Research On Mechanical Response Of Submarine Pipeline

Submarine pipeline is an important part of marine petroleum development. Traditionally, concrete encapsulated steel pipe is used as submarine pipeline, and corrosion, spanning pipeline static breakdown and fatigue breakdown are the main causes of its failure. As a composite pipeline consisting of different functional layers, with the advantages of excellent corrosion resistance, flexibility, fatigue resistance, easy installation, repeated use, etc., submarine oil pipeline is gradually replacing traditional steel submarine pipeline in foreign countries. In view of the use status of submarine oil pipeline in foreign countries and the successful use of imported flexible pipe in Bohai Sea and South Sea of China, petroleum department hopes to independently develop and produce relevant products, in order to save the high-expenditure of imported flexible pipe. Under the demand of field of petroleum, based on GAN Guo-gong's steel-plastic composite water transmission pipeline patent, Chinese rubber industry has developed multilayer spiral-wound steel wire reinforced rubber flexible pipe that is applicable to the field of petroleum. Such submarine oil pipeline is neither different from the flexible pipe of the same name produced abroad, nor different from domestic composite water transmission pipeline, but is a brand-new composite pipeline; moreover, it hasn't been really used, for there are nearly gaps in domestic and overseas relevant researches. This topic is sourced from submarine pipeline damage mechanism and health diagnosis, the State Key Program (50439010) of National Natural Science Foundation, and is entrusted by Research and Design Institute of China Shengli Oil Field to carry out relevant theoretical and experimental researches on the mechanical response of submarine oil pipeline by taking the flexible pipe manufactured by Hebei Ouya Special Rubber Hose Co., Ltd. as prototype specimen.(1) Researches on spanning mechanism and spanning length of submarine pipelineInvestigation data shows that the spanning phenomenon of submarine pipeline is rathercommon and brings the hidden trouble for submarine pipeline's safety run. Based on domestic and overseas literatures, this paper presents the 4 main spanning mechanisms of submarine pipeline with the through comprehensive analysis. Combined with the actual location survey data of Chinese submarine pipeline, taking Chengdao sea area as an example, this paper carries out compartive analysis upon the empirical formulae for the spanning length amd spanning depth of submarine pipelines that were proposed by domestic and overseas scholars.(2) Researches on external load of spanning submarine pipeline under acutal sea conditionSpanning submarine pipeline mainly bears wave-flow hydrodynamic force under acutalsea condition. Based on domestic and foreign literatures, this paper summarizes forms of load of spanning submarine pipeline by wave-flow hydrodynamic force, and presents that the external load system of spanning pipeline in Chengdao sea area can be simplified as horizontal constant tide load and vertical dynamic tide load and horizontal dynamic wave load. Besides, taking Chengdao sea area as an example, through comparative analysis of the velocities and accelerations of wave water particles calculated by using Airy wave theory, five-order Stokes wave theory and cnoidal wave theory, this paper presents that Airy wave theory can be used for calculating the wave forces of submarine pipeline in Chengdao sea area.(3) Researches on constitutive relation for reinforced layers of submarine oil pipeline.Researches on constitutive equation of steel wire wound reinforced single-layer is thefoundation for the mechanical analysis of submarine oil pipeline. According to the features of reinforced layers of flexible pipe, this paper contrastively analyzes domestic and overseas 8 classic prediction models that are applicable to continuous fiber reinforced layer. It is recommended that Chinese engineering empirical formula should be used for predicting the engineering elastic constant in steel wire wound reinforced single-layer under ortho-axis coordinate system of material. Based on the microscopic theory of composite material, Chinese engineering empirical formula is expanded to 3D case; based on tensor theory, constitutive equation of steel wire wound reinforced single-layer board under cylindrical coordinate system of structure is obtained.(4) Researches on mechanical response of submarine oil pipeline under internal pressureOne of functional loads of submarine oil pipeline is internal medium pressure. Previousmechanical researches on steel wire wound composite pipeline under internal pressure generally adopt thin-shell theory. The high internal pressure requirements of submarine oil pipeline make itself a thick-wall structure with multiple reinforced layers, therefore, thin-shell theory is not applicable. This paper adopts flexible pipe as a laminated structure consisting of multiple anisotropic reinforced layers and multiple isotropic rubber layers, and based on 3D anisotropic elastic theory, derives analytic solutions for mechanical response of flexible pipe under internal pressure. The present analytic method is used for computing the displacement, strain and stress of flexible pipe under internal working pressure, and the static loading test and researches upon the flexible pipe prototype specimens under internal pressure is carried out. The calculated values by present method basically coincided with the static loading test results in changing trend, and are close to them in value.(5) Researches on mechanical response of submarine oil pipeline under combined action of internal pressure and transverse loadSpanning submarine oil pipeline has to bear both internal medium pressure and wave-flow hydrodynamic force transverse load under acutal sea condition, and the flexibility characteristics of flexible pipemake itself mostly be in the state of large deformation under acutal sea condition. This paper adopts initial parameter equation of beam deflection curve to determine the axial mean strain of flexible pipe under transverse load, and combined with the 3D anisotropic elastic solutions for infinite small deformation flexible pipe, derives approximate analytic solutions for the mechaincal response of large deformation flexible pipe under combined action of internal pressure and transverse load. By using the present analytic method, this paper computes the mechanical responses of flexible pipe under combined action of internal pressure and transverse load and carries out relevant analysis. The special material and structure of submarine oil pipeline make it difficult to determine its equivalent axial elastic modulus. Through the static loading test and researches of flexible pipe prototype specimens, the qualitative analysis of the equivalent axial elastic modulus of flexible pipe is carried out.(6) Preliminary researches on damage fatigue of submarine oil pipelineThe spanning submarine oil pipeline bears dynamic load of wave-flow hydrodynamic force under actual sea, which causes fatigue failure to flexible pipe throughout the service life. Combined with the fatigue test of flexible pipe prototype specimens under transverse cyclic load, and based on the microscopic damage theory of composite material, this paper carries out relevant micro-analysis upon the damage mechanism of flexible pipe. Submarine oil pipeline has special application fields. Combined with the fatigue test of flexible pipe, this paper presents thee failure criteria including flexible pipe bulging near end, flexible pipe leakage near end and flexible pipe body leakage. Based on Rosen shear-lag model, according to Cox continuous fiber load transfer formula, this paper carries out relevant theoretical analysis upon the failure modes of flexible pipe bulging near end; based on linear elastic fracture theory, this paper carries out qualitative theoretical analysis upon the failure modes of flexible pipe leakage near end; based on phenomenological damage theory, this paper carries out relevant theoretical analysis upon the failure modes of flexible pipe body leakage.(7) Preliminary researches on large deformation dynamic response of submarine pipelineSubmarine oil pipeline generally bears dynamic load under acutal sea condition. Thispaper simplifies spanning flexible pipe as isotropic Bernoulli-Euler elastic curved beam, and based on finite deformation theory of continuous medium mechanics, and in consideration of the effect of internal fluid motion on flexible pipe itself, derives the governing equations of dynamic response of spanning large deformation flexible pipe under dynamic load and carries out relevant analysis.