A Complex Integral Project: Export Cables Crossing the Dutch Wadden Sea
WODCON XXIII - Dredging is changing - The Practice. The Science. The Business.
L.M. de Wit, R. Bijker
"Export cables North of the Wadden Islands To connect a new 700 MW offshore wind farm located approximately 70 km north of the Dutch Wadden Islands to the onshore grid, two subsea 220 kV high-voltage cables are to be installed between an offshore substation and an onshore high-voltage station on the Dutch main land. The cable route crosses the Wadden Sea, before making landfall in the province of Groningen. Figure 1 shows two different routes that are considered for the cables in the Wadden Sea section. Both routes also cross the barrier island Schiermonnikoog. Crossing the Wadden Sea makes this project complex because it is considered pristine and is well protected. The Wadden Sea is one of the largest tidal regions of the world, characterized by dynamic channels and tidal flats. It is a designated Natura 2000 Nature reserve and UNESCO World heritage area. This requires careful installation of the cables, with a strong focus on a minimal impact on the environment. Offshore studies A series of integrated studies has been performed to determine the environmental impact of the installation of the subsea cables. The diagram in Figure 2 shows the individual studies and their interaction. The results of the performed studies resulted in several iterations and optimization of considered routes and installation methods (indicated with dashed arrows in Figure 2). After the initial route development, a risk based burial depth study (RBBD study) and a morphological study were performed. The RBBD study resulted in the minimum required cover above the cable, to comply with the maximum acceptable probability of failure due to shipping induced hazards over the design lifetime of the cable system (40 years). The study makes use of AIS data to determine incident frequencies and the characteristics of local ship traffic. This is used to calculate the local threats, such as sinking and stranding ships, hooking and falling anchors, and lost containers. In the morphological study, historical survey data was analysed to forecast the large- and medium-scale seabed changes during the design lifetime of the cable. Based on these predicted seabed changes, the morphological design basis (MDB) was determined. The MDB is defined as the bed level that is expected (with a high probability) not to be undercut during the design lifetime of the cable system. The minimum required burial depth of the cable was subsequently determined on the basis of the RBBD and the morphological study in combination with specific project and permitting requirements. An important project requirement set by the client is 'bury and would like to forget', which is a CAPEX versus OPEX consideration. 'Bury and would like to forget' results in burial of the cable below the MDB. As a result of the natural morphological dynamics in the Wadden Sea, the required burial depth can be as much as 10 m below the local bed level in sections along the route with dynamic channels. The minimum required burial depth, combined with hydrodynamical data (waves and currents), provided the input to determine feasible cable installation methods, allowing the calculation of dredging volumes. In the shallow Wadden Sea, significant dredging is needed to provide access of cable transport and cable installation barges, depending on the installation method and required burial depth. As the required dredging takes place in a hydrodynamically dynamic area a significant amount of over-dredging also needed to be taken into account. The dilemma encountered during this project is that on the one hand the crossing of the dynamic and sensitive Wadden Sea asks for a tailor-made installation solution. This requires a permit"
Keywords: Offshore export cable, Wadden Sea, integral, risk based, morphology, dredging.