B. Nyhus

SENT Specimens an Alternative to SENB Specimens for Fracture Mechanics Testing of Pipelines

During installation operations offshore pipes are often strained beyond yielding. Due to the high loading condition and the high costs of these operations it is important with accurate defect assessment analysis to avoid delays caused by unnecessary repairs or failure because of flaws that should have been detected and repaired. There is therefore a need for an accurate assessment procedure that can be a tool for defect assessment analysis for this application. It is commonly accepted that the fracture toughness is dependent on the geometry constraint at the crack tip. The traditional single edge notch bend (SENB) specimens have a high geometry constraint, and give lower bound fracture toughness for all geometries. For reeling operations these fracture toughness values are often too low to be used in defect assessment of reeling operations. The same is the assumption of plastic collapse when the net section stress is equal to the average between the yield strength and tensile strength. In this paper, the single edge notch tension specimen (SENT) is presented as an alternative fracture mechanics specimen. This specimen has a geometry constraint that is much closer to flaws in pipes than SENB specimens, which will give more realistic fracture properties of the pipe. In the procedure for defect assessments we present, both the fracture toughness and plastic collapse properties are taken from testing of SENT specimens. FE simulations and full scale testing verify the procedure.Copyright

Guidelines for Engineering Critical Assessments for Pipeline Installation Methods Introducing Cyclic Plastic Strain

Conventional flaw assessment procedures are not explicitly developed for situations with large cyclic plastic strains, e.g. for pipelines installed by the Reeling method, and if used indiscriminately may give un-reliable results. In order to resolve this dilemma DNV, TWI and Sintef conducted a Joint Industry Project (Fracture Control for Installation Methods Introducing Cyclic Plastic Strain – Development of Guidelines for Reeling of Pipelines) to provide guidelines on testing and assessment procedures that can be employed by the industry. The project included Materials Testing, FEM Analyses and Validation Testing of Pipe Segments as well as Full Scale Pipes. This paper summarizes the Guideline document that was developed in the project.Copyright

Hydrogen Induced Stress Cracking (HISC) of Stainless Steels Under Cathodic Protection in Seawater: Presentation of a New Test Method

There has been an increasing trend in the use of stainless steel alloys instead of carbon steel for subsea flowlines and production systems during the last 15 years in the oil industry. Even if this normally is a more robust solution compared to the use of carbon steel insofar as internal corrosion problems are concerned, the use of stainless steels has led to leakage, production shutdown and expensive repair work. The reported failures were associated with hydrogen entrapment resulting from welding and/or external cathodic protection (CP), combined with a certain stress/strain level. Atomic hydrogen entering the alloy can weaken the mechanical strength of the alloy, cause cracks and destroy the integrity of equipment or a system. Such failures attributed to hydrogen induced stress cracking (HISC) are clearly not acceptable from the perspective of safety, environmental hazard and cost. Leading oil and engineering companies and supplier industry have pointed out HISC as one of the major obstacles against safe operation of stainless steel subsea pipelines and production systems. It is important for the oil industry to have design guidelines and reliable test method(s) for qualification and safe utilization of subsea pipelines and components made from the actual stainless steels. This paper describes a test method that has been developed through a Joint Industry Project (JIP) executed by SINTEF and Det Norske Veritas (DNV) with support from leading oil companies and material suppliers. The method has been qualified for use on 13% Cr super martensitic (SMSS) and 22% Cr / 25% Cr duplex stainless steels (DSS/SDSS). The link to DNV-RP-F112 [1] will also be described.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

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

Numerical Investigation on the Influence of Residual Stresses on Ductile Crack Growth Resistance

In this paper, a numerical study was performed to assess the effect of residual stresses on ductile crack growth resistance of a typical pipeline steel. A modified boundary layer model was employed for the analysis under plane strain, Mode I loading condition. The residual stress fields were introduced into the finite element model by the eigenstrain method. The complete Gurson model has been applied to simulate the ductile fracture, which is characterized by microvoid nucleation, growth and coalescence. Results show that tensile residual stresses can significantly reduce the crack growth resistance when the crack growth is small compared with the length scale of the tensile residual stress field. With the crack growth, the effect of residual stresses on the crack growth resistance tends to diminish. It has also been found that the effect of residual stresses on crack growth resistance depends on the initial void volume fraction f0 and hardening exponent n.Copyright

Axisymmetric Modeling of Constraint Effect on the Ductile Crack Growth Resistance of Circumferentially Cracked Pipes

Ductile crack growth plays an important role in the analysis of the fracture behavior of structures. Crack-like defects in pipe systems often develop during fabrication or in-service operation. The standard single edge notched bending (SENB) specimen with crack depth of a/W = 0.5 has a significantly higher geometry constraint than actual pipes with circumferential surface cracks, which therefore introduces a high degree of conservatism in engineering critical assessment (ECA) of pipes. Moreover, it is difficult to know how conservative the results are, because the geometry constraint is highly material-dependent. For circumferential surface flaws in pipes, the single edge notched tension (SENT) specimen has frequently been used because it has a geometry constraint in front of the crack tip that is similar to the cracks in pipes. Much work has been carried out on tensile testing for the SENT specimen as an alternative fracture mechanics specimen of pipes. In studying fully circumferential cracks in pipes, the crack geometry, applied load and boundary conditions are symmetrical about the axis of revolution. A typical radial plane containing the axis of rotational symmetry can represent these axisymmetric bodies; therefore the three-dimensional analysis can be reduced to a two-dimensional problem. This work systemically applies 2D axisymmetric models to study the ductile crack growth behavior of pipes with fully internal and external circumferential cracks under large scale yielding conditions. The complete Gurson model (CGM) developed and implemented by Zhang was utilized to predict the ductile crack growth resistance curves. Pipes with various internal pressure, diameter-to-thickness ratios, crack depths and material properties, as denoted by hardening and initial void volume fraction, have been analyzed. The results have been compared with those of corresponding clamped-loaded SENT (with same crack depth) and standard SENB specimens. It clearly indicates that the SENT specimen is a good representation of circumferentially flawed pipes and an alternative to the conventional standard SENB specimen for the fracture mechanics testing in ECA of pipes.Copyright

NUMERICAL ANALYSES OF DUCTILE FRACTURE BEHAVIOR IN 2D PLANE STRAIN AND AXISYMMETRIC MODELS USING THE COMPLETE GURSON MODEL

Ductile crack growth plays an important role in the analysis of fracture behavior of structures. A strong geometry dependence of ductile crack growth resistance emerges under large scale yielding conditions. This geometry dependence is associated with different levels of crack tip constraint. However, an independent relationship between the fracture resistance and crack tip constraint has also been observed in experimental studies for selected specimen geometries. To verify these results, crack growth resistance curves for plane strain, mode I crack growth under large scale yielding have been computed using the complete Gurson model. Single edge notched bend (SENB) and tension (SENT) specimens with three different crack geometries have been selected for the numerical analyses. Specimen size effect on ductile crack growth behavior has also been studied. SENT specimen appears as an alternative to conventional fracture specimens to characterize fracture toughness of circumferentially cracked pipes due to its similar geometry constraint ahead of the crack tip with that of cracks in pipes. The application of SENT specimen into the axisymmetric model so as to form a pipe segment with a long internal circumferential crack under large scale yielding conditions is examined. The effect of biaxial load conditions (axial tension combined with internal pressure) on the resistance curves is considered.

Constraint Effects in Weld Thermal Simulated Steel Specimens

The brittle fracture constraint effect in two different weld thermal simulated microstructures of a 500 MPa steel is investigated through testing of bending specimens with different a/W ratios. It is found that on average the constraint effect is higher in the CGHAZ microstructure compared to the ICCGHAZ. The procedure for calibration of Weibull parameters proposed by Gao et al. [1] has been applied to the results. It is found that a lower m-value is calibrated for the ICCGHAZ microstructure due to the lower constraint effect observed. Three-parameter representations of the Weibull distribution better represent the distribution of the fracture toughness values compared to two-parameter representations.Copyright

Fracture Mechanics Analysis of a High Strength Steel for Offshore Application

One of the main bearing elements of the Siri jack-up is three tubular legs; outside diameter 3.5m and an overall length of 104m. The main material grade is high strength steel with SMYS≥690MPa. This strength was required due to contact forces during the jacking phase. One of the major issues concerning integrity of the platform was associated with the fatigue life of the legs. The fatigue life of the legs WAS to be 60 years for inaccessible locations, 40 years for accessible submerged locations and 20 years for accessible dry locations. To achieve the required fatigue lives, wall thickness of 65mm, 75mm and 90mm were utilized for the 690 steel. Furthermore, to meet the 40-year requirement, lives for the lower circumferential welds were designed to Class C, i.e. they needed to be improved by grinding. Based on performed research, it was aimed at minimum CTOD values of 0.20mm and 0.25mm in the PWHT and AW conditions respectively. Although the aim was achieved for the base material and sub critical HAZ, the weld metal and the coarse grained HAZ values were significantly below these values, i.e. 0.12mm and 0.02mm respectively. In order to verify the integrity of the platform a fracture mechanics analysis was performed. The assessment comprised evaluation of critical defect sizes and fatigue life calculations using PD6493. Wide plate testing, residual stress measurements and fatigue testing have been performed. The assessment showed that the integrity of the platform was maintained. In order to obtain additional safety an inspection program for the critical welds has been implemented in the operational phase.Copyright