*Head of QM & Application Technology, SalzgitterGroßrohre GmbH, Gottfried-Linke-Str 200, 38239 Salzgitter, Germany

Most large diameter pipe (>20”or 500mm) produced aroundthe world for gas and oilpipelines is made aslongitudinally welded pipe.

However, an alternative is spiral (alsocalled helical) welded pipe which offers theproduction advantages of low capital costsfor the plant and meets the politicallydriven desire for local pipe manufacture.Thus, there have been many spiral pipemills built around the world and more areplanned. Most of the existing plants arecharacterised by the use of simplemanufacturing techniques and qualitytesting procedures making them unsuitablefor application for high-pressures[1,2], asrequired for oil and gas lines.Consequently, companies in charge of theoperation of such pipelines have strongreservations against using spiral weldedpipe or even completely refuse to considersuch pipe for international pipelineprojects. Contrary to this opinion, spiralwelded large diameter pipe forinternational onshore and offshore pipelineprojects have been produced at Salzgitter’slarge diameter pipe mill in Germany forover 35 years, using a two-step method,employed only on a dozen other spiralwelding lines around the world.

STEEL & WIDE STRIP PRODUCTIONThe pipe manufacturing plant at Salzgitter

Two -step spiral welded pipefor pipeline applicationsThe application of a two-step production process in the manufacture ofspiral welded pipe, in which the forming operation and final weldingoperation take place on separate lines, along with the use ofthermomechanically rolled strip, has lead to the internationalacceptance of such pipe for high pressure oil and gas lines in whichlongitudinally welded pipe was formerly specified.BY DR-ING FRANZ MARTIN KNOOP*

is networked with the Salzgitter Group’ssteel plant, hot strip mill and furtherprocessing facilities. Salzgitter’s ownsteelworks exclusively provides the hotrolled coil used in pipe manufacture. Steelproduction is scheduled on an order-oriented philosophy to make steels ofproject-related chemical analysis andmechanical properties. In the wide hot stripmill, continuously cast slabs from the steelplant are rolled under thermomechanicalcontrol to coil. Microalloyed, fine-grained

pipe steels of yield strength up to 555MPa(X80) are produced.

TWO-STEP PIPE MANUFACTURINGSpiral welded pipes are manufactured indimensions from 508 to 1676mm with wallthicknesses up to 22mm. For the past 30years, steel qualities of X65 and X70 havebeen used for more than 90% of the pipeproduced. Salzgitter Großrohre isconcentrating its efforts towards the high-pressure pipeline market. Apart fromreaching the highest possible productquality, continuous improvement in processefficiency with the help of automation, anda high level of training of its workforce tomaintain and gain new accreditations, arethe focus of its efforts.

Unlike most spiral welded pipe workswhich use the conventional one-stepmethod in which pipe forming and seamwelding is completed on the same line usingautomated submerged arc welding on theinside and outside of the pipe, while still onthe pipe forming machine, Salzgitter hasused a modified two-step process since1985. In this continuous tack-welding takesplace on the pipe forming machine,followed by internal and externalsubmerged arc welding off-line at adedicated weldingstation followingcutting the pipes tolength (Fig 1).

37 Steel Times International December/January 2004

1 Two-stage spiral welded

pipe-manufacturing process

2 Three-roll pipe forming of strip followed bycontinuous tack welding

In this method, production is split intotwo distinct steps:

1 Pipe forming combined withcontinuous tack weldingThe hot-rolled wide strip with dressededges, is formed into a pipe in the pipe-forming machine. This forming unitconsists of a three-roll bending system andan external roller cage, the function ofwhich is to set the pipe axis and guaranteethe roundness of the pipe (Fig 2).

The pipe diameter (D) depends on theangle (α) at which the material enters theforming unit and the width (B) of the hot-rolled strip (Fig 3). In the forming unit theconverging strip edges are joined using a

TUBE & PIPETUBE & PIPESpiral welded pipe manufactured by the two-steps process is now accepted on international oiland gas lines

(left) Forming and tack welding (above)submerged arc welding

Continuous tack weld

SALZGITTER P 12/13/03 3:17 AM Page 2

TUBE & PIPE

39 Steel Times International December/January 2004

continuous shielded arc tack weld. As thetack welded pipe leaves the formingmachine, a plasma cutter – moving with thetube – cuts the individual lengths requiredby the customer.

2 Internal and external submerged arcwelding on a dedicated lineThe lengths of formed and continuouslytack-welded pipe are passed to one of threecomputer-controlled, internal/ externalsubmerged arc-welding stations for finalwelding (Fig 4). Using a special roller table,each pipe rotates with a precise screw-likemotion while the submerged arc welding iscarried out – first internally, then externally– using the multiwire technique. A laser-controlled seam tracking system guaranteesexact positioning of the weld seam andoptimum edge alignment. The tack-weldcarried out during the pipe-forming stageserves as a backing for the weld andbecomes fully molten during this second-stage of welding.

Trials carried out in the 1960s revealedthat dressing of the internal weld to reduceturbulence is unnecessary, and that fluidflow through a spiral welded pipe has asimilar impedance to that in alongitudinally welded pipe, the greater weldlength of the spiral pipe being compensatedfor by the smoother surface finish of stripcompared to plate from which longitudinalpipe is made.

IMPROVED YIELDThe automatic submerged arc welding ofthe so-called ‘skelp-end’ weld is anotherparticularity of the manufacturing processat Salzgitter. The skelp-end weld is used tojoin the tail of one coil to the head of thenext to enable a continuous strip to bedrawn through the pipe-forming machine.The quality of the skelp weld joining coilends, which is made on a separate CNCwelding machine placed before the formingmachine, is equivalent to that of the spiralweld itself so enabling the section of pipecontaining the skelp-end weld to be used inthe same applications as all other lengths ofpipe. This is of particularly important forpipes of heavy weight, where the customer’sacceptability of the skelp-end weld withinthe pipe length is essential for commercialefficiency and good process yield.

The two-step process offers qualitativeand economic advantages. Because of theseparation of the pipe forming and finalwelding stages, higher throughput at thepipe-former is possible (up to 12m/min coilfeed speed). Exact geometrical tolerancesare also achieved by concentrating on thepipe geometry within the pipe formingmachine – without being influenced by thesubmerged arc welding stage. In addition,the consistency of quality ofthe subsequent submerged arcweld is improved as weldingconditions can be preciselycontrolled, as welding isindependent of the formingoperation.

TESTING AND QUALITYCONTROL

With regards to the growingdemands for improvedmethods of testing – 100% ofthe weld length in each pipe is

3 Geometric dependence of pipe diameter (D) onrun-in angle �α) and strip width (B)

4 Internal, then external submerged arc weldingof the spiral weld at a dedicated station

tested – major investments have been madein recent years in the range of ultrasonictesting techniques and their automation,such that the plant now has testing facilitiesunique for spiral welded pipes. To ensurethat the coil material is free from injuriouslaminations, such as those produced byrolled in casting defects, the strip body andedge zones are completely tested usingultrasonic equipment in the run-in sectionbefore the pipe forming machine. To inspectthe spiral weld, a sophisticated laser guided,automated weld seam testing system hasrecently been put into operation for the firsttime. Thus, even for such criticalrequirements of offshore projects, eg on thebasis of the DNV rules and other stringentregulations, Salzgitter’s spiral welded pipesare equivalent to longitudinally welded pipe.

All production and testing stages aredocumented for each individual pipe in anelectronic logbook to assurecomprehensive full traceability up to thesteelmaking stage. Strict test requirementsnecessitate that each pipe is tested withregard to its application by means ofextensive individual tests (eg hydrostatic,ultrasonic, and X-ray testing, visual andgeometric inspections). Material properties– strength, toughness, hardness, andductility – are tested on samples at specifiedintervals according to standards and/ orproject specifications in the course of themanufacturing process.

PROPERTIES AND SAFETY OF SPIRALWELDED PIPES

With regard to time and cost optimisationof installation in the field, the requirementsof good forming capability and weldabilityare paramount. Modern thermomechanicaltreated pipe steels are characterised by lowcarbon contents and low carbonequivalents, this providing good weldability.Thus, the application of automated girthwelding in the field to join the lengths ofpipe is possible without restrictions. As withlongitudinally welded pipes, spiral weldedpipes can be cold-bent directly on site or canbe hot-bent within a pipe bending facilitywithout any restrictions. Thanks to its two-

step method of production, the entirelength of the spiral pipe is able to meet closetolerances with regard to diameter, ovalityand straightness. As opposed tolongitudinally welded pipes, no coldexpansion is necessary for spiral weldedpipes and no difference in tolerancesbetween pipe body and pipe ends is needed.

The mechanical properties of spiralwelded pipes exhibit a preferredorientation in the weld seam and in thedirection of rolling of the strip which isfavourable to the main direction of tensilestresses acting in the pipe. The safety ofspiral-welded pipes is thus unquestionablyhigh. Many comparative full scale crackpropagation tests carried out byindependent institutions show that thefracture behaviour and crack propagationresistance of spiral welded pipes are atleast equivalent to those of longitudinallywelded pipes[5]. The same also applies tothe fatigue resistance when the pipes areexposed to dynamically pulsating stress.

Mechanical propertiesModern pipe steels are characterised byexcellent mechanical properties,including those suitable for sour (H2Scontaining) hydrocarbons. To give anexample, the material properties of atypical X65 grade are illustrated in Fig 5by means of cumulative curves andcumulative frequency bars. (The redvertical lines define the maximum andminimum limits). The narrowdistributions seen and actual valuesrecorded far exceed the specified valuesand are proof of an outstanding stabilityin the manufacturing process. Combinedwith high average toughness values of260J at -5°C for the base material, thevalues presented for the yield strength(Fig 5a) and the ultimate tensile strength(Fig 5b) provide proof of the uniformprofile of a modern pipeline material andof the integrated quality controls carriedout in all stages of manufacture. The lowvalues of the yield to tensile ratio (Fig 5c)as well as the high elongation at rupture(Fig 5d) can also be regarded asoutstanding. In particular, in combination

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)

(a) (b) (c) (d)

SALZGITTER P 12/13/03 3:17 AM Page 3

galvanised to give low maintenance for inexcess of 20 years in most situations. CorusTubes’ parapet systems complement otherCorus highways tubular engineeringproducts, which includes a range of lightingcolumns, safety fences and materials forsign gantries, footbridges, bridges andrelated structures.

TUBE & PIPE

40 Steel Times International December/January 2004

Corus Tubes, PO Box 101, Weldon Road, Corby,Northants, NN17 5UA, UK Tel +44 (0)1536 402121Fax +44 (0)1536 404049 www.steelparapets.com

with the high average values of the yieldstrength, a high degree of safety isguaranteed to the customer. This isindependent of the fact whether a stressor strain controlled design has beenselected for the pipeline. This statementcan also be made without any restrictionsfor the properties of the spiral weld itself.

PIPELINE PROJECTSSince 1987, over 4000km of spiral weldedlarge diameter pipes for oil and gas projectshave been made at Salzgitter. The customerlist is long and includes most majorEuropean gas suppliers, such as Ruhrgas,Wingas, Gaz de France, SNAM ReteGas,Enagas, Fluxys and Gasunie – just to namea few. While, for several decades, thecustomer focus has been within Europewhere the highest possible safety isrequired for high-pressure pipelines due tothe high population density, there is agrowing demand for spiral pipe fromSalzgitter in Africa, Asia and the MiddleEast. For example, recent oil and gas lineprojects in Chad-Cameroon, South Africa,Iran and Algeria, are using spiral-weldedpipes from Salzgitter.

Often, after many years of qualificationprocedures, approval to use spiral-weldedpipe has been awarded by a potentialcustomer. This is normally the beginning ofan equally long lasting successful customer-supplier relationship. Thus, since the firstthermomechanically produced X70 hot-rolled wide strip was made at Salzgitter in1972, companies such as Ruhrgas inGermany have been supplied with suchpipe on a regular basis. Fig 6 shows spiral-welded pipe from Salzgitter being installedin a crude oil pipeline in Chad andCameroon by ExxonMobil. More than

50 000 pipes corresponding to a length ofapproximately 650km were produced inSalzgitter for this pipeline which has a totallength of 1070km. Each pipe was coated inSalzgitter, before delivery to the customer.According to the strict projectspecifications, a high-quality, three-layerHDPE (high density polyethylene) coatinghas been applied to the external surface.Pipes for application in gas or water linesare normally also coated internally.

Beyond this pipeline project, there arefurther fascinating examples giving proof ofthe quality and flexibility of the two-stepspiral welded process. Thus, during 2003,Erdgas Zürich AG put the largest pipe gasstorage plant for natural gas into operationnear Zurich with a gas volume of570 000Nm3. For this project, pipes of1524mm diameter and wall thickness19.4mm, in steel grade StE 480.7 TM havebeen used. For the 308 individual pipesdelivered, each of length 17.85m, amaximum deviation in length of just 50mmper pipe was allowed.

In conclusion, for several decades, largediameter spiral welded pipes have beensuccessfully used in oil and gas pipelineprojects and have proven their quality overtime. Employing the two-step manufacturingprocess in conjunction with high quality hotrolled wide strip, the importance of this typeof pipe is growing constantly.

Existing prejudices towards spiral weldedpipes which favour the use of longitudinallywelded pipe, are either due to poormanufacturing quality at certain pipe millsusing the single-step process – suchmanufacturing failings are also possible atother types of pipe mills – or are simplybased on the lack of experience of thecustomer in using spiral welded pipe. When

considering the fundamental possibilities ofthe sophisticated two-step process, furtherimprovements related to product quality andcommercial efficiency are clearly evident.

REFERENCES1 Jones B L; Something to get wound up about ;Petromin, July 1999, 55-602 Jones B L; Large diameter line pipe for high-pressure oil and gas transmission; Petromin, August1999, 44-493 Sommer B; Spiral welded pipe meets high-pressure needs, Oil and Gas Journal, Feb 1, 1982;106-1164 Sommer B; Knoop F M; Manufacturing and use ofSAWH-pipes – 35years of experience; Int Conf Onhelical seam submerged arc welded pipe in oil andgas industries, 16, 17 July 2002, Teheran, 1-255 Pistone V and Mannucci G; Fracture arrestcriteria for spiral welded pipes; Pipeline TechnologyConference 2000, Brugge, 455-469

Corus Tubes’ new bridge parapetsystems have been approved byUK’s Highways Agency asmeeting the new EN1317European Standard. Following

analysis of extensive development crashtesting at MIRA (Motor Industry ResearchAssociation) and computer modelling bythe Corus Automotive Engineering Groupin Coventry, UK’s Highways Agency haspassed the Corus Parapet Systems asmeeting with the containmentrequirements of EN1317.

The Corus Tubes Parapet Systems areaccepted in the Highways Agency’s InterimAdvice Note 44/02 Section 3.3 as areplacement for the now withdrawn list ofapproved systems contained in Annex F ofBS 6779-1. The new systems have beenapproved for containment classes N1(1500kg car at 80km/h) and N2 (1500kg carat 110km/h) with a working width class ofW1 (the narrowest class at <0.6m).

The new systems replaced the existingdesign in June 2003 and Corus has nowwithdrawn support for the old versions(except HBP 100/3 Vertical Infill). The newdesign will only be available via licensedfabricators who will be supplied with thespecial Corus Celsius HBP RHS(Rectangular Hollow Section) from CorusTubes’ Special Projects in Corby. The

Tubular steel bridge parapets approved for UK highwaysparapet system design offers moreadaptable post positioning and easierfabrication than the previous version aswell as improved perfor-mance allround. Certain design features of theCorus Tubes Parapet Systems areprotected by Patents Pending.

The Corus Bridge Parapet Systemscome with a history of successfulapplication stretching back nearly 50years. In this latest improved version, toEN1317, they safely contain vehicleswithin the narrowest working width andare now simpler to fabricate and erect.

The new standard EN1317-5 waspublished in autumn 2003 and willprovide a Europe-wide benchmark forroad restraint systems including highwaybridge parapets. This standard should allowfor a more open market in the provision ofparapets and improve road safety.

The well proven structural properties ofhot finished RHS, which is fully qualityassured, has enabled a product to be madeto meet the stringent new performancerequirements of EN 1317. The special steelgrade material for parapets is branded asCelsius HBP. These are the only steelparapets accepted by UK’s HighwaysAgency in the Normal Containmentcategory.

The tubular steel design is fully hot dip

Corus’ ‘Celsius HBP’ is the only steel parapetmaterial presently in UK that meets the new EN1317 specification for road parapets

6 Construction of oil pipeline using spiral weldedpipe for the Chad - Cameroon DevelopmentProject (photo: ExxonMobil)

STI

STI

SALZGITTER P 12/13/03 3:17 AM Page 4

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.