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Abstract A large-scale model test of a truncated steel catenary riser (SCR) was performed in an ocean basin to investigate the riser responses under pure steady uniform current and pure vessel motions separately. Out-of-plane vortex-induced vibration (VIV) was confirmed to have occurred under both test conditions. A comparative analysis and discussion were carried out on selected cases in terms of out-of-plane VIV responses, VIV developing mechanisms and the fatigue damage contribution. Results indicate that both steady current-induced and vessel motion-induced VIV responses are dominated by strong travelling waves, but vessel motion-induced VIV responses are more 'intermittent' with respect to the response amplitude and frequency owing to its space- and time-varying shedding frequency. 'Power-in' regions are further estimated to understand the VIV developing mechanisms for both test conditions. Finally, fatigue damages are evaluated showing the damage by vessel motion-induced VIV is comparable and at the same level as uniform current-induced VIV, which highlights the great importance of vessel motion-induced VIV, which cannot be neglected in the design and analysis for SCR systems. [ABSTRACT FROM AUTHOR]

This paper proposes the Forgetting Factor Least Squares (FF-LS) method for identification of time-varying hydrodynamics of a flexible riser under multi-frequency vortex-induced vibrations (VIV). Differing from the traditional least squares method used to identify hydrodynamics, with equal weights on all sampled data, this method introduces a forgetting factor to give higher weight to data closer to the present moment. FF-LS allows the possibility of handling the time-varying parameters. By following this procedure for all sampled data of a flexible riser undergoing multi-frequency VIV, the corresponding time-varying hydrodynamics in the Cross-Flow (CF) direction will be obtained considering multi-frequency coupling. The results show that, under multi-frequency coupling, vortex-induced force coefficients of flexible risers change periodically and differ from coefficients at the basic frequency usually used in VIV prediction. This difference is a result of the coupling effect between the basic frequency and high frequency. Time-varying coefficients considering multi-frequency coupling contain steady components and vibration components whose frequencies are the sum of coupling frequencies and the difference of coupling frequencies. To show the advantages of time-varying hydrodynamics, we compare forces reconstructed from different vortex-induced force coefficients, assuming vortex-induced force obtained from the inverse analysis is the real force. The results show that time-varying vortex-induced force coefficients considering a coupling effect between multiple frequencies can accurately reconstruct vortex-induced force, while the sum of vortex-induced force coefficients under multiple frequencies will produce the cross-term that overestimates the vortex-induced force. [ABSTRACT FROM AUTHOR]

Based on the three-dimensional (3D) potential theory and finite element method (FEM), this paper proposes a new numerical method for hydroelastic predictions of floating structures in inhomogeneous seabed and wave field conditions. The continuous floating structure is first discretized into rigid modules connected by elastic beams. The motion equations of the entire floating structure are established according to the six degrees of freedom (6DOF) motions of each module by coupling the hydrodynamics of the modules with the structural stiffness matrix of the elastic beams in the frequency domain. By applying different wave excitation forces onto different modules, this discrete-modules-based method then uniquely realizes application of various wave excitation forces onto different modules of the structures in inhomogeneous waves. The hydroelastic responses of a plate and a Wigley hull under an even and uneven seabed using the proposed method are verified against the results from the published model tests and the conventional 3D hydroelastic method. Finally, the effects of inhomogeneous waves on the distributions of the bending moment, shear force and vertical displacements of the freely floating plate are investigated. The results show that the inhomogeneity of waves may induce about 2 ∼ 3 times increase of the force responses in a specific wave frequency. [ABSTRACT FROM AUTHOR]

In this study, a method to obtain the hydrodynamic forces of a flexible riser undergoing vortex-induced vibration (VIV) based on measured strain is proposed. The tensioned riser is approximated as an Euler–Bernoulli beam, and an inverse method is adopted for the calculation of the hydrodynamic forces in the cross flow (CF) and inline (IL) directions. Based on these hydrodynamic forces, and combined with the VIV velocities and accelerations of the riser, the excitation and added-mass coefficients are obtained through a least-squares method. As an illustration example, the hydrodynamic characteristics of a flexible riser model undergoing VIV in a uniform flow are investigated. Results indicate that VIV leads to non-uniform distribution of the drag coefficient and amplifies the drag coefficient along the riser. Similar distribution of the energy transfer coefficient has been found between the entire riser and that of the CF direction. For synchronized VIV occurring in both CF and IL directions, the energy transferred from the fluid is all dissipated by the structural damping, and hence leads to the energy balance in both CF and IL directions. It further shows that the excitation coefficients on flexible riser undergoing VIV do not agree with those of the forced oscillation tests: excitation coefficients sometimes even become negative within the normally excitation regime, and are related with not only the non-dimensional frequency and amplitude but also the phase angles between the CF and IL vibrations. The added-mass coefficient of flexible risers does not keep a constant value of 1.0 anymore, but depends on the non-dimensional frequency and amplitude of vibration. [ABSTRACT FROM AUTHOR]

The existence of a bidirectionally sheared flow field caused by internal solitary waves has recently been confirmed according to a field survey in the South China Sea. A model test of a tensioned flexible pipe in bidirectionally sheared flow was performed in an ocean basin. The model was 28.41 mm in diameter and 7.64 m in length. The purpose of the model test was to understand the response performance and obtain benchmark data of vortex-induced vibration (VIV) in bidirectionally sheared flow. The test was performed on a rotating test rig to simulate bidirectionally sheared flow conditions. Fiber Bragg Grating (FBG) strain sensors were arranged along the test pipe to measure bending strains, and the modal analysis approach was used to determine the VIV response. Reduced velocities based on the tested first natural frequency in water reached 30.79. The cross-flow and in-line VIV amplitudes, response frequencies in statistics and time-domain analysis, traveling wave characteristics, phase synchronization and trajectories are presented in this article. The maximum root mean square (RMS) VIV amplitudes in the test reached 0.51 diameters and 0.18 diameters in the cross-flow and in-line directions, respectively. A relative low Strouhal number of 0.10 was found in the CF direction. • An experiment of a flexible pipe that was 28.41 mm in diameter and 269 diameters in length under bidirectionally sheared flow was conducted. The bending strain of VIV was measured in the CF and IL directions at 9 and 14 locations, respectively. A total of 58 test conditions with maximum flow velocities varying from 0.30 to 1.39 m/s were used in this experiment. • The RMS VIV response in the CF and IL directions reached maximum levels of 0.51 D and 0.18 D , respectively. The '2n/n' and '2n/2n-1' modal groups, modal groups '3/1' and '4/1' existed under low flow velocity due to the special flow profile. • The Strouhal numbers under the bidirectional sheared flow were approximately 0.10 and 0.24 in the CF and IL directions, respectively, which were smaller than the St numbers under uniform flow and linearly sheared flow. • A strong traveling wave and 'multi-frequency' response phenomenon were found, particularly in the IL directions. [ABSTRACT FROM AUTHOR]

Both amplitude modulation and frequency modulation of Vortex-induced Vibration (VIV) are observed in a recent model test of a flexible cylinder under oscillatory flow, but its hydrodynamics has not yet been broached in detail. This paper employs the Forgetting Factor Least Squares (FF-LS) method for identification of time-varying hydrodynamics of a flexible cylinder under modulated VIV. The FF-LS method's applicability to accurately identify time-varying hydrodynamic coefficients is demonstrated through an elastically mounted rigid cylinder under flow with a given modulated motion. Furthermore, we propose a framework to predict instantaneous amplitude (envelope) and frequency using time-varying hydrodynamic coefficients to establish their analytical relationship. This prediction method is further extended to a highly tensioned flexible cylinder through Fourier series expansion in the spatial domain. By performing the identification procedure for all sampled data of a flexible cylinder undergoing oscillatory flow, we obtain the corresponding time-varying hydrodynamics in the cross-flow direction considering the amplitude and frequency modulation. The results show that, under modulated VIV, hydrodynamic coefficients of the flexible cylinder also show time-varying characteristics. We further investigate differences between identified hydrodynamic coefficients and those obtained from the database of a cylinder with modulated motion under flow. Prediction results using these identified time-varying coefficients reveal that the time-varying excitation coefficients mainly influence the amplitude modulation, and the time-varying added-mass coefficients contain the major information of frequency modulation. These results further suggest including the temporal derivative of the instantaneous amplitude as one determining parameter in building databases to improve the prediction of modulated VIV. [ABSTRACT FROM AUTHOR]

The cross-flow vortex-induced vibration features of a submerged floating tunnel element, which is composed of two rigidly connected cylinders in a tandem configuration, were investigated via a self-oscillation model test in a steady flow. The Reynolds number ranged from 2 × 104 to 9 × 104, and the ratio of the center-to-center distance between the two cylinders and cylinder diameter varied within a range of 2-4. The vortex induced vibration responses and lift forces on the up- and downstream cylinders were studied under different spacing ratios and compared with those on a single cylinder. The results show that the spacing ratio plays an important role in VIV until the ratio reaches 4. For a small spacing ratio, a significant difference between the lift forces on the up- and downstream cylinders appears and induces a large torsional moment. For the convenience of engineering application, a torsional coefficient was proposed. The maximum torsional coefficient can reach 2.9, 1.2 and 0.98 for spacing ratios of 2, 3 and 4 at the reduced velocity of 5, respectively. Considering the vortex induced vibration responses as well as the torsional moment, a spacing ratio of 3 was recommended for tandem floating tunnel design. • Self-oscillation test of a twin-tube submerged floating tunnel segment model. • Lift forces on stationary and self-oscillation twin-cylinder are compared. • A torsional moment is induced due to the difference of lift force on the cylinders. • Spacing ratio heavily affects the vortex induced response, lift and torsion. [ABSTRACT FROM AUTHOR]