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1.
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. 相似文献
2.
Submarine pipelines that transport crude oil and natural gas are often in a complex marine geological environment and may become unstable and fail upon impact by submarine landslides. Previous research has mostly focused on the impact forces exerted by submarine landslides on suspended pipelines, but the impact of submarine landslides on pipelines laid on the seafloor at various impact angles, θ, have been relatively infrequently discussed, and the effects of suspended height, H, on the impact forces exerted by submarine landslides on pipelines have not been thoroughly investigated. In this study, based on the Herschel–Bulkley model, the impact forces exerted by a submarine landslide on laid-on or suspended pipelines at various impact angles θ were simulated using the computational fluid dynamics (CFD) approach. Equations for calculating the axial and normal drag coefficients of a submarine pipeline were proposed. The CFD numerical simulation results were rearranged based on the soil mechanics approach. By comparing the parameters, an essentially corresponding relationship was found between the soil mechanics and CFD approaches when the equations were used to calculate the impact forces exerted by a submarine landslide on a pipeline. In addition, a semi-analytical expression for the failure envelope was provided. Furthermore, the effects of H on the forces on a pipeline were discussed, and an equation for calculating the acting forces on a pipeline along the flow direction of a submarine landslide that comprehensively accounts for the effects of θ and H was proposed. The lift force was discussed preliminarily and the results provide a basis for further investigation. The achievement of this study is applicable for selecting locations of submarine pipeline routes and for designing submarine pipelines. 相似文献
3.
4.
Computational Fluid Dynamics (CFD) numerical analysis was employed to analyze the situations tested experimentally, as described in Part I. The methodology and results of the CFD analyses are discussed and compared with the observations made from the experiments. The numerical model performed satisfactorily with regard to obtaining the impact forces exerted on the model pipe as well as simulating the hydroplaning phenomenon and estimating slurry flow heights. The experimental results were combined with the results of the CFD analyses to develop a practical method to compute the drag force caused by a submarine debris flow impact on a pipeline. The CFD analyses provided some insight to the separated region characterization, but the attempt to analyze the vortex shedding phenomenon as observed in the experiments was unsuccessful. Additional studies are required for better understanding of both the separated region characteristics and vortex shedding. 相似文献
5.
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。 相似文献
6.
Modelling of flow around a near-bed pipeline with a spoiler 总被引：1，自引：0，他引：1
Flow around a pipeline with and without a spoiler near a smooth wall is simulated by solving the Navier–Stokes equations. Finite-difference formulation with a second-order upwind scheme in a curvilinear coordinate system is employed. The influences of the spoiler on hydrodynamic forces, pressure distribution, vortex shedding frequency, velocity profile under the pipe, as well as shear stress on the wall are investigated. The attachment of a spoiler significantly increases drag, root-mean-square (RMS) lift, flow through the gap between the pipe and the wall and shear stress on the seabed around the pipe. The spoiler also generates a non-zero mean downward force on the pipeline, which may enhance the self-burial of the pipeline. 相似文献
7.
The present paper proposes a numerical model to determine horizontal and vertical components of the hydrodynamic forces on a slender submarine pipeline lying at the sea bed and exposed to non-linear waves plus a current. The new model is an extension of the Wake II type model, originally proposed for sinusoidal waves (Soedigdo et al., 1999) and for combined sinusoidal waves and currents (Sabag et al., 2000), to the case of periodic or random waves, even with a superimposed current. The Wake II type model takes into account the wake effects on the kinematic field and the time variation of drag and lift hydrodynamic coefficients. The proposed extension is based on an evolutional analysis carried out for each half period of the free stream horizontal velocity at the pipeline. An analytical expression of the wake velocity is developed starting from the Navier–Stokes and the boundary layer equations. The time variation of the drag and lift hydrodynamic coefficients is obtained using a Gaussian integration of the start-up function. A reduced scale laboratory investigation in a large wave flume has been conducted in order to calibrate the empirical parameters involved in the proposed model. Different wave and current conditions have been considered and measurements of free stream horizontal velocities and dynamic pressures on a bottom-mounted pipeline have been conducted. The comparison between experimental and numerical hydrodynamic forces shows the accuracy of the new model in evaluating the time variation of peaks and phase shifts of the horizontal and vertical wave and current induced forces. 相似文献
8.
A new normal mode spectral analysis method is presented for calculating r.m.s. riser deflections, bending stresses and lower ball joint angles. Forces on the riser consist of: (a) non-linear fluid drag taking account of the relative velocity due to tethered buoyant platform (TBP) motion, riser elastic deflection and wave induced fluid velocity, (b) wave induced fluid acceleration, (c) inertia forces due to TBP acceleration, and (d) buoyancy. The non-linear fluid drag forces are linearized using Tung and Wu's approximation based on the r.m.s. relative fluid velocity and current. A wide range of results is presented for risers in water depths up to 1000 m and it is observed that 6 normal modes are sufficient for calculating bending stresses. A static analysis is also presented for bending stresses due to wave and current induced drag forces and riser offset. 相似文献
9.
Extensive use of autonomous underwater vehicles (AUVs) in oceanographic applications necessitates investigation into the hydrodynamic forces acting over an AUV hull form operating under deeply submerged condition. This paper presents a towing tank-based experimental study on forces and moment on AUV hull form in the vertical plane. The AUV hull form considered in the present program is a 1:2 model of the standard hull form Afterbody1. The present measurements were carried out at typical speeds of autonomous underwater vehicles (0.4-1.4 m/s) by varying pitch angles (0-15°). The hydrodynamic forces and moment are measured by an internally mounted multi-component strain gauge type balance. The measurements were used to study variation of axial, normal, drag, lift and pitching moment coefficients with Reynolds number (Re) and angle of attack. The measurements have also been used to validate results obtained from a CFD code that uses Reynolds Average Navier-Stokes equations (ANSYS™ Fluent). The axial and normal force coefficients are increased by 18% and 195%; drag, lift and pitching moment coefficients are increased by 90%, 182% and 297% on AUV hull form at α=15° and Rev=3.65×105. These results can give better idea for the efficient design of guidance and control systems for AUV. 相似文献
10.
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。 相似文献
11.
Measurements are presented of the effects of corner radius on the hydrodynamic forces experienced by cylindrical bluff bodies in oscillatory flow over the Keulegan Carpenter number range from 1 to 100. The range of Reynolds number covered is from 200 to 2 × 104 and two basic cross-sectional shapes, namely a square and a diamond, were used in the investigation. Drag coefficients in oscillatory flow are found to be more sensitive to corner radius than in steady flow. At a fixed Keulegan Carpenter number the drag coefficient of a diamond section decreases with increasing corner radius and the reduction is more marked at low Keulegan Carpenter numbers. The drag of a square section does not follow a consistent trend with corner radius and for high Keulegan Carpenter numbers the rounded square with a corner radius of 0.265D (where D is the width of the model normal to the flow) can have a drag coefficient equal to half the value for a circular cylinder. 相似文献
12.
Recently, the security and stability of submarine pipelines have attracted much attention in ocean engineering. In this paper, pipelines with a streamlined contour (wedge, airfoil, double-ellipse, and arc-angle hexagon) are designed in hopes of defending against the impact of submarine landslides, and the computational fluid dynamics (CFD) approach is used to investigate the interaction between submarine landslides and streamlined pipelines. The results show that the peak interactional force is more representative of the hazard level of pipelines imposed by submarine landslides. It is also found that the streamlined pipelines possess a significant advantage in reducing the drag force and lift force of landslide–pipeline interaction with a maximum lessening percentage of 66.32 and 40.17%, compared with a conventional circular pipeline. In addition, the influence of applying streamlined pipelines to engineering is briefly discussed, and the empirical equation for estimating the drag force and lift force of streamlined pipelines induced by landslides is recommended based on the numerical test results. 相似文献
13.
鉴于海底管道的服役水深越来越深,主要采用犁式挖沟机对预铺设于海床之上的海底管道采取后挖沟的方式将海底管道埋设于海床之下,以保护其免受不必要的损伤。针对后挖沟深度H是海底挖沟机的重要设计参数,也是影响管道悬跨的重要因素的问题,对SMD(UK)犁式挖沟机展开参数优化,确保作业过程中悬跨段管道在外部静水压力作用下,海底管道不会发生屈曲破坏。采用ABAQUS软件,分别建立了作业前和作业中两种工况下的悬跨模型,分析机械手对接触部分管道的损伤,结果显示,作业中的机械手对悬跨管道的损伤更大;同时,建立了作业中不同管径下,后挖沟深度对管道损伤的安全裕量关系曲线。进一步,结合作业中不同挖沟深度下的管跨段屈曲数值模型,对处于外部静水压力作用下的悬跨管的屈曲失效展开分析,结果显示,随着后挖沟深度的加大,不同管径下的悬跨段管道局部出现塑性压溃的临界压力值不断降低;管道外径的增大,降低了同一后挖沟深度下发生屈曲失效的压力值。最后,在后挖沟深度与外部静水压力组成的区域内,建立屈曲失效临界关系曲线,并划分出工作区和压溃区,为深海管道后挖沟埋管的施工提供工程参考。 相似文献
14.
Owing to the complex environmental conditions, suspension could induce complicated forces on submarine pipelines and even cause vortex-induced vibration, resulting in fatigue damage of pipelines. Through aiming at the 28-inch submarine pipeline in the East China Sea, the pipeline was segmented according to the similarity, considering the factors of pipe assembly, typhoon, current, wave and seabed topography. The effects of span length on natural frequency in each section of submarine pipeline were analyzed by finite element model. The maximum safe span length allowed by each pipeline section was verified by fatigue cumulative damage theory, and the fatigue life of each pipeline section were predicted. The results showed that each order natural frequency of the pipeline decreased with the increase of span length. The calculated results of empirical formulas were much smaller than those of the FEM analysis. The increase of the gap between the suspended pipeline and the seabed was beneficial to enhance the fatigue life of the suspended pipeline. 相似文献
15.
In this paper a nonlinear dynamic PDE formulation for a pipe string suspended from a pipelay vessel to the seabed in a pipelay operation is developed. This model extends a three-dimensional beam model capable of undergoing finite extension, shearing, twist and bending, to apply for marine applications by adding the effects of restoring forces, hydrodynamic drag and seabed interaction. The model is validated against the natural catenary equation and the FEM code RIFLEX. The model is extended to include the pipelay vessel dynamics by applying a potential theory formulation of a surface vessel, suited for dynamic positioning and low speed maneuvering, as a boundary condition for the PDE. This system is found to be input-output passive and stable. Pipeline installation applications where the presented model is suited are e.g., analysis and simulation of the installation operation, operability analysis, hardware-in-the-loop (HIL) testing for vessel control systems, and automation of the pipelay operation. 相似文献
16.
The spectral properties of nonlinear drag forces of random waves on vertical circular cylinders are analyzed in this paper by means of nonlinear spectral analysis. The analysis provides basic parameters for estimation of the characteristic drag forces. Numerical computation is also performed for the investigation of the effects of nonlinearity of the drag forces.The results indicate that the wave drag forces calculated by linear wave theory are larger than those calculated by the third order Stokes wave theory for given waves. The difference between them increases with wave height. The wave drag forces calculated by use of hnear approximation are about 5% smaller than their actual values when measured in the peak values of spectral densities. This will result in a safety problem for the design of offshore structures. Therefore, the nonlinear effect of wave drag forces should be taken into comidemtion in design and application of important offshore structures. 相似文献
17.
In this study an Euler-Euler two-phase model was developed to investigate the tunnel erosion beneath a submarine pipeline exposed to unidirectional flow. Both of the fluid and sediment phases were described via the Navier-Stokes equations, i.e. the model was implemented using time-averaged continuity and momentum equations for the fluid and sediment phases and a modified k−ε turbulence closure for the fluid phase. The fluid and sediment phases were coupled by considering the drag and lift interaction forces. The model was employed to simulate the tunnel erosion around the pipeline laid on an erodible bed. Comparison between the numerical result and experimental measurement confirms that the numerical model successfully predicts the bed profile and velocity field during the tunnel erosion. It is evident that the sediments are transported as the sheet-flow mode in the tunnel erosion stage. Also the transport rate under the pipe increases rapidly at the early stage and then reduces gradually at the end of the tunnel erosion beneath pipelines. 相似文献
18.
Ignazio Maria Viola 《Ocean Engineering》2009,36(12-13):974-984
A computational fluid dynamics (CFD) code was applied to an America's Cup Class Yacht to investigate sailing performance in a downwind configuration. Apparent wind angles at 45°, 105° and 120° are reported, sailed with mainsail and asymmetrical spinnakers. Numerical results are in good agreement with wind tunnel data. A large mesh investigation was performed, ranging from 60,000 elements up to 37 million elements, which shows a converging trend to the experimental values with differences smaller than 3% in both lift and drag. The most commonly used turbulence models in sail applications were tested and the results are presented here in two meshes with 1 million elements and 6.5 millions, respectively. All turbulence models over-estimate forces more than solving the Navier–Stokes system without any additional equations, hence turbulence models do not increase solution accuracy according to these results. 相似文献
19.
考虑深水S型铺设中离散均布托辊对上弯段管道的支撑作用,提出了管道铺设形态和内力的分段解析求解方法,通过迭代搜索算法确定管道最佳反弯点和升离点的位置,结合刚性悬链线理论和欧拉梁弯曲理论,推导了管道的变形和内力的解析表达式。依据所建立的分段解析方法编制了相应的数值计算程序,以某3 000 m水深铺设12英寸管道为例,计算了管道形态和内力响应,与商用有限元软件的数值分析结果进行对比,表明了分段解析方法的准确性和有效性。建立的方法具有较高的计算效率,为进一步开展S型铺设参数的优化或可靠性研究提供了有力的手段。 相似文献
20.
Experimental investigations are carried out on wave-induced pressures and uplift forces on a submarine pipeline (exposed, half buried and fully buried) in clayey soil of different consistency index both in regular and random waves. A study on scour under the pipeline resting on the clay bed is also carried out. It is found that the uplift force can be reduced by about 70%, if the pipeline is just buried in clay soil. The equilibrium scour depth below the pipeline is estimated as 42% of the pipe diameter for consistency index of 0.17 and is 34% of the pipe diameter for consistency index of 0.23. The results of the present investigations are compared with the results on sandy soil by Cheng and Liu (Appl. Ocean Res., 8(1986) 22) to acknowledge the benefit of cohesive soil in reducing the high pore pressure on buried pipeline compared to cohesionless soil. 相似文献