Fatigue Damage Assessment Using a Bandwidth Parameter for VIV Applied to the Foinaven Dynamic Umbilical

Abstract

The application of a method for fatigue damage assessment applied to the relative effects of mooring, wave, and Vortex Induced Vibration (VIV) is presented. It is a simpler procedure than rainflow counting and uses a bandwidth parameter with standard deviation applied to modify narrow band fatigue estimation. This bandwidth parameter has been successfully applied by the authors to investigate in detail the curvature (bending stress) response of the Foinaven lazy-wave umbilical for investigations into VIV. This parameter is valuable since as a single value it may be used to track the variation of bandwidth response with time and against other measures such as varying current speed. The method is conveniently based on a peak counting approach. Low values close to zero indicate a narrow band process whereas values near unity indicate a broad-band process. It is a much better measure than kurtosis for determining bandwidth. Curvature and environmental data were gathered by the Foinaven Umbilical Monitoring System installed on the Foinaven Petrojarl IV floating production unit located in the Altantic margin, West of Shetland. The VIV frequency range owing to current excitation considered is that which is considered as being above consequential wave frequencies (0.2 to 2Hz). This paper presents results of fatigue damage assessment for the periods where a strong variation of bandwidth was observed. 64 records (each lasting about 10 minutes) representing a daily acquisition of about 8 hours were processed showing the correlation between the bandwidth of curvature and the amount of fatigue experienced by the umbilical. Strong VIV regimes are often characterized by an intense activity around a particular frequency and the impact of such narrow-band events on the fatigue life of the structure is clearly demonstrated. This paper also presents the relative contributions of mooring, and waves/vessel motions, and VIV to fatigue damage. Although wave excitation remains the main source of fatigue, VIV appears to potentially contribute to a significant part of the overall fatigue damage.