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When a subsea pipeline is laid on an uneven seabed, certain sections may have an initial elevation with respect to the far-field seabed, eo, and thus potentially affecting the on-bottom stability of the pipeline. This paper focuses on quantifying the effects of the upstream dimensionless seabed shear stress, θ∞, and Reynolds number, Re, on: (1) the maximum dimensionless seabed shear stress beneath the pipe, θmax, to be compared to the critical shear stress in order to determine whether scour would occur and progress towards an equilibrium state; and, (2) the dimensionless equilibrium scour depth beneath the pipe, Seq/D. Using a 2-D Reynolds averaged Navier-Stokes (RANS) approach along with the k-ω Shear Stress Transport (SST) turbulence model, a parametric study involving 243 computational fluid dynamics (CFD) simulations was conducted. The simulation results were used to develop a closed-form equation for the prediction of θmax. Subsequently, experimental measurements of Seq/D have been compiled from published literature, to develop a new closed-form equation for the prediction of Seq/D with a high correlation to the experimental data. In summary, we present two closed-form equations for the prediction of θmax and Seq/D for pipelines with an initial eo/D, which are applicable for both clear-water and live-bed conditions. The effects of θ∞ and Re have been included, albeit Re having a small influence as compared to the other parameters. 相似文献
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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
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Several methods have been employed to install the different types of subsea equipment that are required for offshore oil and gas production systems. More recently, the increasing development of oil fields at remote, ultra-deepwater scenarios has been requiring heavier and more complex subsea equipment. In such scenarios the usual installation methods have been facing increasing challenges, including resonance effects associated to the dynamic behavior of the cable-equipment system. In this context, after presenting a review of the state-of-the-art of installation methods for subsea equipment, this work presents a methodology for the design and evaluation of an installation method based on the combination of steel wire and polyester ropes. The goal is to obtain a feasible and safe installation method that can avoid resonance effects along all phases of the lowering procedure, without the need of specialized devices such as heave compensators, and using only simpler installation vessels such as standard tugboats instead of more expensive and specialized vessels (such as drilling rigs). The method is evaluated by numerical simulations for case studies considering representative metocean data for Brazilian ultra-deepwater scenarios, following a methodology that considers the random characteristic of actual sea states by employing an irregular spectral representation. The results indicate that the method may indeed be feasible for the deepwater installation of heavier equipment, potentially being more cost-effective especially for remote locations where other more complex methods might become inefficient. 相似文献
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Lateral buckling is an important issue in unburied high-temperature and high-pressure (HT/HP) subsea pipelines systems. The imperfection–sleeper method is one of the most well-known methods used to control lateral buckling of HT/HP pipelines. Pipelines–sleeper–seabed numerical models are established and verified to analyze the buckling behavior of pipelines using the imperfection–sleeper method. The influence of six main factors on lateral buckling behavior is investigated in details based on the numerical results. Equations of buckling displacement (buckling displacement is defined by the final displacement of the middle point of the pipelines), critical buckling force, and buckling stress (Mises stress) are proposed using the gene expression programming technique. These equations show good accuracy and can be used to assist in the design of sleepers and assess the compressive and stress levels of pipelines. 相似文献
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The pipeline end manifold (PLEM) is an important subsea facility, which can greatly reduce the cost and risk of the development scenarios of deepwater oil and gas fields by declining the number of export pipelines and risers. However, the employment of PLEMs is a multidiscipline task involving substantial financial and technical factors. Due to various uncertainties of influencing factors, the evaluation process may take several months or years by the engineers with rich project experience. Thus, how to develop quantified reference tools using mathematical models to assist engineers in efficiently making their crucial decisions is essential. In this paper, the optimization of the layout scenarios of cluster manifolds with PLEMs is discussed, where a proposed mathematical model and its dedicated algorithm are illustrated. The optimal solution at the lowest cost can be obtained through in-house routine in MATLAB, including the optimal layout scenario, the number and locations of PLEMs, and the connection relations. Besides, the numerical simulations are performed to demonstrate the validity of the proposed mathematical model and its algorithm. The results show that this optimization layout problem in engineering can be described accurately by the presented mathematical model and the convergence rate of the given algorithm is efficient. 相似文献