NODE 5

TO NODE 10

SOIL MODEL 2

FROM MESH

TO MESH

√

The element 5 to 10 is buried. CAESAR II will generate the soil stiffnesses from user-defined soil dataset #2, and the node 5 end will have a fine mesh so that lateral bearing will be properly modeled.

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Buried Pipe Modeling

CAESAR II - User Guide

Using the Underground Pipe Modeler

Since CAESAR II automatically places lateral bearing meshes adjacent to all buried elbows, the user must only be concerned with the identification of buried tees and points of soil entry or exit. The figure below is illustrative:

Lateral Bearing Mesh Definitions

Please note the following: •

The user has separated the node numbers in the original piping system by 10’s or 20’s instead of the usual 5. This is so that CAESAR II can maintain the normal sequence of node numbers for the added moves.

•

From/To Lateral Bearing mesh specifications are not needed for nodes 30, 110 and 130, since CAESAR II places lateral bearing meshes on each side of a bend by default.

•

A lateral bearing mesh is not needed at 90 because there is no tendency for the model to deflect in any direction NOT axial to the pipe.

•

The tendency for lateral deflection must be defined for each element framing into an intersection (node 50).

Buried Pipe Modeling

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Using the Underground Pipe Modeler

CAESAR II - User Guide

Commands available in this module are File-Open

•

File-Open—Opens a new piping file as the original job.

•

File-Change Buried Pipe Job Name—Renames the buried job (in the event that the user does not wish to use the CAESAR II default of “B” appended to the original job name).

•

File Print—Prints the element description data spreadsheet.

•

Soil Models—Allows the user to specify soil data for CAESAR II to use in generating one or more soil restraint systems. This is described in detail below.

•

Convert Input—Converts the original job into the buried job by meshing the existing elements and adding soil restraints. The conversion process creates all of the necessary elements to satisfy the Zone 1, Zone 2, and Zone 3 requirements, and places restraints on the elements in these zones accordingly. All elbows are broken down into at least two curved sections, and very long radius elbows are broken down into segments whose lengths are not longer than the elements in the immediately adjacent Zone 1 pipe section. Node numbers are generated by adding “1” to the element’s FROM node number. CAESAR II checks before using a node number to make sure that it will be unique in the model. All densities on buried pipe elements are zeroed, to simulate the continuous support of the pipe weight. A conversion log is also generated, which details the process in full.

File Print

Soil Models

Convert

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Buried Pipe Modeling

CAESAR II - User Guide

Notes on the Soil Model

Notes on the Soil Model The following procedures for estimating soil distributed stiffnesses and ultimate loads should be used only when the analyst does not have better data or methods suited to the particular site and problem. COADE’s soil restraint modeling algorithm is generally based on the ideas presented by L.C. Peng in his paper entitled “Stress Analysis Methods for Underground Pipelines,” published in 1978in Pipeline Industry. Soil supports are modeled as bi-linear springs having an initial stiffness, an ultimate load, and a yield stiffness. The yield stiffness is typically set close to zero, i.e. once the ultimate load on the soil is reached there is no further increase in load even though the displacement may continue. The two basic ultimate loads that must be calculated to analyze buried pipe are the axial and transverse ultimate loads. (Many researchers differentiate between horizontal, upward, and downward transverse loads, but when the variance in predicted soil properties and methods are considered, this differentiation is often not warranted. Note that CAESAR II allows the explicit entry of these data if so desired.) Once the axial and lateral ultimate loads are known, the stiffness in these directions can be determined by dividing the ultimate load by the yield displacement. Researchers have found that the yield displacement is related to both the buried depth and the pipe diameter. The ultimate loads and stiffnesses computed are on a force per unit length of pipe basis. Soil Models

Note

The user enters soil data by executing the Soil Models Command. This option allows the user to specify the soil properties for the CAESAR II buried pipe equations. Valid soil model numbers start with 2. Soil model number 1 is reserved for userdefined soil stiffnesses. Up to 15 different soil models may be entered for a single job.

Upon entry, the soil modeler dialog appears. Either the friction coefficient or the undrained shear strength may be left blank. Typically for clays the friction coefficient would be left blank and would be automatically estimated by CAESAR II as Su/600 psf. Both sandy soils and clay-like soils may be defined here.

Buried Pipe Modeling

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Notes on the Soil Model

CAESAR II - User Guide

The soil restraint equations use these soil properties to generate restraint ultimate loads and stiffnesses. (The TEMPERATURE CHANGE is optional. If entered the thermal strain is used to compute and print the theoretical “virtual anchor length.”) These equations are: •

Axial Ultimate Load (Fax) Fax = µD[(2ρsH) + (πρpt) + (πρf)(D/4) ] Where: µ =

Friction coefficient, typical values are:

.4 for silt .5 for sand .6 for gravel .6 for clay or Su/600 Su = Undrained shear strength D = Pipe diameter

•

ρs =

Soil density

H

Buried depth to the top of pipe

=

ρp =

Pipe density

t

=

Pipe nominal wall thickness

ρf

=

Fluid density

Transverse Ultimate Load (Ftr) Ftr = (0.5)(ρs )(H+D)2 [tan(45+φ/2)]2 i OCM

Where:

ϕ = Angle of internal friction, typical values are: 27-45 for sand

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Buried Pipe Modeling

CAESAR II - User Guide

Notes on the Soil Model

26-35 for silt 0 for clay OCM = Overburden Compaction Multiplier If Su is given (i.e. have a clay-like soil), then Ftr as calculated above is multiplied by Su/250psf. Note that since in many cases the stiffer the soil, the more conservative the results, Ftr is multiplied by the OCM as well. Many experienced pipeline engineers do not wish to add this "extra conservatism,"and prefer to use values that are more in line with those that have been used in the past. To do this, the OCM is the parameter that is usually adjusted. Common practice has been to reduce it (from its default of 8) to values from 5 to 7, depending on the degree of compaction of the backfill. Backfill efficiency can be approximated by the Proctor Number, defined in most soils textbooks. (The Proctor Number is a ratio of unit weights.) The standard practice when the Proctor Number is known is to multiply the default value 8 by the Proctor Number. This result should then be used as the compaction multiplier. •

Yield Displacement (yd): yd= Yield Displacement Factor × (H+D)

Note

•

The Yield Displacement Factor defaults to 0.015.

Axial Stiffness (Kax) on a per length of pipe basis: Kax=Fax / yd

•

Transverse Stiffness (Ktr) on a per length of pipe basis: Ktr=Ftr / yd

Once the user clicks OK, the soil data is saved in a file entitled .SOI.

Buried Pipe Modeling

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Recommended Procedures

CAESAR II - User Guide

Recommended Procedures The recommended procedure for using the buried pipe modeler is outlined below: 1. Select the original job and enter the buried pipe modeler. The original job must already exist, and will serve as the basis for the new buried pipe model. The original model should only contain the basic geometry of the piping system to be buried. The modeler will remove any existing restraints (in the buried portion). Add any underground restraints to the buried model. Rename the buried job if CAESAR II default name is not appropriate. 2. Enter the soil data using Soil Models. Soil Models

Convert Input

3. Describe the sections of the piping system that are buried, and define any required fine mesh areas using the buried element data spreadsheet. 4. Convert the original model into the buried model by the activation of option Convert Input. This step produces a detailed description of the conversion.

5. Exit the Buried Pipe Modeler and return to the CAESAR II Main Menu. From here the user may perform the analysis of the buried pipe job. A fairly simple buried-pipe example problem is shown in the following section. This example illustrates the features of the modeler and should in no-way be taken as a guide for recommended underground piping design.

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Buried Pipe Modeling

CAESAR II - User Guide

Original Unburied Model

Original Unburied Model

The following input listing represents the “unburied” model shown above.

Terminal nodes 100 and 1900 are above ground. Nodes 1250 and 1650 (on the sloped runs) mark the soil entry and exit points. Soil Model Number 2, a sandy soil, is entered.

Buried Pipe Modeling

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Original Unburied Model

CAESAR II - User Guide

Elements 1250-1300 through 1600-1650 are buried using soil model number 2. Zone 1 meshing is indicated at the entry and exit points.

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Buried Pipe Modeling

CAESAR II - User Guide

Original Unburied Model

Clicking Convert starts the conversion to a buried model.

The screen listing can also be printed.

Buried Pipe Modeling

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Original Unburied Model

CAESAR II - User Guide

The original unburied model is shown along with the "buried" model below. Note the added restraints around the elbows and along the straight runs.

Note the bi-linear restraints added to the buried model. The stiffness used is based upon the distance to the next node.

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Buried Pipe Modeling

CAESAR II - User Guide

Original Unburied Model

Note that the first buried element, 1250-1251, has no density.

The buried job can now be analyzed.

Buried Pipe Modeling

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Original Unburied Model

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CAESAR II - User Guide

Buried Pipe Modeling