• International Journal of Naval Architecture and Ocean Engineering
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• v.11 no.2
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• pp.970-979
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• 2019

Compliant Vertical Access Risers (CVARs) are compliant systems that incorporate a differentiated geometric configuration that allows the exploitation of oil and gas in deepwater fields and enables a number of operational advantages in the offshore system. One of the main features of CVAR systems is that they allow direct intervention procedures to be applied to the well bore, enabling workover operations to be performed directly from the production platform. Based on the principles of virtual work and variation, a static geometric nonlinear equation of CVARs is derived and applied in this study. The results of this study show that the two ends of the riser as well as the transition region are subject to high stress, while the positions of the floating platform exert significant effects on the geometry of the riser configuration. Compliance and buoyancy factors should be set moderately to reduce the CVAR stress. In addition, the buoyancy modules should be placed in the lower region, in order to maximize the operation advantages of CVAR.

• International Journal of Naval Architecture and Ocean Engineering
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• v.9 no.2
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• pp.127-134
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• 2017

Vortex Induced Vibration (VIV) is a typical flow-structure interference phenomenon which causes an unsteady flow pattern due to vortex shedding at or near the structure's natural frequency leading to resonant vibrations. VIV may cause premature fatigue failure of marine risers and pipelines. A test model was carried out to investigate the role of a stationary fairing by varying the caudal horn angle to suppress riser VIV taking into account the effect of wake interference. The test results show significant reduction of VIV for risers disposed in tandem and side-by-side. In general, fairing with a caudal horn of $45^{\circ}$ and $60^{\circ}$ are efficient in quelling VIV in risers. The results also reveal fairing can reduce the drag load of risers arranged side-by-side. For the tandem configuration, a fairing can reduce the drag load of an upstream riser, but will enlarge the drag force of the downstream riser.

• Journal of the Korean Society for Marine Environment & Energy
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• v.18 no.1
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• pp.29-35
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• 2015

The concept design of a riser for Ocean Thermal Energy Conversion in 10 MW is proposed and its dynamic behaviour characteristics is analyzed with numerical method. A riser pipe with a hollow along its thickness in the cross-section to increase the effective modulus of its cross-section is designed considering the manufacture. The riser pipe without hollows along its thickness needs a lumped weight at the bottom end of a riser in order to keep its vertical hanging configuration from large buoyancy and the strong current. The riser is designed to control its density by inserting materials in high or low density into a hollow. The dynamic behaviour characteristics of the two designed risers is evaluated with the developed numerical analysis tool. The combined stress of the riser with a lumped weight is showed to be dominated by weight of a lumped mass. The riser with no hollow shows large combined stress near sea surface by strong current. Local structural analysis for the cross-section of a hollow riser is needed in detail.

• International Journal of Naval Architecture and Ocean Engineering
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• v.10 no.3
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• pp.348-359
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• 2018

A series of indoor simulation tests on a large-sized shaking table was performed, which was used to simulate the earthquake ground motion for the pipe-soil interaction system to be tested. The purpose of this study is to examine the dynamic characteristic and seismic response of a length of PVC pipeline lay on a clay seabed under seismic load. The pipeline was fully instrumented to provide strain and acceleration responses in both transverse and in-line. Dynamical modal tests show that corresponding mode shapes vertically and horizontally are basically the same. But the absolute values of the natural frequencies vertically are all higher than those corresponding values in transverse. It turned out that the geometry configuration of riser affects its stiffness. Seismic response of pipeline depends significantly on the waveform, and Peak Ground Acceleration (PGA). As the seismic loading progressed, the strain response was severe around both TDZ and catenary zone. Additionally, strain responses in top and bottom positions were more severe than the result in left or right side of the pipeline in the same section.

• Proceedings of KOSOMES biannual meeting
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• 2006.11a
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• pp.239-244
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• 2006

As an effective means to convey crushed materials from seabed to onboard ship, air-lift pump provides a reliable mechanism due to its simple configuration and easy-to-operate principle. The present study is focused on fundamental investigation of related performance through analysis program based on the gas-liquid two-phase flow in circular pipes. It is summarized as important result that an optimum air mass flow rate exists for the maximum lifted liquid mass flow rate in terms of a given submergence rates.

• Journal of Ship and Ocean Technology
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• v.10 no.3
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• pp.17-26
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• 2006

Spar platforms have been installed as a competitive alternative offshore production structure for deepwater oil field. Since the first spar platform was constructed, its configuration has evolved to the so-called the truss spar and then the cell spar. This paper describes the dynamic analysis and fatigue life assessment of steel catenary riser (SCR) connected to cell spar platform. Two different cell spar platforms are considered herein; the original cell spar and the modified one. The original cell spar was modified by introducing an additional damping plate at its bottom in order to reduce wave-frequency motions. Firstly the wave-frequency motions of cell spar platforms are calculated based on the potential theory. Then, the dynamic responses of SCR induced by platform motions are computed. Finally the fatigue life of SCR is estimated by spectral method and the performance of two spar platforms are compared in terms of the fatigue life. Through the present study, it is found that the fatigue life of the modified cell spar increases only slightly.

• Ocean Systems Engineering
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• v.10 no.1
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• pp.67-86
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• 2020

The main goal of this study is to investigate the free vibration analysis of a large sag catenary with application to the jumper in hybrid riser system. The equation of motion is derived by using the variational method based on the virtual work principle. The finite element method is applied to evaluate the numerical solutions. The large sag catenary is utilized as an initial configuration for vibration analysis. The nonlinearity due to the large sag curvature of static configuration is taken into account in the element stiffness matrix. The natural frequencies of large sag catenary and their corresponding mode shapes are determined by solving the eigenvalue problem. The numerical examples of a large sag catenary jumpers are presented. The influences of bending rigidity and large sag shape on the free vibration behaviors of the catenary jumper are provided. The results indicate that the increase in sag reduces the jumper natural frequencies. The corresponding mode shapes of the jumper with large sag catenary shape are comprised of normal and tangential displacements. The large sag curvature including in the element stiffness matrix increases the natural frequency especially for a case of very large sag shape. Mostly, the mode shapes of jumper are dominated by the normal displacement, however, the tangential displacement significantly occurs around the lowest point of sag. The increase in degree of inclination of the catenary tends to increase the natural frequencies.

• International Journal of Naval Architecture and Ocean Engineering
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• v.8 no.4
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• pp.330-343
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• 2016

This paper presents results of an experimental investigation of vortex-induced vibration (VIV) of a flexibly mounted and rigid cylinder with two-degrees-of-freedom with respect to varying ratio of in-line natural frequency to cross-flow natural frequency, $f^*$, at a fixed low mass ratio. Combined in-line and cross-flow motion was observed in a sub-critical Reynolds number range. Three-dimensional displacement meter and tension meter were used to measure dynamic responses of the model. To validate the results and the experiment system, x and y response amplitudes and ratio of oscillation frequency to cross-flow natural frequency were compared with other experimental results. It has been found that the higher harmonics, such as third and more vibration components, can occur on a certain part of steel catenary riser under a condition of dual resonance mode. In the present work, however, due to the limitation of a size of circulating water channel, the whole test of a whole configuration of the riser at an adequate scale for VIV phenomenon was not able to be conducted. Instead, we have modeled a rigid cylinder and assumed that the cylinder is a part of steel catenary riser where the higher harmonic motions could occur. Through the experiment, we have found that even though the cylinder was assumed to be rigid, the occurrence of the higher harmonic motions was observed in a small reduced velocity ($V_r$) range, where the influence of the in-line response is relatively large. The transition of the vortex shedding mode from one to another was examined by using time history of x and y directional displacement over all experimental cases. We also observed the influence of in-line restoring force power spectral density with $f^*$.

• Journal of Ocean Engineering and Technology
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• v.13 no.3B
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• pp.14-21
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• 1999

As an effective means to convey crushed materials from seabed to onboard ship and to raise hazardous or abrasive liquids, air-lift pump provides a reliable mechanism due to its simple configuration and easy-to-operate principle. The present study is focused on investigation of related performance by the analysis program based on the gas-liquid two-phase flow in circular pipes. The program covers pump operating in isothermal and vertical two-phase flow with Newtonian liquids. It is summarized as important result that an optimum air mass flow rate exists for the maximum lifted liquid mass flow rate in terms of a given submergence rates. The comparison between riser performance of the conveyed liquid flow rate calculated by the computer program and measured data with large scale air lift pump system constructed in 200 meter depth vertical tank reveals similar distribution.

• Journal of Ocean Engineering and Technology
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• v.26 no.6
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• pp.1-6
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• 2012

The purpose of this paper is to present a rational method for analyzing, designing, or evaluating the spread mooring systems used with floating drilling units. This paper presents a validated model to calculate the catenary static configuration. A semi-resolution approach is presented in this paper that is capable of predicting the static performance of a caisson mooring system. The solution is derived as a function of only three parameters, which can be solved numerically by implementing different kinds of boundary conditions. The efficiency and accuracy of the method permit quick parametric studies for the optimal selection of the system particle, which is undoubtedly useful for a preliminary design. A number of numerical examples demonstrate the validity of the adopted approach. The paper contains a complete description of the test cases and reports the results in such a way that it can provide a "benchmark" test for users and programmers of computer codes for flexible riser analysis.