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In this paper, a global-local fretting design methodology for the pressure armour layer of flexible marine risers is outlined. This includes global dynamic riser analysis, geometrical and analytical sub-models and local nub-groove contact finite element analysis. Furthermore, a fretting test rig is developed and utilised to quantify coefficient of friction and wear coefficient under representative nub-groove loading conditions. The combination of the global-local computational methodology and experimental characterisation of pressure armour wire material allows for the development of running condition fretting maps. This identifies design criteria for critical riser global curvatures that are associated with minimum number of cycles to failure. The design methodology presented in this paper is applied to a realistic riser design study, using extreme sea-state loading conditions. In this case study, the predicted pressure armour fretting fatigue lives are found to be in the same range as the plain fatigue lives of the tensile armour layer. The authors would like to thank the Irish Research Council and Wood plc. for funding of this project through the Enterprise Partnership Scheme (EPSPG/2013/638), the National University of Ireland for funding through an NUI Travelling Scholarship. The authors also wish to acknowledge the support of the NUI Galway Engineering Building technical staff; particularly Mr. Bonaventure Kennedy and Mr. Patrick Kelly. peer-reviewed 2021-09-19

This paper presents a combined experimental and computational methodology for fretting wear-fatigue prediction of pressure armour wire in flexible marine risers. Fretting wear, friction and fatigue parameters of pressure armour material have been characterised experimentally. A combined fretting wear-fatigue finite element model has been developed using an adaptive meshing technique and the effect of bending-induced tangential slip has been characterised. It has been shown that a surface damage parameter combined with a multiaxial fatigue parameter can accurately predict the beneficial effect of fretting wear on fatigue predictions. This provides a computationally efficient design tool for fretting in the pressure armour layer of flexible marine risers. The authors would like to thank the Irish Research Council and Wood Group for funding of this project through the Enterprise Partnership Scheme (EPSPG/2013/638), and the National University of Ireland for funding through an NUI Travelling Scholarship. We also wish to acknowledge the help and support we have received from Mr. Kieran Kavanagh (Wood Group Kenny), the NUI Galway Engineering Building technical staff; particularly Mr. Bonaventure Kennedy and Mr. Patrick Kelly and the Faculty of Engineering at the University of Nottingham. peer-reviewed

Welded connections are a fatigue sensitive location for offshore steel catenary risers. The susceptibility to fatigue is due to the notch effect of the weld and the gradient in microstructure and material properties across the weld which result from welding thermal cycles and differences in the composition of the parent material and weld metal. In this work, a representative full-scale steel catenary riser girth weld is conducted using X100Q steel. The thermal and strain history in the weld zone are captured using a thermocouple and strain gauge array. A parallel programme of Gleeble thermomechanical simulation is implemented to develop microstructurally uniform heat affected zone (HAZ) test specimens. The parent material, weld metal, simulated HAZ and a cross-weld sample are characterised using a programme of nanoindentation, tensile and fatigue testing. A softened region with microstructure corresponding to intercritical HAZ (ICHAZ) is identified in the girth weld. Tensile and fatigue failures are shown to occur in a representative microstructural region for simulated HAZ specimens, indicating a susceptibility to failure in the ICHAZ for matched or over-matched X100Q welds.

Steel tube umbilical designs are becoming more complex and are being deployed in increasingly severe environments. Umbilical designs can now accommodate up to 3-inch diameter steel tubes for chemical and hydraulic injection, up to four layers of armoring, multiple electrical cables and fiber optic lines. Large power transmission cables are also being incorporated in umbilical constructions. This is leading to ever increasing umbilical mass, radial loads, pressure loads and increased demands on the designer to demonstrate adequate fatigue capacity. A method has been developed for predicting the fatigue response of deepwater umbilicals, particularly for umbilicals subject to high tension and bending loads. The method involves predicting cyclic stress in the umbilical components (steel tubes or power cables) using an industry standard, general purpose numerical tool. It comprehensively accounts for the complex relationships between umbilical curvature, tension, inter-layer contact forces and tube stress variation. This paper describes the validation of the method against measured data obtained from full scale fatigue tests performed on two deepwater umbilicals. A key feature of the method relates to the construction and verification of suitable finite element (FE) models and the simulation of fatigue conditions. For the validation of the models, results from the full scale dynamic umbilical test cases incorporating bend stiffeners and loading arms are used, which demonstrate umbilical hysteretic behavior under cyclic bending loads. Hysteretic stress-curvature loops obtained from strain gauge readings for various tube locations within the bend stiffener regions for two umbilical constructions with significantly different lay-up configurations and fatigue stress regimes are used for comparison to strain predictions from the FE models. A significant advantage of the modelling and analysis methods described in this paper is that it employs the widely-used industry standard numerical FE tool, Abaqus, for the modelling and analysis of the umbilical. Key features and insights from the modelling, analysis approach and validation against the results of a deepwater umbilical dynamic test program are described. A robust and efficient method for the independent assessment of the fatigue capacity of deepwater umbilicals is demonstrated.

This paper presents a combined experimental and computational framework for the design of flexible marine risers against fretting and fatigue. The framework is based on: (i) a suite of laboratory experimental methods for fretting and fatigue tests, and (ii) a global-local methodology for flexible riser analysis including dynamics and frictional contact mechanics. The effect of coefficient of friction on the predicted substrate stress distributions and fatigue life of the nub-groove region of flexibles has been investigated. It is shown that coefficient of friction has a significant effect on predicted trailing-edge tensile stresses in the pressure armour layer and, hence on fretting crack initiation in risers. The framework should assist in the design of pressure armour layers in flexible marine risers against fretting wear and fatigue.

This paper presents a computational methodology for frictional contact mechanics of the pressure armour layer in flexible risers. This will allow, for the first time, quantification of key fretting variables, such as contact pressure, relative slip and sub-surface stresses in this complex geometry, under representative loading conditions. Fatigue lives are calculated using the 3-dimensional critical plane Smith-Watson-Topper multiaxial fatigue parameter. It is shown that COF has a significant effect on predicted trailing-edge tensile stresses in the pressure armour layer and, hence on fretting crack initiation in risers. It is also shown that operating pressure and bending-induced axial displacement significantly affect predicted crack initiation. These results will facilitate representative fretting wear and fretting fatigue testing of pressure armour layer material. Irish Research Council and Wood Group Kenny for funding of this project through the Enterprise Partnership Scheme (EPSPG/2013/638), National University of Ireland for funding through an NUI Travelling Scholarship. peer-reviewed 2018-03-04

Accurate prediction of the load capacity of the constituent components of flexible risers and flowline structures is critical in demonstrating fitness for purpose, particularly as the envelope of application of flexible pipe is being extended to incorporate higher flow rates, greater water depths, more aggressive fluid contents and challenging installation conditions. Analytical techniques and numerical solid modelling methods have evolved significantly in recent years as a means of improving the ability to accurately predict flexible pipe load capacity. Due to the complexity of the flexible pipe structure and the load sources and load paths to which it can be subjected, different analysis methods are required, depending on the type of response being evaluated. Specific effects can be difficult to simulate using analytical approaches and require local stress analysis using numerical techniques. This paper presents numerical approaches to two such effects, the extreme loading of tensile armour wires within a flexible pipe end fitting and the stability of tensile armour wires in axial compression.

During installation and operation a flexible pipe may be subjected to high compressive forces, high cyclic curvatures and external pressures leading to high reverse end-cap loads. Under such loading conditions, which occur particularly in the touchdown region for deep water applications, the limiting condition for the flexible pipe can be the compressive stability of the tensile armour wires. Two potential instability modes are possible: radial mode (birdcaging) and lateral mode (lateral wire disorganization). Previous work on the subject has established the key factors which influence the onset of each buckling mode [1],[2],[3] and [4]. In order to ensure the feasibility of flexible designs for applications with increasing water depth, it is important to improve the knowledge of the mechanisms which can lead to instability of armour wires and enhance the ability to predict with greater assurance, the particular conditions which increase the risk of wire instability. The focus of this work is the comparison of finite element prediction of radial buckling (birdcaging) with physical testing results under loading states that lead a pipe to birdcaging failure. The numerical model incorporates all tensile armor wires and their interactions with each other and adjacent layers. The outer sheath and reinforcing tape layers are explicitly represented, while the inner layers of the pipe (pressure armour and carcass sheath) are idealized using a homogeneous representation. The model also incorporates the effects of manufacturing pre-tension and hoop strength in the anti-birdcaging tape layers which are critical determinants for the onset of buckling. A key aspect of the method presented is the means by which the loading is applied. Specifically, the modeling handles the simultaneous and controlled application of end rotations, axial compression and radial resistance of the tapes through to the point of tape failure, pipe ovalisation and subsequent radial displacement and buckling of individual wires. In summary, in this paper a solid modeling approach is presented, which is compared with full a scale sample test data, that enables the simulation of a flexible pipe undergoing large combined compression, curvatures and pressure loading.

With the move to the development of remote, deepwater fields, increasing use is being made of floating production, storage and offloading (FPSO) facilities from which oil is intermittently offloaded to a shuttle tanker via offloading lines and an anchor leg mooring buoy. The response of the individual components of these systems is significantly influenced by hydrodynamic and mechanical coupling between adjacent components, precluding the use of traditional analysis techniques such as displacement RAOs derived from tank model tests or diffraction/radiation analyses of the independent components. Consequently, the reliable and accurate design of these complex systems requires an analysis tool capable of determining the fully coupled response of each of the individual components of the system. A recently-developed time domain coupled analysis tool has been extended to incorporate a frequency domain coupled analysis capability. This tool combines radiation/diffraction theory with a non-linear finite element (FE) structural analysis technique used for the analysis of slender offshore structures. This paper describes the application of frequency domain analysis to the coupled FE/floating structure problem, with particular consideration given to the linearisation of viscous drag loads on floating structures and the treatment of low-frequency second-order loads in the frequency domain. Results from frequency domain and time domain coupled analyses of a typical West of Africa type offloading system are compared, highlighting areas of application where frequency domain coupled analysis can offer significant benefits when used in conjunction with time domain analysis. Based on this, recommendations are made for the appropriate use of frequency and time domain coupled analysis for this type of system.Copyright © 2005 by ASME