Safety Assessment For Oil Thimble

The oil thimble is very important in the exploitation of petroleum. Defects in a thimble are usually ge (?)erated in construction and operation. The residual strength of the oil thimble defeated must be evaluated so that the correct decisions can be made: the thimble can continue to service, require to be maintained or must to be replaced.Firstly, this paper sets forth in the scientific basis , development background and present research situation of the residual strength assessment technology of oil thimble. Then it puts forward the study intention and detailed study containing and also frames the proper technique courses.The exterior surface crack is the most dangerous among crack defects of oil thimble. The common assessment methods, including the limit load method and fracture mechanics method are discussed and the developing trend of the research on assessment methods is analyzed. Then the energy difference rate method is applied to obtain a simplified closed form solution of the Bernauli's equation about crack opening displacement, which is ultimately deduced in the oil thimble with semi-ellipse outer surface crack or inner surface crack under the a (?)al traction load. For casingΦ5.5"×0.361",Maple language is used to compute non-dimensional values of shape factor of stress intensity factor .On the basis of this, residual strength assessment has been conducted by use of the R6 section l(CEGB). The corresponding security boundary is determined and the influence of pressure and the axial load on the remaining strength has been Systematically analyzed which is convenient for engineering application.Then the API 579 formula Level 1,given by recommendation procedure of fitness for service criterion in petroleum and gas industry by API ,and the finite element analysis were performed on the load-carrying capability of a pressurized oil thimble with defects in the outer surface. Finite element models were established for the pressurized pipe of Φ5.5"×0.361" in outer diameter with pit defects in the outer surface. Strength analysis and calculation of the limit load were carried out based on the pressure vessel code of design by analysis. Finally, difference between API 579 formula and finite element method is proposed and the reason in analyzed. Finally, indefinity of the parameters is considered. Based on the FAD technology and API 579 formula, the .failure probability computation model of the oil thimble containing the crack and the volume defeats are established according to Monte Carlo method. At last, a typical pressure vessel containing complex defeats is involved and the failure probability is calculated. This result really reflected the secure state..