One of the major concerns in offshore structures are the welded connections where fatigue failures are highly relevant. In many cases of offshore welded connections, the fatigue cracks initiated may grow from the weld to the base material, but also from the root of the weld. In this study, the evaluation of design S-N curves for a double-side welded connection made of S355J2 steel used in offshore applications is proposed. The characteristic fatigue curve of the double-side welded connection is obtained using statistical analyses based either in the ASTM E739 standard as well as the probabilistic fatigue model proposed by Castillo & Fernández-Canteli. This study concludes with a comparison between the experimental fatigue curves obtained and the design S-N curves proposed in design codes for offshore structures. Existing code recommendations are very conservative when comparing the design S-N curves with the proposed new characteristic S-N curves. For the joint under investigation the hot spot and nominal stress approaches yield very similar S-N results.

This contribution brings a new insight into pipe cross section ovalisation due to plastic deformation during pipe-lying process to the seabed. Firstly, the influence of material model calibration on ovalization prediction is presented on pure bending case including the Prager model, the Chaboche model and the modified Abdel-Karim–Ohno model. The mechanism responsible for cross section ovalisation was identified as the phenomenon of the accumulation of plastic deformation, the so-called ratcheting. The next part of this contribution presents main results of the pipe-laying process simulation. The pipe cross-section behavior during passing the considered pipe-laying system is studied in detail. A macro based solution makes possible to do a parametric study and to easily apply the offshore standard DNV-OS-F101 in technical practice.

During present offshore gas-condensate production, flow meters, due to its exceedingly high cost, are being substituted by Virtual Flow Metering (VFM) Technology for monitoring total and single-well flow rates through sensor measurements and physical models. In this work, the inverse problem is solved by Data Reconciliation (DR), minimizing weighted sum of errors with constraints integrating multiple two-phase flow models. The DR problem is solved by Parallel Genetic Algorithm, without complex calculations required by conventional optimization. The newly developed VFM method is tested by data from a realistic gas-condensate production system. The results show good accuracy for the total mass flow rate with model calibration. Meanwhile, recommended single-well flow rate can be provided without physical meters. The method is proved of good robustness with individual pressure sensor invalid, even total flow rate measurements unavailable. The time cost of each reconciliation process can meet the demand of engineering application.

Welds are one of the commonly used joint types and are employed extensively in subsea oil and gas production equipment. Commonly used weld joints in subsea components are fillet, butt, full-penetration, plug, and girth. Fatigue is one of the critical failure modes for welded joints. Welded joints are complex to analyze for fatigue loading due to the microstructure change during the welding process. The welding process also induces residual stress in the heat affected zone (HAZ) surrounding the weld. This, in turn, can adversely affects the fatigue life of the joint. The S-N fatigue approach is commonly used for weld fatigue analysis due to the simplicity of this method. Industry standards such as DNV, IIW, BS-7608, and ASME BPVC Sec VIII Div. -2 or -3 are typical references for this type of analysis. For subsea specific applications, DNV-RP-C203 and BS-7608 are generally used because these two standards provide S-N curves for welds in “air” as well as in “seawater with cathodic protection”. These two codes also provide S-N curves for various weld geometries ranging from simple fillet welds to complex tubular joints. Some of the weld fatigue analysis techniques used in the subsea industry are the: nominal stress approach, structural hot spot stress approach, effective notch stress approach (ENS), structural stress method (ASME VIII-2, -3) and the Fracture mechanics based fatigue crack propagation (FCG) approach. This paper presents the fatigue analysis of fillet welds in bore inserts using the ENS method. In the ENS method, a 1mm radius notch is modelled at the weld root or toe, see Figure 1, which yields a finite weld root stress. The stress analysis is carried out using FEA and the stresses on the notch along with the appropriate fatigue curve are used to estimate the weld root fatigue life.

Flanged joints are used in high pressure applications such as process piping, pressure vessels, risers, pipelines and subsea production systems. These flanges are subjected to external loads in addition to pressure. A brief description of high pressure flanges standards is given. Design of high pressure flanged joints are covered in many design codes. A review of allowable stresses, load factors for bolting, flanges and bolt preload requirements has been made for the following codes: ASME VIII-2, ASME VIII-3, ASME B31.3 Chapter IX, API 6A, API 6X, API 17D, API 17TR7, API 17TR8, API 17G, EN 1591-1 and NORSOK U-001. This paper also presents analytically based structural load-capacity (ultimate strength) design equations for flanged joints. The design equations are used to calculate rated working pressure and flange-face separation load-capacity of API 6A type 6BX flanges. Future code recommendations for flange design are provided.

The oil and gas industry is currently drilling into and producing from wells in high-pressure, high-temperature (HPHT) environments. This has created a greater demand to develop more advanced tools and new technology to safely overcome the challenges in these operations. The Bureau of Safety and Environmental Enforcement (BSEE) requires equipment operating in HPHT environments to pass a design verification analysis. The design verification shall be evaluated using finite element analysis (FEA) per ASME Boiler Pressure Vessel Code (BPVC), Section VIII, Division 3 [1] and API 17TR8 [2]. The objective of this study is to generate a pressure-bending-tension (PBT) capacity chart per the elastic-plastic analysis methodology (global collapse criteria) outlined in KD-230 [1] for a subsea tree connector. The PBT capacity chart covers a wide range of normal operating conditions. Results indicate that the structural capacities from the elastic-plastic analysis methodology are higher than those determined by the standard elastic analysis methodology.

Kevlar composite materials are getting scientific interest in repairing of oil and gas pipelines in both offshore and onshore due to their unique properties. Curing is one of the major factor in deciding the final mechanical performance of laminated Kevlar/epoxy nanocomposites. The parameters such as curing time, temperature and applied pressure during the hot pressing will affect chemistry of crosslinking of the epoxy matrix and interaction of epoxy with the Kevlar fiber. The present study is carried out to evaluate the optimal curing conditions of the Kevlar/epoxy nanocomposites. Three different nanofillers (namely Multi walled Carbon nanotubes (MWCNT), Silicon Carbide (SiC) and Aluminum Oxide (Al 2 O 3 )) are incorporated in different weight percentage. Differential Scanning Calorimetry (DSC) and Thermo-Gravimetric Analysis (TGA) tests are carried out to determine the thermal stability and optimal curing conditions. Mechanical performance is investigated by conducting flexure, and drop weight tests. The results show that, the optimal curing temperature for maximizing the mechanical properties is at 170°C. Peeling off the Kevlar layers are observed for nanocomposite samples cured under 100°C. Mechanical strength of the composites is enhanced by optimizing the curing conditions and nanofiller contents.

High pressure high temperature (HPHT) design is a significant new challenge facing the subsea sector, particularly in the Gulf of Mexico. API 17TR8 provides HPHT Design Guidelines, specifically for subsea applications. This paper presents the results of a fatigue based fracture mechanics assessment case study conducted on a fully clad subsea HPHT component. The component was assumed to be constructed from F22 low alloy steel internally clad with alloy 625 and exposed to 20ksi (137.8MPa) and 400°F (204°C) internal pressure and temperature. A number of different assessment methods were evaluated as part of this study, including standard failure assessment diagram (FAD) based assessment methods, such as those found in API 579-1/ASME FFS-1 and BS 7910, as well as finite element (i.e. crack mesh) methods. A detailed description of the finite element analysis (FEA) of the uncracked and cracked component is provided. An internal surface flaw assumed to be exposed to sour production fluids was evaluated. The results of the fatigue and fracture assessments are summarized along with the key differences between the assessment methods adopted. The sensitivity of the assessment results to other variables such as welding residual stresses is also discussed.

As HPHT activity continues to grow and well conditions become more severe, more advanced analysis methods are required. Thermal/heat transfer and structural Finite Element Analyses (FEA) play a fundamental role in the design of subsea equipment for HPHT application. The objective of this study is to evaluate the effect of seawater convective heat transfer coefficient on design verification of a subsea drilling connector in the HPHT environment (bore pressure more than 15,000 psi, and/or bore temperature more than 300°F). The design verification work contains the elastic-plastic assessments including protection against plastic collapse (global), local failure, hydrotest and ratcheting criteria. The fatigue study is also performed per ASME Boiler & Pressure Vessel Code, Section VIII, Div. 3, Article KD-4 in compliance with API 17TR8. The effect of seawater convective heat transfer coefficient on the global, ratcheting and fatigue criteria is studied/presented. Results show that the higher value of heat transfer coefficient (convection) results in more thermal gradients through assembly causing higher stress and plastic strain deformation in the connector components and lower fatigue life.

High pressure high temperature (HPHT) design is a significant new challenge facing the subsea sector, particularly in the Gulf of Mexico. API 17TR8 provides HPHT Design Guidelines, specifically for subsea applications. Fatigue endurance (i.e. S-N) and fracture mechanics design are both permitted, depending on the criticality of the component. Both design approaches require material properties generated in corrosive environments, such as seawater with cathodic protection and/or sour production fluids. In particular, it is necessary to understand sensitivity to cyclic loading frequency (for both design approaches), crack growth rates (for fracture mechanics approach) as well as fracture toughness performance. For many subsea components, the primary source of fatigue loading is associated with the start-up and subsequent shutdown operation of the well, with long hold periods in-between, during which static crack growth could occur. These are the two damage modes of most interest when performing a fracture mechanics based analysis. This paper presents the preliminary results of a novel single specimen test method that was developed to provide fatigue crack growth rate and fracture toughness data in corrosive environments, in a timeframe that is compatible with subsea HPHT development projects. Test data generated on alloy 625+ in seawater with cathodic protection is presented along with a description of how the test method was developed. A crack tip strain rate based formulation was applied to the data to rationalize the effect of frequency, stress intensity factor range (ΔK) and maximum stress intensity factor (K max ).

Traditional design and supply chain processes have created well established approaches to the manufacture of metallic engineering products in the Oil and Gas sector. Normative references such as API 6A (Specification for Wellhead and Christmas Tree Equipment) and API 17D (Design and Operation of Subsea Production Systems - Subsea Wellhead and Tree Equipment), as well as national and company-based specifications and requirements (e.g., Brazilian norm NBR 15827 “Industrial Valves for Installations of Exploration, Production, Refining and Transport of Petrol Products - Requirements for Design and Prototype Test” and Chevron specification SSM - PU - 54.02 - A “Qualification Testing of Subsea Equipment”) and the ASME body of pressure vessel code requirements and derived specifications, are largely predicated on traditional design and supply assumptions with respect to metallic materials and components. The further challenge of recovering oil from deepwater basins has generated initiatives such as “Project 20K”, BP’s project to develop, with programme collaborators, the capability to drill, complete, produce and intervene in deepwater reservoirs with pressures of 20,000 psi at the mudline and temperatures up to 350°F (175°C). Here again, design and supply assumptions to date are predicated on traditional design and supply assumptions for metallic materials and components. Notwithstanding the progress being achieved in meeting performance requirements in these and similar programmes, a global paradigm change is underway in respect of the accelerating front of advanced manufacturing. This has been seen especially in the aerospace and aviation sectors, but significantly less so in the Oil and Gas sector. This paper will contribute to addressing this imbalance by presenting work being done by a major subsea equipment supplier to engage with the challenges of this disruptive manufacturing technology. Specifically, the paper will present the holistic approach and results of innovative work being done in the advanced manufacturing of a subsea metallic component using welding arc additive techniques. The work presented will review and evaluate the impact of this manufacturing technology on the core metallic component domains of Metallurgy, Welding Engineering and NDE, utilizing a robotic system for manufacturing the component. The paper directly contributes, therefore, to the emerging roadmap for advanced manufacturing of metallic components in the Oil and Gas sector.

High Pressure - High Temperature (HPHT) design is a significant new challenge facing the subsea sector, particularly in the Gulf of Mexico. For fatigue-sensitive applications, the current regulations in the Gulf of Mexico will require operators to compare the actual load cycles experienced by the HPHT component to the load cycles used in the design verification analyses. Should the analyses show that the HPHT equipment is experiencing loading combinations that are greater than those anticipated in the original design, a new service life (loads, time in service, and cycles) based on the actual loads and cycles needs to be developed. The condition assessment in other industries is mainly inspection-based, while inspection of subsea equipment at fatigue-sensitive locations during operation may not be practical. Thus, load monitoring and condition assessment of subsea facilities is a major challenge that the subsea industry is facing. Several assessment methods were evaluated as part of this study, including use of available monitoring technologies in combination with analytical methods. The key findings are summarized to help establish the proper Load Monitoring Program (LMP) for HPHT subsea applications.

The objective of this study was to perform comparative weldability evaluation of weld filler metals for an Alloy 230 / P91 steel dissimilar metal weld (DMW) that will be used in once through steam generator (OTSG) on an offshore oil platform. The weldability characteristics of filler metals EPRI P87 and Haynes 230W were evaluated using the cast pin tear test (CPTT) and a stress relaxation cracking (SRC) test in combination with metallurgical characterization using light optical and scanning electron microscopy. Solidification cracking susceptibility rankings generated with the CPTT showed that undiluted P87 filler metal had better resistance to solidification cracking than undiluted Alloy 230W. The SRC testing preformed in the Gleeble 3800™ thermo-mechanical simulator showed that none of the tested welds failed in stress relaxation cracking mode during simulated service at 600°C under constant displacement and tensile load at 90% of the high temperature yield strength. During the SRC testing, filler metal 230W exhibited some level of stress relaxation, while no evidence of stress relaxation was found in filler metal P87.

X65 steel pipes internally clad with Alloy 625 used in subsea oil extraction are normally welded together with Alloy 625 filler metal. For pipe reeling applications, DNV-OS-F101 requires pipe girth welds to overmatch base metal yield strength with 100 MPa. Since Alloy 625 filler metal does not meet this requirement, Ni-base super alloys 718 and 282 were considered as potential welding consumables for reeling applications. The solidification behavior in weld metal of these alloys diluted with Alloy 625 pipe ID cladding was evaluated using thermodynamic simulations. The response to precipitation hardening by multiple reheat cycles was studied by producing multilayer buildups with cold metal transfer (CMT) and pulsed gas metal arc welding (GMAWp) processes. Weld buildup of Alloy 718 exhibited insufficient hardening response and yield strength, while Alloy 282 met the DNV overmatch requirement. Successful narrow groove welding of X65 pipes with Alloy 282 was performed using CMT process. Welding parameter optimization allowed resolving centerline solidification cracking and lack of fusion defects. The weld metal yield strength was lower than in the multipass buildup, which was attributed to lower number of reheats in groove welding. Meeting the overmatch requirement for yield strength in Alloy 282 groove welds requires further parameter optimization.

Pipe-in-pipe (PIP) flowline is a unique solution for long subsea tie-backs in deepwater and ultra-deepwater fields. This is because of its optimum thermal performance over wet insulation. However, pipelines are subjected to the highest loading condition during installation. Significant limitation imposed on existing installation vessel in deepwater, is peculiar to S-lay installation method. Contrary to the level of stress experienced with the S-lay installation method at specific locations such as overbend and sagbend region, this method is still widely utilized because of its high production rate. These regions are dominated by bending curvatures which are defined by different load conditions. Due to the composition of PIP system, it is important to understand the structural response of the flowline, the mechanical interaction occurring between various components and the amount of load transfer at this location. Although, the mechanical interaction within the PIP system are case specific. However, it has been observed that prior to case study analysis; simple pipe models are being developed to assess the mechanical interaction of this system. This paper addresses the impact of the centralizer material on the structural response and load transfer between the outer pipe and inner pipe. The numerical analysis was carried out using Ansys software and was based on Euler Bernoulli bending theory. The centralizer was clamped on to the inner pipe with the clearance between the centralizer and the outer pipe included in the model. The core of the analysis, was modeling the visco-elastic response of nylon rings (Polyamide 6), from which centralizers are made. The centralizer was spaced based on S-lay or J-lay installation criteria against heat sink. The results demonstrated the relationship between spacing of the centralizer and areas of first contact, amount of force transferred through the centralizer material, non-linearity introduced by contact formulation, alongside the time and temperature dependent behavior of visco-elastic material. The result correlated accurately with the bending principle. Different material model was assessed to determine accuracy of results obtained, in the absence of experimental test data to model visco-elastic response. In addition, the bending curvature was used to predict the mechanical interaction in installation and operation analysis, where limitations of explicitly modeling centralizers exist.

FPSO (Floating Production, Storage and Offloading) has become used increasingly for off-shore gas fields. Most tall pressure vessels such as columns installed in the FPSO are surrounded by topside structures. In general, columns in on-shore plants are supported only by bottom skirts; on the other hand, some of the tall columns on FPSO hulls require intermediate supports in addition to bottom skirts because of large accelerations they are subject to due to ship motion induced by wave loading. In this paper, the effects of the support were investigated considering acceleration distributions and structure deformation (drift), then the ideal elevation of the support was determined to minimize bending moment throughout the columns under the combined conditions. A dimensionless controlling parameter related to the acceleration, structure drift, and column properties was proposed and graphs (called N-Charts) were proposed to understand effects in reducing the maximum bending moment with respect to the elevation of the guide.

Subsea steel pipes are often used to form extensive networks for transporting oil and gas over large distances. Such pipes can potentially be subjected to actions characterized by high loading rates and intensities stemming from accidental loads, due to high-mass low-velocity impacts. In order to ensure that such networks can continue to operate even after being subjected to such extreme loading conditions, it is essential that the behavior of the pipes is characterized by a certain level of resilience. The present study aims to investigate numerically, via dynamic nonlinear finite element analysis, the effect of various parameters on the dynamic response exhibited by pipes under impact loading. The considered parameters are associated with the impacting object, the geometry and the support conditions of the pipes, the level of axial loading as well as the internal and the external pressures imposed onto the walls of the pipes. The numerical predictions, which are validated against relevant published test data, reveal that the above parameters, which are associated with the in-situ conditions imposed onto subsea pipes throughout their operational life, can significantly affect, often detrimentally, the behavior exhibited under impact loading. Existing assessment methods employed in practice for predicting the level of damage sustained by pipes during impact do not accurately consider the effect of the above parameters. As a result, questions are raised concerning the ability of such guidelines to realistically predict the level of damage sustained by subsea pipes under impact.

Failure strain at any point on a structure is not a constant but is a function of several factors, such as stress state, strain rate, and temperature. Failure strain predicted from the uniaxial tensile testing cannot be applied to the bi-axial or tri-axial stress state. ASME Sec VIII-Div-2, and −3 codes give methods to predict the failure strain for multi-axial stress state by considering the triaxiality factor, which is defined as the ratio of mean stress to the equivalent stress. Failure strain predicted by the ASME method (based on the Rice-Tracey ductile failure model) is an exponential curve that relates the failure strain to the triaxiality factor. The ASME VIII-3 method also gives procedures to calculate failure strain for various material types: ferritic, stainless steel, nickel alloy, aluminum, etc. Experimental results of failure strain at various stress states show that the failure strain is not only a function of the triaxiality factor, but also a function of the Lode angle. The Lode angle takes on the value of 1, 0, and −1 for tension, pure shear, and compression stress state, respectively. Experimental data shows that the failure strain is a 3D surface which has an exponential relation with triaxiality and a parabolic relation with the Lode angle. To validate the ASME failure strain prediction, this paper compares experimental failure strain test data from literature with the ASME predictions. The ASME predictions are non-conservative especially for moderately ductile materials such as aluminum and high strength carbon steel. A reduction factor on failure strain for low ductile material is presented using the relation between the R (yield/ultimate) and the stress ratio (shear/tensile stress). The ASME method does not account for the environmental effects while calculating the failure strain. High pressure, high temperature (HPHT) subsea components designed using ASME VIII-3 code are subjected to various environments in subsea, such as seawater, seawater with cathodic protection (CP) and production fluid (crude oil). Experimental data shows that the Elongation (EL) and/or Reduction in Area (RA) from tensile testing decrease in these environments. Therefore, to account for any environment effect on the failure strain, reduced EL and RA can be used to predict the failure strain.

The extended finite element method (XFEM) [1, 2] can approximate the discontinuous displacement field near cracks independently of the finite element mesh through the use of interpolation functions, which can describe the displacement field near cracks in the structure. Therefore, XFEM is appearing as an alternative to conventional contour integral method for crack growth analysis. Most benchmarks for XFEM in published literature were mostly for the through-wall crack or loadings were under tension or bending moment. Subsea equipment provided by FMC Technologies is used under the most extreme conditions of pressure and temperature. Due to the fact that most equipment in subsea is pressure containing, assessment of cracks and crack growth under pressure cycles is an essential part of the integrity assessment. The implementation of fracture mechanic based fatigue to analysis can extend equipment life while improving model accuracy. In this paper, crack growth modelling in a thick wall cylinder was performed by XFEM embedded in Abaqus [3]. The case study of semi-elliptical axial surface crack at internal surface was selected. The fracture and fatigue results based on LCF analysis were compared with contour integral approach and published experimental data [4]. One of the advantages of XFEM is it can easily accommodate changes in the crack propagation direction for mixed mode loading.

This paper presents the transference of a well-established nuclear power industry technique to perform environmentally-assisted fatigue monitoring for application to offshore, subsea High Pressure, High Temperature (HPHT) components using existing instrumentation. Applying fatigue monitoring is an effective way to manage these assets against the threat of fatigue failures and meet regulatory expectations during the service life. Our objective is to provide the background and overview of the approach so that design engineers better understand and evaluate its potential in contrast to traditional design changes that may actually produce an unintended consequence that increases the fatigue potential associated with heavy section thicknesses. This paper describes the overall analysis approach that may be used to perform fatigue monitoring of an HPHT component. At the conclusion, recommendations are provided for further research and development to close existing gaps.