crude oil pipeline calculation

7/28/2019 Crude Oil Pipeline Calculation

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This is to certify that:

(i) the thesis compromises only my original work towards the Bachelor Degree

(ii) due acknowledgement has been made in the text to all other material used

Ahmed Essam Khedr

12 July, 07


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Table of Contents

Acknowledgement .................................................................... 6

Chapter 1: Introduction and Literature Review.................... 8

1.1. Introduction .............................................................................. 8

1.1.1. Motivation ................................................................................ 8

1.1.2. Aim of the project ................................................................... 10

1.1.2.1 Stress Analysis .................................................................................... 10

1.1.2.2. Material Selection .............................................................................. 11

1.2. Literature Review ................................................................... 12

Chapter 2: Stress Analysis..................................................... 15

2.1. Allowable Pipe Stress ............................................................. 15

2.2. Wall Thickness Calculation .................................................... 17

2.3. Internal Pressure ..................................................................... 18

2.4. Vertical Earth Load ................................................................ 20

2.5. Surface Live Loads ................................................................. 22

2.6. Ovality and Stress ................................................................... 25

2.8. Ring Buckling ........................................................................ 30

2.9. Fatigue .................................................................................... 31

2.10. Surface Impact Loads ............................................................. 32

2.10.1. Maximum Impact Load ....................................................................... 32

2.10.2. Penetration and PPV ........................................................................... 33

2.11. Buoyancy................................................................................ 35

2.11.1. Applied Load ...................................................................................... 35

2.11.2. Pipe Stress .......................................................................................... 36

2.12. Thermal Expansion ................................................................. 37

2.13. Earthquakes ............................................................................ 37

2.13.1. Seismic Wave Propagation .................................................................. 38

2.13.2. Permanent ground deformation ........................................................... 40


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Chapter 3: Material Selection ............................................... 42

3.1. Metallic Materials ........................................................... 44

3.2. Material Properties of Piping Material .................................... 46

3.3. Chemical properties ................................................................ 48

3.4. Mechanical Properties ............................................................ 49

3.4.1. Strength .............................................................................................. 50

3.4.2. Hardness ............................................................................................. 53

3.4.3. Toughness ........................................................................................... 54

3.4.4. Fatigue Resistance. ............................................................................. 55

3.4.5 Elevated Temperature Tensile and Creep Strength. ............................. 56

3.5. Physical Properties of Metals .................................................. 58

3.5.1. Density ............................................................................................... 58

3.5.2. Thermal Conductivity ......................................................................... 58

3.5.3. Thermal Expansion. ............................................................................ 58

3.5.4. Specific Heat....................................................................................... 59

3.6. Microstructure ........................................................................ 60

3.7. Fabrication Of Steel Pipe ........................................................ 63

3.7.1. Pipe Size ............................................................................................. 63

3.7.2. Seamless Pipe ..................................................................................... 64

3.7.3. Seam Welded Pipe .............................................................................. 64

Chapter 4: Mechanical Design of SUMED Pipeline ............ 67

4.1. Background ............................................................................ 67

4.2. Stress Analysis ....................................................................... 71

4.3. Material Selection for SUMED Pipeline ................................. 90

Chapter 5: Mechanical Design of Arab Gas Pipeline .......... 91

5.1. Background ............................................................................ 91

5.2. Stress Analysis ....................................................................... 93

5.3. Material Selection for Arab Gas Pipeline .............................. 100


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Chapter 6: Conclusion ......................................................... 101

References ............................................................................. 102


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List of Figures

Figure 2-1: Hoop stress and axial stress in a pipe..................................................... 19

Figure 2-2: Soil Prism above the pipe ...................................................................... 21Figure 2-3: Surface Load and Transmitted Pressure ................................................. 24

Figure 2-4: Ovality of Pipe Cross Section................................................................ 27

Figure 2-5: Through-Wall Bending Stress ............................................................... 28

Figure 2-6: Crushing of Side Wall .......................................................................... 29

Figure 2-7: Ring Buckling of Pipe Cross Section..................................................... 31

Figure 2-8: Fall of a Heavy Object on Ground Surface ............................................ 34

Figure 2-9: Resultant Buoyancy Load on Pipe ......................................................... 35

Figure 3-1: Pipe Materials Chart ............................................................................. 45

Figure 3-2: The three most common crystal structures in metals .............................. 46Figure 3-3: Stress-Strain Curve. (1) Ultimate Strength. (2) Yield strength. (3)

Proportional Limit Stress. (4) Rupture. (5) Offset Strain (typically 0.002). ............... 51

Figure 3-4: An Engineering Stress-Strain for Carbon Steel ...................................... 53

Figure 3-5: Transition temperature range and transition temperature in Charpy impact

test ........................................................................................................................... 55

Figure 3-6: Creep time versus elongation curves at a given temperature. ................. 57

Figure 3-7: Growth of Atomic Lattice into Grains ................................................... 61

Figure 3-8: Simplified Phase Diagram of Carbon Steel............................................ 62

Figure 3-9: Atomic Structure of Carbon Steel.......................................................... 62

Figure 3-10: Overview of Seamless Pipe Fabrication .............................................. 65

Figure 3-11: Overview of Seam Welded Pipe Fabrication ....................................... 66

Figure 4-2: A Ship pumping its oil to the pipeline ................................................... 68

Figure 4-3: Pipeline System .................................................................................... 69

Figure 4-4: El Ain El Sukhna Pumping Station........................................................ 69

Figure 4-5: Dahshour Boosting Station .................................................................... 70

Figure 4-6: Tanks in Sidi Kreir ................................................................................ 70

Figure 4-7: Burial of SUMED Pipeline ................................................................... 71

Figure 5-1: Map showing the route of Arab Gas Pipeline ........................................ 92


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Acknowledgement

It is a pleasure to thank the many people who made this thesis possible.

It is difficult to overstate my gratitude to my B.Sc. supervisor, Dr. Hamdy Kandil.

With his enthusiasm, his inspiration, and his great efforts to explain things clearly and

simply,. Throughout my thesis-writing period, he provided encouragement, sound

advice, good teaching, good company, and lots of good ideas. I would have been lost

without him.

I would like to thank the many people who have taught me a lot about pipelines:

SUMED Company Staff (especially Eng.Sherif Haddara). For their kind assistance

with writing letters, giving wise advice, helping with various applications, and so on, I

am indebted to my many student colleagues for providing a stimulating environmentin which to learn and grow.

Lastly, and most importantly, I wish to thank my parents. They raised me, supported

me, taught me, and loved me. To them I dedicate this thesis.


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Abstract

This thesis is intended to study the stresses acting on the pipelines and choose the

most appropriate material for the pipelines so that they can withstand these stresses.

Since many pipelines fail against certain stresses due to choosing inappropriate

material, a stress analysis is made for 2 pipelines SUMED Pipeline and Arab Gas

Pipeline, then choosing materials for both pipelines. The results found that the

materials are safe to withstand the stress that the pipelines are subjected to.


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Chapter 1Introduction and Literature Review

1.1. Introduction

1.1.1.Motivation

Oil and gas collectively provide the world with 55% of its primary energy needs,

either by direct consumption (e.g. natural gas), or by using the fuels to generate

electricity. Consumption of these fossil fuels is staggering: there are over 1 million

tonnes of oil consumed every hour around the world, and 250 million cubic meters of

natural gas consumed every hour around the world [9]. At the end of 2005, world

proven crude oil reserves stood at 1,153,962 million barrels, of which 904,255 million

barrels, or 78.4 per cent, was in the organization of the Petroleum Exporting Countries

(OPEC) (Member Countries. According to the reference case of OPEC's World

Energy Model (OWEM), total world oil demand in 2000 is put at 76 million barrels

per day, as world economic growth continues, crude oil demand will also rise to

90.6m b/d in 2010 and 103.2m b/d by 2020, according to the OWEM. OPEC believes

that oil demand will continue to grow strongly and oil will remain the world's single

most important source of energy for the foreseeable future. The OWEM reference

case sees oil's share of the world fuel mix falling slightly from over 41 per cent today

to just over 39 per cent in 2020. However, oil will still be the world's single largest

source of energy. The reduction in oil's market share is largely due to the stronger

growth enjoyed by other forms of energy, particularly natural gas. Burning crude oil

itself is of limited use. To extract the maximum value from crude, it first needs to be

refined into petroleum products. The best-known of these is gasoline, or petrol.

However, there are many other products that can be obtained when a barrel of crude

oil is refined. These include liquefied petroleum gas (LPG), naphtha, kerosene, gasoil


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and fuel oil. Other useful products which are not fuels can also be manufactured by

refining crude oil, such as lubricants and asphalt (used in paving roads) [10]. A range

of sub-items like perfumes and insecticides are also ultimately derived from crude oil.

Oil must be transported to meet the high demands of the needing countries, but how?

Crude oil is often transported between continents in large tankers, but oil and natural

gas is transported across continents by pipelines. These pipelines are very large

diameter (the Russian system has diameters up to 1422mm), and can be over 1000km

in length. Transmission pipelines are the main arteries of the oil and gas business;

working 24 hours per day, seven days a week, continuously supply our energy needs.

Crude oil can be transported by sea in huge tankers, but most countries in the

developed world have very large, long distance pipelines carrying (transmitting)

crude oil, petroleum products and natural gas around their lands. These pipelines are

usually located under the sea, or in remote rural locations; therefore, the general

public never see them. These transmission pipelines are sophisticated, expensive

energy transportation systems: pipelines are the core of the worlds oil and gas

transportation system. The UK has 40,000km of these transmission pipelines. Natural

gas, crude oil, and petroleum products such as gasoline, would not reach their millions

of consumers without these pipelines [9].

Oil and gas can be transported by various means: road tanker, rail, ship, or pipelines,

but pipeline are usually preferred because:

1. They are by far the more environmentally-friendly: most are buried under

ground, or undersea;

2. They are much safer than transportation by road, rail or sea; consequently,

there is more than 3,000,000 km of transmission pipelines around the world.

3. They are more economical than other means of transportation

Pipelines are important. If we did not have pipelines we would not be able to heat our

homes with natural gas, drive our car, or turn our television on. This is because most

natural gas and gasoline are delivered to consumers using pipelines; also, much of our

electricity is generated by burning oil and gas that is supplied to the power stations.

No pipelines? Then no cars, no heating, no power!


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1.1.2.Aim of the project

The aim of this project is to study and calculate the stresses acting on SUMED

pipeline and to select the material that can withstand this stresses.

1.1.2.1 Stress AnalysisThe piping system must be strong enough to withstand induced stresses, have

relatively smooth walls, have a tight joining system, and be somewhat chemically

inert with respect to soil and water. The piping systems must be designed to perform

for an extended period. The normal design life for such systems should be 50 years

minimum. However, 50 years is not long enough. Governments and private agencies

cannot afford to replace all the buried pipe infrastructures on a 50-year basis. A 100

year design life should be considered minimum. A pipeline system is subjected to

static and dynamic loads due to local environmental and operating conditions, and

provision must be made for the system to have flexibility and expansion capability to

prevent excessive stresses in the pipe or components, excessive bending or unusual

loads at joints, or undesirable forces or moments at points of connection to equipment.

The types of loadings which will affect the flexibility and expansion of the pipeline as

a system include:

Thermal expansion and contraction

Internal pressure

Bending (sag or uplift) due to Dead Loads, including weight of the pipe,

coatings, backfill, and unsupported pipe appurtenances.

Live Loads such as liquid transported, wind, snow, earthquake, waves, or

currents

Earthquakes

Buoyancy


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In this section, these stresses will be studied and calculated to know if the pipe can

withstand these stresses or will fail. If the pipe cannot withstand these stresses,

another stronger material must be chosen to withstand these stresses and this will be

done in the next section (Section 1.2.2).

1.1.2.2. Material SelectionThe selection of materials for piping applications is a process that requires

consideration of material characteristics appropriate for the required service. Material

selected must be suitable for the flow medium and the given operating conditions of

temperature and pressure safely during the intended design life of the product.

Mechanical strength must be appropriate for long-term service, and resist operational

variables such as thermal or mechanical cycling. Extremes in application temperature

can raise issues with material capabilities ranging from brittle fracture toughness at

low temperatures to adequacy of creep strength and oxidation resistance at the other

end of the temperature spectrum. In addition, the operating environment surrounding

the pipe or piping component must be considered. Degradation of material properties

or loss of effective load-carrying cross section can occur through corrosion, erosion,

or a combination of the two. The nature of the substances that are contained by the

piping is also an important factor. In the final count, what will matter is the

performance of the product: its compatibility with the fluid, the environment and the

service in one case; its compatibility with what customer want and like.


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1.2. Literature Review

Many researches have been done in the past years concerning the effect of differentstresses on pipelines and how the pipelines are affected by these stresses. A Failure

analysis of a crude oil pipeline was done by Cesar R.F. Azevedo where the

transversal cracking of a seamed API 5L X46 steel tube belonging to a crude oil

pipeline was investigated. The main cracking nucleated in the internal surface of the

tube, at the boundary between the heat-affected zone (HAZ) and the weld metal,

propagating in a stable mode along the radial and longitudinal directions. Stress

raisers, such as welding defects and corrosion pits, were associated to the cracking

nucleation. The internal surface of the tube and the cracking surfaces presented a

deposit layer, which was rich in Fe, O and S. Diffractometry on the internal identified

the presence of a multi-layered corrosion deposit, formed by iron oxide (Fe2O3 and

Fe3O4) and iron sulphides, such as pyrrhotite, mackinawite and pyrite, indicating the

action of a H2S corrosion assisted mechanism. The crack propagation path did not

depend on the welding macrostructure, growing perpendicular to both the internal

surface and main tensile stresses. Crack propagation was, however, microstructure

sensitive, with a more intense branching occurring inside the base metal rather than

the HAZ region. Both regions presented cracking (blistering) of the sulphide/matrix

interface and microfractographic examination indicated the action of a ductile fracture

mechanism linking the H2 blisters, reinforcing the idea that atomic hydrogen

association rather than hydrogen embrittlement was the active mechanism during the

cracking of the pipeline. These observations indicated that failure of the pipeline

occurred by a stress-oriented hydrogen-induced cracking (SOHIC) mechanism [12].

Another analysis was done by S. de Luna, J Fernndez-Sez, J. L. Prez-

Castellanos and C. Navarroon thestatic and dynamic fracture behavior of a pipeline

steel. This Study deals with the dependence of fracture behavior on the strain rate of a

commercial pipeline steel. Low-blow impact tests, using a Charpy pendulum setup,

and conventional static fracture tests were carried out with this material. Experimental

results showed that the material fracture toughness increases slightly with strain rate.

Numerical analyses of all the experiments were also performed, using a

micromechanical damage model that explains the influence of the strain rate on the


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fracture toughness. Particular attention was paid to the blunting process at the crack

tip under dynamic conditions [13].

Over the past years, greater research emphasis has been placed on the reliability of

offshore pipelines due to potential defects such as flaws in girth welds, damage due to

corrosion, etc. In several situations, pipes can be subjected to very large plastic strains

up to the order of 3%. The extreme loading conditions (high internal pressure

combined with bending/tension) further make the fracture assessment of pipelines a

formidable challenge. Todays design practice for offshore pipelines is commonly

dictated by the local buckling/collapse limit state. Recent research has pushed the

allowable strain limits on the compression side to quite large values, up to the order of

3%. On the other hand, the permissible strain based on fracture on the tension side is

still very restricted.

Current codes and standards (for example, BS 7910: 2000) for fracture assessment are

generally formulated for load-controlled situations. However, there are several

situations, where the pipeline is subjected to displacement controlled loading well into

the plastic regime. In load based approaches it is usually difficult to justify the

utilization of material well above yield. Hence, for fracture assessment of pipelines, a

strain-based approachis advocated. However, these procedures are still based on theexisting crack-driving force equations which are limited to small plastic strains, and

hence, application in structures subjected to large plastic deformation is doubtful.

Hence, an accurate and simple strain-based fracture assessment procedure for offshore

pipelines with the objective of possible further enhancement in deformation capacity

on the tension side is highly desirable [14].

A study was done by Sheng-Hui Wang and Weixing Chen on the pre-cyclic-load-

induced burst of creep deformation of a steel pipeline under subsequent static load

where the room temperature creep of X-52 pipeline as studied under various loading

conditions. Due to cyclic hardening, the steel exhibits cyclic creep retardation, which

is less pronounced at lower stress -ratio and under cyclic load with periodical hold at

peak stress. Pre-cyclic loading has significant effect on subsequent static creep. Up to

40 cycles, pre-cyclic load results in a smaller cumulative creep than that of pure static

creep deformation. This is attributed to the high rate of cyclic hardening during the

initial few cycles, which limits further creep deformation in the subsequent static


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loading. With increasing number of cycles, pre-cyclic loading causes a burst of creep

deformation under subsequent static loading, which results in significantly larger

cumulative creep strain than that of pure static creep. The burst in creep deformation

requires an incubation period that increases with the number of prior load cycles. The

burst strain is dependent on the number of cycles of prior cyclic loading in a more

complicated manner [15].

Earthquakes have been a major concern when designing pipelines and many

researches were done to know the effect of earthquakes on pipelines. In order to

realistically assess the seismic risk of a pipeline system, the accurate estimate of the

pipe strains which depend upon structural details, pipe material, properties of the

surrounding soil, the nature of the propagating wave, etc. is critical. Emphasis in a

study by Yasuo Ogawa and Takeshi Koike has been placed on the analysis of a

structural strain for several types of piping elements unique to the buried pipeline and

also the provision of a simplified design formula which can be used practically. The

purpose of this study is (a) to define the slippage factor in order to estimate the

decrease in pipe strain resulting from the slippage effect, (b) to propose a simplified

method to evaluate the plastic deformation of the pipeline for severe earthquakes, and

(c) to derive a practical design formula for the structural strains of bent pipes [16].

Another Study was on the Seismic response of natural gas and water pipelines in the

Ji-Ji earthquake done by Walter W. Chena, Ban-jwu Shih, Yi-Chih Chen, Jui-

Huang Hung, and Howard H. Hwang where a GIS database and analysis

procedures were established to study the damage patterns of natural gas and water

pipelines in the Ji-Ji earthquake. Repair statistics was obtained from major natural gas

companies and the Taiwan Water Supply Corporation (TWSC), and entered into the

system. Then, repair rates (RR) were calculated. Previously, damage was analyzed

without considering the corresponding pipeline material and diameters. In this study,

new attempts were made to collect more data including those related to the

composition of pipelines to provide a more detailed analysis of the relationship

between earthquake forces and the resulting damage. Statistical analysis was also

conducted to understand the correlation between RR and seismic parameters such as

the peak ground acceleration, peak ground velocity, and spectrum intensity [17].


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Chapter 2Stress Analysis

A pipeline system is subjected to static and dynamic loads due to local environmental

and operating conditions, and provision must be made for the system to have

flexibility and expansion capability to prevent excessive stresses in the pipe or

components, excessive bending or unusual loads at joints, or undesirable forces or

moments at points of connection to equipment. The types of loadings which will

affect the flexibility and expansion of the pipeline as a system include:

1. Internal Pressure

2. Vertical Earth Load

3. Surface Live Loads

4. Ovality and Stress

5. Crushing of side walls

6. Ring Buckling7. Fatigue

8. Surface Impact Loads

9. Buoyancy

10.Thermal Expansion

11.Earthquakes

2.1. Allowable Pipe Stress

Paragraph 402.3.1 of the ASME B31.4 code establishes the allowable stress value, S

in psi (MPa), to be used in the temperature range (-20 oF to 250 oF) (-30 oC to 120 oC)

for design calculations [8]:


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0.72S E SMY S = (Equation 2.1)

Where

0.72 = design factor

E = joint weld factor

SMYS = specific minimum yield strength, psi (MPa)

There are two equally plausible versions of the origin of 0.72 SMYS. The first

explanation is that 0.72 SMYS goes back to the early days of fabrication of steel line

pipe. In the mill, the pipe was tested to a hydrostatic pressure causing a hoop stress

PD/(2t) of 90% SMYS. In service, the pressure was limited to 80% of the mill

hydrotest pressure, or 80% x 90% SMYS = 72% SMYS. The second explanation is

that the 90% SMYS hydrostatic test was reduced by 12.5% for fabrication tolerance

on underthickness, then further divided by 1.1 to compensate for the 110%

overpressure transient allowance (as was the common practice for water pipelines),which leads to 90% SMYS x 0.875 /1.1= 0.72 SMYS [4].

The weld quality or joint efficiency factor E is a factor introduced to account for the

quality of the longitudinal or spiral seam in a pipe. It is a function of the reliability

and quality of fabrication and the extent of inspections performed in the pipe mill. An

electric resistant welded pipe is judged to have a superior seam quality, and its weld

joint efficiency factor is assigned the maximum value 1.0. On the other hand, the

seam weld of a furnace butt-welded pipe was judged to have a seam weld factor of

only 0.6 [4].

For oil and gas pipelines, the thickness of the pipe wall is obtained by assuming that

the hoop stress, which is the largest stress in the pipe, must be limited to a certain

allowable stress S. Using the thin wall approximation, this condition corresponds to


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2

PDS

tp

(Equation 2.2) [4]

Where

P = internal design pressure, psi (MPa)

D = pipe outer diameter, in (m)

t = pipe wall thickness, in (m)

S = allowable stress, psi, (MPa)

2.2. Wall Thickness Calculation

Minimum wall thickness, t, is a function of the internal pressure, P, nominal

diameter, D, and the allowable stress, S, as specified by Sec. 404.1.2 of the ASME

B31.4 code

2

PDt

S=

(Equation 2.3)

Nominal wall thickness, tn, includes an allowance for manufacturing tolerance [3].

nt t allowances= + (Equation 2.4)

The allowances are usually about 12.5% of the calculated thickness and these

allowances are corrosion and tolerance allowances. The actual wall thickness used in


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the system will be equal to or greater than this calculated value according to the

nearest value in the API 5L [7].

In the next part, different stresses and pipe loading will be presented in detail.

2.3. Internal Pressure

To transport a fluid through a pipeline, the fluid must be under sufficient pressure so

that the pressure loss due to friction and the pressure required for any elevation

changes can be accommodated. The longer the pipeline and the higher the flow rate,

the higher the friction drop will be, requiring a corresponding increase in the fluid

pressure at the beginning of the pipeline.

The allowable operating pressure in a pipeline is defined as the maximum safe

continuous pressure that the pipeline can be operated at. At this internal pressure the

pipe material is stressed to some safe value below the yield strength of the pipe

material. The stress in the pipe material consists of circumferential (or hoop) stress

and longitudinal (or axial) stress. This is shown in Figure 2.1. It can be proven that the

axial stress is one-half the value of the hoop stress. The hoop stress therefore controls

the amount of internal pressure the pipeline can withstand. For pipelines transporting

liquids, the hoop stress may be allowed to reach 72% of the pipe yield strength. If

pipe material has 60,000 psi (414 MPa) yield strength, the safe internal operating

pressure cannot exceed a value that results in a hoop stress of

0.7260,000=43,200 psi (297.85 MPa)

To ensure that the pipeline can be safely operated at a particular maximum allowable

operating pressure (MAOP) we must test the pipeline using water, at a higher pressure

and is called the hydrostatic test.


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The hydrostatic test pressure is a pressure higher than the allowable operating

pressure. It is the pressure at which the pipeline is tested for a specified period of

time, such as 4 hr (for aboveground piping) or 8 hr (for buried pipeline) as required

by the pipeline design code API 5L [7].

Generally, for liquid pipelines the hydrostatic test pressure is 25% higher than the

MAOP. Thus, if the MAOP is 1000 psig, the pipeline will be hydrostatically tested at

1250 psig.

Calculation of internal design pressure in a pipeline is based on Barlows equation for

internal pressure in thin-walled cylindrical pipes [5].

Figure 2-1: Hoop stress and axial stress in a pipe

Barlows Equation for Internal Pressure

The hoop stress or circumferential stress, h , in a thin-walled cylindrical pipe due to

an internal pressure is calculated using the formula

2h PD

t = (Equation 2.5)


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Where

h = hoop stress, psi (MPa)

P = internal pressure, psi (MPa)

D= pipe diameter, in (m)

t= pipe wall thickness, in (m)

The Longitudinal stress is half the hoop stress and is calculated using this formula

4l

PD

t = (Equation 2.6)

2.4. Vertical Earth Load

The subject of soil structure interaction has been of engineering interest since the

early 1900s. One major problem existed, however. There was no rational method of

determining the earth load these on buried pipelines. As a result, there were many

failures of pipelines. The loads imposed on buried pipelines depend upon the stiffness

properties of both the pipe structure and the surrounding soil. This results in a

statically indeterminate problem in which the pressure of the soil on the structure

produces deflections that, in turn, determine the soil pressure.

When calculating the earth loads on a buried pipe, a steel pipe is considered flexible

and design procedures for flexible pipes apply. For flexible pipes placed in trench and

covered with backfill, the earth's dead load applied to the pipe is the weight of the

prism of soil with a width equal to that of the pipe and a height equal to the depth of

the fill above the pipe as shown in Figure 2.2 [2].


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When the pipe is above the water table the earth dead load can be obtained from this

equation:

vP C= (Equation 2.7)

Where

3

,

, /

,

vP earth dead load pressure on the pipe psi

unit weight of backfill lb in

C height of fill above the pipe in

=

=

=

(From Table 2.1)

Figure 2-2: Soil Prism above the pipe


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But if the pipe is under the water table, the effect of the soil grain buoyancy will be

included in the earth dead load [2], In this case the earth dead load is calculated using

the following equation:

vP w w w d h R C = + (Equation 2.8)

Table 2.1 Approximate Values of Soil Unit Weight, Ratio of Lateral to Vertical Earth

Pressure, and Coefficient of Friction against Sides of Trench

2.5. Surface Live Loads

Buried pipelines are subjected to concentrated or distributed live loads but we are

concerned about large concentrated loads such as truck-wheel roads, railways and


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aircraft loads at airports. As soil cover decreases, live load pressure on a buried pipe

increases. There is a minimum safe height of soil cover. If the soil cover is less than

the minimum, the surface live load may damage the pipe.

The Live Load effect may be determined based on the Association of State Highway

and Transportation Officials (AASHTO) HS-20 truck loads, E-80 Cooper railroad

loads, or a 180 kip airplane gear assembly as in Table 2.2 [2]. The values of the live

load pressurepP are given in psi and include an impact factor of 1.5 to account for

bumps and irregularities in the travel surface [2].

Table 2.2 Live Loads

For live-loads other than the AASHTO truck, the Cooper rail and the 180 kips aircraft

gear assembly loads, the pressurePp applied to the buried pipe by a concentrated

surface loadPs, without impact, as shown in Figure 2.3, can be calculated using

Boussinesqs equation [2]:


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2

2.52

3

2 1

sp

PP

dC

C

= +

(Equation 2.9)

Where:

Pp= pressure transmitted to the pipe

Ps= concentrated load at the surface, above pipe

C= depth of soil cover above pipe

d= offset distance from the pipe to the line of application of surface load

Figure 2-3: Surface Load and Transmitted Pressure

The pressurepP must be multiplied by a factor called impact factor, Table 2.3, due to

fluctuating nature of the surface line loads


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Table 2.3 Impact Factor (F) versus Height of Cover [2].

2.6. Ovality and Stress

A flexible pipe derives its soil load carrying capacity from its flexibility. Under soil

load, the pipe tends to deflect (reduction of pipe diameter in the vertical direction),

thereby developing passive soil support at the sides of the pipe. At the same time, the

ring deflection relieves the pipe of the major portion of the vertical soil load, which is

then carried by the surrounding soil through the mechanism of an arching action over

the pipe. Allowable limits of deflection have been set by both ASTM (7.5%) and

AWWA (5%).The Earth and live loads can ovalize the pipe, Figure 2.3, and this

ovality can be measured by the modified Iowa deflection equation [2]:

( )3 0.061 '

l

eq

D KPy

D EIE

R

=

+

(Equation 2.10)


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Where:

D = pipe outside diameter, in (m)

y = vertical deflection of pipe, in (m)

Dl = deflection lag factor (~1.0 1.5)

K = bedding constant (~0.1)

P = pressure on the pipe due to soil load Pv plus live load Pp, psi (MPa)

R = pipe radius, in (m)

(EI)eq = equivalent pipe wall stiffness per inch of pipe length, in/ lb

E ' = modulus of soil reaction, psi (MPa)

The bedding constant Kaccommodates the response of the buried flexible pipe to the

opposite and equal reaction to the load force derived from the bedding under the pipe.

The bedding constant varies with the width and angle ofthe bedding achieved in the

installation. Table 2.4 contains a list of bedding factors Kdependent upon the bedding

angle. These were determined theoretically by Spangler and published in 1941. As a

general rule, a value ofK= 0.1 is assumed [1].

Table 2.4 Values of bedding constant K


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Another parameter that is needed to calculate deflections in the Iowa formula is the

deflection lag factor,DL. Spangler recognized that in soil-pipe systems, as with all

engineering systems involving soil, the soil consolidation at the sides of the pipe

continues with time after installation of the pipe. His experience had shown that

deflections could increase by as much as 30 percent over a period of 40 years. For this

reason, If the prism load is used for design, a design deflection lag factorDL =1.0

should be used as a conservative design procedure.

The soil modulus of reaction (E), Table 2.5, is a measure of the embedment material

and surrounding soils ability to support the loads transferred by the deflection of

flexible pipe. A composite E value is used. The composite E value includes several

factors that consider the pipe and trench geometry, the E value of the native soil and

the E value of the embedment material [1].

The deflection of the pipe will cause stress on it and is called the Through wall

bending stress, Figure 2.4, due to earth and surface loads can be calculated using [2]:

4 ( )( )bwy t

ED D

=(Equation 2.11)

Figure 2-4: Ovality of Pipe Cross Section


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Figure 2-5: Through-Wall Bending Stress

Table 2.5 Average Values of Modulus of Soil Reaction


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2.7. Crushing of side walls

Wall crushingis the term used to describe the condition of localized yielding for a

ductile material or cracking failure for brittle materials. This performance limit is

reached when the in-wall stress reaches theyieldstress or the ultimate stress of the

pipe material.

The ring compression stress is the primary contributor to this performance limit.

Figure 2.5, However, wall crushing can also be influenced by the bending stress. Wall

crushing is the primary performance limit or design basis for most "rigid" or brittle

pipe products. This performance limit may also be reached for stiffer flexible pipes

installed in highly compacted backfill and subjected to very deep cover. A quick

check for this performance limit can be made by comparing the ring compression

stress withyieldand/or ultimate strengths.

For Buried pressure-steel pipelines with 100D

t and a yield stress larger than 30,000

psi, crushing of side walls is very unlikely [2].

Figure 2-6Crushing of Side Wall


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2.8. Ring Buckling

Buckling is not a strength performance limit, but can occur because of insufficient

stiffness. The buckling phenomenon may govern design of flexible pipes subjected to

internal vacuum, external hydrostatic pressure, or high soil pressures in compacted

soil, Figure 2.6.

The more flexible the conduit, the more unstable the wall structure will be in resisting

buckling. For a circular ring in plane stress subjected to a uniform external pressure,

the critical buckling pressure is [2]:

3

( )132 ' '

eq

W

EIR B E

FS D(Equation 2.12)

(Equation 2.13)


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Figure 2-7: Ring Buckling of Pipe Cross Section

2.9. Fatigue

This happens when the pipeline is subjected to cyclic surface loads as when the

pipeline is under a railway or highway. Local regulations usually specify a minimum

burial depth which varies from 1 to 6 feet depending on the standard codes [2].

The fatigue performance limit may be a necessary consideration in both gravity flow

and pressure applications. However, normal operating systems will function in such a

manner as not to warrant consideration of fatigue as a performance limit, although

some fatigue failures have been reported in forced sewer mains. Pipe materials will

fail at a lower stress if a large number of cyclic stresses are present. Pressure surges

due to faulty operating equipment and resulting water hammer may produce cyclic

stress and fatigue. Cyclic stresses from traffic loading are usually not a problem

except in shallow depths or burial. The design engineer should consult the

manufacturer for applications where cyclic stresses are the norm [1].

If the pipeline is buried under less than 2 feet of cover, the continual flexing of the

pipe may cause breakage of the road surface [2].


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2.10. Surface Impact Loads

2.10.1.Maximum Impact Load

Another form of dynamic surface loading that has received increasing attention in

recent past is the impact due to falling of heavy objects on the ground surface in the

vicinity of a buried pipeline. Large surface impact can result from dropping of

structural members and equipment during retrofitting or replacement projects. Impact

stresses are also induced during dynamic compaction at a site. An impact at the

ground surface causes a stress wave to travel through the soil. Ground vibration afteran impact can be represented by a single pulse in the time domain which results in

impulsive loading on the buried pipelines. It is evident that the effect of such

vibrations reduces with increasing depth of burial for the pipeline and with increasing

distance from the area of impact. However, instances can be found where existing

pipelines with moderate to large diameter pipes have been laid at very shallow depths.

It appears that observations from blasts and pipe driving may provide good insight as

all these loadings are expected to produce similar effects in a buried pipeline. The

damage in the pipeline from a traveling wave can be expressed in terms of soil strains

which are related to peak particle velocity [6].

The surface impact load due to the weight W of the falling body is given by

max 2

32

(1 )f oWH Gr Pv

=

(Equation 2.14)


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2.10.2.Penetration and PPV

For impact near the pipe location, Figure 2.8, the pressure transmitted to the pipe is

the surface load which considers the ovality and through wall bending and side wall

crushing and ring buckling. Also, the burial depth should be enough to prevent ground

penetration by falling objects. The penetration depth can be calculated from the

following equation [2]:

2

1215,000

p a

Vx kP

= +

(Equation 2.15)

Where:


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For impacts at large distances from the pipe location, the wave propagation causes

deformation in the pipe. The Peak Particle Velocity PPV can be calculated using the

following equation [2]:

1.7

8 fWH

PPVd

=

(Equation 2.16)

Where:

PPV = peak particle velocity, inches per second

W = weight of the falling object, tons

Hf = drop height, feet

d = shortest distance from point of impact to centerline of the pipe, feet

Figure 2-8: Fall of a Heavy Object on Ground Surface


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2.11. Buoyancy

2.11.1.Applied Load

A net upward force is created when the buoyancy force created by the pipe below the

water table exceeds the weight of the pipe and soil combined, Figure 2.9.

Figure 2-9: Resultant Buoyancy Load on Pipe

The upward force on the pipe is calculated using [2]:

( )b w p c v w wF W W W D P h = + + (Equation 2.17)


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Where:

D= pipe outer diameter, in

Fb = upward force due to buoyancy per unit length of pipe, lb

Pv = earth pressure, psi

Ww= weight of water displaced by pipe per unit length of pipe, lb

Wp = weight of pipe per unit length of pipe, lb

Wc =weight of pipe contents per unit length of pipe, lb

2.11.2.Pipe Stress

The buoyancy force causes longitudinal (beam bending) stress which is approximated

by [2]:

2

10b

bf

F L

Z = (Equation 2.18)

Where:

bf = stress caused by buoyancy forces, psi

Z = Section modulus of the pipe cross section

L = length of pipe span in the buoyancy zone, in

To make resistance against Buoyancy, Ballets such as concrete coating, concrete

weights or gravel filled blankets can be used or we can use screw anchors may be

used to anchor the pipe.


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2.12. Thermal Expansion

Buried pipelines are often operated at temperatures that do not significantly differ

from the surrounding soil temperature. In these cases, there will be little or no

differential expansion and contraction between the pipe and soil, and a thermal design

analysis is not required. In cases where the fluid is hot or cold, stresses are generated

as the pipe expansion is constrained by the surrounding soil. For long sections of

straight pipelines, the resulting longitudinal stress is calculated from the following

equation [3]:

(Equation 2.19)

2.13. Earthquakes

In certain critical zones, large ground movement associated with an earthquake may

be devastating to a pipeline. Most buried flexible pipelines can survive an earthquake.

A more flexible piping material with a flexible joint will allow the pipe to conform to

the ground movement without failure. The effects of permanent ground displacement


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produced by an earthquake are best evaluated using finite element analysis

techniques.

2.13.1.Seismic Wave Propagation

Wave propagation provisions are presented in terms of longitudinal axial strain, that

is, strain parallel to the pipe axis induced by ground strain. Flexural strains due to

ground curvature are neglected since they are small for typical pipeline diameters.

The axial strain, _a, induced in a buried pipe by wave propagation can be

approximated using the following equation [2]:

g

a

s

V

C

= (Equation 2.20)

where:

Vg = peak ground velocity generated by ground shaking

Cs = apparent propagation velocity for seismic waves (conservatively assumed

to be 2 kilometers per second

= 2.0 for Cs associated with shear waves, 1.0 otherwise

The axial strains can be assumed to be transferred to the pipeline but need not be

taken as larger than the axial strain induced by friction at the soil pipe interface [2]:

4u

a

T

AE

(Equation 2.21)


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Where:

Tu = peak friction force per unit length at soil-pipe interface

= apparent wavelength of seismic waves at ground surface, sometimes

assumed to be 1.0 kilometers without further information, ft

A = pipe cross-sectional area, in2

E = modulus of elasticity of steel, psi

The peak friction force per unit length at soil-pipe interface is given by the followingequation [2]:

(1 ) tan2

u oT DH K

= + (Equation 2.22)

Where:

D = pipe diameter, ft

H = height of cover + D/2, ft

Ko= coefficient of pressure at rest (~1.0)

= interface angle of friction for pipe and soil


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Table 2.6 Peak Ground Velocity

2.13.2.Permanent ground deformation

Ground deformation from earthquakes includes lateral spread of sloped surfaces,

liquefaction, and differential soil movement at fault lines. Ideally, the routing of a

buried pipe is selected to avoid these seismic hazards. The first step is to establish the

seismic hazard, or design basis earthquake, and predict the corresponding ground

movement. The second step is to establish the performance requirement for the buried

pipe. For example:

1. The pipe may need to remain serviceable and allow, for example, the passage of pig

inspection tools.

2. The pipe may need to remain operational, with valves opening on demand to

deliver flow or closing to isolate a hazardous material.


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3. The pipe may only need to retain its contents, without being operational following

the earthquake.

Based on the performance requirement, an allowable stress or strain limit is

established. The third step is to analyze the pipe response to the postulated movement,

and the resulting tensile, bending, and compressive loads applied to the buried pipe.

This may be done by hand calculations if that the deformations are small. For large

deformations, preferably the calculations should be done by finite element analysis of

the soil-pipe interaction. Finally, the computed stresses or strains are compared to

allowable limits established earlier based on the required performance of the pipe

following the earthquake.


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Chapter 3Material Selection

The selection of materials for piping applications is a process that requires

consideration of material characteristics appropriate for the required service. Material

selected must be suitable for the flow medium and the given operating conditions of

temperature and pressure safely during the planned design life of the product.

Mechanical strength must be appropriate for long-term service, and resist operational

variables such as thermal or mechanical cycling. Extremes in application temperaturecan raise issues with material capabilities ranging from brittle fracture toughness at

low temperatures to adequacy of creep strength and oxidation resistance at the other

end of the temperature spectrum.

In addition, the operating environment surrounding the pipe or piping component

must be considered. Degradation of material properties or loss of effective load-

carrying cross section can occur through corrosion, erosion, or a combination both.

The nature of the substances that the pipelines contain is also an important factor.

The fabricability characteristics of the materials being considered must also be taken

into account. The ability to be bent or formed, suitability for welding or other methods

of joining, ease of heat treatment, and uniformity and stability of the resultant

microstructure and properties all of a given piping material contribute toward or

detract from its attractiveness and economy. The selection process should lead to the

most economical material that meets the requirements of the service conditions and

codes and standards that apply [3].

There are many factors to consider in choosing piping materials. They include such

parameters as availability, type of service, and type of the fluid.

Materials used in piping systems can be classified in two large categories: metallic

and non-metallic. Metallic pipe and fitting materials can in turn be classified as

ferrous (iron based) or non-ferrous (such as copper, nickel or aluminum based).


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This chapter identifies the important metallurgical characteristics of piping materials

and how they can affect or be affected by the operation of all of the other materials

available to the engineer. Carbon and low-alloy steels come closest to being the ideal

construction material. Due to the fact that the majority of piping applications employ

iron-based metals.

There are numerous standards, many of which are interrelated, and they must be

referred and adhered to by design engineers and manufacturers in the process

industry. These standards cover the following:

Material: chemical composition, mechanical requirements, heat treatment,etc.

Dimensions: general dimensions and tolerances.

Fabrication codes: welding, threading.

Standards covering the preceding were drawn up by the following major engineering

bodies:

American Petroleum Institute (API).

American Society for Testing and Materials (ASTM).

American Water Works Association (AWWA).

American Welding Society (AWS).

Manufacturers Standardization Society (MSS).

National Association of Corrosion Engineers (NACE).

Periodically, these standards are updated to bring them in line with the latest industry

practices. Most of the standards have been in circulation for a number of years, and

the changes are rarely dramatic; however, such changes must be incorporated into the

design. It is essential that the latest revision is the final reference point. Other

countries publish comprehensive standards containing data on material, dimensions of

components, and construction procedures.


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American standards are not superior to other national standards, but they are the ones

most commonly used in the process industry. They are based on a long track record

with a very low failure rate, so there is a high degree of confidence in these

publications. Always refer to the latest edition of the relevant standards, and if

necessary, make sure your companys library holds the most current version.

3.1. Metallic MaterialsMetals are divided into two types: ferrous, which includes iron and iron-base alloys;

and nonferrous, covering other metals and alloys. Metallurgy deals with the extraction

of metals from ores and also with the combining, treating, and processing of metals

into useful engineering materials. This section presents the fundamental metallurgical

concepts and practices associated with the most common engineering metals, and

outlines metallurgical considerations appropriate in the selection process of metals for

piping system construction.


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Figure 3-1: Pipe Materials Chart


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3.2. Material Properties of Piping Material

The behavior of piping material can be understood and predicted by studying anumber of properties of the material. Metals are crystalline in structure, composed of

atoms in precise locations within a space lattice. The smallest component of the

crystalline structure is called a unit cell, the smallest repeating building block of the

material. For example, iron and iron-based alloys exist in two unit cell forms, the

body-centered cubic (BCC) and the face-centered cubic (FCC) structure; they are

differentiated in the way the atoms are arranged in repeating patterns. The body

centered cubic structure is represented by a cube with atoms at all eight corners, and

one atom in the center of the cube. The face-centered lattice is represented by atoms at

the eight corners of the cube, plus one atom located at the center of each of the cubes

six faces.

Figure 3-2: The three most common crystal structures in metals

and alloys. (a) Face-centered cubic (FCC); (b) body-centered

cubic (BCC); (c) hexagonal close-packed (HCP).

The crystal structure naturally assumed by a material dictates some of thefundamental properties of the material. For example, FCC materials are generally


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more ductile than BCC materials. This is basically because FCC crystals are the most

tightly packed of metallic structures and, as such, allow for more planes of atoms to

slide across one another with the least amount of resistance (the fundamental atomic

motion involved in what is called plasticity).

Metallic materials consist of these and other ordered crystal structures. Some metals,

most notably iron, change their crystal structure as temperature varies. Structure may

also change as certain other elements are added in the form of alloying additions.

These changes are used to advantage by metallurgists and are the basis for developing

and manipulating important material behavior, such as the heat treatability of carbon

and low alloy steels.

Engineering materials have four essential characteristics that are closely interrelated

and they are:

1. Chemistry: the primary element (iron in the case of ferrous metals), alloying

elements (nickel, chromium, etc. with ferrous metals), incidental elements

(small amount of unintended elements), and impurities (sulfur, phosphorous,

etc.).

2. Mechanical Properties: strength (yield, ultimate, elongation at rupture) and

toughness (Charpy, nil ductility transition temperature, fracture toughness,

ductile vs. brittle appearance of fracture surface).

3. Physical properties: density, modulus of elasticity, coefficient of thermal

expansion, electrical and heat conduction, etc.

4. Microstructure: atomic structure, metallurgical phase, type and size of grains.


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3.3. Chemical properties

Chemical properties are defined as those material characteristics that are dictated by

the elemental constituency of the solid. This is usually measured by the relative

atomic weight percent of the various elements (metals or nonmetals) or compounds

within the material. Metals are not usually used in their pure form. Rather, secondary

elements are purposely added to improve or modify their behavior. This addition of

secondary elements is called alloying, and the elements added fall into two categories,

based on the relative size of the atoms. Atoms significantly smaller than those of the

parent metal matrix fit into spaces between the atoms in the lattices interstices and

are called interstitial alloying elements. Carbon added to iron, creating steel, is the

most common example. Larger-sized atoms will substitute for parent metal atoms in

their matrix locations, thus the name substitutional alloying elements. Examples of

this include zinc substituting for copper atoms in copper, creating brass; and tin

substituting for copper atoms, creating bronze alloys. Pure metals possess relatively

low strength. Adding an alloying element will increase the strength of a metals

atomic matrix because the atomic lattice is strained locally by the foreign atom,

creating a larger impediment for the sliding of planes of atoms across one anotherduring plastic flow. This is true whether the alloying element is interstitial or

substitutional; however, the former generally serve as better lattice strengtheners.

Strength properties are often improved to the detriment of ductility. Proper alloying,

combined with appropriate metal processing and heat treatment, results in

optimization of material properties. Elements are also added to metals to improve or

modify their corrosion or oxidation characteristics, or to improve manufacturability

(e.g., machineability) and/or electrical properties, among other effects. However, it is

important to note that alloying done to optimize one material property may act to the

detriment of others.

Carbon steels, the most common of the construction materials, always contain the

elements carbon, manganese, phosphorous, sulfur, and silicon in varying amounts.

Small amounts of other elements may be found either entering as gases during the

steel-making process (hydrogen, oxygen, nitrogen), or introduced through the ores or


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metal scrap used to make the steel (nickel, copper, molybdenum, chromium, tin,

antimony, etc.). Addition of significant quantities of the interstitial element carbon

will result in high strength and hardnessbut to the detriment of formability and

weldability. A great amount of research has gone into the development of the

principal metals used in piping design and construction; thus the specification limits

must be vigorously adhered to in order to assure reliability, predictability, and

repeatability of material behavior.

The number of elements alloyed with a parent metal, and the acceptable range of

content of each, are identified in the material specification (e.g., ASTM, API, and

ASME). Tests appropriate for determining the elemental constituency of an alloy have

been standardized and are also described in ASTM specifications. The material

specifications also stipulate whether the chemical analysis of an alloy may be reported

by analyzing a sample of the molten metal, or taken from a specimen removed from

the final product. The former is commonly referred to as a ladle analysis, and the

latter as a product or check analysis. This chemistry of a construction material is

reported on a material test report which may be supplied by the material manufacturer

upon request.

3.4. Mechanical Properties

Mechanical properties are critically important to the design process. They are defined

as the characteristic response of a material to applied force. The standardized test

methods for measuring these properties are described in ASTM specifications.

Properties fall into two general categories, strength and ductility. Some properties,

such as material toughness, are dependent on both strength and ductility.


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3.4.1. Strength

Yield stress, ultimate strength and elongation at rupture are the fundamental of the

mechanical properties of pipe and fitting materials. They reflect the ability of the

material to be fabricated and to resist applied loads in service. All three properties are

essential for piping systems.

3.4.1.1. Yield Strength.

It is defined in engineering and materials science as the stress at which a material

begins to plastically deform. Prior to the yield point the material will deform

elastically and will return to its original shape when the applied stress is removed.

Once the yield point is passed some fraction of the deformation will be permanent and

non-reversible.

.Most materials do not abruptly transform from purely elastic to purely plastic

behavior. Rather, a gradual transition occurs as represented by a curve, or knee, in the

stress-strain curve. Lacking an abrupt and easily definable point representing

transition from elastic to plastic behavior, several standardized methods have been

defined by ASTM to determine the yield strength used as the engineering property.

The most common is termed the 0.2 percent offset method. In this approach a line is

drawn parallel to the elastic portion of the curve anchored to a point displaced 0.2

percent along the strain axis.


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Figure 3-3: Stress-Strain Curve. (1) Ultimate Strength. (2) Yield strength. (3) ProportionalLimit Stress. (4)Rupture. (5) Offset Strain (typically 0.002).

The yield strength corresponds to the calculated value of the load indicated at the

intersection point of the drawn line, divided by the original cross-sectional area in the

gauge length of the tensile specimen. By convention, this test is performed at a

constant rate of strain, and is reported as newtons per square meter, or as pounds per

square inch of cross section in English units.

Youngs Modulus is a measure of the elasticity of a material. It varies with

temperature, the higher the temperature the softer the material and the lower its

Young's modulus, as shown in Table 3-1.

Table 3-1 Youngs Modulus E (106) for various Metals at different temperatures


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3.4.1.2. Ultimate Tensile Strength

Upon further increase of applied load under constant strain rate, the specimen will

continue to stretch until the loss of load-carrying cross section caused by specimen

thinning during the test (due to Poissons ratio) cannot withstand further load

increase, Figure 3-3. The ultimate tensile strength constitutes the maximum applied

load divided by the original specimen cross-sectional area.

3.4.1.3. Elongation and Reduction of Area

The ductility of the test specimen can be established by measuring its length and

minimum diameter before and after testing. Stretch of the specimen is represented as a

percent elongation in a given length (usually 2 or 8 in) and is calculated in the

following manner:

(Equation 3-1)


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Figure 3-4: An Engineering Stress-Strain for Carbon Steel

3.4.2. Hardness

This is a measure of the materials ability to resist deformation, usually determined

by a standardized test where the surface resistance to indentation is measured. The

most common hardness tests are defined by the indentor type and size, and the

amount of load applied. The hardness numbers constitute a non dimensioned,

arbitrary scale, with increasing numbers representing harder surfaces. The two most

common hardness test methods are Brinell hardness and Rockwell Hardness, with

each representing a standardized test machine with its own unique hardness scales.

Hardness loosely correlates with ultimate tensile strength in metals. Approximate

hardness conversion numbers for a variety of material types, including steels, can be

found in ASTM Specification E140 (Standard Hardness Conversion Tables For

Metals Relationship Among Brinell Hardness, Vickers Hardness, Rockwell Hardness,

Superficial Hardness, Knoop Hardness, and Scleroscope Hardness).


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3.4.3. Toughness

Toughness is the ability of a material to absorb impact energy prior to rupture. It is

also defined as the material's ability to absorb plastic energy, dynamic or Static. It is a

function of the material, its temperature and, what somewhat complicates things, its

thickness. The thicker the part, the more constrained is the material at its center, and

the lower its toughness. The part is too thick and stiff to deform through the thickness,

it is in a condition called plane strain. On the contrary, a thinner section of the same

material is able to strain outward and the stress is practically constant through-wall, a

condition called plane stress. Under internal pressure, a thicker pipe has more strength

owing to its wall thickness but a thinner pipe of the same material will exhibit largerplastic deformation before rupture. This decrease of toughness with wall thickness

explains why the ASME code specifies minimum operating temperatures as a

function of wall thickness. The thicker the material, the more prone it is to brittle

fracture and the higher its minimum operating temperature. For example, the

minimum operating temperature permitted in ASME B31.3 (Process Piping Design)

for API 5L(Specification for Line Pipe) X42 is +15F for t < 0.394" and +70F for t =

1". For ASTM A 106 Grade B it is -20F for t < 0.5" and +30F for t = 1". For ASTM

A 312 type 304 stainless steel it is -425F regardless of thickness.

The two most common methods used to measure metal toughness are the Charpy

Impact test, defined in ASTM specification E 23(Standard Test Methods for Notched

Bar Impact Testing of Metallic Materials), and the Drop-Weight test, defined in

ASTM E 208 (Nil-Ductility Testing). The Charpy test employs a small machined

specimen with a machined notch that is struck by a pendulum weight. The energy loss

to the pendulum as it passes through and breaks the specimen, measured in kilojoules

or ft / lb of force, is a measure of the toughness of the specimen. Typical impact

behavior versus test temperature is shown in Figure 3-5.

The Drop-Weight test is similar in principle but employs a larger specimen with a

brittle, notched weld bead used as the crack starter. A weight is dropped from a height

onto the specimen, which had been cooled or heated to the desired test temperature.

The test determines the nil-ductility transition temperature (NDTT), defined as the


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specimen temperature when, upon striking, the crack propagates across the entire

specimen width.

3.4.4. Fatigue Resistance.

The ability of a metal to resist crack initiation and further propagation under repeated

cyclic loading is a measure of its fatigue resistance. Several standardized test methods

have been developed to test metals, machined to particular geometries, where

applying a repeating load range. Loads are generally applied through bending,

cantilevered, or push-pull load application in suitably outfitted testing machines.Either constant applied stress or strain ranges can be employed to determine material

response. The most common representation of fatigue test data is an S-N curve,

relating stress (S) required to cause specimen failure in a given number of cycles (N)

[3].

Figure 3-5: Transition temperature range and transition temperature in Charpyimpact test

These tests are generally performed on smooth specimens, but they can also be run

with stress-concentrating mechanisms such as notches machined into the specimen


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surface. The effect of stress concentrations on fatigue life cycles can also be estimated

from the smooth specimen S-N curve by calculating the intensified stress due to the

particular geometry, and intersecting the curve at that point on the stress axis. As the

applied load range decreases, ferritic steels exhibit a point at which an infinite number

of cycles can be absorbed without causing failure. This level of stress is called the

endurance limit. Many of the other metals do not exhibit this behavior, but rather

exhibit an increasing, but finite, number of cycles to failure with decreasing cyclic

load. When considering metal fatigue in design, a further safety margin is often also

applied against the cycles-to-failure at a given stress amplitude. For example, if a

component is continuously cycled over the same stress range, a design limit on

allowable cycles may correspond to the cycle life multiplied by a factor such as

0.8.This is a common safety margin employed in vessel and piping design. As is

normally the case, components may experience a wide variety of cyclic stress ranges,

at various temperatures, over their life. The effect of this array of cyclic parameters on

fatigue life can be estimated by an approach referred to as life fraction summation. In

this design practice, the percentage of life used up in cycling at a certain stress range

is calculated, corresponding to the ratio of the number of actual service duty cycles to

the total number of cycles to failure at that stress range. This calculation is performed

for all of the various stress ranges/duty cycles anticipated. The fractions thereby

calculated are summed and compared to the design limit (1.0 with no safety margin,

or 0.8 or some other value depending on the design safety factor that applies) [3].

3.4.5. Elevated Temperature Tensile and Creep Strength.

Tensile tests are performed at elevated temperatures to characterize the materials

yield and ultimate tensile properties at potential use temperatures above room

temperature. A heating chamber is combined with a conventional tensile testing

machine, and special strain measuring extensometers are used that are capable of

withstanding the test temperatures. Generally, as temperature increases, yield and

ultimate strengths decrease, and ductility increases.

Creep is defined as the time-dependent deformation of a material that occurs underload at elevated temperatures. The test is performed by holding a specimen, similar in


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configuration to a tensile specimen, at a uniform temperature and a constant load

(usually using a dead weight) and allowing the specimen to gradually elongate to

ultimate failure. The practice is defined in ASTM Specification E 139.

The simplest test method records only the applied stress (based on original test

specimen cross section), time to failure, and total elongation at failure. This is called a

stress rupture test. If periodic measurements of strain accumulation versus test

duration are also taken, the test is referred to as a creep-rupture test. A representation

of typical creep strain-versus-time data is shown in Figure 3-6

Figure 3-6: Creep time versus elongation curves at a given temperature.

Three stages of creep behavior are exhibited. Upon initial loading, instantaneous

straining occurs. Almost immediately, the rate of creep strain accumulation (creep

rate) is high but continuously decreasing. The test then progresses into a phase where

the strain rate slows and becomes fairly constant for a long period of time. Finally,

with decreasing load-bearing cross section of the specimen due to specimen stretching

and necking, applied stress begins to increase steadily, as does the creep rate, untilfailure occurs. These three regions are termed the primary, secondary, and tertiary


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stages of creep. The intent of safe design practice is to avoid the third stage, where

strain accumulations are rapid and material behavior less predictable.

3.5. Physical Properties of Metals

Physical properties are those, other than mechanical properties, that pertain to the

physics of a material. Physical properties of importance to the materials and design

engineer are material density, thermal conductivity, thermal expansion, and specific

heat.

3.5.1. DensityDensity is the ratio of the mass of a material to its volume. In vessel and piping

design, the density of a construction material versus its strength per unit area of cross

section is often an important consideration.

3.5.2. Thermal ConductivityThis is the characteristic ability of a material to transmit energy in the form of heat

from a high-temperature source to a point of lower temperature. The ability to

transmit heat is usually expressed as a coefficient of thermal conductivity (k) whose

units are a quantity of heat transmitted through a unit thickness per unit time per unit

area per unit difference in temperature.

3.5.3. Thermal Expansion.

Expressed as the coefficient of linear expansion, thermal expansion is a ratio of the

change in length per degree of temperature, to a length at a given standard

temperature (such as room temperature, or the freezing point of water). The units of


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the coefficient are length of growth per unit length per degree of temperature. The

value of the coefficient varies with temperature. The coefficient of thermal expansion

is not a property specified in ASTM material specifications, but it can be obtained for

different groups of materials, as a function of temperature from the ASME Boiler &

Pressure Vessel Code [ASME II]. The coefficient is critical in the flexibility analysis

of piping systems and is used to calculate the change in length of a material where:

,

exp ,1/

int ,

,

L L T

L change of length in

coeff icient of thermanl ansion of the material F

L ial length of the material in

T change in temprature F

=

== =

=

Table 3-2 Coefficient of Thermal Expansion of Some Metals (10-6 1/oF)

3.5.4. Specific Heat.

This is a measure of the quantity of heat required to raise a unit weight of a material

one degree in temperature.


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3.6. Microstructure

The microstructure of a metal is the structure of its crystals and grains, which is

determined by microscopic examination of a sample of metal. To understand a

material's microstructure, consider first what takes place as steel cools down from a

molten, liquid state. The liquid metal starts to solidify at a number of points

distributed throughout its volume, first at the surface (which is colder) and then

towards the center of the ingot or piece. Around these scattered nuclei of solid metal,

the atoms of iron and alloying elements take their place in a well-structured matrix as

they solidify.

As the temperature continues to drop and more metal solidifies, these well structured

atomic lattices grow into crystals and grains, Figure 3-7, until all the metal has

solidified and the grains have grown to the point where they touch each other,

constituting grain boundaries. The atomic structure within a grain and the grain size

will depend on several factors, including the chemical composition of the material and

its heat treatment.

The equilibrium phase diagram for carbon steel is shown in Figure 3-8. To represent,for example, an ASTM A 106 Grade B pipe material with 0.2% carbon, we place a

point on the bottom horizontal line (which corresponds to the ambient temperature) at

0.2% carbon (a point to the extreme left of the %C axis in Figure 3-8. If the pipe is

now heated to the melting point, for example during welding, we move vertically up

on the phase diagram at 0.2% carbon up to the liquid zone, at approximately 2800F.

As the pipe cools down it will solidify to white metal, which is represented on the

phase diagram by moving vertically down from the liquid zone, down the same

vertical line at 0.2% carbon. As we reach about 2600F, we have entered the zone

noted "austenite". At this temperature, the hot white metal is solid and atoms of iron

in each grain have placed themselves in a face centered cubic arrangement (fcc), as

illustrated in the bottom sketch of Figure 3-6, with an atom at each corner of a cube

(A) and one at the center of each face (C). The carbon atoms locate themselves

between the iron atoms. As the temperature continues to drop, we continue to slide

vertically down on the phase diagram at 0.2% carbon.


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Figure 3-7: Growth of Atomic Lattice into Grains

Below approximately 1600F, part of the austenite atomic structure (FCC) evolves

into ferrite which is body-centered-cubic (BCC), shown as top sketch of Figure 3-9,

with an atom at each corner of a cube (A) and one at the center of each cube (B). The

space between iron atoms is now smaller and some carbon atoms are no longeraccommodated in the crystalline matrix. They combine with iron to form iron carbide

(cementite Fe3C). Steel at room temperature is therefore made of ferrite grains and a

mixture of ferrite and cementite called pearlite.

Below 1333F, and if the cooling process is sufficiently slow (cooling in still air or in

furnace) all the austenite has been converted to ferrite (FCC) and cementite. If this

cooling process is too rapid, the orderly change of atomic structure will not have time

to take place, and a distorted atomic structure, martensite, that is neither BCC nor

FCC, will form. Martensite is hard (in the order of Rockwell C 55 and ultimate

strength as high as 300 ksi) but it is also brittle, prone to cracking. When welding in-

service, the fluid flowing in the line tends to accelerate the cooling process in the weld

bead and heat affected zone, forming martensite, which is prone to brittle cracking,

particularly in the presence of hydrogen.


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Figure 3-8: Simplified Phase Diagram of Carbon Steel

Figure 3-9: Atomic Structure of Carbon Steel

The temperature at which the metal is heated and the speed at which it is cooled down

(form very slow if cooled in furnace, to very quick if dropped in water) will affect its

atomic structure and grain size and, as a result, its weldability, and its mechanical,

metallurgical and corrosion resistant properties. A small grain size results in a more


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ductile material, with better toughness. Another way to affect grain size is by addition

of grain refining elements such as aluminum, columbium (niobium), titanium or

vanadium [ASTM A 941 Standard Terminology Relating to Steel, Stainless Steel,

Related Alloys, and Ferroalloys]. This steel making practice is called "fine grain

practice". Grain size is measured and assigned a grain size number in accordance with

ASTM E 112 (Standard Test Methods for Determining Average Grain Size). The

study of the metal's microstructure, metallography is performed by optical or electron

microscopy. Metallography unveils the metal's microstructure, its grain morphology

as well as its flaws, such as cracks, voids or inclusions. Grain size can be viewed at

magnifications of around 100x and classified according to reference comparison

standards [ASTM E 112] or by computerized imaging techniques.

3.7. Fabrication Of Steel Pipe3.7.1. Pipe SizeCommercial steel pipe is fabricated either by piercing and extruding a hot billet

(seamless pipe) or by bending then welding steel plates or skelp (longitudinal or spiral

seam welded pipe). In either case, the fabricator produces a pipe with dimensions

(diameter and thickness) that comply with a standard, such as ASME B36.10 for

carbon steel pipe, ASME B36.19 for stainless steel pipe, API 5L for line pipe. Pipe

mills also produce custom sizes, typically in the very large diameters. A standard

schedule pipe up to 12" has an inner diameter close to its nominal pipe size (NFS).

Pipe 14" and larger has an outer diameter equal to its NFS. Pipes are specified by

.their nominal size and schedule. Unlike pipes, tubes (or tubing) can have round or

square cross section. Cylindrical tubing generally has an outer diameter equal to its

nominal size, but not in all cases. Pipe schedules were introduced in the 1930's in an

effort to standardize and replace the de

crude oil pipeline calculation

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