Trends in modern manufacturing of oil country tubular goods

PROGRESSING TOWARDS PERFECTION OR THE CORROSION RESISTANT LIFE

Trends in modern manufacturing of oil country tubular goods

Yevgeni KUZNETSOV, doctor of technical sciences, Magnesite Concern

Tube mills of Ukraine mainly specialise in tubes & pipes of low-alloy carbon steels. Drill pipes, casing and oil well tubing, i.e. the oil country tubular goods, have always accounted for a larger share of seamless tubes. Currently, Ukrainian tube mills have found themselves in a difficult financial situation due to antidumping restrictions on exports to developed countries, problems with seamless steel tube exports to Russia and immature domestic market. Therefore, analysis of modern trends in the tube business should suggest a cure to problems.

Exploration of petroleum and natural gas

The sharp increase in prices for crude oil in the second half of 1999 spurred quite a few large oil companies towards greater oil extraction. Certain countries attempt to extract oil and natural gas from extreme depth. Foreign forecasts show that in 2000 through 2004 prospecting and development costs in deep-water petroleum exploration will double to around US$20 billion. The largest multinationals, e.g. BP Amoco Chevron, Exxon-Mobil, Petrobras, Texaco and TotalFin-Elf, have re-oriented their efforts towards development of the shelves of Guinea, the USA, Brazil, Mexico and certain parts of the Far East.

Companies have shifted towards deep-water exploration owing to impairment of mainland and shallow-water wells and due to growing demand for petroleum products. Political instability in the Middle East and Central Asia, together with high cost of construction and maintenance of transcontinental pipelines, adds to those problems. To cope with these issues, the United States and some other oil countries give significant tax incentives to offshore drilling platform exploration.

Offshore exploration has been developing deeper down. For instance, the maximum oil & gas extraction depth was 60 meters until 1960, over 600 m in 1990, while today’s leaders are the 858-meter-deep well on Mexico’s shelf and Petrobras’ 1,850-meter-deep well on the coast of Brazil.

According to certain estimates, petroleum exploration will account for about 90% of the 134 deep-water projects (at the depth of over 300 meters) in 1998 to 2001. Today, the biggest deep-water oilfields are Girasol, Dalia and Agbami in Western Africa, and Crazy Horse and Mad Dog on the US shelf. To reach large-scale extraction at little time, intensive development projects regarding these oilfields provide for application of novel technologies that rest on corrosion-resistant materials.

Corrosion-resistant materials for extreme depth exploration

Quite a few people believe that there are still no such corrosion-resistant materials (CRM) that fully meet the requirements of extreme depth exploration. Reliability and durability of pipelines is of paramount importance in this case due to high environmental risks peculiar to this business.

Over the last 25 years, foreign undersea pipelines have been mainly made of duplex stainless steels and bimetallic substances (previously, martensite steels were made use of). The Norwegians use titanium-based tubes and pipes in the North Sea. However, all these materials have been applied at the depth not exceeding 300 meters.

The greater depths need materials with different qualities. As pipelines in this case are subject to higher stress, the technology of joining pipes together has to be altered. Should the metal in the transverse joint have a lower yield point than the base metal, the main strain will concentrate in that area threatening the tightness of the whole pipeline. Although it is extremely difficult to give the welding metal the same yield strength as to the rest of metal when applying martensite stainless steels, these materials have a limited scope of application. However, these steels are good for operation in environments with high CO2 contents.

Cathodic protection at extreme depth is yet another critical point. The available CRMs provide protection against hydrogen brittleness only in a narrow range of potential differences. However, metal surfaces at extreme depth may be exposed to certain bacteria that alter the properties of metal (at least, this phenomenon is observed in the Black Sea). A number of scientific institutions already work on protection of pipelines against biological effect.

In certain instances, pipelining at extreme depth will require enhanced strength of the body material and will need thick-walled pipes with uniform microstructure and uniform impact strength. That is when bimetallic and clad tubes can be made use of. Certain experts view titanium-alloy tubes as a good material for platform posts due to their nice fatigue properties and strength. Additionally, titanium alloys are not exposed to hydrogen brittleness in the seawater.

Some 8,500 km of tubes for pipeline posts, less than 406 mm in diameter, and 5,800 km of control pipes will be required for the 134 deep-water oilfields that will be developed in the coming future. This means over 500,000 tonnes of tubes and pipes made of high-alloy steels. Meanwhile, Ukraine could have never make that many tubes even of regular stainless steels.

New international standards on oil country tubular goods

Emergence of new international standards evidences higher interest to oil country tubular goods (OCTG).

In June 2000 the International Organisation for Standardisation issued ISO 13680 “Petroleum and natural gas industries – Corrosion-resistant alloy seamless tubes for use as casing, tubing and coupling stock – Technical delivery conditions”. The standard regulates the use of tubes made of new CRMs and expands the series of earlier OCTG standards, like ISO 11960 and ISO 11961.

Extreme depth exploration of petroleum and natural gas poses new requirements to OCTG. A large portion of drill pipes is cold-shaped with a determined set of mechanical properties and impact strength in longitudinal and cross section. Small-diameter tubes of duplex and super-duplex steels, including capillary tubes, find broad application in control lines of deep-water wells. The new ISO defines the procedure of corrosion tests for those and other types of steel.

International standard DIN EN 1026 “Nondestructive testing of steel tubes” was also enforced in June 2000. The standard and its 18 appendices lay down the requirements for nondestructive testing methods and devices for tubes and sheets, testing procedures and evaluation of testing personnel. Notably enough, the standard provides for higher quality of nondestructive testing than Ukrainian statutory requirements.

Manufacture of tubes of CRMs

The technology of making tubes of CRMs is considerably different from that for tubes of carbon and low-alloy steels. The key aspects are the type of CRM, stability and reliability of the manufacturing technology, cost of tubes and delivery terms.

A CRM is selected on the very first stage of the project depending on the operating environment. The principal properties embrace chemical composition, project temperature, pressure and some others. Besides, one should consider the speed of pipelining the liquid or gaseous products, exposure to erosion, temperature fluctuations, etc. Corrosion resistance in aggressive environment is the main performance parameter. These materials need to have high resistance to hydrogen brittleness, saturation with carbon and sulphides, as well as should have certain specific mechanical properties, like ductility at low temperatures, high-temperature creeping, fatigue strength, good weldability and plasticity. Frequently, CRMs are selected regardless limited information on the materials’ properties, thus giving birth to high risk for costly and important equipment. All these matters are not that important for regular tubes of carbon steels.

A wide range of CRMs is mass-produced nowadays, e.g. austenite and martensite stainless steels, alloys based on nickel, titanium, zirconium, etc. The headlong technical progress in the second half of the 20th century posed demand for new materials, including corrosion-resistant steels and nickel-based alloys. Table 1 shows the chronology of appearance of those materials, whereas table 2 ranks them by properties.

If one compares these data with manufacturing plans of Nikopol Pivdennotrubny Works (virtually the sole producer of tubes of CRMs in Ukraine), he can conclude that this mill currently makes the grades that used to be widespread in the world back in the 1960s-1970s.

Super-duplex stainless steels – a new generation of stainless – have risen in the last decade. These steels have higher alloy contents compared to regular stainless steels, as well as contain Cu and W (see table 3 for comparative chemical analysis).

Industrial tests prove that those steels enjoy better corrosion resistance, operate in aggressive environments and have the enhanced mechanical properties. Nitrogen alloying of steels ensures a stable 50 to 50 ratio of austenite to ferrite contents. Chemical and petroleum industries have been using such steels recently. These steels are especially effective in control lines in offshore exploration.

Production of various types of duplex grades has further expanded over the past decade. The world market has seen emergence of titanium, palladium and ruthenium alloys that have high corrosion resistance and nice technological properties and are applied in rolling products, including seamless and welded tubes.

Efforts to create new CRMs are rather intensive. For instance, in 1998 the EU member-states prohibited the use of antirust inhibitors when pipelining petroleum and natural gas. Therefore, tubes made of carbon and low-alloy steels, including the familiar X13 martensite steels, could no longer be used. However, as soon as in mid-2000 there appeared a new martensite stainless steel that hardens when exposed to plastic strain and costs less than nickel-based CRMs. The test batch of tubes manufactured in 2000 showed acceptable performance.

Stable and reliable technologies are the principal factors for tubes of CRMs. The users set exceptionally high requirements to manufacturers due to expensiveness of end equipment and the associated risks of environmental accidents and casualties. A large order for components and structural materials of offshore platforms is normally placed in the following sequence. Firstly, a company announces a tender, defines technical requirements of the project and spells the requirement for international quality certificates issued by the leading global centres. Lately, tube makers have been certifying their own manufacturing facilities, together with their suppliers of billets and hollow sections, machinery and other materials. A small test batch is produced on the next stage. Frequently, the client sends his own experts to observe main technological operations (normally, this is stipulated for in the contract). It is very important for tube mills to have modern equipment and technologies to test technical, structural and corrosion resistant properties of tubes, to have nondestructive testing devices, trained personnel and the like. It is this stage that decides it all for the main order.

Cost and delivery terms have always been a problem for producers of tubes and pipes of CRMs.

This kind of tubes costs several times as high as tubes of carbon steel. Table 4 shows the comparable prices. However, the price difference is incomparable with the spread in manufacturing costs of important equipment. Here is an example: a project of a plant for primary treatment of natural gas. Initial conditions: engineered useful life of the plant is to exceed 25 years, whereas the untreated natural gas contains water and corrosive components, including CO2 and chlorides. There may be applied tubes made of carbon steel and duplex stainless steel, grade UNS S31803. If a decision is based on price only, tubes made of carbon steel are the absolute winners. However, that decision would imply additional maintenance costs due to corrosion impairment of metal, permanent costs of inhibitors and costs of monitoring their effectiveness, regular painting costs. In 10 years all tubes will have to be replaced, which means disassemblage and assemblage costs and losses due to down time of the whole plant.

The Ukrainian domestic prices for tubes made of carbon and simple austenite stainless steels (like 08Х18Н10Т) show a higher difference of 1/8 to 1/10 (see table 4). This mainly happens owing to high normal loss of metal and deficient technologies and equipment applied at Ukraine’s tube mills.

Foreign producers pay special heed to lower manufacturing costs. Production of titanium tubes is the biggest success in that direction.

Long delivery periods are the next major deficiency of tubes made of CRMs. It is regular thing for many foreign firms to deliver tubes in 5 to 6 months after sealing the contract. Reduction in these terms is a weighty competitive advantage. Enhancement and intensification of manufacturing efforts can shorten the delivery terms. There are many technical approaches to that task, with wider use of welded tubes being one of them.

A short while ago, power plants throughout the world used seamless tubes only. Today, one can see a strong shift to welded tubes, largely due to lower cost of such tubes and shorter delivery terms. Currently, up to 60% of all foreign-made boiler tubes made of carbon and stainless steels are welded.

Quality of the welded joint and the size of residual surface irregularities are among the key points in modern production of tubes of CRMs. Barbs and irregularities are removed from the outside and the inside of tubes. In certain cases, additional cold shaping is applied with subsequent heat treatment in order to reach full recrystallisation and homogenisation of the weld. Corrosion resistance of the weld is crucial. Insufficient density of the weld or negligent smoothing of surface irregularities may result in crevice corrosion. Therefore, technologists should prove that a welded joint is corrosion-resistant. To do this, tube mills equip themselves with devices to control temperature, edge compression forces, position of shaping cylinders and some other parameters in the welding area. Complex nondestructive testing is a must.

Summing up this analysis, one may notice that production of tubes of CRMs has a number of technical features different from carbon steels.

Prospects for tubes of CRMs

There is enough ground to anticipate a major breakthrough in production of CRMs in the nearest future. Foreign companies have applied their 15-year developments to create fundamentally new technologies for various types of rolled products. Notably enough, new heat-resistant materials, 6% aluminium contents on the surface, are already produced on the basis of the phenomenon of composite coatings’ appearance on metal surfaces. This scheme can be applied to make a wide range of CRMs, including those with combined properties.

Apparently, the technology of welded tube making shall see some progress soon. At the moment, many foreign firms apply laser welding to tubes of CRMs. Powerful laser machinery and equipment that were been exhibited at the Tubes-2000 Expo back this forecast. Using this technology, one can make thick-walled tubes of high-alloy steels with reliable welds and without any significant surface irregularities. Additionally, the new technology exposes only a small area to heat and reduces normal loss.

Information

Recent publications in the mass media show that the petroleum industry is considered the key consumer of tubes and pipes made of CRMs. Greater efforts in development of CRMs and advanced technologies call for market analysis and timely and reliable information on the qualities of new materials.

A number of countries regularly hold international conferences dedicated to production and use of CRMs that are attended by lead scientists, manufacturers and consumers from the USA, Europe and other regions of the world. During the conferences, skilled professionals share their solutions to and opinions on the main scientific and production issues and certain CRMs.

Organisation and preparation to such events is a matter of extra importance. For instance, preparations to the coming Stainless Steel World – America 2002 Expo and Conference was kicked off in mid-2000. That conference is dedicated to cost saving and enhanced reliability for petroleum and natural gas industries because manufacturers and consumers have to deal with limited budgets and short project execution terms.

Notably enough, potential participators had to submit their reports on production and use of CRMs in advance. Here is a tentative list of issues to be discussed at the conference:

• optimising the chemical composition of alloys;

• methods of surface treatment, application of corrosion-resistant materials in heat exchangers and in petroleum exploration at extreme depth;

• possible replacement of seamless tubes with welded;

• choosing the material, and the cost of durability;

• prospects for international e-commerce;

• global harmonisation of standards and technological conditions;

• globalisation of ASME codes with respect to European practices, etc.

With this principle at the core of specialised conferences, they are doomed to success. Much heed is given to selection of the chairmen who organise discussions and formulate conclusions on the issues discussed.

Widespread use of the Internet resource is also a grand trend of the day.

Altogether, this gives rise to an informal club where tube consumers and manufacturers, designers and producers of new materials share their opinions, set tasks and evaluate the results achieved.

Conclusion

This article is not a call for total alteration of Ukraine’s tube sector towards oil country tubular goods made of corrosion-resistant materials. Apparently, there are some other ways out of the crisis to be analysed. Moving out of the recession requires big investment in research and development and in new machinery for the tube sector and the quality steel sector. These tasks can indeed be completed with success; for instance, back in the 1980s Ukraine established production of tubes of high-alloy steels for use in active zones of nuclear reactors.

Mainly, the current recession in Ukraine’s tube sector has sprung from almost no investment in capital upgrades of manufacturing facilities, while the world market has altered its requirements. Quick and successful decision on the future of tube mills in Ukraine will determine the welfare of workers and economic safety of the whole country.

Table 1. Chronology of austenite steels and nickel-based alloys

Table 2. Gradation of corrosion-resistant steels and alloys

Table 3. Chemical composition of super-duplex stainless steels

Table 4. Prices for tubes & pipes made of different materials