Fatigue fracture resistance of welded pipes made from high strength steels

Funkcionalnost i sigurnost u eksploataciji su dva bitna zahteva za uspešnu konstrukciju, pa se danas uz zahtev čvrstoće pri projektovanju, mora razmotriti i opasnost od zamornog loma, koja je u velikoj meri zavisna od osobina zavarenih spojeva, kao potencijalnih nosilaca greške. Otkazi zavarenih cevi u naftnoj industriji, nastali kao posledica neotkrivenih grešaka u njihovoj izradi ili usled oštećenja tokom eksploatacije, pored uticaja na pouzdanost i bezbednost u radu, imaju i ekološki uticaj na ţivotnu sredinu. Ovo je podstaklo sprovoĎenje intenzivnih istraţivanja u cilju očuvanja celovitosti cevi u bišotinama i cevovodima. Istraţivanja u okviru ove disertacije imala su za cilj da istraţe ponašanje u slučaju oštećenja usled zamora zavarenih cevi izraĎenih od čelika API J55. Čelici povišene čvrstoće mogu biti prilično osetljivi na nastanak i rast zamorne prsline. Uzimajući u obzir uslove agresivne radne sredine, ovi čelici zahtevaju detaljna istraţivanja njihove otpornosti na zamorni lom i procenu integriteta u navedenim radnim uslovima. Primenom parametara mehanike loma, dobijenih na osnovu eksperimentalnih istraţivanja, izvršena je procena preostalog veka zaštitnih zavarenih cevi sa spoljašnjom aksijalnom površinskom prslinom, izraĎenih od API J55 čelika. Analiziran je istovremeno uticaj opsega napona i vrednosti spoljašnjeg aksijalnog oštećenja na preostali zamorni vek. Jedan od ciljeva ove teze bio je da se istraţe nove metode numeričkog modeliranja rasta prsline, a dobijeni rezultati upravo pokazuju efikasnost primene proširene metode konačnih elemenata – PMKE (Extended Finite Element Metхod - XFEM), kao jedne relativno nove metode, u trodimenzionalnoj simulaciji rasta zamorne prsline na geometriji tipa cevi. Rezultati dobijeni u okviru ove teze ukazuju na značajne mogućnosti primene numeričke simulacije potpomognute eksperimentalnim istraţivanjima u analizi integriteta opreme pod pritiskom.The functionality and safety in exploitation are two important requirements for the successful construction and today, together with the strength requirements, the risk of fatigue fracture, which is largely dependent on the properties of welded joints as potential carriers of errors, must be considered. The failures of the welded pipes in the petroleum industry occurred as a consequence of undetected errors in their design or due to the damage during service, have the impact on the reliability and safety at work, as well as an ecological impact on the environment. This encouraged intensive researches in order to preserve the integrity of pipes in the oil rigs and pipelines. The researches in this thesis were aimed to investigate the behavior in case of damage due to the fatigue of welded pipes made of steel API J55. High strength steels can be quite sensitive to the emergence and growth of the fatigue cracks. Taking into account the conditions and aggressive working environment, these steels require detailed researches of their resistance to the fatigue failure and assess the integrity in the specified operating conditions. Through the application of the fracture mechanics parameters, obtained from experimental research, a residual life of welded protective pipes with an external axial surface crack, made of API J55 steel, was estimated. The impact of the stress range and the value of the external axial damage to the remaining fatigue life were analyzed. One of the goals of this thesis was to investigate new methods of numerical modeling of crack growth, and the results just show the effectiveness of the implementation of the extended finite element method - XFEM, as a relatively new method, in three-dimensional simulation of fatigue crack growth on the pipe geometry. The results obtained in this thesis indicate significant opportunities for the application of numerical simulation, supported by experimental studies, to analyze the integrity of the pressure equipment

Mechanical properties of welded pipes produced by high frequency welding of the steel API J55

Izbor čelika, dimenzija i načina proizvodnje zaštitnih zavarenih cevi je deo procesa konstruisanja, jer je u uskoj vezi sa funkcijom cevi kao konstrukcijske celine u određenim uslovima eksploatacije za predviđeni vek trajanja. Kvalitet zavarenih spojeva u procesu proizvodnje cevi definiše se karakteristikama koje one moraju posedovati da bi zadovoljile određene zahteve, što se postiže izborom odgovarajućih postupaka i parametara zavarivanja, sprovođenjem programa kontrole svih tehnoloških operacija u njihovoj izradi i mehaničkim i tehnološkim ispitivanjima otpornosti i deformacije osnovnog materijala i zavarenih spojeva cevi. U radu su prikazani rezultati ispitivanja mehaničkih osobina uzdužno zavarenih cevi, izrađenih visokofrekventnim kontaktnim zavarivanjem od čelika povećane čvrstoće API J55.Selection of the steel, dimensions and manufacturing methods for protective welded pipes is part of the design process, because it is closely related to the function of tube construction as a whole under certain conditions, for a calculated service life. The quality of welded joints in the process of tubes production is defined by characteristics that they must have to meet certain requirements, which are achieved by choosing appropriate procedures and welding parameters, by controlling all unit operations in their preparation and mechanical and engineering stress and strain tests and base material of welded joints. This paper presents the results of the mechanical properties of longitudinally welded pipes. Casing welded pipe intended for oil and gas wells are made by high frequency contact welding of steel API J55, with increased strength

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

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

Fatigue life prediction of casing welded pipes by using the extended finite element method

The extended finite element (XFEM) method has been used to simulate fatigue crack growth in casing pipe, made of API J55 steel by high-frequency welding, in order estimate its structural integrity and life. Based on the critical value of stress intensity factor KIc, measured in different regions of welded joint, the crack was located in the base metal as the region with the lowest resistance to crack initiation and propagation. The XFEM was first applied to the 3 point bending specimens to verify numerical results with the experimental ones. After successful verification, the XFEM was used to simulate fatigue crack growth, position axially in the pipe, and estimate its remaining life