Finite Element Method for Design of Reinforced Concrete Offshore Platforms

A design method of reinforced concrete (R. C.) offshore platforms with nonlinear finite element analysis is proposed. According to the method, a computer program is developed. In this program nonlinear constitutive relationships and strength criteria of concrete and steel bars are included, and the progressive cracking and crushing of the concrete are taken into account. Based on the stress distribution obtained by the nonlinear finite element analysis, the amount of reinforcement in the control sections can be computed and adjusted automatically by the program to satisfy the requirement of the design. The amount of reinforcement required in the control sections, which are obtained with the nonlinear finite element analysis, is agreeable to that obtained in the experiment. This shows that the design method of R. C. offshore platform with the nonlinear finite element method proposed by the authors is reliable for practical use.     

The Stress Concentration of Multiplanar Tubular Joints of Offshore Structure

- The formulation for the stress concentration matrix of multiplanar tubular joint with braces subjected to combined load is established in this paper. The formulations for SCF values of multiplanar tubular joints are compared with those by Wordsworth and Efthymiou. The stiffening effect and load-interaction effect are discussed. A spatial offshore jacket with braces subjected to combined load is computed as a numerical example. The maximum stress of each multiplanar tubular joint and its location are found through SCF matrix. Finally, the difference between the maximum stresses of multiplanar tubular joints and their corresponding uniplanar tubular joints are discussed.     

海上桶形基础平台负压沉贯渗流场有限元分析

讨论了浅海桶形基础平台负压沉贯渗流场有限元分析方法.通过模型实验结果与有限元分析结果的对比,验证有限元法的可靠性.将有限元法用于桶形基础平台负压沉贯渗流场的计算,得到沉贯过程中许用负压随插深的变化,用于指导平台就位施工时的负压控制.不同插深时,相应许用负压下的桶内、外壁渗流水头和水头梯度分布的计算结果,用于沉贯阻力的计算.     

Finite Element Analysis of Deformed Legs of Offshore Platform Structures

The element stiffness matrix of the equivalent beam or pipe element of the deformed leg of the platform is derived by the finite element method. The stresses and displacements of some damaged components are calculated, and the numeri-cal solutions agree well with those obtained by the fine mesh finite element method. Finally, as an application of this method, the stresses of some platform structures are calculated and analyzed.     

A Ring Model for Limit Analysis of Spatial XX Tubular Joints

-A ring model is developed to investigate the ultimate strength of tubular X and XX joints in the case that the brace is compressed. In the present analysis, the tubular joint is made of elasto-perfectly-plastic material, and the arch element of the chord section intersecting with the brace is assumed to be rigid. It is found that when 6 plastic hinges for X joint and 8 plastic hinges Tor XX joint appear in the ring, the limit state is reached, and by means of the equivalent ring width Be formula proposed in a previous paper by the authors, the ultimate strength of tubular X and XX joints subjected to compression can be obtained.     

Structural Analysis of Platforms with the Effect of Joint Flexibility

- A modified space beam element is presented in this paper to consider the local joint flexibility of T, Y tubular joints subjected to axial forces and in- plane bending moments for analysis of platforms. Two numerical examples are shown to verify the efficiency and validity of the method presented here.     

Stress Analysis of Spatial Non-Concurrent Tubular Joints

The formulations of analytic-numerical method for the stress analysis of non-concurrent spatial tubular joints are introduced in the paper. The spatial DT joints with different eccentricity in the vertical diametrical plane of chord are computed. Finally the influence of eccentricity on the stress at possible hot spots is discussed.     

Generation and Application of A Standardized Load-Time History to Tubular T-joints in Offshore Platforms

Marine structures are mostly made of metals and always experience complex random loading during their service periods. The fatigue crack growth behaviors of metal materials have been proved from laboratory tests to be sensitive to the loading sequence encountered. In order to take account of the loading sequence effect, fatigue life prediction should be based on fatigue crack propagation (FCP) theory rather than the currently used cumulative fatigue damage (CFD) theory. A unified fatigue life prediction (UFLP) method for marine structures has been proposed by the authors’ group. In order to apply the UFLP method for newly designed structures, authorities such as the classification societies should provide a standardized load-time history (SLH) such as the TWIST and FALSTAFF sequences for transport and fighter aircraft. This paper mainly aims at proposing a procedure to generate the SLHs for marine structures based on a short-term loading sample and to provide an illustration on how to use the presented SLH to a typical tubular T-joint in an offshore platform based on the UFLP method.     

四阶波动方程的有限体积元法

建立一类四阶色散耗散方程全离散有限体积元格式,并给出格式的收敛性和误差估计,同时还给出数值结果。     

有限元法在桩靴承载力计算中的应用

自升式钻井平台插桩是地基土在桩靴荷载作用下发生连续的塑性破坏的动态过程,当地基极限承载力等于桩靴荷载时插桩完成。经典土力学极限承载力理论对土体潜在滑动面做了假设,无法有效分析土体内部的破坏过程。本研究应用有限元法（FEM ）对插桩过程进行了模拟,得到地基土的破坏机制以及中间荷载下土体的应力、应变情况,通过和各理论公式计算的极限承载力进行对比分析,分析影响地基极限承载力的因素。研究表明,基础宽度与硬土层厚度的比值 B/H越大,下卧软土层越容易发生塑性破坏,极限承载力明显下降,当B/H＜0．286时,可以忽略下卧软土层对地基承载力的影响。     

Computation of Viscous Uniform and Shear Flow over A Circular Cylinder by A Finite Element Method

The incompressible viscous uniform and shear flow past a circular cylinder is studied. The two-dimensional Navier-Stokes equations are solved by a finite element method. The governing equations are discretized by a weighted residual method in space. The stable three-step scheme is applied to the momentum equations in the time integration. The numerical model is firstly applied to the computation of the lid-driven cavity flow for its validation. The computed results agree well with the measured data and other numerical results. Then, it is used to simulate the viscous uniform and shear flow over a circular cylinder for Reynolds numbers from lO0 to lO00. The transient time interval before the vortex shedding occurs is shortened considerably by introduction of artificial perturbation. The computed Strouhal number, drag and lift coefficients agree well with the experimental data. The computation shows that the finite element model can be successfully applied to the viscous flow problem.     

建立海洋平台管节点PSN曲线的贝叶斯方法

针对管节点疲劳试验的小样本特点,探讨了建立ＰＳＮ 曲线的贝叶斯方法。在贝叶斯方法中,ＰＳＮ曲线的统计参数作为随机变量处理,首先根据贝叶斯定理求出参数向量的后验概率密度,然后建立ＰＳＮ曲线的贝叶斯方程,最后再编程计算。算例表明,与传统的ＰＳＮ 曲线相比,贝叶斯ＰＳＮ 曲线更加安全可靠     

非常规管节点疲劳寿命分析与计算

对非常规管节点的疲劳寿命进行了分析研究，用SESAM计算疲劳载荷，应用精细有限元分析计算热点应力，用规范的S—N曲线计算管节点的疲劳寿命。通过实例计算表明，这些处理对非常规管节点疲劳寿命分析与计算是很有效的。     

Nonlinear Finite Element Analysis of Ocean Cables

This study has focused on developing numerical procedures for the dynamic nonlinear analysis of cable structures subjected to wave forces and ground motions in the ocean. A geometrically nonlinear finite element procedure using the isoparamnetrie curved cable element based on the Lagrangian folmulation is briefly summarized. A simple and accurate method to determine the initial equilibrium state of cable systems associated with self-weights, buoyancy and the motion of end points is presented using the load incremental method combined with penalty method. Also the Newmark method is used for dynamic nonlinear analysis of ocean cables. Numerical examples are presented to validate the present numerical method.     

一种适用于网箱耐流特性有限元分析的网目群化方法

养殖网箱耐流特性是网箱工程设计中的重要问题之一,探求对这一问题的数值解法在近几年得到了空前的重视。但是,由于深水抗风浪网箱箱体规模大,如果在有限元分析中单纯以网衣网目脚为单元进行数值计算,其计算量相当庞大。文中介绍1种可供网箱箱体有限元分析的网目群化方法,根据保持群化前后网衣水中重量和投影面积相等以保证网箱网衣总体水动力相同的原则,将若干个真实网目群化为1个虚拟的计算网目,以达到有效减少计算单元、提高运算效率和节省运算时间的目的。通过不同群化条件下数值例的计算结果与实验值的比较,验证网目群化方法的可行性。在一般条件下,8×8群化或6×6群化能够有效实现计算精度与计算效率的兼顾。     

深基坑支护结构设计的平面应变有限元法

采用平面应变有限元法讨论深基坑工程多支撑支护问题 ,与传统的静力平衡法、弹性抗力有限元法相比具有优越性 ,该方法将支护结构与土体作为 1个系统研究 ,充分考虑桩土间的协调作用 ,模拟桩土接触界面 ,能计算出不同开挖阶段坑底及地表的变形、支护结构的应力及弯矩 ,在基坑支护结构设计中具有重要应用价值。     

Model Simplification and Parameter Modification for Offshore Platforms Using Experiment Data

- In this paper, a computation method has been developed so as to compare the finite element method (FEM) with the test results directly. The structure is divided into the "master" and "slave" degrees of freedom. The simplified model can be obtained with modal reduction. Then the design sensitivity analysis of the eigenvalues and eigenvectors has been carried out using the modal frequency and modal shape of the test. A two-story frame structure and a jacket model structure have been calculated. Meanwhile, the modified coefficient, the FEM computational and experimental values have been given. It has been shown that the FEM model modified using the test modal value is efficient.     

深基坑开挖与支护的有限元模拟

采用弹塑性有限单元法对超大基坑的开挖与支护进行数值模拟,据此分析土体的位移、主应力分布、施工过程中支护结构受力,并比较基坑顶部水平位移的计算值与实测值,两者非常接近,计算结果较为理想,可为工程设计和现场施工提供理论参考和指导.     

抗浮爆扩桩抗拔机理有限元分析

高地下水位地区构筑地下建(构)筑物必须解决结构上浮问题,抵抗上拔荷载作用已成为地下工程建设中经常需要面临的问题。试验表明爆扩桩具有很好的抗浮能力,其成桩施工工艺方面的研究也比较完善,但是对爆扩桩的承载机理的研究很少,通过运用NCAP-1有限元计算程序,分析总结爆扩桩的抗拔承载机理。     

Dynamic Finite Element Analysis for Interaction between Two Phase Saturated Soil Foundation and Platform

In this paper, the foundation soil of offshore structure is simulated as a two phase saturated porous medium. The dynamic equations of porous medium and finite element formulation are given. For structural analysis, the technique of multilevel substructure is used, and the saturated soil analysis is set in the highest level substructure model. Based on these theories a dynamic finite element analysis program DIASS for the analysis of interaction between two phase ocean soil foundation and platform structures has been developed. A numerical example is given here to illustrate the influence of the pore water in soil on the structural response of an ocean platform.