Before any pipe was installed, engineers had to successfully plan the project to overcome the unique challenges of the North Sea. For example, the seabed needed to be surveyed. An underwater vehicle called the Hugin was used to give the project design engineers an understanding of the surface the pipeline would have to travel across. The Hugin was supplied by Kongsberg Maritime and the Norwegian Underwater Institute and traveled in depths as much as 4.5 km.
The pipeline was laid across a treacherous seabed. A remote-controlled excavator, the Nexan Spider, was used to prepare the seabed so that the pipeline could traverse steep slopes and uneven terrain. Rocks that were in excess of 60 m high were removed by the excavators. In addition, 2.8 million tons of rock was lowered to the sea floor to level out some of the terrain.
The steel pipeline is 42 inches in diameter along the northern section from Nyhamna to Sleipner Riser, and 44 inches in diameter along the southern section to Easington. In total, 96,600 pipe sections were laid. Two large pipe laying ships, the LB200 and the Soltaire, which is the largest pipe-laying ship in the world, had the task of laying 4 km of pipe per day. After the pipe was laid, divers had to make sure that it was prepared for welding. They did this in a watertight construction area called the Pipeline Repair Habitat where they could make any repairs that were necessary. Hyperbaric welds, a type of underwater welding, were used to join the sections. The submerged pipeline had its share of environmental complications, but integration of Environmental Impact Assessments (EIA) with the engineering developments, time, and resources enabled the working parties to keep a check on the environmental impacts of constructing the pipeline and formulating possible risk mitigation solutions.
