Engineering Critical Assessment Of Non-planar Flaws In Offshore Pipeline

In the welded structure of offshore pipeline used to deliver oil andgas, the welded joints are the weakness location. This is mainly becausethere are inevitable welding defects, residual stresses and inhomogeneousmicrostructures and poor mechanical properties in the welded joint zone.So the safety assessment of weld joint is very important for the structure.The offshore pipeline is made of pipeline steel which has goodplasticity and non-planar flaws are the most common types of flaws.Based on the principle of "fitness for use", the developed countrieshave made great progress in the research of the safety assessment forpipeline and pressure containers and built a series of standard anddocument. Compared to the planar flaws, the research of non-planar flawsis less, and concentrated on the localized thin flaws.In this study, the international safety assessment standards BS7910and SINTAP are introduced systematically. The present state of researchfor the critical assessment and stable analysis of pipeline with non-planarflaws is discussed and observed. A software is designed with VisualBasic6.0 for the assessment of Non-planar flaws. Finally the ECA(Engineering Critical Assessment) which is based on the principle ofBS7910 was carried out to assess the weld joints of the offshore pipelinebetween Lvshun and Dalian with non-planar flaws with the software. Tofinish the assessment, we obtain the tensile property of the weld byperforming the tensile testing of the weld metal and obtain the fracturetoughness of the HAZ and the weld metal by determining the CTOD-Rcure of them. After that, we assess the non-planar flaws in such twosteps:1.The non-planar flaws which meet special requirements areacceptable. 2. Other flaws should be assessed on the basis of theirdimensions as planar flaws. The assessment results are obtained by theiterated calculation on the principle of BS7910 Level 2, which offer thecritical and allowable dimensions of the flaws. The study is of greatsignificance in terms of engineering and economic effectiveness. Thisstudy offers the laws whether the non-planar flaws are acceptable whenthe offshore pipeline is in the laying condition.