Research On Residual Strength Of New Composite With Fatigue Crack Damage

Riser is one of the important equipment of offshore oil and gas resources exploitation,which connects the subsea equipment and platforms at sea level. Compared with shallow water, the environment is worse in the deep-water riser. In the deep sea oil field, the composite riser is more and more widely used due to its structure and good mechanical properties. Riser served at the sea with server environment for a long time, corrosion and crack are the common damage types of riser, which affects the safety and durability, thereby threaten the use of oil resources safety. In order to avoid the occurrence of riser accidents and cause unnecessary economic losses, analyzed the effect of crack damage on the structural strength of riser and evaluate the safety of riser reasonably.A typical deep-water compound oil riser is taken on this paper. Firstly, the overall strength of the riser in typical condition is analyzed to determine the weaken components and joints of riser structure.Based on the principle of fracture mechanics, get the crack size with time varying. On this base, the change of the remaining life and the residual strength of the vertical tube structures are studied by taking into consideration two factors of corrosion and fatigue crack. Following research work have been studied specifically:1、This paper briefly introduces the research status surface cracks and damage marine structures after the ultimate strength, detailing the calculation method of the ultimate of the structure.2、Propagation of fatigue crack based on the theory of fracture mechanic and nonlinear effects are discussed, multi-scale method is introduced into local damage analysis of the new composite rise.3、According to the structural form and parameters of the riser, The formula for calculating the bending rigidity of helical steel layer and analyzed under bending loads by finite element analysis.Comparison between results with finite element analysis and that of theoretical calculation were conducted which verify the correctness of theoretical analysis.4、According to the overall structure of the distribution of the stress riser, get the point of riser, and built of a multi-scale model. Under the action of environmental loads, get the crack size with time varying based on the fracture mechanic. The growth of surface cracks in the length and depth of the two directions, and to explore the surface during the crackpropagation two direction. The controlling effects factors that affect the fatigue life of compound riser are determined.5、The key component with time varying crack is built, get the changing rule of residual strength of compound riser with the time-varying crack. Crack area which had obvious influence on the residual strength of whole structure is selected as analysis object,considering the effects of crack size and uniform corrosion on the ultimate strength of key component, the residual strength of key component with crack is analyzed.