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The conditions to which flexible pipes are being exposed are becoming increasingly severe. In particular, flexible flowlines that are fully or partially restrained by back fill or rock cover can become exposed to large axial compression forces under pressure and temperature loading. A high True axial compression force may lead to a radial outward displacement of the tensile armour which could eventually exceed the allowable limits of API spec 17J if the axial force exceeds a critical level. Furthermore, when the net Effective axial compressive pipe force exceeds a critical level, the pipe may undergo global buckling either upwards (upheaval buckling) or laterally, which needs to be addressed in detail at design stage, typically by using global finite element (FE) simulations in which equivalent mechanical properties are applied to pipe beam elements to capture the overall behaviour of the flexible pipe. The overall global mechanical properties of a flexible pipe, such as axial stiffness and thermal and pressure expansion coefficients, which are important parameters in capturing the global buckling behaviour, are dependent on the local behaviour of the various flexible pipe layers. In this paper an advanced local FE model developed using the industry standard numerical FE tool, Abaqus, together with the assessment methodology is presented in order to study local layer interactions and their influence on the equivalent axial mechanical properties.

Subsea export power transmission cables for offshore wind farms are being installed more extensively year-on-year due to the increasing demands for power output from renewable sources. With the increasing number of installations, the number of cable failures during installation has increased. One failure mode involves the temporary or permanent radial deformation of armour wires otherwise known as armour wire bird caging which occurs from a combination of twist, bending and compression loads which build up in a cable. This failure mode can lead to significant remediation costs and schedule delays for projects affected. In this paper, the authors present a method for predicting armour wire bird caging for generic installation configurations based on a review of the root causes from several historical bird caging failure instances. Various numerical models and analyses which simulate the installation conditions are described. The models simulate key response mechanisms including bending-induced twist and inter-layer separation within a cable. Cable loading conditions are compared with cable bird caging limits and the parameters which influence the onset of bird caging are identified. Based on a range of sensitivity analyses, handling curves to assist with installation are developed and a full-scale test validation programme is proposed. This work was performed for a project which received financial and technical support from The Carbon Trust’s Offshore Wind Accelerator (OWA), a collaborative R&D programme funded by nine leading offshore wind developers (EnBW, Equinor, Innogy, Ørsted, RWE, SPR, Shell, SSE, Vattenfall) and the Scottish Government.

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.

Subsea systems that transport multiphase flow such as wellhead and manifold jumpers, tie-in spools, free-spanning pipelines and risers can all be subject to slug and turbulence loading and response, depending on the characteristics of the reservoir fluids, the pipe geometry and the flow conditions. As part of the SLARP (slug loading and response in pipelines) joint industry project (JIP), a series of tests were undertaken to provide experimental measurements at field-realistic scale dimensions for an investigation into the interactions between slug flow and the dynamics of subsea pipeline jumpers. Advanced numerical methods were validated for simulating the effects of slug flow on subsea pipe structures. The test campaign, pipe configurations, sample test results and numerical correlation of slug flow characteristics with measured pipe dynamics are presented in this paper. The test section consists of a U-bend constructed from 4-inch pipe. Liquid slugs were propelled through the piping by gas pressure and the structural response of the test section was measured as the slugs traversed a series of 90° pipe bends. As a basis for the test programme, slug lengths and velocities of up to 30m and 15m/s, respectively, were used. Frictional slides and wire rope slings were employed to provide differing restraint and damping conditions for the test section. The experimental data during the test campaign were compared to numerical simulations that accounted for load contributors of slugs travelling through the test section. Strong correlations were observed between the slug speed and pipe deflections from the physical tests. The system response was shown to be sensitive to slug velocity, pipe system restraint, pipe system stiffness and the evolution with time of the slug characteristics. The numerical methods employed are shown to be suitable for use in the prediction of dynamic response of subsea pipelines to slug loading where flow conditions are well understood. The novel aspect of this work relates to the successful development of a test piece which has features particular to subsea jumpers and the passing of high velocity slugs through the system combined with the simulation of the test conditions using finite element (FE) software that accounts for all key load contributors. These tests provide key insight to the interaction between slug flow conditions and the dynamics of pipe jumpers at field-realistic scale and in part formed a basis for industry guidelines developed as part of the JIP.

Beam structures undergoing finite deflections and rotations in space have extensive application in the subsea industry particularly for the analysis of holistic systems with larger numbers of mooring and riser components. In using the finite element analysis approach, there is an increasing requirement for large element sizes which preserve accuracy with regard to the coupling of axial, bending and torsion response. The authors outline a method for improving the current state of practice for the analysis of riser systems. The approach draws on the convected coordinates method, Euler–Bernoulli beam theory, the principle of virtual work and the finite element method. Two quasi-rotation measures are developed including a quasi-material rotation definition for rotational deformation relative to the convected axis of a beam and a quasi-space rotation definition to deal with the path dependent nature of rotations in three dimensions. The novel aspect of this work is to relate the rate of change of the quasi-material rotation vector along the beam axis to a linear transformation of the beam axis rate-of-rotation vector through utilising the convected coordinates axes system. In this way, incremental values of quasi-material rotation are directly linked to incremental values of nodal quasi-space rotation and a global Newton–Raphson solution technique for interconnecting beam elements is straightforward to assemble. Furthermore, this leads to accurate definitions of coupled axial, bending and torque response for beams with significant deflection. The approach has particular advantages in the analysis of subsea riser sections. Also, the accuracy of the solution is preserved for a fewer number of elements compared to alternative solutions for computationally sensitive load cases with highly non-linear loading regimes.

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.

Abstract During the life of certain offshore fields, slug flow can lead to alternating internal fluid conditions and irregular flow characteristics in subsea pipelines, risers and spools. These conditions generate time-varying fluid forces at locations of change in flow direction which can lead to significant vibration, fatigue damage and in some cases push structural utilizations above allowable limits. In recent years, slugging related problems have resulted in the change-out of critical subsea equipment. Difficulties exist in predicting and characterizing slug flow in a representative manner due to the complexity of the loading. As a result, the current state of practice with regard to predicting response to slug flow is not considered mature technology and is not dealt with consistently across the industry. Furthermore, subsea design practice and analysis approaches, where they exist, lack sufficient validation. Widespread industry interest in the topic has lead to the requirement to develop enhanced analytical methods, full scale validation and industry guidelines. This paper deals specifically with the mechanics of combining the various slug load contributors, including centrifugal, bend impact and fluid inertial loads in structural models and their effects on stress distributions in pipeline systems. The paper illustrates how enhancements in analytical methods now allow convenient definition of slug flow inputs and automatic generation of dynamic load time histories. Results are presented for a generic subsea piping configuration which is considered particularly susceptible to slug load response. A comparison of responses from different structural software packages is presented for representative design conditions and parameter variations.

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.

Abstract As the offshore industry continues to progress developments in deep and ultra-deep water locations, in many cases the considered or preferred riser solutions are steel catenary risers (SCRs) or lazy wave SCRs. The design of SCRs requires the use of advanced finite element techniques, to be able to accurately represent the complex interaction of the SCR with the environment and support structures, and hence demonstrate their suitability for use. This paper presents new advanced models for the nonlinear analysis of the SCR behavior. For SCRs, the two critical design challenges are the characterisation of the SCR behavior at the vessel hang-off and the modeling of the SCR interaction with the seabed. At hang-off the predominant solution used in the industry is to have a FlexJoint® interface at the vessel. The response characteristics of the SCR at the flex joint and at the seabed are highly nonlinear phenomena. For ultra deepwater in particular it is essential not to restrict the modeling of such effects to linear approximation. To date most analysis models attempt to apply linear solutions to these nonlinear problems. A significant amount of work has been undertaken recently to develop advanced models of the critical interfaces. This paper describes the modeling approaches used and assesses the potential conservatism or unconservatism in the more traditional linear approaches. The use of a fully nonlinear structural model of the flex joint to evaluate and assess equivalent linear modeling approaches is presented and discussed. This highlights a level of conservatism in the current approaches. In addition, advanced seabed modeling features, such as accurate modeling of suction and trenching effects are presented and the effect of these on SCR fatigue performance characteristics are highlighted. The requirement for more accurate modeling techniques is driven by the criticality of SCR design in many applications, particularly with respect to fatigue response characteristics. Issues such as corrosive fluids, forcing the requirement for clad pipe and larger FPSO motions, driving the requirement for very long nipples in the flex joint, are pushing the requirement for more accurate modeling techniques, such as those presented in this paper. Introduction This paper presents advanced analysis methodologies for the global modeling of the non-linear behaviour of SCRs at two critical locations for fatigue, specifically the vessel hangoff and the touchdown region, as part of a three-dimensional time-domain nonlinear beam finite element solution scheme. At vessel hang-off, the dynamic motions of the vessel are transferred to the top of the SCR. SCRs require either a flex joint or tapered stress joint at the vessel/platform connection in order to accommodate the bending moments in this area. The first part of the paper outlines a non-linear methodology for the selection of an accurate stiffness to model the non-linear behaviour of a flex joint, and assesses the conservatism of the approach vis a vis a traditional approach.

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

Abstract The integrated design of deepwater risers and moorings has the potential to bring substantial benefits in terms of overall system response, cost and safety to a offshore development. Existing design methods, with their origin in shallower water design, have typically not considered design integration. The associated design assumptions need to be reassessed in deeper water, where substantial benetits can accrue from integrated design. The justification for integrated design methods lies in the importance of hydrodynamic loads, stiffness, damping, added mass and potential compliant effects of risers as partof a moored system and also in the need to recognise the precise station keeping requirements of riser systems. This paper presents the work of a JIP managed by MCS International and Noble Denton Europe and supported by 20 participant operators, contractors, manufacturers and regulators, which has investigated methods of integrated mooring and riser design over 2 1/2 years between 1997 and 1999. Several altenative innovative riser and mooring analysis methodologies, corresponding to varying levels of design integration, have been developed and investigated for five selected FPS/FPSO systems to evaluate alternative design approaches. The vessels were selected to represent recent state-of-the-art mooring and riser designs in water depths up to 2000m. Introduction A comparison of alternative methodologies for the analysis of integrated mooring and riser system designs, representing several different levels of design integration, are described, together with their design assumptions. Their results and relative merits are evafuated against the results of mooring model tests and original design calculations. Finafly, observations are made on appropriate integrated design methodologies, particularly for deeper waters. Each of the design methods under review represent increasing levels of complexity and integration of the mooring and riser design. The objective of their evaluation was to investigate the analysis methodology which is robust in representing actual conditions while still being a practical design tool. The five vessels considered throughout the investigation include a Central North Sea (CNS) FPSO in 150m water depth, a West of Shetland (WOS) FPSO in 400m water depth, a Northern North Sea (NNS) semi-submersible located in 350m water depth, a Brazilian FPSO in 1000m water depth and a generic Gulf of Mexico (GOM) floating production unit (FPU) located in 1700m water depth. The vessels and their associated mooring and riser systems, which are briefly described in Table 1, incorporate a variety of water depths, mooring system types, riser system designs and environmental conditions.