OFFSHORE VIV MONITORING AT SCHIEHALLION - ANALYSIS OF RISER VIV RESPONSE

During November and December 96, a drilling riser in the Schiehallion field (West of Shetlands in 360m of water) was instrumented with accelerometers to record its Vortex-Induced Vibration (VIV) response in current. A paper has already been presented on the data gathering campaign (Hassanein, 1998). The present paper concentrates on the analysis of selected samples of the recording. This emphasis of the paper is twofold. The first is the investigation of the relationship between the shear of the velocity profile and the probability of single mode dominated response. The second is the determination of the reduced velocity bandwidth, Vr, associated with the fluid excitation of the riser. Little is known about these quantities in the high Reynolds regimes found offshore. The first direct application of this work will be to better calibrate and further develop VIV prediction programs. The results presented in this paper come from 83 samples of 28 minute long records. The raw accelerations were interpreted in terms of modal displacements (the normal modes were found from FE analysis). The current profile data are from acoustic Doppler current profile (ADCP) records that covered all but the top 60m of the water column. INTRODUCTION Risers, in large water depths and strong current environments, are prone to vibrations created by the vortices shed from the structure. These Vortex-Induced Vibrations (VIV) may damage the riser and limit its fatigue life. It is an important design consideration when drilling in high current environments. Existing VIV prediction tools rely on empirical formulations based on the expected Strouhal number and reduced velocity bandwidth. The reduced velocity bandwidth is defined as that range of reduced velocity along the riser, which would result in synchronization of the vortex shedding process with the motion of the riser at a particular riser natural frequency. In other words, it is the approximate range of reduced velocity that could produce lock-in, and is the range of reduced velocity that defines the power-in region for each mode of interest. In present day practice, the expected values of these parameters are usually based on laboratory experiments conducted at subcritical Reynolds numbers. However, for Reynolds numbers above 10 , there is little experimental data on long flexible cylinders such as drilling risers. To overcome this lack of experimental data a month-long offshore monitoring campaign was conducted. The objective of the data analysis was to use the full-scale VIV data to better understand single and multiple mode response behavior and to obtain best fit estimates of the reduced velocity bandwidth for modes observed in the response data. These results are then to be used in to improve VIV response prediction tools. DESCRIPTION OF THE RISER AND THE DATA GATHERING SYSTEM A riser on the Paul B. Loyd rig, drilling in the Schiehallion Field, West of Shetland in 360m of water, was instrumented with accelerometers. In parallel, the current was monitored with an Acoustic Doppler Current Profiler (ADCP), giving a precise description of current velocities through the water column. The data gathering campaign lasted for about 21days. A more detailed description of the full logging system is given in Hassanein (1998). The riser length from the flex joint at the bottom to the ball joint at the top was 368m. The average mass per unit length in the buoyant portion of the riser was 770kg/m,