Structural life assessment of oil rig pipes made of API J55 steel by high frequency welding

Abstract

Predmet rada je analiza integriteta i procjena preostalog vijeka zaštitnih zavarenih cijevi izrađenih od čelika API J55 visokofrekventnim kontaktnim zavarivanjem (VF). Eksperimentalna ispitivanja mehaničkih svojstava osnovnog materijala i ispitivanja na zamor izvršena su na cijevima povučenim iz naftne bušotine nakon 70 000 sati rada. Ispitivanje brzine rasta zamorne pukotine da/dN i opsega faktora intenzivnosti naprezanja na pragu zamora, ∆Kth, izvedeno je na standardnim Charpy epruvetama sa zarezom. Broj ciklusa do loma je izračunat na osnovi početne i krajnje kritične duljine pukotine. Na osnovi broja ciklusa do loma i broja ciklusa opterećenja određen je preostali vijek cijevi s vanjskom aksijalnom površinskom pukotinom. Pokazano je da je preostali vijek osjetljiviji na promjenu opsega opterećenja nego na početnu dubinu pukotine.Structural life of welded pipes made of API J55 steel by high-frequency (HF) welding has been evaluated. Experimental tests of base metal mechanical properties, including fatigue properties, were conducted on pipes after 70 000 hours of service in an oil drilling rig. The fatigue crack growth rate and fatigue threshold is obtained using the pre-cracked Charpy specimens. The number of cycles to the final fracture is then calculated using initial and critical crack depths. Based on the number of cycles to the final fracture and the remaining fatigue lifetime of pipes with axial outer surface crack are determined. It was shown that the remaining fatigue lifetime is more sensitive to the changes in the stress range than to the initial crack depth