| Self-elevating offshore platform is the working and living place where offshore oil is developed. Spud leg is an important part of self-elevating drilling platform, it supports its entire structure at offshore operation. Owing to the limitation of manufacturing process, the structure of platform leg inevitably exists some defects, such as tiny bubbles, slag inclusion and so on. The leg is in a certain bad condition chronically, and constantly suffers from wind, wave, flow and the effect of cyclic loading. Under the influence of the alternating load, tiny defects can easily form and expand crack, and then makes the bearing capacity of pile legs fell sharply, causing serious consequences.Through expanding and simulating crack of weld toe area, this paper finds a rule of structure crack propagation applied to marine environment, in order to take appropriate measures to control the propagation of the crack timely when found the cracks on structure, which can prevent a major offshore platform structure from disasters. It is significant for maritime safety production. Firstly, this paper introduces the main load structure and the working environment of the self-elevating offshore platform, and then studied the structure of initial crack in leg crack propagation process system around the core platform leg structure, combining the theory of fracture mechanics. Specifically, this paper mainly has carried on the exploration and research in the following aspects.Firstly, based on the research and analysis of offshore platform structure, we established finite element model of self-elevating offshore platform, and simplified some of the secondary structure of the platform appropriately. Through " Code for Loads of Port Engineering", "Rules for The Classification and Construction of Fixed Offshore Platform", the environmental load and its own gravity load which the platform received is calculated, and imposed by finite element software, after calculated and got finite element analysis results of the whole offshore platform under the condition of the storm deposit. The results showed that the splash zone of leg structure is weak, it is necessary for key research of this part, the calculation results accord with the actual engineering situation.An offshore platform was established which contains girth weld leg model, and the model of leg vertical welds and reinforcing plate structure is omitted, only keeping the girth weld spatter area. By applying the wind, wave load and gravity load platform, we have carried on the detailed calculation, got leg finite element analysis results, which 1.8 m above sea level in leg force is larger. In order to reduce the unnecessary calculation amount, cutting out a piece of the model from the legs girth weld to make a child model, using professional 3 d crack propagation analysis software, add the oval initial crack on sub models, and the stress intensity factor at the crack front are calculated, the results show that the stress intensity factor at the crack tip, value maximum, constantly promote the crack extension. Using composite crack propagation criterion of the maximum circumferential stress criterion to simulate the crack propagation, by setting the length of crack propagation, and the stress intensity factor as the impetus to simulate crack propagation, got the crack extension after ten step topography. Simulate the crack propagation topography is well represent the course of the actual engineering to crack, to have a certain guiding significance to the engineering detection.Through the relationship between the length of crack propagation and the stress intensity factor of crack, combining with the Paris crack growth rate model to calculate the crack size and the relationship between the applied load cycles, is obtained by calculating the stress cycles, implements the leg fatigue crack propagation life of the calculation. |
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