Hydroelastic analysis of a very large floating plate with large deflections in stochastic seaway

The hydroelasticity of a very large floating plate with large deflections in multidirectional irregular waves is discussed. After a brief introduction on wave loads on a flexible structure, the paper derives the generalised fluid force acting on a floating structure in multidirectional irregular waves. The nonlinear sectional forces induced by the membrane forces in the plate are deduced. The hydroelastic response equations of a floating plate with large deflections in multidirectional irregular waves are established, and a solution method in the frequency domain is discussed including extreme value statistics. A very large floating structure is chosen as an example. The numerical results show that the influence of the membrane forces on the vertical displacements and the bending moments is noticeable but not that large.     

角形刚性阻振结构在船舶舱壁的应用研究

运用波动理论分析了刚性阻振结构对振动波的阻抑特性,基于阻抗失配原理设计了多种刚性阻振结构,并将其应用于船舶舱壁典型的“十”字结构处的刚性阻振隔振设计。在基座处施加激励力,数值计算得到模型在不同刚性阻振设计下的加速度传递函数、振级落差。通过对比分析可知：角形阻振质量带相对于方形阻振质量带具有更宽的阻隔频率以及更高的传递损失,中低频角形阻振质量带的隔振效果好,而高频角形空心阻振质量带的隔振效果好。     

Processing method and governing parameters for horizontal wave impact loads on a semi-submersible

Semi-submersible platform has been widely used in offshore oil exploitation due to its excellent performance, but can be attacked by wave impact loads in extreme ocean environments. Determining wave impact loads accurately is of great significance to the design and operation of offshore structures. An experimental study was carried out to investigate the critical governing parameters for the horizontal wave impact loads on a semi-submersible. The wavelet denoising technique and the frequency response function method are employed successfully to remove the effect of noise and dynamic contamination from the experimental data. The strongly nonlinear characteristics of the wave impact load are demonstrated. The results show that wave impact events are governed by the upwell height and upwell velocity. Most major wave impact events occur where both the two parameters are large, and the upwell velocity is more dominant in the wave impact process. In general, larger parameters tend to result in larger peak pressures and higher probabilities of wave impacts. The motion behaviors of the platform are benefit to reduce the occurrence probabilities of wave impact events and maximum impact pressures, owing to the escape velocities following the wave direction and the rotations leading to the above-water structure away from the waves. The insights given in this study provide a motivation and foundation for developing a sophisticated prediction model of the wave impact load on floating platforms.     

系泊浮体运动响应分析的非线性水动力学方法（英文）

Nonlinear wave loads can induce low-frequency and high-frequency resonance motions of a moored platform in deep water. For the analysis of the nonlinear response of an offshore platform under the action of irregular waves, the most widely used method in practice is the Cummins method, in which the second-order exciting forces in the time domain are computed by a two-term Volterra series model based on incident waves, first-order body motion response, and quadratic transfer functions(QTFs). QTFs are bichromatic waves acting on a body and are computed in the frequency domain in advance. For moving bodies, QTFs are related to the first-order body response, which is to be determined in the simulation process of body motion response but is unknown in the computation procedure of QTFs. In solving this problem, Teng and Cong(2017) proposed a method to divide the QTFs into different components,which are unrelated to the body response. With the application of the new QTF components, a modified Cummins method can be developed for the simulation of the nonlinear response of a moored floating platform. This paper presents a review of the theory.     

Mathematical simulation of steep waves at a focus point

Mathematical models simulating steep waves at a focus point are presented in this paper. Simulations of extreme waves in a model basin were used to determine the loads on floating structures induced by the waves. Based on a new wave theory, numerical test results show that the simulation procedure is effective and the induced motion of water particles in the front of waves is an important factor influencing impact loads on floating bodies.     

Methodology for global structural load effect analysis of the semi-submersible hull of floating wind turbines under still water,wind, and wave loads

In designing the support structures of floating wind turbines (FWTs), a key challenge is to determine the load effects (at the cross-sectional load and stress level). This is because FWTs are subjected to complex global, local, static, and dynamic loads in stochastic environmental conditions. Up to now, most of the studies of FWTs have focused on the dynamic motion characteristics of FWTs, while minimal research has touched upon the internal load effects of the support structure. However, a good understanding of the structural load effects is essential since it is the basis for achieving a good design. Motivated by the situation, this study deals with the global load effect analysis for FWT support structures. A semi-submersible hull of a 10-MW FWT is used in the case study. A novel analysis method is employed to obtain the time-domain internal load effects of the floater, which account for the static and dynamic global loads under the still water, wind, and wave loads and associated motions. The investigation of the internal stresses resulting from various global loads under operational and parked conditions and the dynamic behavior of the structural load effects in various environmental conditions are made. The dominating load components for structural responses of the semi-submersible floater and the significant dynamic characteristics under different wind and wave conditions are identified. The dynamic load effects of the floating support structure are investigated by considering the influence of the second-order wave loads, viscous drag loads induced global motions, and wind and wave misalignments. The main results are discussed, and the main findings are summarized. The insights gained provide a basis for improving the design and analysis of FWT support structures.     

Integrated design optimization of spar floating wind turbines

A linearized aero-hydro-servo-elastic floating wind turbine model is presented and used to perform integrated design optimization of the platform, tower, mooring system, and blade-pitch controller for a 10 MW spar floating wind turbine. Optimal design solutions are found using gradient-based optimization with analytic derivatives, considering both fatigue and extreme response constraints, where the objective function is a weighted combination of system cost and power quality. Optimization results show that local minima exist both in the soft-stiff and stiff-stiff range for the first tower bending mode and that a stiff-stiff tower design is needed to reach a solution that satisfies the fatigue constraints. The optimized platform has a relatively small diameter in the wave zone to limit the wave loads on the structure and an hourglass shape far below the waterline. The shape increases the restoring moment and natural frequency in pitch, which leads to improved behaviour in the low-frequency range. The importance of integrated optimization is shown in the solutions for the tower and blade-pitch control system, which are clearly affected by the simultaneous design of the platform. State-of-the-art nonlinear time-domain analyses show that the linearized model is conservative in general, but reasonably accurate in capturing trends, suggesting that the presented methodology is suitable for preliminary integrated design calculations.     

舰船波浪弯矩响应的频率特性

非直壁式舰船在高浪级和高航速下的弯矩响应中,除了含有低频遭遇频率成分外,还存在着一定数量的遭遇频率的倍频成分及船体各谐调的垂向总振动频率成份,它们都会对舰船波浪弯矩的非线性响应产生相应的影响。如果舰船的第一谐调垂向总振动频率正好是遭遇频率的整数倍,这时弯矩的非线性响应将达到极值。本文从模型试验及理论计算两方面分析了舰船波浪弯矩的频率特性,对船体载荷响应理论的发展及设计载荷的确定具有指导意义。     

多模块MOB连接器动力特性研究

以7模块MOB为研究对象,利用RMFC模型,基于势流理论对7个构成模块单元进行3D水动力性能分析,计算过程中考虑了由于遮蔽效应而导致的浮体间相互作用.编制基于频域的多浮体运动和柔性连接器载荷计算程序,计算在规则波和不规则波下的连接器载荷值,并比较不同连接器刚度下各柔性连接器载荷值.计算结果表明,波浪入射角、波频、连接器刚度和海况对连接器载荷响应具有显著影响.本文的计算结果可为MOB柔性连接器设计与模块耐波性设计提供参考.     

Discussion on the parameters of design waves

In order to respond the discredit on the design wave standard and to recommend new consideration on design wave parameters, based on the long-term distribution of statistic characteristics of waves and the short-term probability properties of sea state defined by giving the return period, the calculation of the return period, the height, the period, and the oceanic wave parameters of the design wave and the forecasting methods are discussed in this paper. To provide references for the operation reliability of floating structures in the extreme sea state, the method of determining the design wave parameters is resurveyed. A proposal is recommended that the design wave, which can be either significant wave with 500-year of the return period, or the maximum wave with 1/N of exceeding probability, 100-year of the return period, can be applied in the engineering design practice.     

Inhomogeneous wave load effects on a long,straight and side-anchored floating pontoon bridge

In this paper, we present a numerical study on the hydroelastic response of a 4.6 km long fjord crossing floating bridge subjected to wave loads. The bridge is straight in design and supported by 35 pontoons along its full length. To limit the response to horizontal loads, four clusters of deep water mooring lines are engaged to increase the transverse stiffness of the bridge. Owing to the very large span across the fjord, inhomogeneity in the wave field exists. This study examines the various effects of inhomogeneous wave loads on the dynamic responses of the floating bridge. These include the spatial variations of the wave direction, significant wave height and peak period as well as the coherence and correlation of waves along the entire length of the floating bridge. For the purpose of comparison, the dynamic bridge responses under homogeneous wave load cases are also studied. In addition, the effects of wave load components and short-crestedness are presented and discussed.     

Wave slamming loads on wave-piercer catamarans operating at high-speed determined by hydro-elastic segmented model experiments

Catamaran vessels operating at high-speed can be exposed to deck diving and bow damage and one resolution of this problem is the wave-piercer design of INCAT Tasmania. Owing to the complexity of the unsteady non-linear flow in the bow area during large wave encounter model testing has been undertaken to identify the peak dynamic slam loads on the ship structure. This paper provides experimental benchmark information relating to the wave slam loads on wave-piercing catamaran ferries. Since the time frames of transient slam loadings and whipping vibration of the entire hull in its first bending mode are similar it is important that the test model replicates the whipping response and therefore needs to be a hydro-elastic model. A 2.5 m hydro-elastic segmented catamaran model has been developed based on the 112 m INCAT Tasmania wave-piercer catamaran to establish the peak wave slamming loads acting on the full-scale vessel. Towing tank tests were performed in regular seas at a maximum full-scale operating speed of 38 knots. The model was instrumented to measure the dynamic slam loads acting on the centre bow and vertical bending moments acting in the demihulls of the catamaran model as a function of wave frequency and wave height. Peak slam loads measured on the centre bow were found to approach the total weight of the model, this being a broadly similar result to the peak loads measured at full-scale. It was found that global dimensionless heave and pitch accelerations peaked in the same range of encounter frequency as did the peak slam load.     

基于浮体Morison方程的半潜式超大型浮式结构波浪力简化计算方法

研究了半潜式超大型浮式结构中移动式海上基地（MOB）在高海况随机波作用下波浪力的简化计算方法。文中基于修正后的浮体Morison方程,经理论推导得出了MOB结构波浪力的计算公式。以MOB结构“三模块模型”为例,研究其在6级海况条件下基于Bretschneider谱模拟的随机不规则波中浪向角变化在0°~90°范围内,各模块的波浪力-历时规律,将本文简化算法的计算结果统计值与势流理论的结果相互对比,并对二者进行误差分析。结果表明：运用本文简化算法得到的MOB波浪力统计结果与势流理论的结果吻合程度高,且二者之间的相对误差在工程允许的范围之内,可充分验证本文方法的正确性、合理性与可行性。本文算法相比于势流理论而言更加简单,建议在结构初步设计阶段运用该方法可高效地评估大量不同工况下MOB结构的波浪荷载,研究成果可为半潜式超大型浮式结构动力响应研究奠定基础。     

斜壁结构的水动力收敛方法研究

在分析波浪中的船舶运动或者计算大型结构的水动力系数时,往往采用时域格林函数方法。时域格林函数的一个重要局限性在于它在计算具有倾斜壁面的水动力系数时,结果很容易发散。时域格林函数本身的奇异性以及高频振动特性显然是水动力发散的一个重要原因。但即使该奇异性通过增加粘性以及表面张力的方式使之消失,计算具有斜壁结构的水动力时,发散现象依然存在。因此,该文提出一种滤波方法,除去时域格林函数的高频部分,留下其低频部分,并定义滤波系数,从而使作用于斜壁结构上的水动力值收敛。通过文中结果与频域兴波格林函数法的比较来确定最优滤波系数。结果表明：最优滤波系数几乎与运动幅值无关,但会受到运动频率以及物面形状的影响。     

Extreme hawser tension assessment for FPSO vessel during offloading operation in Bohai bay

The Floating Production Storage and Offloading Unit (FPSO) is an offshore vessel that produces and stores crude oil prior to tanker transport.Robust prediction of extreme hawser tensions during Floating Production Storage and Offloading (FPSO) operation is an important safety concern. Excessive hawser tension may occur during offloading operations, posing an operational risk. In this paper, AQWA has been used to analyze vessel response due to hydrodynamic wave loads, acting on a specific FPSO vessel under actual sea conditions. Experimental validation of numerical results has been discussed as well.This paper advocates methodology for estimating extreme response statistics, based on simulations (or measurements). The modified ACER (averaged conditional exceedance rate) method is presented in brief detail. Proposed methodology provides an accurate extreme value prediction, utilizing all available data efficiently. In this study the estimated return level values, obtained by ACER method, are compared to the corresponding return level values obtained by Gumbel method. Based on the overall performance of the proposed method, it is concluded that the improved ACER method can provide more robust and accurate prediction of the extreme hawser tension.Data declustering issue has been addressed. Paper highlights ability of ACER method to account for a set of varying sea state probabilities, as required in engineering long term statistical analysis.Described approach may be well used at the vessel design stage, while defining optimal vessel parameters that would minimize potential FPSO hawser tension.     

多体浮式防波堤消波特性数值模拟

多体浮式防波堤消波特性是决定其应用价值的重要因素,基于三维势流理论,通过使用Ansys Workbench软件和AQWA Graphical Supervisor(AGS)图像后处理软件,对多体浮式防波堤在不同入射角和浮筒吃水深度下的消波效果展开研究。结果表明,在入射角度为90°时,多浮筒浮式防波堤能够最大程度地消减波浪。同时,透射系数随吃水深度的增加而不断减小,但结构稳定性会下降。因此,实际工程中应尽量保证浮式防波堤的迎浪面与波浪的来波方向垂直,这样可以获得较好的消波效果,同时应尽量避免选择吃水深度过大的浮式防波堤。     

波能转换型浮式防波堤的水动力性能研究（英文）

The integration of wave energy converters(WECs) with floating breakwaters has become common recently due to the benefits of both cost-sharing and providing offshore power supply. In this study, based on viscous computational fluid dynamics(CFD) theory, we investigated the hydrodynamic performances of the floating box and Berkeley Wedge breakwaters, both of which can also serve as WECs. A numerical wave flume model is constructed using Star-CCM+software and applied to investigate the interaction between waves and wave energy converters while completing the verification of the convergence study of time and space steps. The effects of wave length on motion response and transmission coefficient of the floating box breakwater model are studied. Comparisons of our numerical results and published experimental data indicate that Star-CCM+ is very capable of accurately modeling the nonlinear wave interaction of floating structures, while the analytical potential theory overrates the results especially around the resonant frequency. Optimal damping can be readily predicted using potential flow theory and can then be verified by CFD numerical results. Next, we investigated the relationship between wave frequencies and various coefficients using the CFD model under optimal damping, including the motion response, transmission coefficient, reflection coefficient,dissipation coefficient, and wave energy conversion efficiency. We then compared the power generation efficiencies and wave dissipation performances of the floating box and Berkeley Wedge breakwaters. The results show that the power generation efficiency of the Berkeley Wedge breakwater is always much higher than that of the floating box breakwater. Besides, the wave dissipation performance of the Berkeley Wedge breakwater is much better than that of the floating box breakwater at lower frequency.     

Long-term correlation structure of wave loads using simulation

This paper proposes a new method for combining the lifetime wave-induced sectional forces and moments that are acting on the ship structure. The method is based on load simulation and can be used to determine the exceedance probabilities of any linear and nonlinear long-term load combination. It can also be used to determine the long-term correlation structure between these loads in the form of the long-term correlation coefficients. They are essential part of the load combination procedures in design and strength evaluations as well as in the fatigue and reliability analysis of ship structures.The simulation method treats the non-stationary wave elevations during the ship’s entire life (long-term) as a sequence of different stationary Gaussian stochastic processes. It uses the rejection sampling technique for the sea state generation, depending on the ship’s current position and the season. Ship’s operational profile is then determined conditional on the current sea state and the ship’s position along its route. The sampling technique significantly reduces the number of sea state-operational profile combinations required for achieving the convergence of the long-term statistical properties of the loads. This technique can even be used in combination with the existing long-term methods in order to reduce the number of required weightings of the short-term CDFs. The simulation method does, however, rely on the assumption that the ship is a linear system, but no assumptions are needed regarding the short-term CDF of the load peaks.The load time series are simulated from the load spectra in each sea state, taking into account the effects of loading condition, heading, speed, seasonality, voluntary as well as involuntary speed reduction in severe sea states and the short-crested nature of the ocean waves. During the simulation procedure, special care has been given to maintaining the correct phase relation between all the loads. Therefore, time series of various load combinations, including the nonlinear ones, can be obtained and their correlation structure examined. The simulation time can be significantly reduced (to the order of minutes rather than hours and days) by introducing the seasonal variations of the ocean waves into a single voyage simulation. The estimate of the long-term correlation coefficient, obtained by simulating only a single voyage with the correct representation of seasonality, approaches the true correlation coefficient in probability. This method can be applied to any ship and any route, or multiple routes as long as the percentage of the ship’s total lifetime spent in each of them is known.A study has been conducted to investigate the effects of ship type, route and the longitudinal position of the loads on the values of the correlation coefficients between six different sectional loads; vertical, horizontal and twisting moments, as well as shear, horizontal and axial forces. Three ocean-going ship types have been considered; bulk carrier, containership and tanker, all navigating on one of the three busy ship routes; North America-Europe, Asia-North America and Asia-Europe. Finally, the correlation coefficient estimates have been calculated for five different positions along the ship’s length to investigate the longitudinal variation of the correlation coefficient.     

Response-based extreme value analysis of moored offshore structures due to wave, wind, and current

Floating moored offshore structures have a significant future in offshore operations as an attractive economic alternative to fixed structures in deep waters and/or in areas where there is no existing infrastructure. This paper describes an analysis procedure based on the structure variable approach to estimate load and response values of a moored offshore platform at a given return period by taking into account the joint occurrence of wave, wind, and current. The results show that the most severe mooring loads may not occur when wind, wave, and current are collinear and are at their maximum design values, i.e., the 50- or 100-year case. It is recommended that the extreme mooring design loads for moored offshore systems should be determined through a range of physical or numerical simulations where wave, wind, and current are noncollinear and act with less severe magnitudes than the 50- or 100-year case. This recommendation has also been adopted in the ITTC/Ocean Engineering Committee recommendations to the ITTC Conference held in September 1996.     

预应力混凝土风机结构在环境载荷与5兆瓦负载下的三维分析模拟（英文）

A concrete gravity base structure may not be suitable for offshore weak soil because of its heavy weight. Therefore, a conceptual model for a concrete offshore wind turbine structure suitable for weak soils is proposed. The proposed model is composed of a prestressed concrete(PSC) supported by a pile foundation. For a three-dimensional analysis of the large concrete structure, wave pressures based on the diffraction wave theory are developed using a three-dimensional solid finite element method. Static and dynamic analyses were performed to achieve the conceptual model of a PSC structure subjected to ocean environmental loads and a 5-MW turbine load on southwest coast in Korea. From the analysis, the maximum displacement and stresses of the proposed model did not exceed the allowable values from design standard, and the first mode of natural frequency of the structure was in a safe range to avoid resonance. The proposed model has enough structural stability to withstand external loads, and it is expected to be used in locations suitable for concrete gravity structures.