machine foundation design

MACHINE FOUNDATIONS IN OIL

AND GAS INDUSTRY

Chennai Office By

Varanasi Rama Rao B.E, M.S.(I.I.Sc.)

CONTENTS

1. INTRODUCTION

2. INPUT

3. ANALYSIS&DESIGN

4. REFERENCES

January 2011Engineering Services – Chennai office


1. INTRODUCTION

The analysis and design of structure or foundation subjected to vibratory

loading is a complex problem as it involves interaction of three domains viz.

Structural Engineering, Geotechnical Engineering and Structural dynamics.

The machines used in Oil and Gas industry are usually supported on a

concrete block or a concrete frame and occasionally on steel frames. In

majority of the cases the machines are supported on a simple concrete block.

Types of Machines in Oil and Gas Industry


1.Centrifugal Machines ( Rotating)

2. Reciprocating Machines

Fig-1

Fig-2

Centrifugal Machine ( Rotating)


Fig-3

Reciprocating Machine


Fig-4

Classification of rotating machines based on

Frequency/speed


•Very low speed machines < 100 rpm

•Low speed machines 100 rpm to 500 rpm

•Medium Speed machines 500 rpm to 1500 rpm

•Moderately high speed machines 1500 rpm to 3000 rpm

•High speed machines >3000 rpm

Various Types of Machine Foundations


1.Block (Concrete)

Type2. Frame ( Concrete or Steel)

Type

MACHINE

CONCRETE

BLOCK

SOIL

SOIL

CONCRETE/ST

EEL FRAME

CONCRETE BASE

RAFT

Fig-5


1.Block (Concrete)

Supported by piles

2. Frame ( Concrete or Steel)

supported by Piles

MACHINE

CONCRETE

BLOCK

SOIL

SOIL

CONCRETE/ST

EEL FRAME

Fig-6

2. INPUTS


What inputs are needed for foundation design?

The inputs are broadly categorized as

Project Design basis for Machine foundation

Soil Parameters for foundation Design

Machine vendor inputs

The above are elaborated in the subsequent slides

What inputs are available in Project Design

basis?


The following are the major inputs to be extracted from the project

design basis:

•Criteria for Dynamic Analysis

•Permissible Amplitudes of Displacements and rotation of the foundation in

the absence of Vendor data.

•Grade of Material ( Concrete/Steel ) to be used for the construction of

Machine foundation

•Permissible % of bearing pressures for dynamic loading.

What are the Soil Parameters required?


Primary parameters

1. Dynamic Shear Modulus

2. Poisson’s Ratio

3. Damping factor

Secondary Parameters

1. Coefficient of Elastic uniform compression Cz

2. Coefficient of Elastic uniform shear Cτ

3. Coefficient of Elastic non uniform compression Cθ

4. Coefficient of Elastic non uniform shear Cψ

How to determine the Soil Parameters?


Field Tests :

•Cross Hole Test ( CHT).

•Down hole test (DHT).

•Spectral Analysis of Shear wave ( SASW)

•Block Vibration Tests

Laboratory tests:

•Resonant Column test

•Cyclical Tri axial Test

The above test are conducted usually by a Geotechnical Contractor and

appropriate values are recommended by him in the Geotechnical report

Cross hole test-schematic


Fig-7

Down hole test- Schematic


Fig-8

Spectral Analysis of Shear Wave - Schematic


Fig-9

Block vibration Test


Fig-10

Damping factor determination- Forced/Free

Vibration test


Fig-11

Machine Vendor Input


The following input is required from the machine vendor:

•Geometric configuration of the Machine

•Loads from the Machine: Mass of the stationary as well as rotating parts of

the machine and load transfer mechanism from machine to the foundation

•Critical Machine performance parameters: Critical speeds of rotors, balance

grades and acceptable levels of amplitudes of vibration

•Dynamic forces generated by the Machine: forces generated under various

operating conditions and their transfer mechanism to the foundation for

dynamic response analysis.

•Additional forces generated under emergency or faulted conditions, Test

condition, Erection condition & Maintenance condition of the machine, forces

due to bearing failure ( if applicable) for strength analysis of the foundation.

Typical Vendor Input ( Machine GAD):


Fig-12

3.ANALYSIS & DESIGN


The Analysis of the Machine foundation is done in two stages:

•Dynamic Analysis : Includes determination of the the natural frequencies

of the Machine foundation system and calculation of amplitudes of

displacements and rotations of the foundation under dynamic loading.

•Static Analysis: Includes check for strength of the foundation, stability of

the foundation and check for soil bearing capacity.


The process of evaluating the critical soil properties that influence soil

structure interaction is probably the most difficult part of the machine

foundation design- Dr. K.G.Bhatia

The significant aspects of soil properties which influence soil-structure

interactions are:

•Energy transfer mechanism- Not quantifiable

•Soil mass participation in vibration of the foundation- Not quantifiable

•Effect of embedment of foundation- Approximately quantifiable

•Applicability of Hooke’s law to soil- To some extent

•Dynamic soil parameters-Approximately quantifiable

Deformation modes of soil


Based on above deformable modes of foundation, the following deformable

modes can be anticipated for Soil beneath the block foundation:

• Uniform Compression

•Uniform Shear

•Non Uniform compression

•Non Uniform shear


In the context of Machine foundation design, a Machine would necessarily

include:

•A drive machine

•A driven machine

•A coupling device

Schematically:

Drive

machine

coupling Driven

machine

Fig-13


A typical data set required for each of the components shown in the previous

schematic is:

For dynamic response analysis of foundation:

•Total mass of machine ( including rotating parts), Radius of gyration and its over

all centroid location.

•Mass of rotating parts of the machine, operating speed, height of the centre of the

rotor from machine base frame, etc

•Foot print of machine base frame, details of holding down bolts

•Dynamic forces generated by the machine under operating conditions

For strength design of foundation:

• Static loads from machine

•Equivalent static forces i.e. dynamic forces converted to equivalent static forces

•Forces generated under emergency and faulted conditions eg: bearing failure,

loss of blade, short circuit etc.

•Forces during erection, maintenance and test conditions of the machine.

Parameters for rotary ( centrifugal) machines:


Balancing of rotating machine

Centre of Mass

Centre of Rotation

Balanced condition

Fig-14


Centre of Mass ‘m’

Centre of Rotation

Un- Balanced condition

e

Force Generated due to unbalanced condition F= meω2

The above force is called unbalanced force

Fig-15


Unbalanced forces along the shaft with multiple supports

Fig-16


Picture of Rotor

Fig-17


Picture of Rotor

Fig-18


In every rotating machine there will be certain amount of unbalance(

eccentricity) which is inevitable.

ISO/ Machine manufacturer has set standards for the allowable eccentricity

based on:

•Function of the machine

•Speed of the machine and

•Rotating mass

Many rotating machines are balanced to an initial balance quality as per ISO

standards. This is called the balance quality grade.

Fig-19


Critical speeds: Correspond to flexural frequencies of the rotor. These are

supplied by the vendor. High vibration can occur on account of resonance of

foundation with critical speeds

Forces due to Emergency conditions:

Bearing Failure: Grinding halt of machine due to failure of bearings. Difficult to

quantify and can be taken as an static force equivalent to 3 to 5 times the

rotor weight.

Short circuit force: Furnished by Machine vendor

Loss of parts like blade: Furnished by Machine vendor

Fig-20


Parameters for reciprocating machines:

Typical arrangement for single cylinder reciprocating system

Fig-21


Dynamic forces transferred at C.G of machine at base frame level

Fig-22


Foundation Parameters

Under tuned foundation:

The vertical vibration frequency is < operating frequency of the machine

Preferred for Medium to High speed Machines

Over tuned foundation:

The vertical vibration frequency is > Operating frequency of the machine

Preferred for very low to low speed machines


Vibration Limits in Machine Foundation design

Machine Operating speed

in rpm

Permissible amplitude in

Microns

100 to 500 200 to 80

500 to 1500 80 to 40

1500 to 3000 40 to 20

3000 to 10000 20 to 5

The above are only approximate values. Actual permissible should be

given by the Machine Vendor/Manufacturer

Rotary type machines

Machine Operating speed

in rpm

Permissible amplitude in

Microns

300 to 1500 1000 to 20

100 to 300 1000

Reciprocating type machines


Foundation Sizing ( Block foundation):

Foundation should be dimensioned in such a way that the derived

eccentricity, in both lateral and longitudinal directions is bare minimum.

In no case it should not exceed 5% of the base dimension in the respective

direction

Foundation should extend by at least 150 mm on all sides of machine base

frame

The pressure developed in the soil loads due to static loads should not

exceed 75% of the allowable safe bearing capacity.

Though from strength point of view it may appear adequate to keep

foundation mass slightly above the machine mass, a higher mass ratio

helps to keep the eccentricity of loading within limits.

For rotary machines: foundation mass = 2.5 to 3 times of machine mass

For reciprocating machines: foundation mass= 5 to 8 times of machine

mass


Foundation Stiffness:

Foundation parameters that govern the dynamic response are its mass

and its area of contact with the soil. In specific cases projected parts of

foundation having finite stiffness also influence the dynamic response.

The rigidity of foundation is much higher compared to that of soil

supporting it.

In the case of block foundation the rigidity is so high that under the

influence of static and dynamic forces the deformations of the block are

negligible compared to soil. The block foundation is therefore considered

as rigid body consisting of mass only.

In case the foundation of machine is not a solid block but a frame or some

other kind of structure which has stiffness comparable to that of soil, then

elements of those structures shall be considered to have both mass and

stiffness.


Strength Design:

Since the block foundation behaves like a rigid body supported on a soft media like soil,

invariably the block foundations would turn out to be having adequate strength vis-à-vis

forces imparted by machine.

Strength design is done considering the forces and moments on the foundation due to

static loads, dynamic loads, emergency loads and applicable earthquake/wind loads.

Anchor bolts: All anchor bolts should be checked for pullout force caused due to

Dynamic and Emergency loads

Stability Checks:

The foundation shall be checked for sliding and overturning . But these checks are not

compulsory.

Minimum Reinforcement :

•25 to 50 kg/m3

•Minimum dia. of the bar 12 mm

•Two way reinforcement on all the faces and shrinkage reinforcement ( when thickness

of block exceeds 1m)

Dynamic Analysis ( Block Foundation)


THERE ARE SIX MODES OF VIBRATION OF BLOCK FOUNDATION

WITH REFERENCE TO THE X-Y-Z CO-ORDINATE SYSTEM

VERTICAL VIBRATION

HORIZONTAL VIBRATION ( 2 DIRECTIONS) IN SAME PLANE

ROCKING

PITCHING

YAWING OR TWISTING


Possible Movement for Block foundation:

X

Z

Y

θ

ψ

φ

Fig-23

Single Degree Freedom Mathematical Model


Mass of machine and

foundation

Stiffness of the soil

Soil Damping

Fig-24


Lumped Parameter System

Kx

Z

ψ

KzCz

Cx

Kψ

Cψ/2 Cψ/2

X

)sin(0 tPzkzczm zzz

mkcccD crcr 2

m

kn

mIψ

Fig-25

Lumped Parameter Values

Mode Vertical Horizontal Rocking Torsion

Stiffness k

Mass Ratio

m

Damping

Ratio, D

Fictitious

Mass

1

4Gr

2

8Gr

)1(3

8 3

Gr

3

16 3Gr

5r

I

38

)2(

r

m

34

)1(

r

m

2/1ˆ

425.0

m2/1ˆ

288.0

m2/1ˆ)ˆ1(

15.0

mm m̂21

50.0

m

m

ˆ

27.0

m

m

ˆ

095.0

m

I x

ˆ

24.0

m

I z

ˆ

24.0

m̂

D=c/ccr G=Shear Modulus ν=Poisson's Ratio r=Radius ρ=Mass Density

Iψ,Iθ=Mass Moment of Inertia

58

)1(3

r

I


Closed form solution for free and forced vibration of foundation block subjected

to dynamic loads:

Fig-26


FE Models of Block foundation

Fig-27


Model with Soil and Block foundation

Fig-28


Foundation Sizing ( Framed foundation):

Foundation GA and loading diagram is provided by the Machine

vendor/Manufacturer.

Typical Steel Frame foundation for Turbo Generator

Fig-29


Eccentricity:

For Framed foundation there are two connotations to the term eccentricity:

1. Overall eccentricity: it is defined as the distance between the centre of mass

of the over all system ( machine+ foundation) and C.G of the base contact

area of the foundation with soil. This should be restricted to 5%

2. Top deck eccentricity: It is defined as the distance between the centre of the

Mass Cm ( combined C.G of machine mass, top deck mass and 23% percent

of column mass) and centre of stiffness of frames Ck in the transverse and

longitudinal directions. It is desirable to restrict this eccentricity to 1% of the

respective dimension of the top deck.

Top deck sizing:

Top deck comprises of transverse and longitudinal beams, slab connecting these

beams, projections on all sides of the beams, depressions, cutouts, notches etc.

a. The top deck weight > weight of the machine

b. For beams: Span/Depth = 3 to 5; Depth/width = 1 to 1.5

c. Extent of cantilever projections in plan should not be more than half the width

of the corresponding beam

d. Depth of slab should be invariably same as that of the encompassing beams

except at areas where the recess or depressions are provided to accommodate

machine


Columns:

Total weight of the columns should be close to the weight of the machinery. This is

desirable but not essential condition

But the following should be kept in mind while assessing the sizes of columns

•Centre of stiffness of all the frames should coincide with centre of mass of

machine and top deck.

•Lateral natural frequencies of each of the column ( along transverse as well as

longitudinal directions) considering fixed at both ends should not coincide with

Machinery frequency or its harmonics.

Base raft:

• Raft plan dimensions are selected such that the bearing pressure generated is

less than 70% of allowable bearing pressure.

• Base raft thickness should be such that it acts like a rigid block and undergoes

uniform deformation.

•General guide line is weight of the base raft should be about twice the weight of

the machine


Stiffness of frame foundation:

Unlike block members of frame foundation have finite stiffness and are

subjected to considerable elastic deformations.

Framed foundation is considered as elastic body with both mass and stiffness.

Strength design:

For frame foundation the reinforcement is provided as dictated by the strength

design of the structural members i.e. columns, beams and slabs.

Minimum Reinforcement:

• Reinforcement for top deck and columns to be in the range of 100 to 120

kg/m3

•Reinforcement for base raft shall be in the range of 70 to 80 kg/m3


Framed foundation under construction

Fig-30


Dynamic Analysis ( Framed Foundations)

Unlike block foundations, framed foundation has many modes of vibration.

Closed form solutions for framed foundation:

Before the advent of computers the framed foundations are analyzed manually.

The manual procedure aims at analyzing the frames of the foundation

independently for free and forced vibrations and algebraically summing up the

response for all frames. ( Refer Handbook of Machine foundations by Srinivasulu

and Vaidyanathan for Manual Analysis of framed foundation).


Modeling Methods for Framed foundation:

There are two acceptable methods

Method 1:the loads are applied to the model of the super structure to

determine the foundation response. The computed reactions at the

base of the columns are then used as input forces on the model of the

mat

Method2: in this method both the superstructure and foundation are

incorporated into a single model. This procedure will yield results for

the entire foundation in one analysis.

For both the above methods the beams and columns are modeled

using 3 dimensional beam elements, shear walls ( if any) and mat are

modeled using plate-bending elements.


Models for Dynamic Analysis ( some major points to remember):

The dynamic model should not be created independently of the static model but

should be created from static model by incorporating the following:

•Enough nodes should be specified along the length of beams and columns so

as to capture frequencies of modes which match with machine frequency.

•The nodal masses can be computed either by lumped mass approach or

consistent mass approach

•It is recommended that 2% of critical damping used for concrete elements and

1% for steel elements.

•The dynamic model of foundation should also include the machine. The simplest

method to model this is modeling the machine as series of mass points lumped

with the foundation.


Loads for which Static Analysis is recommended:

•Dead Load

•Live load

•Normal torque load

•Condenser load

•Thermal loading due to machine expansion/contraction

•Piping loads

Loads for which Pseudo-dynamic Analysis is recommended:

•Normal machine unbalanced load for machines mounted on conventional

foundations

•Seismic loading

Loads for which Pseudo-dynamic analysis is acceptable:

•Short circuit load

•Out of phase synchronization

•Loading due to bowed rotor

•Load due to missing rotor blade

Loads for which dynamic analysis is recommended:

•Normal machine unbalance load for machines mounted on low tuned

foundation

•Seismic loading


Typical Free Vibration Response of FE model of Framed foundation

Fig-31


Typical Response of foundation subjected to Dynamic loading

Fig-32


Miscellaneous Topics

1. Soil- Structure Interaction effects:

•The presence of supporting soil affects the static and dynamic

response of the turbine foundation. These effects are termed as

soil structure interaction effects.

•Usually soil-structure interaction effects are considered

negligible for framed foundation under operating conditions.

However, they can be critical under non-periodic forces like

earthquake, short circuit etc.

•Soil structure interaction effects are more predominant in the

case of block foundations.


2. Machine foundations on Piles:

Piles are specifically required for machine foundations in the following

circumstances:

• When soil is weak to withstand the loads

•When it is required to increase the natural frequency of the machine

foundation system.

•When dynamic amplitudes are required to be reduced

•When it is required to stiffen the support system on account of seismic

considerations.

Problems associated with Analysis of machine foundations supported on

piles

•Understanding of dynamic behavior of group of piles is still in its infancy.

•As the reliability of dynamic characteristics of group of piles is faced with

many questions, so shall be the status of computed dynamic response


The following are observations by various researchers with regard to

dynamic behavior of piles:

•Dynamic stiffness of the pile is generally found to be greater than static

stiffness

•Both stiffness and damping of pile are found to be frequency dependent

•Damping increases with pile length

•Embedment of pile cap results in increased stiffness. However, its

quantification is not yet established

•Damping of group of piles is more frequency dependent

•Dynamic group effect considerably differs from static group effect.

•Rocking and torsional stiffness of the pile can be safely ignored.


4.REFERENCES

BOOKS:

1. Barkan D.D.” Dynamics of bases and Foundations”- Mc Grawhill

2. P.Srinivasulu and Vaidyanathan “Hand Book of Machine foundations”- Tata Mc Grawhill

3. “Foundations for Industrial Machines: Hand book for Practicing Engineers”- K.G.Bhatia-

DCAD publishers

4. S.Prakash and V.Kpuri “ Foundation for machines- Analysis and Design”- John Wiley

5. Arya, O Neil and Pincus “ Design of Structure and Foundation for Vibrating Machines”-

Gulf Publishing

6. Indrajit Chowdhury and P.Dasgupta “ Dynamic of Structure and Foundation” –CRC

press

7. “Soil Dynamics and Machine foundations” – Swami Saran- Galgotia

8. “Soil Dynamics “ - Braja M. Das


CODES:

Indian:

IS2974: 5 parts of which parts 1,3 & 4 are for Reciprocating and Rotating machines

IS 5249: Methods of test for determination of Dynamic soil properties

British:

CP 2012

German:

DIN4024 ( Part 1): For framed type ( Flexible) foundations

DIN 4024 ( Part 2): For block type ( Rigid) foundations

American:

ACI 351

Saudi:

SAES-Q-007

ISO:

1S0 10816 ( 7 parts)

machine foundation design

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