Finland is almost as large as Germany, but its population is much smaller, only 5.4 million people. A myriad of lakes and many islands without names give distinction to the country. Not very far away from Suonenjoki, a city with 7,500 inhabitants, approximately 300 km to the north of Helsinki, a very sophisticated drilling operation (in more ways than one) was carried out in a nature sanctuary.
The existing power supply line was to be renewed across a river from one side to the other side including an island in the river. Only manual work and no appliance of construction machines was allowed on the island, therefore the tender for this 294 m long section demanded trenchless underground cabling installation with a cable protection pipe of ND200 using the horizontal directional drilling method. Because the work was carried out from one side of the river, disposal of the drilling fluid volume was easy to perform and according to regulations.
The local fishery supervision authority was rather worried about the breeding area of the fish living in the river, particularly in the riverbank region, and accompanied the drilling operation with a critical eye, particularly where output and whereabouts of the drilling fluid were concerned.
The greatest challenge, however, was the ground itself. At a depth of 10 m, the crew faced hornblende rock and granite with an Unconfined Compressive Strength (UCS) exceeding 300 MPA. This rock is one of the hardest formations known and it dates back to primeval times. The layer close to the surface was also very hard to drill, it being ground moraine rubble generated by ‘erratic glaciers’ during the Ice Age.
All these marginal conditions demanded a high performance, eco-friendly drilling method with a particularly thrifty consumption of resources, no more than 10.4 l/h of diesel fuel and 32 t of drilling fluid at most. Like all Finish enterprises in general, the company manager Ville Husso of Savon Suuntaporaus attached great importance to using a robust drilling technology, given that a technical drop-out in the lonely vastness of Finland would mean many days of shutdown time.
This was found with the for the capabilities of the GRUNDODRILL18ACS rock drilling rig belonging to a boring contractor from Iislami. The pilot bore of 6½ in (165 mm) diameter taxed the patience of the drilling crew time and again, although in the end, the drilling progress was 3 to 4 times as high as that of comparable HDD rigs. The pilot bore was tracked with a DCI F5 tracking system mounted in a boat.
Partially, depths up to 10 m beneath the blind level were reached. The reaming of the pilot bore hole to 12 in (300 mm) diameter was no small ordeal, either. In the end however, the ND200 cable protection pipe was pulled in with a 10 in (250 mm) diameter rock reamer without further ado.
Company manager Ville Husso said: “Drilling surely tested our limits, there were times when I was not so sure that we would make it at all. Now we are happy and very proud of our achievement.”