Bearing points in inspection robots defy the rough North Sea

As the robots work permanently underwater, the components must be resistant to seawater. In addition, the bearings are subjected to a load of up to 500 kg. In order to achieve the longest possible downtime, a wear-resistant bearing is required.

With iglidur® J polymer bearings, SubC has found a very wear-resistant solution that works very well and reliably under harsh conditions. Because they are lubrication-free, the iglidur® bearings can be used underwater without danger to the environment. Even with the occurring loads of 30 MPa in combination with salt water, no significant wear of the bearings could be detected after 200,000 cycles.

Drilling platforms are mounted on jackets - square structures made of steel pipes that are anchored to the seabed. As part of the refurbishment, an inventory had to be made. What is the condition of the steel pipes - especially near the water surface in the so-called splash zone, where the structure is susceptible to corrosion? In the past, divers would have jumped into the water to find the answers. A dangerous task.

One diver compares the mission to a 'trip in the washing machine'. In addition, crew, equipment and support vessels are expensive. Total therefore set out in search of an alternative. The company found what it was looking for in Esbjerg. The Danish town in south-west Jutland is the headquarters of SubC Partner - a company specialising in the development of offshore technologies. SubC develops, produces and operates the robotic crawler for inspecting pipes. A special honour: The robot was honoured with an innovation award from Total.

The crawler has two hydraulically driven metal claws with a total of 12 rollers that clamp around the steel structure with a contact pressure of 500 kg each. The crawler moves around the pipe on 12 rollers. The forward movement is similar to that of a crawler: if it wants to move forwards, the rear claw holds the pipe in place. The front claw pushes forwards a few centimetres using hydraulic cylinders. The front claw then pulls the rear claw back. This allows the robot to reach any point on the pipe.

And the control system? The crawler can be moved using a joystick. It supplies a live image to the command centre via six cameras. From there, the operator also controls the tools with which the robot can be equipped - mainly a wire diamond cutter, which the robot uses to saw off and bring up parts or constructions to be modified on the platform.

Unlike a diver, the robotic crawler does not run the risk of injuring itself during its work. But the salty seawater was still a challenge for the engineers. If the experts had used metal bushes in the robot's joints, there would have been a constant risk of corrosion and unplanned downtime.

The engineers therefore opted for an alternative: bushings made of iglidur® high-performance plastics, which are produced by motion plastic specialist igus® from Cologne. "Thanks to the plastic bushes, we no longer have to worry about corrosion," explains Bjarne Lindquist, the responsible design engineer at SubC Partner. And so it comes as no surprise that the engineers use another igus® product. The wire diamond cutter moves on drylin® linear axes. "Thanks to them, we can position the cutter with millimetre precision."
 
And the plastic components have nothing to hide when it comes to robustness either. Reinforcing materials are added to the plastic compounds, which make the bushings so robust strong that they can withstand high pressures and edge loads. "igus® has developed the plastic products to such an extent that they are an alternative to metal alternatives in professional industrial applications," says Anders Riis Olesen, Technical Sales Consultant at igus®. "That's great." The bushings are also maintenance-free.

As solid lubricants are integrated into the material, dry operation with low coefficients of friction is possible - without additional lubricants such as oils and greases. Last but not least, SubC Partner was able to reduce the costs for the robotic crawler. This is because the iglidur® bushings are up to 40 per cent cheaper than metal bushings.

To prove the durability of its materials, igus operates a 3,800 square metre test laboratory at®, where over 15,000 tribology tests are carried out every year on 300 test systems. The iglidur® Z material used by SubC Partner also had to prove its robustness there. The engineers swivelled a plastic bushing on a gas-nitrided shaft of type ST52 - at a speed of 0.01 metres per second and a load of 30 MPa. Even after 200,000 cycles, no significant wear could be measured on the bushings or the shaft.

The result was different for the metal bushings. The engineers cancelled the test after just 35,000 cycles. The wear on the bushings was too high and the damage to the shaft too severe. iglidur® Z was specially developed for maximum service life under extreme conditions. In addition to its wear resistance to salt water, it is also suitable for high thermal loads and high forces.

To the products used