177 research outputs found
EFFECTS OF TOP-END VESSEL HEAVE ON SUBMARINE RISER VIV OF DEEP WATER PLATFORM
No full textThe dynamic coupling between moving top-end vessel and submarine riser becomes more remarkable for a floating platform in deeper water due to the larger top-end motion amplitude, compared with the fixed platform in shallow water. In this study the impacts of top-end heave on the riser undergoing vortex-induced vibration (VIV) are explored in terms of the parametric excitation and the consequent dynamic behaviors. By using finite element simulations based on a coupled hydrodynamic force approach, the dynamic responses of the integrated system including both a floating top-end and the riser experiencing VIV are examined. Our numerical results show that the riser displacement becomes several times larger than the displacement for the case without top-end motion, and the impact of heave on riser VIV response gets larger as the modal order number dropping. Riser VIV amplitude becomes, almost linearly, more profound when the tension ratio, as one of critical parameters that influence the riser dynamic response, gets larger. Moreover, an interesting phenomenon called mode transition is observed, particularly at lower frequency, during modal dynamics response
DYNAMIC COUPLING BETWEEN TOP-END VESSEL SWAY AND SLENDER RISER VIV IN DEEP WATER
No full textThe impacts of top-end motion on the riser undergoing vortex-induced vibration (VIV) are explored in this study, because the dynamic coupling between moving top-end vessel and submarine riser becomes more remarkable for a floating platform in deeper water due to the larger top-end motion amplitude, compared with the fixed platform in shallow water. A coupled hydrodynamic force approach, involving the vortex-induced lift force along with the fluid drag force, is developed. The dynamic responses of the system including a floating top-end and a riser experiencing VIV are examined by means of finite element simulations. The effects of amplitude and frequency of top-end vessel sway on riser WV are examined. Our numerical results show that the riser displacement becomes several times larger than the displacement for the case without top-end motion. Moreover, the nonlinear response amplification is observed, and the nonlinear amplification gets more pronounced as the number of mode order dropping, while the amplification factor just slightly changes with the increase of sway amplitude
Impacts of top-end vessel sway on vortex-induced vibration of the submarine riser for a floating platform in deep water
No full textThe dynamic coupling between moving top-end vessel and submarine riser becomes more remarkable for floating platform in deep water due to its larger amplitude of top-end motion, compared to fixed platform in shallow water. The impacts of top-end motion on the riser undergoing vortex-induced vibration (VIV) are explored in this study. A coupled hydrodynamic force approach, involving the vortex-induced lift force along with the fluid drag force, is developed, which takes into account the interaction between fluid dynamic force and instantaneous riser motion. Then the dynamic behaviors of the riser suffering both top-end motion and VIV are examined by means of finite element simulations. The effects of the amplitude and frequency of top-end vessel sway on riser's VIV are studied. During the riser's dynamic responses, an interesting phenomenon, called nonlinear response amplification, is observed, which demonstrates that top-end motion may be amplified as the motion propagates along riser length. Our numerical results show that the riser's displacement becomes several times larger than that of the case without top-end motion. Moreover, the nonlinear amplification gets more pronounced as the number of mode order drops, but the amplification factor just slightly changes with the increase of sway amplitude. (C) 2015 Elsevier Ltd. All rights reserved
Motion Reconstruction of Vortex-Induced Vibration of Long Flexible Riser from Experimental and Field Test Data
No full textVortex-induced vibration (VIV) of long flexible cylindrical structures enduring ocean currents is ubiquitous in the offshore industry. Though significant effort has gone into understanding this complicated fluid-structure interaction problem, major challenges remain in modelling and predicting the response of such structures. The work presented in this thesis applies the modal approach to do motion reconstruction of the riser VIV from experimental data at first and then performs some analyses to the riser VIV response based on the reconstructed result. In the first part of the thesis, the modal approach is classified into the frequency domain method and the time domain method according to the types of the measurement data. Two systematic frameworks to do motion reconstruction are built for these two methods. Besides, two factors probably leading to the reconstruction error are proposed. One is using the strain measurement to identify the low modes VIV motion and the other one is unreasonable choice of participating modes. In the second part of the thesis, the riser VIV motion in ExxonMobil VIV test is reconstructed using the frequency domain method and that in the second Gulf Stream VIV test is reconstructed using the time domain method. In the reconstruction process, several problems are needed to be solved, such as the choice of time window, filtering data and the choice of participating modes. And the accuracy of the reconstructed result is verified using the extraction method. Finally, two examples are given to demonstrate the reconstruction errors induced by the above two facors. In the final part of the thesis, some key parameters are extracted out to show the effects of external conditions, e.g. current profile, current speed and strake coverage, on the VIV displacement magnitude and response frequency of the riser. Besides, three methods are provided to identify the travelling wave in the riser VIV response.Mechanical, Maritime and Materials EngineeringMarine and Transport TechnologyOffshore and Dredging Engineerin
Controlling Parameter for Wave Types of Long Flexible Riser Undergoing Vortex-Induced Vibration
No full textThe aspect ratio of slender underwater structures of deep sea platforms such as riser, oil pipelines, tension legs and anchor chains increases with the mining depth increasing, the values of which often approach to 103 order. Investigation results in recent years show that the vortex-induced vibration of the flexible marine risers with large aspect ratio reveals some new phenomena, for example, the vortex-induced wave, multi-mode competition, wide band random vibration, which have brought new challenges to the study of vortex-induced vibration of long flexible risers. In this paper, the dimensionless parameter controlling the wave types of dynamic response of slender risers undergoing vortex-induced vibration is investigated by means of dimensional analysis and finite element numerical simulations (MSC Nastran). Our results indicate that there are three types of response for a slender riser, i.e. standing wave vibration, traveling wave vibration and intermediate state. Based on dimensional analysis the controlling parameter is found to be related to the system damping including fluid clamping and structural damping, order number of the locked-in modes and the aspect ratio of riser. Furthermore through numerical simulations and function fitting, the expression and the critical value of the controlling parameter is presented. At last the physical meaning of the parameter is analyzed and discussed.</span
Ultrafast laser irradiation of spherical nanoparticles: molecular-dynamics results on fragmentation and small-angle scattering
No full textUsing molecular dynamics simulation we study the response of a spherical nanoparticle to a sudden homogeneous energization, such as effected by ultrashort pulse laser irradiation. We consider a Lennard-Jones model system and two different values of the energization. For the smaller one, the sphere expands while a multitude of voids are created inside; the sphere develops finally into an external shell filled with gas and small clusters. For the higher energization, the sphere expands uniformly and no shell structure is formed. An analysis of the pressure generated confirms that in the latter case the pressure is compressive throughout the sphere expansion, while it is temporarily tensile for the lower energization leading to void formation. The final state of both systems shows the fragmentation of the sphere into a multitude of clusters. With increasing fragmentation the cluster distribution becomes shifted to smaller sizes. Simulated small-angle scattering functions of the exploding NP are presented. The distribution of minima allows for an easy determination of the particle size during expansion
High Aspect Ratio (L/D) Riser Viv Prediction Using Wake Oscillator Model
No full textA two-dimensional (2-D) vortex-induced vibration (VIV) prediction model for high aspect ratio (LID) riser subjected to uniform and sheared flow is studied in this paper. The nonlinear structure equations are considered. The near wake dynamics describing the fluctuating nature of vortex shedding is modeled using classical van der Pol equation. A new approach was applied to calibrate the empirical parameters in the wake oscillator model. Compared the predicted results with the experimental data and computational fluid dynamic (CFD) results. Good agreements are observed. It can be concluded that the present model can be used as simple computational tool in predicting some aspects of VIV of long flexible structures. (C) 2008 Elsevier Ltd. All rights reserved
Numerical investigations on gas-liquid distribution characteristics of intermittent flows in a pipeline-riser system
No full textIn this work, VOF model has been introduced to simulate gas-liquid two phase intermittent flow phenomena in a pipeline-riser system. During the flow processing, air and water has been set as the fluid medium, and various operation conditions have been tested in details. As been shown in results, there are mainly air bubbles and slugs in the riser section under different operation conditions. In addition, various fluid medium has been tested for comparison, which will have a positive impact on further research
Cloud-Based Teaching Tool of AlCu Band Gap Simulations Using GPAW: A Python-Driven Approach for Undergraduate Student
No full textThis work introduces a computational teaching module that leverages Python, Google Colab, and the GPAW package to simulate the electronic band structure of AlCu materials. While powerful, traditional Density Functional Theory (DFT) tools like Quantum ESPRESSO or VASP often present steep learning curves and software installation challenges. By contrast, GPAW operating within Python and its seamless integration with Google Colab provides a user-friendly, platform-independent environment for students to explore quantum simulations without local setup requirements. The simulation workflow is highly efficient, with key processes such as structure creation taking only 7 milliseconds, structural relaxation requiring 51.2 seconds, and band structure calculations completing in just 40 seconds. In this educational framework, students model AlCu and its doped variants, visualize band structures, and analyze changes in the electronic properties induced by doping. The approach supports active learning and reinforces core solid-state physics, quantum mechanics, and computational materials science topics. Sample notebooks, learning outcomes, and classroom integration strategies are presented, aiming to democratize access to DFT education through open-source, cloud-based tools
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