Investigation of the influence of waves and vessel motions on the VIV response of flexible risers and umbilicals from a fatigue damage perspective

The influence of waves and vessel motions on VIV is poorly understood. Investigations into this behaviour require substantial data sets in order to carry out a rigorous parametric analysis since many factors can prove the analysis to be very complex. Whilst the effect of waves might be considered to be relatively straightforward to appraise, the effect of vessel motions is a very delicate task indeed. For example, operating conditions can require the vessel to actively weathervane through the action of thrusters which can potentially affect the measurements. Data from the Foinaven Umbilical Monitoring System (FUMS) have enabled a detailed review of more than 22,000 records of curvature and other key parameters, each of about 7 minutes duration from January to May 2001. A strategy had to be established to process in an efficient manner the very many records. This paper presents the method which was followed in order to select suitable records, which could be compared in terms of varying wave height, current speed and direction. Results from spectral analysis are shown in several configurations of wave, vessel motions and current. From a predictive VIV approach, the analysis was performed using VIVALL which has the capability of generating curvature time-series. A comparison of spectra of measured and theoretical records is presented with commentary. In the second part of this paper, a fatigue damage investigation is presented. Trarieux & Lyons previously reviewed several fatigue methods which can be applied to VIV, and developed a simply applied approach as an alternative to the widely-used rainflow counting method. This alternative method is based on a simply calculated bandwidth parameter which has proven to give interesting insight into the frequency content of the VIV response. The application of this method along with rainflow counting is presented for different cases of varying wave height, vessel motions and current. Conclusions on these various influences on VIV and fatigue are provided.