Occurrence of spanning of a submarine pipeline with initial embedment

For better understanding the mechanism of the occurrence of pipeline span for a pipeline with initial embedment, physical and numerical methods are adopted in this study. Experimental observations show that there often exist three characteristic phases in the process of the partially embedded pipeline being suspended: (a) local scour around pipe; (b) onset of soil erosion beneath pipe; and (c) complete suspension of pipe. The effects of local scour on the onset of soil erosion beneath the pipe are much less than those of soil seepage failure induced by the pressure drop. Based on the above observations and analyses, the mechanism of the occurrence of pipeline spanning is analyzed numerically in view of soil seepage failure. In the numerical analyses, the current-induced pressure along the soil surface in the vicinity of the pipe (i.e. the pressure drop) is firstly obtained by solving the N-S equations, thereafter the seepage flow in the soil is calculated with the obtained pressure drop as the boundary conditions along the soil surface. Numerical results indicate that the seepage failure (or piping) may occur at the exit of the seepage path when the pressure gradient gets larger than the critical value. The numerical treatment provides a practical tool for evaluating the potentials for the occurrence of pipe span due to the soil seepage failure.

近海岩土工程

<正>本组的主要内容是围绕近海(Offshore)(又称“离岸”)的勘探、开发和建设中所涉及的岩土力学和基础工程的问题。除了考虑海洋资源的提取(矿物、生物、化学等)、能源的利用(潮汐、风力和海水温差发电、太阳能利用等)以及各种沿海的工程建设外,当前我国正在大力开展近海石油和天然气的勘探和开发工作。

Vertical Bearing Capacity of a Partially-Embedded Pipeline on Tresca Soils

Slip-line field solutions are presented for the ultimate load of submarine pipelines on a purely cohesive soil obeying Tresca yield criterion, taking into account of pipe embedment and pipe-soil contact friction. The derived bearing capacity factors for a smooth pipeline degenerate into those for the traditional strip-line footing when the embedment approaches zero. Parametric studies demonstrate that the bearing capacity factors for pipeline foundations are significantly influenced by the pipeline embedment and the pipe-soil frictional coefficient. With the increase of pipeline embedment, the bearing capacity factor Nc decreases gradually, and finally reaches the minimum value (4.0) when the embedment equals to pipeline radius. As such, if the pipeline is directly treated as a traditional strip footing, the bearing capacity factor Nc would be over evaluated. The ultimate bearing loads increase with increasing pipeline embedment and pipe-soil frictional coefficient.

Effects of the Spudcan Penetration on the Adjacent Foundations

Abstract: To study the effects of spudcan penetration on the adjacent foundations of offshore platforms, experiments and numerical simulations (using business software ABAQUS) are carried out. It is shown that the penetration of spudcan can cause the soil layer affected in an annular zone. The affected zone has a maximum width of one times the diameter of the spudcan. The deflection of the platform’s foundation increases with the penetration of spudcan. The smaller the density of soil layer is, the bigger the displacement of the foundation is. However, the maximum displacement at the top of the foun- dation changes little once the penetration depth is over a critical value. The bigger the diameter and the penetration depth of the spudcan are, the bigger the displacements of the foundation are.

基于海洋结构谱响应波浪载荷参数识别

波浪谱是研究随机波浪的主要方法,也是海洋工程设计的重要依据.介绍在线性波情况下圆柱形结构响应施加了白噪声之后频域内的载荷识别方法,根据谱分析的方法,考虑线性波前后桩腿相位差的影响,比较单个频点反演与采样频率内利用对频率取矩法反演结果的准确度,结果表明在已知波浪谱的情况下利用矩来进行反演识别有明显的优越性.

A Dynamic Model For Ice-Induced Vibration Of Structures

A dynamic model for the ice-induced vibration (IIV) of structures is developed in the present study. Ice properties have been taken into account, such as the discrete failure, the dependence of the crushing strength on the ice velocity, and the randomness of ice failure. The most important prediction of the model is to capture the resonant frequency lock-in, which is analog to that in the vortex-induced vibration. Based on the model, the mechanism of resonant IIV is discussed. It is found that the dependence of the ice crushing strength on the ice velocity plays an important role in the resonant frequency lock-in of IIV. In addition, an intermittent stochastic resonant vibration is simulated from the model. These predictions are supported by the laboratory and field observations reported. The present model is more productive than the previous models of IIV.

Food composition and dietary overlap of macroinvertebrates in a shallow eutrophic lake in China: spatial and temporal variations

The present study was conducted in Lake Donghu, a suburban eutrophic lake arising from the middle reaches of the Yangtze River, China. Food composition of 32 taxa of zoobenthos was analyzed from 1251 gut samples. Macroinvertebrate primary consumers ingested mainly detritus, sand grains and diatoms. The predators primarily preyed on rotifers, crustaceans, oligochaetes and chironomid larvae. The dietary overlap was relatively high among collector taxa but low among other macroinvertebrates. Food composition and dietary overlap of macroinvertebrates changed considerably, both spatially and temporally. Food web structure differed between inshore and offshore regions of Lake Donghu. The inshore web was relatively complex and dynamic whereas the offshore web was simple and stable. Taxon-specific changes of diet seem to have little effect on the benthic food web structure in offshore waters of a eutrophic lake.

On the radiation and diffraction of linear water waves by an infinitely long rectangular structure submerged in oblique seas

The radiation and diffraction of linear water waves by an infinitely long rectangular structure submerged in oblique seas of finite depth is investigated. The analytical expressions for the radiated and diffracted potentials are derived as infinite series by use of the method of separation of variables. The unknown coefficients in the series are determined by the eigenfunction expansion matching method. The expressions for wave forces, hydrodynamic coefficients and reflection and transmission coefficients are given and verified by the boundary element method. Using the present analytical solution, the hydrodynamic influences of the angle of incidence, the submergence, the width and the thickness of the structure on the wave forces, hydrodynamic coefficients, and reflection and transmission coefficients are discussed in detail.

Scattering of linear water waves by a fixed and infinitely long rectangular structure parallel to a vertical wall in oblique seas

The scattering of linear water waves by an infinitely long rectangular structure parallel to a vertical wall in oblique seas is investigated. Analytical expressions for the diffracted potentials are derived using the method of separation of variables. The unknown coefficients in the expressions are determined through the application of the eigenfunction expansion matching method. The expressions for wave forces on the structure are given. The calculated results are compared with those obtained by the boundary element method. In addition, the influences of the wall, the angle of wave incidence, the width of the structure, and the distance between the structure and the wall on wave forces are discussed. The method presented here can be easily extended to the study of the diffraction of obliquely incident waves by multiple rectangular structures.

Response amplitude and hydrodynamic force for a buoy over a convex

A buoy as an offshore structure is often placed over a convex such as a caisson or a submerged island. The hydrodynamic fluid/solid interaction becomes more complex due to the convex compared with that on the flat. Both the buoy and the convex are idealized as vertical cylinders. Linear potential theory is used to investigate the response amplitude and the hydrodynamic force for a buoy over a convex due to diffraction and radiation in water of finite depth. These are derived from the total velocity potential. A set of theoretical added mass, damping coefficient, and exciting force expressions have been proposed. Analytical results of the response amplitude and hydrodynamic force are given. Finally, the numerical results show that the effect of the convex on the response amplitude and hydrodynamic force for the buoy is ignored if the size of the convex is relatively smaller.

大型近海水平轴风力机转轮的空气动力学性能优化判据

以近海风能工程为研究目标,对具有不同特性参数(设计风速、叶尖线速度和转轮实度)的大容量(1~10 MW)风力机转轮的气动性能与几何特性进行分析与研究.首先提出大型机组转轮气动性能优化判据:在其直径最小的前提下具有尽可能高的年可用风能特性因数以及与之相关的风能利用系数,因而可捕获最多风能,使年发电量最大.接着给出影响它的几个主要气动参数,如转轮设计风速、叶尖线速度以及转轮实度,并分析风力机在近海气象条件下运转时上述两个气动指标随这些参数变化的规律.提供的气动分析方法及结果可作为大型近海风力机转轮气动性能的评价基础.

PHYSICAL MODEL FOR SMALL SCALE SANDWAVES MIGRATION IN NORTH GULF OF SOUTH CHINA SEA

The aim of this paper is to investigate the mechanism of small scale sand-wave migration. According to the environmental characteristic of the north gulf of South China Sea, a quasi-3D mechanics model has been built for simulating the small scale sand wave migration. The calculation results are shown to be consistent with the observed data in the trough of sand ridge. Considering the effect of environmental actions and sand wave features, we develop an effective formula to predict sand-wave migration. It is indicated that the physical models should be used to predict the migration of the small scale sand-wave, which is rarely dominated by wave activity.

PREDICTION OF VORTEX-INDUCED VIBRATION OF FLEXIBLE RISER USING AN IMPROVED WAKE-OSCILLATOR MODEL

Based on improving the wake-oscillator model, an analytical model for vortex-induced vibration (VIV) of flexible riser under non-uniform current is presented, in which the variation of added mass at lock-in and the nonlinear relationship between amplitude of response and reduced velocity are considered. By means of empirical formula combining iteration computation, the improved analytical model can be conveniently programmed into computer code with simpler and faster computation process than CFD so as to be suitable to application of practical engineering. This model is validated by comparing with experimental result and numerical simulation. Our results show that the improved model can predict VIV response and lock-in region more accurately. At last, illustrative examples are given in which the amplitude of response of flexible riser experiencing VIV under action of non-uniform current is calculated and effects of riser tension and flow distribution along span of riser are explored. It is demonstrated that with the variation of tension and flow distribution, lock-in region of mode behaves in different way, and thus the final response is a synthesis of response of locked modes.

INVESTIGATING A NEAR-BED SUBMARINE PIPELINE IN A CURRENT

This paper considers the lift forces acting on a pipeline with a small gap between the pipeline and the plane bottom or scoring bottom. A more reasonable fluid force on the pipeline has been obtained by applying the knowledge of modified potential theory (MPT), which includes the influences of the downstream wake. By finite element method, an iteration procedure is used to solve problems of the nonlinear fluid-structure interaction. Comparing the deflection and the stress distributions with the difference sea bottoms, the failure patterns of a spanning pipeline have been discussed. The results are essential for engineers to assess pipeline stability.