Thermoplastic composite pipes (TCPs) are increasingly used to transport hydrocarbons and water in the oil and gas industry due to their superior properties including corrosion resistance, thermal insulation, light weight, etc. The cross-section of TCPs generally consists of three layers: inner liner, composite laminate, and outer jacket. Three layers are bonded together and form a solid-wall construction. Inner liner and outer jacket made of thermoplastic polymer provide protective barriers for the laminate to against the inner fluid and outer environment. The laminate is constructed by an even number of helically wounded continuous fiber reinforced thermoplastic composite tapes. In this study, mechanical behaviors of a TCP under an internal pressure were investigated by using analytical and finite element analysis (FEA) methods. The analytical method which is based on the three-dimensional (3D) anisotropy elastic theory can take account of non-uniformly distributed stress and strain through the thickness of the pipe wall. FEA models were setup by using the software ABAQUS to predict the stress distribution of the pipe. 3D Tsai-Wu failure criterion was used to predict the maximum internal pressure of the pipe. Effects of some critical parameters, such as the winding angle of composite tapes and the number of reinforced plies, on the internal pressure capacity of TCPs were studied. Results obtained from the analytical and FEA methods were fairly agreed with each other, which showed that with the increasing of the number of reinforced plies the internal pressure capacity of a TCP gradually increases and approaches to an extreme value. In addition, the optimal winding angle which results the maximum internal pressure is not a constant value, instead, it varies with the increasing thickness of the laminate layer. This study provides useful tools and guidance for the design and analysis of TCPs, and is currently under validation through experiments.

Subsea pipelines are prone to be damaged by the falling objects from ships or offshore platforms, which may result in economic losses and pollution. The dimensions of dent were commonly used to evaluate the impact resistance of pipes made from carbon steel. Thermoplastic composite pipes (TCPs), due to their superior properties including corrosion resistance, thermal insulation, fast installation, etc., are increasingly used as the subsea pipelines. The TCP is made from thermoplastic resins and reinforced by continuous fibers. Because of the brittle nature of carbon fibers and glass fibers, the dimensions of dent are not suitable for assessment of impact resistance of a TCP. In the present work, a procedure was proposed using the internal pressure capacity as an indicator to evaluate the lateral impact resistance of a TCP. First, the internal pressure capacity of an intact TCP was evaluated. Second, a quasi-static simulation was conducted by applying a lateral compression force on the intact TCP using a rigid ball, until one of the composite plies in the reinforcement layer failed. The quasi-static simulation provided an initial estimate of the minimum energy that causes the start of damage of the TCP. Third, the impact simulations were performed by using a rigid ball hitting the TCP and, then, the internal pressure capacity of the damaged TCP was evaluated. Finally, the internal pressure capacity of the damaged pipe, compared with that of the intact pipe, was used as an indicator to evaluate the lateral impact resistance of the TCP. In this study, a glass-fiber reinforced polyethylene (PE) pipe of an inner diameter of 150 mm was modeled by ABAQUS to illustrate the procedure. A theoretical method was proposed to calculate the impact energy of a dropped object in a shallow water. The example studied in the present work showed that the modeled TCP was not strong enough to survive the lateral impact caused by the dropped object and should be buried to a certain depth beneath the seabed if used as a subsea pipeline.

The combination of high strength and corrosion resistance of duplex stainless steels (DSSs) has promoted their use in subsea environments, particularly for the oil and gas industry. However, in the past 20 years, costly in-service failures of DSS components have frequently been encountered, most of which were attributed to hydrogen induced stress cracking (HISC). This cracking mechanism occurs once a susceptible microstructure coexists with critical levels of hydrogen and stress. In subsea environments, the use of cathodic protection (CP) is a source of hydrogen and the critical stress levels can be reached locally by operation and residual stresses generated during the manufacturing and installation processes. In order to prevent HISC failures, the microstructure, sources of hydrogen and stress levels have to be determined and controlled. An important factor contributing to the total stress experienced by a welded component is the residual stresses induced by welding. In a previous research projects conducted at TWI, residual stresses were measured in a full-scale girth weld between a DSS flange and pipe which had been in service, subsea, for 12 years. This follow-on work, on the same component, tested a number of small-scale cross-weld specimens, under CP to evaluate resistance to HISC of the weld. The observations showed that the cross-weld specimens exhibited superior resistance to HISC than that of the all-parent pipe and flange specimens. This was unexpected, and it was thought that this could potentially be explained by the influence of any compressive residual stresses remained in the small-scale test specimens used for the environmental testing programme. The present work provides the residual stress measurements in small-scale cross-weld specimens, using the neutron diffraction technique, and attempts to explain the relationship between the HISC testing results and residual stresses measured.

Austenitic stainless steels such as SUS 316 LN and austenitic stainless clad steel are used in cargo holds of chemical tankers owing to their corrosion resistance. Recently, the use of duplex stainless steels has been increasing, owing to their better strength and corrosion resistance and lower content of expensive Ni, compared with those features of austenitic stainless steels. However, few duplex stainless clad steels have been approved by classification bodies. Furthermore, the application of duplex stainless steel is not yet mainstream as hull structural materials because a stable supply market has yet to be established. Therefore, when applying cladding steel to construction of chemical tankers, austenitic stainless clad steel is often used at present. The duplex stainless steel and the austenitic stainless clad steel are mixed at construction factories; hence, there is a risk of misuse of welding consumables. If misuse is suspected, it is not possible to judge the erroneous use from visual inspection after construction; therefore it is necessary to uniformly remove the weld and re-weld. However, if universal welding consumables were identified, this might avoid the problems of misuse and simplify the procurement of welding materials. In this paper, we report on our studies to verify welding consumables for use in the hull structures, involving a mixture of duplex stainless steel and the austenitic stainless clad steel. The static strength of the welded joints is a particular focus of this study, from which we confirmed the validity and limitations of welding consumables.

Generally, equipment or mechanical parts are worn out by abrasion, impact, erosion, and environmental corrosion. To reduce these losses, the welding that wears the surface of the material with abrasion or corrosion resistance is called overlay welding. Corrosion and high temperature oxidation are very important factors in overlay welding material selection because they have a great effect on wear rate. Typical equipment is made of carbon steel, stainless steel or a material that is not wear resistant. Therefore, overlay welding is applied to parts where the parts themselves are subject to severe wear or corrosion. The diffusion of chlorine and alkaline ash during the combustion process causes foiling, corrosion, and erosion in the water wall panel & tube which is a heat transfer device. In order to protect the water wall panel & tube from various chemical and physical phenomena occurring during the combustion process, heat-resistant alloy steel such as Inconel is overlay welded on the outer surface of carbon steel pipe. However, since the length of the water wall panel & tube is more than 7 meters, the deformation after overlay welding is very large and the straightening work requires a lot of time and money. Also, since cooling water flows through the pipe during welding, the temperature of the cooling water also affects the welding deformation. In this study, welding process sequence and coolant temperature are optimized to minimize welding deformation during overlay welding.

Titanium alloys are widely used in naval ships due to its high strength, low density, no magnetism, corrosion resistance and so on. However, the material nonlinearity brings new challenges to the ultimate strength evaluation on the Titanium structure. This work is to evaluate the ultimate strength for a stiffened titanium cylinder with consideration of material nonlinearity by numerical analysis and scaled model experiment. Firstly, a series of titanium alloy stiffened cylinder pressure hulls are analyzed for their ultimate strength by non-linear Finite Element Method (FEM). Secondly, model tests are carried out for the above titanium cylinders to obtain their ultimate carrying capacity. Thirdly, the good agreement between experiment and numerical results verify that the numerical simulation method is suitable for ultimate strength evaluation. Finally, some influential factors on the ultimate capacity of the stiffened titanium cylinder are investigated, including stiffeners arrangement, thickness of cylinder hulls, inside diameter. The research work can map the limitations of the current rules and to support the development of ultimate strength assessment guidelines for titanium cylinder pressure hulls.

The marine propeller is regarded as critical component with regard to the performance of the ships and torpedoes. Traditionally marine propellers are made of manganese-nickel-aluminum-bronze (MAB) or nickel-aluminum-bronze (NAB) for superior corrosion resistance, high-yield strength, reliability, and affordability. Since the composite materials can offer the potential benefits of reduced corrosion and cavitation damage, improved fatigue performance, lower noise, improved material damping properties, and reduced lifetime maintenance cost, Many researches on the application of the composite materials for marine propeller had been conducted. In this work, the INSEAN 1619 large screw 7 bladed propeller is analyzed, to explore the hydrodynamic and structural performance of composite materials effect on propeller’s performances, The commercial software ANSYS Workbench was used in this research. The coupled FSI method was used to analysis the dynamic performance of INSEAN 1619 large screw 7 bladed propeller made of different materials. The simulation results show that the effect of fluid–structure interaction in the analysis of flexible composite propellers should be considered.

High tensile strength aluminum offers great potential as a conductor material for Medium Voltage power cables within subsea umbilicals. Its excellent fatigue performance makes it suitable for dynamic umbilicals and its high tensile strength and light weight make it an ideal candidate for deepwater dynamic umbilicals either as an independent load bearing member or as an electrical conductor taking load share in an armoured or steel tube umbilical. Umbilicals, are the bundles of electrical and hydraulic components that connect and control elements of a subsea Oil and Gas production system. 6000 series aluminum conductors, commonly known as AAAC (All Aluminum Alloy) conductors are widely used on high voltage overhead transmission lines, primarily on long spans due to their increased tensile strength. They have been widely used in various environments and possess an excellent track record. However, the use of AAAC 6000 series in a subsea umbilical system is novel. The cable located inside an umbilical bundle is normally of a wet insulation design, and an area of concern is that the seawater may permeate through the cable insulation allowing the conductor material to be in contact with water throughout its design life. Hence the corrosion resistance of the AAAC 6000 series in a seawater environment is of paramount importance and therefore must be assessed. This paper details the corrosion assessment of the 6000 series aluminum power cable conductor at RINA Consulting Ltd’s laboratories together with complimentary field testing. The presented test results are based on long term (6–12 months) laboratory testing. The test programme investigated permeation, impact of temperature, effect of hydrostatic pressure and galvanic corrosion, with the tests being performed on material coupons and cable samples in a simulated seawater environment. Also detailed is the outcome of a full-scale cable field immersion test, 6 months under 1000m depth at seabed temperature of 10°C. The results demonstrated that there was no Cl − and Na + ion permeation through the insulation layer. Also, there is no sign of aluminium conductor corrosion and no drop-in insulation resistance witnessed for all tested samples at temperatures up to 90°C and under a high pressure of 300bar after a one year test period, which is sufficient to qualify use in a subsea application. Based on the results of the presented laboratory testing and field immersion testing, it can be concluded that 6000 series aluminium conductors within ‘wet design’ Medium Voltage power cables will not be susceptible to corrosion in a subsea environment and could be used in other applications such as renewables.

In general, tensile armour wires of flexible pipes that are designed for sour applications have their strength limited to 850 MPa due to the possibility of embrittlement phenomena to occur. A Thermally Sprayed Aluminum (TSA) coating 250 μm thick was applied to high strength steels with UTS of 1470 MPa and YS of 1280 MPa. Three specimens conditions were evaluated: full coating, no coating and coating with a designed defect. The load was applied using a four point bending fixture, maintaining a constant stress of 90% of material’s yield strength. All tests were performed in accordance with recommendations of NACE TM 0177 method B. The test solution was distilled water with NaCl 5.0% saturated with a gas mixture of 10,000 ppm of H2S in balance with CO2 during 720 hours. It was observed that samples without coating were more susceptible to the effect of the environment presenting higher degradation and failure. The fractures presented typical characteristic of the Sulfide Stress Corrosion Cracking (SSCC). Furthermore, it was detected parallel cracks to the surface of the wires indicating the embrittlement phenomenon of Hydrogen-Induced Cracking (HIC). On the other hand, coated samples with and without defects did not fail during the 720 hours of testing. A posterior non-destructive testing and a metallographic analysis did not identify the presence of cracks. These results were attributed to the physical barrier of the aluminum coating and the cathodic protection generated by the preferential aluminum corrosion. This preliminary study shows that TSA coatings can be a good alternative to increase the corrosion resistance of armour wires in sour environments allowing the application of higher strength steels.

Marine hydrokinetic turbines typically operate in harsh, strongly dynamic conditions. All components of the turbine system must be extremely robust and able to withstand large and constantly varying loads; the long and relatively slender blades of marine turbines are especially vulnerable. Because of this, modern marine turbine blades are increasingly constructed from fiber reinforced polymer (FRP) composites. Composite materials provide superior strength- and stiffness-to-weight ratios and improved fatigue and corrosion resistance compared to traditional metallic alloys. Additionally, it is possible to tailor the anisotropic properties of FRP composites to create an adaptive pitch mechanism that will adjust the load on the turbine in order to improve system performance, especially in off-design or varying flow conditions. In this work, qualitative fundamentals of composite structures are discussed with regards to the design of experimental scale adaptive pitch blades. The load-deformation relationship of flume-scale adaptive composite blades are characterized experimentally under static loading conditions, and dynamic loading profiles during flume testing are reported. Two sets of adaptive composite blades are compared to neutral pitch composite and rigid aluminum designs. Experimental results show significant load adjustments induced through passive pitch adaptation, suggesting that adaptive pitch composite blades could be a valuable addition to marine hydrokinetic turbine technology.

The expansion of umbilical functionality to include power cables and high temperature fluid transportation (for applications such as gas lift) has led to the requirement for a high temperature steel tube riser umbilical, as shown in Figure 1. The combination of the elevated temperature and the dynamic service conditions create a unique design brief for steel tube umbilicals. The following paper presents a case study evaluating a steel tube riser umbilical capable of transporting hot gas at 70°C from an FPSO to a well head at approximately 800m water depth. The material selection for the steel tube is super duplex stainless steel (SDSS) and due to the high temperature, the corrosion resistant coating selected to ensure corrosion resistance in seawater at 70°C is fusion bonded epoxy protected with a bonded polypropylene outer layer (3LPP). The fatigue performance of the dynamic steel tube umbilicals is highly dependent on the frictional loads between the components developed due to tension and bending. This loading is most critical in the bend stiffener location at the riser umbilical’s interface with the FPSO structure. The fatigue critical component is usually determined to be one of the super duplex steel tubes within the umbilical. The frictional loads are a function of the coefficient of friction between the interacting components and the contact load developed between the layers of helically wound components. This contact load increases with tension. The paper considers the effects of the polypropylene material selection and the elevated operating temperature on the friction interface between the steel tubes. The work assesses the corresponding change in fatigue damage through the service life in comparison to more common temperatures and materials used in dynamic steel tube umbilicals. Changes in contact load between elements of a friction interface are known to affect the friction coefficient. The contact load across all interfaces has been varied to help understand how the coefficient of friction may be affected by different tensile loads or umbilical designs. Including this variable in the test program also ensures that the friction is quantified at a contact load relative to the design case considered. To assess the differences in friction a scope of component level friction testing is presented and the results are processed through umbilical local fatigue analysis software to establish the implications on fatigue performance. The umbilical structure is designed to free flood in between the components in service, and the bend stiffener region is submerged for the design case in question. Full scale flex-fatigue testing of the dynamic umbilical and the bend stiffener are however conducted in a dry environment. The impact of this on the severity of the test and in comparison to the in-service condition are assessed using component level wet and dry testing to provide changes in friction for sensitivity analysis. In addition to the loading on the tubes, the fatigue performance of the material being loaded is also considered. the fatigue performance of welded super duplex tubing has previously been tested and documented at ambient room temperature conditions and therefore the effect on the welded steel tube fatigue performance due to the increase in temperature for this application has been quantified to ensure the proposed design curve is suitable. cyclic bending stress experienced by the steel tubes during dynamic service (2).

Thermoplastic Control Umbilicals, as shown in Figure 1, have been deployed subsea for decades globally. Typically, these have been installed in harsh dynamic environments such as the North Sea, very cold environments such as the North Atlantic and very warm environments such as the coastal waters of Middle East and Asia Pacific. The inherent fatigue and corrosion resistance of the functional components can offer significant operational advantages while umbilical make-up and manufacturing process can offer significant cost and schedule advantages. As the industry has moved into deeper warmer water regions since the late 1990’s, such as the Gulf of Mexico and West Africa, some of the limitations of conventional thermoplastic umbilicals, such as inherent collapse resistance or design working pressures became barriers for the adoption of the technology. In recent years there have been many new polymer materials developed that provide increased tenacity and temperature stability which subsequently have enabled an evolution in thermoplastic hose technology. This has facilitated the development of the next generation of high temperature, high pressure, collapse resistant hoses that can be deployed in deep water. This paper defines the testing carried out on the constituent parts of the composite hose primarily focusing on the liner and details typical modes of degradation associated with high temperature, pressure or tension. The new material technologies will be benchmarked against conventional materials traditionally used in less aggressive environments. This paper will detail the results of the development program aimed at optimising the hose design process and implementing the cutting edge materials in order to qualify a robust series of hose designs qualified to the stringent requirements of ISO 13628-5 [1]. The paper will also detail the development of the new termination coupling which has been developed in parallel with the next generation hose and which provides a reliable and robust method of coupling the hose to the subsea control system or joining two lengths of hose together. The paper will conclude with a case study comparing a typical deep water installation of a steel tube umbilical with an equivalent thermoplastic umbilical, highlighting the benefits of the new thermoplastic umbilical designs.

In order to improve the corrosion resistance of the drill pipe, the alumina ceramic membrane is coated on the surface of aluminum alloy drill pipe of offshore platform by micro arc oxidation technology. This paper mainly focuses on the experimental work to determine the effect of the nano-MoS 2 content in the solution on the micro morphology, element content and corrosion resistance of the coating. The results show that the amount of nano-MoS 2 particles has big influence on the making of micro-arc oxidation (MAO). The thickness of MAO coating is increased with the adding of nano-MoS 2 particles. Besides, the size of the coating is first increased then decreased, and the surface of coating become more compact and smooth with the increase of the amount of nano-MoS 2 particles. Also, the corrosion resistance of MAO coating is improved as the amount of MoS 2 increases. The corrosion rate is decreased from 0.032g.m −2 .h −1 to 0.024g.m −2 .h −1 when the addition amount of nano-particles increased from 0.5g/L to 2g/L.

Ferritic-austenitic (duplex) stainless steel components are used for oil and gas production duties due to their high strength and corrosion resistance. The material is routinely used for short flowlines, as well as for welded hubs and flanges. Cathodic protection (CP) is employed, via sacrificial aluminium based anodes, which protects ferritic steel parts from seawater corrosion. Whilst CP has proven successful in preventing corrosion, failures have occurred due to the ingress of electrolytically evolved hydrogen. Duplex stainless steel joints become susceptible to environmental cracking under a combination of high stress, hydrogen content, and susceptible microstructures; critical combinations of which may result in hydrogen induced stress cracking (HISC). Successful operation of duplex equipment, in avoidance of HISC, necessitates a good understanding of the total in-service stresses (including from loading applied in service and from residual stresses from manufacture, fabrication, installation and commissioning). One of the key components of understanding the in-service stress at welds is knowledge of the residual stress distribution, following welding. The focus of this paper is to provide an overview of the typical residual stress levels in a welded superduplex stainless steel (SDSS) subsea joint, using neutron diffraction and finite element modelling. The results are presented in the context of current recommended practice, for example DNV RP-F112.

Offshore structural steel design and material requirements continue to present increasing challenges for the steelmaker and fabricators. Niobium-bearing steels currently play a key role in meeting these objectives through the Nb-grain refinement mechanism of the microstructure and cost effective steelmaking. These steels possess a combination of exceptional properties with high strength, excellent weldability, high toughness at low temperature, good ductility, excellent corrosion resistance, and high formability. Reduced variation of Charpy toughness through the thickness of heavy plates is imperative in these offshore platforms to enhance reliability and performance. Toughness variation can be reduced through the proper continuous casting and hot rolling mechanical metallurgy process. These high-performance steels (HPS) possess an optimized balance of these properties to provide maximum cost effective performance in offshore structures at strength levels from 355 to 700MPa with excellent corrosion resistance. This combination of good strength-toughness balance, excellent weathering properties and reduced preheat temperatures for welding in these low carbon Nb-structural steels result in significant cost savings. These enhancements provide structural engineers the opportunity to further improve the structural design and offshore platform performance. Lower carbon Nb-alloy designs have exhibited reduced operational production cost at the steel mill as well, thereby embracing the value-added attribute Nb provides to benefit both the producer and the end user throughout the supply chain.

Umbilicals are important tubes assembly essentially used for subsea equipment controlling functions and fluid injection. Super duplex stainless steel 2507 (UNS 32750 / EN 1.4410) has been the historical grade used for its corrosion resistance and mechanical properties. The Oil and Gas industry is gradually working on the exploration and exploitation of offshore and deep offshore resources. These operating conditions lead to new philosophy in the design of subsea umbilicals, which are evolving to answer the needs of new functionalities. Recent years development supported by O&G operators, have seen innovative super duplex stainless seam welded tube solution being used for their high mechanical characteristics (YS 0.2 ≥ 750 MPa; UTS≥ 900 MPa), tight dimension tolerances, and their ability to generate cost saving during installation. The specific metallurgical manufacturing route for the 2507 strip together with an efficient roll forming process and NDT control allows to obtained tubes for umbilicals which push the usual boundaries of utilization of this super duplex grades. In this paper, Hydrogen Induced Stress Cracking (HISC) behavior, possible failure mode of the super duplex stainless steel tubes exposed to cathodic protection in the termination unit, was extensively studied. A stress load of 110% YS 0.2 was applied using a dead weight bench and the tubes performance was characterized after 500h of exposure in synthetic sea water under cathodic protection. Crevice corrosion is also a reported failure mode for umbilical tubes. Corrosion resistance of the 2507 seam welded tubes was also assessed in natural sea water in crevice conditions at open circuit potential (OCP). No failure was observed during the HISC testing in synthetic sea water after 500hours of exposure and no micro-crack was detected on the tubes after characterization. The metallurgical microstructure of the laser weld is not preferential initiation zone for crevice corrosion.

High pressure tubing and associated tubing couplers are critical components required for the operational control of subsea oil and gas production equipment. Tubing couplers used in subsea oil and gas developments are commonly made from Nitronic 50 HS® (N50HS) due to its high strength, corrosion resistance, and resistance to galling. Nitronic couplers are typically welded to several dissimilar metals including super duplex stainless steel (SDSS) control tubing using SDSS filler metals such as AWS A5.9 ER2594. Recent evaluations have found that sigma (σ) phase forms in N50HS weldments and its effect is not broadly understood by industry. During N50HS solidification, Scheil solidification conditions establish compositional gradients in the unmixed zone located along the fusion line adjacent to the N50HS base metal. This solidification-induced segregation promotes compositions that are susceptible to interdendritic intermetallic compound formation when they are reheated by subsequent weld passes such as in multipass welding or at weld start-stop locations. Decreasing heat input is a common approach to reduce or eliminate the formation of intermetallic compounds in SDSS. Although decreasing heat input can reduce the amount of energy available to drive the solid state transformation from ferrite to σ, it does not change the solidification mode (AF) or solidification conditions from Scheil to para-equilibrium within the range of cooling rates possible with arc welding processes. As such, the compositional gradients that promote intermetallic compound formation along the N50HS fusion line can only be minimized through heat input control and cannot be eliminated in arc welds regardless of the heat input used. The effects of σ on toughness and corrosion resistance of Nitronic weldments were evaluated. N50HS solidified samples with up to 2 volume percent σ were found to have CVN of >40J at −40°C, and no evidence of pitting at 25°C in the ASTM G48 test.

Since the last five years, an innovative technology for umbilical’s seam welded super duplex stainless steel tubes (grade 2507, UNS S32750) has been developed in order to meet new technical challenges. An extensive program of qualification, based on 4 dimensions covering the full manufacturing range, i.e. internal diameter and wall thickness, has been set. More than 11000 test results demonstrated the efficiency and the stability of the manufacturing process. The results were also confirming the expected properties of the tubes, in term of mechanical characteristics, corrosion resistance and manufacturing tolerances. At the end of this qualification phase, a Type Approval Certificate was delivered by an independent and official certifying authority demonstrating then, the suitability of the product for offshore static applications. The next step was to confirm the fatigue performance of the product to meet the requirements of umbilicals for dynamic applications. This paper describes how a dedicated fatigue test protocol has been jointly defined between a Major O&G player and the tube supplier. This protocol has been built in order to test the 3 dimensions already produced during the first step of the qualification, considering tube samples with and without orbital weld. Fatigue test programs are known to be very sensitive to the test parameters, i.e. temperature, test frequency and dimensional properties of the samples. As a consequence, special attention is paid to the description of the testing methodology, the selection of the acceptance criteria and the definition of the testing parameters. Finally, the results of this program are presented and discussed in order to assess the capability of the product with respect to the defined acceptance criteria.

Codes and standards for oil&gas industry, as OS-DNV-F101, recommend the use of single edge cracked plate in tension (SEN(T)) for the experimental determination of the material critical CTOD. For clad pipe welds, this specimen geometry is difficult to be obtained for the weld material or clad corrosion resistance alloy due to the specimen shape and minimum dimensions. Alternatively, circumferential cracked bar geometry, CCB(T) could be used. This geometry configuration can be machined when limited material quantity is available and used for both quasi-static and dynamic fracture characterization. In this paper, an extensive elastic-plastic finite element investigation has been carried out on both SEN(T) and CCB(T) geometries in order to select equivalent configurations in the J-Q space. Ductile crack initiation and growth has been simulated using continuum damage mechanics model. Numerical simulation results indicate that CCB(T) with a crack depth ratio r/a = 0.2 realizes constraint loss similar to that of SEN(T) with a crack depth ratio a/W = 0.5. Similar crack resistance curves have been obtained for these two configurations confirming the equivalence of the selected sample geometries.

Recent extensive use of Corrosion Resistance Alloy (CRA) as internal protection layer of standard carbon-steel pipes (clad and lined pipe) in the oil and gas industry requires an intensive use of bimetallic welds. Since some degree of defects in welds is inevitable, and in codes and standards (such as BS7910) the case of bi-metallic joint is usually not considered, some R&D’s activities are ongoing to define specific design guidance for an Engineering Critical Assessment (ECA) aimed at determining flaw acceptance criteria for fabrication of bimetallic joints. Based on the limited guidance in the literature, proposed procedures for ECA on CRA welds seem not cover the root/hot pass weld region, for which the requirement of “zero defect” became mandatory. As direct consequence, it penalizes the weld fabrication rate, particularly if “J-lay” or “S-Lay” methods are adopted. Furthermore, they are investigating on cases where weld material is overmatching the base metal or for a limited partial overmatching, despite for CRA welds, such conditions, seem quite difficult to be fully met, if current consumable materials present in the marked are selected. Aim of present paper is to describe how any standard ECA procedure (ordinarily used to assess carbon-steel welds) may be alternatively adopted to assess CRA welds for clad & lined pipe material, if specific conditions are respected. For this purpose a few number of elastic-plastic Finite Element Analysis (FEA) is required to identify and/or extends the validity limits which have to be met in order to be conservative in the use of selected standard procedure. Outer, inner and under clad flaws, located along the weld fusion line, were investigated. Such approach, certainly leads to a quite conservatism, but gives the advantage to provide a safe flaws acceptance criterion in root/hot pass weld, and it may be also applied for any level of weld partial overmatching condition. Despite proposed simplified approach is suitable until moderate plastic straining, it may be appropriated for any ECA on CRA pipe when “J-lay” or “S-lay” installation method is adopted, and/or for many riser’s configuration, and/or for several flowline routing also if exposed to post-buckling condition. It is demonstrated that the proposed simplified approach, when applied under moderate plastic strain conditions, provides accurate J-integral solutions compared to the complex method as proposed by current R&D.