Citation:
F. Trarieux, G.J. Lyons and M.H. Patel, Investigations with a bandwidth measure
for fatigue assessment of the Foinaven dynamic umbilical including VIV,
Engineering Structures, Volume 28, Issue 12, , October 2006, Pages 1671-1690.
Abstract:
An innovative use of a method to detect Vortex Induced Vibration (VIV) bandwidth
in the curvature data of subsea flexible risers and umbilicals is presented. The
parameter epsilon gives a valuable estimate of the bandwidth of signals such as
VIV as a single value which may be used to track the behaviour with time and
against other measures such as current speed. The method is conveniently based
on a peak counting approach originally used in a marine context by Cartwright
and Longuet-Higgins. Low epsilon values close to zero indicate a narrow-band
process whereas values near unity indicate a broad-band process. Curvature and
environmental data were gathered by the Foinaven Umbilical Monitoring System
(FUMS) installed on the Foinaven Petrojarl IV floating production unit located
in the Atlantic margin, west of Shetland. The VIV frequency range owing to
current excitation considered is that in the range 0.2–2 Hz. This paper presents
analyses showing the correlation of the epsilon bandwidth parameter with
standard deviation of curvature, current speed and spectral analysis results for
extended periods measured on the Foinaven dynamic umbilical. Examination of the
number of frequencies and their values using spectrograms over long periods
enable interesting interpretation of the build-up and decay of the VIV
behaviour. These data are also reduced to the predominant peak frequencies, and
standard deviation along with epsilon to provide a compact description of the
VIV behaviour over 10 min intervals. Clear frequency and amplitude shifts can be
observed as VIV varies with current ebb and flow. The behaviour is usefully
described using only epsilon, standard deviation, and peak frequencies. The
importance of epsilon for fatigue damage assessment is demonstrated through a
formulation derived by Wirsching and Light. Strong VIV regimes are often
characterized by an intense activity around a particular frequency and the
impact of such relatively 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 damag