PhD, EngD and MSc by research theses (SWEE)
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Browsing PhD, EngD and MSc by research theses (SWEE) by Course name "EngD in Renewable Energy Marine Structures (REMS)"
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Item 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, FeargalMarine 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.Item Open Access A framework for planning of offshore wind energy projects based on multi-objective optimisation and multi-criteria decision analysis.(2018-05) Mytilinou, Varvara; Kolios, AthanasiosThe wind industry is determined to lower the costs of producing energy in all phases of the offshore wind project. During 2015–2016, projects achieved a levelized cost of energy (LCOE) of £97 and more recently it was announced that Ørsted guaranteed £57.5/MWh. Significant price increases on structural materials directly impact on larger scale wind projects, the overall cost of turbines, establishing effective supply chains, improving the consent procedures for new developments, governmental mechanisms and support, improving grid connections and finally reducing overall uncertainty and costs etc. The most important decisions at the planning stage of new investment are the selection of a profitable, cost-effective suitable offshore location and a support structure type, which greatly impact on the overall Life Cycle Costs (LCC). This research aims to introduce and apply a scalable framework to reveal and select the optimal offshore location deployment and support structure in Round 3 zones in the UK by considering the interplay of LCC aspects at the planning stage of development. This research produced a portfolio of five studies while developing the framework above. First, a comparative Political Economic Social Technological Legal Environmental (PESTLE) analysis on wind energy was performed. The analysis focused on Europe, Germany, the UK and Greece, where the UK was selected in this research as the world leader in offshore wind energy. Second, three state-of-the-art Multi-Objective Optimisation (MOO) algorithms were employed to discover optimum locations for an offshore wind farm. The 7-objective optimisation problem comprises of some of the most important techno-economic LCC factors that are directly linked to the physical aspects of each site. The results of Non-dominated Sorting Genetic Algorithm (NSGA II), NSGA III and SPEA 2 algorithms follow a similar trend, where NSGA III demonstrated its suitability by revealing more uniform and clear optimum non-dominated solutions, also known as Pareto Front (PF), because of its main design compared to the other optimisers. Based on their frequency of appearance in the PF solutions, Seagreen Alpha, Seagreen Bravo, Teesside C, Teesside D, and the Celtic Array South West Potential development Area were discovered as the most appropriate. Since PF includes solutions from all regions, this provides the developer with the flexibility to accordingly assign costs in different development phases, as required, and to choose whether to invest the available budget on the installation or the maintenance stage of the project. Third, in order to reveal optimum locations for UK Round 3 offshore zones and each zone individually, three different wind farm layouts and four types of turbines were considered in an 8-objective formulation, where five LCC factors are directly linked to the physical aspects and restrictions of each location. NSGA II discovered Moray Firth Eastern Development Area 1, Seagreen Alpha, Hornsea Project One, East Anglia One and Norfolk Boreas in the PF solutions. Although layouts 1 and 2 were mainly selected as optimum solutions, the extreme case (layout 3) also appeared in the PF a few times. All this demonstrates the scalability and effectiveness of the framework. Fourth, the effectiveness of coupling MOO and Multi-Criteria Decision Making (MCDM) methods is demonstrated, so as to select the optimum wind farm Round 3 location in order to help stakeholders with investment decisions. A process on the criteria selection is also introduced, and seven conflicting criteria are considered by using the two variations of Technique for the Order of Preference by Similarity to the Ideal Solution (TOPSIS) in order to rank the optimum locationsthat were discovered by NSGA II. From the prioritisation list, Seagreen Alpha was found as the best option, three times more preferable than Moray Firth Eastern Development Area 1. Fifth, experts‘ opinions were employed in an MCDM process to select the support structure type in an offshore wind farm. For comparison, six deterministic MCDM methods and their stochastic expansion were employed; WSM, WPM, TOPSIS, AHP, ELECTRE I and PROMETHEE I in order to account for uncertainties systematically. It was shown that the methods can relate to each other and can deliver similar results. The jacket and monopile support structures were ranked first in most deterministic and stochastic approaches. Overall, the effectiveness of the introduced research framework to meet the aim of the research is demonstrated. The framework combines a) a prototype techno-economic model for offshore wind farm deployment by using the LCC and geospatial analysis, b) MOO by using NSGA II and c) survey data from real-world experts within MCDM by using a deterministic and stochastic version of TOPSIS.Item 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, FeargalThe 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.Item Open Access Material pre-straining effects on fatigue and fracture structure behaviour of offshore wind monopile structures.(2021-10) Anandavijayan, Satya Gayathri; Mehmanparast, Ali; Brennan, FeargalAn important issue to be considered in the structural integrity assessment of offshore wind monopile structures is the influence of material pre-straining on the fracture toughnessnd fatigue crack initiation and growth behaviour of the material. Material pre-straining can be introduced into monopile cans during the three-roll bending fabrication process. Literature has shown that in certain steels, this pre-straining effect can affect the behaviour of the material, causing it to behave differently to the parent material. A finite element model has been developed to predict the level of plastic strain present in the material after undergoing the three-roll bending process, with a parametric study conducted to determine the effects of fabrication and geometry factors on plastic strain, which may potentially aid fabricators to control the behaviour of the material when in operation. Thus, the main aim of this project is to investigate the effects of material pre-conditioning on the mechanical response, fatigue, and fracture behaviour of an offshore structural steel. S355 steel which is widely used in offshore wind industry for foundation structures such as monopiles has been used in the experimental test programme involving specimens pre-tensioned to 0%, 5% and 10% to investigate the potentially beneficial or detrimental effects. In addition to this, an experimental study on the effect of fatigue damage on the mechanical properties of S355, across varying levels of pre-straining, has been conducted. The results from this study have been employed in the remaining life analysis of monopile structures, such as failure assessment diagrams, to determine appropriate inspection intervals and recommendations necessary to avoid catastrophic failure.Item Open Access Risk-based methods for the valuation and planning of sustainable energy assets.(2018-08) Ioannou, Anastasia; Brennan, Feargal; Angus, AndrewThis research project aims to develop and apply appropriate methods dealing with risk and uncertainty at a technology and energy system level providing decision support to the various stakeholders involved in the planning, development and operation of sustainable energy investments. The thesis comprises a portfolio of research activities fulfilling the set research objectives. Outcomes of this research portfolio have been either published or are under the peer review process. More specifically, following a systematic literature review to identify the state-of- the-art in risk-based methods for sustainable energy systems planning and feasibility studies, a cluster analysis was applied based on data from existing offshore wind energy installations in the UK, to distinguish investment strategies followed by equity investors. This study has identified three distinct clusters of investors, namely the late entry, pre-commissioning and build-operate-transfer investors. Subsequently, a high-fidelity lifecycle techno economic model was developed allowing for the temporal valuation of a renewable energy investment. This integrated model has allowed for a set of parametric equations to be developed through appropriate selection of approximation models linking global design parameters to key performance indicators. Furthermore, a stochastic extension of the financial appraisal model has allowed for a transition from the conventional deterministic terminology to a stochastic one, assigning confidence levels to key performance indicators (KPIs). Additionally, the development of a purpose-specific tool for the evaluation of the operational phase KPIs, such as the availability, operating cost and energy production losses due to planned and unplanned maintenance has allowed for the development of better-informed risk control policies. Finally, having analysed uncertainties at a technology level, a stochastic optimisation framework was developed for deriving optimal national power generation technology mixes taking into account uncertainties for a series of scenarios linked to national energy strategies through appropriate constraints in the analysis.Item Open Access Structural fatigue assessment and optimisation of offshore wind turbine jacket foundations.(Cranfield University, 2023-08) Marjan, Ali; Huang, Luofeng; Hart, PhilOffshore Wind Turbine (OWT) is an expensive type of renewable energy system, and there is a continuous effort to lower the capital and operational costs. Jacket foundations are increasingly used in offshore wind due to their relatively light weight and adaptability in deep waters. However, the contemporary jacket designs are based on conservative practices from the oil and gas industry. There is still substantial room to optimise the jacket designs for offshore wind usage. This research aims to generate innovative jacket designs by applying a topology optimisation algorithm. The research demonstrates the use of advanced computational techniques to improve existing designs by enhancing structural integrity and fatigue life whilst reducing the mass. The research presents a comprehensive investigation of different parameters and design modifications that impact the design life of an existing jacket. An OC4 jacket foundation is employed, modelled in industrial software from DNV, and transformed into a super element model. The time-series loads obtained from Bladed are used to assess fatigue damages experienced during the foundation's service life. Furthermore, the research presents a topology optimisation method to retrofit an existing jacket foundation design by finding the optimum load path on the structure to enhance fatigue life and lower costs. The jacket's structural optimisation is performed by considering its dynamic response while adhering to the relevant international design standards. In particular, time-domain fatigue simulations were performed to assess the structural integrity of the topology- optimised jacket for the first time. As a result, a range of optimised models with various thickness and diameter options are presented, which are shown to be rational and verify the optimisation procedure. The research contributes a unique integrated topology optimisation framework, dynamic analysis, and time-domain fatigue simulations using industry-standard software tools, and achieved a mass reduction of 35.2% and simultaneously realized a 37.2% better fatigue life compared to the baseline model. The overall environmental load calculation, optimisation procedure and results provide useful practicalities for designing offshore wind turbine foundations and potentially facilitate the relevant industry's structural integrity and cost reduction.