TPi January 2010

The use of different production batches makes everything even more complicated. As they usually exhibit different mechanical properties, the production batch may also have to be taken into consideration during sorting in order to ensure perfect welds. The result is a puzzle of such complexity that a solution is not always possible. The other apparently obvious solution – using thicker pipe material and ensuring the necessary equality of the pipe ends by mechanical machining – entails considerable disadvantages. Besides the cost aspect, there is a weight problem to consider, as the riser would be much heavier. The platform would have to be made larger, at not inconsiderable additional cost, in order to carry the added weight.

Welding the pipe string onshore

A more serious consideration is that the crucial wall thickness of the pipe string

would then be at its lowest in the weld zone. In view of the greater rigidity of the central zone of the pipe, stresses and deformations – especially during bending – are concentrated in the weld zones. The consequence is a disproportional reduction in their dynamic strength. V&M Tubes has worked on technologies for the fit-for-purpose preparation of pipe ends for many years. The range of options includes calibration, internal and external mechanical machining, and thickening by upsetting. Volker Rohden commented, “ Shell wanted to utilise this know-how and therefore in the context of the URSA project, it requested suggestions for a riser end design that would meet the end dimension tolerance for SCR welding without any further machining or sorting of pipes. ”

The PURE concept consists of a single work step in which the pipe end is thickened and exactly rounded at the same time. This is achieved by upsetting the inductively heated pipe ends against a mandrel and outside mould in a controlled process. The inside and outside of the pipe end are then machined. This narrows the tolerance of the inside and outside diameter to an extent that would be unthinkable in conventionally rolled pipes. In comparison to rolled pipes of the same dimensions, the diameter tolerance is just ±0.25mm instead of the usual ±1.6mm. Ovality is no longer a concern. Each pipe can therefore be immediately welded to the following pipe with optimal processing parameters and without prior sorting. The result is a significantly longer service life of the weld under conditions of dynamic stress. The key to the design is a curved outer groove, which ensures optimal stress distribution in relation to the fatigue strength. The calculations of the engineering team headed by Dipl-Ing Marion Erdelen-Peppler of Salzgitter Mannesmann Forschung GmbH in Duisburg played an important role in this project. After completion of the development, Shell purchased a total of 650 PURE pipes and subjected them to its own comprehensive tests. The results were satisfactory and in November/December 2007 two PURE riser strings were produced onshore for the URSA platform and reeled onto the giant drums of the pipe- laying vessel Technip Deep Blue .

Finished riser strings on their way to the drums of the pipe-laying vessel

V&M Deutschland GmbH – Germany www.vmtubes.de

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Tube Products International January 2010

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