Trarieux, Florent Louis-Marie; (2004) An investigation of vortex induced vibrations of subsea risers and umbilicals from offshore field measurements. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis deals with vortex-induced vibrations (VIV) of subsea flexibles (risers and umbilicals) used in the offshore oil industry. VIV results from the interaction of the structure and the fluid and its analysis involves both fluid dynamics cind structural mechanics. An existing semi-empirical model is presented and applied to a steel catenary riser (SCR) and to a lazy-wave umbilical. A comparison between theoretical predictions and measured response is achieved for selected cases where a parametric study could be carried out. The model has been found to provide good predictions when the flow is mainly out-of-plane of the structure but suffers inaccuracies once the flow becomes highly sheared. Experimental data were made available through Phase III of the STRIDE project which was about the instrumentation and data acquisition of an existing steel catenary riser in the Allegheny Field, Gulf of Mexico. STRIDE Phase IV comprised a series of laboratory experiments of a reduced scale SCR. The Foinaven Umbilical Monitoring System (FUMS) for the Foinaven field, west of Shetland, has provided a substantial amount of data for the lazy-wave umbilical and forms the basis for the main core of investigation in this thesis. An innovative method to detect vortex-induced vibration in the curvature data of subsea flexibles is presented. The bandwidth parameter Epsilon gives an indication of the spectral width of a signal and then cein be used to help identify the extent of narrow-bandness within processes such as VIV. The method is based on a peak counting approach formulated by Cartwright and Longuet-Higgins. Low values of Epsilon close to zero indicate a narrow-band process whereas values near one indicate a broad-band process. This method has been applied extensively for a data set covering a few months in the winter and spring of 2001 (25-Jan-Ol to 18-April-Ol) and for a more limited period in the summer of 2000 (15-Jun-OO to 12-Jul-OO). Current profile and wave heights for the 2001 data set have enabled the influence of current on the VIV response to be assessed and also the effect of the wave excitation and wave motions. The technology used by the FUMS to measure curvature is to insert a glass-fiber rod fitted with strain gauges in the hydraulic hose of the umbilical. A substantial amount of friction occurs between the rod and the hose leading to serious rod retrieval issues when the inserted length becomes significant. An analytical model is proposed to represent the magnitude of the friction forces and the influence of the hose pitch and helical radius. Both cases (extraction and insertion) are considered. The model shows clearly the increase of friction during the rod retrieval phase when a tension is applied to the rod.

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