CATS twin technique used in Dinorwig weld inspection

Citation

(1998), "CATS twin technique used in Dinorwig weld inspection", Anti-Corrosion Methods and Materials, Vol. 45 No. 5. https://doi.org/10.1108/acmm.1998.12845eab.006

CATS twin technique used in Dinorwig weld inspection

Methods

CATS twin technique used in Dinorwig weld inspection

Keywords CATS, Epoxy coatings, Inspection

By combining ultrasonic and electromagnetic techniques, CATS International Ltd, of Southport, UK, has been able to carry out complex weld mapping and inspection of epoxy-coated, large diameter tubulars at the Dinorwig standby storage power station in North Wales.

Dinorwig, which is now owned and operated by First Hydro Inc, of USA, features miles of tubulars to carry water from the high level lake, vertically downwards to the under-ground turbine hall and then back up to the lake. All of these tubulars were comprehensively examined when the plant went into service and they have been periodically inspected over the years, using conventional methods.

However, the new owners insisted on an in-service, fingerprinting inspection of the whole plant and this caused particular problems in that area of the pipe network where the high pressure water is delivered to the turbines. The pipework here is of a complex, conchoid geometry, involving a spiral casing which reduces in diameter from 1.4m to 0.8m, with wall thickness ranging from 40mm to 75mm. All the tube is epoxy-coated, internally, and there are no visible welds.

Initially, First Hydro specified that the inspection should be carried out by using edddy currents to detect and map the weld locations and then using time of flight diffraction (TOFD) to examine the welds, and the associated heat affected zones, through the tube walls.

Unfortunately, when this system was applied to a validation block, representing the inspection situation and containing artificially induced, known defects, the limitations of the TOFD technique, in detecting surface-breaking and near surface flaws, were confirmed. This posed a very serious problem, as such flaws can be very important from a fracture mechanics point of view.

This problem was solved effectively by CATS by combining TOFD with Lizard, an alternating current, electromagnetic technique. Lizard is used to inspect the near surface, while TOFD is used for the far surface and to carry out volumetric inspection. After successful validation, the new twin technique was put into service and is now providing reliable and repeatable online reporting.

The new CATS technique has application wherever there is a need for reliable surface and volumetric inspection. For example, it can be used offshore for structures which feature aluminium oxidised protection surfaces. Here, the oxide can mask any surface-breaking defects but by applying the new technique there is no need for sand-blasting off the oxide prior to inspection and, obviously, no need for subsequent re-painting.

Details are available from CATS International Ltd. Tel: 01704 213311; Fax: 01704 25800.