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The Coastal Highway Route E39 (Ferjefri E39) project by the Norwegian Public Roads Administration (NPRA) is aimed at finding solutions for seven the fjord crossings along the 1100 km coastline between Kristiansand and Bergen in Norway. The NPRA now has been collaborating with a group of University of Stavanger (UiS) researchers who have been investigating a new laser-based method for measuring wind behavior in the fjords, which is vital for designing safe structures. The method is now being applied for the first time in relation to bridge design. A fjord is basically a long, narrow, deep inlet of the sea between high cliffs typically formed by submergence of a glaciated valley.
The Bjørnafjord south of Bergen is one of the natural barriers that will be bridged by the Ferjefri E39. With a water depth of up to 550 m and a total width at the crossing point of around 5 km, this is one of the three most difficult fjords to be crossed. The Bjørnafjord crossing will become the longest floating bridge in the world, where the vertical load is supported by floating pontoons.
Traditionally, wind in a fjord is measured by anemometers installed on purpose-built measurement masts on land. But in 2014, wind researchers from UiS started using optical remote sensing to monitor wind conditions. The researchers were introduced to lidar technology during their work in NORCOWE (the Norwegian Center for Offshore Wind Energy). The Lidar approach involves sending a pulsed laser beam through the air and measuring signals backscattered by dust in the wind. The difference between the frequencies of transmitted and received pulses indicates the wind velocity. Inspired by the possibilities offered by lidar, the scientists started a pilot project on the optical wind sensing in the Lysefjord, in cooperation with the NPRA. An extensive system for long-term wind- and vibration monitoring, constructed in 2013, provided an excellent basis for the evaluation of the lidar wind measurements. The 640 m-long suspension bridge with the main span of 446 m, is located at the inlet of the Lysefjord in Norway's Rogaland region. By placing a long-range lidar sensor at Lysefjordsenteret the scientists were able to assess the wind-speeds around the bridge at a distance of almost 2 km. The results from the lidar wind measurements on the Lysefjord bridge confirmed their expectations too.
According to the researchers, Lidar technology enables them to monitor the wind conditions at a number of locations across the entire fjord. The new method gives more detailed knowledge about the conditions in the centre of the fjord, where steady wind and gusts typically have the strongest impact on the bridge. They believe this new measurement method is invaluable for designing bridges across large Norwegian fjords. It is costly, but, it provides important, new data on the wind field around the bridge, so that the structure can be designed optimally.
In spring 2016, Jasna Bogunovic Jakobsen from the Department of Mechanical & Structural Engineering & Materials Science at UiS, who also leads the project on the application of lidar wind measurement to bridge engineering, carried out a six-week measurement campaign in the Bjørnafjord in spring 2016, in collaboration with Technical University of Denmark and Christian Michelsen Research (CMR). Three laser sensors were placed on shore, with beams directed towards the sea. The three lidars were integrated into a wind scanner system, which enables simultaneous, coordinated measurements by the three measurement units.
Due to the large width of the fjord, a specially adapted measurement set-up and subsequent data analysis was required. The aim was to get as detailed and accurate information as possible on the spatial and temporal variation of the wind. Jakobsen and colleague Etienne Cheynet are currently analysing the measurement data from Bjørnafjorden.
NPRA too commented that they will be using this new measurement method to develop design requirements for further bridge projects along Ferjefri E39. In addition to the Bjørnafjord, it includes the Halsafjord and Sulafjord bridges.
