• International Journal of Naval Architecture and Ocean Engineering
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• v.12 no.1
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• pp.367-375
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• 2020

This paper presents an integrated analysis about dynamic performance of a Floating Offshore Wind Turbine (FOWT) OC4 DeepCwind with semi-submersible platform under real sea environment. The emphasis of this paper is to investigate how the wave mean drift force and slow-drift wave excitation load (Quadratic transfer function, namely QTF) influence the platform motions, mooring line tension and tower base bending moments. Second order potential theory is being used for computing linear and nonlinear wave effects, including first order wave force, mean drift force and slow-drift excitation loads. Morison model is utilized to account the viscous effect from fluid. This approach considers floating wind turbine as an integrated coupled system. Two time-domain solvers, SIMA (SIMO/RIFLEX/AERODYN) and FAST are being chosen to analyze the global response of the integrated coupled system under small, moderate and severe sea condition. Results show that second order mean drift force and slow-drift force will drift the floater away along wave propagation direction. At the same time, slow-drift force has larger effect than mean drift force. Also tension of the mooring line at fairlead and tower base loads are increased accordingly in all sea conditions under investigation.

• Ocean Systems Engineering
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• v.9 no.2
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• pp.191-218
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• 2019

The tower-platform interface and mooring system of floating offshore wind turbines (FOWTs) are some of the most critical components with significant influences on overall project costs. In addition to satisfying strength requirements, it is typical and vital to meet fatigue criteria for a service life of 25 years or more. Wind spectra characteristics considered in analysis can penalize fatigue designs, leading to unnecessary costs. The International Electrotechnical Commission (IEC, 2009) recommends the use of site-specific wind data (spectrum, turbulence intensity, etc.) in design of FOWTs, but for offshore sites it is often the case that such data is unavailable and land-based data are used as surrogates in design. For such scenarios, it is worth investigating whether such alternative approach is suitable and accurate, and understanding the consequence of the selection of wind spectral characteristics on fatigue design. This paper addresses the impact of the subsequent selection on fatigue responses of towerbase and mooring system in a FOWT, as a sequel to the paper by Udoh and Zou (2018) which focused on impacts on strength design. The 5 MW semi-submersible FOWT platform with six mooring lines implemented in the preceding study is applied in analysis. Results indicate significant variations in resulting fatigue life with considered wind parameters. Thus, it is critical to apply proper wind spectra characteristics for analysis and design of FOWTs to avoid unnecessary conservatism and costs. Based on the findings of this study, more explicit guidance on the application of turbulence intensities for IEC-recommended models in offshore sites could lead to more accurate load estimates in design of FOWTs.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.5 no.4
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• pp.87-94
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• 1985

Dynamic analysis of guyed tower is presented in this paper. The scope of the study is twofold. The one is to determine an efficient analysis method to include the nonlinearity of the mooring system and the nonlinear hydrodynamic wave forces. The other is to investigate the sensitivity of two major design parameters, that is the stiffness of mooring system and the fixity condition of the tower at mud line. Time history analysis method utilizing mode superposition is mainly considered. However several other methods are also used for the purpose of comparison. Analyses are carried out using the Lena Guyed Tower, which is the first structure of this kind, as a standard structure.

• Journal of Korean Port Research
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• v.10 no.1
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• pp.25-36
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• 1996

Generally, it is pointed out that a nonlinear analysis is needed to estimate accurately the water surface fluctuation and dynamic responses of a floating structure in case of large wave reflection. In this study, a frequency-domain method is applied and newly developed to analyze the nonlinear characteristics of the air-chamber floating breakwater. The air-chamber floating breakwater in this study can control well the wave transformation, motions of the structure and its natural frequency by adjusting the air depth in the chamber. Experiments are carried out to verify the numerical results. It is appeared that the mean water level is setup in the anti-node and setdown in the node, while the nonlinearity in wave profile is larger in the node than in the anti-node. Because of vertical mooring system, the sway, especially the time-independent nonlinear component, plays predominant role in the motion. On the other hand, the time-dependent component, as well as the time-independent one to the tensile force of mooring line contributes greatly, and the time averaged value presents tensional force oriented to the onshore side due drift force.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.31 no.5
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• pp.215-226
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• 2018

In this paper, to estimate the dynamic behavior of a submerged floating tunnel(SFT) by underwater explosion(UE), the SFT is modeled and analyzed by the explicit structural analysis package LS-DYNA. The section of SFT near to explosion point is modeled to shell and solid elements using elasto-plasticity material model for concrete tubular section and steel lining. And the other parts of the SFT are modeled to elastic beam elements. Also, mooring lines are modeled as tension-only cable elements. Total mass of SFT is including an added mass by hydrodynamic effect. The buoyancy on the SFT is considered in its initial condition using a dynamic relaxation method. The accuracy and the feasibility of the analysis model aree verified by the results of series of free field analysis for UE. And buoyancy ratio(B/W) of SFT, the distance between SFT and an explosion point and the arrangement of mooring line aree considered as main parameters of the explosion analysis. As results of the explosion analysis, the dynamic responses such as the dent deformation by the shock pressure are responded less as more distance between SFT and an explosion point. However, the mooring angle of the diagonal mooring system can not affect the responses such as the horizontal displacement of SFT by the shock pressure.

• Journal of Ocean Engineering and Technology
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• v.28 no.4
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• pp.274-279
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• 2014

This paper investigates the line dynamic forces connecting tugs and a floater, where the planar motion of the floater is due to the weather criteria during the position keeping of the offshore structure by tugs. The analysis situation consists of the position keeping states for the seabed mooring line connection work of the offshore structure at the offshore site. Specifically, the decision about the tug power capacity for the position keeping is essential and depends on the weather criteria, line characteristics, length of line, etc. The planar motion of the structure is constrained by the interference of the installation vessel's operational range, behavior of the underwater fairlead, and other surrounding structures. In this paper, the tug line forces and planar motion of an offshore structure are summarized dependent on the tug line length and line material characteristics in the states fora given floater draft and weather criteria. The tug line dynamic forces and planar motion evaluated here will be used to determine the proper tug power and clearance of an offshore structure.

• Journal of Ocean Engineering and Technology
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• v.11 no.4
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• pp.205-212
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• 1997

The major subject of this paper is the conceptual design of a cable-buoy system based on mechanics of floating bodies and mooring lines. The goal is to present a systematic relevant analysis procedure for the conceptual design of oceanographic buoy systems. The dynamics of a free floating body with an anchoring line is studied and a model is proposed through the primary engineering.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.27 no.5
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• pp.369-377
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• 2014

In this paper, to recognize the collision behavior between a submerged floating tunnel(SFT) and underwater navigation vessel(UNV), both structures are modeled and analyzed. The SFT of collision point is modeled tubular section using concrete with steel lining. The other part of SFT is modeled elastic beam elements. Mooring lines are modeled as cable elements with tension. The under water navigation vessel is assumed 1800DT submarine and its total mass at collision is obtained with hydrodynamic added mass. The buoyancy force on SFT is included in initial condition using dynamic relaxation method. The buoyancy ratio (B/W) and the collision speed are considered as the collision conditions. As results, energy dissipation is concentrated on the SFT and that of the UNV is minor. Additionally, the collision behaviors are greatly affected by B/W and the tension of mooring lines. Especially, the collision forces are shown different tendency compare to vessel collision force of current design code.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.22 no.6
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• pp.423-428
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• 2010

This study is aiming to find one of reasonable guidelines to select a proper berthing ship at Kwang Yang harbors for loading/unloading for the POSCO(Pohang Steel Co. Ltd.). For dynamic analysis for the moored ships, the selection of subjected vessels has to be given the priority, so that the motion characteristics are figured out. The calculation of the dynamic fluid forces and wave, wind and current forces in time domain are followed. Then, the dynamic mooring analyses are performed. This study might contribute to make a new guideline by which the proper sized and loaded ships could be moored safety at the berths of Kwang Yang Harbor.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.3 no.4
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• pp.244-254
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• 1991

The numerical analysis on tile behaviour of mooring system in the offshore guyed tower is presented. The governing equilibrium equations are derived by the principle of virtual work. The drag and inertia effects of fluid are included in a Morrison type equation. The finite element method is used in the computation. Geometric nonlinearities for the analysis of the mooring line are considered in which both modified Newton-Raphson method and Newmark-$\beta$ method are employed. Numerical experiments show the validity and the capability of the developed mathematical formulation.