1,721,074 research outputs found
Improve the Understanding of Uncertainties in Numerical Analysis of Moored Floating Wave Energy Converters
No full textThe wave energy industry, still in its infancy compared to similar activities offshore, must look
to the oil and gas industry for guide lines on design criteria for survival, safety and operational
optimisation for installations at sea. Numerical analysis tools for prediction of the response of
floating moored structures have become an important part of the design task for the offshore
industry offering a low cost and low risk option compared to scale tank testing. However, rather
than having only a task of station keeping and survival, the moorings for a wave energy converters
(WECs) would also be required to provide the ability of not adversely affecting the power capture
task. The main aim of this work is to gain an understanding and reduce the uncertainties in the
numerical modelling of WECs.
Experimental work designed and performed under the HydraLab III project of which the author
was a member were used to evaluate the response characteristics of a 1:20 scale “generic WEC”
device with a 3 point mooring system. The investigation was enhanced through further tests
implemented by the author at Heriot-Watt wave tank using a single WEC device. The outcomes
from these experiments were used to aid in the implementation of the aim identified above.
Two numerical model categories were set up to understand the uncertainties apparent to the
mooring simulations. The first category included only the calculation of the mooring line response
using experimental data to inform the motion of the floating body. The second category included
the motion response of the floating body coupling the complex behaviour to the moored system.
The mooring tension results for the first category shows an error between the numerical prediction
and the experimental results up to 16 times that of the experimental value. This was mainly during
slack conditions where the mooring line tension was lower than the pretension in the line at still
water. During the higher tension events the average error was 26%. For the second category it was
found that the numerical predictions of the WEC motion response in six degree of freedom (6DOF)
were generally over predicted. The tension predictions for the coupled simulations identified an
error of between 1.4 and 4.5%.
The work presented here contributed to the understanding of uncertainties in numerical simu-
lations for WEC mooring designs. The disparity between the simulation and experimental results
re-enforced the requirement for a better understanding of highly dynamic responding moored cou-
pled systems. From this work it is clear that the numerical models used to approximate the
response of moored WECs could provide a good first design step. Whilst this work contributed
to the understanding of uncertainties and consequently reduced some of these, further work is rec-
ommended in chapter 6 to investigate the definition of some of the mechanical and hydrodynamic
properties of the mooring line. It is also suggested that external functions should be included
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that would allow to model the coupled effect of Power-Take-Off (PTO) system. It is intended to
conduct future work deriving a fully dynamic mooring simulation including the effects of PTO
Installation Optimisation for Marine Energy Converters to Inform the Designation Process
Full text linkWhilst a number of methods exist for the analysis of site availability and weather downtime via metocean exceedance, there is little available for the detailed analysis of holistic marine energy installation projects. Given the magnitude of expenditure relating to the installation phase of marine energy extraction it is essential that significant cost reduction is achieved in this area.
This thesis presents methods for the analysis of marine operations, considering not just the at site work but the project as a whole. The methods developed consider multiple facets of installation in a geo-spatially diverse environment and utilize multiple resources, for example vessels. Consideration of not only the efficiency of work at site, but also the accessibility of the site due to vessel station keeping, mooring and transit limits is included.
By considering the project in its entirety work may be scheduled in a realistic manner; including simultaneous operations and at site transit to any of multiple working locations. These methods, packaged as a whole, represent a valuable new tool for utilisation in this area.
Novel application of the methods developed is demonstrated and highlights the value, importance and power of this type of analysis. Two marine energy installations are considered as case studies; the Wave Hub in south west England, and a tidal installation at the European Marine Energy Centre in Orkney. These applications demonstrate the knowledge which may be gained and, explicitly in the latter case, the significant cost reductions which may be achieved through the essential optimisation of the installation operations using this newly developed analysis tool.Mojo Maritime Ltd
Understanding Opportunities for South West Businesses to Diversify into the Marine Renewable Energy Supply Chain
No full textIn 2010 it was reported that the south west of England had an abundance of natural marine energy resources with a capacity to deliver 9.2 GW of energy, this combed with the diverse marine skills, services and facilities available in the south west supply chain, gives the south west of England a unique and significant offer to the MRE sector.
This dissertation outlines the opportunities for south west companies to engage with the marine renewable energy, by defining the requirements of the sector.
Potential supply chain company diversification methodology is outlined in a created model, and identification of methods of engagement with the sector was undertaken via the use of a questionnaire; with existing south west based marine renewable energy supply chain companies.
The dissertation is concluded with recommendations for potential supply chain companies in the south west of England wishing to engage with the MRE sector in the following categories: diversification, initial engagement and market entry
A Novel Mooring Tether for Highly Dynamic Offshore Applications
No full textPlease note the following corrections to this text:
Abstract, page 3, para 4: 72 kN should read 172 kN
Page 134, Table 6.7: 74 should read 174, 72 should read 172The mooring of vessels and other floating bodies at sea, such as offshore platforms has necessitated the development of specialised moorings technology. The marine renewable energy (MRE) sector is now at a stage in its development whereby floating devices are adding new challenges to the moorings industries. Floating MRE devices are smaller than, for instance offshore platforms, and are usually targeted for deployment in highly energetic environments. The extreme conditions and the highly dynamic response of an MRE device present challenges in terms of peak loading within the mooring system itself and load transfer to the floating body.
Compliant mooring systems provide advantages by reducing the peak loads and fibre ropes are an important asset in achieving such compliance. However, the extent to which existing fibre ropes can safely extend axially to provide compliance is insufficient and is strongly associated to the minimum breaking load (MBL) of the rope.
A novel fibre rope mooring tether is presented here that provides advantages over existing ropes. The tether employs a hollow fibre rope containing an elastomeric core, this mechanism de-coupling the extension properties from the strength of the line. The load path is carried through the polyester rope which is terminated conventionally by eye splices, thus minimising any new risks to reliability.
Very low axial stiffness is achieved and is shown to be selectable within limits. For comparison, the prototype tether’s MBL of 222 kN is assigned to polyester and Nylon reference ropes. The axial stiifness of these ropes are 590 kN and 463 kN respectively when measured by a secant between the origin and 30% MBL; the novel tether displays an axial stiffness of 72 kN by the same method. This enables the novel tether to achieve more than two and a half times the extension of a comparable Nylon rope in its working range. Numerical modelling of a moored installation demonstrates a threefold reduction in peak load magnitude compared to the existing Nylon rope solution.
The tether exhibits two distinct stages of extension, the first having very low axial stiffness. It is demonstrated that the extent of this soft phase can be selected by design and that this might add another useful element of control to moorings design work
Interdisciplinary study into the effect of a marine renewable energy testing facility on the underwater sound in Falmouth Bay
No full textWave energy has the potential to contribute considerably to the UK's energy mix. The marine environment is already subjected to many anthropogenic pressures. There is a need to develop the industry as sustainably as possible. A key
concern is the potential for underwater noise to affect marine life.
A wave energy converter (WEC; BOLT Lifesaver, Fred Olsen Ltd.) was deployed at the Falmouth Bay marine renewable energy test site (FaBTest). The underwater sound levels were recorded at this site for a two week baseline period, a five-day installation period and intermittent operational and non-operational activity from March 2012 - November 2013.
The recordings were also analysed for the Marine Strategy Framework Directive (MSFD) indicator third octave bands of 63-Hz and 125-Hz for shipping noise.
The median and modal sound levels in Falmouth Bay were found to be loudest in the frequency range 100 Hz - 1 kHz and affected by local shipping activity.
During installation activity, the sound levels were louder at all frequencies recorded as compared to similar periods with no installation activity, with a mean difference of 6.9 dB Hz-1 in the range 10 Hz to 48 kHz. Long term marine renewable energy construction projects may affect the MSFD indicator bands.
There was little overall difference in the average sound levels for the operational and non-operational periods as the median PSD levels were louder by an average of 0.04 dB Hz-1 during the operational activity as compared to the non-operational activity.
The results of this study indicate that the effect of a single WEC device on the overall sound levels in Falmouth Bay is relatively low considering the substantial presence of shipping in the area. However, in the immediate vicinity of the device
(<200 m), the sound produced was found to be of significance to marine animals. It therefore requires considering in future deployments, particularly at a site with little anthropogenic activity.European Social Fun
Analysis of Highly Dynamic Mooring Systems: Peak Mooring Loads in Realistic Sea Conditions
No full textMarine Renewable Energy (MRE) is a promising source of energy for the future. However, it is still under development and many challenges need to be overcome to develop competitive solutions. While the design of the station keeping system of traditional offshore oil and gas structures is driven mainly by their low frequency motions, MRE devices are installed at nearshore locations and move dynamically.
Because of these criteria, MRE mooring systems require novel mooring systems and associated standards. MRE mooring standards need to take into account the highly dynamic behaviour of these systems, which can lead to large mooring loads. The nature of these loads needs to be investigated to improve the confidence in mooring design and to improve cost-effectiveness. The aim of this thesis is to develop the understanding of peak mooring loads on highly dynamic mooring systems, in particular, the environmental conditions associated with the loads. In addition, preliminary research into the response of the mooring systems to environmental conditions is presented.
Both field tests and tank tests have been conducted. Field tests give insight into the behaviour of a dynamic mooring system in real sea conditions. Measuring the mooring loads and the environmental conditions - wave, and current if available – for several months, a methodology has been developed to detect peak mooring loads and identify the associated environmental conditions in order to compare them with the environmental conditions recorded throughout the field tests. The principal finding is that peak mooring loads occur for sea states with large but not always the highest significant wave height HS. The understanding of the effect of tidal conditions on peak mooring loads requires further work.
A tank test of a dynamic mooring system in moderate sea states has been conducted to observe the dynamic behaviour of the mooring system. Tank tests enable detailed observations of the dynamic behaviour of a system in a well controlled environment and allow the calibration of a numerical model. The model can be used to investigate separate physical parameters.
The results from this thesis will assist in the development of specific standards for MRE mooring systems. These standards are essential for the evolution of the MRE industry
Enhancing wave energy developments through mooring system reliability assessment
Full text linkWave energy generation is a promising renewable energy source but it faces certain challenges before it can become commercially viable. In comparison to conventional energy generation it is expensive, furthermore it has been plagued by reliability challenges due to the harsh operating demands of the marine environment.
This Thesis investigates the reliability of wave energy devices, and specifically focuses on mooring system reliability. Two major themes are developed: Firstly, an assessment is conducted on a conventional mooring component, reviewing safety factors suggested in mooring system design guidelines and investigating whether there is a potential to reduce these safety factors (and in so doing, reduce system costs). Numerical modelling, laboratory testing and field testing demonstrate that excessively large safety factors are published in design guidance for static loading scenarios. However, when considering fatigue loading regimes (a critical aspect of wave energy generation), the proposed safety factors are found to be appropriate. In fatigue design, the importance of selecting an appropriate stress concentration factor for use with generic S-N curves is highlighted. These findings indicate the publication of additional stress concentration factors and a standard approach for mean stress adjustment would be a valuable addition to mooring system design guidance for fatigue.
The second theme introduces a novel mooring component, The Exeter Tether, designed to reduce mooring loads and thus reduce system costs. The introduction of any novel technology brings new reliability considerations, and a reliability assessment of the tether and sub-components is presented in this Thesis. Following a failure modes and effects analysis, a bespoke range of physical tests is developed to investigate reliability concerns unique to this novel component. Laboratory testing of the tether assembly shows promising fatigue performance, however field trials highlight concerns regarding bio-fouling and marine debris ingress. Sub-component testing of the EPDM (Ethylene propylene diene monomer) polymer core suggests an increase in material stiffness with both marine ageing and repeated compression cycles. This finding supports results from assembly trials in the laboratory and at sea, where tether assembly dynamic axial stiffness is observed to increase over time. The overarching design philosophy behind the Exeter Tether is to reduce mooring system loads, so establishing the `worked' operating profile of the tether is crucial for the design intentions to be realised without compromising the reliability of the overall mooring system. Trials on the anti-friction membrane establish optimum performance when using two layers of UHMWPE (Ultra high molecular weight polyethylene) tape. Further areas requiring research are highlighted, and suggestions are made to improve the reliability of future design iterations of The Exeter Tether.
The two reliability approaches presented demonstrate the potential for cost reduction in mooring system design and highlight the importance of physical component testing, both in the field and in laboratory conditions, to optimise component design whilst ensuring overall system reliability.The Engineering and Physical Sciences Research Council (EPSRC) through the SUPERGEN UKCMER programme.The European Commission through the MARINET programme also funded two weeks test work to facilitate some of the work presented in this Thesis
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