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1.
A coastal ocean model of semi-implicit finite volume unstructured grid   总被引：1，自引：0，他引：1  
A two-dimensional coastal ocean model based on unstructured C-grid is built, in which the momentum equation is discretized on the faces of each cell, and the continuity equation is discretized on the cell. The model is discretized by semi-implicit finite volume method, in that the free surface is semi-implicit and the bottom friction is implicit, thereby removing stability limitations associated with the surface gravity wave and friction. The remaining terms in the momentum equations are discretized explicitly by integral finite volume method and second-order Adams-Bashforth method. Tidal flow in the polar quadrant with known analytic solution is employed to test the proposed model. Finally, the performance of the present model to simulate tidal flow in a geometrically complex domain is examined by simulation of tidal currents in the Pearl River Estuary.  相似文献   

2.
With the increasing development and utilization of offshore oil and gas resources, global buckling failures of pipelines subjected to high temperature and high pressure are becoming increasingly important. For unburied or semi-buried submarine pipelines, lateral global buckling represents the main form of global buckling. The pipe–soil interaction determines the deformation and stress distribution of buckling pipelines. In this paper, the nonlinear pipe–soil interaction model is introduced into the analysis of pipeline lateral global buckling, a coupling method of PSI elements and the modified RIKS algorithm is proposed to study the lateral global buckling of a pipeline, and the buckling characteristics of submarine pipeline with a single arch symmetric initial imperfection under different pipe–soil interaction models are studied. Research shows that, compared with the ideal elastic–plastic pipe–soil interaction model, when the DNV-RP-F109 model is adopted to simulate the lateral pipe–soil interactions in the lateral global buckling of a pipeline, the buckling amplitude increases, however, the critical buckling force and the initial buckling temperature difference decreases. In the DNV-RP-F109 pipe–soil interaction model, the maximum soil resistance, the residual soil resistance, and the displacement to reach the maximum soil resistance have significant effects on the analysis results of pipeline global buckling.  相似文献   

3.
The buckling of submarine pipelines may occur due to the action of axial soil frictional force caused by relative movement of soil and pipeline, which is induced by the thermal and internal pressure. The likelihood of occurrence of this buckling phenomenon is largely determined by soil resistance. A series of large-scale model tests were carried out to facilitate the establishment of substantial data base for a variety of burial pipeline relationships. Based on the test data, nonlinear soil spring can be adopted to simulate the soil behavior during the pipeline movement. For uplift resistance, an ideal elasticity plasticity model is recommended in the case of H/D (depth-to-diameter ratio)>5 and an elasticity softened model is recommended in the case of H/D≤5. The soil resistance along the pipeline axial direction can be simulated by an ideal elasticity plasticity model. The numerical analyzing results show that the capacity of pipeline against thermal buckling decreases with its initial imperfection enlargement and increases with the burial depth enhancement.  相似文献   

4.
A numerical model for the self-burial of a pipeline trench is developed. Morphological evolutions of a pipeline trench under steady-current or oscillatory-flow conditions are simulated with/without a pipeline inside the trench. The oscillatory flow in this study represents the action of waves. The two-dimensional Reynolds-averaged continuity and Navier-Stokes equations with the standard k-e turbulence closure, as well as the sediment transport equations, are solved with the finite difference method in a eurvilirrear coordinate system. Both bed and suspended loads of sediment transport are included in the morphological model. Because of the lack of experimental data on the backfilling of pipeline trenches, the numericalmodel is firstly verified against three closely-relevant experiments available in literature. A detailed measurement of the channel migration phenomenon under steady currents is employed for the assessment of the integral performance of themodel. The two experimental results from U-tube tests are used to validate the model‘s ability in predicting oscillatory flows. Different time-marching schemes are employed for the morphological computation under unidirectional and oscillatory conditions. It is found that vortex motions within the trench play an important role in the trench development.  相似文献   

5.
A 3D,time-dependent,baroclinic,hydrodynamic and salinity model was implemented and applied to the Oujiang River estuarine system in the East China Sea.The model was driven by the forcing of tidal elevations along the open boundaries and freshwater inflows from the Oujiang River.The bottom friction coefficient and vertical eddy viscosity were adjusted to complete model calibration and verification in simulations.It is demonstrated that the model is capable of reproducing observed temporal variability in the water surface elevation and longitudinal velocity,presenting skill coefficient higher than 0.82.This model was then used to investigate the influence of freshwater discharge on residual current and salinity intrusion under different freshwater inflow conditions in the Oujiang River estuary.The model results reveal that the river channel presents a two-layer structure with flood currents near the bottom and ebb currents at the top layer in the region of seawater influenced on north shore under high river flow condition.The river discharge is a major factor affecting the salinity stratification in the estuarine system.The water exchange is mainly driven by the tidal forcing at the estuary mouth,except under high river flow conditions when the freshwater extends its influence from the river’s head to its mouth.  相似文献   

6.
- Based on the Mohr-Coulomb failure principle and Rankine's theory, the laterally loaded pile ultimate resistance formulas of sand and soft clay proposed by Reese and Matlock respectively are discussed in this paper. The authors put forward the modified ultimate resistance formulas on the basis of which the ultimate resistance formula is developed for horizontally loaded pile in multi-layer soil in consideration of the effect of the overburden soil pressure on the calculation of soil layer. It is significant to the correct application of the ultimate resistance formulas in API and ZCS Rules into offshore engineering.  相似文献   

7.
This paper describes the model tests for determining the axial friction and the lateral resistance of sand to pipeline by using fine sand and prototype pipeline, and the calculation method based on limit analysis theory is verified. The effect of cyclic loading is considered in the test.  相似文献   

8.
Ocean current forecasting is still in explorative stage of study. In the study, we face some problems that have not been met before. The solving of these problems has become fundamental premise for realizing the ocean current forecasting. In the present paper are discussed in depth the physical essence for such basic problems as the predictability of ocean current, the predictable currents, the dynamical basis for studying respectively the tidal current and circulation, the necessity of boundary model, the models on regions with different scales and their link. The foundations and plans to solve the problems are demonstrated. Finally a set of operational numerical forecasting system for ocean current is proposed.  相似文献   

9.
The accumulative shear deformation of soft clays under cyclic loads is considered as pseudo-staticcreep.A pseudo-static elasto-plastic cyclic creep model is developed based on the visco-elasto-plastic theory.Theparameters in the model are determined by cyclic triaxial soil tests.A method for analyzing the stability of offshoresoft foundation under wave loads is given by combining the model with pseudo-static creep analysis.An exampleis analyzed by the method.The results show that the horizontal and vertical stability of foundations under waveloads can be analyzed by it and the analytical results are qualitatively consistent with the observed failure modes ofshallow foundations.  相似文献   

10.
A Note on Soil Structure Resistance of Natural Marine Deposits   总被引：1，自引：1，他引：1  
It has been well documented that natural normally-consolidated marine soils are generally subjected to the effects of soil structure. The interpretation of the resistance of soil structure is an important issue in the theory study and engineering practice of ocean engineering and geotechnical engineering. It is traditionally considered that the resistance of soil structure gradually disappears with increasing stress level when the applied stress is beyond the consolidation yield stress. In this study, however, it is found that this traditional interpretation of the resistance of soil structure can not explain the strength behavior of natural marine deposits with a normally-consolidated stress history A new interpretation of the resistance of soil structure is proposed based on the strength behavior. In the preyield state, the undrained strength of natural marine deposits is composed of two components: one developed by the applied stress and the other developed by the resistance of soil structure. When the applied stress is beyond the consolidation yield stress, the strength behavior is independent of the resistance of soil structure.  相似文献   

11.
The steady flow-induced instability of a partially embedded pipeline involves a complex process of pipe-soil interaction. In accordance with the hydrodynamic loading and the dimensionless analyses, a series of pipe-soil interaction tests have been conducted with an updated pipe-soil interaction facility including a load-displacement synchronous measurement system, to reveal the underlying pipe-soil interaction mechanism. The effects of pipe surface roughness, end-constraint and initial embedment are investigated, respectively. The values of lateral-soil-resistance coefficient for the rough pipes are bigger than those for the smooth pipes. For a fixed value of non-dimensional submerged weight, the values of lateral-soil-resistance coefficient for the anti-rolling pipes are much larger than those for the freely laid pipes. The effects of initial embedment on the ultimate soil resistance get less with the decrease of the submerged weight of the pipe. A comparison is made between the results of the present mechanical-actuator tests and those of the previous water-flume tests, indicating that those results are quite comparable. For the equivalent level of dimensionless submerged weight, the directly laid pipe in currents has higher lateral stability than in waves.  相似文献   

12.
The axial friction response of subsea pipelines in soft clays is a very important aspect for designers of subsea pipelines but the response is not well understood so far. There is a pressing need for the comprehension of the response. In this paper, model tests are performed using full-scale pipes coated with polyethylene (PE) to study the effects of the set-up period, the pipe diameter, the buried depth of the pipe, the shear strength of soft clays and the loading rate on the axial friction response of pipelines in soft clays. The variations of the axial friction coefficient are analyzed using the effective stress method based on model test results. The results show that the axial friction resistance increases with the increasing pipe diameter but the effect of the pipe diameter on the axial friction coefficient can be neglected. The ultimate axial resistance also increases with the increase of the buried depth of pipelines, the undrained shear strength of soft clays and the loading rate. The axial friction coefficient increases with the increasing loading rate. However, the axial friction coefficient decreases with the increasing buried depth. The method to determine the axial friction coefficient is developed by analyzing model test results, which considers the effects of the diameter, the buried depth, the undrained shear strength of soft clays and the loading rate. The study results not only extend the industry data base but also supply a basis to determine the axial friction coefficient of PE-coated pipes in soft clays for ocean engineering geological investigations.  相似文献   

13.
1 .IntroductionThe submarine pipeline is a commonfacility widely usedfor offshore oil and gastransport . Whena pipeline is installed on a seabed and not buried,unsupportedspans may exist insomelocations ,es-peciallyinthe uneven zones of the seabed.The spa…  相似文献   

14.
Subsea pipelines are important facilities in offshore oil and gas industry to transport High-Pressure and High-Temperature (HPHT) hydrocarbon. They are often exerted by cyclic thermal loading through the whole operational life, which may trigger asymmetry in the effective axial force (EAF) profile, leading to a global axial movement, defined as ‘pipeline walking’. It may cause the downtime and structural risks, since this directional accumulation in axial movement results in the overstressing of end connection, loss of tension in a steel catenary riser (SCR) and other issues in the field, therefore subsea pipeline design requires a reliable estimate of the global pipeline walking rate. Current design methods adopt a constant soil friction coefficient and ignore variations in soil-pipeline interaction with time during the heating up and cooling down processes. In reality, the overlooked changing soil friction is important, and may alter the pipeline walking behaviours. To achieve an accurate assessment of pipeline walking, this paper advances the conventional design practice, by introducing a time-dependent axial soil friction function, to examine the pipeline walking behaviour over the operational thermal cycles. A suite of time-dependent matrixes is provided to reveal the development of the expansion/contraction of the pipeline, the corresponding mobilised soil frictions and the EAF profile. The significant nonlinear EAF profile in response to the time-dependent soil friction is examined, and is used to well assess the accumulation in pipeline walking rate. The proposed analytical framework is applied into case studies to demonstrate its validity and applicability in practice.  相似文献   

15.
Tests to measure the soil resistance to lateral pipeline sliding were conducted in 9.15 and 18.3m of water depth in the Gulf of Mexico. The tests were performed with a special vehicle towed from a boat. The vehicle was constructed from two pipeline segments—the one simulating sliding and the other rolling. Tow force data were obtained for sand and clay soils and were analysed with the Coulomb friction model to derive sliding friction coefficients. The data analysis focused on the friction coefficients from inception of vehicle movement to the instant of maximum soil resistance. The coefficients calculated for the maximum soil resistance were 0.45 and 0.75 for the clay and sand, respectively, and these values are consistent with coefficients in the public domain from comparable laboratory tests.  相似文献   

16.
Pipelines are the main element in transporting hydrocarbons from their extraction sites to on-shore or floating facilities, with preference now given to pipelines laid directly on the seabed due to their fast and economic installation. However, these pipelines are exposed and must be stable under all environmental conditions, and therefore, their design for on-bottom stability is of critical importance. Although accurate prediction of the pipe–soil interaction behaviour under hydrodynamic loads from waves and currents is of major concern, limited physical testing of pipes subjected to these cyclic loading conditions has occurred. Tests have concentrated on simpler load combinations in order to develop pipe–soil friction factors or the key parameters in plasticity models that described pipe–soil behaviour. In this paper, results from geotechnical centrifuge experiments of a model pipe on calcareous sand soil collected from offshore on the North West Shelf of Australia are presented. A sophisticated load control scheme allowed complex paths characteristic of hydrodynamic loads to be applied during the testing. Furthermore, pipe testing could be extended to relatively large horizontal movements of up to 5 pipe diameter. The results of the centrifuge testing programme provide improved understanding of the pipe–soil interaction under complex hydrodynamic load paths. They have also been used to assess a state-of-the-art plasticity model describing pipe–soil interaction on calcareous sands.  相似文献   

17.
通过平面拉动法分别测量了5种不同开口比人工鱼礁礁体模型对5种不同粒径大小底质间的最大静摩擦系数,分析了不同粒径底质以及礁体不同开口比的最大静摩擦系数变化规律。试验研究表明:最大静摩擦系数与底质粒径大小呈强负相关(r=-0.964,P<0.01);控制底质粒径时,最大静摩擦系数与礁体质量呈弱负相关(r=-0.265,P<0.01),与礁体开口比呈弱正相关(r=0.245,P<0.01);礁体本身的设计属性如开口比是最大静摩擦系数的主要影响因子。对于新设计礁型在进行礁体稳定性校核时,应根据礁体开口比和实际海域底质泥沙的粒径组成,选取适当的最大静摩擦系数。  相似文献   

18.
Based on similarity analyses, a series of experiments have been conducted with a newly established hydro-elastic facility to investigate the transverse vortex-induced vibrations (VIVs) of a submarine pipeline near an erodible sandy seabed under the influence of ocean currents. Typical characteristics of coupling processes between pipe vibration and soil scour in the currents have been summarized for Case I: pipe is laid above seabed and Case II: pipe is partially embedded in seabed on the basis of the experimental observations. Pipe vibration and the corresponding local scour are usually two coupled physical processes leading to an equilibrium state. The influence of initial gap-to-diameter ratio (e0/D) on the interaction between pipe vibration and local scour has been studied. Experimental results show that the critical values of Vr for the initiation of VIVs of the pipe near an erodible sand bed get bigger with decreasing initial gap-to-diameter ratio within the examined range of e0/D (−0.25<e0/D<0.75). The comparison of the pipe vibrations near an erodible soil with those near a rigid boundary and under wall-free conditions indicates that the vibration amplitudes of the pipe near an erodible sand bed are close to the curve fit under wall-free conditions; nevertheless, for the same stability parameter, the maximum amplitudes for the VIV coupled with local scour increase with the increase of initial gap-to-diameter ratio.  相似文献   

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