Robust robots rule30 July 2001
British Energy’s remote technology team does not always use the most modern or the most expensive equipment vailable.Experience has taught that ‘fit for purpose’ is a more effective and effective philosophy.
Inspecting the lamps, pipework and other components that are awkwardly situated, high up at British Energy’s eight stations would be expensive, time-consuming, and disruptive to other work if scaffolding had to be built and shifted to examine each item. The turbine halls alone have a combined area of over 35,000m2. yet inspection must be carried out, for example, on tens of kilometres of steam pipework hangers and on over a thousand valve snubbers.
The solution at British Energy is to use a telescopic boom camera system that was developed in-house. It comprises a camera with integral LED lighting, power supply, LCD monitor, and all the necessary brackets, cables and leads. Apart from the 5.5m extending carbon-fibre boom, it comes self-contained in its own carry pack, complete with instructions and hints for better operation. Each station now has its own kit. It saves time and money, is easy to use, and it is no more complex than is strictly necessary.
These are the aims of British Energy’s remote technology team at Barnwood, which has become expert at developing or reverse engineering appropriate technologies that can be applied where it is too inaccessible, too ‘hot’, or just not necessary to use manpower. “Fit for its purpose” is the team’s philosophy, once fundamental safety considerations have been addressed.
This philosophy applies equally to the large and complex remote technologies in the original AGR design – inspection manipulators, for example, have six degrees of freedom, weigh up to 20t and have inspection reaches of over 5m – and the team has to ensure best performance from these machines. But at this point in the AGR programme, the reactors are all past their early commissioning stage, and not yet old enough to develop ageing faults, so the team is most often deployed to solve the kind of maintenance problem that would have been slower, less safe, or more expensive to approach using traditional methods.
At this stage in the reactor life cycle, the remote technology team has to solve problems quickly, providing a fast response to meet safety culture and commercial pressures. “That means using off-the-shelf equipment and rebuilding or redeploying it.”
Alan Greenaway, head of the remote technology team, explains: “Taking a range of standard products and reverse engineering them to make them lighter or smaller, or to change their operating principles.”
Typically, the team may be called on to recover debris during an outage: this is often a unique situation and one where delays on the critical path may have severe commercial consequences.
Examples of recovery
At one station, two large pieces of internal baffling from a DC heater had to be recovered from the ‘J’ loop some 23m below the heater. In the same outage, a tape measure had to be retrieved from the top of the A7 impeller, from where the 5m of metal tape could easily become wrapped around the impeller shaft and cause pump running difficulties.
To solve problems like this, the remote technology team may have to begin with a ‘brainstorming’ session, but over several years of experience, a suite of simple but effective retrieval mechanisms have been developed. For ferrous debris, a variety of electromagnets are ready and waiting. In the event, this method was used to retrieve the baffling.
A series of mechanical grippers of different shapes are also well proven. To retrieve the tape measure, a remotely-operated gripper with two opposing jaws with self-aligning spring-loaded contact fingers was used. The contact fingers were mounted on a motor-driven rotating joint, and deployed via a series of interlocking rods (of the type more commonly used to unblock drains). Positioning was carried out with the help of a Pearpoint camera, ideal for this type of work with an extended operating distance of 61m between the camera head and its control station.
For non-ferrous debris or items that are too irregular to be easily retrieved, the team has two systems under test. A glue gun can pick up objects weighing up to several kilograms, and if the pickup is unsuccessful, the glue can be reheated and retrieved. Alternatively, heat-shrink tubing can be used to envelope the debris and retrieve it.
These are all simple solutions, but where a fast and reliable response is required, the simplest solution is often the best solution.
British Energy’s policy is to use remote technologies and reduce dose where possible, so technologies have been developed to carry out routine checks. It is possible for personnel to carry out inspections underneath the reactor vessel, for example, but this would result in dose to the personnel, and to save this, inspection is carried out remotely by camera. It is deployed using a ‘bistable’ boom – one that can be rolled for transport, but which forms a rigid tube when unrolled – with a 5m extension.
Although the team uses ‘off-the-shelf’ technology, re-engineering it to the best advantage requires a high degree of expertise. “If we need tilt and zoom cameras we use them, or whatever is required, but our design philosophy is to aim for reliability, and to improve toughness and robustness”, Greenaway says. “We provide the simplest solution,” he adds, “but that’s not necessarily the cheapest solution.”
That may mean adapting old technology – especially since the team has to deal with components that are not radiation hardened. Fibre optics and modern CCD cameras are efficient and versatile, for example, but in a high radiation environment, the radiation can cause the glass to ‘brown’ and sensors to fail; in these places, old-fashioned 1950s technology tube cameras are more efficient. Sometimes, however, the team can use more modern equipment up to or even beyond its notional capacity. Building up a good relationship with camera suppliers, for example, means that the team can gain access to the best possible information regarding optimal use of the technology, and enables the team to understand the operating principles of the system. This makes it easier for the team to adapt and modify equipment for specific and unique situations.
Recovering Dungeness B
The remote technology team’s experience in providing solutions paid off in 1999 when a crack was discovered in a steam boiler header weld at Dungeness B. Inspection of all similar welds became a necessity, and unless a remote delivery system for the inspection tools could be developed, it might have become necessary to excavate and reweld all the headers. The recovery work was carried out through 2000.
The weld was very difficult to reach. The eight superheater headers in each reactor convey main steam from the boilers to the turbine main steam pipework. The steam passes from a large number of boiler tubes through the tubeplate and into the steam header.
The steam header is manufactured from 3 inch thick 316-grade stainless steel, which is welded to a 10 inch thick tubeplate of the same material, which is in turn welded on the reactor gas side shell at weld A. The weld procedure consisted of a tungsten inert gas root, followed by two runs of manual metal arc welding to protect the root; the remainder of the weld, 55mm, was completed with a submerged metal arc process. The weld metal was also 316-grade stainless steel. The weld was subject to radiography, solution heat treated and finally inspected by surface dye penetrant.
It was during preparations for work on a boiler tube leak at Dungeness B that the welder noticed a crack that was on the adjacent steam header B3 at weld A. Initial NDT investigations revealed that there were indications extending up to 55mm deep in a nominal wall thickness of 79mm. Given the potential threat to the existing safety case, the decision was taken to shut down the other reactor at Dungeness B until a robust safety case could be made. Subsequent volumetric (ultrasonic) inspection into the weld revealed a fully circumferential defect 55mm deep.
Metallurgical analysis revealed that the weld root and manual metal arc runs were of the correct composition, but that there was a submerged metal arc section of rogue weld metal with 17% Cr composition. The problem was confined to a single weld in the single header but a vigourous comprehensive investigation of all welds in both reactors would be required to confirm that the issue was not more widespread and produce a robust safety case.
Access to the gas side of the headers was generally poor and the prospect of in situ excavation and subsequent re-welding of each header weld was daunting in the extreme. Man-access would have required long setup and preparation for suited work including a 2-3 month training programme, and even then, special purpose inspection tools would be necessary. It was not a practicable option.
Overall, the header recovery programme was to take nearly a year, and involve input from hundreds of staff and several large companies. Within this extensive NDT programme the remote technology team played a small part but an important one: with no man access, the team had to deliver.
Following a brainstorming session that took place within the remote technology team, it was proposed to remove one of the plugged holes on the periphery of each of the header tubeplates which had been blanked off following early boiler design modifications. This was a lucky break: the 2 inch diameter hole in the 10 inch thick tubeplate provided a very novel access into the reactor, and if they had not existed, the favoured course of action might have been to drill similar holes. The two welds of most value to the safety case were a fully circumferential weld just beyond the tubeplate, and an axial weld extending from the tubeplate along the header into the reactor.
A cardboard mock up of the header was quickly produced in order to improve visualisation and to trial ideas. Within a few days, a prototype tubular inspection tool had been manufactured, the principle features of which were an end-mounted remotely articulated joint, and a steady, which made use of the thickness of the tubeplate to allow the tool to be manipulated. The articulation was controlled by a bowden wire and derailleur gear lever.
The inspection programme was carried out on both reactors, and only one further instance of rogue material was identified.
It is worth noting that the cost of the ‘production’ inspection pole was somewhat less than £1000. However, it did meet the team’s criteria: it was simple, robust and effective.