Fracture Assessment Optimization Study Of Offshore Pipelines With Circumferential Weld Under Large Strain

S-lay,J-lay and reel-lay are the main laying and installation methods of offshore pipeline.Reel-lay is both efficient and cost-effective.During the process of reel and lay,pipelines are subjected to 1-3%axial large strain.The pipeline will also be subjected to internal pressure during service.Inner clad is also required to transport corrosive media.Fracture assessment is required to assure safety when pipeline contains cracks.This paper optimizes and improves the existing fracture assessment methods.The main research includes the following aspects.Reference stress method is modified,and fracture assessment is conducted for offshore pipelines with equal matching weld and circumferential surface cracks under large-scale axial strain.The newγfactor value is determined by normalizing FAC from R6 option 2 and FEA.The limit load solution of Kastner is modified with newγfactor,so that the modified reference stress method has the same accuracy as the finite element method.The empirical fomula of newγfactor is proposed by regression analysis.The transformation of input parameters from strain to stress is realized by using the derived nominal stress equation.The modified reference strain method is used to conduct fracture assessment of offshore pipelines with equal matching weld subjected to large strain and internal pressure during service.A reference strain theoretical solution that can consider the internal pressure effect and severe cracks of the pipeline is derived.Based on this solution,a failure analysis curve is constructed which can distinguish the influence of internal pressure on fracture assessment.Therefore,the reference strain method is modified.An analytical formula for the J integral of clad pipe with circumferential surface crack and wide range low-match V-shaped groove weld is proposed.A clad pipe model generator based on Python language is developed independently,which is used to build a large number of clad pipes models with circumferential surface cracks.Based on numerical calculation results and hundreds of regression analysis,the empirical formula of the input parameter F of the modified reference strain method is determined.Therefore,the analytical solution of normalized J integral is determined,and the fracture assessment of clad pipes with surface crack and wide range low-match welds is realized.An analytical fracture evaluation method for clad pipes with circumferential embedded cracks and V-shaped groove welds is proposed.Based on the Python language,two plug-ins are developed to build a large number of clad pipe models with embedded cracks cyclically.Based on the numerical calculation results and hundreds of regression analysis,the empirical formula of the parameter F_top,bottom in the modified reference strain method is obtained.Therefore,the analytical evaluation of clad pipes with embedded cracks is realized.In contrast with the method provided by DNV-RP-F108 which transforms embedded cracks into surface cracks in evaluation,the new evaluation method is more direct and accurate,and the influence factors such as the inner clad and the weld bevel angle on the fracture evaluation is considered.The fracture assessment process of offshore oil and gas pipeline under large-scale strain is introduced.The input parameter during evaluation process is explained.The evaluation flow chart during pipeline installation and service are proposed.At last,several evaluation cases are provided for reference corresponding to the flow chart.The analytical methods and experimental results of two evaluation stages are compared.