조관 공정의 수치시뮬레이션에 기반한 해저 파이프라인 최적 설계 절차

학위논문 (박사)-- 서울대학교 대학원 : 건설환경공학부, 2017. 2. 고현무.A pipeline is made of segmented steel pipes connected to form a continuous pipe system for the transportation of oil or gas over a long distance. During the past several decades, UOE and JCO pipes have gained increasing application to produce at lower cost offshore pipelines with diameter larger than 16 inches instead of the conventional seamless pipe. In UOE and JCO pipe forming processes, the steel plate is subject to a series of plastic forming including pressing and spring back and a final welding stage to form the circular pipe. However, the complicated histories of the plastic forming processes executed in the UOE and JCO pipe forming methods involve the following problems. First, the formed pipes develop material properties differing significantly from those of the raw plate. The repeated loading and unloading cycles conducted throughout the forming processes alter the yield strength and curved shape of the stress-strain response of the pipe due to the Bauschinger effect and work hardening. Apart from having critical effect on the structural performance of the steel pipes, these material properties are also representative indicators of their quality. Therefore, the accurate prediction of the material properties will result in non-negligible economy in terms of the cost and time spent for the repeated inspection, design and production performed to secure the strength and structural performance of the formed pipe. The second problem relates to the geometric imperfection and residual stress inherent to the repetition of plastic forming and elastic spring back experienced throughout the UOE and JCO forming processes. Along with the material properties, the ovality and residual stress of the pipe are dominant parameters determining its collapse pressure or bending capacity but occur in such an unpredictable manner that they increase the design uncertainty. The consideration of all these interrelated parameters by means of coefficients as well as the high material and geometrical nonlinearities in the design limits the accuracy of the prediction. This loss of accuracy itself results in excessively conservative design that does not guarantee the pipe to provide consistent quality and satisfactory structural performance. Such situation stresses the pressing need for a method enabling to predict accurately the material properties and structural performance of UOE and JCO steel pipes. This thesis presents an optimal design procedure for offshore pipelines manufactured by UOE and JCO forming processes. The proposed procedure involves (1) the computational simulation of UOE and JCO pipe forming processes by finite element analysis to provide accurate prediction of the parameters of the formed pipe including its material properties, geometrical imperfections, and residual stress(2) the structural analysis of the steel pipe using the results of the simulationand, (3) the maximization of the collapse pressure of the formed steel pipe known to be the main structural performance, while ensuring its producibility and quality. To improve the accuracy of the simulation of the UOE and JCO pipe forming processes, nonlinear combined hardening model is applied to describe the plastic characteristics including yield plateau and evolution of Youngs modulus as well as work hardening and Bauschinger effect. The strain-stress response is obtained by tension-compression cyclic test on the raw material, and the genetic algorithm and RMS method are combined to derive fifteen material parameters. Finite element simulation of the UOE, UOC, JCOE, and JCOC forming processes is performed and the corresponding configuration, material properties, geometric imperfections, and residual stresses are derived for each of the processes. From these results, the yield strength can be predicted directly and the producibility of the steel pipe can be checked by monitoring the shape change of the plate and the reaction force applied to the forming tools. The validity of the numerical simulation of the forming process as well as the derived results are verified by the tensile test conducted on a sample cut from a steel pipe produced by UOE forming. Numerical analyses are then performed to estimate the collapse pressure and bending capacity of steel pipe based on the simulation outputs. Here also, the results are in good agreement with the experimental results of previous studies. Parametric analysis is performed to investigate the effect of the pipe expansion and compression on its material properties and structural performance. It is found that larger pipe expansion increases the tensile yield strength but degrades the collapse performance. Therefore, executing compression instead of expansion can increase significantly the collapse performance but with some loss of the tensile yield strength. However, neither compression nor expression appears to affect relevantly the bending capacity. Finally, the optimal design procedure for UOE and JCO pipes is proposed considering the trade-off effect of the design variables on the yield strength and collapse pressure. The proposed procedure is seen to improve the design consistency and efficiency compared to conventional methods and to achieve maximized collapse pressure while securing the producibility and quality of the UOE and JCO pipes.1 Introduction 1 1.1 Background 1 1.2 Literature Review 4 1.3 Research Objective and Scope 7 1.4 Outline of thesis 10 2 Pipe Forming Processes for Offshore Steel Pipe 12 2.1 Description of the Forming Processes 12 2.1.1 UOE and UOC Forming 12 2.1.2 JCOE and JCOC Forming 26 2.2 Change in Material Properties during the Forming Process 30 2.2.1 Work Hardening 31 2.2.2 Bauschinger Effect 34 2.3 Ovality and Residual Stress after Forming Process 36 2.3.1 Ovality of the Cross Section 36 2.3.2 Residual Stress on Pipe Wall 40 3 Prediction of Yield Strength and Structural Performance of the Pipe 43 3.1 Numerical Material Model for the Simulation 43 3.1.1 Constitutive model 43 3.1.2 Calibration of material parameters using test result 50 3.2 Computational Simulation of Pipe Forming Process 57 3.2.1 Finite Element Modeling Description 57 3.2.2 Calculation of Yield Strengths 70 3.2.3 Results for Ovality and Residual Stress 77 3.2.4 Experimental Verification of the Model 80 3.3 Prediction of Structural Performance of the Pipe 92 3.3.1 Collapse and Bending of the Steel Pipes 92 3.3.2 Details of Finite Element Modeling 98 3.3.3 Calculation of the Structural Performance 101 3.3.4 Verification of the Model 106 4 Investigation of the Influence of Design Variables through Parametric Study 112 4.1 Key Parameter Selection 113 4.2 Influence on the Yield Strengths 114 4.2.1 Compressive Yield Strength in hoop direction for Collapse analysis 114 4.2.2 Tensile Yield Strength in longitudinal direction for Bending Analysis 120 4.2.3 Tensile Yield Strength in hoop direction for Quality Control 126 4.3 Influence on the Structural Performance 132 4.3.1 Collapse pressure 132 4.3.2 Bending capacity 138 5 Optimal design procedure for Offshore Steel Pipes 144 5.1 Definition of the Optimization Problem 144 5.2 Flow for Optimal Pipe Design 146 5.3 Illustrative Example of UOE Pipe Design 149 6 Conclusion 153 References 157 초 록 168Docto

Strain-Based Design Methodology of Large Diameter Grade X80 Linepipe

Continuous growth in energy demand is driving oil and natural gas production to areas that are often located far from major markets where the terrain is prone to earthquakes, landslides, and other types of ground motion. Transmission pipelines that cross this type of terrain can experience large longitudinal strains and plastic circumferential elongation as the pipeline experiences alignment changes resulting from differential ground movement. Such displacements can potentially impact pipeline safety by adversely affecting structural capacity and leak tight integrity of the linepipe steel. Planning for new long-distance transmission pipelines usually involves consideration of higher strength linepipe steels because their use allows pipeline operators to reduce the overall cost of pipeline construction and increase pipeline throughput by increasing the operating pressure. The design trend for new pipelines in areas prone to ground movement has evolved over the last 10 years from a stress-based design approach to a strain-based design (SBD) approach to further realize the cost benefits from using higher strength linepipe steels. This report presents an overview of SBD for pipelines subjected to large longitudinal strain and high internal pressure with emphasis on the tensile strain capacity of high-strength microalloyed linepipe steel. The technical basis for this report involved engineering analysis and examination of the mechanical behavior of Grade X80 linepipe steel in both the longitudinal and circumferential directions. Testing was conducted to assess effects on material processing including as-rolled, expanded, and heat‑treatment processing intended to simulate coating application. Elastic-plastic and low-cycle fatigue analyses were also performed with varying internal pressures. Proposed SBD models discussed in this report are based on classical plasticity theory and account for material anisotropy, triaxial strain, and microstructural damage effects developed from test data. The study results are intended to enhance SBD and analysis methods for producing safe and cost effective pipelines capable of accommodating large plastic strains in seismically active arctic areas