Interaction of oblique waves with infinite number of perforated caissons

An analytic solution based on the division of the fluid domain is developed for the interaction of obliquely incident waves with infinite number of perforated caissons. The whole fluid domain is firstly divided into infinite sub-domains according to the division of structures, and subsequently eigenfunction expansion is employed to represent the velocity potential in each domain. A phase relation is utilized for the analysis of wave oscillation in each caisson, and the character of structure geometry is considered in setting up the mathematical model of reflection waves. The reflection waves from the present analysis include many propagation waves traveling in different directions when the incident wave frequency is high. Benchmark examinations show that the continuous condition of water particle velocity is satisfied at the front walls of caissons, and the reflection coefficients keep agreement with the energy conservation relation very well when porous effect parameter is infinite. Numerical results show that the reflection coefficients of obliquely incident waves are smaller when the length of caissons is shorter at low frequency. The wave reflection coefficients and the wave forces normal to caissons decrease and the wave forces along caissons increase with the increase of the wave incident angle.     

Failure Loci of Suction Caisson Foundations Under Combined Loading Conditions

Suction caissons are widely used to support offshore fixed platforms in coastal areas. The loadings transferred to suction caissons include the eccentric lateral force induced by waves and self weight of the platform structure. However, under this kind of combined loading conditions, the failure mechanism of caissons with shallow embedment depths is quite different from conventional deep foundations or onshore shallow footings. The behaviour of caissons subjected to combined loadings may be described with the "failure locus" in force resultant spaces. Here the failure loci of smooth caissons are studied by use of finite element approach, with the embedment ratio of caissons varying in the range of 0.25～1.0 and eccentricity ratio of horizontal loadings in 0～10. The platform settlement and tilt limits are involved into determination of failure loci, thus the platforms can avoid significant displacements for the combined loadings located inside the failure locus. Three families of loading paths are used to map out the locus. It is found that the shape of failure loci depends on 3 non-dimensional parameters, and the failure locus of a given caisson changes gradually from the elliptical curve to hooked curve with increasing shear strength of soil. The lateral capacity of short caissons may be enhanced by vertical forces, compared with the maximum lateral capacity of long caissons occurring at the vertical force being zero. The critical embedment ratios partitioning elliptical and hooked loci are proposed.     

开孔沉箱在斜向入射波作用下受力研究

应用透空壁内流体速度与壁两侧的压力差成正比的线性模型,研究了斜向波与无限多个开孔沉箱的相互作用.依照结构物的几何形状,把整个流域分成无限多个子域,在每个子域内应用特征函数展开法对速度势进行展开.对于沉箱内的波浪运动,引入相位差概念;在构造反射波模型时,考虑了结构物的几何形状影响.列举出物理模型实验结果与数值实验结果的比较,可以看出两者吻合较好.进一步的数值计算验证表明,当孔隙系数无限大时,开孔墙前后的速度非常接近.在低频入射波作用下,垂直于沉箱的水平力随角度的变大而减小,平行于沉箱排列方向的力则变大.     

Peak Dynamic Pressure on Semi- and Quarter-Circular Breakwaters Under Wave Troughs

A series of physical tests are conducted to examine the characteristics of the wave loading exerted on circular-front breakwaters by regular waves. It is found that the wave trough instead of wave crest plays a major role in the failure of submerged circular caissons due to seaward sliding. The difference in the behavior of seaward and shoreward horizontal wave forces is explained based on the variations of dynamic pressure with wave parameters. A wave load model is proposed based on a modified first-order solution for the dynamic pressure on submerged circular-front caissons under a wave trough. This wave loading model is very useful for engineering design. Further studies are needed to include model uncertainties in the reliability assessment of the breakwater.     

Peak Dynamic Pressure on Semi-and Quarter-Circular Breakwaters Under Wave Troughs

A series of physical tests are conducted to examine the characteristics of the wave loading exerted on circular-front breakwaters by regular waves.It is found that the wave trough instead of wave crest plays a major role in the failure of submerged circular caissons due to seaward sliding.The difference in the behavior of seaward and shoreward horizontal wave forces is explained based on the variations of dynamic pressure with wave parameters.A wave load model is proposed based on a modified first-order solution for the dynamic pressure on submerged circular-front caissons under a wave trough.This wave loading model is very useful for engineering design.Further studies are needed to include model uncertainties in the reliability assessment of the breakwater.     

带横隔板局部开孔沉箱在斜向波作用下的受力研究

基于线性势流假定,对斜向波作用下带横隔板局部开孔沉箱防波堤的水平波浪力进行了理论研究。给出了开孔沉箱法向水平力和横隔板受力的理论计算方法,在极限情况下波浪力的计算结果与文献中的已有结果一致。利用数值算例分析了开孔沉箱总水平力的主要影响因素。开孔沉箱法向总水平力的减小主要集中于结构上半部分波浪影响范围以内。增加单个开孔沉箱的长度有利于减小结构所受总水平波浪力。当波浪入射角或沉箱前开孔墙孔隙影响系数幅值较大时,开孔沉箱横隔板上总水平力的最大值要超过相应的沉箱法向总水平力,此时要注意横隔板的强度问题。     

Experimental and Theoretical Investigation of Wave Forces on A Partially-Perforated Caisson Breakwater with A Rock-Filled Core

1.IntroductionVertical breakwaters are widely used for harbor and coastline protection in coastal engineering.Recently,perforated breakwaters have been often used in practice as they can effectively reduce thewaveforces actingon,the wave reflectionfromand…     

A Prediction Method of Internal Solitary Wave Loads on the Semi- Submersible Platform

An investigation into the prediction method for internal solitary waves (ISWs) loads on the columns and caissons of the semi-submersible platform found on three kinds of internal solitary wave theories and the modified Morison Equation is described. The characteristics of loads exerted on the semi-submersible platform model caused by the ISWs have been observed experimentally, and the inertial and drag coefficients in Morison Equation are determined by analyzing the forces of experiments. From the results, it is of interest to find that Reynolds number, KC number and layer thickness ratio have a considerable influence on the coefficients. The direction of incoming waves, however, is almost devoid of effects on the coefficients. The drag coefficient of columns varies as an exponential function of Reynolds number, and inertia coefficient of columns is a power function related to KC number. Meanwhile, the drag coefficient of caissons is approximately constant in terms of regression analysis of experimental data. The results from different experimental conditions reveal that the inertia coefficient of caissons appears to be exponential correlated with upper layer depths.

     

Undrained Bearing Capacity of Suction Caissons for Offshore Wind Turbine Foundations by Numerical Limit Analysis

Determining the ultimate capacity of suction caissons in response to combined vertical, horizontal, and moment loading is essential for their design as foundations for offshore wind turbines. However, the method implemented for stability analysis is quite limited. Numerical limit analysis has an advantage over traditional limit equilibrium methods and nonlinear finite element methods in this case because upper and lower bounds can be achieved to ensure that the exact ultimate capacity of the caisson falls within the appropriate range. This article presents theories related to numerical limit analysis. Simulations are conducted for centrifuge model tests, the findings of which reveal the ability of numerical limit analysis to deal with the inclined pullout capacity of suction caissons. Finally, this article proposes an estimation of the ultimate capacity of a 3.5 MW offshore wind turbine foundation on normally consolidated clay based on the typical environmental parameters of Bothkennar, Scotland. Undrained failure envelopes and safety factors are obtained for suction caissons with different embedment ratios. Failure mechanisms, plastic zones, clay stress distributions, and the influence of the skin friction coefficients of caissons are discussed in detail.     

重力式海洋平台沉箱的断裂分析

研究重力式海洋平台沉箱的表面裂纹最大深度,通过考虑沉箱有一个内表面裂纹,将问题简化为无裂纹重力式平台问题和带裂纹矩形板问题的叠加,应用有限元方法对重力式海洋平台无裂纹情形进行了静力分析,得到了与裂纹位置对应处的环向拉应力。计算裂纹矩形板的应力强度因子,得到海洋环境荷载下沉箱内表面裂纹的最大深度为0.066 8m,计算结果可供海洋平台的设计参考。     

地震荷载沉箱码头大变形分析的离散元法初探

探讨了应用离散元分析方法求解地震荷载下沉箱和背后填土大变形问题的可能性,开发了能描述沉箱的平面形状及沉箱与土体摩擦特性的矩形单元。并通过简单实例分析了地震时沉箱码头的变形,再现了沉箱码头地震灾害的发生过程,为沉箱在地震情况下的大变形分析提供了一种新途径。比较了两种沉箱模型对分析结果的影响,提出了相应的方法。     

Wave actions on the side caissons of the Venice gates

The present study originates from a construction problem found in the planned deployment of the side caissons of the Venice gates barrier. Each of these caissons is made to float, then sunk into a lateral trench and jointed to the soil at its bottom. As a result, a C-shaped channel forms between the vertical caisson surfaces and the surrounding trench. Incoming storm waves propagating from the sea can then induce forced oscillations in this C-channel. An analytical model based on the method of matched asymptotics is developed in order to obtain the free surface oscillations in the channel, caused by a grazing incident wave. The resonant response of the basin and the amplification factors are then determined. From the free surface elevation, the pressure field and hence the total forces are also found. The analysis moves on to irregular wave motion. The analytical solution enables us to compute the dynamic actions, showing that their magnitude can be significant, due to the excitation of different resonant modes.     

不规则波作用下明基床上开孔沉箱垂直力分析

通过水槽物理模型试验,对不规则波作用下明基床上开孔沉箱所受到的波浪力进行了较为系统的试验研究,分析讨论了开孔沉箱总水平力峰(谷)值时刻对应的总垂直力与基床相对高度、开孔率、波陡、相对水深、消浪室相对宽度等影响因素之间的关系,并与暗基床的研究成果做了对比讨论,给出了明基床上开孔沉箱总垂直力比值(总水平力极值时刻对应的总垂直力/总垂直力极值)与各影响因素之间的计算关系式,研究结果可为工程应用提供参考。     

Finite element modeling of the tensile capacity of suction caissons in cohesionless soil

When suction caissons are used as foundations for jacket structures, the caissons are exposed to significant vertical loads. If the self-weight of the structure is relatively low, a large horizontal load may lead to tension on the foundation on the incoming side. For steady loads, such as the wind load during production, the soil response will be drained. This paper presents the results from a series of finite element analysis (FEA) on suction caissons in cohesionless soil. The analyses are performed on suction caissons with different dimensions and different soil conditions. For normalization, dimensional analyses of the calculated results are performed to create dimensionless groups. The dimensionless groups are used to establish a relation between the normalized tensile capacity and the interface strength. This relation is used to establish two formulations of the drained tensile capacity for suction caissons in cohesionless soils. One for associated plasticity and one for non-associated plasticity with the dilatation angle equal zero.     

Total horizontal and vertical forces of irregular waves on partially perforated caisson breakwaters

In this study, the total horizontal and vertical forces as well as the phase differences of irregular waves on a partially perforated caisson breakwater are investigated. The partially perforated caisson is located on a rubble fill foundation and filled with rock. Based on linear potential theory, a simple semi-analytical solution of the present problem is developed by means of the matched eigenfunction method and the finite element method. Experimental tests are also conducted to validate the theoretical model and examine the wave forces acting on the perforated caisson. The effects of some of the main factors on the total wave forces and the phase difference are examined. Theoretical and experimental studies show that when the total horizontal force reaches its peak, the simultaneous total vertical (upward) force is rather small or even becomes downwards. This is due to the existence of an obvious phase difference between the time histories of the total horizontal and vertical forces, which is an important advantage of perforated caissons over traditional structures.     

不规则波作用下大尺度方形群墩结构的相互影响

本文用谱分析方法对方柱群墩结构在不规则波作用下的相互影响进行了计算。研究了在不同入射波向时每个方墩的群墩影响系数随墩距变化的规律并与折算圆墩的结果进行了比较。入射波谱采用Ｂｒｅｔｓｃｈｎｅｉｄｅｒ谱。计算结果表明方柱墩群与折算圆柱墩群的群墩影响系数较为接近且随着墩间距的增加趋近于孤立墩情形。     

海底管道沉箱基础的复合承载力包络面研究

当负压沉箱被用作深水管汇或管道终端基础时,其长径比常介于1～2之间,而目前的沉箱复合承载力包络面表达式大都针对长径比不超过1的情况,少数覆盖长径比大于1的研究又不适用于土体表层强度非零的情况。采用有限元方法,模拟竖向力、水平力和弯矩共同作用下沉箱基础的响应,采用Probe加载模式获得沉箱的复合承载力包络面。进行大量变动参数分析,针对长径比为1～2的沉箱,讨论了长径比和土体强度分布对单向承载力和包络面的影响,并给出了预测沉箱复合承载力的归一化表达式。     

Investigation of scour effect on tensile capacity of suction caissons considering stress history of sand

Abstract

Since the pull-out response of upwind caissons governs the design of multi-caisson foundations, it is worthwhile to study scour effect on the tensile capacity of suction caissons. The tensile capacity of suction caissons after scour is relevant to the scour depth and the pressure under the caisson lid: the tensile capacity decreases dramatically with increasing scour depth; the smaller the pressure, the stronger the weakening effect of scour. Moreover, the scour effect is investigated in two cases: ignoring stress history and considering stress history. The results show that tensile capacity after scour is larger when the stress history is considered, so ignoring the stress history leads to a conservative design. In order to quantitatively evaluate the effect of scour depth and pressure, an empirical formula for the tensile capacity of suction caissons after scour is proposed based on multiple regression analysis.     

Research on the Effects of in-Line Oscillatory Flow on the Vortex-Induced Motions of A Deep Draft Semi-Submersible in Currents

A Deep Draft Semi-submersible (DDS) under certain flow conditions could be subjected to Vortex-Induced Motions (VIM), which significantly influences the loads on and life fatigue of the moorings and the risers. To investigate the VIM of a DDS with four rectangular section columns in waves coupled with a uniform current, a numerical study using the computational fluid dynamics (CFD) method was conducted. The issues of the VIM of multi-column floaters can be conveniently converted to the issues of oscillating cylinders in fluid cross flows. This paper looks into the CFD numerical simulation of infinite cylinders having rectangular sections in a two-dimensional sinusoidal time-dependent flow field coupled with a uniform current. The resulted hydrodynamic forces and motion responses in different oscillatory flows plus currents both aligned in the same direction for the incidence of 135° of the DDS relative to the flow are compared with the ones in current only cases. The results show that the VIM response of this geometric arrangement of a DDS with four rectangular columns in a current combined with oscillatory flows is more evident than that in the current only case. The oscillatory flows and waves have the significant influence on the VIM response, forces and trajectory, in-plane motions of the DDS.     

Finite Element Analysis of Laterally Loaded Suction Caisson in Anisotropic Clay

1 .IntroductionSuctioncaissons have been widely usedfor offshore oil exploration duetothe advantages of econo-my and simple installation over traditional piles (Huanget al .,2003) .For tensionleg platforms andspar platforms in deep ocean,suction caissons …