The environmental data acquisition and processing from measurements made in the Frigg Field appears to be one of the most extensive and fruitful areas of investigation for a better simulation of environment conditions.
After a brief presentation of acquisition and quality control techniques, very recent results are presented on the following:
validation of stochastic models
verification of design parameters for waves and wind
reliability of extreme value estimation techniques
The stationarity duration is also studied in detail as a random function of H 1/3′ with its influence on extreme value estimates and on relationships between Hmax and H1/3.
This represents a synthesis of advantages and limits of modelling the environment by stationary stochastic process.
After eight years of environmental data acquisition in the Frigg Field, Elf Aquitaine has acquired considerable knowledge of metocean phenomena in the Centrel North Sea. The objective of automatic data gathering of meteorological and oceanographic parameters are to verify the design values in order to accurately estimate the forces on marine structures and to improve the general knowledge of environmental conditions. Further, the aim is that environmental data are to be included in the general collection of data used for systematic mapping of the physical environment, as stipulated by Norwegian rules.
Frigg Field measurements are taken from QP structure, a four-legged jacket located between the Shetland Islands and Norway, in approximately 100m of water Oceanographic data include measurements of waves, surface and bottom currents, water level, and surface and bottom water temperature. Meteorological measurements of wind, air temperature, atmospheric pressure and humidity are also taken.
The present chapter only deals with waves and wind measurements. Waves are measured by a TRT/SYMINES radar distance meter installed on the western corner of the platform. This type of sensor was initially chosen because it is possible to take measurements without interfering with the physical phenomenon. In addition, it has a very small beam width (3 degrees) which eliminates any spreading effect and effects caused by smoothing of the water surface.
In fact, the quality of the data obtained from this sensor has been recognised by all those who have worked on the data and, in particular, the comparison between data measured by this radar wavemeter and data measured by wave data buoys has proved the greater reliability of the fixed instrumentation, which is more accurate for high waves and which reflects the non-Gaussian nature of the waves more precisely.Wind data are collected with a usual Bendix anemometer located at the top of the microwave tower; thus the measurement is not perturbated by the structure (Fig 1).
Fig. 1 Sketch of QP platform with wind and wave sensor location (available in full paper)
All the data are recorded on magnetic tape through a computer unit. Data are filtered, digitized and multiplexed on site. Tune and frequency domain analysis are then applied onshore.
Both wave and wind data re acquired permanently at a 2 Hz sampling frequency.
