Monopile non-destructive testing breakthrough

DFU vac-blast machine DFU vac-blast machine

Greater Gabbard Offshore Winds Ltd (GGOWL), the operators of the Greater Gabbard Offshore Wind Farm (offshore UK) has utilised the latest vacuum blasting techniques from Seawork exhibitor Precision Blast Systems Ltd as part of its maintenance plan.

The wind farm consists of 140 Siemens 3.6MW wind turbines of standard construction with a steel monopile topped with a transition piece which supports the turbine tower, as part of GGOWL’s structural integrity programme the transition pieces are subject to non-destructive testing of the circumferential and longitudinal welds and in order to ensure the most accurate results the paint coating required removal to give consistent surface preparation.

The coating on the transition pieces is a two-component polyamine epoxy in excess of 750μm, under normal on-shore circumstances this could be removed easily with abrasive blasting using a fine abrasive to give a uniform surface finish suitable for NDT, however using this method inside an offshore turbine structure presented a number of problems; dust was the major issue in terms of visibility in such a confined space but the main concerns were the clogging of vent filters and miss-activation of fire alarms in the generator space, there were also issues surrounding the volume of waste from traditional open blasting and the logistics of shipping large amounts of abrasive & waste to/from the turbine. Containment and extraction / filtration of the dust was not practical, there were limitations as to the amount of equipment that could be taken to the installation and this would have also proved very time consuming, the working personnel had limited time once on the turbine.

The only solution seemed to be hand cleaning using power tools with abrasive wheels and this method was initially used but it became clear that this was not satisfactory; it still required containment to prevent dust issues which was not completely effective, paint removal was slow and it left an inconsistent surface finish which often resulted in the contractor or NDT inspector having to make a return visit.

A visit to Seawork International by Charlie Morris, GGOWL’s offshore works project manager found a possible solution.

Precision Blast Systems Ltd were exhibiting their latest model of DFU vacuum blast equipment capable of abrasive blasting with no dust or waste, these compact machines recycle the abrasive resulting in very little waste and the blasting takes place within an enclosed head so they are completely dust-free.

Following a conversation regarding the machine’s capabilities and the offshore application it was agreed that a test should take place at the premises of 3sun Group in Great Yarmouth who were GGOWL’s contractor for these works.

Test blasts were carried out under supervision from GGOWL’s project team and these confirmed that the system removed the coating without problem leaving a uniform surface finish ideal for NDT and more importantly, that it was completely dust free with the relatively small amount of waste contained within the DFU’s waste bag.

Following some training on the equipment, 3sun Group mobilised equipment and staff offshore to start the first package of work removing the internal coating from 500mm sections of the transition piece, the work was completed in good time and with no dust issues.

Precision Blast Systems has reported that GGOWL found major benefits in terms of time saved to complete the work, which has provided a more consistent surface preparation and has resulted in positive financial benefit to the Company’s maintenance programme.

By Jake Frith

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