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TUBE & PIPE Spiral welded pipe manufactured by the two steps process process is now accepted accepted on internation international al oil and gas lines
pipe steels of yield strength up to 555MPa (X80) are produced.
Two -step spiral welded pipe for pipeline applications The application of a two-step production process in the manufacture of spiral welded pipe, in which the forming operation and final welding operation take place on separate lines, along with the use of thermomechanically rolled strip, has lead to the international acceptance of such pipe for high pressure oil and gas lines in which longitudinally welded pipe was formerly specified. BY DR-ING FRANZ MARTIN KNOOP*
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ost large diameter pipe (>20” or 500mm) produced around the world for gas and oil pipelines is made as longitudinally welded pipe. However, an alternative is spiral (also called helical) welded pipe which offers the production advantages of low capital costs for the plant and meets the politically driven desire for local pipe manufacture. Thus, there have been many spiral pipe mills built around the world and more are planned. Most of the existing plants are characterised by the use of simple manufacturing techniques and quality testing procedures making them unsuitable for application for high-pressures [1,2], as required for oil and gas lines. Consequently, companies in charge of the operation of such pipelines have strong reservations against using spiral welded pipe or even completely refuse to consider such pipe for international pipeline projects. Contrary to this opinion, spiral welded large diameter pipe for international onshore and offshore pipeline projects have been produced at Salzgitter’s large diameter pipe mill in Germany for over 35 years, using a two-step method, employed only on a dozen other spiral welding lines around the world. world.
STEEL & WIDE STRIP PRODUCTIO PRODUCTION N The pipe manufacturing plant at Salzgitter
TWO-STEP PIPE MANUFACTURING Spiral welded pipes are manufactured in dimensions from 508 to 1676mm with wall thicknesses up to 22mm. For the past 30 years, steel qualities of X65 and X70 have been used for more than 90% of the pipe produced. Salzgitter Großrohre is concentrating its efforts towards the highpressure pipeline market. Apart from reaching the highest possible product quality, continuous improvement in process efficiency with the help of automation, and a high level of training of its workforce to maintain and gain new accreditations, are the focus of its efforts. Unlike most spiral welded pipe works which use the conventi conventional onal one-ste one-step p method in which pipe forming and seam welding is completed on the same line using automated submerged arc welding on the inside and outside of the pipe, while still on the pipe forming machine, Salzgitter has used a modified two-step process since 1985. In this continuous tack-welding takes place on the pipe forming machine, followed by internal and external submerged arc welding off-line at a dedicated welding station following cutting the pipes to length (Fig 1).
1 Two-stage spiral welded pipe-manufacturing pipe-manufa cturing process process (left) Forming and tack welding (above) submerged arc welding welding 2 Three-roll pipe forming of strip followed by continuous tack welding
Continuous tack weld
is networked with the Salzgitter Group’s steel plant, hot strip mill and further processing facilities. Salzgitter’s own steelworks exclusively provides the hot rolled coil used in pipe manufacture. Steel production is scheduled on an orderoriented philosophy to make steels of project-related chemical analysis and mechanical properties. In the wide hot strip mill, continuously cast slabs from the steel plant are rolled under thermomechanical control to coil. Microalloyed, fine-grained *Head of QM & Application Technology, Salzgitter Großrohre GmbH, Gottfried-Linke-Str 200, 38239 Salzgitter, Germany
In this method, production is split into two distinct steps:
1 Pipe forming combined with continuous tack welding
The hot-rolled wide strip with dressed edges, is formed into a pipe in the pipeforming machine. This forming unit consists of a three-roll bending system and an external roller cage, the function of which is to set the pipe axis and guarantee the roundness of the pipe ( Fig 2). The pipe diameter (D) depends on the angle (α) at which the material enters the forming unit and the width (B) of the hotrolled strip (Fig 3). In the forming unit the converging strip edges are joined using a
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TUBE & PIPE continuous shielded arc tack weld. As the tack welded pipe leaves the forming machine, a plasma cutter – moving with the tube – cuts the individual lengths required by the customer.
2 Internal and external submerged arc welding on a dedicated line
The lengths of formed and continuously tack-welded pipe are passed to one of three computer-controlled, internal/ external submerged arc-welding stations for final welding (Fig 4). Using a special roller table, each pipe rotates with a precise screw-like motion while the submerged arc welding is carried out – first internally, then externally – using the multiwire technique. A lasercontrolled seam tracking system guarantees exact positioning of the weld seam and optimum edge alignment. The tack-weld carried out during the pipe-forming stage serves as a backing for the weld and becomes fully molten during this secondstage of welding. Trials carried out in the 1960s revealed that dressing of the internal weld to reduce turbulence is unnecessary, and that fluid flow through a spiral welded pipe has a similar impedance to that in a longitudinally welded pipe, the greater weld length of the spiral pipe being compensated for by the smoother surface finish of strip compared to plate from which longitudinal pipe is made.
IMPROVED YIELD The automatic submerged arc welding of the so-called ‘skelp-end’ weld is another particularity of the manufacturing process at Salzgitter. The skelp-end weld is used to join the tail of one coil to the head of the next to enable a continuous strip to be drawn through the pipe-forming machine. The quality of the skelp weld joining coil ends, which is made on a separate CNC welding machine placed before the forming machine, is equivalent to that of the spiral weld itself so enabling the section of pipe containing the skelp-end weld to be used in the same applications as all other lengths of pipe. This is of particularly important for pipes of heavy weight, where the customer’s acceptability of the skelp-end weld within the pipe length is essential for commercial efficiency and good process yield. The two-step process offers qualitative and economic advantages. Because of the separation of the pipe forming and final welding stages, higher throughput at the pipe-former is possible (up to 12m/min coil feed speed). Exact geometrical tolerances are also achieved by concentrating on the pipe geometry within the pipe forming machine – without being influenced by the submerged arc welding stage. In addition, the consistency of quality of the subsequent submerged arc weld is improved as welding conditions can be precisely controlled, as welding is independent of the forming operation.
3 Geometric dependence of pipe diameter (D) on run-in angle Ꮙα ) and strip width (B)
4 Internal, then external submerged arc welding of the spiral weld at a dedicated station
tested – major investments have been made in recent years in the range of ultrasonic testing techniques and their automation, such that the plant now has testing facilities unique for spiral welded pipes. To ensure that the coil material is free from injurious laminations, such as those produced by rolled in casting defects, the strip body and edge zones are completely tested using ultrasonic equipment in the run-in section before the pipe forming machine. To inspect the spiral weld, a sophisticated laser guided, automated weld seam testing system has recently been put into operation for the first time. Thus, even for such critical requirements of offshore projects, eg on the basis of the DNV rules and other stringent regulations, Salzgitter’s spiral welded pipes are equivalent to longitudinally welded pipe. All production and testing stages are documented for each individual pipe in an electronic logbook to assure comprehensive full traceability up to the steelmaking stage. Strict test requirements necessitate that each pipe is tested with regard to its application by means of extensive individual tests (eg hydrostatic, ultrasonic, and X-ray testing, visual and geometric inspections). Material properties – strength, toughness, hardness, and ductility – are tested on samples at specified intervals according to standards and/ or project specifications in the course of the manufacturing process.
step method of production, the entire length of the spiral pipe is able to meet close tolerances with regard to diameter, ovality and straightness. As opposed to longitudinally welded pipes, no cold expansion is necessary for spiral welded pipes and no difference in tolerances between pipe body and pipe ends is needed. The mechanical properties of spiral welded pipes exhibit a preferred orientation in the weld seam and in the direction of rolling of the strip which is favourable to the main direction of tensile stresses acting in the pipe. The safety of spiral-welded pipes is thus unquestionably high. Many comparative full scale crack propagation tests carried out by independent institutions show that the fracture behaviour and crack propagation resistance of spiral welded pipes are at least equivalent to those of longitudinally welded pipes[5]. The same also applies to the fatigue resistance when the pipes are exposed to dynamically pulsating stress.
PROPERTIES AND SAFETY OF SPIRAL WELDED PIPES With regard to time and cost optimisation of installation in the field, the requirements of good forming capability and weldability are paramount. Modern thermomechanical treated pipe steels are characterised by low carbon contents and low carbon equivalents, this providing good weldability. Thus, the application of automated girth welding in the field to join the lengths of pipe is possible without restrictions. As with longitudinally welded pipes, spiral welded pipes can be cold-bent directly on site or can be hot-bent within a pipe bending facility without any restrictions. Thanks to its two-
Mechanical properties Modern pipe steels are characterised by excellent mechanical properties, including those suitable for sour (H2S containing) hydrocarbons. To give an example, the material properties of a typical X65 grade are illustrated in Fig 5 by means of cumulative curves and cumulative frequency bars. (The red vertical lines define the maximum and minimum limits). The narrow distributions seen and actual values recorded far exceed the specified values and are proof of an outstanding stability in the manufacturing process. Combined with high average toughness values of 260J at -5°C for the base material, the values presented for the yield strength (Fig 5a) and the ultimate tensile strength (Fig 5b) provide proof of the uniform profile of a modern pipeline material and of the integrated quality controls carried out in all stages of manufacture. The low values of the yield to tensile ratio (Fig 5c) as well as the high elongation at rupture (Fig 5d) can also be regarded as outstanding. In particular, in combination
TESTING AND QUALITY CONTROL With regards to the growing demands for improved methods of testing – 100% of the weld length in each pipe is
(a)
(b)
(c)
(d)
5 Mechanical properties of X65 spiral welded pipes (1220mm od and 18.9mm wall thickness) used for a high pressure gas pipeline in Italy (specified min/ max values shown as red vertical lines)
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TUBE & PIPE with the high average values of the yield strength, a high degree of safety is guaranteed to the customer. This is independent of the fact whether a stress or strain controlled design has been selected for the pipeline. This statement can also be made without any restrictions for the properties of the spiral weld itself.
PIPELINE PROJECTS Since 1987, over 4000km of spiral welded large diameter pipes for oil and gas projects have been made at Salzgitter. The customer list is long and includes most major European gas suppliers, such as Ruhrgas, Wingas, Gaz de France, SNAM ReteGas, Enagas, Fluxys and Gasunie – just to name a few. While, for several decades, the customer focus has been within Europe where the highest possible safety is required for high-pressure pipelines due to the high population density, there is a growing demand for spiral pipe from Salzgitter in Africa, Asia and the Middle East. For example, recent oil and gas line projects in Chad-Cameroon, South Africa, Iran and Algeria, are using spiral-welded pipes from Salzgitter. Often, after many years of qualification procedures, approval to use spiral-welded pipe has been awarded by a potential customer. This is normally the beginning of an equally long lasting successful customersupplier relationship. Thus, since the first thermomechanically produced X70 hotrolled wide strip was made at Salzgitter in 1972, companies such as Ruhrgas in Germany have been supplied with such pipe on a regular basis. Fig 6 shows spiral welded pipe from Salzgitter being installed in a crude oil pipeline in Chad and Cameroon by ExxonMobil. More than
50 000 pipes corresponding to a length of approximately 650km were produced in Salzgitter for this pipeline which has a total length of 1070km. Each pipe was coated in Salzgitter, before delivery to the customer. Accordi ng to the strict project specifications, a high-quality, three-layer HDPE (high density polyethylene) coating has been applied to the external surface. Pipes for application in gas or water lines are normally also coated internally. Beyond this pipeline project, there are further fascinating examples giving proof of the quality and flexibility of the two-step spiral welded process. Thus, during 2003, Erdgas Zürich AG put the largest pipe gas storage plant for natural gas into operation near Zurich with a gas volume of 570000 Nm3. For this project, pipes of 1524mm diameter and wall thickness 19.4mm, in steel grade StE 480.7 TM have been used. For the 308 individual pipes delivered, each of length 17.85m, a maximum deviation in length of just 50mm per pipe was allowed. In conclusion, for several decades, large diameter spiral welded pipes have been successfully used in oil and gas pipeline projects and have proven their quality over time. Employing the two-step manufacturing process in conjunction with high quality hot rolled wide strip, the importance of this type of pipe is growing constantly. Existing prejudices towards spiral welded pipes which favour the use of longitudinally welde d pipe, are either due to poor manufacturing quality at certain pipe mills using the single-step process – such manufacturing failings are also possible at other types of pipe mills – or are simply based on the lack of experience of the customer in using spiral welded pipe. When
6 Construction of oil pipeline using spiral welded pipe for the Chad - Cameroon Development Project (photo: ExxonMobil)
considering the fundamental possibilities of the sophisticated two-step process, further improvements related to product quality and commercial efficiency are clearly evident. REFERENCES
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1 Jones B L; Something to get wound up about ; Petromin, July 1999, 55-60 2 Jones B L; Large diameter line pipe for highpressure oil and gas transmission; Petromin, August 1999, 44-49 3 Sommer B; Spiral welded pipe meets highpressure needs, Oil and Gas Journal, Feb 1, 1982; 106-116 4 Sommer B; Knoop F M; Manufacturing and use of SAWH-pipes – 35years of experience; Int Conf On helical seam submerged arc welded pipe in oil and gas industries, 16, 17 July 2002, Teheran, 1-25 5 Pistone V and Mannucci G; Fracture arrest criteria for spiral welded pipes; Pipeline Technology Conference 2000, Brugge, 455-469
Tubular steel bridge parapets approved for UK highways
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orus Tubes’ new bridge parapet systems have been approved by UK’s Highways Agency as meeting the new EN1317 European Standard. Following analysis of extensive development crash testing at MIRA (Motor Industry Research Association) and computer modelling by the Corus Automotive Engineering Group in Coventry, UK’s Highways Agency has passed the Corus Parapet Systems as meeting with the containment requirements of EN1317. The Corus Tubes Parapet Systems are accepted in the Highways Agency’s Interim Advice Note 44/02 Section 3.3 as a replacement for the now withdrawn list of approved systems contained in Annex F of BS 6779-1. The new systems have been approved for containment classes N1 (1500kg car at 80km/h) and N2 (1500kg car at 110km/h) with a working width class of W1 (the narrowest class at <0.6m). The new systems replaced the existing design in June 2003 and Corus has now withdrawn support for the old versions (except HBP 100/3 Vertical Infill). The new design will only be available via licensed fabricators who will be supplied with the special Corus Celsius HBP RHS (Rectangular Hollow Section) from Corus Tubes’ Special Projects in Corby. The
parapet system design offers more adaptable post positioning and easier fabrication than the previous version as well as improved perfor-mance all round. Certain design features of the Corus Tubes Parapet Systems are protected by Patents Pending. The Corus Bridge Parapet Systems come with a history of successful application stretching back nearly 50 years. In this latest improved version, to EN1317, they safely contain vehicles within the narrowest working width and are now simpler to fabricate and erect. The new standard EN1317-5 was published in autumn 2003 and will provide a Europe-wide benchmark for road restraint systems including highway bridge parapets. This standard should allow for a more open market in the provision of parapets and improve road safety. The well proven structural properties of hot finished RHS, which is fully quality assured, has enabled a product to be made to meet the stringent new performance requirements of EN 1317. The special steel grade material for parapets is branded as Celsius HBP . These are the only steel parapets accepted by UK’s Highways Agency in the Normal Containment category. The tubular steel design is fully hot dip
Corus’ ‘Celsius HBP’ is the only steel parapet material presently in UK that meets the new EN 1317 specification for road parapets
galvanised to give low maintenance for in excess of 20 years in most situations. Corus Tubes’ parapet systems complement other Corus highways tubular engineering products, which includes a range of lighting columns, safety fences and materials for sign gantries, footbridges, bridges and related structures. STI
Corus Tubes, PO Box 101, Weldon Road, Corby, Northants, NN17 5UA, UK Tel +44 (0)1536 402121 Fax +44 (0)1536 404049 www.steelparapets.com
