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ABSTRACT In the present paper, our in-house meshless solver MLParticle-SJTU, based on the improved MPS (moving particle semi-implicit) method, is employed to simulate the liquid sloshing in the tuned liquid damper (TLD). For the validation purpose, parameters from experiments in previous literature are adopted in first numerical simulation, the roll angle and wave shapes inside the TLD show agreement with the experimental data. Then, the influence of various external excitation on the damping characteristic of TLD has been studied through the changing of motion amplitude and frequency of a sliding mass. The numerical results indicate that the damping characteristic is more distinct when the excitation frequency falls near the natural frequency of the TLD system, and the flow in the TLD turns from traveling wave into the quasi-dam-break flow when the excitation amplitude increases from 50 mm to 200 mm. INTRODUCTION In this paper, the sloshing phenomenon in a tuned liquid damper (TLD), which is mainly composed of rectangular tank with the freedom of roll motion, is numerically studied. In the ship engineering, the TLD is commonly employed as the anti-roll device to suppress the roll motion of ship operating in severe sea. Compared with active ship stabilizer, the TLD have advantages of simple structure, low cost and easy maintenance, so a lot of attention has been paid to both experimental (Vera et al., 2010) and numerical researches on the sloshing in TLD (Bulian et al., 2010). The damping characteristic of TLD mainly depends on the sloshing in the tank and the properties of liquid in it. The physical behavior of sloshing flow in the partially filled TLD is similar to shallow water waves (Verhagen and Van, 1965), and shows high non-linear when violent sloshing occurs. Due to the phase lag between the roll motion and the wave movement, the roll motion of tank is damped by the moment induced by the impact loads of waves acting on inner wall of tank.
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Summary Surface-consistent scaling is used in land AVO-compliant processing flows to remove shot-to-shot and receiver-to-receiver amplitude variations. In conventional surface-consistent scaling methods, the scalars are obtained by decomposing the RMS amplitudes of prestack traces that are always contaminated to some extent by noise. The RMS amplitudes are a measure of the total energy on a trace (signal + noise) rather than just signal. Consequently, if noise increases in one area of a survey, the signal in that area is not scaled up enough (the noise biases the scalars). Despite attempts to remove the effects of noise on the derived scalars with various noise attenuation methods, we have observed that data often are poorly scaled at the end of a typical AVO-compliant run stream. As a solution to this problem, Cary and Nagarajappa (2013) proposed an unbiased scaling approach in which the shot and receiver consistent signal estimates were obtained from the RMS amplitudes of the shot and receiver stacks. A restriction of this method is that the data in the analysis window must be nearly flat in order to stack in the shot or receiver domain and obtain accurate scalar estimates. When the data is not flat, then it first needs to be flattened, which can be difficult when geology is complicated. In this paper, we propose an unbiased scaling method that avoids the use of shot and receiver stacks. The proposed method computes the prestack amplitudes from the zero-lag value of the crosscorrelations between each prestack trace and its CDP stack trace. Such a crosscorrelation can provide unbiased prestack amplitudes. In addition, the prestack amplitudes are not dependent on the time variations in the structure. The shot and receiver-consistent averages can then be computed to obtain the scalars in a surface-consistent manner. Introduction In land data, causes of amplitude variations between shots and between receivers include near-surface weathering conditions, coupling, and source/receiver type differences. Surface-consistent scaling is routinely used to estimate and remove these variations. Surface-consistent scaling methods and their applications are discussed in Taner & Koehler (1981), Taner et al. (1991), Yu et al. (1985) and Garceran et al. (2013). In these methods, the RMS amplitudes of the prestack traces are used and a set of equations are formed. The equations are then solved to obtain the surface-consistent scalar estimates. These scalar estimates can do a poor job of balancing the signal in the data because the RMS amplitudes of the prestack traces are biased by spatially-variant noise. Cary and Nagarajappa (2013) showed that unbiased surface-consistent scalar estimates could be obtained by computing the RMS amplitudes of the shot stacks and of the receiver stacks. It was shown that these stack amplitudes are unbiased estimates of the shot and receiver-consistent amplitude variations of the signal in the data. Henceforth in this paper, we refer to this approach as the unbiased scaling method.
ABSTRACT The accurate evaluation of vortex-induced motion (VIM) of semisubmersible platforms is gaining increasing importance with the recent development of deep draft semi-submersible platforms. Recent available field measurements of semisubmersibles indicate that the VIM response is typically found to be smaller than what is predicted in model tests. The possible candidates for the reduction in VIM response in the field include Reynolds number, wave effects, mass ratios, and external damping from mooring lines and risers. Previous researches by CFD investigation and model tests have studied the Froude scaling law, effects of waves and mass ratio on VIM response reduction. In this study, mooring-induced damping effect on the VIM response reduction is focused and investigated. The Finite Analytic Navier-Stokes (FANS) code is coupled with an in-house 6 degree-of-freedom (DoF) floater motion solver and an in-house MOORING3D code, for the time-domain simulation of the VIM response of a semi-submersible with semi-taut chain-polyester rope-chain mooring system. The large eddy simulation (LES) turbulence model is used to provide accurate predictions of hydrodynamic forces. Simulations are performed for the 1:70 model of the platform. Mooring systems for the semi-submersible with different water depth (WD) and line numbers are designed with the prototype scale, to represent mooring damping in different levels. The simulated motions of the semi-submersible with different mooring system are compared to the experimental data, and the impact of mooring-induced damping on the resultant motion characteristics is investigated by the comparison. The comparison shows that mooring damping is probably one critical reason of VIM response reduction in the field. INTRODUCTION The VIM of semi-submersible offshore platforms becomes an important issue with the recent development of deep draft semisubmersibles. At present, the preferred method to estimate the VIM response of a semi-submersible is through model tests. For instance, Waals et al. (2007) performed experimental VIM studies of multicolumn offshore floaters. To model the mooring system of the floater, two soft springs were used to provide the horizontal restoring. However, recent studies suggest that the VIM response in the field is much less than as observed in the model tests, see for studies performed by Irani et al. (2015), Ma et al. (2013), and Rijken et al. (2009). As discussed by Ma et al. (2013), amplitudes of VIM in the field seem much smaller than observed in model tests, with a reduction of as much as 50%. The approach of only using standard model test information gives rise to the overly conservative estimation of VIM amplitudes and the resultant design guidance for moorings and risers, with significant impact on costs.
ABSTRACT The ringing of a two dimensional (2D) floating barge under focused waves has been investigated based on the potential theory. A fully nonlinear numerical wave tank is developed using a higher-order boundary element method (BEM) including a mixed Eulerian-Lagrangian technique. Simulations are made to study focused waves interaction with a floating barge. The focus is on the roll motion principally excited by higher harmonic wave loads. The effect of wave amplitude and wave frequency on the ringing phenomenon is investigated. INTRODUCTION Roll motion has been attracting considerable attention during these years because it is the most critical motion leading to ship or platform capsizing in comparison with other five degrees of freedom motion. The nonlinear roll motion of a 2-D rectangular barge within the framework of potential flow theory has been investigated by Cointe & Geyer (1990) and Koo & Kim (2004). Chen et al. (2016) and Li & Teng (2015) presented the roll motion of a 2-D rectangular barge and twin rectangular barges based on OpenFOAM respectively. A barge is requested to be steady or with limited motion amplitude when handling cargos or performing specific missions. The large-amplitude motion due to resonance will occur when the encounter wave frequency is close to the natural frequency of the barge, which has been investigated extensively by previous studies based on linear theory. However, the nonlinear wave force becomes remarkable when wave amplitude increases. The resonance caused by higher order components has always been ignored. It was observed in model tests and prototype experiments that tension leg platforms (TLPs) and gravity-based structures (GBS) experienced sudden bursts of highly amplified resonant behavior in irregular waves (Report 1993; Chaplin et al., 1997; Scolan et al., 1997). This phenomenon is called as the ‘ringing’, which is closely related to large and steep waves interacting with the structure. This ringing of a structure typically occurs when its natural frequency is about three to five times of the incoming dominant wave frequency and is most likely to be excited by the third harmonic wave force. Typically when a transient wave passes through the structure, it will continue to oscillate over a period of time at its high natural frequency even the wave excitation has diminished. This oscillation can be persistent when the damping level is low. Understanding of such ringing behaviour is important because of the high level of stress generated. So far, most adopted numerical method is based on second-order theory, which is valid only for second-order problem with moderate wave amplitude. Ringing is due to third-order or higher order harmonic components of the nonlinear wave force, a fully nonlinear numerical method is needed. Zhou & Wu (2015) developed three-dimensional fully nonlinear numeical model to simulate the resonance of a TLP excited by the third order wave force component in regular wave. Since irregular wave is believed to be most likely to generate ringing phenomena but not well understood, we propose a fully nonlinear method to study the irregular wave case in this paper.
Abstract This paper presents an overview of prediction of TLP responses: model tests vs. analysis, sponsored by DeepStar Phase V program. ABB and Marintek were invited to carry out the task, which was intended to provide an overall assessment of the current capabilities of the industry in predicting TLP responses and highlight areas of uncertainties and sensitivities. The paper summarizes key results of TLP responses in 6,000 ft water depth of Gluf of Mexico field. Non-linear coupled dynamic analyses were employed indenpently by both ABB and Marintek to model the system in a consistent and accurate manner. The measured hull, tendon and riser configurations, as well as the measured wave elevations, wind loads and mean current velocity profile were applied. The overall correlations between model tests and analyses demonstrated the industry has the analytical capability in predicting the TLP responses. However, analytical tools are not perfect and physical model test is still an important design tool to verify the analyses. Introduction The DeepStar Program sponsored a series of tasks to evaluate the current industry capability in predicting the responses of deepwater theme structures (FPSO, TLP and SPAR). In its Phase IV program, model tests of the FPSO, TLP and SPAR were conducted, and in the Phase V program, engineering companies as well as test basins were invited to evaluate the correlations between the tests and analyses. The TLP physical model tests were carried out in MARIN wave basin tank in January 2001 in water depth of 6,000 feet under Gulf of Mexico hurricane and loop-current conditions. The details of test setups, wave, wind and current generations and cablibrations were described in paper OTC 16582 (ref. 1). In deep or ultra deepwater, the TLP platform tends to interact more pronouncedly to its tendons and risers. The dynamic interactions among platform, tendons and risers cannot be evaluated accurately and consistently by using the conventional uncoupled analysis tools, where the platform, tendons and risers are treated separately. Therefore, analytical capability of fully coupled dynamic analyses was required to complete the project. Both ABB and Marintek performed fully coupled dynamic analyses indenpenly by their own softwares. The purpose of this excerise is to investigate whether the existing numerical tool applying the measured data could reproduce the measured results and identify the gaps for further study. This paper presents an overview of both ABB's and Marintek's work. It includs the following aspects of the study:Model test setup and environmental criteria The state-of-art analytical tools available to the Industry Key results of comparisons - tests vs analyses Sensitivities and uncertainties in predicting TLP responses Assessment of current industry capabilities Areas of future efforts Model Test Setup TLP hull, tendon and riser configurations, tendon numbering, both tendon and riser locations, tendon porch elevation and riser top elevations were illustrated in Figure 1.Hull Parameter Hull configuration, measured hull weight, tendon top tensions and riser top tensions are given in Table 1. Tendon Parameter Tendon configuration, measured tendon dry weight and wet weight are summarized in Table 2.
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