Improving time-domain prediction of vortex-induced vibration for marine risers

Zhang, Boyang (2017) Improving time-domain prediction of vortex-induced vibration for marine risers. Masters thesis, Memorial University of Newfoundland.

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Abstract

As offshore exploration and production is moving into deepwaters, the prediction of the Vortex-Induced Vibration (VIV) of marine risers becomes a critical issue in the design process. VIV can lead to a reduced fatigue life and even the structural failure of the riser. Currently, frequency-domain models are widely used in the offshore industry to predict riser VIV. However, the nonlinearities encountered in complex deepwater environments make linear approaches unreliable. In contrast, time-domain models can capture the nonlinearities in a straightforward manner. In this thesis, a time-domain model was further developed to predict the VIV of both rigid and flexible risers. Through a zero up-crossing analysis of the crossflow displacement, the two state variables, amplitude-to-diameter ratio and reduced velocity, were determined to interpolate the database of hydrodynamic coefficients obtained from forced oscillation tests at high Reynolds numbers. The hydrodynamic forces were then calculated and incorporated into an enhanced global-coordinatebased finite element method program, MAPS-Mooring, to investigate riser behaviours in the time domain. The enhanced program, MAPS-Mooring, comprises a two-stage computation: the riser profile under static equilibrium is first obtained based on a Newton iterative method, and the dynamic profile and tension of the riser are then solved by the second-order semi-implicit Adams method in the time domain. The enhanced program was first validated by using experimental results of mooring line tests in the literature to prove its reliability and robustness. Validation studies were then carried out to the enhanced time-domain VIV model for a rigid riser in a uniform flow and a flexible riser in a step current. Good agreement was observed between the numerical results and the experimental measurements.

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