Offshore Application of HFI-Pipes Production and Properties Hendrik Löbbe Mannesmann Fuchs Rohr Germany Abstract High Frequency Inductive (HFI) welding is a highly productive process for the fabrication of longitudinally welded pipes from hot rolled strip. Longitudinally welded HFI-Pipes are nowadays used in the most diverse range of applications. Typical applications are for example, pipelines for the conveyance of liquid and gaseous hydrocarbons, potable and utility water, brine, district heating systems, hollow sections and Oil Country Tubular Goods (OCTG). HFI-Pipes are increasingly coming into use in the offshore sector, in competition with SAW and seamless pipes. The precondition for their use in this case is the capability for production of pipes with increasing wall thicknesses and very tight and homogenous geometric tolerances. Mannesmann Fuchs Rohr (mills in both Siegen and Hamm, Germany) produces HFI pipes in O.D. 4-20 (114.3-508.0 mm) with wall thicknesses up to t = 0.81 (20.6 mm). Special requirements originating from the offshore sector are thus last but not least fulfilled via the use of high quality primary material with large pipe wall thicknesses. Key steps of the process chain of the production process as well as significant properties of HFI-Pipes from Mannesmann Fuchs Rohr regarding the use in the offshore-sector are discussed. Very important from the production process point of view is the welding technology. A uniform heating of the strip edges is indispensable. Therefore the High Frequency Induction Welding plays a key role since it enables the use of welding frequencies in the lower range of the HF technology. Through this especially in the case of welding higher wall thickness the heating of the entire strip edge at the corners as well as in the middle of the strip is ensured. Another important production step is the sizing of the pipe string. This sizing influences the mechanical values of the pipe as well as the geometrical values. Both result in good properties regarding the use in the offshore sector. The third explained partial stage of the production process is the US testing of the weld seam. Besides the properties of the black pipes Mannesmann Fuchs Rohr offers a special coating system for the offshore sector. This so called rough coating is designed on the basis of the well known 3-layer PE- and PP-coating of Mannesmann (MAPEC) and is furthermore characterised by a rough surface. The rough surface has in this case different advantages regarding the additional application of heavy coating, the forces during pipe laying and safety aspects. HFI-Pipes from Mannesmann Fuchs Rohr have been used in several projects over a period of more than 10 years. Different projects and the experiences of the customers are described.
1 Introduction High Frequency Inductive (HFI) welding is a highly productive process for the fabrication of longitudinally welded pipes from hot rolled strip. Longitudinally welded HFI-Pipes are nowadays used in the most diverse range of applications. Typical applications are for example, pipelines for the conveyance of liquid and gaseous hydrocarbons, potable and utility water, brine, district heating systems, hollow sections and Oil Country Tubular Goods (OCTG). HFI-Pipes are increasingly coming into use in the offshore sector, in competition with SAW and seamless pipes. The precondition for their use in this case is the capability for production of pipes with increasing wall thicknesses and very tight and homogenous geometric tolerances. Mannesmann Fuchs Rohr (MFR) produces HFI pipes in O.D. 4-20 (114.3-508.0 mm) with wall thicknesses up to t = 0.81 (20.6 mm). Special requirements originating from the offshore sector are thus last but not least fulfilled via the use of high quality primary material with large pipe wall thicknesses. This all makes HFIpipes genuine alternative in their wall thickness and diameter range to hot rolled seamless and SAW pipes. 2 Company Mannesmann Fuchs Rohr is a worldwide partner for HFI longitudinally welded steel pipe at the forefront of technology. This includes oil and gas line pipe, pipe for drinking water and sewage systems, tubes for machinery and plant construction as well as oilfield tubulars, pipe for long-distance heating systems and structural tubes. The MFR manufacturing programme is rounded off by a wide range of coating and lining systems, fittings, pipe accessories and project related services. /1/ MFR offers:! over 100 years experience in pipe production,! expertise covering the complete value-added chain from steel production to pipe coating,! a wide range of dimensions and material grades,! short delivery periods through optimized rolling schedules,! heightened flexibility based on two production locations,! optimised delivery capacity through targeted stock management,! technical advice from the planning desk to the construction site and! a worldwide sales network. MFR belongs to the Mannesmannröhren-Werke AG being an affiliate of the Salzgitter AG. MFR with two production sites in Siegen and Hamm, Germany, produces about 300,000 to per year with approx. 590 employees. The export amount is about 70 %.
3 Production Technology The production of HFI-welded pipes is done in a continuous process out of hot wide strip. Multiple production steps have to be executed from hot wide strip to the finished pipe. Against the background of the application of heavy-wall HFI-pipes in the offshore sector essential starting points are given in the context of production technology. 3.1 Welding To achieve a constant high weld seam quality the strip edges must be milled exactly to the necessary size and geometry. Milling heavy-wall hot wide strip a high material removal rate is accumulated. The milled chips have to be removed 100% in order not to damage the already milled strip edges. Regarding the welding technology the current frequency is of vital importance. The heating profile at the strip edges can be manipulated, inter alia, via the selection of frequency (Fig. 1). It must be technologically assured that the skin effect is achieved. The penetration depth of the heat affected zone (HAZ), particularly at the center of the strip, is generally increased as frequency decreases. The irregular configuration of the heat affected zone, in the form of an hour glass, is minimized. This results in the steel's welding temperature also being reached with processing certainty at the center of the wall thickness. Overheated corners of the strip edges possible at higher frequencies are eliminated at lower ones, such as may be used in HFI process. Fig. 1: Schematical displacement of the isotherms with decreasing frequency /2/ Moreover the heat input and the heat profile at the strip edges in HFI welding depend of cause not only on the frequency used, but also on many other influencing factors. Major influencing factors can be subdivided into six groups, i.e., ambient media, energy conversion at the inductor, strip edge condition, impeder state and arrangement, welding speed and welding vee. Frequency is one important factor in welding process control and thus the welding result, but can only be examined in context, together with the other parameters.
3.2 Sizing One crucial production step for the mechanical values is the sizing of the pipe string after seam annealing. Here the pipe diameter is reduced slightly. Due to this production step the compressive yield strength is increased whereas the tension yield strength is decreased due to the Bauschinger Effect. In proportion to the specified tension yield strength longitudinally welded HFI-pipes therefore show arised compressive yield strength. This is advantageous for the collapse behaviour of HFIpipes compared to other pipe producing technologies (see Chapter 4). 3.3 Non Standard Diameter For the use of HFI pipes as jacket pipes e.g. in combination with inner seamless pipes the capability providing pipes in non standard diameters is important. Non standard diameters allow to design optimal distances from the inner pipe to the outer pipe for technical and last but not least economical reasons. MFR can provide numerous non standard diameters in order to offer optimal products for a lot of applications (Fig. 2). Fig. 2: Non standard diameter Standard Diameter [mm] Range of Non Standard Diameter [mm] 168.3 166.0 172.0 219.1 217.0 223.0 244.5 242.0 248.0 273.1 271.0 277.0 323.9 322.0 327.0 339.7 337.0 344.0 355.6 353.0 360.0 382.0 379.0 386.0 406.4 404.0 410.0 419.0 415.0 422.0 445.0 443.0 449.0 457.2 455.0 461.0 473.1 472.1 477.0 508.0 506.0 512.0
4 Collapse behaviour HFI-pipes are appropriate to offshore-application due to two significant properties. On the one hand HFI-pipes have a small variance regarding the geometrical characteristic values. In particular small diameter variances, small ovalities and uniform wall thickness have to be mentioned. The sound geometrical characteristic values regarding diameter and ovality are caused by the sizing of the pipe string. The uniform wall thickness results from hot wide strip as primary material being used. As aforementioned the sizing furthermore arises the compressive yield strength compared to the specified tension yield strength. The higher the wall thickness to diameter ratio is, the higher is the influence of the compressive yield strength on the collapse behaviour. In contrast to this at low wall thickness to diameter ratio the young s modulus is vital instead of the compressive yield strength because of different failure mechanisms. The geometry of the pipe is anyway of utmost importance. Overall investigation at the Salzgitter Mannesmann Forschung, Duisburg, demonstrated the suitability of HFI-pipes from Mannesmann Fuchs Rohr for offshore application (Fig. 3). Collaps pressure requirements of relevant codes are fulfilled. According to DNV OS-F101 the investigations demonstrate that HFI-Pipes moreover fulfil the collapse pressure requirements of seamless pipes. For SAW-pipes the so called fabrication α fab with values of 0.85 or 0.93 depending on the manufacturing technology has to be used. Fig. 3: Results of collapse tests performed on line pipes compared against specifications /3/ Outside Diameter / Wall Thickness (D/t)
5 US-Testing At the end of the production process the entire weld seam is tested by ultrasound. The therefore used test bench features an automated testing process with pipe commissioning, pipe identification, pipe positioning and US-testing of the weld seam with online logging (Fig. 4). The weld seam tracing is done by a pilot line. For the weld seam testing four longitudinal defect levels can be used. Two levels can be equiped with tandem technology. For the transverse defects one level is available. This is arranged on the weld seam. This ultra-modern and flexible testing technology ensures the fulfilment of the required weld seam testing of heavy walled HFI pipe in all respects. Fig. 4: Ultrasonic testing of the weld seam (MFR, Hamm) 6 Coating Upon request the pipes can be supplied with various coatings as 3 layer PE or PP coating (MAPEC), concrete coating, epoxy lining and cement mortar lining. Additionally to these coating systems Mannesmann Fuchs Rohr offers various types of rough coating (Fig. 5). Customers demand more and more rough coating especially in offshore business for several reasons. Main reason is the enhanced bonding due to higher shear forces between PE or PP rough coating and heavy coating if applied. Another major advantage is the reduction of the holding force in the case of offshore laying. Rough coating for instance gives high occupational safety
for the pipeline laying workmanship, and in the case of white PP the optical detection of the pipeline on the sea floor. Mannesmann Fuchs Rohr has supplied rough coating as a part of the well established 3 layer coating system MAPEC with HDPE or PP for offshore projects in the North Sea and Atlantic Ocean. 3-layer coating thickness can be applied from standard to reinforced up to 0.4 /10mm. Additionally the so called MAPEC Rough Light with min. 1.5 mm layer thickness can be supplied. The rough surface is achieved by sintering PE/PP granulate on the hot surface of the extruded smooth PE/PP whereas granulate is partly molten and thus connected to the surface. Up to now the HDPE rough coating of Mannesmann Fuchs Rohr was applied for instance for the West Africa Gas Project (e.g. 508.0 x 11.2 mm; X70) with EP-Flowcoat. In that case, the pipes are used offshore and onshore. For the Total Forvie offshore project (443.0 x 12.7 mm; X65) the PP rough coating was used, too. Fig. 5: PP rough coating for offshore applications 7 Project references Mannesmann Fuchs Rohr delivered during more than 10 years pipes for several offshore projects as jacket pipes for pipe-in-pipe systems and line pipes. The maximum known water depth is about 1300 m. Thereby common lay techniques as J-Lay and S-Lay were used. Also MFR line pipe has been purchased for reeling. According to customer feedback the benefit is among other things the very good roundness of the pipes which enables an economic pipe laying process related to welding process for example. Furthermore the consistent pipe length also enable an efficient pipe laying process and the fabrication of double and quad pipe length in the case of pipe-in-pipe systems e.g. in combination with seamless pipes. Last the consistent wall thickness allows repetitive pipe welding parameters and an automatic UT inspection. Last big projects were for example the West African Gas Pipeline, Anadarko Green Canyon 518 Project, and Total Forvie Project /4/.
8 Summary HFI-Pipes are nowadays widely used in the offshore sector. In this article the most important influencing factors concerning the production technology and the resulting pipe properties are discussed. Relevant production steps in this context are the welding technology to ensure top weld quality even at high wall thicknesses and the sizing of the pipe to improve the mechanical values and to produce constant diameters and small out-of-roundness. Furthermore the primary material supplier is important in this context to ensure a uniform wall thickness over the entire pipe. All these factors lead to a good suitability in different applications in the offshore sector as jacket pipes for pipe-in-pipe-systems or as line pipes. Finally some cases of operation are mentioned were the HFI-pipe from Mannesmann Fuchs Rohr are used for different projects. 9 Literature 1. www.mannesmann-fuchs.com 2. Löbbe, H.: HFI goes Offshore The Influence of Welding Frequency in Production of Thick-Walled HFI Pipe. Tube & Pipe Technology Sep./Okt. 2005, pp. 148-151 3. Zimmermann, B.; Brauer, H.; Marewski, U.: Development of HFIW line pipe for offshore applications. 4th International Conference on Pipeline Technology, 9.- 13.05.2004, Ostende, Belgien 4. Bell, B.; Löbbe, H.; Bick, M.: Offshore Einsatz von HFI-Rohren als Pipe-in-Pipe System in der Nordsee. In: Rohrleitungen Für eine sich wandelnde Gesellschaft. 20. Oldenburger Rohrleitungsforum, 09./10.02.2006, pp. 162-166