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Welding & Preservation

Welding & Preservation

"HINTERSTEIN I" Hindelang Well, Allgäu, Bavaria

Special 25 CrMo 4 steel
Underwater welding using temper-bead
technology in water 36 metres deep
Well depth: 70 metres

Thanks to increasing quality awareness and quality requirements, the quality of weld seams under water has improved considerably. Technical advances have made it possible to perform sophisticated welding work, e.g. repairs on a drinking water well which was still undergoing construction.

The well in question is 70 metres deep and comprises sleeve-coupled and screwed pipe sections 2.5 and 3.0 metres in length. The internal diameter of the well is 1.06 metres and the pipe walls are 20 mm thick.

Before the filtering tube was installed, the well pipe was examined using a conduit camera. The tube push at 33 metres had become completely detached. The 12 conical seal screws were no longer on the tube push, causing the upper section of the pipe at the lower end of the pipe to twist approx. 8 cm upwards to a position on top of the guide projections of the lower inside pipe.

After evaluation of the underwater video and a weld sample of the 25 CrMo 4 tubular steel, Nordseetaucher was commissioned to weld the well pipe.

The level of the ground water is approx. 860 metres above sea level.

Wearing a heated diving suit, the diver was submerged in the well pipe using an assembly cage.

The phase at the external edge of the inside pipe caused a gap of approx. 7 mm. This space was sealed using build-up welds in a temper-bead process. A hydraulic flex (bio-oil viscosity < 1) was then used to clean and smoothen the weld seam and parent metal.

For welding we used a special 3.2 UW electrode which is suitable for applications involving steel with a Cäq >0.40% and for water depths of between 20 metres and 100 metres.

The root layer was applied with a welding current of 170 A and DC voltage of 26 V - 30 V. The final runs were applied with 165 A and V = 26V - 30V.

In order to avoid tension, only a quarter of the root was welded at any given point followed by welding of the same length on the other side (also a quarter). After each complete weld bead - either root or final run - the weld seam was cleaned with a 220-bar high-pressure cleaner and a flex. The seam was cleaned and ground with great care in order to remove even the most minute hydrogen cracks in the surface. This type of weld seam processing results in a relatively small a-dimension in comparison to the number of final runs. This method greatly reduces the error rate in such critical steel. A total of 2 build-up welds, 1 root bead and 8 final run beads were welded in the temper-bead process.

After completion of the welding work, the tube push was also bolted. In the last overlapping section of the pipe ends, approx. 5 cm under the weld seam, a hole approx. 2.5 cm in size was burned through both pipe walls using an underwater fuel electrode. Because the holes were burned, which does not give the accuracy of drilled holes, the bolts had some freedom of movement. A total of 12 bolts made of an extremely hard material were used and secured by welding. As no further repairs could be carried out during or after the sinking process, these additional bolts were fitted as an emergency option to hold the pipe together should the weld seam tear.

The diving work was completed on the 14th November 1998 and the pipe was prepared for sinking.

The laboratory report on test welding of the 25 CrMo 4 pipe material was available on 18th December 1998, whereby C = 0.22% and Cäq = 0.53%. The steel had to be heated to 600° C in order to weld it above water.

Result: due to the high C, Cäq values and pre-heating above water, the report stated that the steel could not be welded in a wet environment.

On 22nd December 1998, the entire 70 metres of pipe were sunk (total weight: over 500 tonnes) and the weld seam did not tear.

It was solely due to the meticulous work carried out by the weld divers - and a smattering of luck - that this drinking water well, which is so important for the region, could be completed.

This example demonstrates how in certain applications underwater welding represents an interesting alternative to other, more expensive, methods, and it proves that underwater welding should not be restricted to secondary components.

The welders:

Andreas Stutz Andreas Schiefer