Investigation On Nonlinear Buckling And Collapse Of Composite Structures For Deepwater Sandwich Pipes

With the expanding of exploration and exploitation of offshore oil-gas resources into deepwater and ultra-deepwater regions, higher demands for offshore pipelines are put forward. The traditional single-walled offshore pipeline is no longer applicable to deepwater and ultra-deepwater regions for the large buoyancy weight and inferior thermal insulation performance. The composite structure for sandwich pipe is a newly-developed technique for offshore pipeline bundles, which shows significant advantages in the exploitation and transportation of deepwater oil-gas resources. Offshore pipeline has to sustain severe external hydrostatic pressure and curvature of various levels during the installation and service life. Meanwhile, the existence of material and geometric nonlinearity further complicates the mechanical properties. Under the deepwater high pressure, offshore pipeline is vulnerable to local buckling, which has the potential of initiating a propagating buckle, resulting in the failure of the whole pipeline, as well as enormous economic and ecological loss. Therefore, it’s of great engineering significance to investigate the nonlinear buckling and collapse mechanisms of deepwater sandwich pipes.Polypropylene was employed as the core material for the superior structural strength and thermal insulation performance. The adhesion behaviours between the core layer and steel pipes were measured by experiments. Afterwards, accurate numerical simulation analyses on the buckling and collapse of sandwich pipes considering various inter-layer adhesion behaviours were conducted. The research work presented in this paper can be summarized as follows.(1) Experimental studies on the inter-layer adhesion behaviours of sandwich pipes. The stress-strain curve of polypropylene was measured by uniaxial tensile test and uniaxial compressive test. Different experiments were designed to investigate the circumferential and longitudinal inter-layer adhesion behaviours, respectively. Besides, the effects of various adhesives and roughness of contact surface on the adhesion behaviours were also studied.(2) Investigation into the nonlinear buckling and collapse of sandwich pipes under external pressure. The buckling and collapse of sandwich pipes under external pressure was simplified into a plane strain problem. The corresponding finite element model was established using the software ABAQUS, in which the inter-layer adhesion behaviours were modeled by nonlinear nodal springs. Based upon extensive parametric studies, the influencing mechanisms of inter-layer adhesion behaviours, initial geometric imperfections, geometric configurations and material properties on the buckling and collapse of sandwich pipes were investigated.(3) Investigation into the nonlinear buckling and collapse of sandwich pipes under pure bending. The three-dimensional finite element model of sandwich pipes under pure bending was established. Furthermore, a series of sensitivity analyses were carried out to explore the effects of pipeline length, inter-layer adhesion behaviours, geometric configurations and material properties on the buckling and collapse of sandwich pipes.The aforementioned results show that the collapse capacity of sandwich pipes is significantly superior to that of traditional single-walled pipes under external pressure or pure bending. The inter-layer adhesion behaviours have major impacts on the collapse capacity of sandwich pipes under external pressure, while the influences are minor under pure bending. Moreover, the geometric configurations of sandwich pipes and material properties of steel pipes have a remarkable influence on the buckling and collapse capacity of sandwich pipes.