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Sub-sea Hot Tap proceeds with total confidence

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A breakthrough project for a hot tap into a major North Sea pipeline has been achieved on time, on budget and without significant problems. In no small part, this was due to full confidence in the integrity of the welds, carried out at some 150 metres depth, thanks to an in situ high-frequency automated ultrasonic inspection, carried out by NDT specialists, Sonomatic.
The project is thought to be unique – the first hot tap carried out at 150 metres depth – and featured new techniques in the inspection regime, developed to a very tight time scale specifically for the project. Further factors were the crucial economic importance of the pipeline and the environmental implications. It had to be 100% right first time with no room for error.

The proposed automated inspection solution was based on the Sonomatic'Nautilus' system, a dual axis, servo-controlled system designed to carry a variety of high-frequency inspection probe heads – in this case, pulse-echo was the chosen technique for the mapping of the weld around the nipple.

Sonomatic was able to demonstrate a dedicated working system, with modified software, within the two-week deadline and supplied equipment in time for the on-shore testing phase.

Nautilus runs on a clamshell collar which is diameter specific to minimise the manipulatorprofile. Its second axis enables scans to be performed at millimetre stages along the nipple toward the nipple weld. Using servo motors, with accurate positional feedback, Nautilus gives very precise movement in two axes for precise data analysis. The pulse echo scans were carried out at three different angles from perpendicular to the pipe giving comprehensive coverage of the fusion face.

The manipulator was controlled from the surface and was interfaced with Sonomatic's Microplus digital imaging system, using specifically written software generating colour images. Each set of scans was successfully analysed before the next was due, despite the considerable pressure on the Sonomatic team on board the diving support vessel (DSV).

Article supplied by Gordon Davidson, technical manager, Sonomatic

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