Characteristics and application cases of titanium alloy pipe for petroleum industry

Titanium (Ti) is an important structural metal developed in the 1950s, due to its high strength, low density. Because of its high strength, low density, super corrosion resistance, non-toxic to human body, and high heat resistance, titanium is widely used in aviation, aerospace, and other industries. Aerospace. Marine. Chemical industry. Medical. Marine engineering and petrochemical and other fields, known as the “space” metal. The “marine” metal and the fourth generation metal. Titanium, like aluminum, can be alloyed with other metallic elements to enhance and improve its mechanical and physical properties to suit different needs.
Usually, people think of titanium as a “rare element”, but it is not. The abundance of titanium in the earth’s crust is 0.61%, ranking 9th among all the elements, and 4th among the metals that can be used as structural materials, second only to Al. Fe. Mg, and its reserves are more than the sum of common metal cu, Pb, zn reserves. More than thirty countries in the world have titanium resources, China is very rich in titanium resources, titanium resource reserves of about 200 million tons, accounting for 28.9% of the total global reserves, titanium ore reserves ranked first in the world.
With the gradual growth of domestic and foreign demand for oil and gas resources, prompting oil and gas exploration and development continue to deepen, the environment and stratigraphic conditions encountered are becoming more and more complex. Such as more than 7000m deep wells are becoming more common; marine drilling is not satisfied with the beach and shallow sea, and gradually marching to deep water; on land, the previous difficult to exploit the three high (high temperature. High pressure. On land, the three high (high temperature. high pressure. high corrosion) gas wells, which were previously difficult to exploit, are also beginning to be overcome. At the same time, drilling and completion tools. Wellhead equipment. Transmission pipelines and other issues will face a variety of Lang. Titanium and titanium alloys have become essential new materials in oil and gas exploration and development due to their excellent properties. Domestic and foreign petroleum industry has developed a number of titanium alloy related standards, such as ASTM B337 seamless and welded titanium alloy pipe standard specification, SY/T 6896.3 “Oil and Gas Industry Special Pipe Technical Specification Part 3: Titanium Alloy Oil Pipe” and so on.

1. Titanium alloy material properties

1.1 Low density. High specific strength

The density of titanium is 4.506g/cm³-4.516g/cm³ (20 ℃), higher than the density of aluminum 2.70g/cm³, lower than the commonly used iron. copper. nickel. The specific strength of several commonly used metal materials for marine vessels (see Table 1). In the conventional temperature range (253℃～600℃), the specific strength (tensile strength/density) of titanium alloy is the highest among all metallic materials.

Table 1 yield ratio and specific strength of metal materials for ships [5]

The advantages of high specific strength are: (1) it can make the equipment design more compact and significantly reduce the structural mass; (2) it can significantly improve the technical level of the equipment and enhance the function; (3) it can improve the safety of the equipment.

1.2 Excellent corrosion resistance

Titanium is a highly reactive metal, while titanium and titanium alloys have good corrosion resistance, which is due to the formation of a dense and stable titanium oxide film on the surface of titanium. Several metal materials in the flow of seawater corrosion rate comparison (see Table 2), from Table 2 can see titanium in seawater almost no corrosion asked, in the more severe high sulfur environment, the corrosion rate of titanium alloy is also extremely low.

Table 2 corrosion rate of several corrosion resistant metal materials in seawater

Good corrosion resistance makes titanium alloy in a variety of corrosive conditions, can be designed with the overall structure of the same life, reduce maintenance costs and improve service capacity; reduce corrosion rich design, significantly reduce the quality of the equipment structure; no coating protection, simplify the equipment manufacturing process, reduce manufacturing costs.

1.3 Fatigue resistance. Impact resistance

Titanium has excellent impact resistance, in the range of -60 ℃ -20 ℃, impact resistance coefficient of 0.63 or more, no obvious brittle transition point, is more than 2.5 times the nickel-based alloy, more than 4 times the B30; fracture toughness is high, fracture toughness in 80 MPa-m1/2-110 MPa-m1/2, from the relationship between the toughness (KJRp ) and (R to ) curve can be seen The fracture toughness of titanium is the highest, steel is the second, aluminum alloy is the worst; fatigue strength is high, for large welded structures and shells, the number of stress cycles when generating high stress low cycle damage generally does not exceed 105 weeks, while titanium are more than 107 weeks.

2. Titanium alloy in the oil industry applications

2.1 Titanium alloy drill pipe

In recent years, in order to improve the recovery rate of crude oil, reduce the cost of oil and gas field exploitation and extend the life of oil and gas field development, there are more and more complex structure wells, such as large displacement wells. Short radius horizontal wells. The old wells are drilled horizontally with open windows. Conventional drill pipe has encountered some difficult problems when applied in these complex structure wells. For example, in ultra-short radius horizontal wells, the borehole curvature is very small (<18 m), which causes large alternating stresses in conventional drill pipe. The fatigue fracture of drill pipe caused by the alternating stress happens from time to time, which brings significant losses to the oilfield. In order to solve these problems, foreign countries have started to develop titanium alloy drill pipe from new materials. Aluminum alloy drill pipe. Composite drill pipe and so on instead of conventional drill pipe.
The laboratory confirmed that when the cyclic stress is at 30 000 psi-40 000 psi, the fatigue life of titanium is 10 times that of ordinary steel, and the elasticity coefficient of titanium alloy pipe is about 16.5 msi, which is better than steel drill pipe and more suitable for high curvature well section. At present, a better economic titanium alloy drill pipe has been produced, which is made of Ti-6AI-4V pipe body after upsetting at both ends. The size of the titanium pipe body meets the API standard. In order to reduce the manufacturing cost, the tool joint is made of steel, the threads of the joint are ground by the upper unloader, and the outer diameter of the joint is welded with wear-resistant tape (to avoid the use of ordinary welding rod after the joint is worn).
Titanium alloy drill pipe is used in ultra-deep wells. It is also used in large displacement wells. Due to the low density of titanium alloy, it can reduce the large hook load by 30%, reduce the torque by 30%～40%, significantly improve the tension margin and safety factor of drilling tools, and improve the ability to deal with accidental complications.

2.2 Titanium alloy oil casing

Most of the natural gas resources in China are sulfur-bearing gas fields, such as Xinjiang Tarim. Sichuan Basin are all high sulfur oil and gas fields. The development of highly corrosive oil and gas fields has become a major problem facing the oil and gas industry. Domestic and international research has been conducted for many years on the corrosion protection of pipes in oil and gas fields containing high H2S2. C2O2. Anti-corrosion coating. Corrosion resistant pipe. The first two can play a role in corrosion protection to a certain extent, but can not solve the high temperature and high pressure conditions of long-term service corrosion. Therefore, the selection of high-quality corrosion-resistant materials for the pipe has become the dominant trend.
Corrosion-resistant metal pipe has two main categories: nickel-based alloys and titanium alloys. At present, nickel-based alloys are more widely used, but nickel-based alloy tubes also have such complex processing technology. High material costs. The surface of the tube is easy to damage and other defects. Moreover, China is one of the countries where laterite nickel ore resources are relatively scarce, the national laterite nickel ore holdings only account for 9.6% of all nickel ore resources, and the domestic laterite nickel ore grade is low, the mining cost is relatively high, which means that China is not competitive in laterite nickel ore. Therefore, in the long run reliance on nickel-based alloys is not conducive to the strategic development of oil and gas. China is rich in titanium resources, with ilmenite reserves of 200 million tons, accounting for 28% of global reserves, ranking first in the world.
With its excellent pitting resistance, titanium alloy has been gradually applied to high sulfur environment in recent years. Especially the working conditions are harsh, CL and H2S content is very high, the general stainless steel, and even nickel-based alloys are subject to corrosion.

Through years of development, titanium alloy materials for oil casing corresponding to common steel have been formed (see below table). The mechanical properties of titanium alloy oil pipe are comparable to those of nickel-based alloy oil pipe, and the joint strength reaches the requirements of standard IS010400 for P110 steel grade.

Titanium alloy seamless steel pipe grade and performance for oil casing

The titanium alloy oil pipe developed has passed the upper unloading buckle test as stipulated in API RP 5C5, and has been subjected to bursting test, with bursting occurring at 173MPa, and the bursting shape is longitudinal cracking, which meets the requirements of use (see below figure).

2.3 Titanium alloy transmission pipeline

Because the titanium alloy pipe is easy to bend, and has a very high resistance to wave impact and platform movement caused by dynamic stress corrosion fatigue strength, titanium alloy pipe stress corrosion fatigue strength than the existing carbon steel more than 10 times, while laying the sea pipe titanium alloy pipe to adapt to the bending and deformation of the laying process conditions. Titanium alloy pipeline welding performance is good, single-sided welding double-sided forming technology to ensure uniformity and reliability of the weld seam (see below figure), and the purpose of titanium alloy pipeline mechanical properties, through the body of the tensile mechanical properties of the test. Wave impact test. Tube body crush test. Fracture toughness KIC test.

2.4 Offshore drilling platform parts. Bulkheads

The requirements of offshore drilling platforms are higher than those of land drilling platforms. As early as the 1990s, foreign countries have applied titanium alloy to various aspects of offshore drilling platforms. The use of titanium alloy as a bulkhead on the Hedwig oilfield semi-submersible floating rig of the Norwegian branch of the North Sea Oilfield reduced the mass of the whole system by 50%, thus reducing the lifting force of the bulkhead by 63%, eliminating the need for huge flexible joints and reducing the system cost by 40%, with an expected service life of up to 25 years (see below figure).

In addition, the heat exchangers in the cooling system on offshore rigs use titanium tubular cooling devices; various types of pumps (such as deep well pumps. Centrifugal pumps) are used to cast titanium pump body and titanium impeller; various valves and other fittings also use titanium products, reducing the maintenance workload (see below figure).

3. Conclusion

• (1) titanium and titanium alloy due to low density. High specific strength. Fatigue resistance. Corrosion resistance and other characteristics, the application of oil and gas exploration in the Jinfa broad prospects, recently developed inside and outside the titanium alloy drill pipe. Oil pipe. Casing. Clamps. Transport pipe. Continuous oil pipe. Valves. Pumps and many other titanium products.
• (2) In the extremely harsh corrosive environment, titanium alloy products can still meet the anti-corrosion requirements, the same length. Diameter. The cost of titanium alloy pipe with the same length, diameter, thickness and strength is lower than that of G3 nickel-based alloy, so there is a trend to replace nickel-based alloy pipe.
• (3) The addition of titanium alloy products reduces the overall mass of the system and makes the equipment more reliable, which allows the offshore drilling platform to enter deeper waters and extend the service life of the platform.

Source: China Titanium Pipes Manufacturer: www.titaniuminfogroup.com
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