“You always have to have a backup plan when working offshore,” Alain Sebregts of Holland-based C-Ventus Offshore Windfarm Services told MJ, and often the best are the simplest.
This proved to be the case with a recent wind tower wall inspection. The metal monopiles in question sit in tidal waters off the Dutch coast, and a check against potential corrosion is a necessity.
However, accuracy demands that the thickness measurements are taken both from outside and inside the tower so a unique approach using mini-ROV to gather the internal wall thickness measurements was developed: this meant the deployment of a dive team inside wind tower was not deemed necessary, reducing both the potential QHSE risks as well as project costs, ushering in a new, more effective approach to wind tower maintenance.
Despite this, the ROV operator and NDT specialist still had to get down via a manhole cover into a space that hadn’t seen the light of day - or for that matter a breath of fresh air - for years. They’d be going into an area normally covered by the tower above and filled with a column of water below, a space effectively isolated since construction. While C-Ventus is familiar with these confined environments “the safety precautions always need serious thought” explained Mr Sebregts.
Therefore the air had to be tested for noxious gas build up before entry and not just one but two teams were readied: as well as the technicians, rope access rescue specialists were necessary both to comply with the safety regulations and to respond, quickly, to any alarm from the manhole watch.
The mini-ROV itself had been kitted out with water analysis capacity and “a very elegant, non-destructive testing device” said Mr Sebregts, able to measure the wall’s thickness on contact rather than needing to penetrate it.
As there had also been some anxiety about the potential presence of corrosion and other fouling, the ROV had also been fitted with a camera to film and take snapshots at regular intervals. However, based upon previous experience and in house knowledge, its high tech GPS, depth gauge and electromagnetic compass were expected to be confused by being completely surrounded by the metal tower and it couldn’t orient itself, so ‘plan B’ was put into action.
“We hung ropes all-round the walls and down into the water to the seabed” explained Mr Sebregts. “One vertical rope every 60 degrees, starting from the stairwell, marked with duct-tape for depth and numbered to give a visual reference.” This meant the ROV remained within sight of a reference point all through the operation.
It worked well: the C-Ventus team were able to complete the inspection scope of the piles within the agreed time line. Interestingly, Mr Sebregts added that the water that had been trapped inside the tower appeared to be clear of growth, although the results from the cameras, water and thickness tests will be tied together and fed back to the engineering team for a full analysis”.
By Stevie Knight
