TABLE OF CONTENTS

61CHAPTER

6INTRODUCTION

82CHAPTER

8LITERATURE STUDY AT SSGC

82.1WELDING

82.2WELDING PROCESS

92.3ESSENTIAL VARIABLES

112.4ELECTRODE CLASSIFICATION

112.4.1Classification Number Designation

122.5WELDING PROCESS

122.5.1Gas Welding

122.5.2Arc Welding

142.5.3Resistance And Thermite Welding

142.6WELDING PASSES

142.6.1Root Pass

152.6.2Hot Pass

152.6.3Filler Pass

162.6.4Cover Or Cap Pass

162.7PIPE FABRICATION THROUGH WELDING

172.7.1Seamless Pipe

182.7.2Electrical Resistance Welded (ERW) Pipe

192.7.3U-O-E (or SAW) Longitudinally Welded Pipe

202.7.4Helical (Spiral) Welded Pipe

212.8WELDING DEFECTS

262.9INSPECTION TESTING OF WELD

262.9.1Radiography (X-rays and Gamma Rays)

292.9.2Ultrasonic Testing

312.9.3Hydrostatic Test

332.10CORROSION & DEGRADATION OF MATERIALS

332.11FORMS OF CORROSION

332.11.1Uniform Attack

342.11.2Galvanic Corrosion

352.11.3Crevice Corrosion

352.11.4Pitting

362.11.5Intergranular Corrosion

372.11.6Selective Leaching

372.11.7ErosionCorrosion

382.11.8Stress Corrosion

392.12CORROSION PREVENTION TECHNIQUES

412.13SELECTION OF MATERIALS

412.14PIPELINE MATERIALS SPECIFICATIONS

422.15MATERIAL PROPERTIES

432.16SUGGESTIONS FOR POSSIBLE FUTURE MATERIALS

442.17CATHODIC PROTECTION

442.18PRINCIPLE OF CATHODIC PROTECTION

452.18.1Electrochemical Concept

452.18.2Thermodynamic Concept

452.18.3Polarization Concept

462.19CATHODIC PROTECTION SYSTEM

462.20CATHODIC PROTECTION WITH GALVANIC ANODES

472.20.1Advantages and Disadvantages of Sacrificial System

482.21CATHODIC PROTECTION WITH IMPRESSED CURRENT

482.21.1Advantages and Disadvantages of Impressed Current System

492.22SACRIFICIAL ANODES

502.22.1Magnesium

512.22.2Zinc

512.22.3Aluminum

512.22.4Calculation For Current Output

522.22.5Calculation For Anode Life

522.23IMPRESSED CURRENT ANODES

522.23.1Graphite Anodes

522.23.2High Silicon Cast Iron Anodes

542.23.3Mixed-Metal Oxide Anodes

542.23.4Platinum Coated Anodes

542.23.5Lead Alloy Anode

542.23.6Current Requirement For Cathodic Protection

562.23.7Power Supply For Impressed Current Cathodic Protection System

602.24TYPES OF GROUND BEDS

612.24.1Deep Ground Bed

612.24.2Shallow Ground Bed

622.25CALCULATION FOR GROUND BED RESISTANCE

632.26CARBONACEOUS BACKFILL

632.26.1Types Of Backfill

642.27FIELD SURVEY AND MEASUREMENTS

642.27.1Soil Resistivity Survey

662.27.2Potential Survey

672.27.3Ohmic Error

682.27.4Line Current Survey

682.27.5Soil Acidity

692.27.6SRB Detection

692.28PROTECTIVE COATING

702.29PREPARATION FOR CATHODIC PROTECTION

702.29.1Sand blasting

702.29.2Coating

712.30HOLIDAY RESISTANCE

723CHAPTER

72EXPERIMENTAL WORK

723.1INTRODUCTION

723.2MATERIAL OF SAMPLE USED

723.3WELDING OF THE PIPE:

733.3.1Welding Procedure

743.4CORROSION PROTECTION

743.4.1Environment

743.4.2Anodes

753.5TESTING

753.5.1Hardness Test

783.5.2Corrosion Penetration Rate:

783.5.3Tensile Testing

803.5.4Impact Testing

823.5.5Metallography

854CHAPTER

85RESULT AND DISCUSSIONS

854.1INTRODUCTION:

854.2Testing of Welding:

854.2.1Tensile Test:

854.2.2Impact Test:

854.2.3Hardness Test:

864.2.4METALLOGRAPHY:

874.3TESTINGS FOR CORROSION:

874.3.1At the time of installment:

884.3.2After four months:

935CHAPTER

93CONCLUSION

94REFERENCES

LIST OF TABLES

10TABLE 21: Filler/Metal Strength Comparison In AWS (Group I)

10TABLE 22: Filler/Metal Strength Comparison In AWS (Group III)

12TABLE 23 The types of electrodes, dia & the type of passes for which it is used

50TABLE 24: The elemental composition of magnesium Anode

51TABLE 25: The elemental composition of Zinc anode.

56TABLE 26: The Current Required for Cathodic Protection

56TABLE 27: Different power supply system for ICCP and their input sources

62TABLE 28: Resistance in ohm of single vertical anode in 1000-ohm/cc Soil

LIST OF FIGURES16FIGURE 21: Showing the Welding Process of Gas Pipeline at SSGC

22FIGURE 22: Showing Welding Defect in Gas Pipeline

27FIGURE 23: X-Rays Film of Weld Taken at SSGC

29FIGURE 24: Equipment for Ultrasonic test at SSGC

31FIGURE 25: Equipment for Hydro testing at Crescent Steel Mills Noriabad

34FIGURE 26: Galvanic corrosion of a magnesium shell that was cast around a steel core.

35FIGURE 27: Plate which was immersed in seawater, crevice corrosion

36FIGURE 28: The pitting of a 304 stainless steel plate by an acid-chloride solution.

37FIGURE 29: Weld decay in a stainless steel. The regions along which the grooves

38FIGURE 210: Impingement failure of an elbow that was part of a steam condensate line.

39FIGURE 211: Photomicrograph showing intergranular stress corrosion cracking in brass.

46FIGURE 212: Cathodic Protection with Galvanic Anode

48FIGURE 213: Cathodic Protection with Impressed Current

58FIGURE 214: Rectifier at SSGC

59FIGURE 215: Shows Electronic Control Unit at SSGC

60FIGURE 216: Shows Modules for Solar System

67FIGURE 217: Shows Potential Survey at Site Visit

73FIGURE 31: Welded sample

76FIGURE 32: Vickers Diamond cone and Vickers Hardness Tester

77FIGURE 33: Hardness Tester

79FIGURE 34: Diagram Of Tensile Test Specimen

80FIGURE 35: Schematic diagram for tensile testing machine

81FIGURE 36: Specimen used for Charpy and Izod impact tests.

82FIGURE 37: Abrasive cutting machine

82FIGURE 38: Mounting Machine

83FIGURE 39: Grinding Machine

83FIGURE 310: Polishing machine

84FIGURE 311: Etching Agents

84FIGURE 312: Metallurgical Microscope

86FIGURE 41: Haz X100

86FIGURE 42: Parent Metal X100

86FIGURE 43: Welded X100

88FIGURE 44: Container & sample of water of unprotected samples (after 4 months)

88FIGURE 45: Container & sample of water in which samples are protected with Aluminum

88FIGURE 46: Container & sample of water in which samples are protected with Zinc

1 CHAPTER

INTRODUCTION

Welding is today used extensively for joining materials together and there is no doubt that it has been the most significant factor in the phenomenal growth of all industries. Very complex geometries can effectively be joined to give complete continuity in the structure and there are very fair fabrications which do not some where, contain a welded joint. Welding products range from the very sophisticated space vehicles to simple garden tools.

Welding applied to structural steel work has supplanted rivets and bolts, resulting in higher and cheaper fabrications with simpler connections. A welded structure because of its cleaner lines is more aesthetically pleasing to the eyes and leads to reduce, since corrosion traps are mainly eliminated. With the more efficient joints obtained by welding, continuous beams and girders may be of higher construction and the rigidity inherent in welded connections simplifies the design of buildings to plastic theory. Welding also allows greater freedom to the designer in that the method of joining; permits a wider choice in component parts and the geometry of the connections, he may require. Welding is also important for the aero planes, space shuttles and gas pipes. The tip or the front face of the space shuttle head is welded, so that it withstands the pressure. [6]Corrosion in Pakistan has not been assessed or documented but it can be assumed that it is not any way less serious than in other industrialized country. Corrosion is defined as the deterioration of a material because of reaction with its environment. To one degree or another, most materials experience some type of interaction with a large number of diverse environments. Often, such interactions impair a materials usefulness as a result of the deterioration of its mechanical properties (e.g., ductility and strength), other physical properties, or appearance. Occasionally, to the chagrin of a design engineer, the degradation behavior of a material for some application is ignored, with adverse consequences. [12]The first and important step for the pipe line network is the selection of material, that it should have the desire properties like toughness, ductility, strength, weldability, wear resistant and corrosion resistant. Furthermore it should also be economically available in the market. [10]Cathodic protection is a technique to protect a buried or immersed metallic structure by making it the cathode of a galvanic cell or by impressing a current from an external power source using an inert anode this technique has been successfully applied for more than 50 years and is now accepted as a proven and an established technology. In order to reduce corrosion to manageable levels, cathodic protection systems must be designed, engineered, installed, operated and maintained to high standards. [2]2 CHAPTER

LITERATURE STUDY AT SSGC

2.1 WELDING

INTRODUCTION XE "INTRODUCTION" Pipelines are constructed from approximately 12 meter double random lengths of pipe joined together by manual, semi automatic or automatic fusion welding. For the pipeline installation, contractor use to accept a range of pipes. The selection of welding method is determined by the contractors capability, the pipe diameter, wall thickness and, to a lesser extent, fabrication location. Welding is the critical step in pipe lying because it dictates the length of time to form the pipeline and a major impact on the cost of the project.

New welding processes are developed nowadays e.g. Friction welding, Flash butt welding and Laser ion beam welding. [5]2.2 WELDING PROCESS

Welding is a process of joining metal which produces coalescence of the material across the joints by heating to a suitable temperature with or without pressure and with or without the addition of filler metal. Coalescence is the growing together of the grain structure of the metals being welded. There are three critical parameters:

Heat input: sufficient energy must be provided to melt the metal and consumable (W/ m2).

Heat input rate: the rate of energy input controls the rate of welding (W / m2/m/s).

Shielding from the atmosphere: prevents oxidation of the molten melt which would produce a weak weldment. [4]

2.3 ESSENTIAL VARIABLESSome essential variables related to welding are:

1. Welding process or method of application:

A change from the welding process or method of application established in the procedure specification constitutes an essential variable.

2. Base Material:

A change in base material constitutes an essential variable. When welding materials of two separate material groups, the procedure for the higher strength group shall be used. For the purpose of this standard, all materials shall be grouped as follows

Specified minimum yield strength less than or equal to 42,000Psi (290MPa).

Specified minimum yield strength greater than 42,000psi but less than 65,00psi (448MPa)

For materials with specified minimum yield strength greater than or equal to 65,000psi (448MPa) each grade shell receive a separate qualification test.

3. Joint Design:

A major change in joint design (for example from V groove to U groove) constitutes an essential variable. Minor changes in the angle of bevel or the land of the welding groove are not essential variables.

4. Position:

A change in position from roll to fixed, or vice versa constitutes an essential variable for welding.

5. Wall Thickness:

A change from one wall thickness group to another constitutes an essential variable for welding.

6. Filler Metal:

The following changes in filler metal constitute an essential variable:

A change from one filler metal group to another.

For pipe materials with specified minimum yield strength greater than or equal to 65,000Psi a change in the AWS classification of the filler metal, changes in filler metal with in filler metal groups may be made.

TABLE 21: Filler/Metal Strength Comparison in AWS (Group I)

TABLE 22: Filler/Metal Strength Comparison in AWS (Group III)

7. Electrical Characteristics:

A change from DC electrode positive to DC electrode negative or a change in current from DC to AC constitutes an essential variable.

8. Time Between Passes:

An increase in the maximum time between completion of the root bead and the start of the second bead constitutes an essential variable.

9. Direction Of Welding:

A change in the direction of welding from vertical downhill to vertical uphill, or vice versa, constitutes an essential variable.10. Shielding Gas And Flow Rate:

A change from one shielding gas to another or from one mixture of gasses to another constitutes an essential variable. A major increase or decrease in the range of flow rates for the shielding gas also constitutes an essential variable.

11. Shielding Flux:

Change in shielding flux is also an important variable.

12. Speed Of Travel:

Change in the speed of travel also constitutes an important variable.

13. Preheat:

A decrease in the specified minimum preheat temperature constitutes an essential variable.

14. Post Weld Heat Treatment (PWHT):

The addition of PWHT or a change from ranges or values specified in the procedure shall each constitutes an essential variable. [15]2.4 ELECTRODE CLASSIFICATION

Electrode classification fall mainly into two categories: the group that will weld satisfactorily in all positions and the group that will weld satisfactorily only in flat and horizontal positions. The E6010, E6011, and E6012 group falls into the all position category. The E7024 type falls into the flat and horizontal welding position category. They are all carbon steel core type electrodes. There are many other classifications and sub classifications, but a good knowledge of the few basic electrodes is sufficient for a well rounded understanding of the different classification working conditions. [4]2.4.1 Classification Number Designation

The letter E always designates electrodes. The first two digits designate the tensile strength in thousands of pounds per square inch (psi). For example, if the first two digits are 60, the minimum tensile strength of the deposited metal would be 60,000 psi. The third digit 1 always designate that the electrode will weld satisfactorily in all positions and a 2 always designates that the electrode will not weld satisfactorily in all positions. The fourth digit 0, 1, 2, or 4designates the type of flux coating and welding current to be used. The fourth digit cannot be considered individually but must be combined with the third digit, not only to help indicate the type of flux coating, but also if more than one type of welding current can be used successfully. It will also give a clue as to the resulting bead face, surface variation, and penetration pattern. [4]Type of electrodeDiaType of passes

E-6010

E-60101/8

5/32Up Hill

Down Hill

E-7010

E-8010

E-90105/32

Hot Pass

E-7010

E-8010

E-90103/16

Filling & Capping

TABLE 2:3: The types of electrodes, Dia & the type of passes for which it is used2.5 WELDING PROCESS

The welding processes most commonly employed today include gas welding, arc welding, and resistance welding. Other joining processes include thermite welding, laser welding, and electron-beam welding.

2.5.1 Gas Welding

Gas welding is a non-pressure process using heat from a gas flame. The flame is applied directly to the metal edges to be joined and simultaneously to a filler metal in wire or rod form, called the welding rod, which is melted to the joint. Gas welding has the advantage of involving equipment that is portable and does not require an electric power source. The surfaces to be welded and the welding rod are coated with flux, a fusible material that shields the material from air, which would result in a defective weld.

2.5.2 Arc Welding

Arc-welding processes, which have become the most important welding processes, particularly for joining steels, require a continuous supply of either direct or alternating electrical current. This current is used to create an electric arc, which generates enough heat to melt metal and create a weld.

Arc welding has several advantages over other welding methods. Arc welding is faster because of its high heat concentration, which also tends to reduce distortion in the weld. Also, in certain methods of arc welding, fluxes are not necessary. The most widely used arc-welding processes are shielded metal arc, gas-tungsten arc, gas-metal arc, and submerged arc.

2.5.2.1 Shielded Metal Arc

In shielded metal-arc welding, a metallic electrode, which conducts electricity, is coated with flux and connected to a source of electric current. The metal to be welded is connected to the other end of the same source of current. By touching the tip of the electrode to the metal and then drawing it away, an electric arc is formed. The intense heat of the arc melts both parts to be welded and the point of the metal electrode, which supplies filler metal for the weld. This process, developed in the early 20th century, is used primarily for welding steels.

2.5.2.2 Gas-Tungsten Arc

In gas-tungsten arc welding, a tungsten electrode is used in place of the metal electrode used in shielded metal-arc welding. A chemically inert gas, such as argon or helium, is used to shield the metal from oxidation. The heat from the arc formed between the electrode and the metal melts the edges of the metal. Metal for the weld may be added by placing a bare wire in the arc or the point of the weld. This process can be used with nearly all metals and produces a high-quality weld. However, the rate of welding is considerably slower than in other processes.

2.5.2.3 Gas-Metal Arc

In gas-metal welding, a bare electrode is shielded from the air by surrounding it with argon or carbon dioxide gas or by coating the electrode with flux. The electrode is fed into the electric arc, and melts off in droplets to enter the liquid metal that forms the weld. Most common metals can be joined by this process.

2.5.2.4 Submerged Arc

Submerged-arc welding is similar to gas-metal arc welding, but in this process no gas is used to shield the weld. Instead, the arc and tip of the wire are submerged beneath a layer of granular, fusible material formulated to produce a proper weld. This process is very efficient but is generally only used with steels.

2.5.3 Resistance And Thermite Welding

In resistance welding, heat is obtained from the resistance of metal to the flow of an electric current. Electrodes are clamped on each side of the parts to be welded, the parts are subjected to great pressure, and a heavy current is applied briefly. The point where the two metals meet creates resistance to the flow of current. This resistance causes heat, which melts the metals and creates the weld. Resistance welding is extensively employed in many fields of sheet metal or wire manufacturing and is particularly adaptable to repetitive welds made by automatic or semiautomatic machines.

In thermite welding, heat is generated by the chemical reaction that results when a mixture of aluminum powder and iron oxide, known as thermite, is ignited. The aluminum unites with the oxygen and generates heat, releasing liquid steel from the iron. The liquid steel serves as filler metal for the weld. Thermite welding is employed chiefly in welding breaks or seams in heavy iron and steel sections. It is also used in the welding of rail for railroad tracks. [16]2.6 WELDING PASSES

Usually welding process is carried out in so many passes some of them are explained below:2.6.1 Root Pass

The root pass is the initial and most critical weld. Because this weld is laid down in a straight line without weaving of the weld bead the root pass is sometimes termed the stringer bead. Conventionally the root pass weld is started at the top of the pipe at the 12 oclock position and is run down to the bottom, 6 oclock position in straight line with the brother in law team of two welders working one on each side of the pipe. This is down hand welding and is the fastest procedure. Unhand welding is slightly slower but uses larger electrodes and higher heat input, which reduces the risk of hydrogen cracking.

For pipes of diameter above 8 in. the welder start and finish together as this keeps the welding stresses in balance. Up to four welding positions may be needed on very large diameter pipes both for speed and stress balance. It is vital that this weld completely fuses the inner faces of the pipe joints without leaving unfused areas or excessive weld metal protruding into the pipe, termed icicles. These metal protrusions may initiate corrosion and will damage pigs. For large diameter pipes it is possible to place the root pass from the inside of the pipe as this avoid formation of the icicles. However this option requires complex equipment and is only cost effective for large diameter pipe and long length pipelines.

Semi automatic equipment is commonly used for the root pass though full automatic equipment is available and is used when the root pass is formed from the inside of the pipe. After the stringer bead or root pass is completed the internal clamp is released and moves through the pipe to position the next joint as the pipe is advanced. [15]2.6.2 Hot Pass

To eliminate any risk of cold cracking of the root pass and the HAZ by hydrogen a second weld is applied as soon as possible over the root pass. The hot pass remelts the first weld slightly and heat treats the HAZ. As a general rule the temperature of the root pass should not be allowed to fall below 100 25 0C if cellulosic electrodes are used to ensure that hydrogen migration is effective. As the pipeline strength increases the inter pass temperature needs to increase, so for welding grade x65 and above the temperature require would be about 150 25 0C. The hot pass is usually placed within 4 to 5 minutes of completing the root pass. The maximum allowable delay to avoid falling below the minimum interpass temperature is about 10 minutes. During the cooling period the root pass must be cleaned to bare metal to remove lateral slag which induces wagon tracks, named because of their appearance in the X-ray film. If the root pass is allowed to cool then it may have strength of 130 MPa above that of the finished joint. If there is an uncontrolled movement of the pipe then the weld can crack. This would necessitate parting the pipe, rebevelling, inspecting and re welding. [15]2.6.3 Filler Pass

The filler weld passes are less critical than the root and hot pass and automatic and semi-automatic welding machines are often used which can lay down rapid volumes of weld metal. The filler passes need to be made with slight weave: the movement of the molten filler metal from side to side. Weaving helps to insure complete fusion of the bevel walls. Between each pas the welds must be cleaned to bare metal. Sometimes additional welds are needed at the sides to build up the weld to the correct thickness. The welding procedure can leave variations in the thickness of the weldment so stripper passes may be needed to even up the thickness of the weldment before the cap pass is applied. [15]

FIGURE 21: Showing the Welding Process of Gas Pipeline at SSGC

2.6.4 Cover Or Cap Pass

The cap or cover pass is the final welds. The cap pass is run around the pipe to fill the residual grove, leaving the weld 1 to 1.5 mm above the pipe surface and with an overlap on the outside surface of the pipe around 1 to 2 mm. If manual arc welding is used a typical electrode size is 5 mm. slightly lower amperage is used to reduce porosity that can occur from overheating of the weld deposits or from excessive weaving. Care is also required to ensure that the overlap of the cap pass is fully fused with the parent pipe. [15]2.7 PIPE FABRICATION THROUGH WELDING

Pipe for the oil and gas industry in largely restricted to four fabrication routes: seamless, longitudinally welded by electrical resistance welding, helical or spiral welded and longitudinally welded using submerged arc welding. Production of pipe by furnace butt-welding of hot plate, though permitted, cannot produce the large diameters required.

Nowadays many pipe fabricators are independent and not part of an integrated steel company. Fabricators also tend to specialize in certain pipe fabrication techniques and few produce pipe material over the full range of diameters and wall thickness. The Oil and Gas Journal make a regular survey of pipe fabricators but this listing is not comprehensive as many of the East European and Far Eastern fabricators are omitted from the list. Independent pipe mills form pipe from plate or coiled plate that is bought from whatever source can provide suitable material, at an acceptable price. For the production of a large quantity of pipe the plate may be sourced from several steel suppliers. The mechanical properties of the pipe will be consistent with specification but caution is necessary regarding the weld procedures as small variations in composition can affect the quality of the weld achieved. [13]2.7.1 Seamless Pipe

Seamless pipe is formed by hot working steel to form a pipe without a welded seam. The initially formed pipe may be subsequently cold worked to obtain the required diameter and wall thickness and heat treated to modify the mechanical properties. A solid bar of steel, termed a billet, is cut from a slab and is heated and formed by rollers around a piercer to produce a length of pipe. The Mannesmann mill is perhaps the best known type of piercing mill. In this mill the steel billet is driven between rotating, barrel-shaped rolls set at a slight angle to each other. The rolls rotate at about 100 150 rpm and the billet also rotates. The piercer is placed just beyond the point where the billet is squeezed by the rolls so that as the formed billet passes through the pinch zone between the two rollers the reducing stress tend to open the metal over the piercer.

The piercing mill produces the primary tube that requires finishing forming the pipe. The wall thickness is further reduced and the pipe finished in plug rolling mills that drive the pipe over long mandrels fitted with plugs of the correct internal diameter between rollers that extrude the tube to the required external diameter. An older process is the Pilger process. This process uses eccentric rolls to form the pipe in discrete stages. A mandrel is inserted into the partly formed pipe from the piercing mill. The assembly is driven into the open rolls and, as the rolls rotate back and forth, sequential sections of the pipe are drawn into the eccentric rolls and outer diameter formed to the required dimension set by the roller eccentricity.

This type of pipe is generally available in diameters up to 16-inch but can be obtained in sizes up to a maximum of 28-inch from a restricted number of suppliers Its principal advantages are its good track record in service and that there are no welds in the pipe sections.

The larger diameter seamless pipe may be more expensive than pipe fabricated by the alternative process. Disadvantages of seamless pipe are a fairly wide variation of wall thickness, typically +15% - 12.5% and out-of-roundness and straightness. [13]2.7.2 Electrical Resistance Welded (ERW) Pipe

ERW pipe is formed from coiled plate steel. The plate is uncoiled and sheared to a convenient workable length, flattened and the edges dressed. The plate is passed through a sequence of rolls to form the pipe. The sequence of rolls crimps the edges of the plate and then progressively bends the body of the plate into a circular form ready for welding of the longitudinal seam. The longitudinal seam weld is made by electrical resistance welding (hence the name of the pipe). When a new coil of plate is started it is welded to the end of the previous coil to allow it to be pulled through the rolling mill. The pipe formed with a joint in the middle (a jointer) is generally not accepted for use for pipelines.

An electrical current is passed across the interface to heat the steel pipe faces that are to be ERW welded. Once molten the faces are pressed together to produce the longitudinal seam weld. The heating may be by low frequency AC current, typically 60 360 Hz, introduced directly into the pipe by rolling contacts or induced into the steel with induction coils operating at high frequencies of above 400,000 Hz. The later process of producing the pipe is term high frequency induced (HFI) ERW pipe.

Pipe for oil and gas pipelines is almost exclusively produce using high frequency induction welding. Two American, three European and four Japanese manufacturers presently produce the majority of this type of ERW pipe.

The pressure exerted on the faces during the weld forming process result in the molten metal at the faces being squeezed outward to form stubs of metal above and below the weld. Any debris or oxides on the steel faces is discharged in the stubs of metal. The stubs of metal are trimmed off and the weld is inspected using ultrasonic probes. The weld is then locally heat treated to anneal the weld and heat-affected zone.

The weld is extremely fine as the bulk of the molten metal is squeezed out. It is not possible to detect the weld by eye and it is prudent to specify that a paint line mark the weld line. Samples of weld are cut from the ends of pipe for metallographic inspection, analysis, tensile, ductile and toughness testing. The invisibility of the weld has lead to attempt to pass off ERW pipe as seamless.

ERW pipe is the main competitor to seamless pipe. It is cheaper than seamless and it can have considerably tighter tolerances on wall thickness. Pipe lengths are typically standard length 50 mm and the pipe can be produced in lengths up to 27. Through API Specification 5L permits a wide tolerance on wall thickness, +19.5/-8%, typical modern wall thickness tolerances are 5 %. It is also claimed that pipe wall thickness can be specified to 0.1 mm and non-API Specification 5L sizes are available. These tight tolerances can have a cost benefit as the smaller tolerance in wall thickness and circularity permit a more rapid set up and lend themselves to semi-automatic and automatic welding processes. [14]2.7.3 U-O-E (or SAW) Longitudinally Welded Pipe

U-O-E pipe is formed from individual plates of steel by firstly forming plate into a U, then into a tube (O). After longitudinally welding, the pipe is then expanded (E) to ensure circularity. Because the longitudinal weld is produced using the submerged arc welding process the pipe is sometimes termed SAW pipe.

Submerged arc welding is process where an electric arc is submerged or hidden beneath a granular material. The electric acc provides the necessary heat to melt and fuse the metal. The granular material, called flux; completely surrounds the electric arc, shields the arc and the metal from the atmosphere. A metal wire is fed into the weld zone underneath the flux.

Tab plates are fixed to the steel plate and the plate cut to exact size and the edges dressed. The edges are then crimped and the complete plate is progressively bent into a U-shape and then into a tube in presses. The O-press leaves a residual 0.2 0.4% compression in the pipe. A higher compression is provided for pipe for sour service. The butting edges are tack welded at the tab plates to prevent movement during the main welding. The butting edges of the tube are then welded along using submerged arc welding with multiple head welding devices. At least two welding passes are made. First the internal weld is formed and then the pipe is rotated through 180 and a second external weld pass made. The tab plates are provided to allow the weld to start and finish beyond the end of the pipe to ensure a quality weld at the start and end of the pipe.

U-O-E pipe is used for the larger diameter pipelines. It is competitive with seamless pipe for the intermediate diameters (14-inch to 28-inch). For the smaller diameter pipes the pipe fabricator may use cut down plate because producing narrow plate is less economical. Because inclusions and segregation tend to concentrate at the centre of the plate a pipe formed from a split plate may have inclusions and segregation adjacent to the weld. Such pipe may be unsuitable for sour service. If split plates are to be used then an odd number should be cut to avoid the centre line of the original plate abutting the longitudinal weld. [13]2.7.4 Helical (Spiral) Welded Pipe

A coil of hot-coiled plate is uncoiled, straightened and flattened and the edges dressed. The plate is then helical wound to form a pipe. The width of the strip and the angle of coiling determine the pipe diameter. As the pipe is formed the helical seam is welded using inert gas welding or submerged arc welding (SAW) first internally and as the seam rotates to the top position the external weld is made. A continuous length of pipe is produced. After forming the pipe is passed through a sequence of rollers to ensure circularity.

The pipe weld is tested using radiography or ultrasonic testing and the pipe is then cut to the required lengths. The pipe joints are hydrostatically tested before being re-inspected. If the pipe passes inspection then it is end-faced or beveled, the end protector caps are fitted and pipe transported to the pipe racks.

The end of the coiled plate is welded to the start of the next pate coil and this result in a weld perpendicular to the helical weld forming the pipe. This weld joint should not be less than 300 mm from the end of the pipe. This weld may not receive the same degree of scrutiny as the helical welds and the pipe specification may need to call for an additional inspection of these welds after the hydro testing stage.

Helical welded pipe can be made in a wider range of diameters and wall thickness than nominal API Specification 5L sizes. It can also be produced in long lengths above the normal double random length of 12 m. because the pipe is formed from late the wall thickness tolerances are good being similar to U-O-E pipe though there may higher out-of-roundness. It has been used for large diameter pipelines, both crude oil and gas, but is generally considered a less reliable material than U-O-E formed pipe. It is cheaper than U-O-E and is widely used for caissons, sleeves, low-pressure hydrocarbon service, dried gas service and water transportation where the service is moderate. [13]

2.8 WELDING DEFECTS

Different welding defects are listed below

1. Inadequate Penetration without High-low (IP):

Inadequate penetration without high-low (IP) is defined as the incomplete filling of the weld root. IP shall be considered a defect should any of the following conditions exist:

a) The length of an individual indication of IP exceeds 1 in. (25mm).

b) The aggregate length of indications of IP is any continuous 12 in. (300) length of weld exceeds 1 in. (25mm).

c) The aggregate length of indications of IP exceeds 8% of the weld length in any weld less than 12 in. (300mm) in length. [6]2. Inadequate Penetration Due to High-low (IPD):

Inadequate penetration due to high low (IPD) is defined as the condition that exists when one end of this is exposed (or unbonded) because adjacent pipe or fitting joints are misaligned. IPD shall be considered a defect when any of the following conditions exist:

a) The length of an individual indication of IPD exceeds 2 in. (50mm).

b) The aggregate length of indications of IPD in any continuous 12 in. (300)

c) Length of weld exceeds 3 in. (75mm). [6]3. Inadequate Cross Penetration:

Inadequate cross penetration is defined as face imperfection between the first inside pass and the first outside pass that is caused by inadequate penetrating the vertical land faces. ICP shall be considered a defect if any of the following conditions exists:

a) The length of an individual indication of ICP exceeds 2 in. (50mm).

b) The aggregate length of indications of ICP in any continuous 12 in. (300) length 2 in. (50mm). [6]4. Incomplete Fusion:

Incomplete fusion is defined as a surface imperfection between the weld metal and the base material that is open to the surface. Incomplete fusion is considered a defect if any one of the following conditions exists:

a) The length of an individual indication of IF exceeds 1 in. (25mm).

b) The aggregate length of indications of IF in any continuous 12 in. (300mm) length of weld exceeds 1 in.

c) The aggregate length of indications of IF exceeds 8% of the weld length in any weld less than 12 in. in length. [15]5. Incomplete Fusion Due to Cold Lap:

Incomplete fusion due cold lap is defined as an imperfection between two adjacent weld beads or between or between the weld metal and the base metal that is not open to the surface. . IFD shall be considered a defect if any of the following conditions exists:

a) The length of the individual indication of IFD exceeds 2 in. (50mm).

b) The aggregate length of indications of IFP in any continuous 12 in. length of weld exceeds 2 in.

c) The aggregate length of indications of IFD exceeds 8% of the weld length.

6. Internal Concavity:

Internal concavity is defined as that a bead that is properly fused to and completely penetrates the pipe wall thickness along both sides of the bevel, but whose center is somewhat above the inside surface of the pipe wall. The magnitude of concavity is the perpendicular distance between an axial extension of the pipe wall surface and the lowest point on the weld bead surface. [15]

FIGURE 22: Showing Welding Defect in Gas Pipeline

7. Burn Through:

A burn through is defined as a portion of the root bead where excessive penetration has caused the weld puddle to be blown into the pipe. For pipe with an outside diameter greater than or equal to 2.375 in. (60.3mm), a BT shall be considered if any of the following conditions exist:

a) The maximum dimension exceeds in. (6mm) and the density of the BT image exceeds that of thinnest adjacent thinnest material.

b) The maximum dimension exceeds the thinner of the nominal wall thickness joined, and the density of the Butt image exceeds that of the thinnest adjacent parent material.

c) The sum of the maximum dimensions of separate BT whose image density exceeds that of the thinnest adjacent parent material exceeds in. (13mm) in any continuous 12 in. length of weld or the total weld length, whichever is less.

For pipe with an outside diameter less than 2.375 in. BT shall be considered a defect when any of the following conditions exists:

a) The maximum dimension exceeds in. (6 mm) and the density of the BT image exceeds that of the thinnest adjacent parent material.

b) The maximum dimension exceeds the thinner of the nominal wall thicknesses joined, and the density of the BT image exceeds that of the thinnest of the adjacent material.

c) More than one BT of any size is present and the density of the more than one of the images exceeds that of the thinnest adjacent parent material. [15]8. Slag Inclusions:

A slag inclusion is defined as a non metallic solid entrapped in the weld metal or between the weld metal and the parent material. Elongated slag inclusions (ESI) e.g. continuous or broken slag lines or wagon tracks are usually found at the fusion zone. Isolated slag inclusions (ISI) are irregular shaped and may be located anywhere in the weld. For evaluation purposes, when the size of a radiographic indication of slag is measured, the indication maximum dimension shall be considered its length.

For pipe with an outside diameter greater than or equal to 2.375 in. (60.3 mm), slag inclusions shall be considered a defect should any of the following conditions exist;

a) The length of an ESI indication exceeds 2 in. (50 mm).

b) The aggregate length of ESI indications in any continuous 12 in. length of weld exceeds 2 in.

c) The width of an ESI indication exceeds 1/16 in. (1.6 mm).

d) The aggregate length of ISI indications in any continuous 12 in. length of weld exceeds in. (13 mm).

e) The width of an ISI indication exceeds 1/8 in. (3 mm).

f) More than four indications of an ISI with the maximum width of 1/8 in. are present in any continuous 12 in. length of weld.

g) The aggregate length of ESI and ISI indications exceeds 8% of the weld length.

For pipe with an outside diameter less than 2.375 in. ( 60.3 mm ), slag inclusion shall be considered a defect if any of the following conditions exist;

a) The length of an ESI indication exceeds 3 times the thinner of the nominal joined.

b) The width of an ESI indication exceeds 1/16 in. (1.6 mm).

c) The aggregate length ISI indications exceeds 2 times the thinner of the nominal wall thicknesses joined and the width exceeds the thinner of the nominal wall thicknesses joined.

d) The aggregate length of ESI and ISI indications exceeds 8% of the weld length. [15]9. Porosity:

Porosity is defined as gas trapped by solidifying weld metal before the gas has a chance to rise to the surface of the molten puddle and escape. Porosity is generally spherical but may be elongated or irregular in shape, such as piping (wormhole) porosity. When the size of the radiographic indication produced by a pore is measured, the maximum dimensions of the indication shall apply to the criteria a through b. Individual or scattered porosity shall be considered a defect should any of the following conditions exist;

a) The size of an individual pore exceeds 1/8 in. (3 mm).

b) The size of an individual pore exceeds 1/8 in. of the thinner of the nominal wall thicknesses joined.

Cluster porosity (CP) that occurs in any pass except the finish pass shall comply with the criteria of a CP that occurs in the finish pass shall be considered a defect should any of the following conditions exist:

a) The diameter of the cluster exceeds in. (13 mm).

b) The aggregate length of CP in any continuous 12 in (300mm) length of weld exceeds in (13mm).

c) An individual pore with in a cluster exceeds 1/16 in (2mm) in size.

Hollow bead porosity is defined as elongated linear porosity that occurs in the root pass.

HB shall be considered a defect if any one of the following conditions exists:

a) The length of individual indication of exceeds in (13mm).

b) The aggregate length of indications of HB in any continuous 12 in (300mm) length of weld exceeds 2 in. (50mm).

c) Individual indications of HB, each greater than inches (6 mm) in length, are separated by less than 2 in. (50mm).

d) The aggregate length of all indications of HB exceeds 8% of the weld length.

10. Cracks:

Cracks shall be considered a defect should any of the following conditions exists:

a) The cracks, of any size or location in the weld, are not shallow crater crack or star crack.

b) The crack is the shallow crater crack or star crack with a length that exceed 5/32 in (4mm). [4]11. Undercutting:

Undercutting is defined as a groove melted into the parent material to the toe or roots of the weld and left unfilled by weld metal. Undercutting adjacent to the cover pass (EU) or root pass (IU) shall be considered a defect if any of the following conditions exists:

a) The aggregate length of indications of EU and IU in any combination, in any continuous 12 in. (300mm) length of weld exceeds 2 in. (50mm).

b) The aggregate length of EU and IU, in any combination, exceeds 1/6 of the weld length. [4]

12. Accumulation of Imperfections:

Excluding incomplete penetration due to high-low and under cutting, any accumulation of imperfections (AI) shall be considered a defect if any of the following conditions exists:

a) The aggregate length of indications in any continuous 12 in. (300mm) length of weld exceeds 2 in. (50mm).

b) The aggregate length of indications exceeds 8% of the weld length.[4]13. Pipe or Fitting Imperfections:

Imperfections in pipe or fittings detected by radio-graphic testing shall be reported to the company. Their disposition shall be as directed by the company. [4]2.9 INSPECTION TESTING OF WELD

2.9.1 Radiography (X-rays and Gamma Rays)

X-rays are an electromagnetic radiation delivered in quanta or parcels of energy is opposed to continuous delivery. They move at the speed of light in straight lines; are invisible; are not deviated by a lens; ionize or liberate electron from matter through which they can pass and they destroy living cells.

Gamma rays are similar to X-rays but differ in wavelength, X-rays having a continuous or broad spectrum while Gamma rays are made up of isolated wavelengths and have a line spectrum depending upon the element used. Iridium has two distinct types of atoms, one with a mass number of 191 and the other with a mass number of 193. The later has extra two neutrons in its nucleus. These are stable isotopes suffer radioactive decay or change into the stable form over a period of time and the type of radiation and the period of time for which it is given out determines its suitability for a particular use. [11]2.9.1.1 X-rays Method

X-rays are produced by an X-ray tube which consists of an evacuated glass bulb with two arms. One arm houses the cathode, a filament which is heated by an electric current as in electric light bulb, and this heated filament gives off a stream of an electrons.

FIGURE 23: X-Rays Film of Weld Taken at SSGC

In the other arm is the anode, which is a metal stem. By placing a high voltage of the order of 30 to 500 KV and upwards between anode and cathode the electrons are attracted at high speed to the anode and are focused into a beam by means of a focusing cup. Fixed in the anode at an angle to the electron beam is the anti cathode. This is a dense, high melting point slab of metal such as tungsten, on to which the electron beam impinges and is arrested. The resulting loss of kinetic energy appears as heat and X-rays and the later emerge from the tube at right angles to its axis. The tube current, which indicates the intensity of flow of the electrons, is in mill amperes and the intensity of the radiation is somewhat proportional to this MA value.

The rays can penetrate solid substances but, in doing so, a certain portion of the rays is absorbed and the amount of absorption depends upon the thickness of the substance and its density. The denser and thicker the substance, the smaller the proportion of X-rays that will get through. X-ray film is made many layers on a base of cellulose triacetate or polyester, the small silver halide crystals which are sensitive to the X-rays being suspended in a gelataine.

The film is placed in a rigid or flexible cassette with intensifying screen on either side so as to improve the image. The weld or object to be radiograph is placed in the cassette in the path of the rays. And after exposure for a short time, depending upon the thickness or object, the film is developed either manually or automatically. The weld will appear as a light bend across the X-ray negative. Any defect in the weld can be seen as a dark area of faults such as blow holes, porosity, and slag inclusion. [15]2.9.1.2 Gamma-Ray Method

Like X-rays gamma rays show a shadow graph on a sensitized film and are interpreted in the same way. The advantages of radioisotope sources for radiographic purposes are that they need no power supply or cooling system. Their small focus makes them very suitable for weld inspection in narrow pipes and because some radio isotopes have high power of penetration, thick specimens can be radio graphed at shortened exposure time. They have, however, harder radiation than an X-ray tube so that the image has less contrast and interpretation is more difficult. Also the activity decreases appreciably with those radio isotopes that have a short half life so that their radioactivity depends upon the time, since renewal and a time- activity curve must be consulted when using them. The radioactivity of the source cannot be varied or adjusted and since it cannot be switched off, it has to be effectively shielded.

The radioactive source is a pellet of a substance in a welded stainless steel container about 15 mm long by 5mm diameter. The pellet is a cylinder of the pure metal cobalt-60 and iridium-192 and a pressed and sintered pellet of thulium dioxide thulium -170. These radioactive pellets dont induce radioactivity in the container and the source can be returned. After a certain period depending upon its half life, to the makers to be re energized in an atomic reactor.

The source must be stored inside a container with a dense radiation shield, usually made of lead, tungsten or even depleted uranium where it is kept until actually in use. One type has shutter mechanism for exposure, another type has the source mounted inside the removable portion of the shield, which can be detached and used like a torch so that the radiation appears forwards, away from the operators body and shielded in the backwards direction. This type is useful for most work, including pipe welds.

A third type has the radio isotope mounted on a flexible cable and contained within a shielded container. It can be pushed along the guide tube by remote control and can be positioned in otherwise awkward places. With this type, positioning and source changing is easily performed. Pipeline crawlers for various diameter pipes are used, carrying the radio isotope and enabling it to be positioned in the pipe center to give a radial beam of radiation when exposed. The film is placed around the outside of the pipe enabling the radio inspection at that point to be performed with one exposure. The crawler can be battery operated and travels on wheels with forward, reverse, expose and stop controls, the positioning within the pipes being controlled to a few millimeters accuracy. [15]2.9.2 Ultrasonic Testing

Ultrasonic testing applies waves above the frequency limit of human audibility and usually in the frequency range 0.6 to 5 MHz. A pulse consisting of number of these waves is projected into the specimen under test. If a flaw exists in the specimen an echo is reflected from it and from the type of echo the kind of flaw that exists can be deduced.

FIGURE 24: Equipment for Ultrasonic test at SSGC

The equipment consists of an electrical unit which generates the electrical oscillations, a cathode ray tube on which pulse and echo can be seen, and probes which introduce the waves into the specimen and receive the echo. The electrical oscillations are converted into ultrasonic waves in a transducer which consist of piezo-electric element mounted in a Perspex block to form the probe, which in use, has its own face pressed against the surface of the material under test. When a pulse is injected into the specimen a signal is made on the cathode ray tube. The echo from a flaw is received by another probe, converted to an electrical e.m.f. (which may vary from micro volts to several volts) by the transducer and is applied to the cathode ray tube on which it can be seen as signal displaced along the time axis of the tube from the original pulse.

The first applications of ultrasonic to flaw detection employed longitudinal waves projected into the specimen at right angles to the surface. This presented problems because it meant that the weld surface had to be dressed smooth before examination, and more often than not the way in which the flaw oriented, as for example lack of penetration, made detection difficult with this type of flaw. The type of wave used to overcome these disadvantages is one which is introduced into the specimen at some distance from the welded joint at an angle to the surface (e.g. 20degree) and is known as shear wave. The frequency of the waves (usually 2.5 and 1.5 MHz for butt welds), the angle of incidence of the beam, the type of surface and the grain size, all affect the intensity of the echo which is adjustable by means of a sensitivity control. The reference standard on which the sensitivity of the instrument can be checked consists of steel block 300*150*12.7mm thickness with a 1.6 mm hole drilled centrally and perpendicularly to the largest face, 50.8mm from one end.

Echoes are obtained from the hole after 1, 2 or 3 transverses of the plate and from the amplitude of the echo the intensity from a hole of known size can be checked. There are three types of probes which are available: the single probe, the twin transmitter receiver probe, and the separate transmitter and receiver probe.

To make a length scan of the weld the transmitter-receiver unit is moved continuously along a line parallel to the welded seam so that all points of the whole area of the welded joint are covered by the scanning beam, double echoes are not obtained from a single flaw. It is evident that varying the distance from the weld to the probe varies the depth at which the main axis of the beam crosses the welded joint and moving the probe at right angles to the line is thus known as depth scan. A spherical flaw will have no directional characteristics and a wave falling upon its center will be the size of flaw. Cylindrical flaws behave in the same way but in the case of a narrow planar flaw it is evident that optimum echo will be received when the crack is at right angles to the wave and there will no echo when the crack lies along the wave, but if the probe is moved to the first echo position the crack is no longer lying along the beam.

The probes must make good contact with the specimen and on slightly curved surfaces a thin film of oil is used to improve the contact. On surfaces with greater curvature, as for example when investigating circumferential welds on drums, curved probes are used. [13]2.9.3 Hydrostatic Test

Hydrostatic Inspection:

Each length of pipe shall withstand, without leakage, an inspection hydrostatic test to at least the pressure specified. Test pressures for all sizes of seamless pipe and for welded pipe in sizes 18 in. and smaller, shall be held for not less than 5 sec. Test pressures for welded pipe in sizes 20 in. and larger shall be held for not less than 10 seconds. For threaded-coupled-pipe, the test shall be applied with the couplings made up power tight make-up is specified. Except sizes greater than 12 in. OD may be tested in the plain- end condition. For threaded pipe furnished with couplings made up handling tight, the hydrostatic test shall be made on the pipe in the plain-end or threads only condition or with couplings applied. FIGURE 25: Equipment for Hydro testing at Crescent Steel Mills Noriabad

Verification of Test:

In order to insure that every length of pipe is tested to the required test procedure, each tester, except those of those of which butt welded pipe is tested, shall be equipped with a recording gauge that will record the test pressure and duration of time applied to each length of pipe, or equipped with some positive and automatic or interlocking device to prevent pipe from being classified as tested until the test requirements (pressure and time) have been complied with.

Test Pressures:

The minimum test pressure shall be standard test pressure or alternate test pressure or intermediate or higher pressure at the discretion of the manufacturer unless specifically, or a higher pressure as agreed upon between the purchaser and manufacturer. The minimum test pressure for grades, diameters, and wall thicknesses for all sizes Grades A25 smaller than 5 9/16 in. and those of grade A and B smaller than 2 3/8 in., the test pressure has been arbitrarily assigned. The test pressure for the intermediate wall shall be equal to the next heaviest wall. The computed pressures are not an exact multiple of 10 psi (100 k pa), they shall be rounded to the nearest 10 Psi (100 KPa). [15]

2.10 CORROSION & DEGRADATION OF MATERIALS

INTRODUCTION

Corrosion is defined as the deterioration of a material because of reaction with its environment. To one degree or another, most materials experience some type of interaction with a large number of diverse environments. Often, such interactions impair a materials usefulness as a result of the deterioration of its mechanical properties (e.g., ductility and strength), other physical properties, or appearance. Occasionally, to the chagrin of a design engineer, the degradation behavior of a material for some application is ignored, with adverse consequences.

Deteriorative mechanisms are different for the three material types. In metals, there is actual material loss either by dissolution (corrosion) or by the formation of nonmetallic scale or film (oxidation). Ceramic materials are relatively resistant to deterioration, which usually occurs at elevated temperatures or in rather extreme environments; the process is frequently also called corrosion. For polymers, mechanisms and consequences differ from those for metals and ceramics, and the term degradation is most frequently used. Polymers may dissolve when exposed to a liquid solvent, or they may absorb the solvent and swell; also, electromagnetic radiation (primarily ultraviolet) and heat may cause alterations in their molecular structure. [8]2.11 FORMS OF CORROSION

It is convenient to classify corrosion according to the manner in which it is manifest. Metallic corrosion is sometimes classified into eight forms: uniform, galvanic, crevice, pitting, intergranular, selective leaching, erosioncorrosion, and stress corrosion.

2.11.1 Uniform Attack

Uniform attack is a form of electrochemical corrosion that occurs with equivalent intensity over the entire exposed surface and often leaves behind a scale or deposit. In a microscopic sense, the oxidation and reduction reactions occur randomly over the surface. Some familiar examples include general rusting of steel and iron and the tarnishing of silverware. This is probably the most common form of corrosion. It is also the least objectionable because it can be predicted and designed for with relative ease. [7]2.11.2 Galvanic Corrosion

Galvanic corrosion occurs when two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte. The less noble or more reactive metal in the particular environment will experience corrosion; the more inert metal, the cathode, will be protected from corrosion. For example, steel screws corrode when in contact with brass in a marine environment; or if copper and steel tubing are joined in a domestic water heater, the steel will corrode in the vicinity of the junction. Depending on the nature of the solution, one or more of the reduction reactions

The rate of galvanic attack depends on the relative anode-to-cathode surface areas that are exposed to the electrolyte, and the rate is related directly to the cathodeanode area ratio; that is, for a given cathode area, a smaller anode will corrode more rapidly than a larger one. The reason for this is that corrosion rate depends on current density, the current per unit area of corroding surface, and not simply the current. Thus, a high current density results for the anode when its area is small relative to that of the cathode. A number of measures may be taken to significantly reduce the effects of galvanic corrosion. These include the following:

1. If coupling of dissimilar metals is necessary, choose two that are close together in the galvanic series.

2. Avoid an unfavorable anode-to-cathode surface area ratio; use an anode area as large as possible.

3. Electrically insulate dissimilar metals from each other.

4. Electrically connect a third, anodic metal to the other two; this is a form of cathodic protection. [7]

FIGURE 26: Galvanic corrosion of a magnesium shell that was cast around a steel core.2.11.3 Crevice Corrosion

Electrochemical corrosion may also occur as a consequence of concentration differences of ions or dissolved gases in the electrolyte solution, and between two regions of the same metal piece. For such a concentration cell, corrosion occurs in the locale that has the lower concentration. A good example of this type of corrosion occurs in crevices and recesses or under deposits of dirt or corrosion products where the solution becomes stagnant and there is localized depletion of dissolved oxygen.

Corrosion preferentially occurring at these positions is called crevice corrosion. The crevice must be wide enough for the solution to penetrate, yet narrow enough for stagnancy; usually the width is several thousandths of an inch.

Crevice corrosion may be prevented by using welded instead of riveted or bolted joints, using non-absorbing gaskets when possible, removing accumulated deposits frequently, and designing containment vessels to avoid stagnant areas and ensure complete drainage. [7]

FIGURE 27: Plate which was immersed in seawater, crevice corrosion

has occurred at the regions that were covered by washers.

2.11.4 Pitting

Pitting is another form of much localized corrosion attack in which small pits or holes form. They ordinarily penetrate from the top of a horizontal surface downward in a nearly vertical direction. It is an extremely insidious type of corrosion, often going undetected and with very little material loss until failure occurs.

The mechanism for pitting is probably the same as for crevice corrosion in that oxidation occurs within the pit itself, with complementary reduction at the surface.

It is supposed that gravity causes the pits to grow downward, the solution at the pit tip becoming more concentrated and dense as pit growth progresses. A pit may be initiated by a localized surface defect such as a scratch or a slight variation in composition. In fact, it has been observed that specimens having polished surfaces display a greater resistance to pitting corrosion. Stainless steels are somewhat susceptible to this form of corrosion; however, alloying with about 2% molybdenum enhances their resistance significantly. [7]

FIGURE 28: The pitting of a 304 stainless steel plate by an acid-chloride solution.2.11.5 Intergranular Corrosion

As the name suggests, intergranular corrosion occurs preferentially along grain boundaries for some alloys and in specific environments. The net result is that a macroscopic specimen disintegrates along its grain boundaries. This type of corrosion is especially prevalent in some stainless steels. When heated to temperatures between 500 and 8000C (950 and 14500F) for sufficiently long time periods, these alloys become sensitized to intergranular attack. It is believed that this heat treatment permits the formation of small precipitate particles of chromium carbide (Cr23C6) by reaction between the chromium and carbon in the stainless steel. Both the chromium and the carbon must diffuse to the grain boundaries to form the precipitates, which leaves a chromium-depleted zone adjacent to the grain boundary. Consequently, this grain boundary region is now highly susceptible to corrosion.

Intergranular corrosion is an especially severe problem in the welding of stainless steels, when it is often termed weld decay. Figure shows this type of intergranular corrosion. Stainless steels may be protected from intergranular corrosion by the following measures:

a. Subjecting the sensitized material to a high-temperature heat treatment in which all the chromium carbide particles are re-dissolved,

b. Lowering the carbon content below 0.03 wt% C so that carbide formation is minimal,

c. Alloying the stainless steel with another metal such as niobium or titanium; which has a greater tendency to form carbides than chromium so that the Cr remains in solid solution. [7]

FIGURE 29: Weld decay in a stainless steel. The regions along which the grooves

have formed were sensitized as the weld cooled.

2.11.6 Selective Leaching

Selective leaching is found in solid solution alloys and occurs when one element or constituent is preferentially removed as a consequence of corrosion processes. The most common example is the dezincification of brass, in which zinc is selectively leached from a copperzinc brass alloy. The mechanical properties of the alloy are significantly impaired, since only a porous mass of copper remains in the region that has been dezincified. In addition, the material changes from yellow to a red or copper color. Selective leaching may also occur with other alloy systems in which aluminum, iron, cobalt, chromium, and other elements are vulnerable to preferential removal. [7]2.11.7 ErosionCorrosion

Erosioncorrosion arises from the combined action of chemical attack and mechanical abrasion or wear as a consequence of fluid motion. Virtually all metal alloys, to one degree or another, are susceptible to erosioncorrosion. It is especially harmful to alloys that passivate by forming a protective surface film; the abrasive action may erode away the film, leaving exposed a bare metal surface. If the coating is not capable of continuously and rapidly reforming as a protective barrier, corrosion may be severe. Relatively soft metals such as copper and lead are also sensitive to this form of attack. Usually it can be identified by surface grooves and waves having contours that are characteristic of the flow of the fluid. The nature of the fluid can have a dramatic influence on the corrosion behavior.

Increasing fluid velocity normally enhances the rate of corrosion. Also, a solution is more erosive when bubbles and suspended particulate solids are present. Erosioncorrosion is commonly found in piping, especially at bends, elbows, and abrupt changes in pipe diameterpositions where the fluid changes direction or flow suddenly becomes turbulent. Propellers, turbine blades, valves, and pumps are also susceptible to this form of corrosion.

One of the best ways to reduce erosioncorrosion is to change the design to eliminate fluid turbulence and impingement effects. Other materials may also be utilized that inherently resist erosion. Furthermore, removal of particulates and bubbles from the solution will lessen its ability to erode. [7]

FIGURE 210: Impingement failure of an elbow that was part of a steam condensate line.2.11.8 Stress Corrosion

Stress corrosion, sometimes termed stress corrosion cracking, results from the combined action of an applied tensile stress and a corrosive environment; both influences are necessary. In fact, some materials that are virtually inert in a particular corrosive medium become susceptible to this form of corrosion when a stress is applied. Small cracks form and then propagate in a direction perpendicular to the stress; with the result that failure may eventually occur. Failure behavior is characteristic of that for a brittle material, even though the metal alloy is intrinsically ductile. Furthermore, cracks may form at relatively low stress levels, significantly below the tensile strength. Most alloys are susceptible to stress corrosion in specific environments, especially at moderate stress levels. For example, most stainless steels stress corrodes in solutions containing chloride ions, whereas brasses are especially vulnerable when exposed to ammonia. The stress that produces stress corrosion cracking need not be externally applied; it may be a residual one that results from rapid temperature changes and uneven contraction, or for two-phase alloys in which each phase has a different coefficient of expansion. Also, gaseous and solid corrosion products that are entrapped internally can give rise to internal stresses. Probably the best measure to take in reducing or totally eliminating stress corrosion is to lower the magnitude of the stress. This may be accomplished by reducing the external load or increasing the cross-sectional area perpendicular to the applied stress. Furthermore, an appropriate heat treatment may be used to anneal out any residual thermal stresses. [7]

FIGURE 211: Photomicrograph showing intergranular stress corrosion cracking in brass.2.12 CORROSION PREVENTION TECHNIQUES

Some general techniques include material selection, environmental alteration, design, coatings, and cathodic protection. Perhaps the most common and easiest way of preventing corrosion is through the judicious selection of materials once the corrosion environment has been characterized.

1. Material Selection:

Standard corrosion references are helpful in this respect. Here, cost may be a significant factor. It is not always economically feasible to employ the material that provides the optimum corrosion resistance; sometimes, either another alloy and/or some other measure must be used. [9]2. Environmental Alteration:

Changing the character of the environment, if possible, may also significantly influence corrosion. Lowering the fluid temperature and/or velocity usually produces a reduction in the rate at which corrosion occurs. Many times increasing or decreasing the concentration of some species in the solution will have a positive effect; for example, the metal may experience passivation.

Inhibitors are substances that, when added in relatively low concentrations to the environment, decrease its corrosiveness. Of course, the specific inhibitor depends both on the alloy and on the corrosive environment. There are several mechanisms that may account for the effectiveness of inhibitors. Some react with and virtually eliminate a chemically active species in the solution (such as dissolved oxygen). [9]3. Design:

Several aspects of design consideration have already been discussed, especially with regard to galvanic and crevice corrosion, and erosioncorrosion. In addition, the design should allow for complete drainage in the case of a shutdown, and easy washing. Since dissolved oxygen may enhance the corrosivity of many solutions, the design should, if possible, include provision for exclusion of air. [3]4. Coatings:

Physical barriers to corrosion are applied on surfaces in the form of films and coatings. A large diversity of metallic and nonmetallic coating materials is available.

It is essential that the coating maintain a high degree of surface adhesion, which undoubtedly requires some pre-application surface treatment. In most cases, the coating must be virtually nonreactive in the corrosive environment and resistant to mechanical damage that exposes the bare metal to the corrosive environment. All three material typesmetals, ceramics, and polymersare used as coatings for metals. [3]

5. Cathodic Protection:

Cathodic protection is a technique to protect a buried or immersed metallic structure by making it the cathode of a galvanic cell or by impressing a current from an external power source using an inert anode this technique has been successfully applied for more than 50 years and is now accepted as a proven and an established technology. [7]Selection of material and cathodic protection are discussed below in detail:

2.13 SELECTION OF MATERIALS

INTRODUCTION

The first and important step for the pipe line network is the selection of material, that it should have the desire properties like toughness, ductility, strength, weldability, wear resistant and corrosion resistant. Furthermore it should also be economically available in the market.

For economic reasons carbon-manganese steels are used whenever possible for the fabrication of pipelines for production and transmission of oil and gas and also for water injection systems. Pipeline engineers need to be familiar with the modern methods of fabrication of pipe and also be aware of the limitations of particular steels to the type of product that can be safely transported. In this Section the manufacture of carbon-manganese steel pipelines is described including the compositions and fabrication methods of the steel plate used for forming pipe. Corrosion, calculation of corrosion allowances and corrosion limitations of the carbon-manganese steels are also discussed.

The steels used form the pipe joins are low-carbon carbon manganese structural steels. The higher strength grades are micro-alloyed and are often termed high strength low allows (HSLA) steels. Similar types of steels are used for ships, pressure vessels, pump bodies and OCTG tubular goods.

From a materials and corrosion viewpoint it is generally the case that pipeline service is becoming more severe both for new pipelines and pipelines in service. For example there are new multiphase pipelines operating at temperatures above 125 C at very high shut-in pressures and high concentrations of carbon dioxide. Sometimes abundant water is present in the region. The pipelines production systems operated with dry hydrocarbons thus avoiding corrosion problems. As a consequence of the more severe service higher quality of pipe is required both for new fields and for replacements in the older fields, which are now in the refurbishment phase. To meet these demands the steel and pipe production processes have become much more complicated. [15]2.14 PIPELINE MATERIALS SPECIFICATIONS

In most parts of the world the pipe joins for oil and gas will conform to the American Petroleum Institute API Specification 5L. In 1999 this Specification was converted into an international standard, ISO 3183, which covers the selection and use of seamless, longitudinal welded and helical (spiral) welded line pipe. In comparison to API 5L, ISO 3183 is in three parts with the various steel grades divided between Parts 1 and 2. Part 3 is based on the EEMUA Publication 166 and deals with both compositional and sour service requirements and is only relevant to submarine pipelines.

Though the API 5L Specification dated back to the 1920s it became the basic international specification in about 1948. At that time the highest strength grade was X42. The ISO Standard now includes pipe grades up to X80. Despite the recent conversion to SI units it remains common parlance in the Oil and Gas Industry to use the feet pound second (FPS) units for general discussion. To accommodate previous and present design terminology reference is made here to API 5L and API 5LX though it is to be understood that the same comments relate to ISO 3183. European Standard EN 10208, derived from BS EN 1028, Parts 1 & 2, is related to the new ISO 3183; EN 10208 gives compositional specifications for pipe usually as the maximum compositional values only; if considered relevant minimum values need to be additionally specified. EN 10208 is only used within Europe; elsewhere ISO 3183 will be the primary document. [15]2.15 MATERIAL PROPERTIES

Pipeline steel requires having high strength whilst retaining ductility and fracture toughness and weldability. Strength is the ability of the pipe steel (and associated welds) to resists the longitudinal and transverse tensile forces imposed on the pipe in service and during installation. Ductility is the ability of the pipe to absorb over stressing by deformation. Toughness is the ability of the pipe material to withstand impacts or shocks loads. Metallic engineering materials are generally tough and fall in a ductile manner, i.e. they yield before they break. In comparison non-ductile or brittle materials are glass-like and fail suddenly by brittle fracture.

Weldability is the ability and ease of production of a quality weld and heat affected zone of adequate strength and toughness. Most metals can be welded but not all have good weldability. For example the parts of an aluminum alloy aero plane are held together with bolts, rivets and adhesive rather than by welding.

The balance of properties (strength, toughness and weldability) required depends on the intended use of the pipeline. An example of a severe service pipeline would be a high-pressure sour gas/condensate pipeline in Arctic conditions; such a pipe would require heavy wall thickness with high toughness at low temperatures whilst having resistance to sulphide cracking. The heavy wall thickness would complicate the welding process. To obtain both high strength and toughness without sacrifice of weldability requires limited alloying combined with complex thermo-mechanical treatment of the steel combined with micro alloying. [15]2.16 SUGGESTIONS FOR POSSIBLE FUTURE MATERIALS

1. The modified 13% Cr materials, often termed weld able 12 Cr; will be widely used as confidence is gained. To date about 300 km of pipe has been installed.

2. The super-austenitic materials are possible materials for use either as solid pipelines or, more likely, as a cladding material. The lower nickel content would reduce the cost of the steels to midway between the type 300 austenitic steels and the high nickel allows. Super-austenitic steels have high PREN values and hence good resistance to pitting, crevicing and stress corrosion cracking. These materials are also readily wieldable compared to the duplex stainless steels.

3. Seamless pipes are without any welds; in spiral welded pipes line pipes are joined through different welding processes. As weld itself and the heat affected zone has short life span than the base material. So it better to use seamless pipe instead of spiral welded pipes. The only one drawback of seamless pipe is that they are costly.

4. If we carryout comparison between PSL1 and PSL2 pipes, we will reach at the conclusion that PSL2 is better than PSL1.The one drawback is that PSL2 pipes are costly than PSL1. Testing requirement for PSL2 is more than PSL1. The plate uses for PSL2 pipes do not contain any weld repair. [15]2.17 CATHODIC PROTECTION

INTRODUCTION

Cathodic protection is a technique to protect a buried or immersed metallic structure by making it the cathode of a galvanic cell or by impressing a current from an external power source using an inert anode this technique has been successfully applied for more than 50 years and is now accepted as a proven and an established technology. In order to reduce corrosion to manageable levels, cathodic protection systems must be designed, engineered, installed, operated and maintained to high standards.

Sacrificial anode may be magnesium, zinc or aluminum alloys. The material for impressed current anode may be silicon iron, graphite, mixed metal anode and titanium mesh.

Underground steel pipelines are commonly protected by the application of cathodic protection which provides corrosion control to buried or submerged areas and where the coating rapping is damaged. Over recent years the use of cathodic protection to protect reinforcement steel within concrete structures has become increasingly important. Cathodic protection can be applied to most steel structures that are either buried or immersed in soil, sand or water. Above ground reinforced concrete structures can be protected using recently developed anodes which are incorporated within a sprayed or poured cementitious mortar. Such systems can be fitted most economically during construction or even to existing structures to prevent further deterioration of the reinforcement. [3]2.18 PRINCIPLE OF CATHODIC PROTECTION

The principle of cathodic protection is to make the potential of the whole surface of the steel structure sufficiently negative with respect to the surrounding medium to ensure that no current flows from the metal into the medium. This is done by forcing an electric current to flow through the electrolyte towards the surface of the metal to be protected, thereby eliminating the anodic areas. The current may be obtained from any convenient external source, such as a rectified alternating current supply, direct current generator or by galvanic action.

The principle when impressed current is used is illustrated below figure in which a DC current source is shown connected to the structure to be protected and to an auxiliary anode buried in the electrolyte. The auxiliary anode is arranged to be at a higher potential than metal structure to protected, so that current will flow from the former to the latter.

Corrosion of steel pipeline in normally aerated soils and waters can be entirely prevented if the pipeline is maintained at a potential minus 0.85 volts (-0.85V). Under anaerobic conditions where sulfate-reducing bacteria are present, this potential will increase to minus 0.95 volts (-0.95 V). [1]2.18.1 Electrochemical Concept

Corrosion means a process of metal dissolution and generation of electrons at anodic side. This anodic reaction can be represented by the following reaction

Fe = Fe2+ + 2e

The reaction indicates that if a flow of electrons is applied from an external source towards the anodic sites, the generation of electrons and hence the metal dissolution can be stopped at the anodic site. In order to achieve this; the corroding must be made cathode in the electrical circuit and this is exactly what cathodic protection is. [1]2.18.2 Thermodynamic Concept

Corrosion will not occur unless there is a thermodynamic possibility for the corrosion to occur. This thermodynamic concept is illustrated by the potential- pH of the metal. Such concept is also called pourbaix concept. This concept defines domains of corrosion, passivity and immunity. This concept also illustrates how the corrosive condition can be converted to protective condition by changing the corrosion potential of the structure. This change in potential is achieved by making the structure cathode. [1]2.18.3 Polarization Concept

Polarization may be defined as change in electrochemical potential due to flow of current. In each corrosion cell we have an anodic site and a cathodic site. The potential of anodic site is called anodic potential and the potential of cathodic site is called cathodic potential. When the anodic site is connected to the cathodic site a certain corrosion current starts flowing between the anodic and the cathodic site. The open circuit potential values of the anodic and cathodic sites will change with a flow of this current and tend to shift towards each other. The anodic potential shifts towards the cathodic potential and the cathodic potential shift towards the cathodic potential. The resulting potential in this situation is called mixed potential or corrosion potential. The change in potential of anodic side from the open circuit to the cathodic site from the open circuit to the steady corrosion potential is termed as cathodic polarization. [1]2.19 CATHODIC PROTECTION SYSTEM

Basically two types of cathodic protection systems are available:

Sacrificial anode system; which make use of the protective current generated by galvanic action of the sacrificial anodes system. The sacrificial anodes are made from magnesium, zinc, aluminum, or their alloy.

Impressed current system where an External DC source is used to provide the required amount of current in impressed cathodic protection system current is provided by suitable Rectifier through inert type of anode. Impressed CP system current must be discharged from ground bed. The sole purpose of this ground bed is to discharge current. In this process of discharging current anodes (ground bed) are consumed by corrosion. It is desirable to use materials for ground bed that are consumed at a much lower rate than are usual pipeline metals. This will ensure reasonable long life for anodes. [15]2.20 CATHODIC PROTECTION WITH GALVANIC ANODES

In a corrosion cell of two dissimilar metals, one metal is active with respect to the other and corrodes faster. In a CP with galvanic anodes, this effect establishes a dissimilar metals cell strong enough to counteract corrosion cell normally existing on pipelines. This is done by connecting a very active metal to pipeline. The metal will corrode and discharge current to the pipeline and reduce its corrosion.

FIGURE 212: Cathodic Protection with Galvanic Anode

In case of CP with galvanic anodes, CP does not eliminate corrosion. Under normal conditions, the current available from galvanic anodes is limited. Similarly, the driving voltage existing between steel pipe and galvanic anode metals is limited. Therefore, the resistance between the anodes and the earth must be low for the anodes to discharge useful amount of current. A normal installation is one in which the current from galvanic anode is expected to protect a substantial length of pipeline. [3]2.20.1 Advantages and Disadvantages of Sacrificial System

Advantages:

1. No main power is required.

2. Can be fitted on needed basis.

3. Is practically self regulating on current output?

4. Does not usually cause interference effects on neighboring structures thus reducing the possibility of stray current corrosion.

5. Anodes can be bolted, welded or brazed directly on to the structure to be protected

6. It can be designed up to the required design. If there are no weight limitation.

7. It cannot be incorrectly connected.

8. It can be manufactured up to any size or shape which suits the installation.

9. It does not require specialists to install system.

10. Once installed, limited inspection is required for performance checkups.

11. It is relatively easily to design and install. [3] Disadvantages:

1. Anodes have limited current output therefore can only be used for certain applications in low resistivity electrolytes.

2. Difficult to monitor effect because the anodes usually cannot be disconnected.

3. Usually requires to be replaced at intervals.

4. Difficult to monitor the effect because the anodes usually cannot be disconnected

5. Often awkward anode size and shapes may have extra weight or may be affected by liquid. [3]

2.21 CATHODIC PROTECTION WITH IMPRESSED CURRENT

In this system current from some outside source is impressed on the pipeline by using a ground bed and a power source the most common power source is the rectifier. This device converts alternating currents to low voltage direct current. Schematically Cathodic protection system with impressed current is shown in the fig. below

FIGURE 213: Cathodic Protection with Impressed Current

Because of the high current requirement underground transmission pipeline system are always protected by impressed current system. However in certain areas it may necessary to enhance it locally with sacrificial anodes.

Rectifiers usually are provided with the means for varying the DC output voltage, in small increments, over a reasonably wide range. Most pipeline rectifiers operate in the range between 10 and 50v and can be obtained with maximum current outputs ranging from less than 10 A to several hundred amperes. This serves to explain the flexibility in choice of power source capacity available to the corrosion engineer when planning an impressed current CP system. [1]2.21.1 Advantages and Disadvantages of Impressed Current System

Advantages

1. Good throwing power so can be used to protect a wide variety of structures.

2. Grounds can be installed at remote sites.

3. Variables voltage overcomes high resistivity circuits.

4. Output currents can be individually very accurately controlled.

5. Operates with high output density there by reducing the anodes overall size and weight.

6. It evolves a smaller number of anodes than a sacrificial system.

7. Can be self regulating using suitable electronics circuits.

8. Systems parts can be replaced for example; the system can be rehabilitated by installing new ground bed but utilizing the remaining original components. [3] Disadvantages1. Requires continuous alternating current main

2. More complicated design than sacrificial anode system are more system components required.

3. Requires specialist contractors for installation.

4. Requires cabling and high integrity insulation.

5. Regular inspection and maintenance

6. Incorrect operation can cause damage to coating

7. Requires interference effects on neighboring structures to be checked.