Researches On The Buckling Behaviors Of Deepwater Sandwich Pipes Filled With Composites

The development of advanced petroleum equipment matching the requirements of exploration and development of deep-water oil and gas resources is the cornerstone of successful implementation of marine oil and gas resources exploitation strategy.With the exploitation of offshore oil and gas gradually moving towards deep water and even ultra-deep water area,the subsea severe high pressure and low temperature cold environment has put forward more stringent requirements on the bearing capacity and thermal insulation performance of oil and gas transmission pipelines.However,the application of conventional single-wall steel pipe in deep water has brought about more and more problems,and it is difficult to meet the application requirements.Under this background,sandwich pipe has become a very competitive solution for deep-sea oil and gas development and transportation pipeline due to its excellent structural strength and relatively good thermal insulation performance,and it has aroused the attention of many scholars and researchers.The complex and severe stress environment and the nonlinearity of sandwich pipe structure and material make the sandwich pipe easy to buckle and lose stability in the process of laying and long-term service,which leads to the failure of the whole pipeline and huge economic loss and environmental pollution.Therefore,it is very important to study the buckling behavior of sandwich pipes.In this paper,several kinds of fiber-reinforced cement-based composite materials were designed and prepared as the sandwich pipe core material,and the interlaminar bonding properties of sandwich pipes were studied experimentally.Based on these study,the buckling instability mechanism of sandwich pipe subjected to external pressure,pure bending or the combination load of the former two were studied using finite element numerical simulation method.The effects of core materials,interlaminar bonding properties and initial ovalities on buckling instability of sandwich pipes were discussed.The main contents of the study are as follows.(1)Experimental studies on core layer materials and interlayer bonding properties.The core material experiments were conducted.Nine different core material ratio scheme were designed by orthogonal experiment method.Totally 27 uniaxial tensile and compressive specimens for various materials were fabricated through the mold designing and casting experiments.On this basis,the uniaxial tension and compression experiments of the core materials were completed,and obtained the corresponding stress-strain curves.The interlaminar shear stresses of sandwich pipes with different core materials and core thicknesses were measured by 4 pipes push-out tests.Secondly,the coefficients of friction between two different core materials and inner and outer tube steel material were also measured by slope experiment.(2)Studies on materials constitutive models and interlayer contact model of sandwich pipes.Firstly,the material constitutive theories of sandwich pipes were deeply analyzed.And the improved Ramberg-Osgood constitutive model was built to describe the stress-strain relationship of inner and outer steel tubes.The concrete damage plastic constitutive model was built to describe the stress-strain relationships of core layer materials.Secondly,the interlayer contact theory of sandwich pipes were analyzed,and the corresponding interlayer bonding models were established based on the interlayer bonding performance experiments.In addition,the structural stability theories were studied.(3)Numerical simulations of buckling instability of sandwich pipes.The finite element models of sandwich pipes under external pressure,pure bending and their combined loads were established by ABAQUS finite element software.Then the buckling mechanism of sandwich pipes were simulated.The sensitivity analysis of characteristic parameters were carried out to analyze the effects of characteristic parameters such as 3 different interlaminar bond properties,9 kinds of core materials and 5 initial ovalities on buckling performance of sandwich pipes were studied.The results show that the interlaminar bonding properties,different core materials and initial ovalities of the pipes have significant effects on the buckling performance of the sandwich pipes subjected to external pressure.The better the interlayer bonding behavior is,the stronger the buckling performance of the sandwich pipes will be.The critical buckling pressure of sandwich pipe filled with PVA6 FRCC was increased by 18.27% with the increase of interlayer bond performance.The greater the initial ovality of the pipe,the weaker buckling performance the sandwich pipe would have.When the initial ovality increased from 0.3% to 0.7%,the compressive capacity of the sandwich pipe decreased by 12.15%.And the buckling performance of the sandwich pipe is positively related to the tensile and compressive performance of the core materials.Under bending,good interlaminar bonding performance is beneficial to strengthen the buckling performance of sandwich pipes,but it is not as obvious as that under external pressure.The influences of interlaminar bonding performance and initial ovalities of pipelines is negligible with only 0.26% decrease in resistance to bending while the ovality increased from 0.3% to 0.7%.Subjected to the combined action of external pressure and bending load,the influences of the three characteristic parameters on buckling behavior of sandwich pipes were similar to that under external pressure.It is important to note that the buckling behavior of sandwich pipes will be weakened with the increase of initial external pressure in the combined loading condition.