Crude Oil Pipeline Calculation

Engineering and Material Science Faculty German University in Cairo

Mechanical Design of Crude Oil and Petroleum Products Pipelines Bachelor Thesis

Author:

Ahmed Essam Khedr

Supervisor:

Dr. Hamdy Kandil

Submission Date:

12 July, 2007

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

1

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.

1.2.

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

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 2

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 3

Chapter 6: Conclusion ......................................................... 101 References............................................................................. 102

4

List of Figures Figure 2-1: Hoop stress and axial stress in a pipe..................................................... 19 Figure 2-2: Soil Prism above the pipe ...................................................................... 21 Figure 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 .............................. 46 Figure 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

5

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 environment in 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.

6

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.

7

Chapter 1 Introduction 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

8

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 world’s 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!

9

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 Analysis

The 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

10

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 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 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.

11

1.2. Literature Review

Many researches have been done in the past years concerning the effect of different stresses 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 Fernández-Sáez, J. L. PérezCastellanos and C. Navarro on the static 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 12

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. Today’s 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 approach is advocated. However, these procedures are still based on the existing 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-loadinduced 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 13

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, JuiHuang 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].

14

Chapter 2

Stress 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 Buckling 7. 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 o F to 250 o F) (-30 o C to 120 o C) for design calculations [8]:

15

S = 0.72 × E × SMYS

(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 16

PD pS 2t

(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

t=

PD 2S

(Equation 2.3)

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

t n = 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 17

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 0.72×60,000=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. 18

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 Barlow’s equation for internal pressure in thin-walled cylindrical pipes [5].

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

Barlow’s Equation for Internal Pressure

The hoop stress or circumferential stress, σ h , in a thin-walled cylindrical pipe due to an internal pressure is calculated using the formula

σh =

PD 2t

(Equation 2.5) 19

Where

σ 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

σl =

2.4.

PD 4t

(Equation 2.6)

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].

20

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

Pv = γ C

(Equation 2.7)

Where

Pv = earth dead load pressure on the pipe , psi γ = unit weight of backfill , lb / in 3

(From Table 2.1)

C = height of fill above the pipe , in

Figure 2-2: Soil Prism above the pipe

21

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:

Pv = γ w hw + Rw γ d 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 22

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 pressure P p 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 pressure Pp applied to the buried pipe by a concentrated surface load Ps, without impact, as shown in Figure 2.3, can be calculated using Boussinesq’s equation [2]:

23

Pp =

3Ps   d 2  2π C 1 +      C  

2.5

2

(Equation 2.9)

Where: P p= pressure transmitted to the pipe P s= 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 pressure P p must be multiplied by a factor called impact factor, Table 2.3, due to fluctuating nature of the surface line loads

24

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]:

∆y D l KP = D  ( EI )eq + 0.061E   R3 

 '  

(Equation 2.10)

25

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 K accommodates 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 of the bedding achieved in the installation. Table 2.4 contains a list of bedding factors K dependent upon the bedding angle. These were determined theoretically by Spangler and published in 1941. As a general rule, a value of K = 0.1 is assumed [1].

Table 2.4 Values of bedding constant K

26

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 factor DL = 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]:

σ bw = 4E (

∆y t )( ) D D

(Equation 2.11)

Figure 2-4: Ovality of Pipe Cross Section

27

Figure 2-5: Through-Wall Bending Stress

Table 2.5 Average Values of Modulus of Soil Reaction

28

2.7.

Crushing of side walls

Wall crushing is 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 the yield stress 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 with yield and/or ultimate strengths. For Buried pressure-steel pipelines with

D ≤ 100 and a yield stress larger than 30,000 t

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

Figure 2-6Crushing of Side Wall

29

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]:

1 FS

32RW B ' E '

(EI )eq

(Equation 2.12)

D3

(Equation 2.13)

30

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].

31

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 after an 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

Pmax =

32 WH f Gro π 2 (1 −v )

(Equation 2.14)

32

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]:

 V2  x p = kPa  1 +   215, 000  Where:

33

(Equation 2.15)

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

 WH f  PPV = 8    d 

(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

34

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]:

Fb =Ww − W p +Wc + D ( Pv − γ w hw )

35

(Equation 2.17)

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]:

σ bf =

Fb L2 10Z

(Equation 2.18)

Where:

σ 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.

36

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 37

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]:

εa =

Vg

(Equation 2.20)

αC s

where: Vg = peak ground velocity generated by ground shaking Cs = apparent propagation velocity for seismic waves (conservatively assumed to be 2 kilometers per second

α = 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]:

εa ≤

Tu λ 4AE (Equation 2.21)

38

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

λ = 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 following equation [2]:

Tu =

π DH γ (1 + K o ) tan δ 2

(Equation 2.22)

Where: D = pipe diameter, ft H = height of cover + D/2, ft Ko= coefficient of pressure at rest (~1.0)

δ = interface angle of friction for pipe and soil

39

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.

40

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.

41

Chapter 3 Material 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 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 loadcarrying 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).

42

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. 43

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 company’s library holds the most current version.

3.1.

Metallic Materials

Metals 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.

44

Figure 3-1: Pipe Materials Chart

45

3.2.

Material Properties of Piping Material

The behavior of piping material can be understood and predicted by studying a number 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 cube’s 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 the fundamental properties of the material. For example, FCC materials are generally 46

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.

47

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 metal’s 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 another during 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 48

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 hardness—but 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.

49

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.

50

Figure 3-3: Stress-Strain Curve. (1) Ultimate Strength. (2) Yield strength. (3) Proportional Limit 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. Young’s 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 Young’s Modulus E (106) for various Metals at different temperatures

51

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 Poisson’s 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)

52

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

3.4.2. Hardness

This is a measure of the material’s 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).

53

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 larger plastic 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 +15°F for t < 0.394" and +70°F for t = 1". For ASTM A 106 Grade B it is -20°F for t < 0.5" and +30°F for t = 1". For ASTM A 312 type 304 stainless steel it is -425°F 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

54

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 Charpy impact 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 55

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 material’s 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 under load at elevated temperatures. The test is performed by holding a specimen, similar in 56

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, until failure occurs. These three regions are termed the primary, secondary, and tertiary 57

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. Density Density 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 Conductivity This 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 58

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:

∆L = α L ∆T ∆L = change of length ,in α = coefficient of thermanl exp ansion of the material ,1/ °F L = int 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.

59

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 2800°F. 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 2600°F, 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.

60

Figure 3-7: Growth of Atomic Lattice into Grains

Below approximately 1600°F, 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 longer accommodated 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 1333°F, 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 inservice, 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.

61

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 62

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 Pipe

3.7.1. Pipe Size Commercial 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 designations of Standard (STD), Extra Strong (XS), and Double Extra Strong (XXS), in use since the late 1800's. The schedule number of stainless steel pipe (ASME B36.19) is followed by the letter S, and includes lower schedules with thinner walls than carbon steel pipe (such as schedule 5S and 10S) for low-pressure corrosive service. 63

3.7.2. Seamless Pipe Seamless pipe is fabricated by piercing a hot cylindrical billet and forming a seamless tube. The "seamless" fabrication process follows several steps, depending on the applicable material specification. These steps typically include the following operations: (a) a forging is heated to white metal temperature, (b) the white-hot ingot is forged and elongated into cylindrical bars, (c) the white-hot bars are pierced and sized to the right diameter and thickness, (d) the pipe is hydrotested, (e) mechanical properties are verified against the material and procurement specifications, (f) the ends are beveled or threaded, and (g) the pipe is cleaned, marked and readied for shipment. Alloys such as stainless steel would also be passivated (pickling, descaling) by immersion into a warm acid bath, followed by water rinse and drying in air (passivation), the pipe is then measured, weighed and marked.

3.7.3. Seam Welded Pipe

Seam welded pipe is made from skelp (the name given to plate used in pipe fabrication) deformed in an O-shape or spiral shape, then welded along a longitudinal or spiral seam. There are several types of seam welded steel pipe: Electric resistance welded pipe, furnace butt welded pipe, arc welded pipe, electric flash welded pipe, and double submerged arc welded pipe. Electric resistance welded pipe (ERW) is made from plate, longitudinally butt welded by heat from electric current, without filler metal. Furnace butt-welded pipe is made from a heated plate drawn through a welding bell and butt welded by compression of the plate edges in the hot furnace. Arc welded pipe is made from plate butt welded by manual or automatic arc, with single or multiple passes on the outside diameter (OD) and inside diameter (ID), with or without filler. Electric flash welded pipe is made from plate, longitudinally butt welded by heat from electric resistance. Double submerged arc welded pipe is made by the submerged arc welding process, typically with passes on the ID and OD. The fabrication process of seam welded pipe follows the steps illustrated in Figure 3-3: (a) steel plates (skelp) are welded end-to-end and rolled into coils, the outer edges are beveled, (b) plates are bent progressively on a pipe mill into a U then O shape, (c) the seam is welded and heat treated (typically by induction or gas furnace annealing), (d) 64

the weld is inspected (depending on the material specification, seam inspection may be as little as visual or as much as 100% radiography), (e) the pipe is hydrotested, (f) mechanical properties are verified for weld and base material, including tensile properties (yield stress, ultimate strength and elongation at rupture) and ductility by ring crush, (g) the ends are beveled or threaded, and (h) the pipe is cleaned (pickling for stainless steel), measured, weighed and marked. Note that for 6" and larger pipe, a seam welded pipe with 100% radiography can be a cost-effective alternative to a seamless pipe. From the 1920's to as late as the 1980's in some pipe mills, line pipe (API 5L) was seam welded using low frequency ac current (360 Hz) or dc current. Under these conditions, there must be a very close contact between the electrode and the skelp to achieve continuous fusion. That is why some pipes fabricated during that period exhibit lack of fusion along the seam, referred to as cold welds or stitched welds [Kiefner]. This condition is practically inexistent in modern seam welded pipe using high frequency ac current (in the order of 450 kHz).

Figure 3-10: Overview of Seamless Pipe Fabrication

65

Figure 3-11: Overview of Seam Welded Pipe Fabrication

66

Chapter 4 Mechanical Design of SUMED Pipeline 4.1. Background The Sumed pipeline (also known as Suez-Mediterranean pipeline) is a 320 km long oil pipeline in Egypt, which runs from Ain Sukhna terminal on the Gulf of Suez to Sidi Kreir on the Mediterranean. It provides an alternative to the Suez Canal for transporting oil from the Persian Gulf region to the Mediterranean. Sumed consists of two parallel lines with the 42-inches diameter having a flow rate of around 2.5 million barrel per day (Fig 4-1).

SUMED Pipeline Location Map

Figure 4-1:SUMED Twin Pipeline route

The pipeline is owned by Arab Petroleum Pipeline Company, a joint venture of EGPC (50%, Egypt), Saudi Aramco (15%, Saudi Arabia), ADNOC (15% the United Arab Emirates), three Kuwaiti companies (each of 5%), and QGPC (5%, Qatar). 67

The pipeline was constructed to transport oil instead of Suez Canal because it was closed during 1973 war in Egypt. The construction began 1974 and was completed December 1976 and the first oil moved through the pipeline and shipped was in January 1977. The pipeline transport more than 40 types of crude oils from Saudi Arabia, Iran, Egypt, Kuwait, UAE, Iraq, Qatar, Oman, Yemen and Russia. There are four terminals at El Ain El Sukhna where the oil ships can put in their oil and there are other 6 terminals at Sidi Kerir where the oil ships can receive the transported oil and this is shown in Figure 4-2

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

68

Natural GasSupply 20.000CU.M/ HR

3Tanks

shoreline Deballasting plant

Relief tank 500

320

Relief tank

48

constant speed

52

285

relief tank

48

42 M

M

M

M

42

M M

M

M

42

M

M

150

3+3Gas Turbine pumps

42 M

M

GULFOFSUEZ

shoreline

FromQarun

M

M

42

42

42

M

48

M

M

M

Nile river Mainlines

variablespeed

15tanks

M

M

2*42inch320KM M

SPMSIZEIN,000DWT

Meters

M

320

M

15tanks

4tanks

Throughput 117MTA

150

M

42

48

150

MEDITERRANE 150

48 285

M

fromSUeztoMed.

350 SPMSIZEIN,000DWT

ToMidtap100.000Bpd

Meters

S-35 S-36

Figure 4-3: Pipeline System

There is a main pumping station in El Ain El Sukhna as shown in Figure 4-3 and there is a boosting station in Dahshour as shown in Figure 4-5 which is located at 140 km from El Ain El Sukhna to allow the flow of the crude oil till Sidi Kreir by giving it a boost.

Figure 4-4: El Ain El Sukhna Pumping Station

69

Figure 4-5: Dahshour Boosting Station

There are 22 tanks in Sidi Kreir to store the oil till the ships receive it, the tanks are shown in Figure 4-6. In El Ain El Sukhna there are 15 tanks.

Figure 4-6: Tanks in Sidi Kreir

The pipeline is buried under one meter of soil to protect it and this is shown in Figure 4-7 70

Figure 4-7: Burial of SUMED Pipeline

The stresses acting on SUMED pipeline in different regions will be calculated and see whether it is safe or not. The material of the pipeline is steel API 5L X60 and has a yield strength of 60,000 psi (413.7 MPa) and the pipe diameter is 42 inches (1.07 m) with an internal maximum allowable pressure of 1334.35 psi (9.2 MPa). The pipeline is buried 3 ft under the soil and has a modulus of elasticity of 29 ×106 psi (GPa ) The Pipeline will pass in 3 different regions across Egypt. The 1st region is in the eastern desert, then the pipeline will cross the Nile river which is the 2nd region. The 3rd region is when the pipeline passes through the Delta (agricultural area).

4.2. Stress Analysis

For calculating the allowable pipe stress, a design factor of 0.72 will be used for new seamless pipes with a joint weld factor of 1.0, the allowable pipe stress is calculated from the following equation (Equation 2.1): 71

S = design factor × SMYS × jo int weld factor = 0.72 × 60000 × 1 = 43200 psi = 297.85 MPa

When calculating the minimum wall thickness of SUMED pipeline, the internal pressure, the diameter and the pipe allowable stress must be known and the thickness is calculated using the following equation (Equation 2.3)

t=

PD 1334.3472*42 = = 0.649 in = 16.48 mm 2S 2* 43200

But corrosion allowance and fabrication tolerance allowance must be added (12.5%) [2], so the total allowances are calculated from Equation 2.4: allowances = 0.125 × 0.649 = 0.081 in

t n = t + allowances = 0.649 + 0.081 = 0.73 in = 18.54 mm

But when using the API 5L Table 6C page 61, the nearest thickness is 0.750 in (19.05 mm). So this thickness will be used.

Moment of Inertia: The moment of inertia of the pipe is calculated using the following equation: Moment of Inertia I =

t 3 0.753 = = 0.0352 12 12

72

Internal Pressure: The internal pressure acts in both the axial and longitudinal directions. The hoop stress which is in the axial direction is calculated using Equation 2.5:

σh =

PD 1334.35* 42 = = 37361.8 psi = 257.6 MPa 2t 2*0.75

While the longitudinal stress is calculated using Equation 2.6:

σl =

PD 1334.35*42 = = 18680.9 psi = 128.8 MPa 4t 4*0.75

This is half the hoop stress

The internal pressure is the same in all the 3 regions. Since the hoop stress is less than the pipe allowable stress then the pipe is safe for this stress which is the largest among all other stresses acting on the pipeline SUMED Pipelines passes in 3 regions as shown in Figure 4-1, so the stress analysis will be done in each region separately

Regions: 1st Region

When the pipeline passes in the desert, then there will be no live loads, the modulus of soil reaction E’ for moderate coarse grained soil is 2000 psi, from Table 2.5. The specific weight of sand is γ sand = 100lb / ft 3 , from Table 2.1.

73

Dead Load The Sand Pressure on the pipe (Vertical Earth Load) which is buried under 3 ft of sand is calculated using Equation 2.7:

Pv sand = 100 × 3 = 300 psf = 2.08 psi = 1.43 × 10 −2 MPa

Ovality and stress The pipe ovality due to the earth’s dead load is calculated using Equation 2.10:

∆y 1*0.1* 2.08 = = 0.000896 6 29*10 *0.0352 D + 0.061(2000) 213 The through wall bending stress is calculated using Equation 2.11:

σ bw = 4(29*106 ) (0.000896)(

0.75 ) = 1856 psi = 12.8 MPa 42

Ring Buckling Pressure

The critical ring buckling pressure is calculated using Equation 2.12 and Equation 2.13:

74

1

B'= 1 + 4e

( −0.065

36 ) 42

= 0.209

Pc = 32(1)(0.209)(2000)

(29 × 106 × 0.0352) = 429.3 psi = 2.96 MPa 423

Surface Impact Load An assumption that a load of 420 tons (840,000 lb) fall from a height of 15 ft (4.57 m) causing an impact area of 6 ft (1.82)in diameter is made. To calculate the maximum impact load, The mass density which is ρ = γ / g must be known, The soil’s Poisson ratio is 0.37, the shear wave velocity of near surface soils is Vs = 10,000 inches per second [2].

g = 9.81*100*2.5 = 2452.5 in / sec2 γ sand = 100lb / ft 3 = 0.05787lb / in 3

(from table 2.1)

ρ=

0.05787 = 0.0000236 lb .sec 2 / in 4 2452.5

G=

0.0000236 × (10000)2 = 236 psi 10

The surface impact load due to the weight of the falling body using equation 2.14 is

75

Pmax =

32(840, 000)(180)(236)(36) π 2 (1 − 0.33)

= 2493278.9 lb = 11090657.09 N Where the coefficient of penetration, k, is 0.0367 for sandy soil [2] Pa =

W 840, 000 = = 206 psi = 29664 psf 2 (π r0 ) (π 362 )

Now we have to determine the impact velocity V = (2 gH f )0.5 = ( 2 × 32.2 × 15 )

0.5

= 31 ft / sec

The penetration depth using equation 2.15 is x p = (0.0367)(29709) log(1 +

312 ) = 2.1 ft = 0.64 m 215000

Which is safe as the pipe is buried under 3 ft (0.91 m) in soil

Buoyancy Since the pipeline is in the desert there is no water in the 3 ft where the pipeline is buried so there is buoyancy stress on the pipeline

Thermal Stress An assumption is made that the maximum operating temperature is 86 F (30o C) and the installation temperature is 59 F (15o C) which gives a maximum compressive thermal stress that is calculated using Equation 2.19:

76

σ c = (29 × 106 )(6.345 × 10 −6 )(86 − 59) − 0.3(37361.8) = − 6240.41 psi = − 43.03 MPa

The negative sign means that this stress is compressive stress

Earthquakes The moment magnitude Mw is taken as 6.5, a source to site distance as 20 km and the sediment type as soft soil. The ratio of Peak Ground Velocity (cm/sec) to Peak Ground Acceleration (g) will be 140 according to Table 2.6

Vs = PGV = 140 cm / sec.g (0.74g ) = 103.6cm / sec

The induced axial strain using equation 2.20 then becomes: εa =

1.036 = 0.000518 2000

But ε a must be ≤

Tu λ 4AE

The peak friction force per unit length at soil-pipe interface is calculated using Equation 2.22 π DH γ (1 + K o ) tan δ 2 π 3.5 = (3.5)(3 + )(100)(1 + 1) tan 26.4o 2 2 = 2592.7 lb / ft

Tu =

77

With Tu = 2592.7 lb/ft, λ = 1.0 km = 3278 ft, E = 29 106 psi, and A = (42) (0.75) = 98.96 in2, the axial strain induced by friction at the soil pipe interface is 0.00074

Since ε a ≤

Tu λ , then the pipeline is safe against this earthquake 4AE

Conclusion Total Longitudinal Stresses (128.8) + (0.0143) + (12.8 ) + ( 2.96 ) + ( −43.03) = 101.5 MPa Total Hoop Stresses 257.6 MPa

Total Stress acting on the pipe

( 257.6 )

2

+ (101.5) 2 = 276.87 MPa

Since the total stress acting on the pipe is less than the allowable pipe stress then the pipe is SAFE

2nd Region

When the pipeline passes under the Nile River (7.28 m), then there will be neither live loads nor surface impact loads, the modulus of soil reaction E’ for moderate fine grained soil (less than 25% coarse grained particles) is 400 psi, Table 2.5. Since the pipe is under the Nile River, the height of water in the soil above the pipe is 3 ft (36 in). The specific weight of sand is γ sand = 100lb / ft 3 , Table 2.1.

78

Modulus of Soil Reaction E'

400 psi (2.76 MPa)

Height of water above the pipe hw

36 in (0.91 m)

Water buoyancy factor Rw

Rw = 1 − 0.33(36 / 36) = 0.67

Modulus of pipe elasticity E

29 ×106 psi (200GPa )

Dead Load

The Sand Pressure on the pipe (Vertical Earth Load) which is buried under 3 ft of sand is calculated using Equation 2.8

Pv soil = (62.4*3) + (0.67*120*3) = 428.4 psf = 2.975 psi = 2.05 ×10−2 MPa

Ovality and stress The pipe ovality is calculated using Equation 2.10

∆y 1*0.1*(2.975) = = 0.0022 6 29*10 *0.0352 D + 0.061(400) 213

79

The through wall bending stress is calculated using Equation 2.11

σ bw = 4(29*106 ) (0.0022)(

0.75 ) = 4557.14 psi = 31.42 MPa 42

Ring Buckling Pressure

The critical ring buckling pressure is calculated using Equation 2.13

1

B'= 1 + 4e

( −0.065

36 ) 42

= 0.209

Pc = 32(0.67)(0.209)(400)

(29 × 106 × 0.0352) = 157.15 psi = 1.08 MPa 423

Buoyancy The weight of water displaced by the pipe

Ww = 62.4* π *

3.52 = 600.358 lb / ft = 8760.1 N / m 4

80

The weight of the pipe

W p = 330.72 lb / ft = 4826.5 N / m

The effective weight of soil above the pipe

Ws = D (Pv − γ w hw ) = 3.5(428.4 − 62.4 × 3) = 844.2 lb / ft = 12320.2 N / m

Since the sum of the pipe weight and the effective weight of the soil above the pipe is greater than the weight of the water displaced by the pipe, then there is no net buoyancy force acting on the pipe.

Thermal Stress An assumption is made that the maximum operating temperature is 59 F (15o C) and the installation temperature is 50 F (10o C) which gives a maximum compressive thermal stress that is calculated using Equation 2.19:

σ c = (29 × 106 )(6.345 × 10 −6 )(86 − 59) − 0.3(37361.8) = − 6240.41 psi = − 43.03 MPa

The negative sign means that this stress is compressive stress

81

Earthquakes By taking the moment magnitude Mw as 6.5, a source to site distance as 60 km and a sediment type as soft soil. The ratio of Peak Ground Velocity (cm/sec) to Peak Ground Acceleration (g) will be 142 according to Table 2.6

Vs = PGV = 142 cm / sec.g (0.74g ) = 105.08cm / sec

The induced axial strain then becomes: εa =

1.0508 = 0.0005254 2000

But ε a must be ≤

Tu λ 4AE

The peak friction force per unit length at soil-pipe interface is calculated using Equation 2.22:

π DH γ (1 + K o ) tan δ 2 π 3.5 )(100)(1 + 1) tan 26.4o = (3.5)(3 + 2 2 = 2592.7 lb / ft

Tu =

With Tu = 2592.7 lb/ft, λ = 1.0 km = 3278 ft, E = 29 106 psi, and A = (42) (0.75) = 98.96 in2, the axial strain induced by friction at the soil pipe interface is 0.00074

Since ε a ≤

Tu λ , then the pipeline is safe against this earthquake 4AE

82

Conclusion Total Longitudinal Stresses (128.8) + (0.0205) + ( 31.42 ) + (1.08 ) + ( −43.03) = 118.3MPa Total Hoop Stress 257.6 MPa

Total Stress acting on the pipe

( 257.6 )

2

+ (118.3)2 = 283.47 MPa

Since the total stress acting on the pipe is less than the allowable pipe stress then the pipe is SAFE

3rd Region

When the pipeline passes in an agricultural area and under a highway, there will be earth and surface loads, the modulus of soil reaction E’ for moderate fine grained soil is 1000 psi, from Table 2.5. I assume that the height of water in the soil above the pipe is 2 ft (24 in). The specific weight of sand is γ soil = 120lb / ft 3 , from Table 2.1.

Modulus of Soil Reaction E'

1000 psi

Height of water above the pipe hw

12 in (0.305 m)

Water buoyancy factor Rw

Rw = 1 − 0.33(12 / 36) = 0.89

83

Dead Load

The Sand Pressure on the pipe (Vertical Earth Load) which is buried under 3 ft of sand is calculated using Equation 2.8

Pv soil = (62.4*1) + (0.89*120*3) = 382.8 psf = 2.658 psi = 1.83 × 10−2 MPa

Surface Live Load The surface load due to the highway is 4.17 psi (0.0288 MPa) from table 2.2

Ovality and stress The pipe ovality is calculated using Equation 2.10

∆y 1*0.1*(2.658) = = 0.00155 6 29*10 *0.0352 D + 0.061(1000) 213

84

The through wall bending stress is calculated using Equation 2.11

σ bw = 4(29*106 ) (0.00155)(

0.75 ) = 3210.7 psi = 22.14 MPa 42

Ring Buckling Pressure

The critical ring buckling pressure is calculated using Equation 2.13

1

B'= 1 + 4e

( −0.065

36 ) 42

= 0.209

(29 × 106 × 0.0352) Pc = 32(0.89)(0.209)(1000) = 286.4 psi = 1.97 MPa 423

Surface Impact Load An assumption that a load of 367.5 tons (700,000 lb) will fall from a height of 10 ft (3.05 m) causing an impact area of 6 ft (1.83 m) in diameter is made. To calculate the maximum impact load, the mass density which is ρ = γ / g must be known, the soil’s Poisson ratio which is 0.37, the shear wave velocity of near surface soils which is Vs = 10,000 inches per second [2].

g = 9.81*100*2.5 = 2452.5 in / sec2

85

γ soil = 120lb / ft 3 = 0.06944lb / in 3

(from table 2.1)

ρ=

0.06944 = 0.0000283 lb .sec 2 / in 4 2452.5

G=

0.0000283 × (10000) 2 = 283 psi 10

The surface impact load due to the weight of the falling body using equation 2.14 is

Pmax =

32(700, 000)(120)(283)(36) π 2 (1 − 0.37)

= 2098644.9 lb = 9335237.6 N

The coefficient of penetration, k, is 0.0482 for soil with vegetation Pa =

W 700, 000 = = 171.93 psi = 24757.44 psf 2 (π r0 ) (π 362 )

Now we have to determine the impact velocity V = (2 gH f )0.5 = ( 2 × 32.2 × 10 )

0.5

= 25.38 ft / sec

The penetration depth is calculated using equation 2.15 x p = (0.0482)(24757.44) log(1 +

25.382 ) = 1.55 ft = 0.47 m 215000

86

Which is safe as the pipe is buried under 3 ft (0.91 m) in soil

Buoyancy

The weight of water displaced by the pipe

Ww = 62.4* π *

3.52 = 600.358 lb / ft = 8760.1 N / m 4

The weight of the pipe

W p = 330.72 lb / ft = 4826.5 N / m

The effective weight of soil above the pipe

Ws = D (Pv − γ w hw ) = 3.5(405.6 − 62.4 ×1) = 1201.2 lb / ft = 17530.2 N / m

Since the sum of the pipe weight and the effective weight of the soil above the pipe is greater than the weight of the water displaced by the pipe, then there is no net buoyancy force acting on the pipe.

87

Thermal Stress An assumption is made that the maximum operating temperature is 86 F (30o C) and the installation temperature is 59 F (15o C) which gives a maximum compressive thermal stress that is calculated using Equation 2.19:

σ c = (29 × 106 )(6.345 × 10 −6 )(86 − 59) − 0.3(37361.8) = − 6240.41 psi = − 43.03 MPa

The negative sign means that this stress is compressive stress

Earthquakes By taking the moment magnitude Mw as 6.5, a source to site distance as 40 km and the sediment type as stiff soil. The ratio of Peak Ground Velocity (cm/sec) to Peak Ground Acceleration (g) will be 102 according to Table 2.6

Vs = PGV = 102 cm / sec.g (0.74 g ) = 75.48cm / sec

The induced axial strain then becomes: εa =

0.7548 = 0.0003774 2000

But ε a must be ≤

Tu λ 4AE

88

The peak friction force per unit length at soil-pipe interface is calculated using Equation 2.22: π DH γ (1 + K o ) tan δ 2 π 3.5 = (3.5)(3 + )(120)(1 + 1) tan 26.4o 2 2 = 3111.2 lb / ft

Tu =

With Tu = 2592.7 lb/ft, λ = 1.0 km = 3278 ft, E = 29 106 psi, and A = (42) (0.75) = 98.96 in2, the axial strain induced by friction at the soil pipe interface is 0.000888

Since ε a ≤

Tu λ , then the pipeline is safe against this earthquake 4AE

Conclusion Total Longitudinal Stresses (128.8) + (0.0183) + ( 0.0288 ) + ( 22.14 ) + (1.97 ) + ( −43.03) = 109.93MPa Total Hoop Stresses 257.6 MPa

Total Stress acting on the pipe

( 257.6 )

2

+ (109.93)2 = 280.08 MPa

Since the total stress acting on the pipe is less than the allowable pipe stress then the pipe is SAFE. 89

4.3. Material Selection for SUMED Pipeline In the selection of the suitable material for SUMED pipeline, API 5L will be used. Since the total stress calculated on the pipe is 281.74 MPa, then a material is needed must have a yield strength more than 281.74 But there must be a safety factor (assuming a safety factor of 1.4). So the total stresses including the safety factor is 394.4 MPa, when looking in the API 5L we find that the material grades that can be used are steel grades of X60, X65, and X70. Economically X60 is the best material and it is safe as it has minimum yield strength of 414 MPa and ultimate yield strength of 517 MPa. So the material that will be used in SUMED Pipeline is Steel API 5L grade X60

90

Chapter 5 Mechanical Design of Arab Gas Pipeline

5.1. Background The Arab gas pipeline is a pipeline that exports Egyptian natural gas to the Middle East and, once it has been extended, even to Europe. The first pipeline section runs from Al Arish in Egypt to Aqaba in Jordan. This section was completed in July 2003, costing $220 million. The annual capacity of this section is 1.1 bcm. The second section extended the pipeline from Aqaba to El Rehab in Jordan (24 km from the Syrian border). This pipeline, which cost $250 million, is 393 km long with a diameter of 36 inches. The second section was commissioned in 2005.The Egyptian consortium that developed this section included EGAS, ENPPI, PETROGET and GASCO.

The third section has a total length of 324 kilometers from Jordan to the Deir Ali power station in Syria. From there it would extend to the town of Rayan, Syria. In March 2006, Egypt, Syria, Jordan, Turkey, Lebanon and Romania reached an agreement to build the pipeline's extension through Syria to the Turkish border. From there, the pipeline will be connected to the planned Nabucco Pipeline, a proposed natural gas pipeline that is planned to transport natural gas from Turkey to Austria, for the delivery of gas to Europe. Turkey expects to buy 2-4 bcm of gas annually by the Arab Gas Pipeline. The pipeline route is shown in Figure 5-1, and includes all the 3 phases.

91

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

In the following section, a mechanical design will be made for the second phase of the Arab Gas Pipeline which is from El Aqaba to El Rehab in Jordan (24 km from the Syrian border).

92

5.2. Stress Analysis

For calculating the allowable pipe stress, a design factor of 0.72 will be used for new seamless pipes with a joint weld factor of 1.0, the allowable pipe stress is calculated from the following equation (Equation 2.1): S = design factor × SMYS × jo int weld factor = 0.72 × 65000 × 1 = 46800 psi = 322.7 MPa

When calculating the minimum wall thickness of Arab Gas pipeline, the internal pressure (100 bar), the diameter (0.914 m) and the pipe allowable stress must be known and the thickness is calculated using the following equation (Equation 2.3)

t=

PD 1450*36 = = 0.5577 in = 14.17 mm 2S 2*46800

But corrosion allowance and fabrication tolerance allowance must be added (12.5%) [2], so the total allowances are calculated from Equation 2.4: allowances = 0.125 × 0.5577 = 0.0697 in

t n = t + allowances = 0.5577 + 0.0697 = 0.627 in = 15.85 mm

But when using the API 5L Table 6C page 61, the nearest thickness is 0.625 in (15.875 mm). So this thickness will be used.

93

Moment of Inertia: The moment of inertia of the pipe is calculated using the following equation: Moment of Inertia I =

t 3 0.6253 = = 0.0203 12 12

Internal Pressure:

The internal pressure acts in both the axial and longitudinal directions The hoop stress which is in the axial direction is calculated using Equation 2.5:

σh =

PD 1450*36 = = 41760 psi = 287.9 MPa 2t 2*0.625

While the longitudinal stress is calculated using Equation 2.6:

σl =

PD 1450*36 = = 20880 psi = 143.95 MPa 4t 4*0.625

This is half the hoop stress

Since the hoop stress is less than the pipe allowable stress then the pipe is safe for this stress which is the largest among all other stresses acting on the pipeline

94

When the pipeline passes in the desert, then there will be no live loads, the modulus of soil reaction E’ for moderate coarse grained soil is 2000 psi, from Table 2.5. The specific weight of sand is γ sand = 100lb / ft 3 , from Table 2.1.

Dead Load

The Sand Pressure on the pipe (Vertical Earth Load) which is buried under 3 ft of sand is calculated using Equation 2.7:

Pv sand = 100 × 3 = 300 psf = 2.08 psi = 1.43 × 10 −2 MPa

Ovality and stress

The pipe ovality due to the earth’s dead load is calculated using Equation 2.10:

∆y 1*0.1*2.08 = = 0.00112 6 29*10 *0.0203 D + 0.061(2000) 213

The through wall bending stress is calculated using Equation 2.11:

σ bw = 4(29*106 ) (0.00112)(

0.625 ) = 2255.6 psi = 15.6 MPa 36

95

Ring Buckling Pressure

The critical ring buckling pressure is calculated using Equation 2.12 and Equation 2.13:

1

B'= 1 + 4e

( −0.065

36 ) 36

= 0.211

Pc = 32(1)(0.211)(2000)

(29 × 106 × 0.0203) = 412.8 psi = 2.85 MPa 363

Surface Impact Load An assumption that a load of 420 tons (840,000 lb) fall from a height of 15 ft (4.57 m) causing an impact area of 6 ft (1.82)in diameter is made. To calculate the maximum impact load, The mass density which is ρ = γ / g must be known, The soil’s Poisson ratio is 0.37, the shear wave velocity of near surface soils is Vs = 10,000 inches per second [2].

g = 9.81*100*2.5 = 2452.5 in / sec2

96

γ sand = 100lb / ft 3 = 0.05787lb / in 3

(from table 2.1)

ρ=

0.05787 = 0.0000236 lb .sec 2 / in 4 2452.5

G=

0.0000236 × (10000)2 = 236 psi 10

The surface impact load due to the weight of the falling body using equation 2.14 is

Pmax =

32(840, 000)(180)(236)(36) π 2 (1 − 0.33)

= 2493278.9 lb = 11090657.09 N

Where the coefficient of penetration, k, is 0.0367 for sandy soil [2] Pa =

W 840, 000 = = 206 psi = 29664 psf (π r02 ) (π 362 )

Now we have to determine the impact velocity V = (2 gH f )0.5 = ( 2 × 32.2 × 15 )

0.5

= 31 ft / sec

The penetration depth using equation 2.15 is x p = (0.0367)(29709) log(1 +

312 ) = 2.1 ft = 0.64 m 215000

Which is safe as the pipe is buried under 3 ft (0.91 m) in soil

97

Buoyancy Since the pipeline is in the desert there is no water in the 3 ft where the pipeline is buried so there is buoyancy stress on the pipeline

Thermal Stress An assumption is made that the maximum operating temperature is 86 F (30o C) and the installation temperature is 59 F (15o C) which gives a maximum compressive thermal stress that is calculated using Equation 2.19:

σ c = (29 × 106 )(6.345 × 10 −6 )(86 − 59) − 0.3(37361.8) = − 6240.41 psi = − 43.03 MPa

The negative sign means that this stress is compressive stress

Earthquakes The moment magnitude Mw is taken as 6.5, a source to site distance as 20 km and the sediment type as soft soil. The ratio of Peak Ground Velocity (cm/sec) to Peak Ground Acceleration (g) will be 140 according to Table 2.6

Vs = PGV = 140 cm / sec.g (0.74g ) = 103.6cm / sec

98

The induced axial strain using Equation 2.20 then becomes: εa =

1.036 = 0.000518 2000

But ε a must be ≤

Tu λ 4AE

The peak friction force per unit length at soil-pipe interface is calculated using Equation 2.22:

π DH γ (1 + K o ) tan δ 2 π 3 = (3)(3 + )(100)(1 + 1) tan 26.4o 2 2 = 2105.3 lb / ft

Tu =

With Tu = 2105.3 lb/ft, λ = 1.0 km = 3278 ft, E = 29 106 psi, and A = (36) (0.625) = 70.7 in2, the axial strain induced by friction at the soil pipe interface is 0.000842

Since ε a ≤

Tu λ , then the pipeline is safe against this earthquake 4AE

Conclusion Total Longitudinal Stresses (143.95) + (0.0143) + (15.6 ) + ( 2.85) + ( −43.03) = 119.4 MPa Total Hoop Stresses 287.9 MPa

99

Total Stress acting on the pipe

( 287.9 )

2

+ (119.4) 2 = 311.7 MPa

Since the total stress acting on the pipe is less than the allowable pipe stress then the pipe is SAFE.

5.3. Material Selection for Arab Gas Pipeline In the selection of the suitable material for Arab Gas Pipeline, the API 5L (Specification for Line Pipe) will be used. Since the total stress calculated on the pipe is 311.7 MPa, then a material is needed must have a yield strength more than 311.7 But there must be a safety factor (assuming a safety factor of 1.4).So the total stresses including the safety factor is 436.4 MPa, when looking in the API 5L we find that the material grades that can be used are steel grades of X65, and X70.

Economically X65 is the best material and it is safe as it has minimum yield strength of 448 MPa and ultimate yield strength of 531 MPa. So the material that will be used in Arab Gas Pipeline is Steel API 5L grade X65

100

Chapter 6 Conclusion

This project was about studying and calculating the different stresses acting on the pipelines and selecting the best material that can withstand these stresses. This Project has 2 cases studies. The first was SUMED pipeline and after performing stress analysis on the pipeline, a material was chosen from the standard API 5L which was API 5L X60 and this material can withstand the different stresses acting on SUMED pipeline. The second case study was Arab Gas pipeline (Second Phase) where a stress analysis was done and according to that analysis a material was chosen form the standard API 5L and this material was API 5L X65. The materials for both pipelines can withstand the different stresses that act on both pipelines so the mechanical design is safe.

101

References [1] Moser, A. P. Buried Pipe Design, 2nd Edition. [2] Alliance, A. L. (2001). Guidelines for the Design of Buried Steel Pipe. [3] Nayyar, M. L. Piping Handbook (7th Edition). McGraw-Hill. [4] Antaki, G. A. (2003). Piping and Pipeline Engineering. Dekker. [5] Menon, E. (2004). Liquid Pipeline Hydraulics. Dekker. [6] Abhinav Gupta, M. Response of Buried Pipelines to Surface Impact Loads. Department of Civil Engineering, NC State University. [7] Institute, A. P. (2000). API 5L (Specification for Line Pipe)(42nd Edition). [8] Engineers, T. A. (2002). ASME B31.4 (Pipeline Transportation Systems For Liquid Hydrocarbons and other Liquids). [9] http://www.ncl.ac.uk/marine/pipelineeng/whypipelines.htm. Retrieved from Centre for Pipeline Engineering. [10] OPEC. Retrieved from http://www.opec.org/library/FAQs/PetrolIndustry/q6.htm. [11] Retrieved from http://www.aopl.org/go/doc/888/57662/&printerfriendly=1. [12] Azevedo, C. R. (2006). Failure analysis of a crude oil. Engineering Failure Analysis , 978-994. [13] S. de Luna, J. F.-S.-C. (2000). An analysis of the static and dynamic fracture behaviour of a pipeline steel. International Journal of Pressure Vessels and Piping , 691-696. [14] Erling Østbya, K. J. (2005). Fracture response of pipelines subject to large plastic deformation under bending. International Journal of Pressure Vessels and Piping , 201-215. [15] Chen, S.-H. W. (2002). A study on the pre-cyclic-load-induced burst of creep deformation of a pipeline under subsequent static load. Material Science and Engineering , 144-151. [16] Yasuo Ogawaa, T. K. (2001). Structural design of buried pipelines for severe earthquakes. Soil Dynamics and Earthquake Engineering , 199-209. [17] Walter W. Chena, B.-j. S.-C.-H. (1209-1214). Seismic response of natural gas and water pipelines in the Ji-Ji earthquake. Soil Dynamics and Earthquake Engineering , 2002.

102

103