台风浪下海上机场VLFS动态响应与能量转换机理

超大浮式结构(very large floating structure,简称VLFS)是集空港和海港为一体的大型海上多功能浮式结构,现有研究大多关注规则波下刚性模块柔性连接模型的动力响应,忽略了台风浪极端环境下海上机场自身柔性引起的非线性振动特性。针对此问题,提出了一种新型多柔-刚性混合模块建模方法,采用Jonswap谱特征参数对台风“鲇鱼”过境实测海浪谱开展了精细化仿真模拟,分析了台风浪下海上机场VLFS整体和局部非线性动态响应特性,揭示了海上机场与环境荷载之间的能量转换机理。结果表明:海上机场多柔-刚性混合模块模型可以较好地反映此类VLFS结构动力响应特性;海上机场超长柔性及台风浪场不均匀性使其结构呈现显著非线性,位移、转角和水弹性变形分别以沿波向、绕展向和沿垂向为主,极值应力主要分布于撑杆附近;环境荷载能量和结构重力势能在初始阶段主要转换为系泊势能,稳定阶段则主要转换为结构动能和弹性势能。

VLFS的新型结构设计

作为一种新型的海洋浮式结构,将网架结构的设计理念应用于超大型海洋浮式结构物VLFS的设计。网架结构具有结构强度高、工业化程度高、制造难度低、安装便利和制造工期短等优点。运用ABAQUS有限元软件对新型VLFS浮式结构在海洋环境条件下的应力和应变进行分析,新型VLFS浮式结构的强度、稳定性和耐波性满足工程要求。

An Improved Simplified Method for Predicting the Hydroelastic Response of Mat-Like VLFS

Very Large Floating Structures (VLFS) have received considerable attention recently. Efficient and accurate estimation of their hydroelastic responses in waves is very important for the design. The most efficient approach would obviously be the analytical one, Within the category of analytical approaches, the simplified method proposed by Ohkusu and his colleague are of special characteristics. However, when one studies their methods, several questions arise. The purpose of this paper is to critically study the simplified methods proposed by Ohkusu and his colleague in order to answer these questions. Some problems in their original methods have been found and possible improvements are suggested. It is concluded that the improved simplified method using the same idea of Ohkusu and his colleague could provide a reasonable estimate of the hydroelastic response of mat-like VLFS in a certain range of incident angles of waves.

Numerical and Experimental Investigation of Hydrodynamic Interactions of Two VLFS Modules Deployed in Tandem

This paper numerically and experimentally investigates the hydrodynamic interaction between two semi-submersible type VLFS modules in the frequency domain. Model tests were conducted to investigate the relationship between interactions and wave headings. Numerical studies were performed by solving the radiation-diffraction problem and were validated against the experimental results. Motion Response Amplitude Operators (RAOs) were obtained from numerical and experimental studies. The dependency of the hydrodynamic interaction effect on wave headings is clarified. The influence of different wave periods on the motion responses of two-module VLFS and wave elevations in the gap is studied. The results indicate that the hydrodynamic interactions of the two modules are directly related to the wave headings and the periods of the incident wave. The shielding effect plays an important role in short wave, and the influence decreases with the increase of the incident wavelength. The numerical results based on the diffraction-radiation code can give a relatively good estimation to the responses in short wave while for long wave, it would over-predict the response.

Seismic ground amplification induced by box-shaped tunnels

The growing use of underground structures,specifically to facilitate urban transportation,highlights the need to scrutinize the effects of such spaces on the seismic ground response as well as the surrounding buildings.In this regard,the seismic ground amplification variations in the vicinity of single and twin box-shaped tunnels subjected to SV waves have been investigated by the finite difference method.To evaluate the effects,generalizable dimensionless diagrams based on the results of parametric numerical analysis considering factors such as variations in the tunnels′depth,the distances between the tunnels,tunnel lining flexibility,and input wave frequency,have been presented.In addition,to assess the effects of underground box-shaped tunnels on the response spectrum of the ground surface,seven selected accelerograms have been matched based on a specific design spectrum for the stiff soil condition of Eurocode 8(CEN,2006).The results underline the significant amplification effect of the box-shaped tunnels on the ground motions,specifically in the case of horizontal twin tunnels,which should be given more attention in current seismic design practices for surface structures.

A method to Estimate Dynamic Responses of VLFS Based on Multi-Floating-Module Model Connected by Elastic Hinges

Based on the elastic foundation beam theory and the multi-floating-module hydrodynamic theory,a novel method is proposed to estimate the dynamic responses of VLFS(Very Large Floating Structure).In still water,a VLFS can be simplified as an elastic foundation beam model or a multi-floating-module model connected by elastic hinges.According to equivalent displacement of the two models in static analysis,the problem of rotation stiffness of elastic hinges can be solved.Then,based on the potential flow theory,the dynamic responding analysis of multi-floatingmodule model under wave loads can be computed in ANSYS-AQWA software.By assembling the time domain analysis results of each module,the dynamic responses of the VLFS can be obtained.Validation of the method is conducted through a series of comparison calculations,which mainly includes a continuous structure and a three-part structure connected by hinges in regular waves.The results of this paper method show a satisfactory agreement with the experiment and calculation data given in relative references.

Hydroelastic Response of VLFS with An Attached Submerged Horizontal Solid/Porous Plate Under Wave Action

The hydroelastic responses of a submerged horizontal solid/porous plate attached at the front of a very large rectangular floating structure(VLFS)under wave action has been investigated in the context of linear water wave theory.Darcy’s law is adopted to represent energy dissipation in pores.It is assumed that the porous plates are made of material with very fine pores so that the normal velocity across the perforated porous is linearly associated with the pressure drop.In the analytic method,the eigenfunction expansion-matching method(EEMM)for multiple domains is applied to solve the hydrodynamic problem and the elastic equation of motion is solved by the modal expansion method.The performance of the proposed submerged horizontal solid/porous plate can be significantly enhanced by selecting optimal design parameters,such as plate length,horizontal position,submerged depth and porosity.It is concluded that good damping effect can be achieved through installation of solid and porous plate.Porous plate has better damping effect at low frequencies,while solid plate has better damping effect at high frequencies.The optimal ratio of plate length to water depth is 0.25-0.375,and the optimal ratio of submerged depth to water depth is 0.09-0.181.

Oblique Wave-Induced Responses of A VLFS Edged with A Pair of Inclined Perforated Plates

This paper is concerned with the hydroelastic responses of a mat-like, rectangular very large floating structure(VLFS) edged with a pair of horizontal/inclined perforated anti-motion plates in the context of the direct coupling method. The updated Lagrangian formulae are applied to establish the equilibrium equations of the VLFS and the total potential formula is employed for fluids in the numerical model including the viscous effect of the perforated plates through the Darcy’s law. The hybrid finite element-boundary element(FE-BE) method is implemented to determine the response reduction of VLFS with attached perforated plates under various oblique incident waves.Also, the numerical solutions are validated against a series of experimental tests. The effectiveness of the attached perforated plates in reducing the deflections of the VLFS can be significantly improved by selecting the proper design parameters such as the porous parameter, submergence depth, plate width and inclination angle for the given sea conditions.

Analysis on the applicability of VLFS for China

With the advantages on rapid construction,no limitation of extent,less influence on environment, the VLFS has been turned into the focus of coastal and offshore engineering studying.As the platform,VLFS was built as airport,huge oil house etc,and would play the role of ocean economics,politics and military.The restrictive condition of VLFS is strong wave by monsoon,seismic,serious bomb etc.The floating breakwater should be put forward to shelter VLFS.The analysis of wave condition including typhoon route,archipelago and water depth is performed in this paper.The advantage and disadvantage are compared between VLFS and the marine structure from land.

A Novel Conceptual Telescopic Positioning Pile for VLFS Deployed in Shallow Water:Structure Design

A conceptual design of using novel telescopic piles to position a multi-modular very large floating structure(VLFS),which is supposed to be severed as a movable floating airport,is proposed.The telescopic piles can automatically plug in the soil to resist the environmental loads and pull out from the soil to evacuate or move on to the next operational sea.The feasibility demonstration of the conceptual design includes two parts:function verification and structure design.In the latter part of the conceptual design,a time-domain structural analysis is firstly conducted by using Abaqus software.The simulation results suggest that the preliminary structure scheme is not optimum due to the insufficient structure utilization,although both structure safety of the piles and positioning accuracy are guaranteed.To realize a cost reduction of construction and installation,a Genetic Algorithm-Finite Element Analysis(GA-FEA)method is employed to perform structural optimization.After optimization,31 percent of the weight of each pile is reduced and higher structure utilization is maintained.The difference of the self-weight and allowable buoyancy of a single module(SMOD)of a semisubmersible-type VLFS is much larger than the weight of the piles.Combined with the function verification in our previous work,the conceptual design of using the novel telescopic pile to position VLFS is demonstrated to be feasible.

Experimental Study of Mooring Type Effect on the Hydrodynamic Characteristics of VLFS

Mooring system is a significant part of very large offshore floating structures(VLFS).In this paper,a single module pontoon type VLFS model considering four mooring types is studied through physical model tests to determine the effects of mooring conditions on the hydroelastic response,mooring force,incident coefficient,reflection coefficient and energy dissipation coefficient.Eight mooring cables are symmetrically arranged on both sides of the model.The floating body model satisfies the similarity of stiffness and gravity,while the cable satisfies the similarity of elasticity and gravity.The results show that the effect of mooring type on mooring force is greater than that on hydroelastic response.Increasing the initial tension of the mooring cable will reduce the amplitude of the leeward of the VLFS model.The mooring angle of the mooring cable will affect the maximum mooring force and the initial tension of the mooring line will affect the wave period in which the maximum mooring force occurs.The transmission coefficient and wave energy dissipation coefficient will change regularly with different mooring types.These results may provide a reference to facilitate the mooring design of VLFS.

Variations of Atmospheric ELF/VLF Radio Noise Due to Seismogenic Modifications in Tropospheric Conductivity

We suggest a possible explanation of the influence of pre-seismic activity on the registration rate of natural ELF(extremely low frequency)/VLF(very low frequency) pulses and the changes of their characteristics. The main idea is as follows. The distribution of the electric field around a thundercloud depends on the conductivity profile of the atmosphere. Quasi-static electric fields of a thundercloud decrease in those tropospheric regions where an increase of air conductivity is generated by pre-seismic activities due to emanation of radioactive gas and water into the lower atmosphere. The electric field becomes reduced in the lower troposphere, and the probability decreases of the cloud-to-ground (CG) strokes in such “contaminated” areas. Simultaneously, the electric field grows inside and above the thunderclouds, and hence, we anticipate a growth in the number of horizontal and tilted inter-cloud (or intra-cloud) (both termed as IC discharges) strokes. Spatial orientation of lightning strokes reduces vertical projection of their individual amplitudes, while the rate (median number strokes per a unit time) of discharges grows. We demonstrate that channel tilt of strokes modifies the spectral content of ELF/VLF radio noise and changes the rate of detected pulses during the earthquake preparation phase.

基于VLF/LF三维闪电定位系统的吉林省闪电特征分析

利用2018—2020年吉林省VLF/LF三维闪电定位资料,分析了吉林省闪电的时空分布和雷电流强度特征。结果表明:吉林省闪电集中发生在5—9月,峰值在6月;日闪电高发时段在13—19时;总闪、地闪和云闪频数时变化特征趋于一致,均呈双峰型,主峰值出现在16—17时,次峰值出现在23—24时;地闪以负闪为主,云闪中的正、负闪比例相近。吉林省总闪密度高值区主要集中在辽源北部、四平东部、长春南部至吉林中西部一带,东部延边州的龙井、延吉、图们一带,松原南部至长春西部一带;云闪和地闪高密度区域也基本出现在这三个高值区内;闪电高值区均处于山区及半山区地带。雷电流强度主要集中在10~50 kA,正闪的平均雷电流强度高于负闪。云闪多发生在2~5 km的高度,高度小于16 km时,平均雷电流强度随高度的增加而增大;高度大于16 km时,高度越高平均雷电流强度越小。

汶川8级地震前空间电离层VLF电场异常现象

基于法国DEMETER卫星记录的空间电场频谱资料,提取分析了汶川地震前可能存在的电场异常信息。首先利用轨道的重访特性分析了四川汶川8级地震前震中上空固定纬度点(31°N)的VLF电场信号2年多的正常频谱变化背景,结果显示1～6月各频点变化比较稳定,但在7～10月份各频点谱值扰动比较剧烈,存在整体幅度上升或者下降的现象,因此电场可能存在年周期变化。在背景分析的基础上,收集了2008年4月1日～5月12日震中区上空2000km范围内所有夜间的降轨轨道在北纬31°记录的电场所有频点资料,计算结果显示震前1周内即5月6～10日相对4月份呈现明显的电场能量增强现象,增强幅度达到3σ,频率集中分布在2～6kHz范围内,异常区域的空间统计分布反映该电场增强现象位于震中以南较大范围内。这次异常发生在空间电磁活动平静时期,可能与汶川地震的孕育过程有关。最后讨论了空间地震电场异常的可能形成机理,对地震电离层耦合机理的深入认识还需要地基和空间的联合立体观测来支持。

用VLF、EH4和CSAMT方法寻找隐伏矿——以赤峰柴胡栏子金矿床为例

VLF、EH4和CSAMT均为基于电磁原理的地球物理手段,但它们在探测深度、测量精度和野外操作的复杂程度上有较大的差异.因而不同探测目标和精度要求,尽可能使用不同的方法或方法组合.VLF一般用于矿区及外围的地球物理扫面,以快速确定和发现覆盖层(不超过50～60m)之下矿化构造系统的空间展布和基本规模.CSAMT具有探测深度大(>2000m)、工作效率高和抗干扰能力强等特点,一般用于确定矿化系统的深部的宏观几何形态和产状变化.EH4是这三种方法中对地下地质结构解析度最高、有效探测深度适中(800～1200m)的一种地球物理测深方法,它可以解析较为精确地探测矿体的三维形态、规模、产状变化和细部结构但采集数据时对近场电磁干扰要求较高.这三种地球物理方法在赤峰柴胡栏子金矿区的联合应用,为寻找覆盖层之下隐伏矿体取得了良好的效果.

强震前ELF/VLF磁场的扰动特征统计研究

本文利用DEMETER卫星记录的变化磁场数据统计研究了2005—2009年北半球7级以上强震前后空间磁场的扰动特征.在震中±10°范围内利用震前2个月至震后1个月的5年同期观测的ELF/VLF[370～897Hz]频段磁场功率谱密度数据构建了背景场,定义了表征空间磁场在地震时段相对于背景场的扰动幅度指标.统计研究发现26个强震中42%的地震主要表现为震前磁场扰度幅度逐渐上升,超过3倍标准差,随后在扰度幅度下降过程中发震.35%强震在地震前10天内最大扰动幅度超过3倍标准差,在扰度幅度处于最高值期间发震,震后磁场扰动幅度逐渐回落.23%的强震在震前震后无明显规律.随机事件测试发现在空间天气平静且无强震条件下随机地点上空的磁场无明显的时空演化规律,且随机地点上空的空间磁场相对于背景场的平均扰度幅度很小,最大扰度幅度从未超过2倍标准差,并且没有明显的随时间变化特征.

ZH-1卫星观测的VLF人工源信号特征分析与全波模拟

中国地震电磁监测试验卫星张衡一号(ZH-1)已于2018年2月2日成功发射,正在开展卫星数据在轨测试,并对卫星数据质量进行判定.本文对ZH-1卫星2018年5月至6月夜侧的VLF频段电场功率谱数据进行了分析.通过分析位于不同L值、具有不同发射频率的多个VLF人工源上空的卫星重访轨道观测数据,发现ZH-1卫星记录的人工源信号电场变化标准差与DEMETER卫星记录电场变化标准差几乎一致,说明ZH-1卫星观测数据具有较好的稳定性.通过重访轨道均值与全波模型计算结果对比,发现两者在数值上较为接近,在形态上较为一致,说明ZH-1卫星VLF频段电场功率谱数据具有一定的可靠性.此外,研究了VLF人工源上空及共轭区的电场分布特征和电波传播规律,并与DEMETER卫星的结果进行了对比,结果表明VLF人工源产生的电磁辐射穿透电离层后以导管或者非导管的哨声波模向共轭区传播,因为传播过程中的朗道阻尼,共轭区的电场能量比辐射源顶空更小.VLF人工源位于L<1.5时,电磁波传播更容易发生非导管传播,VLF人工源信号导管传播模式在共轭区的电场响应相对于共轭点会发生一定程度北向偏移.

地震电离层VLF电磁场频谱特征研究

对2006年3月—2009年2月DEMETER卫星VLF电磁场频谱数据的分析发现,全球地震电离层电磁场频谱有以下特征:50～300 Hz为地震电离层效应的优势频段;距离地震空间位置越近,地震异常越显著;不同半球的地震,异常方向和方位不同;对地磁夏季北半球地震的研究表明,海洋地震主要为正异常,陆地地震为负异常;地磁夏季,不同半球地震的磁场频谱异常显著不同,北半球的正异常偏南,而负异常偏北,南半球的负异常偏东北,而正异常偏西南。

地闪和云闪初始击穿VHF/VLF辐射特征观测和比较

分析了合肥地区负地闪和云闪初始击穿过程产生的VLF大双极性脉冲序列及其同步的VHF爆发特征。地闪初始击穿大脉冲序列极性通常与后面的回击极性相同,相邻大脉冲之间的平均间隔为160μs,大脉冲序列持续时间平均为2.2ms。云闪初始击穿的大双极性脉冲序列由正极性大脉冲开始,相邻脉冲间隔时间平均为1.3ms,平均持续时间为11.5ms,明显区别于地闪中的情况。显著的VHF爆发几乎在大双极性脉冲序列出现的同时启动,它与VLF大脉冲一起是初始击穿过程中一种最强烈的特征性事件,在合肥地区出现频率极高。地闪初始击穿期间VHF包络(观测系统带宽100kHz)呈现准连续特征,而云闪初始击穿期间VHF包络则呈现分立特征,显示两者VHF辐射爆发的特征有所不同。初始击穿期间VLF大脉冲辐射与VHF爆发之间是相关的,但是二者强度之间不存在唯一的大小对应关系。同时分析了合肥地区地闪初始击穿大脉冲序列活动相对于地闪回击的一些特征,序列中最大脉冲幅度与随后的首次回击幅度之比平均为0.43,有94%的初始击穿在首次回击启动前40ms内出现第一个可以辨别的VLF脉冲(105例的平均结果是21.5ms)。

地面VLF波穿透电离层的能量衰减变化

地面VLF人工源可以导致辐射带高能粒子沉降.为研究辐射带粒子加速和沉降机制,乃至实现对辐射带电子"人工控制"的设想,需要在精确计算得到VLF人工源在电离层激发电磁场能量分布的基础上,计算哨声波对辐射带电子的调制作用.以往计算VLF人工源在电离层中激发的电磁场能量分布多基于Cary给出的波导中VLF波衰减率和Helliwell提出的经典电离层吸收曲线,但近期研究表明,这些模型结果存在较为明显的误差.本研究建立了地面VLF信号穿透电离层传播的全波计算模型,将计算结果与DEMETER卫星记录的NWC通讯台激发的电磁响应进行对比.虽然模型没有考虑电离层参数水平不均匀性,但模型计算结果与卫星观测值也有较好的一致性.利用经过验证的全波模型计算了不同地磁参数、电离层参数情况下,不同辐射特性的地面VLF辐射在波导中的衰减和穿透电离层时D/E区的吸收值,探讨了上述参数对电磁波能量在波导和D/E区中衰减的影响规律.