Application of HFI-pipes for a pipe-in-pipe system in the North Sea

The gas transportation takes place from Forvie North via the different stations up to the Alwyn North Platform over a distance of about 32 km (Fig. 1). For the transportation a pipe-in-pipe system consisting of 15“ seamless pipes with 34.5 and 38.7 mm wall thickness and outer 17.5” HFI-pipes with 12.7 mm wall thickness is installed. While the inner seamless pipes are stressed essentially by the transported material with pressure from the inside, the outer HFIpipes have to withstand the water pressure from the outside. Therefore HFI-pipes from Mannesmann Fuchs Rohr are well suitable due to their excellent collapse behaviour [1].

The pipe-in-pipe system was laid by Allseas laybarge Solitaire in a campaign of 22 days and then the structures’ installation, tie-in spools, etc. completed by Stolt Offshore (now Acergy) all with no significant problems.

TOTAL E&P UK PLC announced that the Forvie North Field commenced production on December 29, 2005. A fast track development planning and the use of innovative technology, such as a high-pressure flexible riser and the pipe-in-pipe system have brought this field on stream only 9 months after Field Development Approval was received from the UK DTI.

The Forvie subsea system has also been designed with significant expansion capacity to allow the tie in of additional wells from nearby UK exploration prospects and, subject to commercial agreement, from fields in the nearby Norwegian sector.

HFI-Pipes – Production and Properties

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, and brine, district heating systems, hollow sections and Oil Country Tubular Goods (OCTG). 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 last but not least fulfilled via the use of high quality primary material in combination with high pipe wall thicknesses [1].

The HFI pipe production is characterised by a continuous manufacturing process. The high constant product quality of HFI-pipes from Mannesmann Fuchs Rohr is the result of the application of modern production methods and machine tools, the high qualified staff and the usage of numerous testing methods [2], [3]. HFI-Pipes have an economical advantage due to the high productive manufacturing methods out of hot wide strip. Typical are good mechanicaltechnological properties as well as accurate geometrical values. High compressive yield strength and narrow wall thickness tolerances can be given as examples. Last but not least these properties result in the good collapse performance and thus a good behaviour against outside pressure as necessary for the outer pipes in pipe-in-pipe systems.

Experimentally verified were these collapse values by means of tests at the Salzgitter Mannesmann Forschung and at the Technical University of Clausthal, both located in Germany. Important for the achievement of a high collapse pressure is the outer diameter to wall thickness ratio D/t. Investigations showed overall that the fabrication factor α^Fab = 1.0 according to DNV-Standard OS-F101 for seamless pipes is also fulfilled by HFI-pipes. [1]

HFI-Pipe Requirements

Black Pipe
Especially for the Forvie project experience from previous offshore projects with the pipe-inpipe manufacturer ITP were helpful. Special design requirements from Total and ITP demanded the production of the non-standard diameter 443.0 mm, which is one of the several possible non-standard diameters from Mannesmann Fuchs Rohr. The inserting of the seamless pipes and the thermal isolation required narrow pipe tolerances over the entire pipe length and even smaller tolerances of the inner diameters at the pipe ends. The straightness of the pipes with maximum 0.15 % enables an accurate inserting of the inner pipe. Furthermore HFIpipes in the non-standard outer diameters 477.0 and 510.0 mm were provided as so called sleeve pipes in fix length tolerance.

Coating
Upon request Mannesmann Fuchs Rohr can supply pipes with various coating systems 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. Within the here presented project a white PP-rough coating was demanded (Fig 3).

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. 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.

Project appropriation

In the Total Forvie Project the HFI-pipes were used as the outer pipe of the pipe-in-pipe system. The space between the outer- and inner-pipe is filled with the porous insulating material Izoflex™ (Fig. 3). At the pipe ends the HFI-pipe is reduced to the outer diameter of the inner seamless pipe (Fig. 4). This is done to prevent water from penetrating into the space between the two pipes. The HFI-pipes therefore underlie in essence the described stress during the fabrication of the pipe-in-pipe system, the stress during the offshore laying and the outer water pressure in a water depth of approx. 120-140m during the service of the pipeline.

Summary

HFI-pipes are well suited for the use as outer pipes in pipe-in-pipe systems due to there good geometrical and mechanical-technological properties. Especially the high geometrical accuracy is advantageous for the production of the pipe-in-pipe system as well as for the offshore laying of the pipeline. By the possible application of the PP-rough coating based on the well known 3-layer-coating also in this case all specified requirements are fulfilled.

Literature

[1] Zimmermann, B.; Brauer, H.; Marewski, U.: Development of HFIW line pipe for offshore applications. “4th Int. Conf. on Pipeline Technology”, 9.-13.05.2004, Ostende, Belgium

[2] Löbbe, H.: HFI goes Offshore – The Influence of Welding Frequency in Production of Thick-Walled HFI Pipe. Tube & Pipe Technologyy Sep./Okt. 2005, pp. 148-151

[3] Löbbe, H.; Zimmermann, B.: Qualitätssicherung von HFI-Rohren. 3R international (43) 3/2004, pp. 180-184