It sounds simple: lower 89 concrete elements to the seabed and connect them to form an 18-kilometre-long tunnel between Lolland and the German island of Fehmarn. In reality, it is one of the most complex maritime operations in the world right now - and a process where even small deviations can delay the entire project.
The Fehmarn tunnel will be the world's longest immersed tunnel for cars and trains. Once completed, trains will be able to travel between Copenhagen and Hamburg in under three hours, and motorists will cross the belt in about ten minutes. But behind the vision lies a series of challenges that only appear in the internal technical and environmental documents used by the authorities to manage the work.
FemernBusiness has obtained access to more than 700 pages from German authorities through freedom of information requests. These include environmental assessments, noise calculations, logistics plans, technical drawings, and detailed procedures for the work at sea. Together, they provide a rare insight into how many frameworks and restrictions govern the immersion of the enormous tunnel elements.
They reveal, among other things, that the work is caught between two major constraints: Summer, when environmental requirements are tightened, and winter, when the weather shuts down operations. Between these two periods, there are only a few months with both calm weather and freer working conditions - and even then, the work can be halted from day to day.
Noise Requirements in Natura 2000
The German authority conditions establish two central limits for underwater noise. At most, 20 percent of the Fehmarn Belt's cross-section may at any time be exposed to noise over 144 dB. In the summer months from June to September, no more than one percent of the Natura 2000 area may be exposed to noise over 140 dB. The limits are monitored with the Quonops model, which calculates noise distribution throughout the entire belt sea every quarter of an hour. If the values are exceeded, the work must be altered or halted. This means that even in periods of perfect weather, the immersion can be paused.
Underwater noise is not the same as in the air. The scale is the same, but the reference point is different: in the air, it is measured relative to 20 µPa, while underwater it is 1 µPa. Therefore, the numbers seem higher, and at the same time, sound propagates much further in water than in air. By comparison, porpoises themselves emit click sounds of up to 205 dB re 1 µPa. This shows how powerful sound pressures naturally exist in the marine environment - but also why the limits are set significantly lower than the porpoise's own signals. The requirements are not about avoiding individual powerful clicks, but about preventing prolonged and extensive noise impact on endangered species.
Breeding Season and Species Protection
In addition to the noise limits, species protection plays a central role. The German Planfeststellung, which outlines the construction requirements, obliges the builder to ensure that the Fehmarn Belt continues to function as a breeding area, nursery, and migration corridor for porpoises. This means that construction activities must be adapted to the animals' sensitive periods. During the birds' breeding season, typically from May to July, noisy work must not take place in certain zones. At the same time, contractors must use more gentle methods when particularly sensitive species are present. These seasonal closures restrict the working year and make planning more complicated.
Protected Marine Mammals
The established noise limits have been introduced precisely to protect marine mammals such as porpoises, which are covered by the EU's Habitats Directive. The documents make it clear that the porpoise's function as a breeding and nursery species in the Fehmarn Belt is a main reason for the restrictions. Therefore, operational stop mechanisms are built into the work. If measurements or model calculations show that the limits are exceeded, the lowering must be stopped immediately. The same applies if marine mammals are observed close to the work area. Even short observations can therefore interrupt the work and shift the schedule.
Wind and high seas
In addition to the environmental requirements, the weather is an entirely unpredictable factor. Femern A/S itself describes the immersion as the most complex and weather-sensitive work in the entire construction process. To carry out a sinking, several consecutive days of calm weather are required because the whole process involves towing, precise placement, sinking, and underwater coupling. Strong winds and high seas can halt work in the middle of an operation, and bubble curtains, used to dampen noise, function poorly in rough weather.
“No spill”
Another requirement, which came through the approval of the German authorities, is the so-called “no spill” principle. In the summer months of June, July, and August, all material from marine work must be completely collected so that nothing escapes into the marine environment. This means that the immersion becomes more technically demanding and time-consuming precisely in the high season, where the weather otherwise provides the best opportunities for offshore work. Thus, some of the flexibility that one would normally have in the summer months disappears.
The smaller work area
The work corridor in the Natura 2000 zone has been significantly narrowed. The contract assumed that the work could take place at a width of 1,100 meters, but in the authority's approval, the width was reduced to 648 meters. The narrower corridor means less maneuvering space for the large vessels, tighter logistics, and less space for simultaneous operations. The result is that the pace is further slowed down because much more precise planning is required, and fewer activities can take place in parallel.
