The Fedje HF observatory

Pipeline overview map The Norwegian Meteorological Institute (met.no) is involved in a project where three high-frequency (HF) radars are being installed to monitor the surface currents and the waves in the fairways to the petroleum terminals Sture and Mongstad west of Bergen, Norway. The current maps and wave observations will be transferred in real time to the Vessel Traffic Service (VTS) located at Fedje. The observatory will become an integrated part of the monitoring capabilities of the VTS and will assist in guiding the pilots and the captains as they traverse the fairways.

The two petroleum terminals Mongstad and Sture are situated to the North-East and South-East of Fedje. Together they comprise the largest petroleum harbour in Europe. The speed of the Norwegian Coastal Current occasionally exceeds 1.5 m/s (3 kn) in this region. The strength and direction of the current changes quickly and its full extent can only be mapped with area-covering remote sensing devices. The unpredictable current combined with the rugged coastline and narrow fairways makes maneuvering to and from the terminals a challenge. The Vessel Traffic Service located at Fedje is responsible for managing the ship traffic through the fairways. This is done in conjunction with mandatory pilot service, also operated through the Fedje VTS.

The EuroROSE project

The European project EuroROSE (European Radar Ocean Sensing Experiment) demonstrated the value of real time monitoring of surface currents and waves in the fairways connecting the terminals to the open ocean during a six-week quasi-operational phase in Feb-Mar 2000. A two-array HF radar of the German WERA type was used for the experiment.

WERA real time current map
A surface current map from the German WERA HF-radar during the EuroROSE real time campaign in Feb 2000. The fairways to the terminals are shown in pale blue colour. The current speed reaches almost 3 kn (1.5 m/s) in the most intense region of the Coastal Current.

A permanent observatory

Following the success of the EuroROSE project, the petroleum terminals run by Statoil and Norsk Hydro decided to provide funding for a permanent HF observatory which would equip the Vessel Traffic Service (VTS) in Fedje with real time observations of surface currents and waves to further improve the ship traffic management. The Norwegian Coastal Authorities (Kystverket) through Fedje VTS are responsible for maintenance and inspection of the equipment.

The data will be archived and quality-checked by The Norwegian Met. Institute. The observations will also be made freely available in real time through the web page hf.met.no.

Technical details

The observatory comprises three compact Codar SeaSonde HF radars (see the picture below).

Range, resolution and precision

The radars have a nominal range of about 50 km. The integration time is one hour and the horizontal resolution is about 2 km near the radar, decreasing with radial distance. The carrier frequency is 13 MHz.

Currents are measured indirectly through the Doppler shift of the echoed signal. The scattering agent according to first order Bragg scatter theory is ocean waves half the electromagnetic wavelength travelling towards or away from the radar (approximately 12 m in our case). The dispersion relation of surface waves is well known. From this a theoretical Doppler shift can be estimated. The waves ride on the surface currents which add a small additional Doppler shift. It is this additional frequency shift that allows us to calculate the component of the surface currents in the direction towards or away from the radar. A minimum of two radars is necessary to estimate the total current vector.

The measurement precision of the radars has been estimated to lie in the range 10-20 cm/s.

Effective deph of HF current measurements

The fluid particles engaged in wave motion follow circular orbits in deep water. The longer the wavelength, the greater the radius of the orbit. Hence the resonant ocean wave is affected by the mean current over a water column whose depth is proportional to the radar wavelength. The "effective" depth of HF current measurements at 13 MHz is approximately 0.5 m.

An introduction to the theoretical underpinnings of the SeaSonde measurement technique can be found at here.

Naut�y radar

The Rx antenna in Naut�y, the northern site. A representative of Codar Ocean Sensors is acting as yardstick.

Theoretical Codar coverage map
The theoretical coverage of a three-radar configuration. Note that the southern radar is not yet installed.

Project status

The first two radars were installed in December 2002 and started reporting total vectors in August 2003. The third radar is still pending as of September 2003.

We invite scientists in the field of oceanography to take advantage of this new source of surface current data and hope that the observatory will advance our understanding of the Norwegian Coastal Current.

Updated 2003SEP10 by Oyvind.Breivik@met.no