HDD Mongstad

HDD Mongstad

Visser & Smit Hanab carried out a unique project for Statoil in Norway. Wind, rain, snow and rock had to be battled. The assignment involved detailed engineering and horizontal directional drilling from the site of the oil refinery in Mongstad, Norway, to the adjacent fjord.

The drilling in this project was not a standard drilling, the angle at the point of entry being 45 degrees. This wide angle was necessary because the exit point was at a depth of approximately 234 metres below sea level. However, continuing the wide angle to the bottom of the fjord would have made it impossible to pull the 12'' gas pipe from the floor of the fjord into the drilled hole. Therefore, the design of the horizontal directional drilling was worked out in more detail and, because the rig is not designed to work at a 45 degree angle, adjustments to the drilling rig were explored. It also had to be taken into account that the Norwegian soil near Mongstad consists of very solid rock (granite and gneiss).

The client had constructed a heavy concrete slab with anchor bolts at the proposed drilling rig site based on Visser & Smit Hanab's design. A support construction was brought from the Netherlands and installed, after which two mobile cranes placed the drilling rig on the slab. The rest of the equipment was then installed and connected near the drilling rig.

After preparations on the site were completed the actual pilot drilling could start. It was agreed with the customer that the drillstring including the drill-head was to be pulled out of the drill hole after every 100 metres of drilling in order to inspect the bored section by means of a gyro and to compare the results with the position tracking system. After 37 days the point of exit was reached. The main question then was whether the exit location and exit angle were acceptable to the client. The exit point was measured with a remote offshore vessel (ROV) and approved by Statoil. After completing the pilot drill hole and as was agreed in advance, a video inspection was carried out in order to detect and assess any cracks in the rocks.

The diameter of the pilot hole, about 350 mm, was not big enough to pull the product pipe from the floor of the fjord to the surface. The diameter had to be enlarged to approximately 500 mm. A hole opener was used to ream the bore hole in rock. Another video inspection was carried out after reaming. As no unacceptable cracks were detected the last stage of the project could be carried out: inserting a 24 mm nylon pull cord. The cord was connected to a type of stopper. The stopper was then thrust through the drill hole to the bottom of the fjord by means of high water pressure. The 24 mm nylon cord was used to insert a steel pull cord into the drill hole, which in turn is used to pull the 12'' gas pipe onto land.