North East Frigg Articulated Column Behaviour

ABSTRACT Measurements of movements were carried out on board a compliant platform in the North Sea during one year. Data processing established relationships between platform response parameters and environmental parameters. Comparisons between the measured results and the predicted platform design behaviour show that observed movements are similar to calculation predictions. INTRODUCTION In order to acquire better understanding of the dynamic behaviour of a compliant structure, and to confirm the validity of calculation tools used for this behaviour prediction during design, a mean is to analyse measurements of movements carried out on board an existing compliant platform. This paper is related to the description of movements recorded from March 1984 to March 1985 on board an articulated column, installed in a -depth of 102 m of water in the North Sea, statistical treatments carried out on movements and environmental conditions. The concept of articulated column which is being developed by ELF AQUITAINE since 1964 has been described several times (Ref. 1). DESCRIPTION OF THE PLATFORM, DATA ACQUISITION AND DESIGN BEHAVIOUR CALCULATIONS NORTH EAST FRIGG Field Control Station is an articulated column operating since 1983 in a depth of 102 m of water, 16 km from the FRIGGFIELD in the North Sea. It supports the production control system of a subsea well cluster (see figure 1). A concrete gravity base is connected to the column by a universal joint. The column is a steel cylinder (Ø 8 m) with a float (Ø 14.6 m) at the upper part, and supports an unmanned deck (650 t) at + 131 m above mud-line. A solid ballast in the lower part of the column body counteracts the uoyancy tank (see figure 2) (Ref. 2–3). Two linear accelerometers on the methanol deck and one roll and pitch sensor in the living quarters measure inclination and acceleration along North-South and East-West directions with a 4 hz frequency, for 17 minutes, every 20 minutes. Some of these "raw data" sequences were recorded. For each sequences of 17 minutes, the following reduced data are computed and recorded on board: Mean Inclination of the column (I mean), Root Mean Square Inclination (IRMS), Maximum Inclination (Imax), Root Mean Square Acceleration (ARMS), Maximum Acceleration (Amax), and their respective geographical directions. This gives us a large data base of around 19000 reduced data for each parameter over one year. This data base was completed with the environmental data for the same period of time: Significant Wave Height (Hs), Significant Wave Period (Ts), Mean value of Wind speed for a 10 minute reading (W), Mean value of Current speed for a 20 minute reading (C), and their geographical directions.