1,721,134 research outputs found
Structural reliability assessment of complex offshore structures based on non-intrusive stochastic methods
Get PDFKolios, Athanasios - Associate SupervisorOffshore Wind Turbines (OWTs) are deployed in harsh environments often characterised
by stochastic loads and resistance properties. It becomes necessary to propose an
accurate and efficient approach for the assessment of uncertainties in material properties
and operating environments. Structural Reliability Assessment (SRA) as a form of
uncertainty analysis is a useful tool in the design of structures because it can directly
quantify how uncertainty about input parameters can affect structural performance. First,
this thesis developed a novel non-intrusive SRA method for an OWT jacket structure
which maps the response of the structure through a finite number of simulations to
develop a response surface and then employ First Order Reliability Methods (FORM) to
evaluate the reliability index. This method was validated against a commercial FEA
package (DesignXplorer© from ANSYS) which employs direct simulations to predict the
probability of failure. The method developed was used in performing stochastic sensitivity
analysis of the variables imposed on the OWT support structure. The results from this
study, reveals that the uncertainties in the design wind speed is a design driving factor
and the hydrodynamic load effects are secondary to this, for the ultimate (ULS), and
fatigue limit states (FLS), among others. Second, the SRA of the same structure
subjected to pitting corrosion-fatigue was assessed using a damage tolerance modelling
approach. The non-intrusive formulation in this study used an Artificial Neural Network
(ANN) response surface modelling technique instead of the Multivariate (Quadratic)
Polynomial Regression (MPR) method used previously apart from the FEA to represent
the crack propagation regimes. 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, among others. The benefit of this approach is that it allows for
high fidelity computational tools to be employed for the analysis, hence extending its
applicability to various specialist engineering problems through the advanced modelling
techniques.PhD in Energy and Powe
A framework for planning of offshore wind energy projects based on multi-objective optimisation and multi-criteria decision analysis.
Get PDFThe 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.EngD in Renewable Energy Marine Structures (REMS
A damage detection and location scheme for offshore wind turbine jacket structures based on global modal properties
Get PDFAbstract Structural failures of offshore wind substructures might be less likely than failures of other equipments of the offshore wind turbines, but they pose a high risk due to the possibility of catastrophic consequences. Significant costs are linked to offshore operations, like inspections and maintenance activities, thus remote monitoring shows promise for a cost-efficient structural integrity management. This work aims to investigate the feasibility of a two-level detection, in terms of anomaly identification and location, in the jacket support structure of an offshore wind turbine. A monitoring scheme is suggested by basing the detection on a database of simulated modal properties of the structure for different failure scenarios. The detection model identifies the correct anomaly based on three types of modal indicators, namely, natural frequency, the modal assurance criterion between mode shapes, and the modal flexibility variation. The supervised Fisher's linear discriminant analysis is applied to transform the modal indicators to maximize the separability of several scenarios. A fuzzy clustering algorithm is then trained to predict the membership of new data to each of the scenarios in the database. In a case study, extreme scour phenomena and jacket members' integrity loss are simulated, together with variations of the structural dynamics for environmental and operating conditions. Cross-validation is used to select the best hyperparameters, and the effectiveness of the clustering is validated with slight variations of the environmental conditions. The results prove that it is feasible to detect and locate the simulated scenarios via the global monitoring of an offshore wind jacket structure
Probabilistic modelling of geotechnical conditions for offshore wind turbine support structures
Get PDFThe geotechnical conditions of the soil can fluctuate greatly across the wind
farm. This is an issue since geotechnical modelling is the base of the structural
design of an offshore wind farm, and the efficient installation of the wind
turbines depends on its accuracy. This paper deals with the characterization of
the seabed, predicting the soil properties over the total affected area by a wind
farm, with the challenge to reduce the required data samples in the site
investigation under the number of installed wind turbines, to reduce its cost.
It is compared the prediction outcome from two different interpolation methods,
kriging and radial basis function, assessing their accuracy by the Mean-Squared
Error and the Goodness-of-Prediction Estimate, as well as with a visual
examination of their mapping; obtaining higher accuracy for radial basis function
and reducing to half the required sample points, from the initial value of installed
wind turbines.
In a second stage it is studied the soil effect over the foundation, analyzing the
results from a FEA, where different geometries of the structure are compared
submitted to different load cases to check its limit states. Those results show
that the foundation cost can increase four times due to the soil conditions,
taking into account only the steel volume, and demonstrating how important is
the soil characterization in the foundation design, as it gives the chance to
relocate those wind turbines that require more expensive foundations
Environmental impact assessment and optimisation of commercial aviation
Get PDFThe 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
Fatigue and fracture mechanics of offshore wind turbine support structures
Get PDFWind 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
Investigation of the reliability deterioration of ageing marine structures
Get PDFIn the present work, an investigation of the fatigue life benefits emerging from
fillet weld geometries optimization has been carried out.
At first, an introduction to ageing mechanisms, corrosion and especially fatigue,
acting on operating marine structures has been made. Residual stresses at
weld toes, stress modes, and types, geometrical factors (weld angle, toe radius,
leg length), welding techniques selected, post-welding treatment and plate‟s
material are some of the principal factors affecting the fatigue life of a fillet weld
joint.
Especially, the accuracy of various approaches in fatigue life estimation of
specific geometries under pre-set types and levels of stress is studied. It is
evident so far that even the notch stress concept is the most accurate method
based on S-N curves, the Fracture Mechanics approach can offer more
accurate solutions of a crack development through the material. Towards this, a
literature review on crack evolution aspects in welded and non-welded plates
under bending and tension was performed; substantial parameters were
determined and finally implemented in the LEFM model which was used for the
simulation purposes of Chapter 6.
As far as the crack aspect ratio evolution is concerned, an extensive reference
is available in literature since many researchers have investigated its
contribution to the determination of geometrical paths, commonly known as
“Preferred Propagation Paths”. Their significance is related with our ability to
determine accurate SIF solutions leading to precise fatigue life estimations.
A typical fillet weld joint 2-D model has been developed in CAE Abaqus
software and a Finite Element Analysis of subject T-profile has been carried out.
Through this analysis, the fillet weld angle, the weld leg length, the weld toe
curvature radio ρ and the carrying load plate thickness are examined for their
impacts on the maximum surface stress. Finally, a number of stress mitigating
measures are proposed and their effects are analyzed.
Undoubtedly, the notch stress concept today is gradually gaining more and
more acceptance among other fatigue analysis practices, hence the need for an
estimation of the actual surface stresses along fillet weld toes, has become
imperative. Towards this, different 2-D geometries are tested against stress
concentration factors developed at weld toes, which are calculated on the basis
of maximum in-plane principal stresses over nominal stresses in mode I pure
bending and pure tension respectively. Moreover, validation with corresponding
results from literature is provided. Finally, three different concepts for reducing
the maximum surface stresses are presented. The first one proposes grinding
of the weld toe area and formulation of an artificial U-notch or a part- circular
profile. The second one applies to non-penetrating welds and assumes the
existence of a root gap of a specific geometry which is related to the fatigue life
and stress concentration factor of the fillet weld joint. Last but not least, the
relatively recent concept of the variable radius notch is discussed, even though
it is applicable mostly to notched bodies, not weld joints.
Afterwards, a Linear Elastic Fracture Mechanics analysis of reference 2D fillet
weld model is demonstrated. A number of geometrical parameters considered
at previous stage for their impact on surface Stress Concentration levels at the
weld toe region, have been correlated to fatigue life benefits in terms of
increased number of stress cycles till failure.
An extensive analysis of 9 different T-butt weld joint geometries has been
provided in order to investigate how positively a possible SCF reduction can
affect the fatigue life of a weld joint. Essential geometric variations (weld angle,
length, toe radius, root slot) were considered in the 2D model. All calculated
benefits both in pure bending and pure tension cases have been reported
accordingly.
Based on a linear interpolation of the points scatter (SCF, N-cycles) both in
banding and tension, it was observed that a surface stress mitigation of 1%
could lead to 1,33 up to 2,5% fatigue life benefit in the range of SCF=2 – 2,5. It
is evident so far that the geometrical optimization of a weld joint in respect of
notch stress mitigation can be a powerful tool both in shipbuilding and
maintenance practice in the future. However, technically wise their application
may incur high initial costs of improved tools of welding and post welding
treatment and robots even though it would consist a cost effective solution in a
medium/long term basis.
Finally, the above process is followed by a reliability analysis of the most critical
geometrical parameters affecting the fatigue life of a fillet weld joint. Reliability
assessment results concerning medium, high and low cycle fatigue are provided
and a comparative analysis of each factor‟s impact on fatigue life has been
carried out
Multi criteria risk analysis of a subsea BOP system
Get PDFThe Subsea blowout preventer (BOP) which is latched to a subsea wellhead is
one of several barriers in the well to prevent kicks and blowouts and it is the
most important and critical equipment, as it becomes the last line of protection
against blowout. The BOP system used in Subsea drilling operations is
considered a Safety – Critical System, with a high severity consequence
following its failure. Following past offshore blowout incidents such as the most
recent Macondo in the Gulf of Mexico, there have been investigations, research,
and improvements sought for improved understanding of the BOP system and
its operation. This informs the need for a systematic re-evaluation of the Subsea
BOP system to understand its associated risk and reliability and identify critical
areas/aspects/components.
Different risk analysis techniques were surveyed and the Failure modes effect
and criticality analysis (FMECA) selected to be used to drive the study in this
thesis. This is due to it being a simple proven cost effective process that can
add value to the understanding of the behaviours and properties of a system,
component, software, function or other. The output of the FMECA can be used
to inform or support other key engineering tasks such as redesigning, enhanced
qualification and testing activity or maintenance for greater inherent reliability
and reduced risk potential. This thesis underscores the application of the
FMECA technique to critique associated risk of the Subsea BOP system.
System Functional diagrams was developed with boundaries defined, a FMECA
were carried out and an initial select list of critical component failure modes
identified. The limitations surrounding the confidence of the FMECA failure
modes ranking outcome based on Risk priority number (RPN) is presented and
potential variations in risk interpretation are discussed.
The main contribution in this thesis is an innovative framework utilising
Multicriteria decision making (MCDA) analysis techniques with consideration of
fuzzy interval data is applied to the Subsea BOP system critical failure modes
from the FMECA analysis. It utilised nine criticality assessment criteria deduced
from expert consultation to obtain a more reliable ranking of failure modes. The MCDA techniques applied includes the technique for order of Preference for
similarity to the Ideal Solution (TOPSIS), Fuzzy TOPSIS, TOPSIS with interval
data, and Preference Ranking Organization Method for Enrichment of
Evaluations (PROMETHEE). The outcome of the Multi-criteria analysis of the
BOP system clearly shows failures of the Wellhead connector, LMRP hydraulic
connector and Control system related failure as the Top 3 most critical failure
with respect to a well control. The critical failure mode and components
outcome from the analysis in this thesis is validated using failure data from
industry database and a sensitivity analysis carried out. The importance of
maintenance, testing and redundancy to the BOP system criticality was
established by the sensitivity analysis. The potential for MCDA to be used for
more specific analysis of criteria for a technology was demonstrated.
Improper maintenance, inspection, testing (functional and pressure) are critical
to the BOP system performance and sustenance of a high reliability level.
Material selection and performance of components (seals, flanges, packers,
bolts, mechanical body housings) relative to use environment and operational
conditions is fundamental to avoiding failure mechanisms occurrence. Also
worthy of notice is the contribution of personnel and organisations (by way of
procedures to robustness and verification structure to ensure standard expected
practices/rules are followed) to failures as seen in the root cause discussion.
OEMs, operators and drilling contractors to periodically review operation
scenarios relative to BOP system product design through the use of a Failure
reporting analysis and corrective action system. This can improve design of
monitoring systems, informs requirement for re-qualification of technology
and/or next generation designs. Operations personnel are to correctly log in
failures in these systems, and responsible Authority to ensure root cause
analysis is done to uncover underlying issue initiating and driving failures
Life cycle assessment of composites and aluminium use in aircraft systems
Get PDFAs a consequence of the gradually expanding aviation network, civil aircrafts
are occupying an increasingly high proportion of the transport industry. Air
transport now dominates the intercity rapid transit, long-distance passenger
transport, international passenger and freight transport, and specific regional
transport, advantaged as it is by fast, convenient, comfortable and safe options.
Nevertheless, the potential adverse impact on the environment of air transport,
specifically, in the case of this research, the pollutants generated during aircraft
production remain a concern.
Using the A319 as the main research object, this thesis will conduct a life cycle
assessment research about its environmental impact. Moreover, it will focus on
the impact brought by the application of composite materials to the entire life
cycle environmental influence of the aircraft, particularly the material production
and disposal process. At the same time, a contrast with the B737-800 aircraft
will be made due to their different composite material use rate.
Firstly, the inventory list is formed by collecting data about the weight and
material of every component in the aircraft, the input and output information of
the composite material manufacturing process, the disposal situation of the
aircraft and the treatment of composite material. Secondly, the impact
assessment of the aircraft is conducted to examine their environmental
influence. During the assessment, each life stage and the whole life cycle of the
aircrafts is assessed, and a comparison between these two aircraft types is
made. Finally, according to the impact assessment result, the environment load
increase brought by the manufacturing of composite material and the decrease
of the environment impact due to the weight reduction character of composite
material is calculated and compared.
From this research, the conclusion that the use of composite material has a
positive effect on decreasing the environmental impact of the whole life cycle of
the aircraft is obtained. This will enable aircraft manufacturers to target these
reas for improvement, to produce more comfortable, environment friendly and
market competitive aircraft
Carbon dioxide storage in the UK southern north sea: experimental and numerical analysis
Get PDFThis thesis contributes to the significant portfolio of research on carbon capture and storage (CCS) in general, and the potential for CO₂ storage with impurities within the UK Southern North Sea (UKSNS) to meet the global greenhouse gas emission reduction targets. First, this thesis extensively reviews the current developments in carbon dioxide storage, highlighting major options for CO₂ sequestration, storage site evaluation criteria, behaviour of CO₂ in the reservoir, methodologies for estimating storage capacity, appraisal of the major storage projects, and a projection of the future outlook for CO₂ storage. The review draws attention to the fact that although a high-quality knowledge base has been developed through CCS research, the main hinderance to CO₂ storage deployment is associated with public acceptability of the technology. Second, this thesis involves laboratory experimental investigation of the effect of impure CO₂ on reservoir grain size distributions and permeability using rock samples from the Bunter saline aquifer. The thesis shows that the presence of impurities in the CO₂ stream can affect the grain size distribution and fluid transmissivity. Third, this thesis uses numerical modelling to evaluate the effect of impure CO₂ on reservoir performance with a case study from the Bunter saline aquifer. The results show that depending on the impurities present in the CO₂ stream, the limits of stability during storage operations in saline aquifer varies, however, the variation does not affect reservoir performance negatively during long-term injection and storage
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