To identify the characteristic microstructural length determining the mechanical properties of a quenched and tempered medium-C steel and its dependence on the prior austenite grain size, different tempering treatments have been carried out after a fully martensitic quenching. The resulting microstructures have been analyzed by Orientation Imaging Microscopy (OIM) and two kind of features have been taken into consideration: packets (i.e. domains delimited by high-angle boundaries) and cells (domains bounded by low-angle grain boundaries). The main results can be summarized as follows: 1. A very weak effect of austenite grain size on packet size was found. 2. A finer packet size was measured at mid-thickness with respect to surface after external and internal water quenching process. This phenomenon was attributed to the effect of the strain path on the phase transformation during quenching. 3. The through-thickness microstructural gradient remains substantially unchanged after tempering. 4. Grains with high-angle boundaries do not significantly grow after tempering; on the contrary, low-angle grain boundaries move, fully justifying the hardness evolution with the tempering temperature.
The collapse of tubular members under external pressure has been the subject of intense research during the last years. Currently the industry is analyzing several collapse prediction procedures that include the effects of, not only the basic geometrical parameters (nominal or average dimensions), but also of the amplitudes of deviations from the theoretical cylindrical shape. However, different measurement procedures currently used in the estimation of the collapse properties of pipes may yield different estimations of the pipe imperfection, for the same pipe. In order to establish a new collapseprediction formula, a reliable measurement procedure must be assured. Introduction Since the work of Timoshenko1 it has been acknowledged that the most important geometrical parameters affecting the collapse resistance of a circular ring (or a pipe) are the slenderness (OD/t) and the lack of circularity of the section. The current work on the collapse prediction of OCTG pipe uses the same geometrical parameters. Since an important part of the prediction is still based on regressions (an complete discussion on this issue can be found in Ref.2), it is important to properly define these variables. The definition of slenderness of the pipe is straightforward, and there has been almost no problems with its interpretation3. For the lack of circularity of the section, Timoshenko considered in his analyses a perfectly oval initial defect, what we are going to call "mode 2". The following definition (generally normalized by the average or nominal diameter) has normally been used in the collapse prediction formulas4: (mathematical equation)(available in full paper) The above expression gives a complete description of the geometry for a perfectly oval pipe. However Yeh and Kyriakides5 showed that also higher order contributions are needed for a complete geometrical description of a "real" pipe. In the present paper we will show the effect of different representations of the lack of circularity of a pipe on the collapse prediction and its relation to different measurement procedures. "Mode 2" approach to collapse prediction During 1997, a research program was started at FUDETEC on the improvement of finite element predictions for the collapse resistance of OCTG pipes6. The previous work was performed using nonlinear elasto-plastic 2D finite element models, which showed which variables had the greater effect. However, when comparing actual collapse data with the predicted values, it was observed that FEA results tended to underestimate the collapse resistance. The observed underestimation (less than 10%), while acceptable on an "academic basis", was insufficient for practical applications. It is important to note that a similar "bias" was observed when comparing data from our database with the Tamano formula. Following this observations it was proposed that higher order 2D effects (mode 3, 4 etc.) in the 2D representation of the geometry, as well as 3D effects could be responsible of the observed discrepancies6,7. In order to measure the full geometric representation an Imperfection Measurement System (IMS) was developed7, following the original ideas of Arbocz et al.8 and Yeh et al.5.
Use of slotted liner as a sand control device is widespread in SAGD operations in Western Canada. Operating temperatures in such thermal EOR wells can be extreme, sometimes exceeding 270 °C (518 °F), and the associated compressive axial mechanical strain imposed by constrained thermal expansion can load the pipe material beyond its proportional limit. Selection of an appropriate slotted liner configuration is critical to ensure structural stability (and hence sand control) is reliably maintained during operation. Mechanical properties of the tubular material at elevated temperature strongly influence the compressive stability of the liner structure, but lower-temperature properties also affect the ease of pipe slotting on a production scale, which is typically achieved by plunging thin saw-blades through the pipe wall. Common slotting issues include breakage or unacceptably high blade wear rates. This paper describes the development basis for a new tubular material formulation that is specifically optimized for thermal structural stability in SAGD applications without compromising slotting performance. Elevated-temperature mechanical properties are designed to prevent compressive buckling failures and to minimize strain localization potential. Results of analytical and experimental work (including stability analysis of the liner structure, thermo-mechanical material testing, and bench-scale slotting trials) are described. Introduction Ongoing rapid development of bitumen reserves in Western Canada has led to an increased focus on developing robust tubular design bases for extreme service conditions in in-situ recovery schemes such as SAGD. Specifically, cemented or frictionally constrained tubulars are subjected to thermallyinduced, deformation-controlled loading that leads to a unique set of design challenges and more stringent requirements for post-yield material response than those employed in traditional elastic design. Slotted liner is used as a sand control device in a majority of SAGD wells. Slotted liners used in thermal applications must provide a stable structure under extreme thermally-induced compressive loading in order to maintain wellbore access and to prevent excessive sand from entering the wellbore. While the deformation resistance of slotted liners depends on geometric attributes such as pipe wall thickness, slotting configuration, and slot geometry, material properties have a considerable impact on structural stability and localization resistance. Materials typically employed in slotted liner installations include API grades such as K-55 and L-80. However, the API mechanical property requirements for such materials are defined at room temperature, and the variability permitted by the API specification is large. Efforts to develop engineering design bases for liners in specific fields must consider the operating conditions of the application, and generally result in a more specific requirement for post-yield material properties than is offered by API. Hence, an API grade designation is not considered to be a sufficient description of material requirements for such tubulars. The manufacturing process for slotted liner involves plunging a series of thin sawblades through the wall of the tubular. Typical slot quantities are in the hundreds per metre of pipe length, and with individual well lengths of 600 m to 1000 m, an efficient slotting process is desirable.
Use of slotted liner as a sand control device is widespread in SAGD operations in Western Canada. Operating temperatures in such thermal EOR wells can be extreme, sometimes exceeding 270°C (518°F), and the associated compressive axial mechanical strain imposed by constrained thermal expansion can load the pipe material beyond its proportional limit. Selection of an appropriate slotted liner configuration is critical to ensure that structural stability (and hence sand control) is reliably maintained during operation. Mechanical properties of the tubular material at elevated temperature strongly influence the compressive stability of the liner structure, but lower-temperature properties also affect the ease of pipe slotting on a production scale, which is typically achieved by plunging thin saw blades through the pipe wall. Common slotting issues include breakage or unacceptably high blade wear rates. This paper describes the developmental basis for a new tubular material formulation that is specifically optimized for thermal structural stability in SAGD applications without compromising slotting performance. Elevated-temperature mechanical properties are designed to prevent compressive buckling failures and to minimize strain localization potential. Results of analytical and experimental work (including stability analysis of the liner structure, thermo-mechanical material testing, and bench-scale slotting trials) are described. Introduction Ongoing rapid development of bitumen reserves in Western Canada has led to an increased focus on developing robust tubular design bases for extreme service conditions in in-situ recovery schemes such as SAGD. Specifically, cemented or frictionally-constrained tubulars are subjected to thermally-induced, deformation-controlled loading that leads to a unique set of design challenges and more stringent requirements for post-yield material response than those employed in traditional elastic design. Slotted liner is used as a sand control device in a majority of SAGD wells. Slotted liners used in thermal applications must provide a stable structure under extreme thermally-induced compressive loading in order to maintain wellbore access and to prevent excessive sand from entering the wellbore. While the deformation resistance of slotted liners depends on geometric attributes such as pipe wall thickness, slotting configuration, and slot geometry, material properties have a considerable impact on structural stability and localization resistance.
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