Erling O̸stby

Fracture Control — Offshore Pipelines: New Strain-Based Fracture Mechanics Equations Including the Effects of Biaxial Loading, Mismatch, and Misalignment

In this paper a framework for a strain-based fracture mechanics crack driving force methodology for pipes with surface cracks, are presented. The model addresses the effects of crack depth, crack length, pipe diameter, wall thickness and yield to tensile ratio. Based on FE simulations, an equation to calculate the applied crack driving force, either through CTOD or J, has been derived. The equation is intended for use in cases where global plastic deformation occurs. A general approach to introduce the effects of biaxial loading, yield stress mismatch, and misalignment on the driving force, through definition of an effective wall thickness and an effective crack ligament height, is outlined. Models to quantify the effects of the different parameters are also derived. Finally, results are presented from comparison between 2D and 3D FE simulations and the predictions made by the proposed driving force equations.Copyright

Simulations of Ductile Tearing at Large Strains of Biaxially Loaded Pipes

The purpose of the present paper is to present results from analyses of ductile tearing of biaxially loaded pipes subjected to large scale yielding. The paper deals with three dimensional finite element modelling of pipes with a circumferentially orientated surface crack, where the analyses aim to reproduce the crack propagation behaviour of six full scale bend tests of x-65 seamless pipes with different levels of internal overpressure. The tests were performed as a part of the joint industry project Fracture Control - Offshore Pipelines. Ductile tearing is taken into account by using the Gurson-Tvergaard-Needleman formulation, where calibration of the material model parameters is done by reproducing the fracture toughness test of a SENT-specimen of the same material with finite element modelling. The following simulations of the pipes show a good correspondence with the full scale test results, where both the global response and the ductile tearing from the crack are captured. One important result of the study is that the Gurson-Tvergaard-Needleman parameters that were calibrated against the SENT-specimen could successfully be used for the ductile tearing simulation of the full scale pipes.Copyright

Burst Tests for High-Pressure Hydrogen Gas Line Pipes

Burst tests of line pipes containing high-pressure hydrogen and methane gas were conducted. The pipes tested were X65 ERW pipes with 267mm outer diameter and 6mm wall thickness. Applied pressures were 12 and 15MPa. A 300mm long crack was initiated by a shaped charge. Unstable shear fracture propagated at velocity of approximately 200m/s and then arrested. It was demonstrated that the arrested crack lengths were shorter in the hydrogen gas burst tests than methane gas burst tests. Pressure measurement indicated that decompression wave in hydrogen gas propagated faster than that in methane gas. This is the primary reason to the shorter arrested crack lengths for the hydrogen gas burst tests. The test results are discussed based on numerical analyses of gas decompression behaviors.Copyright

Large Scale Tests of Strain Capacity of Pipe Sections With Circumferential Defects Subjected to Installation-Induced Plastic Strain History

Installation of offshore pipelines by reeling introduces plastic pre-straining. The pre-strain history is not homogenous and it will vary around the circumference of the pipe. The pre-strain history will modify the yield and flow properties. Also, the fracture toughness may be influenced by the pre-straining. The result is that the bending strain capacity of pipelines during operation will differ depending on how the bending moment coincides with pipe orientation during installation. Three full scale tests of 12″ x-60 pipes with wall thickness 19.3mm and a 3×100 mm outer surface defect were performed to investigate the effect of pre-strain history. Two pipes were pre-strained in bending to 2% strain in the outer fibre and then straightened to simulate the reeling. The final tests to establish the strain capacity during operation as a function of strain history were performed in four point bending with an internal pressure of 325 bar. The strain capacity for the side of the pipe that ends in tension and the side that ends in compression from pre-straining was 1.7% and 2.6% respectively. The strain capacity of the third test without pre-straining was 5.7%. The results show that pre-straining will modify the strain capacity and the effect must be taken into account in engineering critical assessment of pipes during operation. The effect of prestraining should be evaluated for all installation methods that involve plastic deformation during installation, and not only reeling. It is important to note that the notch size in the full scale tests was larger than what would normally be accepted for reeling. In addition the notch was positioned in base material and not in weld metal, which is a more realistic position for a notch. The welds are normally overmatched and this might reduce the effect of prestraining.Copyright

Fracture Control — Offshore Pipelines: Probabilistic Fracture Assessment of Surface Cracked Ductile Pipelines Using Analytical Equations

Since modern pipelines usually display ductile fracture behaviour, fracture assessments accounting for ductile tearing should be used. In this work we use a simplified strain-based fracture mechanics equation in the probabilistic fracture assessments. Furthermore, we use the traditional tangency criterion between the crack driving force and the crack growth resistance, in calculation of the onset of critical ductile tearing. Additionally, two types of external load on the line-pipe are considered, namely strains due to external bending moments and internal pressure. We establish the probability of fracture for line-pipes with relevant diameter to thickness ratios, and thicknesses, for J-laid or S-laid offshore pipelines. The distinction between system effects, in which all defects are likely to be subject to the same loading, and cases where only a small part of the pipeline will experience high loading, is also discussed.Copyright

Full-Scale Burst Test of Hydrogen Gas X65 Pipeline

Full-scale burst test of X65 UOE linepipe, with 559mm outer diameter and 13.5mm wall thickness, pressurized at 16MPa by hydrogen gas was conducted. A 735mm long crack was introduced by explosive shaped charge over circumferential weld. The cracks were initiated and propagated in the both directions. The propagated crack lengths were 600mm and 270mm. J integral resistance curves were obtained from drop-weight as well as quasi static tests for the tested pipe material which was subjected to hydrogen charging. The tested steel showed little change in the resistance curves under realistic charging condition. Numerical simulation model of dynamic crack propagation, coupled with gas decompression behavior considering gas escape from opened crack, showed that an initiated crack was arrested at shorter distance in hydrogen gas pipelines than in methane gas pipelines, primarily due to earlier gas decompression in the former. The present results, together with the earlier full-scale burst tests conducted by the authors, demonstrated that hydrogen gas pipelines can be operated safely by using modern high-strength and high-toughness steel linepipes.© 2010 ASME

PREDICTION OF HYDROGEN EMBRITTLEMENT IN 25% CR DUPLEX STAINLESS STEEL BASED ON COHESIVE ZONE SIMULATION

Laboratory experiments and cohesive zone simulation of Hydrogen Induced Stress Cracking in SENT tests specimens of 25% Cr duplex stainless steel have been performed. A polynomial formulation of the traction separation law and hydrogen dependent critical stress was applied. Best agreement with the experiments was found for an initial critical stress of 2200 MPa and a critical separation of 0.005 mm. Proposed threshold stress intensity factor and lower bound net section stress is 20 MPa√m and 480 MPa. High crack growth rates and typical hydrogen influenced fracture topography suggest large influence of the stress and strain in the fracture process zone on the hydrogen diffusion rate.Copyright

Assessment of Fracture Integrity for Trawl Impact of the Ormen Lange SFD Pipelines

Ormen Lange Southern Field Development (SFD) is part of the phase 2 development of the Ormen Lange gas field located about 120 km offshore the coast of Norway. The SFD includes an 8 slot template, two 16 inch infield flowlines, one 6 5/8 inch MEG line and one umbilical located at about 850 m water depth. Although there are presently no fishing activities at the development area, the pipeline design has included a design case with evaluation of the structural integrity and potential for failure caused by future interaction with fishing gear such as trawl impact/pull-over and hooking. In contrast to the MEG line and the umbilical, which will be trenched and buried along the whole pipeline route, the 16 inch production flowlines will be left exposed on the seabed and may therefore be subjected to interference with trawl equipment in the future. It was therefore decided that pipeline engineering shall document that impact from trawl equipment during operation will not cause detrimental damage to the exposed flowlines, resulting in leakage of hydrocarbons to the environment and/or high cost of repair. In the event of impact from trawl equipment, it is likely that the pipe will be operating and thus be in a state of internal overpressure. Recent research has shown that the effect of internal pressure can be detrimental to the fracture response of pipelines with circumferential flaws subjected to bending or tensile loading. Today’s analytical equations that are the basis for most engineering critical assessments (ECA) are not capable of accounting for the effect of internal pressure when elastic-plastic fracture mechanics is considered. LINKpipe, which is a special purpose finite element program for assessing the fracture integrity of pipelines, is capable of accounting for the effect of internal pressure and was therefore chosen for the fracture integrity assessment. The flowline was analyzed for a range of defect sizes and material stress-strain behaviors. The finite element model was subjected to bending while under internal pressure, and both surface breaking defects and embedded defects have been assessed to ensure that the Ormen Lange SFD flowlines are capable of withstanding impact from trawl equipment during operation. The analyses were used to determine safe operational windows regarding acceptable defect sizes for both surface breaking and embedded defects for the parameters analyzed.Copyright

A Cohesive Zone Modeling Approach to Hydrogen Induced Stress Cracking in 25%Cr Duplex Stainless Steel

Hydrogen influenced cohesive zone elements are implemented in finite element (FE) models of rectangular U and V notched tensile specimens. The material description outside the cohesive zone is representative of a fine grained 25% Cr duplex stainless steel, UNS32760-S. A three step procedure consisting of conventional elastic plastic stress analysis, stress driven diffusion analysis and finally cohesive zone fracture initiation analysis makes the basis for the presented work. The applied boundary conditions are representative of mechanical stresses and environmental loads on an oil and gas pipeline in subsea conditions. A linear traction separation law gives reasonably good fit with experimental results for gross stress levels of 0.85–0.9 times the material yield stress. Hydrogen concentration of 40 ppm at the surface and 1 ppm in bulk always gives crack initiation at the surface despite the peak normal stress localized in front of the notch tip.Copyright

Effects of the Yield Criterion on Predicted Fracture Responses of Pipelines Subjected to Large Plastic Deformation With and Without Internal Pressure

The structural integrity of offshore pipelines is of vital importance for safe oil and gas transport. To ensure the required safety level, non-linear Finite Element (FE) analyses are necessary to perform fracture assessment of pipes under various, realistic loading conditions. Many standard material models, as found in commercial FE codes, pre-suppose the yield criterion of von Mises. This choice provides in many cases reasonable accuracy, certainty and engineering designs, but for some materials and application areas, it is much too inaccurate. In this work, 3D elastic–plastic FE simulations of pipes with internal surface cracks have been carried out. The aim of the work is to evaluate the influence of the yield criterion on the predicted fracture response. Analyses are performed on pipes loaded in tension, with and without internal pressure. The model shows that the yield surface shape may have a significant effect on the predicted evolution of Crack Tip Opening Displacement (CTOD). If the internal pressure is weak, a reduction in strain capacity is observed when the yield surface shape is varied from the rounded von Mises towards the cornered Tresca-like yield surface.Copyright