CERES :: Browsing by Author "Brennan, Feargal"

Browsing by Author "Brennan, Feargal"

Now showing 1 - 20 of 42

Open Access
Cash flow at risk of offshore wind plants
(IEEE, 2017-08-18) Vaienti, Claudio; Ioannou, Anastasia; Brennan, Feargal
Offshore wind power plants might be seen as high risk investments. Their risk depends on technical and financial elements. When some corporations decide to invest in a plant, they decide to take all above-mentioned risks. The question “Given a specific investor, a specific plant, etc., how big are the investment risks?” has not a clear answer. In fact, the impact of the previous risk factors on cash flows is not completely quantified, mainly because all the risks are related, but the dependency structure is difficult to be modelled. Hence, it is important to have a measure of the impact of the risks into the cash flows. Due to the lack of knowledge in this quantification, we have decided to investigate it more in the detail. The paper aims to measure the variability of cash flows and how effective are the strategies for locking electricity prices, ship freight rates, or both in the reduction of this variability. We adopt the Monte Carlo approach for simulating all the possible cash flows and for measuring all the uncertainties. The output shows that seasonal and uncertain cash flows. The strategies, for reducing the probability of negative cash flows, work only with locked electricity prices.
Open Access
CFD modelling of VAWT wake effects
(Cranfield University, 2016-12) Huang, Xiaojian; Brennan, Feargal
Wake effects are important to wind turbine design and wind farm design, because they will affect the aerodynamic performance and structural loads of wind turbine operating in a wind farm. Wake effects were investigated extensively for horizontal axis wind turbine(HAWT) in the past, but there has been very limited work done for the vertical axis wind turbine(VAWT), whose wake effects are unique because the blades will go through their own wake region during the operation. The presented thesis aims to bridge this knowledge gap by modelling the VAWT wake effects using CFD. As for the general wind turbine wake effects study, four key aspects can be identified: wake models, aerodynamics, structural dynamics, and structural integrity. Relevant literature is reviewed in the thesis, and a comprehensive framework of studying the VAWT wake effects is proposed. The framework covers all the four key aspects of the wind turbine wake effects study, and two of them will be addressed in the presented thesis, wake models and wake aerodynamics. CFD modelling in the thesis is based on RANS method. The near wake modelling focuses on the aerodynamics prediction and the far wake modelling focuses on the wake structure prediction. As for the near wake study, wake effects of a circular cylinder at Re=140000 is studied and validated. the aerodynamic performance of NACA0015 airfoil at various angle of attack at Re=2000000 is modelled using different turbulence models, dynamic stall effects of the airfoil at three different regimes are investigated. They form the basis of analysing the aerodynamic performance of VAWT rotor. A 17m 2-bladed VAWT designed based on such geometries (circular cylinder and NACA0015 airfoil) is modelled thereafter, simulated aerodynamic performance under different tip-speed ratios are compared with experiment data. As for the far wake study, both rotor simplification using porous disk and full rotor simulation are presented. A persistent symmetric wake region is observed from the porous disk modelling while the full rotor modelling predicts an asymmetric wake region. The wake interaction is then studied in a two turbine VAWT array, the influence of wake effects on the performance of VAWT at 3 diameters downstream is investigated. Overlapping of wake region is analysed.
Open Access
Design and testing of a novel human-powered generator device as a backup solution to power Cranfield´s Nano-Membrane Toilet
(Cranfield University, 2014-04) Perez Lopez, Eloy; Kolios, Athanasios; Brennan, Feargal
In today’s world there are 2.6 billion people that lack basic sanitation (37% of world inhabitants). In August of 2012, Cranfield University was awarded by the Bill & Melinda Gates Foundation with $810,000 to produce a prototype of the Cranfield’s innovative Nano-membrane Toilet (NMT). Finally, the prototype is going to be exhibited at the “Reinvent the Toilet Fair” during 21st and 22nd of March 2014 in the Taj Palace Hotel, New Delhi (India). Cranfield’s NMT demands electricity for its daily performance. Nevertheless, it is targeted to off-grid communities. Consequently, a human-powered generator (HPG) was selected as a backup solution. The current MSc by Research aimed to design and test of a prototype of the aforesaid HPG. Moreover, to promote its usage, a portable power supply unit is designed to store energy and power small-loads like charging mobile phones and electric lighting. To select the most suitable design for our case study, a methodology using the Technique for Order of Preference by Similarity to the Ideal Solution has been developed. As a result the plugged-in bike HPG alternative was selected. Next, prototypes of this generator and the portable power supply unit were developed, tested and shipped for display. While testing of the plugged-in generator and portable power supply unit, 26 Watt-hours (Wh) were harvested over 15 minutes, with its corresponding average charging power of 105 Watts. Nevertheless, the present study concludes 96 Wh as a more accurate energy level to be harvested during one hour of pedalling. Considering 96 Wh of energy, a round-trip battery efficiency of 70% (lead-acid), and a NMT’s demand of 283 Wh; a 10 people household needs to pedal the HPG over 4 hours and 20 minutes. Nevertheless, if considering an 85% inverter efficiency, 57.12 Wh are available to fully charge one mobile phone (5.6 Wh) and provide 4.5 hours of room and desk lighting (11 Watts bulb).
Open Access
Design, construction and testing of an ascending micropenetrometer to measure soil crust resistance
(Cranfield University, 2014-02) Lorentz, Andrew; Brennan, Feargal; Collu, Maurizio; Ritz, K.
The increasing world population is putting pressure on global food production. Agriculture must meet these growing demands by increasing crop yields. One phenomenon which prevents seedling emergence and damages crop yield is soil crusting. Understanding of soil crusting and the factors which influence it is fundamental to ensuring good crop production. An instrument which will test soil crust strength in a novel way, mimicking seedling growth, may lead to pre-emptive agricultural soil management which could increase crop production. This work details the process of design, construction and testing of an ascending penetrometer to measure soil crust strength. The full design process is discussed from concept generation and evaluation, using experimental methods and a multi-criteria decision making tool, through to final design configuration, specification, manufacture and testing. Traditionally, soil penetrometers measure soil strength by forcing a probe from the surface of the soil into the bulk soil below. To more accurately measure the direct impedance a seedling would experience a device should measure impedance from the bulk soil upwards and into the soil crust, mimicking what a growing seedling would experience. Results prove that the manufactured ascending penetrometer with a force resolution of 0.01N and displacement resolution of 0.0004mm is capable of detecting differences in soil crusts. At these resolutions and accuracy to 0.1N and 0.1mm excellent repeatability was achieved. The machine is therefore a useful and realistic tool for quantitatively comparing soil crusts in soil. It is hoped that being able to compare soil crust strength will lead to improved soil management techniques.
Open Access
Development of damage tolerant composite laminates using ultra-thin interlaminar electrospun thermoplastic nanofibres
(European Society for Composite Materials, 2018-06-30) Li, Danning; Prevost, Raphael; Ayre, David; Yoosefinejad, Ata; Lotfian, Saeid; Brennan, Feargal; Yazdani Nezhad, Hamed
Carbon fibre-reinforced polymer (CFRP) composites are extensively used in high performance transport and renewable energy structures. However, composite laminates face the recurrent problem of being prone to damage in dynamic and impact events due to extensive interlaminar delamination. Therefore, interlaminar tougheners such as thermoplastic veils are introduced between pre-impregnated composite plies or through-thickness reinforcement techniques such as tufting are employed. However, these reinforcements are additional steps in the process which will add a degree of complexity and time in preparing composite lay-ups. A novel material and laying-up process is proposed in this paper that uses highly stretched electrospun thermoplastic nanofibers (TNF) that can enhance structural integrity with almost zero weight penalty (having 0.2gsm compared to the 300gsm CFRP plies), ensuring a smooth stress transfer through different layers, and serves directional property tailoring, with no interference with geometric features e.g. thickness. Aerospace grade pre-impregnated CFRP composite laminates have been modified with the TNFs (each layer having an average thickness of <1 micron) electrospun on each ply, and autoclave manufactured, and the effect of the nanofibers on the fracture toughness has been studied. Interlaminar fracture toughness specimens were manufactured for Mode I (double cantilever beam) and Mode II (end notched flextural) fracture tests. Such thin low-density TNF layers added an improvement of 20% in failure loads and fracture toughness in modes I and II.
Open Access
Effect of electricity market price uncertainty modelling on the profitability assessment of offshore wind energy through an integrated lifecycle techno-economic model
(IOP Publishing, 2018-10-10) Ioannou, Anastasia; Angus, Andrew; Brennan, Feargal
According to the Contracts for Difference (CfD) scheme introduced to support the deployment of offshore wind installations, an electricity generation party is paid the difference between a constant "strike price" (determined be means of a competitive auction) and the average UK market electricity price for every MWh of power output produced. The scheme lasts for 15 years, after which the electricity output is sold on the average market price. To this end, estimating the long term profitability of the investment greatly depends on the forecasted market prices. This paper presents the simulation results of future electricity prices based on three different simulation methods, namely: the Geometric Brownian motion (GBM), the Autoregressive Integrated Moving average (ARIMA) and a model combining Mean-Reversion and Jump-Diffusion (MRJD) processes. A number of simulation paths are generated for a time horizon of 10 years and they are introduced to a fully integrated techno-economic model developed by the authors. As a result, joint probability distributions of the NPV derived from the three different methods are presented. This study is relevant to investors and policy makers to check the viability of an investment and to predict its stochastic temporal return profile.
Open Access
Effects of structural steels microstructure and waveform on corrosion-fatigue behaviour of offshore wind turbine foundations.
(2019-10) Igwemezi, Victor Chinedu; Mehmanparast, Ali; Brennan, Feargal
Marine structures in the offshore environment are subjected to constant cyclic wave and wind forces. Also, the foundations are in direct contact with seawater (SW), hence introducing corrosion damage in the structure. Due to the cyclic loading cracks could nucleate and grow, or existing cracks could propagate at loads far less than the maximum design load, and the problem is worsened in marine environment. Offshore Wind Turbines (WTs) are relatively new structures and their long-term corrosion-fatigue performance data are very scarce. In fact, presently there are little or no public data on corrosion-fatigue performance of these sub-grades of S355 steel used in the design of extra-large (XL) Wind turbine support structures (WTSSs). This experimental research primarily seeks to understand the rate at which a crack grows in the modern normalised-rolled (NR) and thermomechanical control process (TMCP) S355 steel subgrades designated as S355G10+M, S355G8+M and S355J2+N under the influence of: fatigue waveforms, change in fatigue load level and material microstructure in seawater (SW) at frequencies within the range of commercial Wind Turbine (WT) operating condition. The test programme employed a soft-stiff frequency range of commercial WTs in the North Sea. The waveforms considered in this study are constant amplitude sinewave and trapezoid waveform (generally referred to here as hold-time) and the experiments were conducted under frequencies of 0.2Hz, 0.3Hz, 0.5Hz. The stress ratio in all tests was 0.1 and the maximum applied loads were 9kN and 10kN. All calculations where done under Linear Elastic Fracture Mechanics (LEFM) and all tests and investigations were limited to the Paris Region or the Stage II of the da/dN vs. ΔK plot, where LEFM applies. All the tests have been performed in accordance with BS 7910:2013+A1:2015 and ASTM E647-15. This study found that frequency has no obvious effect on the FCGR of ferrite-pearlite steels in air. There was enhancement of the FCGR in seawater by a factor that depended on the waveform and load level. The CFCGR of the sine waveform was found to be higher than that of the holdtime for all the load levels and frequencies used. Using the mean curve, SW enhanced the FCGR by an average factor of 1.48 under hold-time and 2.17 under sinewave throughout theParis Region in the range of 20 - 35 MPa√m with reference to the air mean curve. Similar trend was obtained if mean +2SD is used, giving an average factor of increase of 1.48 under hold-time and 2.30 under sinewave. Comparison of the experimental results on S355G8+M, S355G10+M and S355J2+N has shown that under both sinewave and hold-time waveforms, the CFCG trend in normalised steels (e.g. S355J2+N) is consistently higher than that of the TMCP steels (e.g. S355G8+M, S355G10+M) in both air and sea environments. Change in load level and frequency did not affect the CFCGR of the TMCP steels in SW under sinewave, but slight change was observed with decrease in the load level for the holdtime. The Pmax is found to have a profound influence on CFCGR than the cyclic frequency (in the range of 0.2Hz to 0.4Hz). Decreasing the load level reduces the effect of frequency in SW and the difference in CFCGR for waveforms diminishes with increasing frequency and ∆K. In SW, increase in the fatigue load and decrease in the frequency, especially for holdtime has the highest retarding effect under corrosion dominated regime. This is a consequence of what we referred to here as the microplastic zone size. High load level increases the microplastic zone size closest and ahead of the propagating crack tip and opens up the crack tip region to allow the entry of more damaging chemical species into the plastically deformed regions causing fast attack by the time-dependent corrosion process resulting to rapid crack tip dissolution and blunting. When the main active crack tip is blunted the crack propagation process is considerably reduced or even arrested for a very long time as observed especially for the hold time test. This appears to suggest that for same ∆K value, combination of low stress range and long crack length, a will be more damaging than combination of high stress and short a in SW. Corrosion products are also found to build up in and around the blunted crack tip leading to further retardation of the crack growth. Extensive blunting of the crack front due to availability of time explains why the CFCGR is generally lower in holdtime as compared to sinewave. Fractographic and metallographic analyses were carried out to understand the disparity in the CFCGR between the waveforms under the test conditions. Fractographic examination of the fractured surface of the corrosion-fatigue specimens showed that the CFCG mechanism is by ductile striations both in air and SW. The crack path was always non-planar with complex crack front. Generally, three phenomena were identified that primarily retarded crack growth in the ferrite-pearlite steels in air. These are crack diversion, crack bifurcation and metal crumbs. The three factors retarded the crack growth by reducing or re-distributing the effective driving stress at the main propagating active crack tips. It was found that the main crack tip blunting process is the primary factor controlling the CFCGR of steel at high ∆K and low frequency in a ferrite-pearlite steel in SW. Other fundamental factors are crack angle diversion, branching of crack front and formation of metal crumbs along the crack path. The extent of formation of the aforementioned phenomena is a strong function of the steel microstructure. This implies that microstructure has a strong effect on the FCGR of ferrite-pearlite steels in the Paris Region, both in air and SW. This conclusion is in contrary to current theory that microstructure has little or no effect in the Paris Region of the da/dN vs ∆K sigmoidal curve. Transgranular and quasi-intergranular modes of propagation in both environments were observed. The quasi-intergranular mode is a situation where the fatigue-crack propagated through a thin layer of high solute ferrite ribbon, 𝛼𝐻𝐴 adjacent to the low relief, αLR or the pearlite, P phases. The morphology and chemistry of the phases local to the main crack front and the load level appeared to determine which mode the crack growth would adopt. The angle the crack front made with the least resistant path ahead of it seemed to determine if it would propagate by transgranular or quasi-intergranular mode.
Open Access
Environmental impact assessment and optimisation of commercial aviation
(Cranfield University, 2011-11) Howe, Stuart; Kolios, Athanasios; Brennan, Feargal
The aviation industry represents approximately 3% of global greenhouse gas emissions, however with significant growth expected over the coming decades this proportion is expected to increase. Continued governmental and social pressure to reduce global emissions is posing a challenging question to the industry; how to improve environmental efficiency and reduce emissions with increasing industry growth. The environmental impact of aviation globally is discussed, examining the significant emissions and protocols that exist and their relative impacts both environmentally and economically. The viability of alternative biofuels is discussed, determining the life cycle environmental impact of future replacements to kerosene based jet fuel. This thesis therefore aims to provide an understanding of the fundamentals of aviation emissions but also most importantly provide possible solutions to assist the industry in reducing its emissions ‘footprint’. An important factor in determining efficiency improvements is to understand the impact of particular stages of an aircraft life and the impact they have individually. This was achieved using an established methodology called Life Cycle Assessment (LCA), which is an efficient tool for the analytical consideration of the environmental impact of manufacturing, operation and decommissioning. The results of a comprehensive LCA study of an Airbus A320 are documented considering all phases of the service life. The study draws useful conclusions, indicating the significance of special materials such as carbon fibre reinforced plastic (CFRP) on the total manufacturing emissions of the aircraft and indicating its operational phase as the one contributing most in its environmental performance breakdown. The thesis also examines short-term efficiencies for emissions reduction in commercial aviation, focussing on improvements in aircraft routing. The initiation of the EU emissions trading system (ETS) within European aviation willincentivise airlines to reduce their annual CO2 emissions. An alternative routing strategy is proposed for selected long haul routes, which introduces multiple stages into the route utilising two aircraft and is shown to reduce total CO2 emissions by up to 13.7%. Combined with blended biofuel, this reduction was estimated to increase to 16.6% with a reduction in ticket fares estimated to be as high as $19 per passenger per flight.
Open Access
Fatigue and fracture mechanics of offshore wind turbine support structures
(Cranfield University, 2015-07) Lozano Minguez, Estivaliz; Brennan, Feargal; Kolios, Athanasios
Wind power, especially offshore, is considered to be one of the most promising sources of ‘clean’ energy towards meeting the EU targets for 2020 and 2050. However, its popularity has always fluctuated with the price of fossil fuels since nowadays wind electricity production cannot compete with nuclear or coal electricity production. Support structures are thought to be one of the main drivers for reducing costs in order to make the wind industry more economically efficient. Foundations and towers should be fit for purpose, extending their effective service life but avoiding costs of oversizing. An exhaustive review of the background and state of the art of the Fatigue-Life assessment approaches has been carried out, combining analysis of the gathered experimental data and the development of Finite Element models based on contemporary 3D solid models with diverse Regression Analyses, in order to identify their weakness and evaluate their accuracy. This research shows that the guides and practices currently employed in the design and during the operation of the offshore wind turbine support structures are obsolete and not useful for optimisation, which generally leads to conservationism and an unnecessary increase in costs. The basis for a comprehensive update of the Girth Weld and Tubular Joint S-N curves and the Stress Concentration Factors of Tubular Joints has been set out. Furthermore, a reliable methodology for deriving the Stress Intensity Factor at the deepest point of a semi-elliptical surface saddle crack in a tubular welded T-joint has been proposed.
Open Access
Fatigue Crack Growth in Complex Stress Fields
(Cranfield University, 2008-08) Chahardehi, Amir Ebrahim; Brennan, Feargal
Fatigue crack growth has been traditionally modelled using LEFM through the use of the Paris law. This requires an accurate method for stress intensity factor (K) calculation. Weight functions have been developed for one-dimensional cracks (e.g. edge and through cracks); these are functions that enable separation of the loading and geometry and considering the effect of each one of these two factors on the stress intensity factor (SIF) separately. They have been proven to be useful for arbitrary stress distributions where an accurate empirical formula for the stress intensity factor does not exist. Such cases include residual stress fields due to surface treatments or welds. However, in the case of surface cracks, or part-through cracks, the problem of modelling the growth of these cracks poses two main questions, namely, how should the Paris law be generalised to suit the two-dimensional scenario, and under arbitrary loadings, how can the SIFs be calculated for these cracks. Current solutions involve tedious mathematical calculations and are complicated functions. In this thesis, the concept of root mean square (RMS) SIF is examined and by drawing mathematical analogy with the one-dimensional case, a novel weight function is derived which enables calculation of RMS SIF values for a range of semi-elliptical surface cracks under arbitrary loadings. The accuracy of the weight function is verified through comparisons with finite elements results for a variety of loadings/geometries. The simplicity of the weight function construction method makes it a useful tool for fatigue life predictions where incremental recalculations of SIF is required as the crack grows. Surface treatments such as shot peening and laser peening are used for crack growth retardation. It is generally believed that it is through the introduction of what is termed ‘beneficiary compressive residual stresses’ that crack retardation occurs. The compressive residual stresses are superimposed on the ‘detrimental tensile stresses’ due to loading and hence lead to a lower SIF level. By having such a strong tool as weight functions, this general belief can be put to test. To this end, a set of experiments were carried out to study the behaviour of cracks in residual stress fields arising from laser peening. Edge cracks were grown in partially-peened specimens. Neutron diffraction stress measurements were taken and stress profiles were obtained for these specimens. Measurements of strain fields near the crack show the interaction between the crack and the stress field induced by the peening process. The effect of laser peening on crack growth is discussed and recommendations for future work are proposed. Overall the thesis proposes a weight function for surface cracks the uniqueness of which is in its simplicity, and develops an understanding of the nature of induced and transient stresses in laser-peened components. The concept of ‘effective fatigue stress’ is introduced and its calculation is described, and conclusions are drawn from the nature of this stress distribution.
Open Access
Human-free offshore lifting solutions
(2018-10-10) Leimeister, Mareike; Balaam, T.; Causon, Paul Douglas; Cevasco, Debora; Richmond, M.; Kolios, Athanasios; Brennan, Feargal
With single elements weighing up to hundreds of tonnes and lifted to heights of 100 meters, offshore wind turbines can pose risks to personnel, assets, and the environment during installation and maintenance interventions. To increase safety during offshore lifts, this study focuses on solutions for human-free lifting operations. Ideas in the categories of logistics, connections, as well as guidance and control, were discussed and ranked by means of a multi-criteria decision analysis. Based upon 38 survey responses weighting 21 predefined decision criteria, the most promising concepts were selected. Logistically, pre-assembled systems would reduce the number of lifts and thus reduce the risk. A MATLAB-based code has been developed to optimise installation time, lifted weight, and number of lifts. Automated bolting and seafastening solutions have high potential to increase safety during the transport of the wind turbine elements and, additionally, speed up the process. Finally, the wind turbine should be lifted on top of the support structure without having personnel being under the load. A multi-directional mechanical guiding element has been designed and tested successfully in combination with visual guidance by cameras in a small-scale experiment.
Open Access
Improvement of structural health monitoring performance for offshore wind turbines subjected to fatigue and pitting corrosion.
(2019-07) Khodabux, Mohammud Waseem; Mehmanparast, Ali; Brennan, Feargal
The offshore wind industry is growing fast in the UK. Structural health monitoring has been employed to assess the loads experienced by the structures but also to assess the damage for more intelligent inspection visits, thus reducing cost of maintenance and risk of injuries. Damage can exist in various forms but the two most detrimental ones are fatigue and corrosion. This study delves into the technical aspect of SHM and applies interpolation (longitudinal and circumferential interpolation respectively) techniques to enable data fusion across the structure for fatigue damage assessment from bending strain gauge sensors (on offshore wind turbines). This gives a more refined interpretation of the most damaged locations, which can be used as a guide for targeted inspection rather than the traditional form. Also, it can be used in the design phases for improvement to consolidate the location the highest damage is more prone to. Further analysis has been done to improve the confidence in the readings and to reduce the sampling rate based on damage assessment. Interpolation techniques have also been applied to quantify the damage with respect to pitting corrosion as a form of marine localised corrosion, which is vicious as it can be a prominent initiator for pit to crack transition. The algorithm developed marries for the first time in a data-driven approach pitting corrosion and a data-driven Structural Health Monitoring system. The main transitions from pit initiation to propagation with growth and an interface to capture the pit to crack transition and crack growth using linear elastic fracture mechanics have been developed. To improve the model, a field experiment has been done to express pit characteristics in a statistical fashion with respect to depth, which have been quantified using mass loss techniques, laser scanning and image processing.
Open Access
The influence of microstructure on the fatigue crack growth rate in marine steels in the Paris Region
(Wiley, 2020-07-28) Igwemezie, Victor C.; Mehmanparast, Ali; Brennan, Feargal
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
Open Access
The influence of partial peening of fatigue crack growth
(2017-05) Al-Turaihi, Ali Sabah; Mehmanparast, Ali; Brennan, Feargal
Fatigue crack growth rate is an important factor for the life assessment of engineering components and structures. Various surface treatment techniques have previously been developed and employed in industrial applications to extend the fatigue life of engineering structures by implementing compressive residual stresses on material surfaces. Residual stresses can decelerate or accelerate fatigue crack growth in engineering structures, depending on their distribution profiles (i.e. tension or compression). In this thesis, the influence of partial surface peening on the fatigue crack growth of a high strength steel (HSS) material has been investigated by performing laboratory tests. The effects of partial peening on the fatigue crack growth behaviour of Optim700QL ferritic HSS have been experimentally investigated. Dog-bone shaped specimens were tested under tension-tension fatigue load and specimens with semi-circular notches were tested under 4-point bending conditions. An initial notch was machined in the middlewidth of all specimens to create a starter crack for fracture mechanics studies. Three distinct extents of partial shot peening and cavitation shotless peening, with respect to the crack tip and specimen symmetry line, were applied on the specimen geometry. The finite element modelling (FEM) was used to first calculate the shape function, and then the stress intensity factor (SIF), to develop a new model for 4-point bend specimen geometry with the dimensions considered in this project, the solutions of which are not available in the literature. Moreover, FEM was used to predict the stress concentration factor (SCF) for a dog-bone specimen to avoid reaching plasticity at the crack tip in load calculations for tension-tension tests. The fatigue crack growth results from the partially peened specimens have been compared with those obtained from similar specimen geometry but with no peening. The results show that the residual stress fields formed ahead of the initial notch tip due to the peening process play a significant role in the fatigue crack growth behaviour of the material, indicating that partial surface peening can be used as an effective method to decrease the fatigue crack growth rate under pure bending fatigue loading conditions, but is harmful under tension-tension loading conditions.
Open Access
The influence of partial surface shot peening on fatigue crack growth behaviour of a high-strength ferritic steel
(Wiley, 2017-10-27) Al-Turaihi, A; Mehmanparast, Ali; Brennan, Feargal
The effects of partial surface shot peening on the fatigue crack growth behaviour of a ferritic steel have been experimentally investigated in this paper. Dog-bone specimens fabricated from Optim700QL were tested under tension-tension fatigue loads. Three distinct extents of partial shot peening, with respect to the crack tip and specimen symmetry line, were tested. The fatigue crack growth results from these experiments have been compared with those obtained from the same specimen geometry but with no peening. The results show that the residual stress fields formed ahead of the initial notch tip due to the partial peening process play a significant role in the fatigue crack growth behaviour of the material and effectively result in accelerated crack propagation at the midwidth of the specimens. It has been shown in this study that partial peening can lead to a fatigue crack growth rate around twice as fast as that of the unpeened specimen.
Open Access
The influence of residual stresses on structural integrity of renewable energy marine structures.
(2019-07) Jacob, Anais Louise Melanie; Mehmanparast, Ali; Brennan, Feargal
Offshore wind turbines operate in harsh environments and are subjected to severe cyclic loading conditions which result in corrosion and fatigue damage particularly in the support structures which are predominantly made of monopile type foundations. A reliable assessment for the fatigue life of offshore wind turbine monopile foundations is significantly dependent on the level of locked-in welding residual stresses at circumferential weld regions. In this work, fatigue crack growth tests have been conducted in air and seawater on S355 G10+M structural steel which is widely used in the fabrication of offshore wind turbine foundations. Fracture mechanics tests have been performed on compact tension specimens with the crack tip located in the heat affected zone. All tests were performed at room temperature and the obtained results are compared with the literature data available on a range of offshore structural steels and also the recommended trends in BS7910 using the 2-stage law and simplified law. Moreover, the specimen orientation, with respect to the weld geometry, has been examined and discussed in this work. This study presents, for the first time, residual stress characterisation in compact tension specimens extracted from monopile weldments using three different methods; 1- Neutron diffraction, as a non-destructive technique which is widely used to measure lattice spacing from which residual strains and subsequently residual stresses can be calculated, 2 Neutron imaging, which is a relatively new non-destructive technique that enables residual stresses to be measured through strain mapping of the area of interest, 3- Contour method, as a destructive technique which can be used to measure residual stresses in engineering components and structures. Neutron diffraction and neutron imaging are two complementary techniques which have been employed in this work by performing measurements on the Engin-X and newly developed IMAT instruments, respectively, at the Rutherford Appleton Laboratory. Neutron diffraction residual strain measurements were conducted along all three directions (i.e. transverse, longitudinal and normal with respect to the weld geometry) on compact tension specimens with the crack tip located in the heat affected zone whilst neutron imaging technique was used to measure residual strains in the longitudinal direction. A comparison of the obtained results from neutron diffraction and neutron imaging techniques has shown that neutron imaging can provide an acceptable measure of residual strains, and subsequently residual stresses, if an accurate value of strain-free lattice spacing, dₒ, is employed in data analysis. Moreover, it has been shown that the contour residual stress measurement results are in good agreement with the neutron diffraction results. The residual stress measurement results have been discussed in terms of the possible sources of error encountered in each technique and the accuracy of each method against the others. The residual stress measurement results showed notably significant remnant residual stresses in compact tension specimens that could have an impact on subsequent fracture and fatigue test results. In addition, the measured 2D map of transverse residual stresses, acting normal to the crack plane, exhibited variations in through thickness direction. This implies that the residual stresses in small laboratory samples extracted from large scale weldments should be carefully characterised and taken into account in interpretation of the structural integrity test results. In order to examine the specimen size effects on welding residual stress profiles, the contour method has been applied to a compact tension specimen as well as a large welded mock-up, typical of the weldment used in fabrication of offshore wind monopiles. The measurement results on the welded mock-up showed that the level of damaging tensile residual stress in large-scale mock-ups, hence real size structural welded monopiles, is considerably larger than residual stresses in extracted laboratory samples. This means that the welding residual stresses play an even bigger role in structural integrity assessment of full scale offshore wind turbine monopiles. Finally, a numerical model has been developed to implement the residual stress profile in ABAQUS finite element simulations and calculate the effective stress intensity factor range in the presence of residual stresses for accurate characterisation of fatigue crack growth behaviour, particularly in the near threshold region. The results have shown that when the transverse residual stresses ahead of the crack tip are predominantly compressive, the values of effective stress intensity factor range in compact tension specimens are less than the applied stress intensity factor range by around 10 MPa√m. This indicates that residual stresses play a key role in the fatigue life of welded structures, especially in the near threshold region.
Open Access
Informing parametric risk control policies for operational uncertainties of offshore wind energy assets
(Elsevier, 2019-03-01) Ioannou, Anastasia; Angus, Andrew; Brennan, Feargal
The aim of this paper is to investigate uncertainties present during operation of offshore wind (OW) energy assets with a view to inform risk control policies for hedging of the incurring losses. The parametric framework developed is subsequently applied across a number of different locations in the South East Coast of the UK, so as to demonstrate the effect of weather conditions and resulting downtime on a number of operational Key Performance Indicators (KPIs), such as downtime due to planned and unplanned interventions, wind farm availability, Operation and Maintenance (O&M) costs and power production losses. Higher availability levels were observed in areas closer to shore of the specified region, while the distribution of O&M cost per MWh generated demonstrated a general trade-off of higher power generation in locations farther from shore due to better wind speed profiles and higher O&M costs, as a result of the decreasing vessels accessibility. The proposed methodology aspires to contribute to the development of better-informed risk control policies, through parametrically estimating the probability of exceedance curve of the production losses of an OW farm and indicating appropriate thresholds to be considered, so as not to exceed a maximum level of risk.
Open Access
An integrated approach to microalgae biomass generation and processing
(Cranfield University, 2016-03) Pearce, Matthew William; Brennan, Feargal
Liquid combustible fossil fuel empowers global society, yet is a non-renewable entity with time-constrained limits to supply. Advanced generation biofuel derived from microalgae could feasibly yield more than conventional biofuel crops, utilise non-agricultural land or the sea and remediate atmospheric carbon dioxide and anthropogenic waste. However, technical and economical limits have so far prevented the successful implementation of microalgae biofuels. This thesis exemplifies how apparently disconnected technologies are able to become united in their provision for the growth and processing of microalgae. In so doing, it employs unique experimental methodology which unites inter- disciplinary themes with the proposition to cultivate and process microalgae biomass in a manner which has never been done before. The novelty of this endeavour presents a unique set of challenges, reasoning and results with implications for future creative research and investigation. The philosophical approach to conception and achievement of the laboratory work intercedes with entirely new methodology. Selected examples of such methodology follow. In chapter 3, a newly developed bio-composite gel disk was processed aligning a new design of apparatus for a geotextile puncture resistance test. In chapter 3, a novel formulation for harvesting microalgae is described. In chapter 5, a modified methodology of the preceding chapter is used to investigate seawater ion remediation via ionic and density phase separation. Chapter 6 integrates waste components from 5 different industries, namely dairy farming, anaerobic digestion, brewing, steel slag aggregates and coal power combustion with no previously known unification of such technologies in scientific literature. Chapter 7 assesses the lipid quality of biomass harvested by the novel formulation of chapter 3, before and after exposure to hydrothermal liquefaction. Chapter 8 extrapolates findings from the thesis to define an economic appraisal and suggest a cost saving process.
Open Access
An integrated structural health monitoring approach to composite-based pipeline repair
(Cranfield University, 2011-06) Abd Murad, Mahadi; Brennan, Feargal
One of the most common problems in the structural integrity of industrial pipelines is external corrosion. Composite materials have been accepted as a repair option to restore the original strength of the effected damaged section but the approach to analyse how well the composite repair has influenced the stress and strain distributions between the reinforced steel and the reinforcing composite is yet to be determined. This approach is required since it can increase the confidence level among pipeline operators or manufacturers in terms of the composite repair solution that they have chosen or offered. The aim of this study was to develop an integrated approach that could provide a complete methodology to assess the integrity of pipeline systems, especially in quantifying the level of reinforcement provided by the repair in the elastic working region. This cannot be achieved by the traditional method that only relies on classical mechanics derived from strength of materials and compatibility relations. To fulfil this aim, efforts included the simulation of a fibre glass composite repair system using finite element analysis (ABAQUS) software, design and fabrication of the pressure and temperature test rig, installation of fibre glass composite material, and data acquisition. The arc-shaped notch defect that has never been studied before was chosen in the study of stress concentration and bonding integrity of the composite repair system and further validated experimentally using the test rig. The numerical results of this work demonstrated different stress concentration values as the arc-shaped defect size increased. With a proper test plan, the optimum repair system that contains variable lengths of the arc-shaped defect and different lengths as well as thicknesses of the composite material was achieved in the stress distribution study. The bonding integrity via strain distribution study was further validated experimentally. The experimental pressure test results showed some good agreement, especially in the hoop load transfer and limitations of the simulation works were adequately addressed. The experimental temperature results managed to show how the thermal strain behaved in the anisotropic Triaxial Woven Fibre composite repair system. This approach is useful for a better acquisition of information that enables the diagnosis and prognosis of the pipeline repair system prior to installation on site. The novelty of this technique is in its ability to analyse the load transfer in a composite repair system in a more efficient way which can save time and cost.
Open Access
Integrity of offshore structures
(Cranfield University, 2015-08) Adedipe, Oyewole; Brennan, Feargal
Corrosion and fatigue have been dominant degradation mechanisms in offshore structures, with the combination of the two, known as corrosion fatigue, having amplified effects in structures in the harsh marine environments. Newer types of structure are now being developed for use in highly dynamic, harsh marine environments, particularly for renewable energy applications. However, they have significantly different structural details and design requirements compared to oil and gas structures, due to the magnitude and frequency of operational and environmental loadings acting on the support structures. Therefore, the extent of corrosion assisted fatigue crack growth in these structures needs to be better understood. In this research, fatigue crack growth in S355J2+N steel used for offshore wind monopile fabrications was investigated in air and free corrosion conditions. Tests were conducted on parent, HAZ and weld materials at cyclic load frequencies similar to what is experienced by offshore wind monopile support structures. The seawater used for testing was prepared according to ASTM D1141 specifications and was circulated past the specimens through a purpose designed and built corrosion rig at a rate of 3 l/min, at a temperature of 8-100C and at a pH of 7.78-8.1. A new crack propagation method accompanied by constant amplitude loading was used. Crack growth rates in parent, HAZ and weld materials were significantly accelerated under free corrosion conditions, at all the stress ratios used compared to in air environment. However, in free corrosion conditions, crack growth rates in the parent, HAZ and weld materials were similar, particularly at a lower stress ratio. The results are explained with respect to the interaction of the loading condition, environment and the rate of material removal by corrosion in the weldments. A new model was developed to account for mean stress effects on crack growth rates in air and in seawater, and was found to correlate well with experimental data as well as with the other mean stress models tested.