Hydrodynamic research: waves at the push of a button

Marginal Column

March 2014

 

The world’s largest oceanic laboratories replicate the waves found on the open seas. This is done to make drilling ships and offshore platforms ready for marine use.

 
 

Captain Nemo tells his guest, Professor Aronnax, about the treasures hidden below the depths of the sea: “First off, I’ll mention that at the bottom of the sea there exist veins of zinc, iron, silver, and gold whose mining would quite certainly be feasible.” Jules Verne’s novel, Twenty Thousand Leagues under the Sea, described the submarine Nautilus and appeared in 1870. Then, of course, that was a bold and far-fetched idea. Today, extracting oil or natural gas from the seabed has become a matter of routine. And nonetheless, technical challenges appear again and again, triggering an ongoing search for new solutions.

Trials in a miniature sea

To make this possible, the designs of drilling ships and platforms and other offshore equipment have to be tested using scale models – under the most realistic possible conditions. This is done, for example, at LabOceano, the wave research laboratory at the National University in Rio de Janeiro. There are two other nearly identical wave tanks – one in Wageningen, the Netherlands, the other in Shanghai, China – and these are among the world’s largest systems for conducting research on deep-sea waves.

In Brazil, the tank’s surface area is 40 by 30 meters. It is 15 meters deep and in the center there is a pit 25 meters in depth. The basin holds 23 million liters of water – ten times the volume of an Olympic pool.

Out in the real oceans, waves are generated by a highly complex combination of wind, tides, currents and climate differentials. Corino Corver, responsible for the worldwide sales of hydrodynamic research systems and located at Bosch Rexroth in the Netherlands, explains: “Actual wave creation for this kind of research cannot be replicated in the laboratory. But we can simulate wave behavior. The water’s movement is as close as possible to nature.”

Like a giant water snake

In the Netherlands, Bosch Rexroth offers a complete wave generator, including all the mechanical structures, drive and control systems, and software to compute and generate the waves and collect and analyze the data. Paddle segments made of stainless steel form the heart of the wavemaker. 75 of these paddles are lined up at the edge of the basin in Rio. They are each 400 millimeters wide, about 1.80 meters high, and two-thirds are under water.

When hydrodynamic testing starts, the paddles tilt forward and backward independently. The motion in this row of paddles resembles a huge water snake. The first waves roll through the basin. Gradually they are generated one after another, and if required from several directions – the so called 3D waves. Crests and peaks form, i.e. long crested and short crested waves. Horizontal fans simulate the wind. Inside the tank, the waves crash against the swaying model of an oil drilling platform. Sensors measure movement, force, and displacement.

“Generating a specific wave form at a specific time and at a specific location in the basin – that is the challenge,” says Corver. The research institutes are interested in exposing their models to scenarios incorporating differing sea conditions – and wind strengths close to the real environment. “To do this, Rexroth starts the design process at the model and translates the desired wave back through to the wavemaker. The wave computation software calculates the waves and the wave generation software figures out the angles and frequencies with which the paddles have to move.” The electric servo drives for the paddles execute the motions.

To match the model’s physical size, the software can simply scale waves up or down as needed. “One principle applies, of course: the larger the scale of the model, the more relevant the results of testing,” according to Corver. “That is why we expect even larger wave research laboratories to be built in the future.”