SCR立管打底焊根部成形及其对疲劳性能的影响研究

Investigation of the Root Formation of SCR Root Welding and Its Effect on Fatigue Performance

浮式平台配合钢悬链线立管(SCR)是深海油气资源主流开发方式之一,具有很高的经济价值和战略意义.但是由于深海浮式结构在风/浪/流作用下将产生较大漂移运动,使得SCR系统在服役期间需要承受巨大极限载荷与平台运动/涡机振动所产生的交变荷载,致使其环焊缝容易产生严重的断裂危险和疲劳失效问题.以Φ355 mm×19.1 mm的X65管线钢为试验材料,对比分析了冷金属过渡(CMT)、表面张力过渡(STT)两种单面焊双面成形打底焊工艺下焊接接头的疲劳性能,发现单面打底焊容易因为焊根余高过大或者未熔合、未焊透等缺陷导致接头疲劳性能显著降低.在此基础上,进一步对比了7种常见的单面打底焊工艺下的接头疲劳性能,并对疲劳断口进行了分析,结果发现所有单面打底焊工艺均难以稳定地保证焊根位置余高偏小且无未熔合和未焊透的缺陷.基于此,提出了一种熔化极气体保护焊/脉冲钨极氩弧焊(GMAW/GTAW-P)双面打底焊工艺来保证SCR立管焊缝的内壁成形质量.试验结果发现,该工艺下焊根余高尺寸可以限制在200μm以内,焊根过渡角θ≤3°,焊根熔宽在6~8 mm,从工程应用角度有效地解决了传统单面打底焊工艺下焊根成形不良的问题,并将焊接接头的疲劳寿命提高至1×10^(7)周次以上,为高疲劳寿命的SCR立管打底焊提供了一种新思路.

In deep-sea oil and gas resource development,the combination of a floating platform with a steel catenary riser(SCR)is the primary method,offering substantial economic value and strategic significance.However,the significant drift motion experienced by deep-sea floating structures owing to wind,waves,and currents necessitates the SCR system to withstand immense ultimate and alternating loads generated by platform motion and turbomachinery vibration throughout its service life.Consequently,the circumferential welds are susceptible to serious fracture hazards and fatigue failure issues.This study focused on the use of X65 pipeline steel with dimensions ofΦ355 mm×19.1 mm,assessing the fatigue performance of welded joints using two types of single-sided welding processes:cold metal transfer(CMT)and surface tension transfer(STT).The analysis revealed that the fatigue performance of the welded joints under the single-sided root welding process was significantly reduced owing to defects such as excessive weld root reinforcement,incomplete fusion,or incomplete penetration.Building on this observation,the fatigue performances of joints formed via seven common single-sided root welding processes were further compared,and the resulting fatigue fractures were carefully analyzed.The experimental results indicated that all single-sided root welding processes faced challenges in consistently ensuring minimal reinforcement and freedom from unfused and unwelded defects at the weld root position.In response to these challenges,a novel welding process involving gas metal arc welding(GMAW)/gas tungsten arc welding-pulsed(GTAW-P)for double-sided root welding was proposed.This process aimed to ensure the quality of the inner-wall formation of the SCR weld.The experimental data demonstrated that this process effectively restricted the size of the weld root reinforcement within 200μm,the transition angle of the weld rootθ≤3°,and the weld root width to 6—8 mm.Importantly,it addressed the issue of poorly shaped weld roots associated with traditional single-sided root welding processes,enhancing the fatigue life of the welded joints to more than 1×10^(7)cycles.Furthermore,it presents a novel approach toward achieving high fatigue life for the root welds of the SCR in engineering applications.