Flexible pipes are complex multi-layered structures used mainly in the offshore oil and gas industry in deep and ultradeep-water applications. In such environments, these flexible structures are subjected to multiple loads, from internal pressure of the transported fluid and high hydrostatic pressure applied by the water column to tensile or compressive axial loads and other stresses like torsion and bending generated by the platform's movements. A flexible pipe is essentially non-serviceable component and consequently the design should provide adequate margins against failure for the specified service life, based on the known degradation and failure mechanisms. Since the mechanical and structural properties are given mainly by the tensile and pressure armor layers, it is crucial to study and analyse the mechanical behaviour of these materials correctly estimating the end of their life and avoiding possible catastrophic consequences. Starting on these premises, the aim of this work is the experimental evaluation of fatigue properties (da/dN vs. ΔK curves and ΔKth) of four carbon steels used for tensile and pressure armors. In this work two types of FCG tests have been performed in air and at room temperature. For minimizing possible crack closure influence in the experimental results, the tests were performed at R = 0.6 load ratio. First class of tests have been done at Constant-Amplitude-Load ΔP. Afterward, the K-decreasing tests have been performed at the same load ratio. All these tests have been done in accordance with ASTM E647 standard. The results of these tests in air at room temperature have been plotted in a da/dN vs ΔK curves and the Paris law constants, C and m, have been found. From the second type of tests, ΔKth values have been obtained. Finally, the experimental constants of the Paris Law and the ΔKth values were compared with results of similar test reported in open literature.
I tubi flessibili sono strutture multistrato impiegati principalmente nell’industria petrolifera e del gas, soprattutto sulle piattaforme petrolifere offshore. Questi tubi possono essere utilizzati a profondità molto elevate e quindi soggetti a diversi sforzi e carichi come la pressione interna causata dal liquido trasportato, la pressione esterna dell’acqua, carichi di tensione e compressione, la flessione e la torsione causate dal movimento della piattaforma petrolifera. Queste strutture sono componenti sulle quali si può intervenire poco durante il loro ciclo di vita. Per questo motivo i possibili difetti e le loro conseguenze devono essere tenuti in considerazione in fase di progettazione. Dato che le proprietà meccaniche e strutturali sono date essenzialmente dalle armature di tensione e di pressione, è molto importante analizzare questi materiali e studiare il loro comportamento meccanico. Partendo da queste premesse, l’obiettivo di questo lavoro è di valutare sperimentalmente le proprietà meccaniche a fatica (curve da/dN vs. ΔK e ΔKth) di quattro acciai usati per armature di trazione e di pressione. Due tipi di test a fatica sono stati eseguiti in aria e a temperatura ambiente. Per minimizzare possibili effetti del meccanismo di crack closure nei risultati sperimentali, i test sono stati svolti sotto un rapporto di tensioni R = 0.6. I primi test sono stati svolti in modalità a carico ΔP costante. I secondi test sono stati eseguiti in modalità ΔK-decrescente sotto lo stesso rapporto di tensioni. L’intero processo è stato svolto seguendo la normativa standard ASTM E647. Dopo che le curve sperimentali da/dN vs ΔK in aria a temperatura ambiente sono state acquisite, anche le costanti di Paris, C ed m, sono state ottenute. Dalla seconda tipologia di test sono stati acquisiti anche i valori di ΔK di soglia nelle stesse condizioni. Infine, tutti questi risultati sono stati confrontati con risultati di test simili riportati in letteratura scientifica.
Determination of da/dN vs. ΔK curves and ΔKth of carbon steels for flexible pipes
Luchin, Vasile
2020/2021
Abstract
Flexible pipes are complex multi-layered structures used mainly in the offshore oil and gas industry in deep and ultradeep-water applications. In such environments, these flexible structures are subjected to multiple loads, from internal pressure of the transported fluid and high hydrostatic pressure applied by the water column to tensile or compressive axial loads and other stresses like torsion and bending generated by the platform's movements. A flexible pipe is essentially non-serviceable component and consequently the design should provide adequate margins against failure for the specified service life, based on the known degradation and failure mechanisms. Since the mechanical and structural properties are given mainly by the tensile and pressure armor layers, it is crucial to study and analyse the mechanical behaviour of these materials correctly estimating the end of their life and avoiding possible catastrophic consequences. Starting on these premises, the aim of this work is the experimental evaluation of fatigue properties (da/dN vs. ΔK curves and ΔKth) of four carbon steels used for tensile and pressure armors. In this work two types of FCG tests have been performed in air and at room temperature. For minimizing possible crack closure influence in the experimental results, the tests were performed at R = 0.6 load ratio. First class of tests have been done at Constant-Amplitude-Load ΔP. Afterward, the K-decreasing tests have been performed at the same load ratio. All these tests have been done in accordance with ASTM E647 standard. The results of these tests in air at room temperature have been plotted in a da/dN vs ΔK curves and the Paris law constants, C and m, have been found. From the second type of tests, ΔKth values have been obtained. Finally, the experimental constants of the Paris Law and the ΔKth values were compared with results of similar test reported in open literature.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/181552