Lidar to protect world’s longest floating bridge against high winds

13 Jun 2017

Norway's new 1100km coastal Euro-highway will rely on understanding wind behavior using new lidar-based measurement method.

The Norwegian Public Roads Administration's giant project Coastal Highway Route E39 (Ferjefri E39) includes finding solutions for seven fjord crossings along the 1100km coastline between Kristiansand and Bergen.

The NPRA 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.

The Bjørnafjord south of Bergen is one of the natural barriers that will be bridged by Ferjefri E39. With a water depth of up to 550m and a total width at the crossing point of around 5km, this is one of the three most difficult fjords to be crossed.

“This will be the first time we have to develop a bridge of these dimensions,” said project manager Mathias Egeland Eidem from the NPRA. “Gathering as accurate information as possible about waves, currents and wind conditions is therefore crucial.”

The Bjørnafjord crossing will become the longest floating bridge in the world, that is a bridge where the vertical load is supported by floating pontoons. “We need to collect as accurate information as possible, for example about the wind conditions above the sea surface,” added Eidem.

A lot of wind

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 UiS 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 640m-long suspension bridge with the main span of 446m, 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 2km. Happily, the results from the lidar wind measurements on the Lysefjord bridge confirmed their expectations.

Good correlation

“It was encouraging to see that wind velocities recorded at such a large distance corresponded with the data from the anemometers on the bridge,” said Jasna Bogunovic Jakobsen from the Department of Mechanical & Structural Engineering & Materials Science at UiS, who leads the project on the on the application on lidar wind measurement to bridge engineering.

“Lidar technology enables us to monitor the wind conditions at a number of locations across the entire fjord. The new method gives us 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,” said Jakobsen.

She believes this new measurement method is invaluable for designing bridges across large Norwegian fjords. “This new measurement method is costly. But, it provides important, new data on the wind field around the bridge, so that the structure can be designed optimally.”

Measurement campaign

In spring 2016, Jakobsen carried out a six-week measurement campaign in the Bjørnafjord, 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.

Mathias Egeland Eidem from NPRA commented that he will be using this new measurement method to develop design requirements for further bridge projects along Ferjefri E39. In addition to the Bjørnafjord, this includes the Halsafjord and Sulafjord bridges.

“As lidar wind sensing is a relatively new technology in connection with bridge construction, it is important for us to collaborate with wind expertise at UiS. They can assist in developing a measurement system suited our needs”, says Eidem. Construction of the Bjørnafjord bridge, with its 5000 metre span, could start as soon as 2023.

Video

The ambitious scope of the E39 coastal road building program is shown in the following video: