Beulverhalten von ringversteiften Kreiszylinderschalen unter Axialdruck – Teil 1: Versuche und geometrische Imperfektionsanalyse/Buckling behavior of ring-stiffened cylinder shells under axial pressure – Part 1: Tests and geometric imperfection analysis

Moderne Behälter werden oft als ringversteifte Kreiszylinderschalen ausgeführt. Der traglaststeigernde Effekt eng liegender Ringsteifen unter Axialdruck erfuhr in der Forschung bis heute nur geringe Beachtung. Er ist bisher ungeregelt und infolge unnötig hohen Materialeinsatzes bleiben Stahlbehältern Marktanteile und bessere Nachhaltigkeit verwehrt. Anhand von Versuchen und einer numerischen Studie werden die neuesten Forschungsergebnisse des Einflusses der Ringsteifen auf das Axialbeulverhalten von Kreiszylinderschalen erläutert. Die Ergebnisse zeigen, dass die ringversteiften Kreiszylinder unter Axialdruck nach der aktuellen Bemessungspraxis unwirtschaftlich bemessen werden.   Im Teil 1 dieses Beitrags werden Versuche im verkleinertem Maßstab durchgeführt, um den Einfluss der Ringsteifen auf das Beulverhalten der Kreiszylinderschalen unter Axialdruck zu erforschen. Gemäß einem Vergleich von Versuchsergebnissen wird eine mehrfach höhere Tragfähigkeit ringversteifter Schalen gegenüber unversteiften Schalen ermittelt. Die geometrischen Imperfektionen des Probekörpers werden durch eine 3D-Scan-Technologie gemessen und danach mittels der Methode der Fourier-Reihen analysiert. Im Teil 2 folgen die numerische Simulation und das Ableiten eines Ingenieurmodells.

Evaluation of collapse moment of pressurized 30°–180° bend pipes subjected to out-of-plane bending moment

The present paper determines collapse moments of pressurized 30°–180° pipe bends incorporated with initial geometric imperfection under out-of-plane bending moment. Extensive finite element analyses are carried out considering material as well as geometric nonlinearity. The twice-elastic-slope method is used to determine collapse moment. The results show that initial imperfection produces significant change in collapse moment for unpressurized pipe bends and pipe bends applied to higher internal pressure. The application of internal pressure produces stiffening effect to pipe bends which increases collapse moment up to a certain limit and with further increase in pressure, collapse moment decreases. The bend angle effect on collapse moment reduces with the increase in internal pressure and bend radius. Based on finite element results, collapse moment equations are formed as a function of the pipe bend geometry parameters, initial geometric imperfection, bend angle, and internal pressure for elastic-perfectly plastic material models.

EXPERIMENTAL STUDY OF THE INTERACTION OF CORROSION DAMAGE AND OUT-OF-CIRCULARITY IN THE COLLAPSE OF SUBMARINE PRESSURE HULLS

The effect of corrosion damage on overall collapse strength of submarine pressure hulls was studied experimentally. Ring-stiffened cylinders were machined from aluminium tubing and loaded to collapse under external pressure. In selected specimens, some of the outer shell material was machined away in large single patches, representing general corrosion. Other specimens had many smaller patches, representing corrosion pitting from the outside of the hull, followed by grinding. Large-amplitude out-of-circularity (OOC) was introduced by mechanically deforming selected cylinders. Clusters of artificial corrosion pits were found to have approximately the same effect on collapse pressure as equal-depth general corrosion covering the same region of plating. General corrosion was found to be most severe when it was “in-phase” with OOC, since, during pressure loading, high compressive stresses resulting from corrosion were compounded by compressive bending stresses associated with OOC, and furthermore, the corrosion tended to increase the geometric imperfection itself. On the other hand, out-of-phase corrosion reduced the effect of OOC, while at the same time the thinning-associated compressive stresses were counteracted by local tensile bending stresses associated with OOC, so that strength reductions were correspondingly smaller. Overall collapse pressures for corroded specimens were reduced by, on average, 0.85% for each 1% of shell thinning. That result is based on a linear approximation of the nonlinear relationship between thinning and collapse pressure. The linear trend-line, which was used to account for the experimental scatter, is based on specimens with 13 to 27% shell thinning, and with a variety of corrosion areas and OOC amplitudes.

Equivalent Geometric Imperfections for Local Buckling of Slender Box-section Columns

Determining the plate or the local buckling resistance is highly important in designing steel buildings and bridges. The EN 1993-1-5Annex C provides a FEM-based design approach to calculate the buckling resistance based on numerical design calculations (geometrical and material nonlinear analysis - GMNIA). Within the GMNIA analysis-based stability design, the application of the imperfections has a special role. Thus, the applicability of the EN 1993-1-5 based buckling curve (Winter curve) has been questioned for pure compression, and previous investigations showed the buckling curve of EN 1993-1-5 Annex B is more appropriate for the design of slender box-section columns subjected to pure compression, the magnitude of the equivalent geometric imperfection to be applied in numerical models for local buckling is also questioned and investigated by the authors within the current paper. The aim of the current research program is to investigate the necessary equivalent geometric imperfections to be applied in FEM-based design calculations using GMNIA calculations. A numerical parametric study is executed to investigate the imperfection sensitivity of box-section columns having different local slenderness. The necessary imperfection magnitudes are determined to each analyzed geometry leading to the buckling resistance predicted by the standardized buckling curves. Based on the numerical parametric study, a proposal for the applicable equivalent geometric imperfection magnitude is developed, which conforms to the plate buckling curves of the EN 1993-1-5 and giving an improvement proposal to the local buckling imperfection magnitudes of the prEN 1993-1-14, which is currently under development.

Influence of Heavy Weight Drill Pipe Material and Drill Bit Manufacturing Errors on Stress State of Steel Blades

Drilling volumes should be increased in order to increase hydrocarbon production, but this is impossible without the usage of high-quality drilling tools made of modern structural materials. The study has to analyze the design, technological and operational methods to increase the performance of drilling tools made of various materials and has highlighted prospects of technological method applications. The scientific novelty of the study consists in the development of a new analytical model of PDC drill bit–well interaction. The developed model takes into account the drill bit manufacturing errors in the form of bit body–nipple axes misalignment on the drill bit strength. This result makes it possible to determine the permissible manufacturing errors to provide safe operation of the drill bit. It is established that there is an additional transverse force that presses the drill bit to the well wall in the rock due to manufacturing errors. It is determined that the magnitude of this clamping force can be significant. The material effect has been analyzed on additional clamping force. It is established that geometric imperfection of the drill bit causes the minimal effect for the elastic system of the pipe string, which includes a calibrator and is composed of drill pipes based on composite carbon fiber material, and the maximal effect—for steel drill pipes. Polycrystalline diamond compact (PDC) drill bit and well wall contact interaction during operation in non-standard mode is considered. Non-standard stresses are determined, and the strength of the blades is estimated for different values of drilling bit manufacturing error.

Determination of collapse moment of different angled pipe bends with initial geometric imperfection subjected to in-plane bending moments

From the recent literature, it is revealed that pipe bend geometry deviates from the circular cross-section due to pipe bending process for any bend angle, and this deviation in the cross-section is defined as the initial geometric imperfection. This paper focuses on the determination of collapse moment of different angled pipe bends incorporated with initial geometric imperfection subjected to in-plane closing and opening bending moments. The three-dimensional finite element analysis is accounted for geometric as well as material nonlinearities. Python scripting is implemented for modeling the pipe bends with initial geometry imperfection. The twice-elastic-slope method is adopted to determine the collapse moments. From the results, it is observed that initial imperfection has significant impact on the collapse moment of pipe bends. It can be concluded that the effect of initial imperfection decreases with the decrease in bend angle from 150∘ to 45∘. Based on the finite element results, a simple collapse moment equation is proposed to predict the collapse moment for more accurate cross-section of the different angled pipe bends.