Properties of Pipe

Welding of thick wall pipes has become a key process in several important industrial areas, including offshore and onshore pipelines, vessel fabrication, and nuclear industry [29]. The majority of welded steel pipe is produced from coil or plate [55].

Choosing proper pipe for special purposes is a factor that influences the whole job by minimizing the possible generation of any defects. The first and foremost step is selecting material of pipe. Factors in choosing ideal pipe for special application are, pipe wall thickness, outside diameter, and cleanliness. Storage and handle of pipe in welding industry are important as well and pipes should be stored in a proper weather condition. Also, dust, moisture, and dirt inside of tube or pipe, makes welding difficult and costly [14].

The result of experimental test done by UNAMON in 1996s [24] showed orbital welding are more efficient for the small diameters and thicknesses. Measuring proper thickness in pipe welding is essential to prevent hardening of the structure in the HAZ and cold cracking. The greater thickness of the pipeline causes higher cooling rates [56].

3.3.1. Possible Pipe Materials in Orbital welding

Controlling of certain elements, such as sulfur in the composition of material is very important in welding procedure [14]. Generally, weldability of 300-series stainless steels with orbital system which used prevalently are good with the exception of the types 303 which contain high sulfur and 303SE which contain selenium for ease machining. Weldability of 400-series stainless steels is good, however possibility of requiring post-weld heat treatment is also high [3]. Recently, usage of high strength low alloy (HSLA) steels, greatly increased in welding of pressure vessels, tube and pipes with orbital systems [57].

Titanium and its alloys pipe are using in applications that corrosion resistant and particular strength are required and can be welded by orbital pipe welding systems [58, 59]. In Gulfstream pipeline project [60], grades were used for the majority of the offshore pipeline was APL 5L Grade X70. American Petroleum Institute (API) 5L X60 pipe with diameter of 406.4 mm was successfully laid recently in the UK sector of the North Sea [61].

High strength X80 pipe were used for the first time in Europe by 1980s in Germany using SMAW welding and at almost the same time, Nova Company in Canada used mechanized pipe welding for X80 pipe [21]. Common X80 steels used in industries has properties of: 615 Megapascal (MPa) of yield strength, 680 MPa of tensile strength, 21 % of elongation, 226 HV10 of hardness, and 330 J of Charpy impact energy at 20 ℃ [37]. Other high strength steels, such as X90, X100, and X120 are also possible to be welded by orbital pipe welding system.

3.3.2. Materials Weldability for Oil and Gas Applications

There is a strong demand for gas and oil in the world especially in the fast growing economies like China and India, which implies continued growth of oil and gas pipeline installation [62, 63, 64]. Worldwide gas consumption is predicted to be almost double over 25 years, from almost 2540 trillion liters in 2000 to 4980 trillion

liters in 2025 [16]. The main problem of using gas is that many of gas resources situated in remote locations and transportation of gas from those resources needs long distance pipelines transmission [62, 63].

Crude oil is a heterogeneous mixture of hydrocarbons with non-hydrocarbon components which includes alcohols, phenols, sediments, water, salts, sulfur compounds, acid gases (such as Hydrogen Sulfide H S), carbon monoxide (CO), etc.

[65, 66]. There is always high level of corrosion possibility in inner surface of pipe by oil during its movement due to the corroding agents (water and oxygen). Settled water which contains dissolved salts and acids, in bottom of pipe has corrosion effects on those parts in the pipeline [66]. Therefore, high level of safety and trust direct to reduction of cost, highest efficiency, and lowest defects, required in oil and gas pipeline distribution. So, special attention required in material selection of pipe and recent research focuses on the fracture toughness property which is a main factor in the design of oil and gas pipelines [67, 68, 69, 70, 71].

Demand of liquefied gases worldwide, such as Liquid Natural Gas (LNG) which may be obtained by cooling down methane gas to temperature below -163 ℃ has increased [72]. Higher pressures and flow levels towards using line pipe of larger diameter and/or higher operation pressure. To improve operational efficiencies, avoid large wall thickness of pipe, and cost saving, development of higher strength steel grades with relatively thinner wall pipe started more than 30 years age [16, 37, 47, 62, 73, 74].

Thermo-mechanical rolling (TM treatment) method which provides possibility of producing material from steels up to X70 was invented in the early seventies to replace with hot rolling and normalizing method. X70 is micro-alloyed consists of niobium and vanadium with lower amount of carbon. Figure 8 depicts the historical development of the pipe steels [73]. X70 steel showed great welding result in China's East-to-West gas transmission project [37]. As can be seen from the figure, X80 steel were invented by combining TM rolling and subsequent accelerated cooling (Acc

Cooling) which lead to higher strength, lower carbon content and as a result better field weldability rather X70 steel [73].

Figure 8 Development of pipeline steels [73]

X80 steel is purer and consists of lower sulfur compared with X70 steel and can be adapted to different welding heat input [37]. In the study [37] welding of X80 with submerged arc welding shows the suitability of this steel for welding at large heat input even for four wires SAW method. X100 steel produced by adding molybdenum, copper, and nickel and is processed to plate by combining TM rolling and modified accelerated cooling [73]. The first X100 steel section installed by TransCanada in September 2002 [16, 55] and X100 steels can tolerate pressure from 7 to 20 MPa [75]. Figure 9 compares microstructures of three typical X70, X80, and X100 pipeline steels. As shown in the figure, X70 steel has uniform grains and has within ferrite grains. X80 is obtained by changing the microstructure of the steel matrix from ferrite-Pearlite to ferrite-bainite. In X80 and X100, with accelerated cooling that follows TM rolling, more uniform and extremely fine microstructure is obtained [73].

Figure 9 Microstructure of high strength pipeline steels: (a) X70, (b) X80, (c) X100 [73]

The X120 steel, with yield strength of 827.4 MPa, is 50 % stronger than standard gas transmission pipe. It means more and higher gas pressure can be transferred but requiring special welding procedure is its problem. Successful use of X120 in the project of transferring natural gas from northwestern Canada to markets all over North America, by TransCanada Pipelines Ltd shows bright future for this type of steel [76]. Also, high strength low micro-alloyed steel has been using in high pressure operation of oil and gas transportation for a while [34]. In designing of oil and gas pipelines, fracture toughness property should be considered as a major factor [68].