Experimental and Numerical Investigation of Local Scour Around Submarine Piggyback Pipeline Under Steady Current

As a new type of submarine pipeline, the piggyback pipeline has been gradually adopted in engineering practice to enhance the performance and safety of submarine pipelines. However, limited simulation work and few experimental studies have been published on the scour around the piggyback pipeline under steady current. This study numerically and experimentally investigates the local scour of the piggyback pipe under steady current. The influence of prominent factors such as pipe diameter, inflow Reynolds number, and gap between the main and small pipes, on the maximum scour depth have been examined and discussed in detail. Furthermore, one formula to predict the maximum scour depth under the piggyback pipeline has been derived based on the theoretical analysis of scour equilibrium. The feasibility of the proposed formula has been effectively calibrated by both experimental data and numerical results. The findings drawn from this study are instructive in the future design and application of the piggyback pipeline.     

Scour development around submarine pipelines due to current based on the maximum entropy theory

This paper presents the results from laboratory experiments and theoretical analysis to investigate the development of scour around submarine pipeline under steady current conditions. Experiments show that the scour process takes place in two stages: the initial rapid scour and the subsequent gradual scour development stage. An empirical formula for calculating the equilibrium scour depth (the maximum scour depth) is developed by using the regression method. This formula together with the maximum entropy theory can be applied to establish a formula to predict the scour process for given water depth, diameter of pipeline and flow velocity. Good agreement between the predicted and measured scour depth is obtained.     

A Field Investigation on the Effects of Background Erosion on the Free Span Development of a Submarine Pipeline

The safety of submarine pipelines is largely influenced by free spans and corrosions. Previous studies on free spans caused by seabed scours are mainly based on the stable environment, where the background seabed scour is in equilibrium and the soil is homogeneous. To study the effects of background erosion on the free span development of subsea pipelines, a submarine pipeline located at the abandoned Yellow River subaqueous delta lobe was investigated with an integrated surveying system which included a Multibeam bathymetric system, a dual-frequency side-scan sonar, a high resolution sub-bottom profiler, and a Magnetic Flux Leakage(MFL) sensor. We found that seabed homogeneity has a great influence on the free span development of the pipeline. More specifically, for homogeneous background scours, the morphology of scour hole below the pipeline is quite similar to that without the background scour, whereas for inhomogeneous background scour, the nature of spanning is mainly dependent on the evolution of seabed morphology near the pipeline. Magnetic Flux Leakage(MFL) detection results also reveal the possible connection between long free spans and accelerated corrosion of the pipeline.     

Local scour and the laws of scour pit’s shape downstream of debris flow sabo dam

The erosion shape and the law of development of debris flow sabo dam downstream is a weak part in the study on debris flow erosion. The shape and development of scour pit have an important effect on the stability and safety of debris flow sabo dam, which determines the foundational depth of the dam and the design of protective measures downstream. Study on the scouring law of sabo dam downstream can evaluate the erosion range and reasonably arrange auxiliary protective engineering. Therefore, a series of flume experiments are carried out including different debris flow characteristics （density is varying from 1.5 t/m3 to 2.1 t/m~） and different gully longitudinal slopes. The result shows that the scour pit appears as an oval shape in a plane and deep in the middle while superficial at the ends in the longitudinal section, the position of the maximum depth point moves towards downstream with an increase of flume slope angle. The maximum depth of scour pit is mainly affected by the longitudinal slope of gully, density of debris flow, and the characteristics of gully composition （particle size and the viscosity of soil）. The result also indicates that the viscosity of soil will weaken the erosion extent. The interior slopes of scour pit are different between the upstream and the downstream, and the downstream slope is smaller than the upper one. For the viscous and non-viscous sands with the same distribution of gradation, the interior slope of non- viscous sand is smaller than the viscous sand.According to tbe regression analysis on the experimental data, the quantitative relationship between the interior slope of scour pit, slope of repose under water and the longitudinal slope of gully is established and it can be used to calculate the interior slope of scour pit. The results can provide the basis for the parameter design of the debris flow control engineering foundation.     

Experimental Investigation of Local Scour Around Artificial Reefs in Steady Currents

Artificial reef is a man-made object that is deployed purposefully on the seafloor to restore the offshore fishery resources and the ecological environment.To secure its ecological effects,it is important to study the possible instability of artificial reefs,like drifting and reversing caused by burial and scour in different seafloor conditions.In the present study,experiments of local scour around an artificial reef are carried out in steady currents.The effect of the open-area ratios and the open-hole heights of the cubic reefs,and the bottom angles of the triangular reefs on the time-scale of the scour process and the equilibrium scour depth are investigated.The results indicate that for the cubic artificial reef,the scour depth decreases with the increasing open-area ratios,and increases with the increasing open-hole heights.In the present study,the optimal prototype of the cubic reef with an open-area ratio of 0.49 and open-height of 0.7 m produces the minimum scour depth.For the triangular reef,the scour depth increases when the bottom angle increases.Moreover,based on the experimental results,empirical equations of the effects of the cut-opening and the bottom angle on the maximum equilibrium scour depth are proposed.The formulas will provide theoretical support and practical guidance for the optimized design and construction of artificial reefs.     

Effects of spillway types on debris flow trajectory and scour behind a sabo dam

Debris flows are one of the common natural hazards in mountainous areas. They often cause devastating damage to the lives and property of local people. The sabo dam construction along a debris flow valley is considered to be a useful method for hazard mitigation. Previous work has concentrated on the different types of sabo dams such as close-type sabo dam, open-type sabo dam. However, little attention has been paid to the spillway structure of sabo dam. In the paper, a new type of spillway structure with lateral contraction was proposed. Debris flow patterns under four different spillway structures were investigated. The projection theory was employed to predict trajectory of debris flow out from the spillway and to estimate the incident angle and terminal velocity before it plunged into the scour hole behind the sabo dam. The results indicated that the estimated data were in good agreement with the experimental ones. The discrepancy between the estimated and experimental values of main parameters remained below 21.82% (relative error). Additionally, the effects of debris flow scales under different spillway structures were considered to study the scour law. Although the debris flow pattern and scour law behind the sabo dam under different operating conditions was analyzed in this paper, further study on the scour mechanism and the maximum scour depth estimation based on scour theory is still required in the future.     

基于BDS-3 PPP-B2b服务的实时PPP性能分析

基于精密单点定位(PPP)原理及BDS-3 PPP-B2b电文改正模型,分析在使用电文改正信息进行PPP过程中需要注意的2个改正参数间匹配性问题,以及静态、仿动态条件下经过PPP-B2b电文信息改正后B1C+B2a、B1I+B3I模式定位服务性能。结果表明,在静态条件下,2种定位模式水平、高程方向定位精度均优于11 cm;仿动态条件下,水平、高程方向定位精度均优于22 cm,在15 min内均能达到水平、高程方向分别不超过0.3 m、0.6 m的精度要求;B1C+B2a组合在定位精度和收敛速度方面均略优于B1I+B3I。     

The influence of upstream slope on the local scour at drop structure

The drop structure will fail as a result of local scoring downstream. This paper discusses the influence of a drop structures’ upstream slope to local scour. Empirical equations of the scour hole were developed by laboratory experiment, theoretical assumptions, and regression analysis. These equations include the maximum scour depth and length during the scouring period, the maximum equilibrium scour depth and length, and the unit width scour rate. The four channel slopes (0%, 2%, 4%, and 6%) before the drop structure has been included in the analysis. A series of laboratory experiments were conducted to obtain 48 groups of experiments and 419 scour hole profiles during the scouring period. The material used in the scour section is uniform non-cohesive and with a median diameter of d ₅₀ = 0.5 mm. The results have been used to develop empirical equations via regression analysis to determine the coefficients of theoretical equations. The high correlation coefficient indicates that the equations developed in this study are suitable for verifying the characteristics of a scour hole at drop structure in the sloped channel. The semi-empirical equation is more accurate than the empirical equation. Compared to a horizontal channel, a sloped channel tends to cause a greater equilibrium maximum scour length, shorter equilibrium maximum scour depth, and faster unit-wide scour rate.     

枣庄市台儿庄区小龚庄岩溶地下水水源地允许开采量计算评价

科学有效的地下水资源评价是可持续开发地下水资源的关键,其中地下水资源可开采量的评价是地下水资源评价的核心内容。该文在对枣庄市台儿庄区小龚庄水源地水文地质条件、岩溶塌陷发育情况等进行详细分析的基础上,采用开采试验法对小龚庄水源地岩溶地下水允许开采量进行了计算,计算结果为11354m~3/d。根据已发生岩溶塌陷情况,设定岩溶塌陷预警水位,利用非稳定流Theis公式计算了安全开采量范围在11187～13150m~3/d之间,对开采试验法计算结果进一步进行评价。根据评价结果最终确定以11354m~3/d作为小龚庄水源地的允许开采量是可靠的,可以保证水源地最大限度的开采地下水资源,同时又保证水源地运行时不易引发岩溶塌陷的环境地质问题。该方法为同类型水源地地下水允许开采量的计算评价提供了一定的借鉴。     

沿山脊走向长输油气管道强震动力响应特征

沿山脊走向的长输油气管道在我国西部山区常有分布,强震作用下其动力响应直接关系到管道的安全运营,实际管道工程建设中亟需相关研究结果提供指导。依托云南玉溪龙马槽村段此类输油管道工程,考虑山坡的工程地质特征,对管道与坡体进行整体三维数值建模,采用有限差分方法FLAC3D进行数值模拟,基于汶川地震波,计算得到了水平地震加速度峰值、震后等典型时刻的管道位移、轴力、剪力和弯矩及坡体稳定性等地震动力响应特征。结果表明,管道内力最大值出现于震后时刻,地震作用使管道轴力达到较高水平,而剪力与弯矩值的量级仅占最大轴力的约1%;竖向地震波对管道内力影响较小,管道受力的不利部位出现在其与断层交界附近。地震过程中管道水平方向存在较多的弹性变形,竖向则存在较多的塑性变形,管道累计变形是影响其内力的主要因素,管道内力最大值出现于震后时刻。      

Thermal state of soils in the active layer and underlain permafrost at the kilometer post 304 site along the China-Russia Crude Oil Pipeline

On-site monitoring is very important for understanding formation mechanisms of frost hazards frequently occurring in pipeline foundation soils and for designing and deploying according mitigative measures in permafrost regions.Significant thaw subsidence of ground surfaces along the ChinaRussia Crude Oil Pipeline(CRCOP) from Mo'he to Daqing,Heilongjiang Province,Northeast China have been observed at some segments underlain by ice-rich warm(1.0°C) permafrost since the official operation in January 2011.Recent monitoring results of the thermal states of foundation soils at the kilometer post(KP) 304 site along the CRCOP are presented in this paper.The results indicate that during the period from 2012 to 2014,shallow soils(at the depths from0.8 to 4.0 m from ground surface) has warmed by approximately 1.0°C in the lateral range of 1.2 to 2.1 maway from the pipeline axis,and deeper permafrost(such as at the depth of 15 m,or the depth of zero annual amplitude of ground temperatures) by 0.08°C per year 4 m away from the pipe axis,and 0.07°C per year 5 m away from the pipeline axis.The results indicate an all-season talik has developed around and along the CRCOP.The thaw bulb,with a faster lateral expansion(compared with the vertical growth),enlarges in summer and shrinks in winter.This research will provide important references and bases for evaluating thermal influences of warm pipeline on permafrost and for design,construction,operation and maintenance of pipelines in permafrost regions.     

Numerical study on global motion of Truss Spar in frequency and time domains for the Liwan 3-1 area

Using frequency and time domain analysis, the authors analyzed the hydrodynamics and motion behavior of a Truss Spar platform at a water depth of 1500 m in the Liwan 3-1 area of the South China Sea. Firstly, the seakeeping ability is acquired in the frequency domain by calculating the hull’s hydrodynamics and comparing with a semi-submersible platform. The random wave analysis for 100-year, 10-year and 1-year return periods in Liwan 3-1 distinctly shows lower heave but larger surge and pitch re-sponses of the Truss Spar than those of a semi-submersible. Secondly, 3-hour motions of the Truss Spar are predicted and compared in the time domain under 100-year return period conditions in Liwan 3-1 and the Gulf of Mexico. Thirdly, the hull/mooring line cou-pled and uncoupled models are compared. Finally, the responses of the Truss Spar under 10-year and 1-year return period conditions are assessed. The results reveal that the mooring line damping reflected by the coupled model distinctly decreases the low frequency motion. The maximum heave response for 100-year return period waves is 1.23m and below 0.1m for the case of 1-year return period.     

Quality Assessment of Frozen Solenocera crassicornis Treated with Sodium Metabisulphite by Soaking or Spraying

The present work was carried out to evaluate the safety of shrimp(Solenocera crassicornis)treated with different concentrations of sodium metabisulfite(SMB)by soaking or spraying during frozen storage.Shrimps soaked in higher concentrations of SMB showed higher sensory scores,lower total color differences,and better anti-melanosis effects than shrimps in the control and other treatment groups throughout frozen storage(−18℃).Lower total volatile basic nitrogen and thiobarbituric acid reactive substances and higher salt soluble protein contents were detected in shrimp soaked with high doses of SMB compared with other samples.In addition,lower counts of total aerobic plates and psychrotrophic bacteria were observed in shrimp treated by soaking with higher doses of SMB than those in control shrimp and shrimp treated with other methods during frozen storage(−18℃).However,the SO2 content of 5%SMB-soaked samples exceeded the maximum allowable limit of 100mgkg−1.Overall,the use of 1.5%SMB soaking to treat shrimp results in good antioxidant and antimicrobial effects and,thus,may be suggested to preserve S.crassicornis under frozen conditions.The results of this study present important guidance on the use of SMB to maintain the quality of marine-trawling shrimp from manufacturing to consumption.     

从研究岩溶地下水角度分析评价岩溶塌陷的危险性——以南宁坛洛塌陷为例

南宁市是广西岩溶地面塌陷危害十分严重的城市之一。2012年6月2日突发的岩溶地面塌陷主要集中分布在坛洛镇南西方向,形成塌陷坑19个,地裂缝17条,威胁62户共232人,潜在经济损失约870万元。坛洛塌陷多形成于松散盖层厚度小于15m,地下水位埋深小于20m,地下岩溶强发育的区段。碳酸盐岩岩溶发育、松散盖层厚度较小和水动力条件(降水、地表水和地下水)复杂为岩溶塌陷产生的必要条件,尤以地下水的作用对岩溶塌陷的形成起至关重要的作用。通过对坛洛地下水动态观测与岩溶塌陷的调查研究确定地下水强径流带,分析评价岩溶地面塌陷的危险性,为岩溶地区的岩溶塌陷危险性勘察、评价工作提供一定指导意义。     

Numerical modeling of local scour around hydraulic structure in sandy beds by dynamic mesh method

Local scour, a non-negligible factor in hydraulic engineering, endangers the safety of hydraulic structures. In this work, a numerical model for simulating local scour was constructed, based on the open source code computational fluid dynamics model OpenFOAM. We consider both the bedload and suspended load sediment transport in the scour model and adopt the dynamic mesh method to simulate the evolution of the bed elevation. We use the finite area method to project data between the three-dimensional flow model and the two-dimensional (2D) scour model. We also improved the 2D sand slide method and added it to the scour model to correct the bed bathymetry when the bed slope angle exceeds the angle of repose. Moreover, to validate our scour model, we conducted and compared the results of three experiments with those of the developed model. The validation results show that our developed model can reliably simulate local scour.     

Dynamic performance of a semi-submersible platform subject to wind and waves

By applying experimental and numerical simulations, the motion performance of a semi-submersible platform with mooring positoning system under combined actions of wind and waves is studied. The numerical simulation is conducted by the method of nonlinear time domain coupled analysis, and the mooring forces are calculated by the piecewise extrapolating method. The scale in the model experiment is 1:100, and the mooring system of the model is designed with the method of equivalent water-depth truncation by comparing the numerical and the experimental results, the platform motion and mooring forces subject to wind and waves are investigated. The results indicate that the numerically simulated mooring forces agree well with the experimental results in static equivalent field, but show some difference in dynamic equivalent field; the numerically simulated platform motions coincide well with the experimental results. The maximum motion of the platform under operating conditions is 20.5 m. It means that the horizontal displacement is 2% less than the water depth, which satisfies the operating requirements.     

Liquefaction of sand under low confining pressure

Undrained behaviour of sand under low cell pressure was studied in static and cyclic triaxial tests. It was found that very loose sand liquefies under static loading with the relative density being a key parameter for the undrained behaviour of sand. In cyclic triaxial tests, pore water pressures built up during the cyclic loading and exceeded the confining cell pressure. This process was accompanied by a large sudden increase in axial deformation. The necessary number of cycles to obtain liquefaction was related to the confining cell pressure, the amplitude of cyclic loading and the relative density of sand. In addition, the patterns of pore water pressure response are different from those of sand samples with different relative densities. The test results are very useful for expounding scour mechanism around coastal structures since they relate to the low stress behaviour of the sand.     

Numerical Simulation for Shallow Strata Stability of Coral Reef in the Southwest of Yongshu Reef (South China Sea)

In recent years, infrastructure construction on coral reefs has been increasingly developed. Therefore, the shallow strata stability of coral reefs in the South China Sea should be evaluated. This study aims to investigate the profiles for the shallow strata of coral reefs in the southwest of the Yongshu Reef, particularly in the hydrodynamic marine environment, and to establish a geological model for numerical simulation using Geo-Studio. The shallow strata of the coral reefs include mass gravel, sand gravel, mid-coarse sand, silty sand, fine sand, and reef limestone. The shallow reef slope near the lagoon is similar to a ‘layer cake', in which the side close to the sea is analogous to a ‘block cake'. The simulation results showed that coral reef stability depends on wave loads and earthquake strength and on the physical properties of coral reefs. The factor of safety for the outer reef is greater than 10.0 under static, wave, and seismic conditions; this indicated that the outer reefs were less affected by waves and earthquakes. However, the factor of safety next to the lagoon varied from 0.1 to 5.3. The variation was primarily caused by the thick strata of coral reefs close to the sea(reef limestone, typically with the thickness 10 m and equivalent to a block). The soil and rock layers in the coral reef strata with thicknesses 10 m showed weak engineering geological characteristics. Our findings can provide useful information to future construction projects on coral reefs.     

Extraction of astaxanthin from Euphausia pacific using subcritical 1, 1, 1, 2-tetrafluoroethane

Euphausia pacific is an important source of natural astaxanthin.Studies were carried out to assess the extractability of astaxanthin from E.pacific using subcritical 1,1,1,2-tetrafluoroethane(R134a).To examine the effects of multiple process variables on the extraction yield,astaxanthin was extracted under various conditions of pressure(30-150 bar),temperature(303-343 K),time(10-50 min),flow rate(2-10 g min-1),moisture content(5.5%-63.61%),and particle size(0.25-0.109 mm).The results showed that the extraction yield increased with temperature,pressure,time and flow rate,but decreased with moisture content and particle size.A maximum yield of 87.74% was obtained under conditions of 100 bar,333 K,and 30 min with a flow rate of 6 g min-1 and a moisture content of 5.5%.The substantial astaxanthin yield obtained under low-pressure conditions demonstrates that subcritical R134a is a good alternative to CO 2 for extraction of astaxanthin from E.pacific.     

Evaluation of the stability of vegetated slopes according to layout and temporal changes

Vegetation in slopes can effectively improve slope stability.However,it is difficult to estimate the effects of vegetation on slope stability because of variations in plant species and environmental conditions.Moreover,influences of plant growth on slope stability change with time,resulting in changes in the safety factor.This study was conducted to evaluate the stability of vegetated slopes with time and investigate the effects of different layouts of plant species on slope stability.Here,we used a plant growth model and slope stability analysis to build an evaluation model.To accomplish this,one species of tree,shrub and grass was chosen to set six layout patterns.A slope with no vegetation served as a control.The safety factors of the seven slopes were then calculated using the developed evaluation model and differences in the safety factors of slopes were compared and discussed.The slope vegetated with Platycladus orientalis reached the most stable state at the age of 60 years.Shrub slope(Vitex negundo)had the maximum safety factor after 20 years.Overall,the safety factor of vegetated slopes increased from 12.1%to 49.6% compared to the slope with no vegetation.When wind force was considered,the safety factor value of the slope changed from 3.5%to 43.5%.Vegetation mixtures of trees and grasses resulted in the best slope stability.Planting grasses on slopes can improve slope stability of trees to a greater degree than that of slopes with shrubs in the early stage of growth.