Sea legs testing the waters

The Delft start-up companies 3Delft and BMO Offshore tested a model of an anemometer tower for its seaworthiness at Deltares’ Atlantic basin last week.

BMO Offshore is a start-up company, which offers wind and wave measurements on location to prospecting wind park developers. BMO developed the so-called FlexMetMast to perform the measurements at sea. Instead of hammering a steel monopile 30 metres into the ground, BMO developed a set of sea legs for the mast. After adding ballast, the sea legs will sink 4 to 5 metres into the soil. After the measurements, the whole structure can be lifted, removed and reused elsewhere.
Last week, a 1:50 model of the FlexMetMast was tested in the Atlantic basin at Deltares. “We will test the structure with waves equivalent to 19 metres,” says Tim Raaijmakers (MSc), from Deltares. Such waves on the Atlantic correspond to a once in a hundred years’ storm. The running up of the waves onto the structure is measured, just as are the impact and the erosion at the structures’ feet. “The combination of waves and current can wash away the sand around the pillars,” Raaijmakers explains.

Traditionally, the scale-models for such tests are made of steel in a long, labour-intensive process – just take a look at the details of the mast. This model, or at least the difficult parts of it, was made through 3D printing by another start-up: 3Delft. Master student Hans van Toor (Aerospace Engineering) explains that 3D printing offers a more efficient way of model building. The tower and the conical feet have been made by 3D printing of nylon. The straight tubes were bought and cut to size. Lead ballasts, both in the feet and the platform, correct both the mass and the centre of gravity. “People say skilled labourers are hard to come by,” says Van Toor. “We say: just have your parts and forms 3D-printed. Even functional metal parts can be 3D printed these days.”

BMO’s Dr Bart Ummels quickly lists the benefits of 3D printing of scale models: “It’s a fast, flexible and precise technique. And it’s quicker and cheaper than conventional model making.”

His 3Delft colleague Jan Verschoor MSc. recalls that 3D printing has a history of already thirty years. “The first prints were made in plaster, which became a horrible mess pretty soon afterwards”, he says. Printed objects would fall apart at the least touch. “Nowadays, we have Rapid Manufacturing”, says Verschoor. Functional objects can be printed from various materials including metals. “People have printed functional hinges and moving gear wheels within a closed volume. 3D printing offers huge opportunities for designers.”