| Lift points are the joints between the lifting equipment and the lifting objects, they are discretely distributed at the truss intersections of the main beam with a great number and a wide span. Lifting point design is a discrete variable optimization problem that emerged together with multi-cranes cooperative lifting, which should take into account the lifting capacity of the cranes, the strength of the structure as well as the number and location of the lift points, etc. Thus, Lift points design is an important aspect in the process of lifting scheme making.The offshore platform deck of Qingdao Offshore Engineering CO., LTD. was analyzed in this paper. After a good mastery of FEM parametric technology, sub-structure technology, enlighten optimization and MATLAB programming technology, a scheme to search the best lift points using advanced optimization technologies was proposed, which was based on finite element numerical method to figure out the structure strain energy in different lift points system and take the minimum strain energy as the objective. The main research work and results of the paper are as follows.(1) Features of the long span steel structures like the offshore platform deck are analyzed and different model features are extracted into different categories. That is to classify all offshore platform decks into different module categories. Each module can be parameterized and the finite element model can be established by assembling these parameterized modules. Simultaneously, considering the computing time will account for a very big proportion of the whole optimization period, the sub-structure technology was used to condense the DOFs of each module and reduce the number of nodes and the work amount of computing. This modeling method based on parameterized sub-structure assembling can realize a swift finite element modeling process and a shorter optimization period, as well as a high computing accurateness.(2) Mathematical model of lift points and punishment function of restriction condition was set up based on PSO (Particle Swarm Optimization), which takes the effect of stress concentration in structural finite element calculation into account, takes the total strain energy as the objective function and the lifting capacity of each crane as a constraint condition. Considering the particularity of lifting point optimization, that is the comprehensive design of numbers and locations and the discrete distribution characteristics, this paper upgraded the algorithm. A hierarchic optimization scheme based on local search was proposed, which applied hierarchical search in design of numbers and locations, generally adopted a comprehensive optimization principle of rough search in numbers of outer layer lift points and precise search in locations of inner layer lift points. The dichotomy was applied to accelerate the convergence of the program.(3) Take the advantage of high efficiency of scientific computing in MATLAB language, a hierarchic optimization program based on PSO is programmed and it was compared with Enumeration Algorithm, Genetic Algorithm, Simulated annealing Algorithm and the Zero Order Optimization affiliated to ANSYS. The superiority of the method proposed in this paper in search efficiency and optimization degree was verified. All the optimization methods mentioned previously have been written into APDL files, for the program to call to solve the objective function.(4) Established the finite element model, carried out case analysis of lift points optimization and verified the effectiveness of the method proposed in this paper.The method made out in this paper was highly important to amend traditional lift points design process and cranes selection. The result also proved that the lifting scheme set lift points system as system objective is more rational due to the exact theory for computing and a way to overall design.
|
PDF Full Text Request