Welding

Welding is a process in which two or more parts are joined permanently at their touching surfaces by the force of heat and/or pressure. Often a filler material is added that helps in the merging. The parts that are joined together by welding are called a weldment. Factors affecting Weld-ability 1. Melting Point 2. Thermal conductivity 3. Thermal Expansion 4. Surface condition 5. Change in Micro structure. Welding is mainly used in metal parts and their alloys. Welding is of two types : Fusion welding and Solid state welding.1 Fusion welding : In this the base metal is melted by means of heat. Ina fusion welding operation, a filler metal is added to the molten pool to facilitate the process and provide bulk and strength to the joint. The most commonly used fusion welding processes are: arc welding, resistance welding, oxyfuel welding, electron beam welding and laser beam welding. 2. Solid-state welding: In this method joining of parts takes place by the application of pressure or a combination of heat and pressure. No filler metal is used in this . The most commonly used solid-state welding processes are: diffusion welding, friction welding, ultrasonic welding. Arc welding is a method of permanently joining two or more metal parts. It consists of combination of different welding processes wherein merging is done by heating with an electric arc, (mostly without the application of pressure) and with or without the use of filler metals that again depends upon the base plate thickness. A joint is achieved by melting and fusing the adjacent portions of the separate parts. The finally welded joint has a strength approximately equal to that of the base material. 

The arc temperature is maintained approximately at 4400°C. To prevent oxidation, a flux material is used which decomposes under the heat of welding and releases a gas that shields the arc and the hot metal. Precautions in Arc welding courses in kerala 1. Due to the intensity of heat and light rays from the electric arc, the operator’s hand face and eyes are to be protected while arc is in use 2. Heavy gloves are worn 3. Hand shield or a helmet with window of coloured glass must be used to protect the face 4. The space for the electric arc welding should be screened off from the rest of the building to safeguard other workmen from the glare of the arc.The second basic method employs an inert gas to form a protective envelope around the arc and the weld. The commonly used gases are Helium, argon, and carbon dioxide . Other processes that are used in the industry are as follows: 1. Diffusion bonding (DB): in this method parts are pressed together at an elevated temperature below the melting point for a period of time. 2. Explosion welding (EXW): In this method the parts to be welded are driven together at an angle with the help of an explosive charge and fuse together from the friction of the impact. 3. Ultrasonic welding (USW) for metals: This process makes use of transverse oscillation of one part against the other to develop sufficient frictional heat for fusion to occur. 4. Electro slag (ESW) and Electro gas (EGW) processes: In these a molten pool of weld metal contained by copper “shoes” is used to make vertical butt welds in heavy plate.

RADIOGRAPHIC TESTING

Different forms of Radiographic testing are :

Fluoroscopy: In a fluorescent salt screen  the image of the test specimen can be visually seen. The X rays passing through the object excite the fluorescent material producing bright spots in the more heavily irradiated areas. The fluorescent screen may be viewed directly or by means of a mirror or by using a camera and a closed circuit television. t 10 mm thickness, thin metal parts, welded assemblies and coarse sandwich constructions are screened by this method and castings with obvious large defects are rejected before usual inspection using film radiography.

Micro radiography: Micro-radiography is mainly applied in metallurgical studies. The radiograph when enlarged gives the structural details of the specimen.

Enlargement radiography: In some situations an enlarged image of an object is desired. To get the enlargement of the image the object to film distance is increased. To overcome the penumbral effects a source of an extremely small size is used.

High speed or flash radiography : for the radiography of moving objects, the exposure time should be very small and, at the same time, the intensity of the X rays should be extremely high. This is achieved by discharging huge condensers through special X ray tubes which give current of the order of thousands of amperes for a short time (of the order of a millionth of a second). This technique is normally applied in ballistics.

Auto radiography: In this case the specimen itself contains the material in radioactive form. When a film is placed in contact with the specimen, an autoradiograph is obtained showing the distribution of the radioactive material within the specimen. The technique is mainly used in the field of botany and metallurgy.

Electron transmission radiography : a beam of high energy X rays is used to produce photoelectrons from a lead screen. These electrons after passing through the specimen (of very low absorption like paper, etc.) expose the film and an electron radiograph is obtained. 

 Electron emission radiography: In this case a beam of X rays is used to produce photoelectrons from the specimen itself. These electrons expose the film which is placed in contact with the specimen. Since emission of electrons depends upon atomic number of an element, the electron emission will give the distribution of elements of different atomic numbers. 

Neutron radiography: In this case a neutron beam is used to radiograph the specimen. The recording system will, therefore, not be a photosensitive film since it is insensitive to neutrons. The following methods are used to record the image

Proton radiography : For special type of studies a proton beam can also be used. The number of protons transmitted through a specimen whose thickness is close to the proton range is very sensitive to exact thickness. This helps in detecting very small local variations in density and thickness.

 Stereo radiography : Two radiographs of the specimen are taken from two slightly different directions. The angle between these directions is the same as the angle subtended by the human eyes while viewing these radiographs. In the stereo viewer the left eye sees one radiograph and the right eye the other. In this way a realistic three dimensional effect is obtained giving the visual assessment of the position of the defect.

Xeroradiography : This is considered as a "dry" method of radiography in which a xerographic plate takes the place of X ray film. The plate is covered with a selenium powder and charged electrostatically in the dark room. Exposure to light or radiation causes the charge to decay in proportion to the amount of radiation received and a latent image is formed.

NON – DESTRUCTIVE TESTING

Adequate NDT and inspection by suitably qualified personnel is very much essential during all stages of pipe manufacture, construction and operation. The newly used inspection methods and equipment assist in obtaining the maximum life expectancy from a pipeline reducing the overall operating costs. Fluoroscopy, computed radiography, digital radiography and automated ultrasonic testing helps in improving the probability of detection (POD) of discontinuities. Modern NDT methods have become more quantitative and less obtrusive, which many a time, results in saving over time. Thus it can be said these advanced NDT methods have the potential that could lead to significantly lower repair rates while maintaining existing safety standards .

The Radiation detectors that used are image intensifiers in fluoroscopic and real time imaging systems The equipment that is used to perform Radiographic inspection can be either an X-ray machine or a radioactive isotope that produces gamma radiation. The isotopes help in increased portability as no electrical power supply is required. Electronic imaging panels and phosphorescent imaging screens are used to create digital images for computed and digital radiography. Real time imaging can be used close to the welding station and can detect all the defects at an early stage thus reducing the number of faulty welds produced. Phosphorescent imaging plates in digital radiography replaces X-ray film and processing chemicals. They can be used again and the X-ray images are stored electronically on optical disc. These images can be electronically enhanced to increase or reduce density leading to discontinuities which may have been previously . The phosphorescent imaging screens are flexible and are used in conventional x-ray film. The phosphorescent screens store a latent image which is scanned with an infrared laser scanner and are then viewed on a monitor. Magnification and measuring tools are then used for further evaluation of images. The use of phosphorescent screens require shorter exposure time which can amount to considerable savings.

The primary benefit of UT is that it is a truly volumetric test which means it is capable of determining not only the approximate dimensions and location of a defect, it also provides information to the testing technician regarding the type of defect. Another major advantage of UT is that it requires access to one side of the material to be tested and it will best detect crack’s and incomplete fusion which may not be possible with Radiographic testing. Since a variety of beam angles can be used, UT can detect defect which may not be detectable by radiography .UT requires highly skilled technicians because interpretation of indications are difficult. Reference standards are used for calibration and setting up of the equipment. Test scans can be recorded by equipments providing automated scanning. This test method is generally limited to the inspection of butt welds of materials that are thicker than 6 mm. Automated UT is used in pipe mills where the welds are inspected by a multiple array of probes, scanning the entire weld and detecting any discontinuities at an early stage. AUT is an in-field test method. An array of probes mounted in a scanner are placed on the pipe and the weld area is scanned. An encoder records the probe position with respect to the distance traveled, which allows the weld to be tested in a shorter period of time, resulting in a complete volumetric test of the weld and reducing the number of errors.

IMPACT OF CRUDE OIL TRANSPORT

The increase in crude oil shipments results in environmental and safety risks from accidents that may occur along pipelines, rail lines, waterways and at transshipment sites. All of these modes pose certain risks and each has certain advantages compared with the other modes All the modes of crude oil transport pose potential risks to the environment, public health and safety. All modes of crude oil transport have advantages and disadvantages based on a range of operational, economic and environmental factors and considerations. Modes of transport like railroads, vessels, barges and trucks can carry less of oil in comparison to the pipelines, but their routes are more flexible, allowing oil industry shippers to respond more quickly to changing production locations and volumes and changes in demand from coastal refineries Risk may range from a modest spill on isolated rural land in the winter (limiting ground contamination) to a major catastrophic spill in large water bodies or a derailment-produced spill and fire in a major urban area. Different types of oil influence the mode or modes of transportation chosen and the risks associated with those choices .Manufacturing industries that rely both on oil for their operations and water for their industrial processes and could be impacted by oil spills . A spill in an important and sensitive region can have far-reaching consequences, including both the damage created by the oil itself and the effect of intensive cleanup efforts, which can compound the environmental impacts in ecologically sensitive areas.

A pipeline oil spill, when one occurs, can have severe and long lasting impacts on public health, the environment and regional economy .The age and quality of the pipeline infrastructure are important factors in assessing oil spill risk from this mode .Pipelines result in fewer oil spill incidents and personal injuries than road and rail, and large spills in the recent past shows that the overall impact of a spill on the environment, economy and human health can be serious. Over time the quality of pipeline declines due to structural degradation, cracks caused by corrosion, defective welding or incidental damage from third-party activities. Damages to pipeline infrastructure may contribute to increased risks of a pipeline spill. Pipelines require regular maintenance inspections and constant monitoring during operation. Accidents may occur from undetected structural or mechanical failure and made worse by insufficient or delayed monitoring. Ships and barges pose fewer risks in transporting hazardous liquids than trains and trucks, and have economic advantages over these modes of transport, as well .A barge or tanker ships containing crude oil may undergo severe structural damage and spill cargo as the result of a collision with another ship. The increased volume of rail transport has also led to a rise in oil spill incidents involving trains .Unmonitored crossing points are special risk zones where accidents with automobiles can increase the risk of an oil spill or explosion. Even improperly assembled trains are more susceptible to derailment .Trucks are primarily used to transport oil for relatively short distances because long distance transport by truck is not ordinarily an economical option. Since large trucks are used to transport oil to and from railway transshipment facilities and pipelines, poorly maintained and monitored infrastructure at delivery points and fuel loading terminals could contribute to accidents, including fire and explosion.
Diploma in Oil and Gas Engineering