146 research outputs found
Kathy Riser and Nancy Perez on Utah State Capitol steps for 1991 Utah Pride March
No full textColor photograph Kathy Riser and Nancy Perez standing on the steps of the Utah State Capitol wearing Queer Nation Utah t-Shirts other marchers gather in the backgroun
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
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
Studi Kasus Tegangan Riser Saat Proses Upending Dengan Bantuan Sling Pada Davit Lift
No full textKebutuhan minyak dan gas mengalami peningkatan setiap tahunnya sehingga proses pengeboran dan transportasi minyak dan gas harus dipercepat. Moda transportasi minyak dan gas yang paling efektif dan optimal yaitu pipa bawah laut. Riser merupakan bagian penting pada proses transportasi, karena sebagai pipa penghubung antara fasilitas diatas permukaan air dan pipa bawah laut. Proses yang paling penting dalam menggunakan riser maupun pipa bawah laut adalah proses instalasi, karena saat instalasi pipa akan mengalami kenaikan tegangan. Pada tugas akhir ini penulis akan melakukan studi kasus tegangan riser saat proses upending (perputaran riser dari posisi horizontal ke posisi vertikal) dengan variasi kondisi sling normal pada davit lift dan kondisi beberapa (dua dari tiga) sling terhenti, bisa dikarenakan kesalahan operator atau kesalahan mesin dan menginput beban lingkungan beruada gelombang dan arus dari arah 0°, 45°, 90°, dan 135°. Code yang digunakan untuk validasi tegangan riser yaitu ASME B31.4 (2019) dan code untuk proses instalasi riser yaitu DNVGL-ST-N001 (2016). Software yang digunakan untuk simulasi selama 800 detik, pemodelan dan mengetahui tegangan riser saat upending yaitu Microsoft Excel, MOSES, dan Orca Flex. Tegangan yang digunakan adalah tegangan von misses maksimum, tegangan saat kondisi normal sebesar 168,353 MPa atau 37,412% SMYS riser dan Unity Check (UC) sebesar 0,416 dengan beban lingkungan dari arah 90°. Sedangkan saat kondisi beberapa (dua dari tiga) sling terhenti pada detik ke 300 sampai 400 tegangan yang terjadi sebesar 216,166 MPa atau 48,037% SMYS riser dan Unity Check (UC) sebesar 0,534 dengan beban lingkungan dari arah 90°. Terhentinya sling mengakibatkan terjadinya kenaikan tegangan pada riser, tetapi riser saat upending masih tergolong aman karena nilai Unity Check (UC) masih dibawah 1 (satu).
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The need for oil and gas increases every year so that the process of drilling and transportation of oil and gas must be accelerated. The most effective and optimal mode of oil and gas transportation is the submarine pipeline. The riser is an important part of the transportation process, because it acts as a connecting pipe between above-water facilities and subsea pipelines. The most important process in using a riser or subsea pipe is the installation process, because during installation the pipe will experience an increase in stress. In this final project the author will conduct a case study of the riser stress during the upending process (rotation of the riser from a horizontal position to a vertical position) with variations in normal sling conditions on the davit lift and the condition of several (two of three) slings stopping, it could be due to operator error or machine error. and input the environmental load in the form of waves and currents from the direction of 0°, 45°, 90°, and 135°. The code used to validate the riser voltage is ASME B31.4 (2019) and the code for the riser installation process is DNVGL-ST-N001 (2016). The software used for simulation for 800 seconds, modeling and knowing the riser voltage when upending are Microsoft Excel, MOSES, and Orca Flex. The voltage used is the maximum von misses stress, the stress under normal conditions is 168,353 MPa or 37,412% SMYS riser and Unity Check (UC) is 0.416 with an environmental load of 90°. Meanwhile, when the condition of several (two of three) slings stops at the 300th to 400th seconds, the stress is 216,166 MPa or 48,037% SMYS riser and Unity Check (UC) is 0.534 with environmental load from 90°. Stopping the sling causes an increase in the voltage on the riser, but the riser when upending is still relatively safe because the Unity Check (UC) value is still below 1 (one)
Vibration Control of Marine Top Tensioned Riser with a Single Tuned Mass Damper
No full textThe study of the Tuned Mass Damper (TMD) on Top Tensioned Risers (TTRs) through the application of numerical analysis is of great significance for marine engineering. However, to the best knowledge of the author, neither the in-field riser data nor the ocean current data used in published papers were from engineering design, so the research results provide limited guidance to the actual engineering project. In view of this problem, this study designed a single TMD to suppress the vibration of the engineering TTR under the action of the actual ocean current. First, the dynamic model of a riser-TMD system was established, and the modal superposition method was used to calculate the model. The non-resonant modal method of the flexible structure was used to design the TMD parameters for the engineering riser. Ocean current loading in the South China Sea was then applied to the riser. The vibration of the riser without and with TMD was compared. The result showed that TMD could effectively reduce the vibration response of the riser. When compared without TMD, the maximum value of displacement envelope and the RMS displacement were reduced by 26.70% and 17.83% in the in-line direction, respectively. Moreover, compared to without TMD, the maximum value of displacement envelope and RMS displacement were decreased by 17.01% and 22.05% in the cross-flow direction, respectively. In the in-line direction, the installation position of TMD on the riser was not sensitive to the effect of the displacement response; meanwhile, in the cross-flow direction the installation position of TMD on the riser was more sensitive to the effect of the displacement response
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