Project design

Femern A/S was responsible for designing and providing the basis for the official approval of the coast-to-coast section of the Fehmarnbelt Fixed Link. After a series of investigations between alternative solutions (an immersed tunnel, a bored tunnel, a cable stayed bridge and a suspension bridge), an immersed tunnel was recommended as the preferred technical solution for the Fixed Link. Femern A/S, advised by a consortium of consulting engineers consisting of Ramboll, Arup and Tunnel Engineering Consultants (TEC), provided a basic design of both the tunnel trench and all the facilities associated with the early construction works of the FBC scope of works. These included work harbours, land reclamations, coastal protection works, site roads and several large stockpiles. These designs were detailed by FBC, thus allowing for optimisation of the design towards, for instance, the chosen work methods and the results of detailed field investigations.

To align with upcoming (strict) legislative and regulatory arrangements that logically come with such bilateral project development, minimisation of environmental impact was an important design requirement, with an aim of creating added value. This focus in the initiation phase resulted, for instance, in the requirement to reuse all dredged material onshore to minimise spreading of sediments in the Fehmarnbelt and to use these materials to create natural value within reclamations, predominantly on the Danish side.

Since physical parameters such as strength or bulking of material after dredging are depending on methods chosen for dredging, transport or placement of the same material, involvement of the contractor in defining these parameters is of utmost importance to assure that such value creation stays economical. Therefore, design of some horizontal and all vertical dimensions of land reclamations and stockpiles were the contractors’ responsibility. In this arrangement, optimised storage and reuse of dredged material was achieved in the most sustainable way. At the same time, the land reclamations were to be constructed using dredged material usually considered unsuitable for such construction works in view of poor remoulded strength of the materials. As the ultimate use for the land reclamations was to create natural values, at least for the majority of its areas, playing with strength requirements in alignment with final use became another aspect of increasing the project’s sustainability.

Environmental Impact Assessment

The Fehmarnbelt is an area of outstanding natural beauty and importance. The tunnel trench dredging works were executed in the vicinity of several Natura 2000 sites and other nature protection sites. The trench is running parallel to one of the most important bird migration routes between Scandinavia and mainland Europe. The belt itself is home to protected species, such as harbour porpoise, harbour seal and grey seal. Seagrass meadows and boulder reefs are located at some distance from the works.

To protect these nature values, an extensive environmental impact assessment was undertaken, satisfying both the Danish and German authorities responsible for the protection of the environment as well as international treaties for the protection of marine environments, such as the Helsinki Convention and OSPAR. This assessment resulted in extensive requirements related to the execution of the works, including reporting requirements, training requirements for crews operating the dredging equipment, thresholds and limits to be adhered to, and calendars governing actual operations. In this way, the stakeholders’ needs (with stakeholders being broadly defined as both natural, socio-economic and legislative stakeholders) could all be addressed, while still allowing the execution of the construction works.

In order to effectively manage the operations while complying with these requirements, a method of adaptive execution management was selected by FBC. The contractual arrangements between Femern A/S and FBC allowed for such an approach.

Adaptive execution management

Fine sediment control

In view of previous experiences with large scale marine infrastructure development projects in Denmark, the proper control and management of fine sediments originating from the dredging and reclamation activities were a crucial aspect of all preparations and execution of the project, aligning with expectations from both countries’ legislative and regulatory context. Following from extensive environmental research performed during the environmental impact assessment, a total sediment spill budget was defined in advance. This sediment spill budget was split into smaller parts, distributed over time (months and seasons) and space (eight spill monitoring areas along the alignment of the tunnel trench, including nearshore areas at Lolland and Fehmarn). Spill budgets range from negligible or zero in certain environmentally sensitive areas in the summer season to allowable levels in less sensitive areas over the winter period. Overall compliance is monitored at all these levels and at the totals. FBC had the responsibility to ensure compliance using extensive field monitoring and numerical modelling, including verification of achieved accuracies at the end of all dredging operations.

To achieve this, FBC set up a robust management process to control spill: the Plume Clarity Management Programme. This management framework consists of several feedback loops connecting spill monitoring and spill modelling as well as execution of the works and mitigating measures. All dredging activities were combined into 35 scenarios consisting of combinations of dredging methods and soil units. Each scenario was extensively monitored using specialised vessels and equipment. A specific spill impact reduction and identification tool was developed to take account of the spill generated by dredging operations on a daily basis based on monitoring as well as modelling. Continuous reporting using internal and external dashboards, including a traffic light strategy, was part of the spill management framework.

In practice, sediment spill control governed all dredging and reclamation operations in detail. Dredging planning over weeks and seasons was based on predictive modelling of spill amounts as well as f ield monitoring results. By adapting the planning based on actual data, the entire operation was optimised by FBC to minimise spill. Ultimately, the dredging works were concluded well within the overall spill budget.

Underwater noise management

Underwater noise is a significant theme in the Fehmarnbelt, potentially impacting local marine species as harbour porpoise, grey seal and harbour seal as it could mask biological signals, changing animal behaviour and causing physiological impacts in extreme situations. Underwater noise is subject to EU legislation, Danish and German legislation as well as several international conventions, including the Helsinki Convention and OSPAR. The topic was extensively discussed during the planning stage of the project because of a lack of national and internal standards to measure and assess noise from shipping and dredging. These discussions resulted in specific requirements that were verified through modelling by independent experts.

In practice, compliance with the underwater noise requirements did impact the construction works. These impacts ranged from the need to take precautionary measures, such as the installation and deployment of acoustic pingers during piling operations to significant planning impacts on dredging operations. FBC provided results of detailed underwater noise measurements of all major equipment involved in the project, measured onsite to ensure relevance to local conditions. This data was used to update underwater noise modelling, which in turn resulted in the need for adaptive management of dredging operations in order to secure compliance with the requirements.

Environmental management

Bird protection measures

The Fehmarn belt is the bottleneck of Scandinavian bird migration. Every spring and autumn large numbers of birds migrate across the belt and large areas in and around the belt are of international importance for water birds.

One of the key elements in protecting birds from the possible negative impacts of the works is related to light emission from the vessels. Requirements included the shielding of lighting, installation of f ilters on deck lighting and the timely switching on of lights during twilight to avoid shock effects for birds and attraction or barrier effects for fish. In practice, specific lighting plans were produced for each vessel involved in the construction operations. All vessel crews were trained to be aware of requirements and instructed on dealing with fatigued birds resting on board and disoriented birds. This included shutting down of lights in case of birds perching on the vessel. The crews were required to report daily on this topic during the bird migration seasons.

Greenhouse gas emissions

In an effort to reduce greenhouse gas emissions, FBC took several noteworthy steps in addition to common measures, such as the deployment of well-maintained equipment, selection of fuel-efficient equipment and optimised working practices and office management. These additional steps included the application of an energy regeneration system on a wire crane deployed for dredging of the deeper parts of the tunnel trench. This system stores a surplus of energy during the lowering of the crane’s bucket, to be used for the hoisting process. Another optimisation aimed at reducing greenhouse gas emissions included the contractor induced adoption of a sand borrow area closer to the project site than the area included in the impact planning process, significantly reducing greenhouse gas emissions during transport. Last but not least, greenhouse gas emissions were reduced through the use of biofuels during the mobilisation of the trailing suction hopper dredger Willem van Oranje.

Record-breaking project

The Fehmarnbelt tunnel construction project is about to set records in many different ways. Upon completion, the tunnel will be the longest immersed tunnel worldwide. The dredging of the tunnel trench itself also required record-breaking achievements such as the dredging accuracies that met unprecedented requirements. At the same time, the construction has to adhere to the strictest environmental standards as set out in the environmental impact assessment and German plan approval process. As such, the tunnel trench dredging project can be seen as a state-of-the-art example of the integration of technical challenges with construction methods optimised to minimise the impact on the environment. These achievements were made in close collaboration between FBC as the contractor, the project owner Femern A/S and the consortium of consulting engineers Ramboll, Arup and TEC. Key elements of this collaboration included the establishment of joint project objectives, open communication in the best interest of the project and mutual trust in the capacity and capabilities of each partner. By applying these principles, each partner was tasked with the management of risks and challenges closest to its own field of expertise and sphere of influence. This resulted in the successful completion of this challenging dredging project.

Critical success factors for this collaboration included the early involvement of all parties during the design process to enable the integration of construction methods with added value creating design objectives, such as storage of all dredged material onshore while creating land reclamations using materials usually considered unsuitable for such purposes. Adaptive execution management was implemented during the execution of the dredging operations to safeguard adherence to the strict environmental requirements related to f ine sediment dispersion and underwater noise generation.