Extreme Bottom Currents Along a Deep Fjord Pipeline Route

McClimans, Thomas A. (SINTEF Fisheries and Aquaculture, Trondheim, Norway) | Eidnes, Grim (SINTEF Materials and Chemistry, Trondheim, Norway) | Moshagen, Hermann (BHM Engineering Services, Trondheim, Norway)


Horizontal density changes in the sea have the potential to drive extreme bottom currents. The focus in this paper is on sill overflows of Atlantic water from the Norwegian Trench into Fensfjorden in Western Norway. Fensfjorden is used for routing pipelines to the oil terminal at Mongstad. The paper describes the total study program that was carried out over the past years and outlines the challenges met in establishing a bottom current data basis for pipeline design and integrity management applications. The work included a study of large scale oceanographic and meteorological conditions believed to influence the actual area, field measurements and a thorough evaluation of the current and density data describing the flow system in the fjord and finally an advanced laboratory testing program in a rotating (Coriolis) basin. A theoretical model was set up connecting large scale ocean dynamics and extreme events of bottom current inflows. The model was tested and confirmed as a valid extreme event forecast model. The model is believed to be of considerable value for a variety of future purposes along the actual part of Fensfjorden. The total work is believed to be relevant for similar coastal locations around the globe.

Sudden increases of near-bottom currents are known to occur in Norwegian fjords, as well as other places around the globe, and must be taken into account in the design of submarine power or communication cables, pipelines and other seabed installations. Causes of these events include suspension and turbidity currents on slopes and horizontal density differences creating sill overflows to closed basins. The focus here will be on sill overflows of Atlantic water from the Norwegian Trench into Fensfjorden in western Norway. The outer part of Fensfjorden has water depths varying between about 250 m at the sill and down to 540 m at a location about 3 km further into the fjord.