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1.
It is well known that a submerged cable, under hydrostatic pressure, will shrink in diameter due to radial compressive stresses. Less widely appreciated is that the cable will also shorten due to axial compressive stresses acting along the length of the cable. This paper offers an explanation and presents numerical examples to illustrate this shortening effect. It is demonstrated that a submerged cable can support axial compressive stresses without buckling because the fluid pressure on the lateral surface provides a stabilizing force.  相似文献   

2.
Submarine pipelines are the primary component of an offshore oil transportation system. Under operating conditions, a pipeline is subjected to high temperatures and pressures to improve oil mobility. As a result, additional stress accumulates in pipeline sections, which causes global buckling. For an exposed deep-water pipeline, lateral buckling is the major form of this global buckling. Large lateral displacement causes a very high bending moment which may lead to a local buckling failure in the pipe cross-section. This paper proposes a lateral global buckling failure envelope for deep-water HT/HP pipelines using a numerical simulation analysis. It analyzes the factors influencing the envelope, including the thickness t, diameter D, soil resistance coefficient μ, calculating length Lf, imperfection length L and imperfection amplitude V. Equations to calculate the failure envelope are established to make future post-buckling pipeline failure assessment more convenient. The results show that (1) the limit pressure difference pmax (the failure pressure difference for a post-buckling pipeline when it suffers no difference in temperature) is usually below the burst pressure difference pb (which is the largest pressure difference a pipeline can bear and is determined from the strength and sectional dimensions of the pipeline) and is approximately 0.62–0.75 times the value of pb and (2) thickness t has little influence on the normalized envelopes, but affects pmax. The diameter D, soil resistance coefficient μ, and calculating length Lf influence the maximum failure temperature difference Tmax (the failure temperature difference for a pipeline suffering no pressure difference). The diameter D also significantly affects the form of the normalized envelope.  相似文献   

3.
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   

4.
Results of a numerical and experimental study into buckling performance of multi-segment pressure hull subjected to uniform hydrostatic pressure are discussed. Constituents of multi-segment configurations are bowed-out cylindrical shells with, and without flanges. Details about five collapse tests of laboratory scale mild steel, CNC machined models are given. Segments were about 200 mm diameter, 100 mm long and had uniform wall thickness of 3 mm. Experimental collapse pressures were in the range from 12 to 20 MPa. Numerical collapse pressures agreed well with those obtained during experiments.  相似文献   

5.
This paper presents analytical and numerical researches on the buckling or collapse of offshore pipelines under external hydrostatic pressure. Firstly the case of homogeneous ring model is investigated followed by a detailed study on corroded rings. The elastic-plastic collapse pressure could be treated as the least root of an elementary function. We prove that collapse pressure is a strictly increasing function of mode number in this paper and present some interesting structures of the roots. Partially corroded ring is parametrized by corrosion depth and angle extent. A comprehensive comparison shows that plasticity should not be neglected when the ring is thick-walled. Moreover, a study on large deflection deformation of 3D cylindrical shells quasi-statically dented under constant external pressure is carried out theoretically and numerically. The buckle propagation pressure is shown to be a meaningful value to normalize external pressure. This paper serves to enhance the understanding of destabilizing effect of external pressure mainly applicable and relevant to subsea offshore industry.  相似文献   

6.
Numerical ocean modelling is computationally very demanding. Traditionally, the hydrostatic approximation has been applied to reduce the computational burden. This approximation is valid in large scale studies with coarse grid resolution. With faster computers and gradually smaller grid sizes, we may expect that more studies will be performed with non-hydrostatic ocean models. In recent papers several methods for including non-hydrostatic pressure in σ-coordinate models have been suggested. In this paper the sensitivity of the non-hydrostatic pressure field, the velocity fields, and the density fields to changes in the method for computing non-hydrostatic pressure in σ-coordinate ocean models is addressed.The first test case used involves the propagation and breaking of an internal wave at an incline in a tank. The other test case concerns tidally driven flow over a sill in a stratified fjord. The results from our numerical exercises suggest that the velocity and density fields are very robust to the model choices investigated here. The differences between the model results are of the same order as the uncertainty due to the internal pressure gradient error, and they are smaller than an estimate of the uncertainty due to subgrid scale closure.  相似文献   

7.
基于振动的水下环肋圆柱壳临界载荷的预报具有结构无损的优势。以Flügge壳体理论和基于正交各向异性理论的环肋圆柱壳自由振动方程为基础,运用波传播法得出耦合系统的频率方程,并求出对应外压下的固有频率,通过最小二乘法线性拟合获得临界载荷、外压及固有频率的关系表达式。绘图得知任意边界的同一模态下,水下环肋圆柱壳的固有频率平方与静水压力成正比。结构失稳时,刚度丧失,固有频率降为零,据此求出水下环肋圆柱壳的最小弹性临界载荷。分析了不同边界条件对环肋圆柱壳临界载荷的影响,得知边界约束越强,环肋圆柱壳的临界载荷越大。结果对比表明了本方法的正确性,为水下环肋圆柱壳的固有频率及临界载荷的理论计算提供了一种新的方法。  相似文献   

8.
The paper reports on a theoretical and an experimental study into the collapse of three thick-walled circular conical shells, which were tested to failure under external hydrostatic pressure. All three vessels failed by plastic non-symmetric bifurcation buckling. Two theoretical analyses were carried out, both based on the finite element method. One of the theoretical analyses was based on inelastic non-symmetric bifurcation buckling and the other analysis was based on plastic axisymmetric buckling. Both of these theoretical analysis and the experimental observations appeared to indicate that there is a link between plastic non-symmetric bifurcation buckling and plastic axisymmetric buckling.  相似文献   

9.
The paper describes experimental tests carried out on three ring-stiffened cones that were tested to destruction under external hydrostatic pressure. The cones were carefully machined from EN1A Steel. All three cones failed by plastic non-symmetric bifurcation buckling in a mode commonly known as general instability. In this mode the entire ring-shell combination buckles bodily.The paper also provides a design chart using the results obtained from these three vessels, together with the results of six other vessels obtained from other tests. The design chart allows the possibility of obtaining a plastic knock down factor, so that the theoretical buckling pressures, based on elastic theory, can be divided by the plastic knockdown factor, to give the predicted buckling pressure. This method can also be used for the design of full-scale vessels.  相似文献   

10.
A flat plate in pitching motion is considered as a fundamental source of locomotion in the general context of marine propulsion. The experimental as well as numerical investigation is carried out at a relatively small Reynold number of 2000 based on the plate length c and the inflow velocity U∞. The plate oscillates sinusoidally in pitch about its 1/3 − c axis and the peak to peak amplitude of motion is 20°. The reduced frequency of oscillation k = πfc/U∞ is considered as a key parameter and it may vary between 1 and 5. The underlying fluid-structure problem is numerically solved using a compact finite-differences Navier–Stokes solution procedure and the numerical solution is compared with Particle Image Velocimetry (PIV) measurements of the flow field around the pitching foil experimental device mounted in a water-channel. A good agreement is found between the numerical and experimental results and the threshold oscillation frequency beyond which the wake exhibits a reverse von Kármán street pattern is determined. Above threshold, the mean velocity in the wake exhibits jet-like profiles with velocity excess, which is generally considered as the footprint of thrust production. The forces exerted on the plate are extracted from the numerical simulation results and it is shown, that reliable predictions for possible thrust production can be inferred from a conventional experimental control volume analysis, only when besides the wake's mean flow the contributions from the velocity fluctuation and the pressure term are taken into account.  相似文献   

11.
The paper reports on the buckling of three ring-stiffened prolate domes under external hydrostatic pressure. The study was partly theoretical and partly experimental, where in the case of the latter, the finite element was used. Comparison between experiment and theory was good. The effect of ring stiffening the domes was to increase their buckling resistances by factors varying from 4.43 to 5.72.  相似文献   

12.
A theoretical and an experimental investigation was carried out, where a carbon fibre corrugated circular cylinder was tested to destuction under external hydrostatic pressure. The theoretical investigation was via the finite element method, where the structure was modelled with several orthotropic axisymmetric thin-walled shell elements. The experimental observations were aided with strategically placed strain gauges. Comparison between theory and experiment showed that the experimentally observed buckling pressure was a little lower than the theoretical prediction. This may have been due to the fact that the model had slight initial geometrical imperfections in the circumferenential direction.  相似文献   

13.
This paper reports on experimental work carried out on nine thin-walled circular cylinders which were tested to destruction under external hydrostatic pressure. Seven of the cylinders failed through non-symmetric bifurcation buckling and two failed through axisymmetric collapse. The results were used from these tests, together with the results from other experiments, to produce a design chart which could be used for designing against the occurence of elastic and inelastic shell instability.  相似文献   

14.
Submersible pressure hulls with fiber-reinforced multilayer-sandwich constructions have been developed in recent years as substitutes for classical metallic ring-stiffened pressure hulls. This study aims to optimize the design of filament-wound multilayer-sandwich submersible pressure hulls, taking into consideration the shell buckling strength constraint, the angle-ply laminated facing failure strength constraint and the low-density isotropic core yielding strength constraint under hydrostatic pressure using the hybrid genetic algorithm (HGA). The thickness of the facing, the thickness of the core layer, the orientation angle of the fibers in the facings and the shear modulus of the core material are taken as design variables. A sensitivity analysis is performed to study the effects of the operational depths and the hull shell geometry parameter, the length-to-diameter ratio (L/D), on the optimal design of filament-wound multilayer-sandwich submersible pressure hulls with graphite/epoxy, glass/epoxy and boron/epoxy composite facings. The results reveal that the optimal weight of various sandwich pressure hulls increases linearly with the operational depth, but it is almost unchanged as the geometry paramter. Furthermore, Graphite/Epoxy is the best choice for the material of the facings in a light-weight design. With reference to wall design, Boron/Epoxy is the best choice for the material of the facing at shallow depths, but Graphite/Epoxy is the best choice at extreme depths. Results of this study provide a valuable reference for designers of underwater vehicles.  相似文献   

15.
The multiple intersecting spheres (MIS) pressure hull is a logical derivative of the single unstiffened sphere, which is frequently used for deep operating, small submersibles because of its attractive low buoyancy factor. This paper investigates the optimum design of an MIS deep-submerged pressure hull subjected to hydrostatic pressure, using a powerful optimization procedure combined the extended interior penalty function method (EIPF) with the Davidon–Fletcher–Powell (DFP) method. In this study, the thickness of the shell, the width of the rib-ring, the inner radius of the rib-ring and the angle of intersection of the spherical shell are selected as design variables, and structural failure and human requirements are considered to minimize the buoyancy factor. Additionally, a sensitivity analysis is performed to study the influence of the design variables on the optimal structural strength design. The results reveal that the shell thickness is most important to lobar buckling strength, and that rib-ring width, rib-ring inner radius and spherical shell intersection angle are most important to rib-ring hoop strength. Optimization results may provide a valuable reference for designers.  相似文献   

16.
Considering the shear deformation and thickness stretching of large deformation, a modified numerical calculation method based on the thick shell theory is established to determine the collapse pressure of thick-walled pipes. Verification experiments are conducted on ten pipe specimens in hyperbaric chambers. The good agreement between experimental results and numerical predictions shows the validity and reliability of the new numerical calculation method. Combining DNV specification, the characteristic collapse pressure is also calculated for comparison. The difference between experimental results and DNV calculations illustrates the latter one is much conservative in predicting collapse pressure for thick-walled pipes. Sensitivity analysis on manufacturing imperfections and material properties is investigated for pipes with different D/t ratios. Thick-walled pipes are easier to be affected by initial ovality, residual stress and hardening factor. Based on the stress distribution at the moment of collapse, a novel discovery is found that the collapse pressure of thick-walled pipes is dominated by material plastic behavior.  相似文献   

17.
This paper revisits the classic seamount test used in numerous previous studies to evidence the sigma errors of the pressure gradient force (PGF) and their long-term effects on circulation. Two kinds of analysis are developed. We first consider the initial PGF errors. Then, the global level of erroneous kinetic energy is computed along a 180-day simulation. The long-term circulation appears to be better correlated to the initial vorticity errors than to the initial error diagnostics.The original feature of this study is to reconsider the currently admitted idea that Density-Jacobian type PGFs perform better than the primitive sigma formulation discretized in a straightforward way (hereafter Straightforward-Primitive PGF). Errors on the discrete hydrostatic pressure are actually closely related to the way the density field is initialized. If a mass conserving method is preferred to a straightforward initialization, the rectangular integral of the Straightforward-Primitive PGF is likely to be more accurate than the trapezoidal rule usually involved in Density-Jacobian PGFs. Errors on the vorticity field of the Straightforward-Primitive PGF depend on the discretization of the hydrostatic correction term. A modified version of the Straightforward-Primitive PGF is shown to be in better agreement with the concept of bottom torque consistency. The seamount tests show that this so-called Modified-Primitive PGF performs globally better than the current low-order Density-Jacobian PGFs.  相似文献   

18.
A pressure relief technique has been proposed to reduce hydrostatic pressures by opening drainage holes around a water tank. This solution is evaluated by laboratory experiments and numerical parametric investigation. After seepage flow is allowed, the hydraulic head acting on the water tank is reduced due to seepage losses. The drainage holes should be opened on the base slab near the sidewalls, and at a certain height on the sidewalls. Given a fixed total opening area, the number of drainage holes with smaller diameter should be allowed near the edge of the water tank to increase the efficacy of the approach. The properties of the surrounding soil influence the results significantly, where a cushion layer with a higher hydraulic conductivity and a greater thickness is beneficial to the stability of the water tank, and a backfill layer with adequate thickness and hydraulic conductivity should be selected. An illustrative example is given in the end to demonstrate the advantage of the proposed antiflotation design strategy compared to the conventional enhanced self-weight method, and more economic design using less reinforcement and concrete can be achieved.  相似文献   

19.
《Ocean Modelling》2011,40(3-4):248-261
Accurate representation of geostrophic and hydrostatic balance is an essential requirement for numerical modelling of geophysical flows. Potentially, unstructured mesh numerical methods offer significant benefits over conventional structured meshes, including the ability to conform to arbitrary bounding topography in a natural manner and the ability to apply dynamic mesh adaptivity. However, there is a need to develop robust schemes with accurate representation of physical balance on arbitrary unstructured meshes. We discuss the origin of physical balance errors in a finite element discretisation of the Navier–Stokes equations using the fractional timestep pressure projection method. By considering the Helmholtz decomposition of forcing terms in the momentum equation, it is shown that the components of the buoyancy and Coriolis accelerations that project onto the non-divergent velocity tendency are the small residuals between two terms of comparable magnitude. Hence there is a potential for significant injection of imbalance by a numerical method that does not compute these residuals accurately. This observation is used to motivate a balanced pressure decomposition method whereby an additional “balanced pressure” field, associated with buoyancy and Coriolis accelerations, is solved for at increased accuracy and used to precondition the solution for the dynamical pressure. The utility of this approach is quantified in a fully non-linear system in exact geostrophic balance. The approach is further tested via quantitative comparison of unstructured mesh simulations of the thermally driven rotating annulus against laboratory data. Using a piecewise linear discretisation for velocity and pressure (a stabilised P1P1 discretisation), it is demonstrated that the balanced pressure decomposition method is required for a physically realistic representation of the system.  相似文献   

20.
This paper presents a procedure to calculate the design pressure distributions on the hull of a wave energy converter (WEC). Design pressures are the maximum pressure values that the device is expected to experience during its operational life time. The procedure is applied to the prototype under development by Martifer Energy (FLOW—Future Life in Ocean Waves).A boundary integral method is used to solve the hydrodynamic problem. The hydrodynamic pressures are combined with the hydrostatic ones and the internal pressures of the large ballast tanks. The first step consists of validating the numerical results of motions by comparison with measured experimental data obtained with a scaled model of the WEC. The numerical model is tuned by adjusting the damping of the device rotational motions and the equivalent damping and stiffness of the power take-off system. The pressure distributions are calculated for all irregular sea states representative of the Portuguese Pilot Zone where the prototype will be installed and a long term distribution method is used to calculate the expected maximum pressures on the hull corresponding to the 100-year return period.  相似文献   

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