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The aim of this work was to examine uniaxial, torsion, and multi-axial fatigue characteristics of ER100S-1 low carbon steel specimens fabricated with wire arc additive manufacturing (WAAM) technique, a subcategory of directed energy deposition (DED). Two distinct specimen orientations were tested—vertical and horizontal, extracted perpendicular and parallel to the WAAM deposited layers, respectively. Fracture surfaces of the tested specimens were analysed under scanning electron microscope (SEM) to observe fracture mechanisms corresponding to different specimen orientations, different fatigue loading conditions, and to interpret the fatigue results obtained from the tests. Finally, the obtained stress–life results were compared with the fatigue data available in the literature for a series of wrought and WAAM-built structural steel specimens. Moreover, the S–N curves obtained in this study were evaluated against the fatigue design curve recommended for offshore marine welded structures in DNV standard. Test results have shown advantageous characteristics of WAAM-built ER100S-1 specimens compared with behaviours of other structural steels and conservative prediction of its fatigue life by the design curve available in the DNV standard.

Wire arc additive manufacturing (WAAM), also known as directed energy deposition (DED) process, is an efficient additive manufacturing technology, offers high potential to rapidly fabricate large-scale parts with complex geometries layer-by-layer. However, the fundamental understanding of the fatigue behaviour of such parts and the material requirements need to be significantly improved at all levels before this unique technology can be implemented for critical applications. This work aims to investigate the fatigue behaviour of WAAM built ER70S-6 steel under uniaxial, torsion and multiaxial loading conditions. Specimens were extracted in two different orientations: vertical and horizontal, to explore if the orientation direction has any effect on the fatigue results. Scanning Electron Microscopy (SEM) was conducted to examine the fracture surface of broken specimens and identify crack initiation regions and fracture mechanisms. The obtained results were compared with the fatigue data available in the literature on common structural steels fabricated using conventional welding and WAAM technique, showing similar fatigue behaviour with wrought S355 specimens. Moreover, the uniaxial data set on ER70S-6 WAAM specimens was evaluated according to the DNV RP-C203 standard for continuous welds, demonstrating advantageous fatigue resistance in the examined material.

A crucial part of the structural integrity assessment of marine structures is the analysis of the fatigue crack growth behaviour of the welded joints in seawater environments, where the cracks often initiate and propagate under corrosion-fatigue loading conditions. In recent years, technological developments have facilitated the fabrication of steel components and structures using additive manufacturing technologies. Among the existing technologies, the Wire Arc Additive Manufacturing (WAAM) technique has proven to offer great potentials for fabrication of large-scale structures. The present study investigates the corrosion-fatigue crack growth (CFCG) behaviour of the WAAM parts fabricated using ER70S-6 low carbon steel wire to assess the suitability of this technology for future marine structures. In this experimental study, the cracking behaviour and test duration in corrosion-fatigue tests were investigated and analysed in conjunction with the microstructural examination of the tested specimens. Moreover, the obtained results were compared with the recommended trends available in BS7910 standard for conventional welded joints and the data available in the literature on widely used offshore structural steel weldments. The CFCG results obtained from this study contribute to the overall knowledge and design requirements for the new optimised functionally graded structures made with WAAM technology for marine applications.

Wire arc additive manufacturing (WAAM) is an advanced fabrication technology for the rapid and efficient production of large-scale engineering structures. In order to design WAAM components for a given loading condition, it is essential to characterize the mechanical and failure behavior of the parts. In this study, the performance of ER100S-1 low carbon steel has been investigated by performing fatigue crack growth tests on compact tension specimens extracted from a WAAM built wall. The experimental results have been compared with the recommended trends in the BS7910 standard and with data available in the literature. Metallurgical investigations have been carried out to explore the microstructural effects on the fatigue behavior of the WAAM built components. The specimen location and orientation effects were comprehensively examined, and the results are discussed in terms of the influence of macroscopic and microscopic deformation on the overall response of the WAAM built components under fatigue loading conditions.

A study has been conducted to evaluate the mechanical and fatigue crack propagation properties of wire + arc additively manufactured ER70S-6 components. A parallel-built deposition strategy was employed to fabricate the additively manufactured wall. The hardness values were slightly higher at the bottom and top of the wall due to the presence of Widmanstätten ferrite and carbides. The characterization of mechanical properties in both orientations; parallel and perpendicular to the deposition direction showed a marginal difference in yield strength and ultimate tensile strength. The crack growth rates were correlated with linear elastic fracture mechanics parameter ΔK and compared with an oscillation-built deposition strategy from the literature. The crack growth rates of both deposition strategies were found to be very similar to each other. Furthermore, it has been demonstrated that the variability in the crack growth histories can be reasonably well captured by using the NASGRO crack growth equation.

The influence of prior creep damage on the subsequent fatigue cracking behaviour of thick-walled pressure vessels has been thoroughly investigated in this study. Creep damage was introduced into fracture mechanics specimens by performing creep tests at 550 °C and stopping them after reaching a desired crack length. Using this experimental methodology, creep damage was successfully introduced ahead of the crack tip in the form of intergranular voids and microcracks. In order to assess the creep damage effects, in the absence of significant plasticity, on the structural integrity of thick-walled pressure vessels the material was uniformly pre-strained under compressive plastic deformation prior to the specimen manufacture, thus hardening the material. Fatigue tests were performed on specimens with and without creep damage and the results were compared with each other. Moreover, metallurgical investigations were conducted to understand the influence of microstructural damages, as a result of creep loading conditions, on the fatigue cracking behaviour of the material. The results from this study show a clear reduction in the fatigue cracking rate in the presence of creep damage local to the crack tip. The findings have been discussed in terms of the effects of static creep loading conditions and microstructural damages on the subsequent fatigue cracking behaviour under cyclic loading conditions.

A purpose-developed structural reliability assessment (SRA) framework for the evaluation of horizontally curved aluminium alloy bridge decks on steel I-girders of centre subtended angle, \theta\le34.4 with precinct on the American association of state highway and transportation officials (AASHTO) load resistance factor design (LRFD) specification is presented. The finite element analysis (FEA) simulations were performed in the ABAQUS© CAE, whilst the probabilistic assessment model was developed using the first-order reliability method (FORM). Besides performing detailed design checks and a validation exercise, the developed SRA framework was used to examine the structural behaviour of the bridge assembly in the presence of stochastic design truck axle loads, structure’s self-weight, among other loads, whilst varying geometric properties. The most critical structural responses chosen at salient points obtained via FEA simulation is applied in deriving the limit state function (LSF), which is then substituted into the stochastic model within the purpose-developed iterative FORM algorithm to calculate the reliability index (RI), \beta of the structure. It is shown that the proprietary AlumadeckTM system conforms with the LRFD specification, which stipulates that the target value of RI is 3.5 for resistance factor (RF) of 1.0 (assuming 80% composite action compression flange). The result also reveals that the RI shows a strong dependence on the composite action between the deck and the girder for RF of 1.0 full composite action (considering the failure of the bottom flange), indicating the safety index is within acceptable limits. Furthermore, it is revealed that the minimum composite action for safety is 40%, corresponding to a safety index of 1.16. Hence, it can be inferred from the foregoing that the AlumadeckTM can withstand the stochastic axle load it is subjected to considering the HL-93 load design condition and satisfies all design criteria considered from a stochastic perspective based on the AASHTO LRFD guidelines, provided the minimum stiffner thickness t_{stiffner} of 7mm is adopted (based on the FEA simulation results). A case study conducted herein established that the structural configuration selected (i.e. depth at 2.4m, flange thicknesses at 21mm, flange width at 50cm and web thickness at 16mm) demonstrates the structural safety and durability of the bridge system coupled using the AlumadeckTM.

The need for increased manufacturing efficiency of large engineering structures has led to development of wire arc additive manufacturing (WAAM), which is also known as direct energy deposition (DED) method. One of the main barriers for rapid adoption of the WAAM technology in wider range of industrial applications is the lack of sufficient performance data on the WAAM components for various materials and operational conditions. The present study addresses this essential need by exploring the effects of laser shock peening surface treatment on corrosion-fatigue crack growth (CFCG) life enhancement of WAAM components made of ER70S-6 and ER100S-1 steel wires. The experimental results obtained from this study were compared with the CFCG trends from nominally identical specimens without surface treatment and prove the efficiency of the examined surface treatment method for corrosion-fatigue life enhancement and crack growth retardation of WAAM built steel components, regardless of the material type and specimen orientation. Furthermore, the residual stresses in the WAAM built specimens with and without surface treatment were measured to validate the influence of beneficial residual stresses, arising from surface treatment, on subsequent CFCG behaviour of the material. The residual stress profiles show the beneficial compressive stress fields in the surface treated areas which result in CFCG life enhancement. The results from this study make significant contribution to knowledge by evaluating the suitability of WAAM built steel components for application in offshore environments. Engineering and Physical Sciences Research Council (EPSRC): EP/L016303/1

With the advancements in additive manufacturing (AM) technologies, it is expected that fast and efficient production using the AM techniques will gradually replace the traditional manufacturing processes. An important consideration in the design and life assessment of AM-built parts is the asset integrity management and in particular fatigue life enhancement of such components. Surface rolling treatment is known to be an efficient way to introduce deep compressive residual stresses into engineering components, and therefore it has been considered as a practical and effective life enhancement technology. In this study, the surface rolling effects on the fatigue life enhancement of wire arc additively manufactured (WAAM) parts have been investigated. For this purpose, a compact tension specimen geometry was modeled in ABAQUS and the rolling process was simulated by means of finite element simulations to predict the extent of compressive residual stresses induced into a WAAM built part. The simulation results have been discussed in terms of the beneficial effects of surface rolling on fatigue life enhancement of WAAM built parts.

This paper investigates from a structural reliability assessment (SRA) perspective the fatigue reliability using the S-N curve approach compared with the fracture mechanics (FM) approach for a typical welded offshore wind turbine (OWT) jacket support structure. A non-intrusive formulation was developed for an OWT jacket support structure in 50 m deep water, consisting of a sequence of steps. First, stochastic parametric 3D (three-dimensional) Finite Element Analysis (FEA) simulations are performed, taking into account stochastic variables such as wind loads, wave loads and soil properties using facilities within the software package ANSYS. Secondly, the FEA results are post-processed using an Artificial Neural Network (ANN) response surface modelling technique deriving the performance functions expressed in terms of stochastic variables. Finally, the First Order Reliability Method (FORM) is applied in calculating the reliability index values of components. The developed framework was applied to elucidate the fatigue damage process, including the small to long crack transition amongst other stages, for structural steels used for OWT jacket applications. The FM formulation investigated includes a crack growth formulation based on the bilinear crack growth law, considering both segments of the crack growth law as non-correlated and correlated in calculating the reliability index (RI). Sensitivity analysis results showed a strong dependence of the structure's reliability levels on the uncertainties of the crack growth law constants measured in terms of coefficient of variation (COV). Also investigated, was the reliability of the structure reassessed and updated in the presence of assumed structural health monitoring/ condition monitoring (SHM/CM) data. The results from the case study revealed that fracture reliability is highly sensitive to the initial crack size. It is recommended to apply the S-N curve method at the design stage while the FM approach applied towards the end of the design life as the structure approaches failure.

This study aims to investigate the influence of welding process on the elastic lattice deformation and its effects on fatigue and fracture behavior of S355 G10+M steel, which is widely used in fabrication of offshore wind turbine monopile structures. In situ neutron diffraction measurements were taken on cross-weld test samples at room temperature to monitor the evolution of intergranular strains under static and cyclic loading conditions. Both static and cyclic test results have shown that the {200} orientation exhibits the least load carrying capacity while {211} had the maximum stiffness. The hkl-specific response predicted using Reuss and Kröner model were found to agree well with experimental values obtained for the heat-affected zone for all the orientations; however, discrepancies between the experimental and model predictions have been observed for the base metal and weld metal. Moreover, the microstructural differences between the weld metal and heat-affected zone resulted in the maximum elastic–plastic strain mismatch at the interface of the two regions. The results from this experiment would be useful to understand the role of crystal-specific microstrains and lattice deformation on fatigue and fracture behavior of thick-walled monopile weldments.

Monopile foundations contain welding residual stresses and are widely used in industry to support offshore wind turbines (OWTs). The monopiles are subjected to hammering loads during installation and cyclic loads during operation, therefore the influence of residual stress redistribution as a result of fatigue cycles must be evaluated in these structures. The existing empirical models to predict the residual stress redistribution in the presence of cyclic loading conditions are strongly dependent on the material, welding process and loading conditions. Hence, there is a need to predict the residual stress redistribution using finite element simulations. In this study numerical analyses have been conducted to predict the initial state of residual stress in a simplified weld geometry and examine the influence of subsequent cyclic loads on the relaxation behavior in residual stress profiles. The results have shown that fatigue cycles have a severe effect on residual stress relaxation with the greatest reduction in residual stress values observed in the first cycle. Moreover, the numerical prediction results have shown that the stress amplitude plays a key role in the extent of residual stress relaxation in welded structures.

Thick weldments used in offshore structures frequently act as fatigue crack initiation sites due to stress concentration at weld toe as well as weld residual stress fields. This paper investigates the cyclic deformation behavior of S355 G10+M steel, which is predominantly used in offshore wind applications. Owing to the vast size difference of monopile structure and weld cross‐section, a global–local finite element (FE) method was used, and the weld geometry was adopted from circumferential weld joints used in offshore wind turbine monopile foundations. Realistic service loads collected using supervisory control and data acquisition (SCADA) and wave buoy techniques were used in the FE model. A nonlinear isotropic–kinematic hardening model was calibrated using the strain controlled cyclic deformation results obtained from base metal (BM) as well as cross‐weld specimen tests. The tests revealed that the S355 G10+M BM and weld metal (WM) undergo continuous cyclic stress relaxation. Fatigue damage over a period of 20 years of operation was predicted using the local stress at the root of the weldments as the life limiting criterion. This study helps in quantifying the level of conservatism in the current monopile design approaches and has implications towards making wind energy more economic.

In some industrial applications, welding is the only alternative to join different parts. However, the major problem in welded structures is the tensile residual stresses that are inevitably produced during welding process. Surface tensile stresses can threaten the performance of the weldments as they act as an accelerant in fatigue crack initiation and failure. Shot peening is a well-known method which can enhance weldments' fatigue performance and longevity by inducing surface compressive residual stresses in order to eliminate or limit tensile residual stresses. In this study, the effect of shot peening on redistribution of multi-pass welding residual stresses was numerically and experimentally investigated on very large components typical of welded joints used in offshore wind turbine monopiles. In experimental part of the study, the residual stresses were measured using the Incremental Centre Hole Drilling (ICHD) method before and after shot peening. Finite element studies were carried out using 3D welding models and random shot peening analyses. Moreover, extensive finite element analyses were conducted to study the effect of model dimensions and the number of passes on prediction of welding residual stresses. Interesting set of results obtained from both the numerical studies and the ICHD measurements were in good agreements and showed that shot peening can be advantageous even for large components with multi-pass welded joints. Additionally, reducing the number of weld passes in finite element models could considerably lower the computational time without affecting the accuracy of results at surface regions of the models.

This paper presents a critical assessment of the effect of microstructure on the fatigue crack growth rate (FCGR) in ferrite-pearlite (α-P) steels in air and marine environment. This was done by evaluating the resistance of three subgrades of S355 steel, produced by normalized-rolling, NR (S355J2+N) and thermo-mechanical control process, TMCP (S355G8+M and S355G10 + M), to fatigue crack growth (FCG) in air and seawater. The data from the tests were then compared with results of several studies on α-P steels of similar compositions and microconstituents in the literature. It was found that microstructure strongly affected the Paris region of the da/dN vs. ΔK sigmoidal curve in both media. This is contrary to the current belief that microstructure has little or a negligible effect in the linear region of the FCGR curve in air. TMCP and quenched-and-tempered (QT) steels offered better resistance to FCG than NR steel in both media. TMCP steel offered the highest resistance to FCG. The crack path was non-planar with a complex crack front. The tendency for crack deviation, branching, wedging action by metal crumbs and perhaps interlocking majorly explained why the FCGRs of the TMCP and QT steels are lower than that of NR. The result also suggests that ductility and yield stress do not have a strong retarding effect on FCG in α-P steels at low stress intensity factor range (SIFR). This finding has a fundamental implication for analytical and numerical models that wholly depend on the mechanical properties of the steel and assumes plane horizontal crack propagation front in predicting FCGR in steels. Such models are likely to fail in predicting FCGR even in a subgrade of the same grade of steel.

At present, the UK government is driving the survival of the wind energy industry by using interventions that encourage investment in the sector. The use of a Contract for Difference (CfD)/Strike price model by the UK government supports the wind industry and guarantees that wind energy generators have a stable premium over a period of 15–20 years; however, this may not last forever. The growth and stability of the wind industry will depend essentially on continued reductions in wind energy cost, even below that of fossil-fuel based energy sources. Huge cost reduction beyond the present strike price of £ 57.50/MWh for some projects to be delivered in 2022/2023 may be achieved quickly through efficient and optimized turbine support structure. Consequently, the offshore wind industry is currently making enormous efforts to upscale wind turbines (WTs) from 8 MW to 9.5MW,10MW and then 12 MW HAWT (Horizontal Axis Wind Turbine). This level of upscaling no doubt creates tough challenges because the mass of the turbine increases linearly with the cube of the rotor radius. Monopiles having diameters larger than 7 m have been proposed, with a wall thickness section in the range of 70–110 mm. It is generally thought that Thermo-Mechanical Controlled Process (TMCP) steels are well suited for extra-large (XL-WTs). This paper reviews the present status of WTs and critically assesses the material factors in the structural integrity concerns that may confront the use of XL steel plates in the design of XL-WT support structures.

Offshore wind turbines (OWT) are subjected to harsh environmental conditions in addition to the variable service loads. The present study is aimed at performing a realistic fatigue life estimation of the monopile structure using operational service loads recorded by online monitoring systems. Fatigue damage analysis has been conducted at the circumferential weld joints using finite element (FE) method by considering geometrical and material property discontinuities. Global-local modelling of the OWT was performed in as-welded condition to capture the local stress range at the weld toe, which acts as the critical site where cracks are most likely to initiate and propagate. The S-N fatigue design approach and maximum stress range at the weld toe have been used to determine the fatigue crack initiation life in monopiles. The results from the proposed approach show that a realistic life assessment can be made on monopile structures by accounting for the geometrical effects at the circumferential welds.

Offshore wind turbines are subjected to cyclic loading conditions during their operational lifespan which is typically between 20 and 25 years. An important issue in fatigue design and integrity assessment of offshore wind turbine foundations is the examination of the long-term fatigue and corrosion-fatigue behaviour of steel structures in the high cycle region. High cycle fatigue tests, particularly at low frequencies in a seawater environment, are time-consuming and costly. Therefore, there is an essential need to perform accelerated tests to predict the long-term behaviour of the structures under realistic operational loading conditions. In this work, the existing fatigue acceleration mechanisms have been reviewed and a novel methodology has been proposed for accelerated testing and analysis of fatigue data in different environments (i.e. air, salt-spray and seawater) at higher temperatures. Two distinct equations have been developed and proposed for the calibration and prediction of S-N fatigue life and crack growth behaviour of steels in different environments. The proposed methodology has been validated through comparison with the existing data in the literature and predictions have been made at operational temperatures using high temperature data. The proposed approach is relatively simple to calibrate for a material of interest and enables accelerating S-N fatigue and crack growth testing of the examined materials by a factor of two and three, respectively. The proposed methodology and the obtained results have been discussed in terms of the need for accelerated testing for fatigue design and integrity assessment of offshore wind monopiles, especially those which are close to the end of initial design life and need a comprehensive engineering analysis for life extension or decommissioning.

It is vitally important to measure the brittle crack arrest properties of shipbuilding steels to ensure that accidental damage will not result in total structural failure. Wide-plate test methods allow for direct measurement of the crack arrest toughness but this kind of testing is incredibly expensive. Therefore, there is a need for cheaper and simpler test methods which are able to measure a material’s brittle crack arrest toughness. In this work, Compact Crack Arrest (CCA) testing, which is standardised in ASTM E1221, has been successfully used to measure the crack arrest toughness of thick sections of EH47 shipbuilding steel. The results from this study have been compared to small-scale test methods. It was found that instrumented Charpy testing gives an overprediction of the CCA results, and nil-ductility transition temperature (NDTT) from Pellini tests gives a conservative estimate. The results presented in this study are discussed in terms of the effectiveness of the CCA test method for measurement of brittle crack arrest toughness and integrity assessment of large-scale structures.

A commonly used material in offshore structures is S355 structural steel. For example, during the monopile fabrication process, the material is pre-strained to different levels at different depths through the thickness. Therefore, the influence of pre-straining on fatigue life and crack growth behaviour of the material needs to be examined and considered for design and life assessment procedures. In the present study, uniaxial fatigue and fatigue crack growth tests have been conducted on materials with different pre-strain levels and the results are compared with the un-strained material state. From the test data, it has been seen that the S-N fatigue life will reduce with increasing pre-straining level, while the fatigue crack propagation rate remains largely unchanged in pre-strained material. The results from this study are compared with the recommended S-N fatigue and fatigue crack growth trends available in standards and are discussed in terms of the applicability and level of conservatism in the recommended curves to account for the material pre-straining effects on the fatigue life assessment of offshore structures.

The aim of this study is to look into the current information surrounding decommissioning and life extension strategies in the offshore wind sector and critically assess them to make informed decisions upon completion of the initial design life in offshore wind farms. This was done through a two-pronged approach by looking into the technical aspects through comprehensive discussions with industrial specialists in the field and also looking into similar but more mature industries such as the Offshore Oil and Gas sector. For the financial side of the assessment, a financial model was constructed to help portray a possible outcome to extend the life for a current offshore wind farm, using the existing data. By employing a techno-economic approach for critical assessment of life extension strategies, this study demonstrates the advantages and disadvantages of each strategy and looks to inform the offshore wind industry the best course of action for current wind farms, depending on their size and age.

Determination of long-term creep rupture properties for 316H steel is both costly and time-consuming and given the level of scatter in the data would need substantial number of tests to be performed. The primary objective of this study is to estimate the long-term creep properties of cross-weld (XW) and as-received (AR) 316H stainless steel by performing accelerated tests on pre-compressed (PC) material. In this work, uniaxial creep rupture tests have been performed on XW specimens and the results have been used to establish a correlation with accelerated test results on the PC material. Moreover, tensile tests have been performed on XW specimens at room temperature and 550 °C to examine the pre-conditioning effects on the mechanical response of the material. Similar power-law creep properties have been found for the creep strain rate and rupture time behaviour of the XW and PC specimens. It also has been found that the creep ductility data points obtained from XW and PC specimens fall upon the estimated trend for the AR material at 550 °C when the data are correlated with the applied stress normalised by 0.2% proof stress. The results show that the long-term creep properties of the XW and AR material can be estimated in much shorter time scales simply by performing tests on the PC material state.

Reheat cracking in an ex-service Type 316H stainless steel steam header component has been investigated in this study. The examined steam header was in service for 87,790" role="presentation" style="display: inline; line-height: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border-width: 0px; border-style: initial; position: relative;">h and the cracks in this component were found in the vicinity of the weld toe. The root cause of this type of failure was due to the welding residual stresses. The welding-induced residual stresses had been present in the header at the early stage of the operation and were released during service. In this paper, a novel technique has been proposed to simulate the residual stress distribution normal to the crack direction by applying remote fixed displacement boundary conditions in an axisymmetric model. This approach can simulate the presence of residual stresses in actual components without the need to develop full weld simulation to quantify them. The predicted residual stress levels and distributions normal to the crack direction have been found in good agreement with the measured residual stresses available in the literature for a similar header. The creep crack growth (CCG) rates have been characterized using the fracture mechanics C∗" role="presentation" style="display: inline; line-height: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border-width: 0px; border-style: initial; position: relative;">C∗C∗ parameter and estimated using predictive models.

This paper presents the state of the art in Structural Reliability Analysis (SRA) methods with a view of identifying key applications of each method and its proposed variations, qualifying characteristics, advantages, and limitations. Due to the increasing complexity and scale of modern offshore jacket structures, it becomes increasingly necessary to propose an accurate and efficient approach for the assessment of uncertainties in their material properties, geometric dimensions, and operating environments. SRA, as a form of uncertainty analysis, has been demonstrated to be a useful tool in the design of structures because it can directly quantify how uncertainty about input parameters can affect structural performance. Herein, attention was focused specifically on the probabilistic fracture mechanics approach because this accounts accurately for fatigue reliability mostly encountered as being dominant in the design of such structures. The well-established analytical/approximate methods such as the First- and Second-Order Reliability Methods (FORM/SORM) are widely used as they offer a good balance between accuracy and efficiency for realistic problems. They are, however, inaccurate in cases of highly non-linear systems. As a result, they have been modified using methods such as conjugate search direction approach, saddle point approximation, subset simulation, evidence theory, etc. in order to improve accuracy. Initially, direct simulations methods such as the Monte Carlo Simulation Method (MCS) with its various variance reduction techniques such as the Importance Sampling (IS), Latin Hypercube Sampling (LHS), etc. are ideal for structures having non-linear limit states but perform poorly for problems that calculate very low probabilities of failure. Overall, each method has its own merits and limitation, with FORM/SORM being the most commonly used, but recently, simulation methods have increasingly been used due to continuous advances in computation powers. Other relevant methods include the Response Surface Methods (RSM) and the Surrogate Models/Meta-models (SM/MM), which are advanced approximation methods and are ideal for structures with implicit limit state functions and high-reliability indices. Combinations of advanced approximation methods and reliability analysis methods are also found in literature as they can be suitable for complex, highly non-linear problems.

Creep crack growth is known to be the dominant failure mechanism in high-temperature components. Particularly in welded structures operating at elevated temperatures, cracks are often found to initiate and propagate in the vicinity of the weld region which can eventually penetrate into the base material after a long period of operation. In this study, creep crack growth tests have been performed on specimens extracted from an ex-service 316 H welded component to examine the crack initiation and growth behaviour in near-weld regions. The results show that the cracking behaviour of the base metal in near-weld specimens is similar to the as-received 316 H data set, suggesting that the material inhomogeneity would not influence the crack propagation behaviour in service-exposed components. Moreover, the test results show that the crack initiation and growth behaviour of the HAZ specimens can be estimated in much shorter time scales by performing tests on pre-compressed material.

In this study corrosion-fatigue tests have been conducted on fracture mechanics specimens extracted from an S355 G10+M structural steel welded plate. The tests have been performed on compact tension specimens with the crack tip located in the heat affected zone. The corrosion-fatigue test results from this study have been compared with the data available on the base metal as well as air tests on the same material. Moreover, the obtained results have been compared with the corrosion-fatigue data available in the literature on a wide range of steels and also the fatigue trends for welded joints in free-corrosion condition recommended in the BS7910 Standard. The effect of the specimen orientation, with respect to the weld region, is also examined in this study and it has been found that higher corrosion-fatigue crack growth rates are generally observed in the tests with 0° orientation. The results have also shown that the corrosive environment has significant effects on the fatigue crack growth acceleration at the beginning of the tests; however, as the crack propagates, the environmental damage effect on crack growth behaviour becomes less pronounced. The results presented in this study are discussed in terms of improvement in the structural integrity assessment of offshore wind turbine monopiles.

The structural integrity of offshore wind turbine (OWT) support structures is affected by one of the most severe damage mechanisms known as pitting corrosion‐fatigue. In this study, the structural reliability of such structures subjected to pitting corrosion‐fatigue is assessed using a damage tolerance modelling approach. A probabilistic model that ascertains the reliability of the structure is presented, incorporating the randomness in cyclic load and corrosive environment as well as uncertainties in shape factor, pit size and aspect ratio. A non‐intrusive formulation is proposed consisting of a sequence of steps. First, a stochastic parametric Finite Element Analysis (FEA) is performed using SMART© crack growth and Design Xplorer© facilities within the software package ANSYS. Secondly, the results obtained from the FEA are processed using an Artificial Neural Network (ANN) response surface modelling technique. Finally, the First Order Reliability Method (FORM) is used to calculate the reliability indices of components. The results reveal that for the inherent stochastic conditions, the structure becomes unsafe after the 18th year, before the attainment of the design life of 20 years. The FEA results are in very good agreement with results obtained from analysis steps outlined in design standard BS 7910 and other references designated as ‘theoretical analysis methods’ in this study. The results predict, for the case study, that the pit growth life is approximately 56% of the total pitting corrosion fatigue life. Sensitivity analysis results show that the aspect ratio of pits at critical size plays a significant role on the reliability of the structure.

This paper presents a study on the effect of microstructure on the fatigue crack growth (FCG) rate in advanced S355 marine steels in the Paris Region of the da/dN versus ΔK log–log plot. The environments of study were air and seawater (SW), under constant amplitude sinewave fatigue loading. Fundamentally, three phenomena (crack tip diversion, crack front bifurcation and metal crumb formation) were observed to influence the rate of FCG. These phenomena appear to be a function of the material microstructure, environment and crack tip loading conditions. The three factors retarded the crack growth by reducing or redistributing the effective driving force at the main active crack tip. A crack path containing extensively the three phenomena was observed to offer strong resistance to FCG. In SW, the degree of the electrochemical dissolution of the microplastic zone appears to be an additional primary factor influencing FCG in the steels.

This paper investigates the cyclic deformation behaviour of S355 G10+M steel which is predominantly used in offshore wind applications. The thick weldments were identified as regions prone to fatigue crack initiation due to stress concentration at weld toe as well as weld residual stress fields. The monopile structure was modelled using a global-local finite element (FE) method and the weld geometry was adopted from circumferential weld joints used in offshore wind turbine monopile foundations. Realistic service loads collected using SCADA and wave buoy techniques were used in the FE model. A non-linear isotropic-kinematic hardening model was calibrated using the strain controlled cyclic deformation results obtained from base metal as well as cross-weld specimen tests. The tests revealed that the S355 G10+M base metal and weld metal undergo continuous cyclic stress relaxation. Fatigue damage over a period of 20 years of operation was predicted using the total elastic-plastic strain energy accumulated at the root of the weldments as the life limiting criterion. This study helps in quantifying the level of conservatism in the current monopile design approaches and has implications towards making wind energy more economic.