Ministry of Education
HRAKLEITOS – Funding of PhD dissertation research
Aglaia E. Pournara – PhD candidate
The present research aims at estimating the residual structural capacity of oil and gas transmission pipelines of steel grade X52, which contain wall defects in the form of wall wrinkling (i.e. local buckling) or denting (Fig.1). The necessity of inspection and examination of pipe wall deformations, caused by external forces, is crucial especially for old pipeline networks, in order to safeguard their structural integrity and the unhindered flow of energy resources. Experimental and numerical research is being carried out in order to investigate the residual structural integrity of buckled steel tubes (Fig.2 and Fig.3). A series of experiments on steel tubes are being carried out, where steel tubes of diameter-to-thickness ratio equal to 30 and 65 have been buckled or dented first (Fig. 4 and Fig. 5) using the appropriate mechanical equipment, and subsequently, the deformed tubes have been subjected to further monotonic and cyclic (bending and pressure) loading in order to estimate their residual strength and remaining fatigue life (Fig. 6 and Fig. 7). Furthermore, a series of small-scale tests on 30cm-long longitudinal strips extracted from the X52 pipes have been performed towards the thorough investigation of the cyclic behavior along the compression fiber of a dent/buckled region (Fig. 8 and Fig.9). Non-linear numerical analysis is already in progress in order to simulate the experimental procedure for each type of deformation and loading case in order to verify the experimental results and estimate the level of residual stress and strain distributions around the deformed area of the pipe wall (Fig.10 and Fig.11). Furthermore, numerical models have been developed in order to estimate the stress-strain concentration factors at the critical region of buckled and dented pipelines under cyclic bending and pressure conditions. More efficient material models are being implemented in the finite element analysis in order to simulate the cyclic response of the dented/buckled pipes.
The ultimate objective of this present research emphasizes on the utilization of the experimental and numerical results for the development of design guidelines and methodologies, which will be employed for the inspection, estimation and possible repair of hydrocarbon steel pipelines with structural damages in the form of wall wrinkles.
Spyros A. Karamanos
36 months (2010 - 2013)
Figure 1: A locally-damaged pipeline with wall wrinkles (buckles).
Figure 2: Test set-up of denting procedure.
Figure 3: Test set-up of monotonic and cyclic 4-point bending.
Figure 4: Buckled specimen SP3b (D/t=65).
Figure 5: Dented region and instrumentation of Specimen SP6b (D/t=30).
Figure 6: Fatigue crack due to cyclic 4-point bending.
Figure 7: A buckled pipeline segment, failed under cyclic pressure loading due to fatigue.
Figure 8: Finite element model; Buckle development
Figure 9: Finite element model; Dent development
Figure 10: Dented model with d/D=6% (a) before pressure application
and (b) after the pressure application
Figure 11: “Smoothening” of the dent profile with the increase of internal pressure