The British Nuclear Group awarded the construction contract for an outfall system for the Hinkley Point Nuclear Power Plant to specialist contractor Visser & Smit Hanab UK Ltd. (VBMS was part of Visser & Smit Hanab until 2012.) The method that had been proposed by Visser & Smit Hanab in the preceding design study combined horizontal directional drilling with marine construction experience and skill, pairing a cost-effective solution with a tight programme of works.
The outfall is designed to discharge rainwater collected within the Radioactive Controlled Area (RCA) into the Bristol Channel. The Hinkley Point outfall consists of a PE 100 SDR 11 sleeve pipe, with an OD of 200 millimetres, and a 125-millimetre ID inner pipe of the same PE grade and quality. This pipe-in-pipe system provides a secondary safety: any leakage at the shore-based tie-in point will be contained within the pipe system. The discharge point of the system is located some 590 metres offshore, approximately 40 metres west of the existing water intake within the Bristol Channel. This necessitated a horizontal directional drilling with a total length of 670 metres that passed completely through Bridgewater’s solid rock cliffs.
The pipe strings were welded by means of automated butt fusion welding and were completed within two weeks. Because of their lengths, the pipe strings had to be looped around the plant area, causing them to run through the radioactive controlled area. While this has no impact on the pipe quality, it necessitated additional cleansing of the string prior to placement. Started at the same time as the horizontal directional drilling, this was completed within the allotted time.
Upon completion, the pipe strings were pulled into place. A cable was attached to the extremity of the drill string. Upon retraction to the drill rig, the cable could then be connected to the pull head of the PE pipe. Once this operation was completed, the cable, and subsequently the PE string, could then be pulled through the rock towards the exit point. This was accomplished with Visser & Smit Hanab’s 30-tonne single drum winch, which was installed aboard the Multicat vessel the Sealion. A 5-tonne anchor was embedded in the seabed to cope with the forces active during the pull-out.
A diffuser system, comprised of a stainless steel diffuser with two horizontal outlets, was connected to the outfall at the exit point. The conditions under which this diffuser had to be installed in the Bristol Channel, which has the second largest tidal range in the world and a visibility of zero metres, had a major impact on dive operations. The tidal range of 12 metres to 13 metres, producing water depths of between 2-3 metres at low tide and 16 metres at high tide, required rigid planning. Special procedures were put in place to ensure the safety of the divers.
The entire project was carried out in just three months, from August through October.
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