Efficient J-Based Failure Assessment Diagrams for Engineering Critical Assessments of Circumferentially Cracked Pipes Subjected to Axial Force, Pressure, and Bending

Engineering critical assessments (ECAs) of cracked pipes increasingly involve situations of high strains (e.g., reeling and ratcheting fatigue), multiple loads (combined bending, axial forces, and internal pressure), and multi-axial stressing (due to pressure). In this paper, some of the implications of these loading conditions on ECAs are investigated by generating BS 7910 Level 3C Failure Assessment Diagrams (FADs) from the results of a large matrix of finite element analysis (FEA) J computations for circumferentially cracked pipes. The Level 3C (J-based) FADs (which provide the most accurate FAD approach to ECAs) are compared with the corresponding and more widely employed (but less accurate) Level 2B (material dependent) FADs in order to assess the accuracy of the latter. Use of FEA J solutions in a Level 3C FAD ensures that the effects of material behavior, load type, crack type, crack geometry, and pipe geometry are accurately captured whereas a Level 2B FAD only attempts to accurately capture the effects of material stress-strain behavior. It is demonstrated that under some circumstances a Level 2B assessment will result in non-conservative results compared to the corresponding Level 3C assessment. The current comparison between Levels 3C and 2B addresses the mechanics involved in these approaches and does not take into account the possible differing treatments of material property uncertainties on ECAs within the two approaches. Based on the current results, an efficient J formulation is described that facilitates the practical implementation of a J-based ECA. The novel approach used is based on determining material dependent shift factors that transform Level 3C FADs derived from the fully plastic components of J solutions into Level 3C FADs that represent J behaviors in the linear elastic and fully plastic regimes, and the transition region in-between. This new J formulation treats combined axial forces, pressure, and bending when applied proportionally or non-proportionally and forms the basis of the monotonic and cyclic crack tip driving forces employed in the program FlawPRO. This program performs comprehensive conventional and high strain J-based ECAs that involve reeling, arbitrary strain cycling, ratcheting fatigue, and ductile tearing that are equivalent to a Level 3C FAD approach.

Fatigue Crack Growth Rate and Fracture Toughness of API5L X65 in Sweet Environments

Corrosion fatigue and fracture toughness in sour environments of APIX65 5L have typically been studied in relatively severe environments like NACE A and NACE B solutions. There are very limited data in sweet and mildly sour environments that are of interest in various applications. This paper presents fatigue crack growth frequency scans in a range of sweet and mildly sour environments as well as on different microstructures: Parent Pipe, Heat Affected Zone (HAZ) and Weld Center Line (WCL). The fatigue crack growth rate (FCGR) increased with decreasing frequency and reached a plateau value at low frequencies. FCGR in the sweet environments that were investigated did exhibit a frequency dependence (increasing with decreasing frequency) and had plateau FCGR in the range of 10–20× the in-air values. In the mildly sour environments that were investigated, FCGR was found to be about 25 to 30× higher than the in-air values. By comparison, in NACE A environments the FCGR is typically about 50× higher than the in-air values. The FCGRs of parent pipe and HAZ were found to be similar over a range of environments, whereas the WCL FCGR data were consistently lower by about a factor of 2×. The lower FCGR of the WCL is likely due to the lower concentration of diffusible hydrogen in the weld. FCGRs as a function of ΔK (stress integrity factor range) were measured on parent pipe at the plateau frequency. The measured Paris law curves were consistent with the frequency scan data. Rising displacement fracture toughness tests were performed in a range of sweet and sour environments to determine the R-curve behavior. Tests were performed in-situ at a slow K-rate of 0.05Nmm−3/2/s over a range of environmental conditions on parent pipe. The initiation toughness and the slope of the R-curve decreased sharply in the sour environments. The initiation toughness and slopes were largely independent of the notch location as well as environmental conditions. Typical values of initiation toughness were in the range of 90–110N/mm.

Employing Visual Image Correlation for the Measurement of Compressive Strains for Arctic Onshore Pipelines

The Arctic onshore environment contains regions of discontinuous permafrost, where pipes may be subject to displacement-controlled bending in addition to high hoop stresses due to the pressurized fluids being transported. Considering the displacement-controlled nature of the deformations, strain-based design methodologies have been developed for permafrost pipelines when they are subject to bending and tension, which limit the longitudinal compressive and tensile strains. The widely accepted methodology in the industry to obtain the compressive strain capacity of line pipes subject to bending is to conduct Finite Element Analysis, incorporating material and geometrical nonlinearity calibrated against benchmark full-scale tests (bend tests) [1,2]. During these tests, compressive strains can be measured by various methods. The seemingly obvious choice is to apply strain gauges along the compression face of the specimen with respect to bending (intrados). This method will provide reasonable results until the compressive strain pattern begins to vary due to the initiation of buckle formation, which typically occurs shortly after yield. In order to measure average compressive strain beyond yield and up to buckling, the method used by C-FER Technologies (C-FER) involves using rotation measurement devices (inclinometers) to calculate the strain change between the most compressive and tensile fibres of the specimen (intrados and extrados, respectively) with respect to the bending direction. This value is then subtracted from the tensile strain gauge readings as measured by the strain gauge(s) located on the extrados of the specimen. The average compressive strain values derived from the inclinometer and extrados strain gauge measurements are based on the assumption that the plane sections remain plane. Recently, five large diameter pipes were bend-tested at C-FER’s testing facility in Edmonton, Alberta. In addition to the compressive strain measurement method used by C-FER described above (C-FER method), a visual image correlation (VIC) camera system was used to survey the strain distribution on the compressive face of the specimens. This paper gives a brief description of the test setup and instrumentation of this test program. The VIC camera setup and measurement technique are described and the overall strain distribution on the bending intrados as measured by the VIC cameras is presented. Strain measured by the VIC system is compared with gauge measurements at local points as well as the average compressive strain behaviour of the specimens obtained through the C-FER method described above. The results show that the VIC system can be a candidate to replace the conventional measurement techniques employed for compressive strain limit testing in support of strain-based design of arctic pipelines.

Laboratory Experimentations for New Hydrothermal Monitoring Systems Using ADCPs

Deep ocean mining in a hydrothermal area needs careful environmental impact assessments in terms of preservation and mitigation of biodiversity. The General Environmental Technos Co. Ltd., or KANSO TECHNOS, for short, has participated in environmental impact assessments in hydrothermal areas in the Izu-Ogasawara and the East China Sea areas (Ishida et al., 2011). Through the experience, we suggest a method of using acoustic systems such as acoustic Doppler current profilers (ADCPs) for monitoring of suspended matters and benthos in hydrothermal areas. Thus, we try to do in-situ observations, called Tow-yo (or Towing) observations with ADCPs (Komaki and Ura, 2009; Komaki et al., 2010). This system has a great advantage in enabling the measurement of great environmental factors, echo intensity and current velocity in a large range. To confirm exactly what the substances are and how large they are from the measured echo intensity data, we tried laboratory experiments in water tanks with echo sounders and turbidity sensors. These results will finally be integrated in a simulation model to predict substances from in-situ data in deep water for future monitoring systems.

Characterization of the Uncertainties in the Inspection Results of Ultrasonic Intelligent Pigs

In-Line Inspections using magnetic flux leakage (MFL) and the Ultrasonic (UT) intelligent pigs are the most common tools used to assess the integrity of pipelines. But, both MFL and UT inspection results are subject to various sources of uncertainties which must be quantified and accounted for in the integrity assessment of the inspected pipeline. A series of pull-through tests (PTT) of seven MFL tools and two UT tools from five service providers was performed on a 12-inch diameter pipe containing pre-existing internal corrosion defects of various length, width, and depth, and located in a variety of circumferential and longitudinal positions. The results of these tests are used to quantify the detectability statistics and the sizing uncertainties of the different tools for future use in developing calibrated probabilistic models for reliability based inspection, quantitative risk assessment and life extension studies for pipelines. The results of the MFL tools were presented in 2012 OMAE conference and this paper presents the results of the two UT tools.

Effects of Crack Front Curvature on J and CTOD Determination in Fracture Toughness Specimens by FEA

This paper investigates the effect of crack curvature on a single edge notched bend (SENB) specimen using finite element analysis (FEA). The variations of J and CTOD were studied through the thickness of the specimen for different levels of crack curvature. Five different curved crack fronts were considered, with the same weighted average crack length based on BS 7448. The levels of curvature varied from a straight crack front to a maximum deviation of 25% from the average crack length. The findings can be used to recommend more comprehensive crack curvature qualification checks for SENB tests and potentially provide improved estimates of fracture toughness when curvature is greater than allowed by the standard.

Subsea Pipelines: The Remotely Welded Retrofit Tee for Hot Tap Applications

Over the last 10 years Statoil has developed subsea remote hot tap technology. The first use of the remote hot tap technology was the Tampen Link hot tap in 2008. The hot tap cutting operation itself was performed without the use of divers; however, the preparatory works including hyperbaric welding of the hot tap tee was performed with saturation divers. The world’s deepest hot tap operations on a pressurized pipeline were performed on the Ormen Lange field in the Norwegian Sea in August 2009. Two Hot Taps were conducted in a water depth of 860 meters on pre-installed tees. The development of the Pipeline Repair System Remote Hot Tap equipment is the result of targeted research and development effort and extensive qualification work over many years. The Retrofit Tee remote installation, welding and cutting equipment has now been employed as part of a production installation: the Åsgard Subsea Compression project to extend the Åsgard gas-field lifetime. This paper describes the dry hyperbaric GMA weld procedure development and qualification performed in the laboratory, using the Corrosion Resistant Alloy consumable filler material Alloy 59, for the Remote Hot Tapping application using the Retrofit Tee. Specific attention was paid to areas of concern prior to performing the formal Weld Procedure Qualification work, particularly with regard to the effects of moisture and temperature upon the resultant weld quality. Weld procedures developed in the laboratory were successfully tested subsea in representative environments using the Remote Tee Welding Tool (RTWT) equipment in the Sognefjord, Norway, with offshore tests performed at 265 and 350msw in 2011. Further tests were performed in 2012 at 265msw and in the Retrofit Tee itself at 316msw in the Nedstrandfjord. The hyperbaric GMA weld procedures have now been applied for the production application for the Åsgard Subsea Compression project, with the installation of a Retrofit Tee, associated valve module and gooseneck spool to enable a cost-effective modification to the existing pipeline network without interfering with ongoing production. The Retrofit Tee will be connected with a Subsea Compressor station to extend the gas-field lifetime. This milestone represents the world’s first application of diverless dry hyperbaric GMA welding for a subsea production application.

Structural Integrity Control of Ageing Offshore Structures: Repairing and Strengthening With Grouted Connections

Structural integrity control (SIC) is an increasingly important element of offshore structures. Not only is it used in newly built and existing offshore structures (e.g. oil and gas (O&G) production & process facilities (P&PFs), wind turbine installations, etc.), but SIC is also essential for ageing offshore platforms which are subjected to an extension of their design service life. In these cases, SIC programs must be performed to assess the platforms. If any significant changes in structural integrity (SI) are discovered, then it is essential to implement an appropriate strengthening, modification and/or repair (SMR) plan. Currently, welded and grouted repairs are mostly used for SMR. Although a welded repair may typically restore a structure to its initial condition, if the damage is due to fatigue loading and welded repairs have been carried out, then historical evidence reveals that there is a high potential for the damage to reappear over time. On the other hand, mechanical connections are significantly heavier than grouted connections. Consequently, grouted repairs are widely used to provide additional strength, for instance, to handle situations such as preventing propagation of a dent or buckle, sleeved repairs, leg strengthening, clamped repair for load transfer, leak sealing and plugging, etc. This manuscript examines current developments in grouted connections and their comparative pros and cons in relation to welded or mechanical connections. It also provides recommendations for future research requirements to further develop SMR with grouted connections.

Crack Extension Effects on Welding Residual Stress in Fitness for Service Assessment

In the fitness for service assessment of a welded component with crack like defect, the current industry practices treat the welding residual stress in the same way as the applied operating stress in calculating the crack driving force. Although a growing crack should release the welding residual stress, the same initial residual stress is used in calculating the stress intensity factor solution regardless of the crack depth. Such procedures should result in too conservative solutions, since the crack extension effect on welding residual stress release is ignored in the crack driving force calculation. The present paper discusses the current state of the industry code and standard procedures for welded component defect assessment with a practical example problem solution, along with detailed discussions on the effects of crack growth on the welding residual stress. The objective of the paper is to motivate the industry to improve the code and standard procedures for the weld defect assessment. Also it presents a comparison of crack-like defect assessment results between the current industry procedure and one of newly proposed crack driving force calculation procedures for a crack growing in welding residual stress field, considering the welding residual stress release effects of growing crack. Recommendations for improving the industry code and standard procedures for the weld area defect assessment are provided based the investigation.

Sour Resistant High Strength Seamless Pipes for Risers

In case of a Weld On Connector’s riser using ASTM A182 F22 forged joints, high strength (SMYS of 80 ksi) steel pipes for sour service (hardness below or equal to 250 HV10) suitable for welding to the connectors are required. Welding is challenging because of the Post Weld Heat Treatment (PWHT) needed to reduce the hardness in the F22 HAZ while maintaining the required strength in the pipe. Theoretical evaluations were performed by means of metallurgical models and a potential solution was identified in grade P22-like steel (2¼ Cr - 1 Mo), with minor modifications with respect to the standard ASTM A335 and supplied in Q&T condition. A trial heat was cast and hot-rolled into pipes. After water quenching, the response to tempering was assessed by means of laboratory heat treatments and subsequent mechanical testing, together with metallurgical examination. Simulated PWHTs were also performed on Q&T material. 80 ksi grade P22 seamless pipes were finally produced and qualification involved mechanical testing before and after simulated PWHT: SMYS of 80 ksi and HV10 ≤ 250 requirements were met. The material also exhibited excellent toughness and resistance to HIC and SSC cracking.