海底滑坡对置于海床表面管线作用力的CFD模拟

海底管线是海洋工程中用于传输原油和天然气等的重要通道,通常放置在海床表面或处于悬跨状态。本文采用计算流体动力学CFD法模拟了不同冲击角度下海底滑坡对置于海床表面的海底管线的作用,得到了管线所受的轴向荷载和法向荷载与滑坡冲击角度之间的关系。同时分析了沿冲击方向管线截面形状与管线所受阻力之间的关系。对已有研究进行拓展延伸,丰富了不同工况下轴向阻力系数和法向阻力系数的计算成果,得出了海底滑坡对置于海床表面管线冲击力的计算公式。     

有限高度海底流滑体冲击桩基的数值模拟

海底滑坡长距离流滑过程中,对桩基础的冲击会威胁上部结构的安全.用幂律流体表征流滑体的特性,在雷诺数为0.1～120范围内,采用有限体积法,在三维数值模型中模拟流滑体对桩基础的冲击.考虑流滑体自由表面和自重,推导了新的包含拖曳力项、承载力项和自重项的冲击力表达式,并分析了三项在对冲击力的影响,结果表明:流滑体高度与桩体直...     

深海滑坡灾害的物质点法模拟

深海滑坡可以沿着斜坡搬运大量沉积物,是危害巨大的海洋地质灾害之一。评估深海滑坡的流滑机制并量化分析其对海底结构物的冲击对海洋资源的开发利用至关重要。采用物质点法对滑坡的滑动特性进行了总应力分析。设计了三组不同的土体、海床参数,展现了三种不同的滑动形态:延展、块体和扩张。随后复现了流滑体对部分暴露管线的冲击过程,并对冲击力的稳定值进行了分析,分析中考虑了流滑体的惯性、抗剪强度和静压力的影响。研究表明,物质点法能够用于模拟深海滑坡的流滑过程及其对海底结构物的冲击,可以为海洋工程中的实际设计工作提供服务。     

海底滑坡作用下滩海管道结构安全分析

海底滑坡作为常见的海洋地质灾害,对海洋油气工程安全产生巨大威胁。海床土体失稳引起滑坡体滑动,会对海底管道产生拖曳作用。基于计算流体动力学方法(CFD)建立海底滑坡体对管道作用的评估模型,采用H-B模型描述块状滑坡体并与试验比较验证,分析不同海床倾斜度滑坡对管道的作用并拟合表达式;研究了海底管道在滑坡作用下的力学响应,并采用极限状态方法开展海底滑坡作用下管道结构极限安全分析,探讨了管道埋地状态时的极限安全界限,建立滑坡作用下管道结构安全分析方法。研究表明:滑坡对管道作用力与海床倾角呈现正相关,而覆土层厚度对作用力影响较小;随着不排水抗剪强度的减小,允许的滑坡宽度和速度均增加,表明土体不排水抗剪强度与引起的拖曳力呈正相关;滑坡土体宽度对极限安全速度影响较大。     

珠江口盆地海底滑坡发育特征及成因机制

海底滑坡属于海洋地质灾害的一种主要类型。国内外海洋领域学者对不同地理位置的海底滑坡做过一些建设性的研究工作,国内学者在南海北部海底滑坡的研究中主要关注的是灾害分区和成因分析,而对南海北部珠江口盆地海底滑坡的形态、类型、规模、分布特征等方面的基础性工作还缺乏系统的科学分析。通过2012年以来对珠江口盆地滑坡区海洋土的静力触探试验、十字板剪切试验及室内分析试验,揭示出珠江口盆地海洋土的垂直分布特征,根据不同深度海洋土的工程性质差异可将该区土体自上而下划分为5层。按照海底滑坡所处的地貌单元不同,把珠江口盆地水深160~1280 m范围内发育的滑坡分为4处规模较大的滑坡带,均呈NE-NEE向展布,主要为顺层型滑坡、圆弧型滑坡和崩塌型滑坡3种基本类型,规模从4 km2到800 km2不等,集中分布在北纬19°-22°,东经113°-117°范围内。通过海底地质环境、水文气象环境及力学平衡分析,指出了影响海底滑坡稳定性的主要因素包括地震活动、海流影响及沉积物质的快速堆积3个方面。     

基于SPH深度积分模型的海底滑坡数值分析

海底滑坡会对海洋工程结构物造成严重破坏。滑移速度和距离是量化和分析海底滑坡的两个重要参数。目前BING等计算方法在模拟水下土体流动方面存在局限性,因此通过建立考虑土体固结和侵蚀效应的控制方程,选用摩擦流变模型,采用SPH深度积分算法,对海底滑坡进行了模拟研究。对比不同水深、坡度、接触摩擦系数和侵蚀率条件下的滑移体的速度、高度、长度的时程曲线,整理了最大滑移距离和速度,讨论变化规律。研究成果可为海底滑坡灾害预警和海底管线路由选址提供技术参考。     

XCP投弃式海洋探头阻力系数数值计算及试验研究

以XCP探头为模型,采用有限体积法结合κ-ε两方程湍流模型,对XCP探头阻力系数进行了数值计算,研究了XCP探头的阻力特性。获得了不同雷诺数下阻力系数的变化趋势和水下稳定运动时的极限速度。结果表明阻力系数随着雷诺数的增加而减小,其数值计算结果与海上实测数据基本吻合,验证了该数值计算方法的正确性,可为XCP探头结构设计提供参考。     

海洋激流的观测实验及分析讨论

海洋中存在着冲击力极强的海洋激流,这已是不争的事实.从20世纪70年代起,一些国家就开始对大洋海底激流进行现场观测.据报道,他们所观测到的异常的底层流速的最大值仅为0.4m/s,却被视之为海底激流加以分析研究[1～5].     

海底滑坡与天然气水合物之间的相互关系

海底滑坡是具有巨大危害作用的海洋地质灾害之一,近来发现其与海底天然气水合物的分解有重大的关系。详细分析了天然气水合物的形成和分解对海底沉积物的影响;提出了4种自然因素引起水合物分解,导致海底滑坡的观点;列举了国内外滑坡研究的具体事例;从另外一方面讲,海底滑坡对寻找天然气水合物又具有重要的指示作用。     

小波浪高雷诺数时垂直桩柱的波流力

本文在0.45×105≤Re≤5.5×105,2.94≤KC≤7.85的范围内实验研究了作用于垂直桩柱上的波浪力,比较了阻力系数和惯性力系数在不同雷诺数区域之中的异同,分析了桩柱升力的频率特性,给出了考虑桩柱振动影响在内的桩柱阻力及升力自功率谱的数学表达式.     

An Analytical Solution for the Run-Out of Submarine Debris Flows

Submarine debris flows have a significant impact on offshore and coastal facilities. The unique characteristics of submarine debris flows involve large mass movements and long travel distances over very gentle slopes. To improve our insight and knowledge of the basic mechanism behind submarine debris flows, an analytical model was derived for the mobility of submarine debris flows. This model takes into account the mass change of debris flows induced by deposition, stagnation pressure, and the topography of the depositional area. One case study on the Palos Verdes debris flow proves its ability to predict the run-out distance of a submarine debris flow to a reasonable level of accuracy. On the gentle slopes, the submarine debris flow progressively loses mass due to deposition, which in turn influences the flow velocity. In addition, the results show that the slope angle and spreading angle of the debris depositional zone play important roles in the sliding process.     

Numerical analysis of a cylinder moving through rate-dependent undrained soil

Geo-hazard assessment of the potential damage to a pipeline caused by a submarine landslide requires a quantitative model to evaluate the impact forces on the pipeline. In contrast with typical geotechnical problems, the strain rate within the fast moving, flow-like submarine landslide is typically far higher, which will lead to enhancement of the soil strength and therefore result in larger impact forces. Generally, there are two possible predictive frameworks for strain-rate dependence: a fluid dynamics framework and a geotechnical framework. By comparison of common rheological models adopted in these two different approaches, a unified additive power-law model, a normalised form of the Herschel-Bulkley model from fluid mechanics, is explored in this paper. This model has been used in conjunction with a large deformation finite element approach to investigate the undrained limiting loads on a cylinder moving steadily through inertia-less soft rate-dependent material, in order to quantify the strain-rate effects.The flow mechanism and the effects of the shear-thinning index and Oldroyd number on the shear zones are explored. The calculated resistance factors are compared with the drag coefficients obtained from computational fluid dynamics analysis. The average rate of strain experienced by the soil flowing past the cylinder is estimated for a given flow velocity and an expression in the form of a conventional bearing capacity equation, but with shear strength linked directly to the normalised flow velocity, is proposed to predict the magnitude of the viscous force exerted by the debris flow.     

Submarine debris flow impact on pipelines — Part I: Experimental investigation

Estimating the impact forces exerted by a submarine debris flow on a pipeline is a challenge, and there is room for considerably more work to advance the state of the art. To this end, an experimental program was performed to investigate the impact on two pipeline installation scenarios: 1) suspended pipeline and 2) laid-on-seafloor pipeline. The results and observations from the experimental investigation are discussed. The definition of Reynolds number was modified for non-Newtonian fluids and an ad hoc method was developed to estimate the drag force exerted by an impact perpendicular to the pipe axis. The method may be used in prototype situations to estimate the drag force from submarine debris flow impact on pipelines. The experimental program was complemented by Computational Fluid Dynamics (CFD) analyses, the details of which are discussed in the accompanying paper.     

高雷诺数下近壁圆柱绕流数值模拟与分析

近壁圆柱绕流问题在海底悬跨管道的研究中具有重要的意义。在绕流阻力、升力以及海底土壤的耦合作用下,海底管道所发生的移位、悬跨等现象对于海底管道的安全运行构成了很大的威胁。正确预测各种绕流条件下管流之间的作用力是保证油气管道安全的首要任务。海底管道在极端海洋环境条件下的管、流相互作用为高雷诺数绕流问题,处于高雷诺数下的绕流模拟比处于低雷诺数下的绕流模拟要复杂很多,它需要更精细的网格以及合适的湍流模型。此文对处于悬跨状态下的海底管道进行数值研究,给出不同间隙比下海流绕流海底管道的流场结构形态,分析了间隙比对绕流阻力和绕流升力的影响,为进一步研究海底悬跨管道的受力和变形提供载荷边界数据。     

Safety Assessment of Buried Pipeline during Pile Driving Vibration in Offshore Engineering

To assess the impact of pile driving on adjacent submarine pipelines during the reconstruction of a pier berth, the local damage model of submarine pipelines is established to explore the safety thresholds of the particle peak velocity and horizontal displacement. The results are analyzed and adjusted by the existing standards and the corresponding literatures. Then, a three-dimensional numerical model is presented to assess the feasibility of the construction of piles by the obtained safety limits, in which the nonlinear behavior of the soil and stress–seepage coupling analysis are considered. After the construction, the safety of submarine pipelines is rechecked by the measured value of the particle peak velocity and horizontal displacement. Meanwhile, the propagation law of vibration, the horizontal displacement of underground soil, and the pore pressure during pile driving are explored. The results indicate that the construction of piles of 2# mooring pier did not cause damage to adjacent submarine pipelines. However, the construction of piles of 1# mooring pier which is nearer may cause damage to submarine pipelines.     

Centrifuge study on the runout distance of submarine debris flows

ABSTRACT

In this paper, a series of mini-drum centrifuge experiments on motion of submarine debris flow, which are able to correctly reproduce the self-weight stresses and gravity-dependent processes, are presented. These tests were performed using artificial submarine clay with high water content, from 93 to 149%. The extremely low shear strength made the debris material behave as idealistic lubricating material when it was deposited, resulting in a linear relationship between water content and runout distance of strongly coherent debris flow. On the other hand, the dilation of the flow body and hydroplaning was observed for weakly coherent debris flow, which further increased the mobility of flow body. A densimetric Froude number Fr_d was used to indicate the threshold of hydroplaning, which occurs if the Fr_d is greater than 0.2. Finally, two simple analytical models based on prototype debris flow under 1?g condition was used to validate the experiment results, which further prove the effects of soft marine clay on the high mobility of submarine debris flow. On the other hand, when the water content exceeded 120%, the experiment results deviated from the analytical solution due to the effects of hydroplaning.     

An improved analytical approach for simulating the lateral kinematic distress of deepwater offshore pipelines

Offshore pipelines are critical infrastructures and any possible damage may have devastating financial and environmental consequences. Earthquake-related geohazards (such as strong ground motion, active seismic faults, submarine landslides and debris flows) consist crucial threats that an offshore pipeline has to overcome. The main aim of the current study is to examine analytically a seabed-laid offshore pipeline subjected to a lateral kinematic distress due to a submarine landslide or a debris flow. Extra emphasis is given on the impact of pipe-soil interaction on the pipe response, by the realistic representation of the soil resistance via a tri-linear model. Firstly, the proposed analytical model is validated with a numerical model utilizing the finite-element method. Subsequently, various combinations of soil parameters and loading conditions that affect the examined problem are investigated with realistic input data taken from the offshore section of the high-pressure natural-gas pipeline TAP (Trans Adriatic Pipeline) in the Adriatic Sea. Finally, useful conclusions are drawn regarding the applicability and the efficiency of the proposed approach.