CFD approach to roll damping of ship with bilge keel with experimental validation

The most common method of reducing roll motion of ship-shaped floating systems is the use of bilge keel which act as damping elements. The estimation of the damping introduced by bilge keel is still largely based on empirical methods. The present work adopts the CFD approach to the estimation of roll damping, both without and with bilge keel and validates the results with experiments conducted in a wave flume. Specifically, free oscillation tests are conducted at model scale to obtain roll damping, both by experiments and CFD simulation and reasonably good comparisons are obtained. The experiments also include PIV study of the flow field and attempt has been made to correlate the measured flow field with that obtained by CFD. The CFD methodology has the potential to determine rationally the size and orientation of bilge keels in design with reasonably accurate estimate of the additional roll damping that it provides to ship's roll motion.     

Study on Global Performances and Mooring-Induced Damping of A Semi-Submersible

The harsh environmental conditions bring strong nonlinearities to the hydrodynamic performances of the offshore floating platforms, which challenge the reliable prediction of the platform coupled with the mooring system. The present study investigates a typical semi-submersible under both the operational and the survival conditions through numerical and experimental methods. The motion responses, the mooring line tensions, and the wave loads on the longitudinal mid-section are investigated by both the fully non-linearly coupled numerical simulation and the physical experiment. Particularly, in the physical model test, the wave loads distributed on the semi-submersible’s mid-section were measured by dividing the model into two parts, namely the port and the starboard parts, which were rigidly connected by three six-component force transducers. It is concluded that both the numerical and physical model can have good prediction of the semi-submersible’s global responses. In addition, an improved numerical approach is proposed for the estimation of the mooring-induced damping, and is validated by both the experimental and the published results. The characteristics of the mooring-induced damping are further summarized in various sea states, including the operational and the survival environments. In order to obtain the better prediction of the system response in deep water, the mooring-induced damping of the truncated mooring lines applied in the physical experiment are compensated by comparing with those in full length. Furthermore, the upstream taut and the downstream slack mooring lines are classified and investigated to obtain the different mooring line damping performances in the comparative study.     

Sensitivity Analysis of Air Gap Motion with Respect to Wind Load and Mooring System for Semi-Submersible Platform Design

A design of semi-submersible platform is mainly based on the extreme response analysis due to the forces experienced by the components during lifetime. The external loads can induce the extreme air gap response and potential deck impact to the semi-submersible platform. It is important to predict air gap response of platforms accurately in order to check the strength of local structures which withstand the wave slamming due to negative air gap. The wind load cannot be simulated easily by model test in towing tank whereas it can be simulated accurately in wind tunnel test. Furthermore, full scale simulation of the mooring system in model test is still a tuff work especially the stiffness of the mooring system. Owing to the above mentioned problem, the model test results are not accurate enough for air gap evaluation. The aim of this paper is to present sensitivity analysis results of air gap motion with respect to the mooring system and wind load for the design of semi-submersible platform. Though the model test results are not suitable for the direct evaluation of air gap, they can be used as a good basis for tuning the radiation damping and viscous drag in numerical simulation. In the presented design example, a numerical model is tuned and validated by ANSYS AQWA based on the model test results with a simple 4 line symmetrical horizontal soft mooring system. According to the tuned numerical model, sensitivity analysis studies of air gap motion with respect to the mooring system and wind load are performed in time domain. Three mooring systems and five simulation cases about the presented platform are simulated based on the results of wind tunnel tests and sea-keeping tests. The sensitivity analysis results are valuable for the floating platform design.     

Added mass and damping of a sphere section in heave

The Norwegian wave-power buoy¹ consists of a half-immersed floating sphere which is open to the sea at the bottom end. It is a two degree-of-freedom device involving the independent motion of the outer rigid sphere and the pressure across the internal free surface. A simpler model of the device is to represent the oscillatory flow through the bottom opening as another rigid body motion being that of the curved surface which would complete the sphere.The wave-induced forces on this surface and also the outer spherical surface due to independent oscillations of either are determined semi-analytically using a simple extension of the method of Havelock,² recently simplified and generalised by Hulme,³ for the complete half-immersed sphere. In particular the 2 × 2 added mass and damping matrices are determined as a function of frequency and relative size of bottom opening to sphere radius. These quantities are essential in any theoretical analysis of a multi-degree of freedom wave-energy device.     

两层流体中矩形箱浮体的附加质量和阻尼系数

研究了两层流体中矩形箱浮体的辐射问题。基于特征函数匹配理论,针对矩形箱浮体的三种振荡运动模式(横荡、垂荡和横摇),建立两层流体中矩形箱浮体辐射势的求解方法,导出矩形箱浮体附加质量和阻尼系数的计算公式。对所建立的求解模型进行了数值计算分析,结果表明在矩形箱浮体的某个振荡频率范围内,流体的分层效应对其附加质量和阻尼系数有显著影响的。     

Hull/mooring/riser coupled dynamic analysis and sensitivity study of a tanker-based FPSO

A computer program is developed for hull/mooring/riser coupled dynamic analysis of a tanker-based turret-moored FPSO (Floating Production Storage and Offloading) in waves, winds, and currents. In this computer program, the floating body is modeled as a rigid body with six degrees of freedom. The first- and second-order wave forces, added mass, and radiation damping at various yaw angles are calculated from the second-order diffraction/radiation panel program WAMIT. The wind and current forces for various yaw angles of FPSO are modeled following the empirical method suggested by OCIMF (Oil Company International Marine Forum).

The mooring/riser dynamics are modeled using a rod theory and finite element method (FEM), with the governing equations described in a generalized coordinate system. The dynamics of hull, mooring lines, and risers are solved simultaneously at each time step in a combined matrix for the specified connection condition. For illustration, semi-taut chain-steel wire-chain mooring lines and steel catenary risers are employed and their effects on global FPSO hull motions are investigated. To better understand the physics related to the motion characteristics of a turret-moored FPSO, the role of various hydrodynamic contributions is analyzed and assessed including the effects of hull and mooring/riser viscous damping, second-order difference-frequency wave-force quadratic transfer functions, and yaw-angle dependent wave forces and hydrodynamic coefficients. To see the effects of hull and mooring/riser coupling and mooring/riser damping more clearly, the case with no drag forces on those slender members is also investigated. The numerical results are compared with MARIN's wave basin experiments.     

Sensitivity of AUV added mass coefficients to variations in hull and control plane geometry

The sensitivity of the added mass coefficients of a typical autonomous underwater vehicle (AUV) to changes in geometric parameters was investigated. Qualitative deductions were made concerning the effect of geometric variations. Then the added mass coefficients for several configurations of body geometry were generated for the Canadian Self-Contained Off-the-shelf Underwater Testbed (C-SCOUT) vehicle using the computer program Estimate Submarine Added Mass (ESAM). The changes in the added mass coefficients have direct relationships to the varied parameter. The results presented here are specific to the C-SCOUT, but may be extended to similar axisymmetric bodies.     

Design analysis of a truss pontoon semi-submersible concept in deep water

Truss pontoon semi-submersible (TPS) is a new offshore structure concept in industry, where a truss spar is used to create the added mass by the heave plates. In the present paper, the effect of the heave plates on the vertical motion of the floating structure is demonstrated. A TPS is analyzed by utilizing the linear diffraction theory as well as the linear part of the Morison equation. The close agreement of the analysis results with the experimental results suggests that the simplified Morison equation can be used for the present analysis without sacrificing the quality of the results. However, good engineering judgment is required for estimating the values of the hydrodynamic coefficients as well as the amount of damping introduced in the structure. It is also found that the heave plates indeed introduce large added mass and considerable damping in the system motion in the vertical direction such that the resonant oscillation becomes less of a problem. This suggests that the TSP concept may have merits as a heave-controlled floating production structure in the deepwater development.     

Wave radiation by a uniform cylinder in front of a vertical wall

In this paper, an exact analytical method is developed for the problem of wave radiation by a uniform cylinder in front of a vertical wall. Based on the image principle, the hydrodynamic problem of a cylinder in front of a vertical wall is transformed into the equivalent problem of double cylinders in unbounded fluid domain. Consequently, an analytical method of eigenfunction expansion is adopted to calculate the radiation of the cylinder due to the motion in surge, sway, roll and pitch, respectively. Moreover, numerical analysis has been carried out in detail in order to discuss the influences of the distance between the cylinder and the vertical wall and water depth on the added mass and radiation damping of the cylinder. It is shown that added mass and damping of the cylinder in front of a vertical wall are evidently different from those in case of the cylinder in unbounded fluid domain from the numerical results. It is also found that the added mass and radiation damping oscillate with wave number, and the oscillating frequency increases with the increasing of the distance between the cylinder and the wall.     

土工膜-土工布界面动力剪切特性试验研究

糙面土工膜（GMX）和无纺土工布（GT）是垃圾填埋场中周边衬垫系统的重要组成部分,其界面特性对于整体填埋场的稳定性尤为重要。但是,目前对GMX-GT界面的动力剪切特性研究较少。为此,利用大型直剪仪,开展了一系列全饱和条件及干燥条件下GMX-GT界面的循环剪切试验,主要研究了竖向应力、位移幅值和循环次数等对GMX-GT界面动力剪切特性的影响,并对比分析了全饱和及干燥两种条件下GMX-GT界面动力剪切特性的差异。研究结果表明,随着位移幅值的增加,GMX-GT界面呈现出由剪切硬化向剪切软化转变的特性。循环剪切作用使得界面的内摩擦角随着位移幅值的增加而增大。GMX-GT界面主要表现为剪缩特性,且总剪缩量随竖向应力、位移幅值和循环次数的增加而增加。剪切刚度随竖向应力和循环次数的增大而增大,随位移幅值的增大而减小。阻尼比随位移幅值的增大而增大,随循环次数的增加而减小,说明位移幅值会增加GMX-GT界面的能量的耗散。GMX-GT界面在干燥条件下的破坏模式与饱和条件下的存在明显差异,干燥条件下GT内部的破坏更加显著,全饱和条件下GMX表面的破坏更加明显。