The specialised vessel that will lower the precast sections of the Fehmarnbelt immersed tunnel into place is undergoing final tests in Denmark after carrying out a first full connection to a finished tunnel element.

Over the weekend the immersion pontoons IVY 1 and IVY 2 were coupled to a completed standard tunnel element in the large work harbour on Lolland as part of a final series of trials ahead of approval by the Danish maritime authority. The standard elements are 217m long, 47m wide and weigh 73,000t.

The contractor group Femern Link Contractors (FLC) said initial tests, in which the pontoons operated both separately and together while holding the element, confirmed that the systems functioned as intended.

FLC plans a concluding phase of testing in which ballast will be added to the element to simulate the weight it will carry once filled with concrete. The IVY units will then lower and raise the loaded element inside the harbour to demonstrate their ability to control immersion and retrieval; the company expects these trials to finish this month. Following successful tests and formal approval, IVY 1 and 2 will transport elements from the quay to the prepared tunnel trench and submerge them with millimetre precision, the contractor said.

Commencement of the immersion of the elements has been long anticipated. Excavation of the 18km trench was completed in April 2024, the first tunnel element was cast in May 2024 and the immersion vessels arrived in October 2024. Yet, well over a year later, not a single element has been submerged – and they have been piling up on site, with 14 standard and three “special” elements (39m long, 47m wide, weighing 21,000t) having been cast.

Danish state client organisation Femern A/S said the delays were due to issues with the immersion vessels. The project is now two years behind schedule and it recently had to cancel procurement of two major contracts on the project.

Revelations about the tensions between the state-owned client and the contractor were revealed in recent weeks. Correspondence between the director of FLC (featuring Vinci, Aarsleff, Wayss & Freytag, Max Bögl and others) and the Danish transport minister – seen by NCE – revealed a blame game about the cause of the delays and the extent of the issues.

All parties are now positive that immersion will start in the coming months.

The IVY vessels carry 23km of steel wire across 66 winches

How the immersion system works

The Fehmarnbelt immersed tunnel, which will link Denmark and Germany beneath the Baltic, is designed as an 18km concrete tube assembled from 79 standard elements and 10 special elements that house technical systems.

The immersion will see the standard elements kept afloat by being connected at each end to IVY 1 and IVY 2, while special elements are moved with the two pontoons joined to form a single vessel.

The IVY vessels carry 23km of steel wire across 66 winches. These winches lower elements into a pre-dredged trench at depths of up to 40m and control their placement to very tight tolerances.



Project context and challenges

The Fehmarnbelt link is among Europe’s largest ongoing infrastructure projects, intended to cut travel time between Copenhagen and Hamburg and to strengthen freight and passenger links between Scandinavia and continental Europe. The tunnel’s construction has involved large-scale offshore dredging, production of massive concrete elements in a purpose-built quay and the development of specialised immersion vessels and equipment.

Immersed tunnels require precise control during placement because incorrect positioning can compromise water-tightness and structural integrity. The IVY pontoons’ winch and ballast systems are therefore critical to the project’s schedule and to meeting Danish maritime regulations before seabed operations commence.

Next steps

After the maritime authority’s approval, work at the Lolland harbour will continue with final concreting of elements to reach their sinking weight. The IVY units will then begin transporting and immersing production elements into the prepared trench. FLC’s timetable suggests that the harbour tests will be completed in March, after which offshore immersion operations can start, subject to regulatory sign-off and weather conditions.

Lasse Vester, deputy contract director at Sund & Bælt said: “IVY 1 & 2 is a completely unique vessel, built specifically for the tunnel elements used for the Fehmarnbelt tunnel. It has taken longer than expected to complete and test the vessel. FLC is now close to reaching the finish line, enabling the vessel to be approved to play the key role in the immersion of the tunnel’s many elements. We expect the first element to be immersed and connected to the tunnel portal at Rødbyhavn later this spring.”

Like what you’ve read? To receive New Civil Engineer’s daily and weekly newsletters click here.