Research On Stability Of Decommissioned Offshore Jacket Platforms During Cutting

Nowadays, the removal of the abandoned offshore jacket platform has been listed in the domestic relevant laws and regulations and international conventions. At present, in our country there are more than100offshore platforms entering into the abandonment stage, which makes the removal more imperative. The key part of the removal is the jackets, which is influenced mostly by cutting.Jacket as the mainly part for supporting the upper structure of the offshore platform, most of which consists of cylindrical steel tube by the way of welding according to definite structure schema. So at present the stability calculation of the jacket is mainly in terms of the classical stability theory of cylindrical shells under axial compression, which neglects the damage of jacket. Actually the propagation of notch has a great influence on the stability of jacket during cutting process. And at present the stability analysis of perforated structure is usual on the basis of the Donnell hypothesis, which is not available in the field application. Combined with the classical stability theory of cylindrical shells, the stability formula of jacket in the removal process suitable for the field application is deduced through numerical analysis and experimental verification in this paper. Specific content as follows:(1) On the base of classical stability theory of cylindrical shells, the working condition of jacket in the removal process is analyzed; the reasonable physical model is built as well. Through analyzing the mainly influencing factor, the analysis plan is determined by orthogonal test. The sensitivity of varying factors to the stability of jacket is analyzed by the finite elements software.(2) The theory of linear small deflection and large deflection buckling on cylindrical shells without notch is analyzed, as well as the stability theory of cylindrical shells with notch. This is applied to establish the mathematical model of buckling critical load calculation. And the mathematical model is fitted by differential evolution (DE). And the reasonable justification is made in view of the practical application.(3) According to the theoretical calculation, the stability test designed and implemented. The stability of cylindrical shells under different incision is tested with the seamless steel tube as the research object by using pressure testing machine, strain tester, resistance strain gauge and other instruments and equipment. The buckling shape and strain rule of the steel tube in the test are analyzed. And the results are coMPared with the theoretical value, which is applied to revise the stability formula of the jacket deduced in former. (4) Set one abandoned offshore jacket platform at South China Sea as the model, both the stability formula and finite element method are used to analyse the stability. And the results verify the accuracy of the stability formula of the jacket in the field application.In this research, the calculation method of bulking critical load of the jacket during cutting process is deduced through studying the development of jacket removal, the cutting method of jackets in domestic and foreign and determining the load types and boundary conditions. Using the similarity theory, the calculation method of jacket during cutting process is perfected and revised by test to assure the accuracy and practicability, which can provide the reference for guaranteeing the cutting process security.