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ABSTRACT: In this paper a probabilistic model for ductile fracture assessment of pipelines is established. A specially designed program based on shell and line-spring elements for fracture mechanics analyses of surface cracked pipes in different loading scenarios, is applied to establish a set of deterministic response/capacity calculations. Variables are pipe and defect geometry, material properties and ductile tearing resistance. The pipes are subjected to a combination to tension and pressure loads. Some of the results are compared with 3D FEM results obtained from Abaqus/Explicit analyses. The deterministic calculations are used to establish so-called response surfaces suitable for reliability calculations. The proposed methodology is illustrated with examples, where the probability of failure is determined using different solution methods. The results illustrate the simplicity, robustness and efficiency of the proposed probabilistic fracture mechanics model. INTRODUCTION To ensure safe and cost effective design and operation of structures like pipelines, it is required to have simple and reliable standardized assessment procedures. This is specifically important when the capacity utilization of the pipeline is high, which typically can be the case when using highly ductile pipeline steel materials. Large deformation may occur in different scenarios like pipeline laying or during operations. A pressurized pipe may be subjected to external loads for example in free spans due to irregular seabed topography or lateral/upheaval buckles caused by thermal loads. Modeling of three-dimensional ductile tearing analyses of pipes with defects is challenging and still not common due to complex modeling, lengthy solution time and extensive post-processing. However, 3D models are important to perform in order to investigate the detailed physics of fracture mechanics problems, see e.g. (Sandvik et al. 2007, 2008). This is neither suitable in engineering fracture mechanics assessment nor as a basis in probabilistic models for pipes where numerous analyses are needed.
- Europe (0.47)
- North America (0.46)
The Effects OfWeld Metal Mismatch And Crack Position On the Strain Capacity In SENT Specimens In an X65 Material
Østby, Erling (SINTEF Materials and Chemistry) | Nyhus, Bård (SINTEF Materials and Chemistry) | Sandvik, Andreas (StatoilHydro ASA) | Levold, Erik (StatoilHydro ASA) | Thaulow, Christian (Norwegian University of Science and Technology)
In this paper the results from SENT testing of two different welding procedures using an X65 base material is presented. The first welding procedure yields close to evenmatch conditions, whereas the second welding procedure gives 10-15% overmatch compared to the base material. Both defects lying in the weld metal and on the fusion line are investigated. It is observed that the ductile tearing resistances in both weld metals are significantly lower than for the base material. The resistance curves measured for the fusion line defects are more similar to the base material curve, however, slightly different crack growth is obtained depending on which side of the defect the measurements are performed. The crack driving force and strain capacity are on average higher in the overmatch specimens. However, a significant scatter is observed, especially for the weld metal defects. For the fusion line defects the scatter is smaller. For the material systems investigated the strain capacity will on average not depend strongly on the crack position. INTRODUCTION Defects may limit the tensile strain capacity of pipelines. Such defects are mainly found in relation to girth welds. Mismatching in weld metal (WM) stress-strain properties compared to the base material will lead to a modification of the crack driving force as a function of the applied strain. It is common practice to specify overmatch conditions in the weld metal in order to shield or reduce the deformation in this region. However, overmatch can be difficult to obtain in some cases (e.g. for very high strength steels). Another aspect is related to the larger scatter in material properties usually found in weld metals. Also, the ductile crack growth resistance will in many cases differ between the weld metal and the base material. Although not without exceptions, the metallurgical conditions in the weld metal will usually lead to a reduced crack growth resistance compared to the base material of the pipe.
- Europe (0.70)
- North America > United States (0.29)
Strain Capacity of SENT Specimens - Influence of Weld Metal Mismatch And Ductile Tearing Resistance
Østby, Erling (SINTEF Materials and Chemistry, Trondheim, Norway) | Nyhus, Bård (SINTEF Materials and Chemistry, Trondheim, Norway) | Hauge, Mons (StatoilHydro ASA, Trondheim, Norway) | Levold, Erik (StatoilHydro ASA, Trondheim, Norway) | Sandvik, Andreas (StatoilHydro ASA, Trondheim, Norway) | Thaulow, Christian (Norwegian University of Science and Technology, Trondheim, Norway)
ABSTRACT In this paper results from experimental investigations and numerical modeling of the strain capacity in SENT (Single Edge Notched Tension) specimens are presented. Both SENT specimens with defects in the pipe base material and in weldments are considered. The weld metal is about 10% overmatched compared to the base material. The results show that the overmatching on average leads to an increase in strain capacity, however, a much larger scatter in the results is observed for specimens with the defect in the weld material. The paper also demonstrates that 3D FE simulations are very well capable of reproducing the physics observed in the experiments. INTRODUCTION Prediction of strain capacity of pipelines with defects is an important part of strain-based fracture assessment. The strain capacity can be influenced by several different parameters, e.g. defect size, weld metal mismatch, and crack growth resistance. The scatter in these parameters also plays an important role, and will have an effect on how to define appropriate safety factors. A thorough understanding of the influence played by the different factors is vital in order to obtain robust strain-based fracture assessment schemes. SENT specimens have recently been proposed as an interesting candidate for fracture mechanics testing of pipelines and associated weldments (see Nyhus et al. (2003)). The SENT specimen displays a similar ligament deformation pattern and constraint level as found for cracks in pipes. In this paper we apply testing of SENT specimens to investigate some of the above mentioned factors, comparing results from pure pipe material specimens with specimens with defect in a slightly over-matched weld metal. The effect of weld metal overmatch on the crack driving force has been discussed by several authors, and examples can be found in Kim et al. (2000), Liu et al. (2007), Gioielli et al. (2007), and Motohashi and Hagiwara (2007).
- North America (0.46)
- Europe > Portugal (0.15)
- Research Report > New Finding (1.00)
- Research Report > Experimental Study (0.86)
Fracture Control - Offshore Pipelines JIP Use of Abaqus/Explicit to Simulate Ductile Tearing In Pipes With Defects Loaded Beyond Yielding
Sandvik, Andreas (StatoilHydro ASA, Trondheim, Norway) | Østby, Erling (SINTEF Materials and Chemsitry, Trondheim, Norway) | Thaulow, Christian (Norwegian University of Science and Technology, Trondheim, Norway)
ABSTRACT In this paper three dimensional finite element modeling of pipes with circumferentially oriented surface cracks has been carried out to simulate experiments with combined bend loading and internal pressure. The models are solved using Abaqus/Explicit. Quasi-static solutions were obtained by controlling the loading rates. Ductile tearing is taken into account using the Gurson-Tvergaard-Needleman model, and the constitutive model represents a realistic model for X-65 pipeline steel. In Part I the applicability of the solution method is demonstrated by comparing FEM-simulation results with results from full scale experiments of a pipe in four-point bending, with and without internal pressure. New results from a full scale experiment are also presented. The results show that both the global structural response and the local ductile fracture, are captured in the simulations. In Part II, a parametric study on the effects of defect dimensions and internal pressure, assuming tension loading, is presented. The simulations illustrate how the strain capacity depends on the pressure level, with decreasing strain capacity with increasing internal pressure. Additionally, the strain capacity decreases as the crack depth increases. The effect of crack length in the circumferential of the pipe becomes more important as the crack depth increases. INTRODUCTION Pipelines may be exposed to large variation of loads, depending on the surroundings and area of application. These loads may result in global deformations well into the plastic region. If a crack is present in a tensile region it may grow and possibly lead to a fracture and collapse. The strain capacity of ductile pipes with defects is limited by the amount of tearing that can be tolerated before the maximum load capacity is reached. In the Fracture Control - Offshore Pipelines JIP the focus has been on ductile steel materials and the effect of internal pressure.