Elastoplastic analysis of tubular joints of offshore platforms

The subject of this paper is the analysis and design of complex tubular joints, eventually including internal and external gussets and stiffener rings, corresponding to fixed and mobile offshore structures.Two different joints are analysed. In the first case an X joint is studied for elastic and elastoplastic behaviour, loading up to collapse in order to determine ultimate strength and safety factor. Finite elements which can reproduce the elastic and plastic singularities of the stress and the strain fields in the crack tip, are then used for the analysis of a T joint. Both direction and rate are considered in the crack propagation, and an elastoplastic analysis is carried out, to determine the crack opening displacement (COD).Finally, the consideration of fatigue effects in tubular joints is discussed, and techniques for evaluating fatigue life are outlined.     

Fatigue crack in large-scale tubular joints for offshore structures

Tubular members have been used for constructing offshore structures. Large-scale fatigue tests of welded tubular K-joints under the balanced in-plane bending brace were carried out to investigate the fatigue behavior of API 2W Gr.50 steel produced by POSCO. The experimental results were verified by numerical approaches and compared with the IIW, DnV RP-C203 and API RP 2A-WSD design curves. The test results based on the hot spot stress were in agreement with the design curves. The SCF factor for tubular K-jo...     

A computer program for the structural analysis of welded tubular joints with RHS chords

A computer program is presented for the analysis of statically loaded structural hollow section (SHS) lattice girder joints having one compression bracing member and one tension bracing member welded to a rectangular hollow section (RHS) chord member. A set of failure modes has been established for such joints, having either gapped or overlapped bracings at the connection to the chord face. To these failure modes the yield line method has been applied in order to provide an assessment of the limiting joint strength. The computer program, based upon the analyses of these failure models and written in Fortran, can hence be used to estimate the yield or ultimate joint strength, with a data input of 30 joint parameters. The program and the theories proposed have been verified by comparison with the results of 150 joint tests, conducted both in isolation and in complete girders, at testing centres in three different countries.     

An error estimation method in finite element structural analysis

In the paper a new interpretation of the finite element approach is described and illustrated numerically. The conventional shape functions of the displacement—and stress—type finite element models are treated as constraints imposed on the continuous medium considered. This enables a consistent error estimation analysis based on a concept of so-called reaction forces and deformation incompatibilities.     

An elastoplastic cracked-beam finite element for structural analysis

A finite element model has been developed in this paper to analyse statically indeterminate skeletal cracked structures. The model is based on elastic-plastic fracture mechanics techniques in order to consider the crack tip plasticity in the analysis. Stiffness matrices for single-edge and double-edge cracked structural elements have been derived using transfer matrix theory. These matrices take into account the effects of axial, flexural and shear deformations due to crack presence. The present model has been applied to investigate the effects of crack size, structure cross-section depth and crack tip plasticity on the redistribution of internal forces in structures. Hence, this analysis can be employed to identify the overstressed regions in cracked structures.     

An adaptive finite element technique for structural dynamic analysis

An adaptive finite element discretization technique, which utilizes specially derived Ritz vectors, is presented for solving structural dynamics problems. The special Ritz vectors are applied as the bases of transformation in geometric coordinates for mode superposition dynamic analysis. To capture the low frequency response and the high frequency response using multigrid principles, a hierarchical formulation for the formation of the coefficient matrices is proposed and it is utilized in the framework of the adaptive h-refinement. Assuming that the solution can be resolved into a set of orthogonal vectors and the refined mesh which passes the refinement criteria for all the vectors can satisfy the refinement criteria for the solution, the Ritz vectors are used as sources to discretize the continuous spatial domain. An a posteriori energy norm of residual error serves as the error measure. Finally, the performance and the efficiency of the proposed technique is demonstrated by solving several examples.     

An efficient flexibility analysis of structures

An algorithm is developed for generating suboptimal cycle bases corresponding to localized statical bases, leading to highly sparse flexibility matrices. For this purpose, an expansion process is used, in each step of which an independent optimal cycle having the least possible overlap with the neighbouring cycles is generated. The elements of the statical basis are then formed on the selected cycles. These cycles are also ordered to reduce the bandwidth of the corresponding flexibility matrix. This is achieved by ordering the nodes of the interchange graph, defined for the selected cycle basis.     

Real-time finite element structural analysis in augmented reality

Conventional finite element analysis (FEA) is usually carried out in offsite and virtual environments, i.e., computer-generated graphics, which does not promote a user’s perception and interaction, and limits its applications. With the purpose of enhancing structural analysis with augmented reality (AR) technologies, the paper presents a system which integrates sensor measurement and real-time FEA simulation into an AR-based environment. By incorporating scientific visualization technologies, this system superimposes FEA results directly on real-world objects, and provides intuitive interfaces for enhanced data exploration. A wireless sensor network has been integrated into the system to acquire spatially distributed loads, and a method to register the sensors onsite has been developed. Real-time FEA methods are employed to generate fast solutions in response to load variations. As a case study, this system is applied to monitor the stresses of a step ladder under actual loading conditions. The relationships among accuracy, mesh resolution and frame rate are investigated.     

Vessel routing with pickups and deliveries: An application to the supply of offshore oil platforms

This paper presents a new routing problem, the Vessel Routing Problem with Selective Pickups and Deliveries (VRPSPD), an extension of existing pickup and delivery problems that arises in the planning of logistics operations in the offshore oil and gas industry. The VRPSPD is a single-vessel model that can lead to significant economic improvements to the current planning scheme without having a very large impact on the operations. In addition, we formulate a Multi-Vessel Routing Problem with Pickups and Deliveries (mVRPPD) that leads to even larger economical gains, but also entails more important changes in the current planning and operations. To quantify and justify the benefits of the VRPSPD and mVRPPD, an industry case based on real data was constructed and solved for 300 days. The VRPSPD is solvable with a commercial solver for most real-size instances. However, for the mVRPPD on the largest instances, it was necessary to develop a state-of-the-art adaptive large neighborhood heuristic search to reduce computational time.     

An effective method and its application in assembling the structural stiffness matrix in material-nonlinear finite element analysis

A very effective technique for assembling the structural stiffness matrix in material-nonlinear finite element analysis is presented. Its application to the stable crack growth analysis is described in detail. A finite element analysis of the process of slow crack growth for a center-cracked specimen subjected to monotonically and slowly increased load until the point of fast fracture is reached is made by means of a material-nonlinear and quasi-three-dimensional finite element program developed by the technique, as an example to illustrate that the technique has been successfully applied to a practical problem.     

An analysis of the flexibility of Asymmetric Power GARCH models

The Asymmetric Power GARCH (APGARCH) model allows a wider class of power transformations than simply taking the absolute value or squaring the data as in classical heteroscedastic models. A dynamic estimation is used to compare the three GARCH families and examine their forecasting performances in a value-at-risk setting. The results suggest that the optimal power transformation obtained with the APGARCH model is virtually never statistically different from 1 or 2. Moreover, although some indices switch between these two values over the time, the measures of accuracy and efficiency used to assess the performance of VaR forecasts indicate that the additional flexibility brought by the APGARCH model provides little, if any, improvements for risk management.     

Choosing among weight-estimation methods for multi-criterion analysis: A case study for the design of multi-purpose offshore platforms

Application of the sustainability concept to environmental projects implies that at least three feature categories (i.e., economic, social, and environmental) must be taken into account by applying a participative multi-criterion analysis (MCA). However, MCA results depend crucially on the methodology applied to estimate the relative criterion weights. By using a logically consistent set of data and methods (i.e., linear regression [LR], factor analysis [FA], the revised Simos procedure [RSP], and the analytical hierarchy process [AHP]), the present study revealed that mistakes from using one weight-estimation method rather than an alternative are non-significant in terms of satisfaction of specified acceptable standards (i.e., a risk of up to 1% of erroneously rejecting an option), but significant for comparisons between options (i.e., a risk of up to 11% of choosing a worse option by rejecting a better option). In particular, the risks of these mistakes are larger if both differences in statistical or computational algorithms and in data sets are involved (e.g., LR vs. AHP). In addition, the present study revealed that the choice of weight-estimation methods should depend on the estimated and normalised score differences for the economic, social, and environmental features. However, on average, some pairs of weight-estimation methods are more similar (e.g., AHP vs. RSP and LR vs. AHP are the most and the least similar, respectively), and some single weight-estimation methods are more reliable (i.e., FA > RSP > AHP > LR).     

An exploration of structural refinement using structural matrices of local elements as design variables

A structural refinement method is introduced in detail in this paper. The refinement of the structure is at the local modeling uncertainties which are roughly found by residual force vectors. The design variables used to change the initial design of the structure are the stiffness and mass matrices of those structural elements which were identified with modeling errors. An iteration loop is established to perform the modification of the structure. The refined structure has greatly improved the correlation between simulated test data and analytical data.     

组合结构等效阻尼比的确定及在有限元计算中的应用

针对组合结构动力响应计算中,一般根据工程经验确定结构阻尼比,往往导致结构动力响应计算结果偏小,不利于结构安全评估的问题,首先根据复阻尼理论与结构动力学方法,推导出组合结构的等效阻尼比计算公式;然后,以某体育场组合结构为工程背景,在有限元法基础上使用ANSYS软件建立该结构的有限元模型.根据计算出的自振特性以及利用ANSYS二次开发功能所形成的阻尼刚度矩阵,实现等效阻尼比的有限元法求解.计算结果表明,等效阻尼比能够正确反映组合结构的动力特性,从而可以科学地计算其动态响应.     

The equivalent solid plate concept in the finite element analysis of tube bundle heat exchangers

The equivalent solid plate concept as used previously in analytical approaches to the mechanical design of tube bundle heat exchangers is derived within the context of finite element analysis. The concept is then applied to the finite element analysis of a typical chemical reactor, and the results are compared with some extensive measurements made during proof testing.     

Perturbation finite element method of structural analysis under fuzzy environments

A preliminary analysis of the perturbation fuzzy finite element was provided by Yang et al. (Appl. Math. Mech. 20(7) (1999) 795). In this paper, we provide a detailed analysis of the perturbation fuzzy finite element method based on variational principle. Firstly, on the basis of the second-order perturbation principle of small parameter, the fuzzy functional of total potential energy and the definite perturbation expansions are proposed. Secondly, definite recursion equation of fuzzy variational principle is deduced and fuzzy finite element recursion functional is presented based on fuzzy variational principle. Thirdly, the proposed approach is compared with the conventional fuzzy finite element method. Finally, a numerical example is given to illustrate the method.     

Modified models for revolute joints coupling flexibility of links in multibody systems

Multibody System Dynamics - This paper develops three different types of finite element models for revolute joints in flexible multibody systems, in which the dry clearance revolute joints have...     

Evaluation of the local quality of stresses in 3D finite element analysis

This article deals with the local quality on stresses produced during finite element analysis in 3D linear elasticity. We use an estimation technique based on the concept of error in constitutive relation, which yields excellent estimates of the local errors without requiring the approximate calculation of Green functions.     

Neurocomputing method based on structural finite element analysis of discrete model

Based on structural finite element analysis of discrete models, a neurocomputing strategy is developed in this paper. Dynamic iterative equations are constructed in terms of neural networks of discrete models. Determination of the iterative step size, which is important for convergence, is investigated based on the positive definiteness of the finite element stiffness matrix. Consequently, a method of choosing the step size of dynamic equations is proposed and the computational formula of the best step size is derived. The analysis of the computing model shows that the solution of finite element system equations can be obtained by the method of neural network computation efficiently. The proposed method can be used for parallel computation of structural finite element in a large-scale integrated circuit (LSI).