Padraic E. O’Donoghue

Global and local fatigue analysis of X100 and X60 steel catenary riser girth welds

Abstract The use of steel catenary risers in offshore oil and gas production has increased significantly in recent years, in particular for deepwater applications. This paper presents a detailed assessment approach with respect to welded steel catenary risers, to reduce the requirement for over conservative factors of safety. The paper presents a global–local finite element modelling approach, with the Flexcom offshore dynamic analysis package used to determine the global load quantities for two ultra-deepwater free hanging steel catenary risers. These results act as stress boundary conditions in a local model using Abaqus, focusing on the failure susceptible regions of a girth weld. The methodology accounts for geometrical discontinuities of the weld and the different mechanical properties of the parent material, weld metal and heat affected zone, which result in fatigue hotspots for dynamic structures such as steel catenary risers. Two line pipe steels, a current generation X60 and a next generation X100, are investigated in the local analysis. Two pipe wall thicknesses are examined, with the weld geometry being different for each. It is shown that weld detail can lead to significantly different stress concentrations. X100 is shown to be superior to X60 with respect to yielding and fatigue.

Finite Element Modelling of the Thermo-Mechanical Behaviour of a 9Cr Martensitic Steel

A multi-axial, unified sinh viscoplastic material model has been developed to model the behaviour of advanced materials subjected to high temperature cyclic loading. The material model accounts for rate-dependent effects related to high temperature creep and cyclic plasticity effects such as isotropic and kinematic hardening. The material model, which is capable of simulating both isothermal and anisothermal loading conditions, is implemented in multi-axial form in a material user subroutine and validated against uniaxial test data. The results validate the implementation for both isothermal and anisothermal uniaxial loading conditions for as-new P91 steel.

Influence of material inhomogeneity on the mechanical response of a tempered martensite steel

Failure in steel weldments operating at high temperatures often occurs in the heat-affected zone adjacent to the weld. Such failures can be a result of material inhomogeneity within the heat-affected zone and in the case of tempered martensite steels have been linked with regions of untransformed α (ferrite) phase or over-tempered martensite within the intercritical region of the heat-affected zone. In this work, two-dimensional Voronoi tessellation is used to construct polygonal Voronoi cells to represent the microstructure of the heat-affected zone of a weld in a tempered martensite steel. The Voronoi construction is treated as a representative volume element of the material and is discretised by 8-node linear brick elements, with periodic boundary conditions. The lattice orientation at each material point is specified by three Euler angles, which are assumed to be randomly distributed, to represent the initial lack of texture in the intercritical region of the heat-affected zone. The constitutive response is represented by a nonlinear, rate-dependent, finite-strain crystal plasticity model. The results indicate that small amounts of ferrite can induce significant enhancements in stress and inelastic deformation at the interface of the ferrite and martensite grains. This localisation of stress and strain may be critical for microcrack and/or void formation and may be a contributory factor to Type IV cracking.

A precipitate evolution-based continuum damage mechanics model of creep behaviour in welded 9Cr steel at high temperature

A multiaxial, physically based, continuum damage mechanics methodology for creep of welded 9Cr steels is presented, incorporating a multiple precipitate-type state variable, which simulates the effects of strain- and temperature-induced coarsening kinematics. Precipitate volume fraction and initial diameter for carbide and carbo-nitride precipitate types are key microstructural variables controlling time to failure in the model. The heat-affected zone material is simulated explicitly utilising measured microstructural data, allowing detailed investigation of failure mechanisms. Failure is shown to be controlled by a combination of microstructural degradation and Kachanov-type damage for the formation and growth of creep cavities. Comparisons with experimental data demonstrate the accuracy of this model for P91 material.

High Temperature, Multi-Material, Cyclic Plasticity of a P91 Welded Branch-Header Connection Under Cyclic Pressure

This paper presents a study on high temperature cyclic plasticity of a welded P91 T-joint under cyclic internal pressure, in the context of high temperature low cycle fatigue (HTLCF) performance of such connections. In the present work, attention is focused on the development of a multi-material model for high temperature cyclic plasticity, including the effects of the different weld-related material zones, namely, parent metal, weld metal and heat-affected zone. The cyclic plasticity behaviour of the three zones is identified from previously-published high temperature, low cycle fatigue test results on uniaxial test specimens, including parent metal, weld metal and cross-weld specimens, obtained from a specially fabricated pipe girth weld, using ex-service P91 material. The cyclic plasticity material model includes the effects of kinematic hardening and cyclic softening. A three-dimensional finite element model of the welded T-joint is developed, incorporating the three sets of identified cyclic plasticity constants. The study is limited to isothermal conditions of 500°C, with a view to understanding the complex effects of multiple material zones with inhomogeneous cyclic plasticity behaviour. The heat affected zone is shown to play a key role in the development of plastic strains and localised stresses. The particular T-joint geometry is the subject of an investigation due to premature failure in a combined cycle gas turbine plant.Copyright

High Temperature Low Cycle Fatigue Behaviour of MarBN at 600 °C

The changing face of fossil fuel power generation is such that next generation plants must be capable of operating under (i) flexible conditions to accommodate renewal sources of energy and (ii) higher steam pressures and temperatures to improve plant efficiency. These changes result in increased creep and fatigue degradation of plant components. The key limiting factor to achieving more efficient, flexible plant operation is the development of advanced materials capable of operating under such conditions. MarBN is a new precipitate strengthened 9Cr martensitic steel, with added boron and tungsten, designed to provide enhanced creep strength and precipitate stability at high temperature. Accurate characterisation of this material is necessary so that it can be used under flexible plant operating conditions with high temperature fatigue.This paper presents a combined work program of experimental testing and computational modelling on a cast MarBN material. To characterise and assess the fatigue performance of MarBN, an experimental program of high temperature low cycle fatigue (HTLCF) tests is conducted at a temperature of 600 °C. MarBN is found to give an increased stress range compared to previous P91 steel experiments, as well as considerable cyclic softening. To characterise the constitutive behaviour of the cast MarBN material, a recently developed unified cyclic viscoplastic material model is calibrated and validated across a range of strain-rates and strain-ranges, with good correlation achieved with the measured data throughout.Copyright