Abstract
To gain insight into riser motions and associated fatigue damage due to vortex-induced vibration (VIV), data loggers such as strain sensors and/or accelerometers are sometimes deployed on risers to monitor their motion in different current velocity conditions. Accurate reconstruction of the riser response and empirical estimation of fatigue damage rates over the entire riser length using measurements from a limited number of sensors can help in efficient utilization of the costly measurements recorded. Several different empirical procedures are described here for analysis of the VIV response of a long flexible cylinder subjected to uniform and sheared current profiles. The methods include weighted waveform analysis (WWA), proper orthogonal decomposition (POD), modal phase reconstruction (MPR), a modified WWA procedure, and a hybrid method which combines MPR and the modified WWA method. Fatigue damage rates estimated using these different empirical methods are compared and cross-validated against measurements. Detailed formulations for each method are presented and discussed with examples. Results suggest that all the empirical methods, despite different underlying assumptions in each of them, can be employed to estimate fatigue damage rates quite well from limited strain measurements.
Highlights
Current velocity flow fields on marine risers can lead to the formation and shedding of vortices downstream; fluctuations in resulting hydrodynamic pressures can cause such risers to experience sustained vortex-induced vibration (VIV), which can lead to fatigue damage
(ii) Fatigue damage rates estimated by the proper orthogonal decomposition (POD) and modal phase reconstruction (MPR) methods are quite similar since both methods are affected by the quality of the unavoidable interpolation—it may be noted that the presence of sensors close to a target location leads to good estimation of the fatigue damage there; when nearby sensors are not present, as is the case for sensor no. 9 indicated by the red ellipses in Figure 8c,d, fatigue damage estimates are less accurate
There are many factors that may affect the accuracy of riser response reconstruction and the selection of a method for assessing fatigue damage; these include characteristics of the motion, the arrangement of sensors, non-stationary characteristics of the riser response, complex configurations of the riser, unknown or uncertain boundary conditions, etc
Summary
Current velocity flow fields on marine risers can lead to the formation and shedding of vortices downstream; fluctuations in resulting hydrodynamic pressures can cause such risers to experience sustained vortex-induced vibration (VIV), which can lead to fatigue damage. Accumulated fatigue damage may cause riser failure, a shortened service life, and even ocean pollution To prevent such catastrophic consequences, it is useful to be able to accurately estimate the rate of fatigue damage accumulation and the expected service life of a marine riser at the design stage, or to measure and monitor the accumulated fatigue damage and predict the remaining life for an installed riser. Ocean current profiles are measured at a nearby location over the period of the riser monitoring campaign. Such in situ full-scale measurements of riser VIV response are extremely valuable in the study of riser VIV response and the estimation of fatigue damage
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