Reliability-based Optimization Research On Tower Of Topside Lifting System

With rapid development of global economy, the amount of oil and gas required becomes much larger. Meanwhile, inland and offshore oil and gas resources are exhausted. However, there is abundant oil in deep sea, so the oil extraction will be moved toward the deeper waters, and large tonnage offshore platforms can provide the necessary technical support. The construction process of platforms has become a hotspot in the field of marine engineering, and how to complete the assembly of platforms by lifting topside to a certain height is the key process. By the research on specific structure of a platform, a set of large tonnage topside lifting system is designed, in order to reduce the whole construction period and improve the engineering quality.Considering the expensive production cost of offshore platform topside, the accuracy and safety of lifting process must be ensured. Hence, in view of structural design of lifting system, the reliability of every part must be guaranteed.The tower is very important for sustaining loads in lifting system, and its stress status is usually complicated. In addition, design quality of the tower has a great influence on working performance of the whole lifting system. It is impossible to ensure the reliability of the tower by taking the inherent uncertainty of geometric parameters, material properties and the loads into account, as well as to obtain the model with minimum weight by conducting optimization to its multiple parameters structure at the same time, with the only method of reliability analysis or the certain optimization. To solve this problem, the two methods must be integrated, in other words, running the reliability-based optimization to the tower.First of all, based on the secondary exploitation technology of ANSYS program, parametric model of the tower is established in APDL, and the model is assigned as the analysis file for certain and reliability-based optimization. The working condition for optimization is confirmed by comparing the analysis results of the tower in several conditions. Then the mathematic model of optimization design is set up, in which structural parameters with the greatest influences on invalidation of the tower are assigned as optimization variables, strength and stiffness design requirements are assigned as constraints, and minimum weight is the objective. The certain optimization is carried out by OPT module in ANSYS, and finally, the certain optimal tower structure is obtained. Secondly, considering the inherent randomness of design information, reliability analysis to the certain optimal tower model is executed with Monte Carlo method. The criteria that probabilities of maximum Von Mises stress and maximum horizontal displacement both less than respective allowable values are no less than95%is quoted to judge whether the structure meets reliability design requirement. The flow of reliability-based optimization is brought forward. According to the sensitivity analysis results and the scatter drawings of design parameters, the value ranges of important random input variables are optimized, in order to improve the structural reliability. So far, the random optimal structure with enough reliability and minimum weight is obtained.At last, based on the similarity theory, the prototyping model of large tonnage topside lifting system is designed and manufactured. Structural, hydraulic and control tests on the model are conducted, and stresses on the important parts of the tower are measured. By comparing and analyzing the results between model testing and numerical simulation, the rationale of finite element modeling and loading methods of the tower is verified. Further more, the veracity of optimization design result is strongly evidenced.