© Daniel Zupanc

A stroll among sharks and rays

Tunnels at the bottom of aquariums are no longer hard to find. The “Haus des Meeres” in Vienna takes this one step further – visitors can walk through the marine aquarium protected by an enclosed PLEXIGLAS® tube.

The three-year-old boy cheers rejoices with excitement. He keeps trying to grab the passing cownose rays with his little hands – but his efforts end at the virtually invisible acrylic glass panel. Meanwhile, the ray turns its body with effortless elegance to show its underside, which looks like a smiling face. Adults are also fascinated by new Atlantic Tunnel at the Haus des Meeres. Here, they get a first-hand experience of what it would feel like to be in the midst of an underwater world.

“Our goal was to create an experience much like a dive in the Atlantic,” says Hans Köppen, Managing Director of the Haus des Meeres, describing Vienna’s new attraction. And we have clearly succeeded – at least telling from the visitors’ reactions. But the diverse range of animals is not the only thing that’s impressive. The feeling of strolling through the underwater world in the marine aquarium and experiencing the marine life in 3D is truly unique – below, above and right next to fish, plants and the sparse rock formations of the Atlantic.

The next step in the evolution of large-scale aquariums

Safety first

The PLEXIGLAS® tube has a dead weight of 15 tons. This allows the tube to withstand pressures of up to 70 megapascal; but in reality, it must withstand only a maximum of three megapascal. In other words: If you were to place two fully laden trucks on the tube, it would yield by just 1.5 millimeters. “It was in particular the brand acrylic glass by Evonik that allowed us to play it safe,” says Thilo Üblagger, who implemented the project.

This is only possible thanks to a design that is second to none. Unlike the commonly used aquarium tunnels, the ten-meter-long all-glass tube is not anchored to the floor, but was rather designed to float in the tank and is held at two fixed points. “This was necessary because the tank depth would not have allowed for a barrier-free construction,” Köppen explains. However, this requirement proved to be a major challenge during construction. In a tank with a depth of six and a half meters, a ten-meter-long air-filled tube floating two meters below the surface tends to do only one thing – rise. In the case of the Atlantic Tunnel, this means that a total weight of 65 tons is pushed to the surface. It is an enormous challenge not only for the support structure – in this case milled holes in an old bunker wall made of concrete – but also for the tube’s material. It must withstand huge pressure, may not bend and in no case break.

The designers tried to counteract this buoyancy by using PLEXIGLAS® with a thickness of 120 millimeters. The brand acrylic glass by Evonik is extremely robust and retains its shape; and the right know-how allows for almost invisible connections. After all, the all-glass tube was to be built from two half-cylinders. “Many companies know how to connect smaller components of acrylic glass. However, no one has ever attempted to build a closed tube system with a length of ten meters floating horizontally underwater,” says Thilo Üblagger, Managing Director of k-tec GmbH in Salzburg, which implemented the project.

One year spent on development

But no one has ever attempted to build a closed tube system with a length of ten meters floating horizontally underwater.

Thilo Üblagger
Managing Director of k-tec GmbH

To achieve this, Üblagger and his team worked together with specialists for specialty glazings at Evonik to test all relevant processes on numerous smaller components during the one-year development phase. For example, they determined how to form the two ten-meter-long blocks of PLEXIGLAS®, calculated the optimum bonding temperature for the polymers and analyzed the required width of the glue joints and other conditions. The day on which both preformed half-cylinders were connected to one another, however, was still marked by considerable tension. “We only had one try,” Üblagger recalls. “The upper and lower halves of the tube had to be bonded in perfect synchronicity and with a precision of a millimeter.” If this had not succeeded, the three-million-euro project would have failed.

A distortion-free view

The interdisciplinary team of thermoforming and sealing experts, structural engineers, marine biologists and process engineers delivered perfect results during the planning, manufacturing and assembly phases – the tube splices merged seamlessly. This creates a perfect illusion of walking through the underwater world. The most important requirements for the tube material were that it had to withstand the extreme forces in the tank while also boasting a great visual appearance. “PLEXIGLAS® enables a distortion-free view,” explains Üblagger. The fascinating underwater world is just inches away and the animals in the water appear like they really are – without a green tint, no cloudiness and in the right proportions and shapes.