• International Journal of Ocean System Engineering
• /
• 제3권4호
• /
• pp.188-208
• /
• 2013

The motion response of floating structures should be adequately low to permit the operation of rigid risers along with dry well heads. Though Spar platforms have low heave responses under lower sea state, could become unacceptable in near resonance region of wave periods. Hence the hydrodynamic response, heave in particular, must be examined to ensure that it is minimized. To reduce heave motions, external damping devices are introduced and one such effective damping device is heave plate. Addition of heave plate can provide additional viscous damping and additional added mass in the heave direction which influence the heave motion. The present study focuses on the influence of heave plate on the hydrodynamic responses of Classic Spar in regular waves. The experimental investigation has been carried out on a 1:100 scale model of Spar with single and double heave plates in regular waves. Numerical investigation has been carried out to derive the hydrodynamic responses using ANSYS AQWA. The experimental results were compared with those obtained from numerical simulation and found to be in good agreement. The influence of disk diameter ratio, wave steepness, pretension in the mooring line and relative spacing between the plates on the hydrodynamic responses of Spar are evaluated and presented.

• International Journal of Ocean System Engineering
• /
• 제4권1호
• /
• pp.1-18
• /
• 2014

Heave plates have been widely used to enhance viscous damping and thus reduces the heave response of Spar platforms. Single heave plate attached to the keel of the Spar has been reported in literature (Tao and Cai 2004). The effect of double heave plates on hydrodynamic response in random waves has been investigated in this study. The influence of relative spacing $L_d/D_d$ ($D_d$-the diameter of the heave plate) on the hydrodynamic response in random waves has been simulated in wave basin experiments and numerical model. The experimental investigation has been carried out using 1:100 scale model of Spar with double heave plates in random waves for different relative spacing and varying wave period. The influence of relative spacing between the heave plates on the motion responses of Spar are evaluated and presented. Numerical investigation has been carried out to investigate effect of relative spacing on hydrodynamic characteristics such as heave added mass and hydrodynamic responses. The measured results were compared with those obtained from numerical simulation and found to be in good agreement. Experimental and numerical simulation shows that the damping coefficient and added mass does not increase for relative spacing of 0.4 and the effect greater than relative spacing on significant heave response is insignificant.

• Structural Engineering and Mechanics
• /
• 제65권5호
• /
• pp.513-521
• /
• 2018

In this paper, the resonance response of spar-type floating platform in coupled heave and pitch motion is investigated using a CPU time-effective numerical method. A coupled nonlinear 2-DOF equation of motion is derived based on the potential wave theory and the rigid-body hydrodynamics. The transient responses are solved by the fourth-order Runge-Kutta (RK4) method and transformed to the frequency responses by the digital Fourier transform (DFT), and the first-order approximation of heave response is analytically derived. Through the numerical experiments, the theoretical derivation and the numerical formulation are verified from the comparison with the commercial software AQWA. And, the frequencies of resonance arising from the nonlinear coupling between heave and pitch motions are investigated and justified from the comparison with the analytically derived first-order approximation of heave response.

• Coupled systems mechanics
• /
• 제1권3호
• /
• pp.247-268
• /
• 2012

A finite element model is developed for dynamic response prediction of floating offshore wind turbine systems considering coupling of wind turbine, floater and mooring system. The model employs Morison's equation with Srinivasan's model for hydrodynamic force and a non-hydrostatic model for restoring force. It is observed that for estimation of restoring force of a small floater, simple hydrostatic model underestimates the heave response after the resonance peak, while non-hydrostatic model shows good agreement with experiment. The developed model is used to discuss influence of heave plates and modeling of mooring system on floater response. Heave plates are found to influence heave response by shifting the resonance peak to longer period, while response after resonance is unaffected. The applicability of simplified linear modeling of mooring system is investigated using nonlinear model for Catenary and Tension Legged mooring. The linear model is found to provide good agreement with nonlinear model for Tension Leg mooring while it overestimates the surge response for Catenary mooring system. Floater response characteristics under different wave directions for the two types of mooring system are similar in all six modes but heave, pitch and roll amplitudes is negligible in tension leg due to high restraint. The reduced amplitude shall lead to reduction in wind turbine loads.

• 한국해양공학회지
• /
• 제22권5호
• /
• pp.138-141
• /
• 2008

This paper treats the development of a new type of deep draught semisubmersible offshore structural shape that has excellent heave motion response and less down-time. This new type of semisubmersible shape is characterized by heave dampers at each pontoon corner and appropriate column size adjustment to achieve the desired cancellation and resonance motion period. It was found that the final model had efficient workability, based on the wavescatter diagrams for the GoM and North Atlantic ocean, due to less heave motion, although the specific heave response at the 100 year return period wave condition in the North Atlantic was more or less increased.

• 해양환경안전학회지
• /
• 제21권4호
• /
• pp.449-456
• /
• 2015

본 연구에서는 다양한 감쇠판을 원통형 부유체에 부착하고 수치 시뮬레이션을 통해 원통형 부유체의 heave RAO 및 고유주기의 변화를 고찰하였다. 먼저 감쇠판을 부착하지 않은 원통형 부유체의 heave RAO 및 고유주기를 파악하였고, 원통형 부유체에 부착한 감쇠판의 크기와 형상을 바꿔가며 각 경우에 대한 heave RAO 및 고유주기를 평가하였다. 수치 시뮬레이션 결과, 감쇠판의 모든 면적에 대해서 감쇠판을 부착한 원통형 부유체의 고유주기는 증가하였고, 감쇠판의 크기가 1.30D부터 입사파의 피크주기에서 heave RAO는 감소하였다. 감쇠판의 모든 면적에 대해서 원형 감쇠판이 Y자형 감쇠판보다 고유주기가 길게 나타나며, 고유주기의 차이는 감쇠판의 면적이 커질수록 그 차이가 커졌다. 운동응답 스펙트럼 계산 결과, 원형 감쇠판과 Y자형 감쇠판을 부착한 원통형 부유체의 heave 운동이 입사파의 피크주기에서 현저하게 감소하였다. 또한 원형 감쇠판을 부착한 원통형 부유체의 고유주기가 Y형 감쇠판을 부착한 경우보다 입사파의 피크 주기에서 더 멀리 이동하였다.

• 한국전산구조공학회논문집
• /
• 제26권4호
• /
• pp.247-254
• /
• 2013

본 논문에서는 해상 시추작업을 위한 heave compensation system의 시뮬레이션 모델을 개발하였다. 우선 시뮬레이션을 위하여, 다물체계 동역학 커널을 개발하였다. 다물체계 동역학 커널은 입력 받은 heave compensation system 시뮬레이션 모델의 운동학적 정보를 이용하여 recursive Newton-Euler formulation 방법을 기반으로 운동방정식을 자동으로 구성하고, 수치적으로 해를 계산하는 기능을 한다. 그리고 해상 시추선에 작용하는 외력을 계산하기 위하여 유체 정역학적 힘과 유체 동역학적 힘을 계산하는 모듈을 개발하였다. 이와 같이 개발한 커널과 모듈들을 적용하여 해상 시추선의 hoisting system 동적거동 해석을 수행하고, 관절에서의 구속력을 계산하였다.

• 한국해양공학회지
• /
• 제27권5호
• /
• pp.43-50
• /
• 2013

In the present investigation, the hydrodynamic characteristics of a vertically floating hollow cylinder in regular waves have been studied. The potential theory for solving the diffraction and radiation problem was employed by assuming that the heave response motion was linear. By using the matched eigenfunction expansion method, the characteristics of the exciting forces, hydrodynamic coefficients, and heave motion responses were investigated with various system parameters such as the radius and draft of a hollow cylinder. In the present analytical model, two resonances are identified: the system resonance of a hollow cylinder and the piston-mode resonance in the confined inner fluid region. The piston resonance mode is especially important in the motion response of a hollow circular cylinder. In many cases, the heave response at the piston resonance mode is large, and its resonant frequency can be predicted using the empirical formula of Fukuda (1977). The present design tool can be applied to analyze the motion response of a spar offshore structure with a moon pool.

• Ocean Systems Engineering
• /
• 제9권3호
• /
• pp.219-240
• /
• 2019

This paper presents the comparison between SFT response with linear and nonlinear cables. The dynamic response analysis of submerged floating tunnel (SFT) is presented computationally with linear and nonlinear tension legs cables. The analysis is performed computationally for two wave directions one at 90 degrees (perpendicular) to tunnel and other at 45 degrees to the tunnel. The tension legs or cables are assumed as linear and non- linear and the analysis is also performed by assuming one tension leg or cable is failed. The Response Amplitude Operators (RAO's) are computed for first order waves, second order waves for both failure and non-failure case of cables. For first order waves- the SFT response is higher for sway and heave degree of freedom with nonlinear cables as compared with linear cables. For second order waves the SFT response in sway degree of freedom is bit higher response with linear cables as compared with nonlinear cables and the SFT in heave degree of freedom has higher response at low time periods with nonlinear cables as compared with linear cables. For irregular waves the power spectral densities (PSD's) has been computed for sway and heave degrees of freedom, at $45^0$ wave direction PSD's are higher with linear cables as compared with nonlinear cables and at $90^0$ wave direction the PSD's are higher with non-linear cables. The mooring force responses are also computed in y and z directions for linear and nonlinear cables.

• Structural Engineering and Mechanics
• /
• 제63권1호
• /
• pp.37-46
• /
• 2017

This paper is concerned with the numerical study on the resonance response of a rigid spar-type floating platform in coupled heave and pitch motion. Spar-type floating platforms, widely used for supporting the offshore structures, offer an economic advantage but those exhibit the dynamically high sensitivity to external excitations due to their shape at the same time. Hence, the investigation of their dynamic responses, particularly at resonance, is prerequisite for the design of spar-type floating platforms which secure the dynamic stability. Spar-type floating platform in 2-D surface wave is assumed to be a rigid body having 2-DOFs, and its coupled dynamic equations are analytically derived using the geometric and kinematic relations. The motion-variance of the metacentric height and the moment of inertia of floating platform are taken into consideration, and the hydrodynamic interaction between the wave and platform motions is reflected into the hydrodynamic force and moment and the frequency-dependent added masses. The coupled nonlinear equations governing the heave and pitch motions are solved by the RK4 method, and the frequency responses are obtained by the digital Fourier transform. Through the numerical experiments to the wave frequency, the resonance responses and the coupling in resonance between heave and pitch motions are investigated in time and frequency domains.