Worldwide, there are more than 2000 offshore drilling rigs and many more individual wells from which oil and gas are extracted on a continuous basis. The technical equipment for these installations places extreme demands on the hand-picked suppliers in the stainless steel industry. Germany’s Schoeller Werk took up the offshore challenge 35 years ago and has long been among the kingpins of the industry. Schoeller Werk has supplied over 500,000 m of piping to one company alone – TCO Norway, a service provider for Norway’s state-owned Statoil enterprise – since winning their custom in 2014. At the core of this partnership is a plug-drawn annular pipe made from high quality, nickel based alloys nos. 825 and 625. In addition, there are austenitic pipes in class 316 Ti stainless steel. The pipes delivered so far have impressed Statoil so much that they have defined them as standard for their own specifications. Apart from a variety of materials, a range of diameters and wall thickness are also vital – plug-drawn Schoeller pipes cover all the possibilities. The pipe design and corresponding quality testing ensure that the finished solution can withstand internal pressure of up to 2500 bar with ease. Moreover, the high quality materials, in conjunction with the improved surface quality derived from the drawing process, are able to resist the effects of salt water and other aggressive media.
Smoothing plugs
Among the features of these plug-drawn pipes are their geometrically precise curvature and high weld quality. In practice, these are every bit as good as the basic material and it is possible to manufacture individual pipes up to 2000 m in length. An internal mandrel (plug) smoothens the inside of the longitudinal weld. In combination with an external mandrel, the initial cross-section of the pipe can be halved in this way. Overall, this longitudinally welded solution gives the impression of being a seamless pipe. A glance at the microstructure of the material reveals that the weld is virtually invisible once the pipe has been drawn.
Careful testing
The offshore industry uses these pipes as hydraulic control lines for safety valves as well as for pumping chemicals into an oil tank, in other words they support the entire extraction process. The injection pipes allow rig operators to use chemicals in a targeted fashion to liquefy the oil and thus improve its flow properties. During the complex manufacturing process, the pipes undergo various tests to ensure the special quality of the products before they are installed. Strips of metal are welded with a longitudinal seam using the tungsten inert gas (TIG) welding process and rolled into tubes. Apart from the obligatory eddy-current testing, these tubes subsequently undergo an air under water (AUW or ‘bubble’) test, in which they are submerged in water and filled with air to a pressure of up to 210 bar. A visual inspection is carried out along the entire length of the tubes to check that they are airtight.
To enable Schoeller Werk to supply its customers with the required lengths of 15,000 m and more, individual pipes are orbitally welded together and an X-ray is taken to check that the orbital weld is airtight and free from pores. Before being delivered to customers, the control and injection pipes are also subjected to a hydraulic test. This involves filling the finished coils with hydraulic fluid and applying pressures of up to 2500 bar in order to simulate the extreme conditions sometimes experienced in offshore work.
Service complements manufacturing
The comprehensive service package offered by Schoeller Werk includes the encapsulation of pipes with plastic sheathing in so-called flat packs. This means that bundles of pipes can be attached to the extraction tubes and protected from buckling and squashing. Flushing and filling of pipes are other possibilities: in this case, the insides of the pipes are flushed with hydraulic fluid until the latter conforms to the specified ISO or SAE purity classification. The fluid filtered in this way can remain in the pipes if the customer so requires – as a ready-to-use product. Furthermore, the pipe bundles can be equipped with electrical wiring or stainless steel support cables. And thanks to the smooth internal surface, plug-drawn pipes are also ideal as ducting for optical fibre cables transmitting light signals.
Online search: cppPC117schoeller
Author : Markus Zimmermann
Head of Team Energy,
Schoeller Werk
