Norway is using extensive steer core drilling for the first time to substantially mitigate project risk by probing geology along the entire length of the subsea section of a planned road tunnel as it passes below a fjord.
The national roads authority – Statens Vegvesen (NPRA) – awards the NOK59.98 million (US$7.35 million) contract to local company Entreprenørservice AS to undertake the core drilling task on its 15.5km long Romsdal subsea tunnel project, located in the north of the country. Subcontractor for the SI work is Asera Mining AB.
Entreprenørservice beat two rival bidders for the contract – Zublin-Foralith JV and Rockma Exploration Drilling AB, respectively.
Romsdal is to be a twin tube tunnel, linked by cross passages, although its final design length depends on how the data from the SI influences the depth and grade of the proposed vertical alignment. At this stage the tunnel grades are planned to be no more than 5%, and the deepest part of the crossing would be 350m below water level with about 50m rock cover to the fjord bed. Approximately half of the alignment between Otrøya and Vik – on separate islands – is below the fjord.
The core drilling is to be undertaken over the next few months. The geology, as presently known, is strong bedrock but includes some weak zones. The core drilling will help identify the rock surfaces, the client said.
Four main core drilling holes are planned from three different locations, and while their lengths will be determined as work proceeds it is expected that the longest could be up to 2.2km long. The 50mm diameter drilling is to be within 10m-20m above the proposed tunnel alignment.
Drill steering is to be performed by Norwegian firm Devico AS under a separate contact with Statens Vegvesen. This type of drilling method takes much longer than the more typical type of drilling from ships to the seabed.
Romsdal is the only project in Norway, so far, to employ core drilling as a key aspect of its SI work for the full length of the subsea road tunnel. Statens Vegvesen explained that the possibility comes from there being an island in the middle of the fjord crossing, allowing extensive core drilling coverage of the tunnel alignment from both of the main shores as well as the island, Tautra. The technique is mainly used in the offshore oil and gas industry.
Other SI work for the tunnel has included geomagnetic and seismic surveys. Depending on the results of the core drilling the client will decide whether further SI is needed.
After completion of all the SI work and refinement of the tunnel designs, Statens Vegvesen envisages that the next stages of approvals and procurement will enable main construction to begin around 2018. Presently the client anticipates there will be at least two contract packages for the tunnelling works.
The Romsdal scheme is part of the country’s strategic E39 Coastal Highway, which features many fjord crossings – some built, others under construction, and more planned. The Romsdal project will include both tunnel and suspension bridge construction to link the island of Otrøya across neighbouring fjords to two different parts of the mainland.
Romsdal tunnel is longer than the 14.3km Ryfylke tunnel, currently under construction near Stavanger in the south west of Norway and due to be Norway’s longest subsea road tunnel. Although not part of the E39, the tunnel is part of the Ryfast scheme – which also includes the 5.5km long Hundvåg tunnel – which will be a strategic connection to the coastal route and will be a major new road (Rv13) for the Stavanger region.
Eventually both Romsdal and Ryfast will be dwarfed by the 25.5km long E39 Rogfast scheme, also to be built near Stavanger. The Rogfast project is planned for construction in the 2020s. Its SI work has involved some drilling from the shore, but the tunnel’s considerable length under open waters has meant more traditional seabed drilling from ships has also been used.