A rupture of buckled steel pipes on the tensile side of a cross-section is studied in this paper as the most plausible case of ultimate failure for the pressurized buried pipelines under monotonically increasing curvature. Finite element simulation of full-scale bending tests on two pressurized X80 pipes with different yield-to-tensile strength (Y/T) ratios were conducted. The Y/T ratio and internal pressure were identified as the crucial factors that have a coupled effect on the ultimate failure mode of buckled pipes. That is, the high values of Y/T ratio and internal pressure mutually trigger the rupture of buckled pipes on the opposite side of the wrinkling.
Steel pipelines are so ductile and can accommodate a large amount of post-buckling deformations while preserving their operational safety and structural integrity. To benefit from this outstanding quality and prevent the buckled (wrinkled) pipelines from premature rupture, the postbuckling behavior of the steel pipes should be well understood.
Rupture is one of the major failure limits to the integrity of pipelines that endangers the environment as well as the public safety and property. Comprehensive experimental and numerical studies on the fracture of buckled steel pipes (Das, 2003; Sen, 2006; Mohajer Rahbari, 2017) show that under increased monotonic curvature, successive buckles (wrinkling) are formed on the compressive side of the wall, and the occurrence of rupture at the wrinkling location is unlikely because of the ductile nature of steel material. Rupture of wrinkling can occur once buried pipelines are subject to a very rare and changing boundary conditions accompanied by extremely large plastic deformations toward tearing the wrinkled wall (Ahmed, 2011). However, experiments have shown that the increasing curvature can easily trigger the postbuckling rupture of the tensile wall on the opposite side of the wrinkling (Sen, 2006; Mitsuya et al., 2008; Tajika and Suzuki, 2009; Igi et al., 2011; Tajika et al., 2011; Mitsuya and Motohashi, 2013; Mitsuya and Sakanoue, 2015). This mode of failure seems very likely to be the rupture limit of the wrinkled pipes, as it occurs following the same regime of monotonic bending deformations that have previously made the pipe buckle.
Austenitic-ferritic (super)duplex stainless steels, (S)DSSs, are of particular interest in oil and gas applications, due to their combination of high strength and corrosion resistance. However, (S)DSSs are known to be susceptible to hydrogen embrittlement, via a mechanism commonly referred to as hydrogen-induced stress cracking (HISC). In subsea environments, (S)DSS components are often exposed to cathodic protection (CP), which is mainly applied to protect structural steel components, to which (S)DSS components are connected. CP can introduce hydrogen to metallic surfaces that are exposed to seawater and, in the case of (S)DSSs, the absorbed hydrogen can cause embrittlement via HISC, which has been found to be responsible for a number of subsea failures.
During failure investigations of (S)DSS components, it has been observed that the microstructure, i.e. as manifested by the size, spacing and distribution of the austenite, ferrite and chromium nitride precipitates, obtained by various manufacturing and fabrication processes, is a key factor in resistance to HISC. However, because of the complexity of (S)DSS microstructures, the micro-mechanisms of hydrogen embrittlement have remained largely unknown. This work aims to evaluate and compare the resistance to HISC for two types of DSS products with significantly different microstructures: rolled and hot isostatically-pressed (HIPed), using the conventional fracture toughness test methods, i.e. unloading compliance testing of single edge notched bend specimens (SENBs). The two materials have been characterized in terms of composition, phase balance, austenite spacing and mechanical properties. An environmental-mechanical test program, largely based on fracture toughness testing, has been developed and performed to investigate the effect and significance of specimens’ notch geometry, as well as hydrogen-charging conditions, highlighting the difficulties of evaluation of resistance to cracking of DSSs, using fracture toughness based tests in hydrogen-charging environments.
(S)DSSs, composed ideally of 50% austenite and 50% ferrite, are of particular interest in oil and gas applications. However, (S)DSSs are known to be susceptible to hydrogen embrittlement, via a mechanism commonly referred to as HISC. In subsea environments, CP is a potential source of hydrogen, and HISC has been found to be responsible for a number of catastrophic in-service failures 1, 2. HISC is controlled by a combination of appropriate hydrogen concentration and a sufficiently high stress, in a susceptible microstructure. Those three factors are interdependent: high stresses reduce the critical hydrogen concentration necessary in a particular microstructure for HISC failure to occur.
Kwietniewski, Carlos Eduardo Fortis (LAMEF - UFRGS) | Renck, Tiago (LAMEF - UFRGS) | dos Santos, Fabrício Pinheiro (PETROBRÁS) | Scheid, Adriano (PGMec-UFPR) | Sartori, Marcelo (LAMEF - UFRGS) | Reguly, Afonso (LAMEF - UFRGS)
Super duplex stainless steels (SDSS) combine excellent corrosion resistance especially to the localized forms of corrosion with medium to high mechanical strength. This unique combination of properties has made these alloys very successful in the oil and gas industry. However, in some specific scenarios, even these steels have to be protected against corrosion, which is usually accomplished by cathodic protection. The synergic effect of hydrogen produced during cathodic protection with service mechanical loads may produce the embrittlement phenomenon known as hydrogen induced stress cracking (HISC). Indeed, documented cases of failure due to HISC have somehow deteriorated the image of the SDSS and raised some concerns. The aim of this investigation is to evaluate through fracture toughness tests the susceptibility of SDSS to HISC and more specifically to determine the effect of the cathodic protection potential and the stress intensity factor rate (K-rate) on the results produced. Within the range of parameters studied here, the degradation of fracture toughness due to HISC is strongly dependent on the testing parameters employed, especially the cathodic potential with a less pronounced effect of the K-rate. The results also suggest that the issue of HISC might not be a material’s problem but instead can be mitigated by the optimization of cathodic protection design.
Oil and gas floating production units fundamentally depend on the performance of their devices, components and structures. Rigid pipelines are essential equipment used in the offshore industry, commonly employed as flow-lines and risers. Carbon steels such as API1 5L X65 are the material of choice for such applications due to their low relative cost and availability. However, in the case of the Brazilian pre-salt, it seems unlikely that carbon steels can be applied, since the oil contains high concentrations of CO2, which causes generalized corrosion.1-3 Therefore, operators in Brazil are compelled to consider alternative solutions, such as lined or clad pipes as well as corrosion resistant alloys like duplex and super duplex stainless steels.
Super duplex stainless steels strategically combine the properties of ferritic and austenitic steels. The superior corrosion resistance of SDSS, especially for the localized forms of corrosion, is attributed to the use of high amounts of Cr and Mo.4 The addition of Ni and N, on the other hand, stabilizes the microstructure at the ideal volume fraction of austenite and ferrite phases, thus providing the best combination of mechanical properties and corrosion resistance.2, 5-7 Applications for SDSS include sea water systems, flow-lines, risers, pressure vessels and pipelines in general, where optimized mechanical properties are required along with high corrosion resistance in different media.5, 7, 8
Although SDSS present excellent corrosion resistance in general, in some cases to operate satisfactorily these alloys need to be cathodically protected against corrosion. That is the case for environments containing concentrated chloride solutions (severe salinity) and high temperature beyond their critical pitting/crevice temperature (CPT/CCT). Even when the operating conditions are not particularly severe, equipment and components made of SDSS may still be under cathodic protection due to occasional electric contact (galvanic coupling) with other structures that must be protected against corrosion.7, 9-11
Any catastrophic rupture scenarios of a steel pipe should be taken into considerations in the design and during the maintenance stage as the loss-of-containment may be accompanied by either property damage or fatal accidents. Ductile fracture of wrinkled (buckled) steel pipes on the tensile side of the cross-section is studied in this research as the most plausible case of ultimate failure for pressurized buried pipelines being subjected to monotonically increasing curvature. The results from two full-scale bending tests on X80 line pipe specimens that are pressurized up to 60% of specified minimum yield strength (SMYS) are considered as an input for the current study. The specimens possess the same dimensions and are made of X80 steel grade with different yield strength to tensile strength ratios (Y/T) of 90% and 83%. The specimen with higher Y/T ratio ruptured on the tensile side of the cross-section while experiencing post-buckling deformations. However, the specimen with lower Y/T ratio was unloaded after the formation of the local buckling.
Finite element analysis (FEA) of the full-scale tests were conducted and verified using the experimental data. The power law is calibrated to model the post-necking plasticity of steel using material test data, and, cumulative fracture criterion in conjunction with general fracture strain locus for the pipelines’ high-strength steel is implemented to predict the ductile fracture initiation in the pipe's wall. It is shown that the FE model accurately reproduces the load-displacement response and final rupture of the specimen with the higher Y/T ratio. For the other specimen, numerical simulation shows no rupture until the inner surface of the buckle comes into contact with itself which reveals that the lower Y/T ratio reduces the chance of rupture. Further numerical studies postulate that both Y/T ratio and internal pressure have a coupled effect on the rupture of wrinkled pipes and play a key role in triggering that kind of failure. That is, higher values of Y/T ratio and internal pressure increases the probability of the rupture of wrinkled pipes.
The fabrication of structures for Arctic applications is expected to face major challenges when it comes to the fracture toughness of the heat affected zone and the weld metal. Although the initial base metal toughness may be excellent, a severe toughness deterioration normally occurs as result of the rapid heating and cooling cycles in welding. The present investigation addresses tensile behavior and toughness properties of 32 and 50 mm thick 420 MPa plates, including tensile testing at both room temperature and −60°C, and Charpy V impact toughness and CTOD fracture toughness at −60°C. The welds were deposited by gas shielded flux cored arc welding using a heat input of 2-2.4 kJ/mm. The results showed a dramatic reduction in the fracture toughness after welding, i.e., from CTOD level above 2.5 mm to below 0.25 mm for the 50 mm plate, and from ~ 2 mm to the lowest value of 0.12 mm for the 32mm plate. The Charpy V toughness appeared to be good for the 50 mm, both for the heat affected zone and the weld metal, while the 32 mm plate suffered from low values in the weld metal root area. The results for the 50 mm thick plate are very promising, particularly for use in the temperature range down to −20 to −40°C.
The oil and gas industry is moving north due to the large oil and gas reserves. For example, a preliminary assessment by the US Geological Survey suggests the Arctic seabed may hold as much as about 30% of the world's undiscovered gas and 13% of the world's undiscovered oil (Gautier et al, 2009), mostly offshore under less than 500 meters of water. In these areas, the temperature may occasionally fall below −30 to −40°C, which represents new challenges to the materials. Normally, structural steels and pipelines may easily satisfy toughness requirements at such low temperature. However, welding tends to be very harmful to low temperature fracture toughness. Both the heat affected zone (HAZ) and the weld metal may fail in providing sufficient toughness (e.g., Akselsen et al, 2011; Østby et al, 2011; Akselsen et al, 2012; Akselsen and Østby, 2014).
The authors propose a cleavage fracture initiation model for bainite steels. The authors considered three stages of fracture initiation in the model: stage I, microcrack initiation in the brittle phase; stage II, propagation of the microcrack to a neighbor matrix; and stage III, propagation of the cleavage crack across the packet boundary. The cleavage fracture is assumed to initiate when all three stage conditions are satisfied in any one of the volume elements surrounding a notch tip. The authors applied this assumption to bainite steels and validated this model by comparing results of the analysis and toughness test results.
Recently, the strength of steel plates used for offshore structures and container ships has been increased, satisfying the demand for the increase in structural size scale and for performance in more severe operating conditions. Bainite steels have been used for various structures for achieving higher strength. However, the mechanism of cleavage fracture initiation in bainite steels remains unclear.
Beremin (1983) proposed a stochastic model of brittle fracture, assuming that brittle fracture follows, essentially, a weakest-link mechanism. He assumed multiple volume elements in a material and that each volume element contains one microcrack. In Beremin’s model, the critical stress of the microcrack in a volume element controls the fracture of the whole of the material. Beremin’s model led to Weibull stress, and the Weibull stress parameters can be obtained from multiple fracture toughness tests. These parameters are considered as material properties that control the fracture toughness distribution of the material. However, this model requires many fracture toughness tests to derive these parameters and does not explain the details of the fracture initiation mechanism of complex microstructures like bainite.On the other hand, Martin-Meizoso et al. (1994) proposed a model for predicting the fracture toughness of bainite steels. In that model, the process by which a carbide crack grows into a bainitic packet and propagates across a packet boundary was formulated. Lambert-Perlade et al. (2004) assumed the same process as in the model by Martin-Meizoso et al. (1994) and modeled fracture initiation of the simulated heat-affected zone (HAZ) containing an upper bainite microstructure. However, formulations for critical conditions of the fracture initiation process in the models are too simple to predict the toughness of the bainitic steels.
Akselsen, Odd M. (SINTEF Materials and Chemistry) | Lange, Hans I. (Norwegian University of Science and Technology (NTNU)) | Ren, Xiaobo (SINTEF Materials and Chemistry) | Nyhus, Bård (SINTEF Materials and Chemistry)
For steel structures to be installed in the Arctic region, the risk of brittle fracture represents a primary concern due to the ductile to brittle usually transition taking place at sub-zero temperatures. Therefore, the present investigation addressed the heat affected zone and weld metal toughness of two extra low carbon steels of 420 MPa yield strength grade, supplied in 20 and 50 mm thickness. The testing included tensile, Charpy V and CTOD. The results obtained showed that the Charpy V toughness was relatively high at -600C, but that some low values may occur for the fusion line position. The fracture toughness at -600C, based on SENB05 (a/t=0.5) geometry, appeared to be low for both weld metal and fusion line positions. More specific measures may be taken into account in welding procedure qualification of the current steels, such as using lower crack length (e.g., a/t=0.2), tension instead of bending (SENT testing) or a full engineering critical assessment.
The oil and gas industry has been gradually moving towards the north. In Norwegian waters, the Goliat field was recently set in production by ENI. The design temperature for this field was -200C, which is somewhat lower than previously experienced, and below the lowest design temperature in the NORSOK standard (2014), which is currently -140C. Not far from the Goliat, Johan Castberg may be the next field of exploration, and is now under evaluation by Statoil. When going further north and east, the ice edge is approached, and the design temperature may fall down to -300C, or even below. This represents huge challenges to the materials which are to be used. Normally, e.g. structural steels and pipelines may easily satisfy toughness requirements at such low temperature. However, welding tends to be very harmful to low temperature fracture toughness. Recent results have demonstrated that the toughness may be on the borderline for both the heat affected zone and the weld metal (e.g., Akselsen et al, 2015; Akselsen & Østby, 2014; Akselsen et al, 2012; Akselsen et al, 2011), indicating that required robust solutions are not yet available for the most challenging part of the Arctic region, unless some constraint loss corrections are applicable.
Explorations of new oil and gas fields in harsh arctic surroundings and the possibility of passing through the Northeast Passage lead to new challenging requirements for offshore constructions. Further developments of steel grades are of basic importance to meet the increasing demand on plates for structural applications.
The challenging new requirements for existing steel grades are an improved toughness at lower temperatures without decrease of strength and an adapted weldability.
Therefore, advanced base material properties of heavy plates as well as HAZ (Heat Affected Zone) behaviour have been developed by optimizing of the chemical composition and utilization of TMCP (Thermomechanical Controlled Process) technology.
In this paper, the development of high-strength heavy plates with improved low temperature toughness is described and illustrated with a recent application. Plates of grade S420G1/2 up to 65 mm thickness and Charpy impact properties down to -80 °C and CTOD (Crack Tip Opening Displacement)-properties down to -50 °C were successfully produced and delivered for the construction of an offshore loading tower in Siberia.
During the last years, the demands on constructions and therefore on steel plates have been increased. Because of the low actual oil-price the investigations in the arctic region were reduced (Reuter, 2015; Meinert, 2015). However, there is still a rising potential for more export terminals in Arctic areas due infrastructure constraints over land because of the possibility of passing through the Northeast Passage. Thus more offshore constructions like terminals and platforms can be expected for the future (Fig. 1). Lower ambient and service temperatures have to be considered. Lowest Anticipated Service Temperature (LAST) in the North Sea is -10 °C, whereas structures in the arctic region should anticipate for a service temperature between - 50 and -60 °C. In order to ensure safe service in winter periods, sufficient fracture toughness shall be proven for highest thickness involved at the anticipated LAST.
Separation is often observed after Charpy V-notch tests or Drop Weight Tear Tests (DWTT). Separation is defined as the fracture morphology whereby many cracks were formed parallel to the rolling plane. On the other hand, an arrowhead fracture is often observed near the surface of DWTT. The morphology of the arrowhead fracture is similar to that of separation. In this study, the relationship between arrowhead fracture and microstructure was investigated as compared with that between separation and microstructure. From this study, it was thought that the mechanism of arrowhead fracture formation was the same as that of separation formation although stress constraint in arrowhead fracture was different from that in separation.
High strength line pipe steels, with API (American Petroleum Institute) X80 grade yield strength or higher, have been used for many pipeline projects because of the lower costs to transport natural gas. Crack arrestability of brittle and running ductile fractures is one of the required properties for high strength line pipe steels as cracks must be arrested, even if a brittle fracture occurs from welds such as girth welds. Cracks must also be arrested if the line pipe body is subject to ductile fractures.
The DWTT (Drop Weight Tear Test) (Eiber, (1979)) is a primary test method that evaluates the crack arrestability of brittle fractures. This test evaluates whether a ductile crack is transferred from a brittle fracture after a brittle crack is initiated just under the notch. Previous results (Amano, (1986)) indicated that the crack speed fell below 450 m/s and that the crack was subsequently arrested in the full crack burst test, for line pipes with a DWTT shear area of more than 40%. However, a DWTT shear area of 85% or higher is required for such specifications as those of the API because DWTT shear area scattering is taken into account in a circumferential direction.
Recently, the requirements of oil and gas drilling in arctic regions have led to a significant increase in vessel size. This trend has led to increased safety requirements for materials such as high strength, good toughness at low temperature, and good weldability. Furthermore, crack arrestability has been a long-standing key issue for large container ships. Full-thickness weld joints of steel plates with 80-mm thickness were prepared through the use of two welding processes, namely Flux-Cored Arc Welding (FCAW) and a combined welding process of Electro Gas Welding (EGW) and FCAW. The effect of the joint design on crack arrestability was investigated to prevent a catastrophic failure along the block joint of the hatch side coaming in the container ship. A brittle crack-arrest technique was developed without a block joint shift on the basis of an arrest weld at the end of the hatch side coaming weld line.
Recently, the size of the ship required to explore and produce oil and natural gas in the arctic offshore region has greatly increased the demand for large vessels. High performance steel plates are required by these industrial trends (Yamaguchi et al., 2006). As the usage of large-scale, high-strength metallic structures in various civil engineering constructions, shipbuilding, and other industries increases, higher standards and assessments are required for the integrity and performance of the components (Masubuchi, 1980; Ouchi, 2001). It is even more critical when the heavy-section or thick steel plate and pipes are welded because the inherently generated large residual stresses are detrimental to the safety of the structure and can lead to an abrupt crack initiation and fracture (Withers and Bhadeshia, 2001; Webster and Ezeilo, 2001).
In the shipbuilding industry, the container ship size has gradually increased for mass transportation and cost reduction in the shipping industry. Thus, thick and high-strength steel plates are used for the upper deck structure of container ships because of their large hatch openings (see Fig. 1) (Yamaguchi, 2006). It is expected that EH47 grade steel with minimum yield strength of 460 MPa and more than 80-mm thickness will be used to build the container ship above 18,000 TEU (Twenty-foot Equivalent Unit). In addition, offshore structures have been constructed with around 100-mm-thick steel plates. Due to the rapid increase in vessel and offshore structural size, the applied steel plate thickness has also increased. Thicker steel plates are usually used for the upper structure of a ship including the hatch side coamings, sheer strakes, and longitudinal bulkheads of large container ships. This is because of the restrictions on designing the hull girder strength for their large hatch openings.The 147th Research Committee of The Shipbuilding Research Association of Japan (147th Research Committee, 1978) had investigated crack-arrest toughness of high heat input less than 30 kJ/mm welds with the thickness below 40 mm. They concluded that a long brittle crack can be arrested when the brittle crack that initiated from the weld joint propagates into the base metal. In addition, they concluded that plates of more than 40-mm thickness are more prone to fracture.