Annual report 2019

35

channel De Honte is the deepest waterway in the Netherlands with a depth of sixty meters. “To cope with the hard soil here, the jumbo hopper dredger Prins der Nederlanden was ftted out with an extended suction pipe and the heaviest possible draghead,” Veth explained. “Right next to De Honte is a sandbank that falls dry at low water. Here, dredging could only be done with a small cutter suction dredger.” EQUIPMENT Prior to the dredging work, extensive surveys were conducted to determine the status of the seabed. Subsequently dredging work was carried out by the trailing suction hopper dredgers Causeway and Prins der Nederlanden, the cutter suction dredger Rhone and the backhoe dredger Manu Pekka. A wide strip of mud was removed in the so-called mud flat near Borssele and replaced with a sand package, so that the cables can dissipate their heat better. To avoid the risk of unstable soil, Boskalis subsidiary Cofra compacted the soil using vibro-compaction. The fallpipe vessel Rockpiper was mobilized to protect the cables with rock in the fairway. To this end almost 150,000 tons of rock will be brought in, of which a part is scheduled to be installed in the second quarter of 2020. Equipment of Boskalis Nederland covered the cables with rock in the shallow sections. NEARSHORE SECTION A critical part was the nearshore installation of the cable through the sandbank in the Western Scheldt. “This area in particular required a seamless coordination of different execution methods. A new tool was developed in-house specifcally for the simultaneous laying and burying of cables,” Veth said. This BSS3 (Burial Sledge System) tool is equipped with a jet pipe and a cutter tool, with which it buried the cables at depths ranging from 3 to 8 meters. The barge Giant 7 was converted into a cable-laying vessel specifcally for this project. The vessel installed the 19-kilometer cables, each weighing 1,600 tons, in the nearshore section. The BSS3 and the Giant 7 installed the cables in single runs. OFFSHORE SECTION The frst two cables were installed by the cable-laying vessel Ndurance and the multi-purpose trenching vehicle Trenchformer. The third and fourth are being installed by the Ndeavor and the new burial tool CBT2400. The cables are installed over distances of about 30 kilometers (Alpha cables) and 35 kilometers (Beta cables). The work in the offshore section also included installing a 7-kilometer-long interlink cable between the Alpha and Beta platforms. After laying the cables, the Giant 7 was converted in the port of Flushing to make the subsea cable connections. The total distance covered by the cables was more than the length of cable that could be carried on the ship at once. “As a result, the separate cable sections on the seabed had to be connected at eight different locations. Each connection took about ten days to make,” Veth said. LANDFALLS Boskalis is completing a total of four landfalls to connect the cables to the high-voltage grid. “This required the temporary lowering of the Borssele dike. In case of an unexpected storm, the sea defenses had to be completely intact. In all cases, our team had to be able to restore the dike within twenty-four hours,” Veth explained. The cables were pulled more than a kilometer and a half over the mud flat. After the completion of a landfall, the Giant 7 crossed the waterway to lay the cables at a range of depths, for which the shipping route to and from the port of Antwerp had to be completely closed. The team managed to stick to the time limit of ten hours on the frst two occasions. The fnal installation activities for cables three and four will take place early March 2020.