The heavy tunnel elements float like a hollow ship hull
Each tunnel element will be towed by four tugboats from the production site to a holding area near the tunnel trench in the Fehmarnbelt.
Because of this, each tunnel element is suspended from sophisticated immersion pontoons. From the navigation bridge of the pontoons it is possible to fine-tune the element's position in the water and control the subsequent immersion.
In the factory, the tunnel element is equipped with watertight steel bulkheads at each end. The bulkheads keep the inside of the element completely dry. They also ensure that its approximately 73,000 thousand tonnes float in spite of the enormous weight. In principle, each floating element is a large, hollow, air-filled ship’s hull.
Two submersible pontoons have been attached to a tunnel element and the element exits the production site. Photo: The construction of The Øresund Bridge.
Prior to towing, the tunnel element is also equipped with a system of internal ballast tanks, which are typically filled about halfway with water. The volumes in the ballast tanks can be remotely controlled from the bridge on the immersion pontoons. This allows the manager responsible to continuously fine-tune the element's position in the water according to the waves and currents.
THE GREEN LIGHT IS GIVEN ONLY WHEN WEATHER CONDITIONS ARE OPTIMAL, ALL PREPARATORY WORK IS COMPLETELY FINISHED AND BOTH FEMERN A/S AND THE CONTRACTORS AGREE THAT THE TIME IS RIGHT FOR IMMERSION TO BEGIN.
Did you know that...
... the approximately 73,000 thousand tonnes of a heavy element can float despite its enormous weight.
... the steel bulkheads keep the element completely dry inside and ensure that it can float.
... the tunnel element has to be towed by four strong tugs.
Strong tugs for specialised navigationFemern A/S anticipates using up to four strong tugs to tow each tunnel element from the production site to the tunnel trench. By its nature, this is a specialised form of navigation and means that a great number of people from Femern A/S, the contractors, the German and Danish maritime authorities, as well as the specially designed co-ordination centre at the Fehmarnbelt are involved for everything to run smoothly. Guard ships will monitor shipping traffic throughout the operation.
Four strong tugboats is towing the tunnel element towards the tunnel trench
The tug captains need to be aware of the busy traffic in the Fehmarnbelt while towing the heavy elements. The large load limits the tugs’ ability to manoeuvre, which means it is about sailing safely and carefully. The element will usually protrude half a metre above sea level during towing.
The element is temporarily anchoredWhen the tunnel element has been towed out to the tunnel trench, it will be anchored temporarily in a holding area near the tunnel trench.
The element is anchored temporarily to await the green light for final immersion. This is given only when the weather conditions are optimal, all preparatory work is completely finished and with the agreement of both Femern A/S and the contractors. Then the immersion can begin. It takes between 24 and 72 hours to lower the tunnel element safely and precisely into place. The placement of each 217 metre-long element has to be accurate to within approximately 5 centimetres.