• Journal of Korean Society of Coastal and Ocean Engineers
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• v.5 no.1
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• pp.25-38
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• 1993

Numerical models of directional wave spectra for the analysis of offshore structural cable responses are verified. Alternative spreading models are used to predict wave-induced flows in water and for mooring systems. Hydrodynamic wave forces upon cable are estimated. using a Morison formula encompassing considerations for drag and for inertial forces both parallel and tangential to the slope of the cable. Numerical analysis for directional random waves. including consideration of displacement and velocity, trajectory, phase plane response. and tension are shown for mooring system cable responses at both the tether point for a buoy and at the anchor point. The effects of wave forces far different drag coefficients, various significant wave heights, and selected wave parameters are considered in the analysis. For the specific systems considered in the examples, it is demonstrated that wave period and height as well as wave spreading function parameters and drag coefficients, have an important effect upon the dynamic responses of the cable-buoy systems.

• International Journal of Naval Architecture and Ocean Engineering
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• v.6 no.3
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• pp.685-699
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• 2014

In this study, the dynamic responses of a riser under the combined excitation of internal waves and background currents are studied. A modified Taylor-Goldstein equation is used to calculate the internal waves vertical structures when background currents exist. By imposing rigid-lid boundary condition, the equation is solved by Thompson-Haskell method. Based on the principle of virtual work, a nonlinear differential equation for riser motions is established combined with the modified Morison formula. Using Newmark-${\beta}$ method, the motion equation is solved in time domain. It is observed that the internal waves without currents exhibit dominated effect on dynamic response of a riser in the first two modes. With the effects of the background currents, the motion displacements of the riser will increase significantly in both cases that wave goes along and against the currents. This phenomenon is most obviously observed at the motions in the first mode.

• Journal of Advanced Research in Ocean Engineering
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• v.3 no.1
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• pp.15-24
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• 2017

This paper explores a series of numerical simulations of dynamic responses of multi-piles (dolphin) type substructures for 2.5MW class offshore wind turbine. Firstly computational fluid dynamics (CFD) simulation was performed to evaluate wave loads on the dolphin type substructures with the design wave condition for the west-south region of Korea. Numerical wave tank (NWT) based on CFD was adopted to generate numerically a progressive regular wave using a virtual piston type wave maker. It was found that the water-piercing area of piles of the substructure is a key parameter determining the wave load exerted in horizontal direction. In the next the dynamic structural responses of substructure members under the wave load were calculated using finite element analysis (FEA). In the FEA approach, the dynamic structural responses were able to be calculated including a deformable body effect of substructure members when wave load on each member was determined by Morison's formula. The paper numerically identifies dynamic response characteristics of dolphin type substructures for 2.5MW class offshore wind turbine.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.11 no.3
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• pp.141-148
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• 1999

The magnitude of breaking wave forces given by plunging breakers incident on a pile structure is much greater than the forces calculated by Morison's formula, but those forces may act on pile for very short duration in the range of a few multiples of 0.01 second. Hence, a dynamic analysis for the impact forces of breaking waves may be necessary for the accurate determination of pile displacements in the first stage of design. The time series of the impact force along the pile length is thus required, which may be estimated from the pressure distribution. In the present study, breaking wave pressures are measured for a vertical pile at real field which is easily subjected to plunging breakers in stormy weather conditions. The measured data are analyzed and compared with other results to quantify the characteristics of breaking wave pressures in real fields.

• Bulletin of the Society of Naval Architects of Korea
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• v.23 no.2
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• pp.33-45
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• 1986

In the present paper, the nonlinear analysis of dynamic response of the jacket type offshore structures subject to nonlinear fluid force is performed. Furthermore, several analysis methods, such as quasi-static analysis, Newmark-$\beta$ method and state vector time integration technique, and described and compared with each others in order to investigate the efficiency numerical of the schemes for this kind of nonlinear structural analysis. In the problem formulation, various environmental forces acting on the jacket type offshore structure have been studied and calculated. Particularly, hydrodynamic forces are calculated by using the Morison type formula, which contains the interaction effect between the motion of the structure and the velocity of fluid particles. Also, Stokes' 5th order wave theory and Airy's linear wave theory are used to predict the velocity distribution of the fluid particles. Finally, the nonlinear equation of motion of the structure is obtained by using three-dimensional finite element formulation. Based on the above procedures, two examples, i.e. a single pile and a typical offshore jacket platform, are studied in details.

• Journal of the Society of Naval Architects of Korea
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• v.39 no.3
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• pp.57-63
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• 2002

This paper describes a control algorithm for the buffer of a deep-sea mining system, in which the buffer is connected to a long slender pipe and then to a surface ship on one end, and to a collector on sea floor through a flexible hose on the other end. A mathematical modeling is established for designing the controller for buffer thrusters, in which the dynamic response of the long pipe is taken into account based on the mode superposition method. The fluid loading acting on the pipe is estimated by using Morison's formula. For simplicity, the surface ship is assumed to be kept stationary, the reaction from the flexible hose is ignored and only the lateral motions are considered. In order to guide the buffer to react only to the low-frequency motion of the surface vessel, the FIR digital filter is introduced to a PID-based controller It can be shown numerically that the high frequency component of the ship's motion can be effectively filtered out by using the FIR low pass filter.