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Extreme Loads on the Mooring Lines and Survivability Mode for the Wave Dragon Wave Energy Converter
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... Results showed that by adopting such a survivability mode, extreme forces in the main mooring line can be reduced in the order of 20%. A correct floating position has also shown to be relevant for the force-reduction, as by ensuring the device is not trimmed to the back extreme loads could also be reduced by a similar percentage [6]. ...
... Pre-tension provided by the rear mooring line is not accounted in the given figures.Figure 6 shows that by using a more compliant system a reduction in the extreme mooring tension can be achieved. In the case of rigid moorings a reduction in the extreme mooring tension can be achieved also by decreasing the floating level, as was already shown in previous tests where the mooring stiffness was comparable to k strong used here [6]. This behavior, referred to as survivability mode, brings the opportunity for a reduction of the design requirements (and capital cost) of the mooring system with virtually no added cost, but it loses effectiveness as the compliance of the mooring system increase. ...
... On the other hand, the survivability mode is relevant only for cases in which it has not been possible to provide the mooring system with a sufficiently large compliance already since the design stage (e.g., due to limited water depth at the deployment location). In these conditions it allows for average reductions in the mooring tensions in the order of 15%, being very safe to put in place and maintain as it is a passive control strategy Previous findings, based on tests with a comparatively scaled mooring stiffness of 133 N/m (even less compliant than k strong used in these study), have shown that in these conditions reductions in the order of 20% are possible by adopting a survivability mode [6]. The adoption of a further negative trim position has also shown to be efficient only in the case of low compliance. ...
The paper presents the results of an experimental study identifying the response of a 1.5 MW Wave Dragon to extreme conditions typical of the DanWEC test center. The best strategies allowing for a reduction in the extreme mooring tension have also been investigated, showing that this is possible by increasing the surge natural period of the system. The most efficient strategy in doing this is to provide the mooring system with a large horizontal compliance (typically in the order of 100 s), which shall be therefore assumed as design configuration. If this is not possible, it can also be partly achieved by lowering the floating level to a minimum (survivability mode) and by adopting a negative trim position. The adoption of the design configuration would determine in a 100-year storm extreme mooring tensions in the order of 0.9 MN, 65% lower than the worst case experienced in the worst case configuration. At the same time it would lead to a reduction in the extreme motion response, resulting in heave and pitch oscillation heights of 7 m and 19° and surge excursion of 12 m. Future work will numerically identify mooring configurations that could provide the desired compliance.
... The occurrence of extreme, or peak, loads on WEC mooring systems must be carefully evaluated, to ensure a robust and efficient mooring design [213]. One of the main challenges, for a commercially successful WEC, is to ensure survivability in extreme conditions, at a reasonable cost [214]. Calculating the maximum tension in the mooring system requires a dynamic analysis; however, historically, quasi-static mooring analysis is used, and phenomena affecting the maximum line tension, that are neglected by this modelling procedure, were accounted for by an overall safety factor, with a typical value of three, for operational conditions, and two for survival conditions [101]. ...
... The objective of minimising mooring fatigue using control strategies is similar to the work of Ferri et al. [228] and Tom et al. [229], who investigate minimising structural fatigue of a WEC via control. Parmeggiani et al. [214] investigate using control strategies to reduce the peak mooring loads in extreme wave conditions, and presents a method which is able to reduce the extreme loads in the main mooring line by 20-30% in wave conditions over a 100 year return period; however, this is an exclusively experimental investigation and no MMs are used. ...
Mathematical analysis is an essential tool for the successful development and operation of wave energy converters (WECs). Mathematical models of moorings systems are therefore a requisite in the overall techno-economic design and operation of floating WECs. Mooring models (MMs) can be applied to a range of areas, such as WEC simulation, performance evaluation and optimisation, control design and implementation, extreme load calculation, mooring line fatigue life evaluation, mooring design, and array layout optimisation. The mathematical modelling of mooring systems is a venture from physics to numerics, and as such, there are a broad range of details to consider when applying MMs to WEC analysis. A large body of work exists on MMs, developed within other related ocean engineering fields, due to the common requirement of mooring floating bodies, such as vessels and offshore oil and gas platforms. This paper reviews the mathematical modelling of the mooring systems for WECs, detailing the relevant material developed in other offshore industries and presenting the published usage of MMs for WEC analysis.
... Moreover, sea waves involve low-frequency and alternating high forces, making it necessary to use strong structures and heavy conversion systems and therefore increasing the technology costs. Other problems to be faced are related to corrosion of components in contact with the sea water, possible leakage of oil (if hydraulic conversion systems are used), survivability in case of extreme events, maintenance, and environmental and visual impacts [12][13][14][15]. ...
... Adding (13) in (9) ...
The ISWEC (inertial sea wave energy converter) is presented, its control problems are stated, and an optimal control strategy is introduced. As the aim of the device is energy conversion, the mean absorbed power by ISWEC is calculated for a plane 2D irregular sea state. The response of the WEC (wave energy converter) is driven by the sea-surface elevation, which is modeled by a stationary and homogeneous zero mean Gaussian stochastic process. System equations are linearized thus simplifying the numerical model of the device. The resulting response is obtained as the output of the coupled mechanic-hydrodynamic model of the device. A stochastic suboptimal controller, derived from optimal control theory, is defined and applied to ISWEC. Results of this approach have been compared with the ones obtained with a linear spring-damper controller, highlighting the capability to obtain a higher value of mean extracted power despite higher power peaks.
... They are also subjected to other oceanic forces such as wind and tide. Survivability of the mooring system is essential, as failure could not only be very costly, but could cause harm to both animals and humans alike, and be a serious hazard to shipping [8][9][10][11][12][13][14][15]. ...
... The waves repeatedly compress air trapped within the ducts which is directed to drive an air turbine that will generate electricity. At the end of each duct, behind the air-take-off, is a baffle system which is essential for energy conversion, Figure 2. The OWEL WEC is designed such that the buoyancy tanks can be flooded, so as to lower the device in the water during periods of extreme wave conditions -a not uncommon survival protocol for floating WECs [11]. ...
Floating wave energy converters are surface based thus facilitating installation and maintenance. They tend to be moored offshore and consequently have less of an impact than other devices both visually and audibly. Mooring these devices is a challenging task, as not only are they subject to drift forces due to the aggressive environment, but they are also designed to operate at their resonant frequency in order to obtain as much power as possible. Such operational parameters require heavy duty mooring systems, capable of coping with the dynamic environment. These moorings will, in turn, affect the performance of the device by restraining the motions and thus modifying the energy absorption characteristics. In this paper a free floating representation of the Offshore Wave Energy Ltd. device (OWEL) has been modeled in RANS CFD in order to obtain initial mooring loads. Subsequently, a preliminary mooring arrangement for OWEL was developed, and using these loads it was modeled using OrcaFlex. The dynamic, non-linear loads were then coupled to a fully transient, multiphase CFD analysis of the device in order to obtain performance characteristics for further detailed design. The numerical results have been compared to results obtained through physical model scale tests of the device and show a good degree of correlation.
... Hydraulic and Coastal Laboratories of Aalborg University (AAU) on a 1:51.8 scale model of Wave Dragon [5] ...
... Other non-linear features, such as the overtopping, are disregarded in the analysis; this is not considered a major problem at this stage, as previous studies have shown that in extreme conditions the device should be set to the lowest floating position possible to increase its survivability, a condition in which it is almost totally submerged and the nonlinearities due to the overtopping are very much limited [5]. ...
The Wave Dragon Wave Energy Converter is ready to be up-scaled to commercial size. The design and feasibility analysis of a 1.5 MW pre-commercial unit to be deployed at the DanWEC test center in Hanstholm, Denmark, is currently ongoing. With regard to the mooring system, the design has to be carried out numerically, through coupled analyses of alternative solutions. The present study deals with the preliminary hydrodynamic characterization of Wave Dragon needed in order to calibrate the numerical model to be used for the mooring design. A hydrodynamic analysis of the small scale model in the frequency domain is performed by the software HydroD, which uses WAMIT as core software. The quadratic damping term, accounting for the viscous effect, is determined through an iterative procedure aimed at matching numerical predictions on the mooring tension, derived through time domain coupled analysis, with experimental results derived from tank tests of a small scale model. Due to the complex geometry of the device, a sensitivity analysis is performed to discuss the influence of the mean position on the quality of the numerical predictions. Good correspondence is achieved between the experimental and numerical model. The numerical model is hence considered reliable for future design applications.
... Such tests are very important steps for developing prototypes of WECs, or combined concepts. a series of tests of the Wave dragon concept from conceptual model to model scale tests and then to full scale tests was performed from 1986 to 2006 [202,203] and a strategy for the survival condition was proposed by lowering the floating level under extreme conditions [204]. Model tests of the Pelamis concept with the focus on the hydrodynamics was performed with a scaling ratio of 1:20 [205], and model tests with the focus on the hydraulic PTO system in the scaling ratio of 1:7 and in full scale were carried out [206] in which the feasibility and efficiency of the PTO is verified. ...
... The study achieved a 50% reduction in device response with a need for only a 10% change in mass. Terminator devices are often designed to change their angle to incoming waves or increase their draft to reduce energy absorption, and therefore loading, in large waves [19][20][21][22]. Pecher et al. [22] detailed 1/15-scale testing of the Weptos WEC to assess mooring force and structural bending moments in five extreme wave conditions. ...
A wave energy converter must be designed to survive and function efficiently, often in highly energetic ocean environments. This represents a challenging engineering problem, comprising systematic failure mode analysis, environmental characterization, modeling, experimental testing, fatigue and extreme response analysis. While, when compared with other ocean systems such as ships and offshore platforms, there is relatively little experience in wave energy converter design, a great deal of recent work has been done within these various areas. This paper summarizes the general stages and workflow for wave energy converter design, relying on supporting articles to provide insight. By surveying published work on wave energy converter survival and design response analyses, this paper seeks to provide the reader with an understanding of the different components of this process and the range of methodologies that can be brought to bear. In this way, the reader is provided with a large set of tools to perform design response analyses on wave energy converters.
... Previous studies have looked at the effects of extreme responses on the mooring lines, but few studies have been concerned with the extreme response of the device itself, e.g. see Parmeggiani et al. (2011); Muliawan et al. (2013a,b); Ambühl et al. (2014). ...
This paper presents both linear and nonlinear analyses of extreme responses for a multi-body wave energy converter (WEC) in severe sea states. The WEC known as M4 consists of three cylindrical floats with diameters and draft which increase from bow to stern with the larger mid and stern floats having rounded bases so that the overall system has negligible drag effects. The bow and mid float are rigidly connected by a beam and the stern float is connected by a beam to a hinge above the mid float where the rotational relative motion would be damped to absorb power in operational conditions. A range of focussed wave groups representing extreme waves were tested on a scale model without hinge damping, also representing a more general system of interconnected cylindrical floats with multi-mode forcing. Importantly, the analysis reveals a predominantly linear response structure in hinge angle and weakly nonlinear response for the beam bending moment, while effects due to drift forces, expected to be predominantly second order, are not accounted for. There are also complex and violent free-surface effects on the model during the excitation period driven by the main wave group, which generally reduce the overall motion response. Once the main group has moved away, the decaying response in the free-vibration phase decays at a rate very close to that predicted by simple linear radiation damping. Two types of nonlinear harmonic motion are demonstrated. During the free-vibration phase, there are only double and triple frequency Stokes harmonics of the linear motion, captured using a frequency doubling and tripling model. In contrast, during the excitation phase, these harmonics show much more complex behaviour associated with nonlinear fluid loading. Although bound harmonics are visible in the system response, the overall response is remarkably linear until temporary submergence of the central float (‘dunking’) occurs. This provides a strong stabilising effect for angular amplitudes greater than and can be treated as a temporary loss of part of the driving wave as long as submergence continues. With an experimentally and numerically derived response amplitude operator (RAO), we perform a statistical analysis of extreme response for the hinge angle based on wave data at Orkney, well known for its severe wave climate, using the NORA10 wave hindcast. For storms with spectral peak wave periods longer than the RAO peak period, the response is controlled by the steepness of the sea state rather than the wave height. Thus, the system responds very similarly under the most extreme sea states, providing an upper bound for the most probable maximum response, which is reduced significantly in directionally spread waves. The methodology presented here is relevant to other single and multi-body systems including WECs. We also demonstrate a general and potentially important reciprocity result for linear body motion in random seas: the averaged wave history given an extreme system response and the average response history given an extreme wave match in time, with time reversed for one of the signals. This relationship will provide an efficient and robust way of defining a ‘designer wave’, for both experimental testing and computationally intensive computational fluid dynamics (CFD), for a wide range of wave–structure interaction problems.
In this study we compare wave climates and their potential for wave energy conversion for the two energetic but quite different sites of Albany and Orkney. Energy capture is based on the M4 machine with well defined characteristics. The M4 machine is a self reacting system with 3 floats, each float with a circular cross-section when viewed from above. The smaller two floats are rigidly connected by a beam, and the largest float is connected to the mid float by a beam with a hinge. The machine generates power through the relative angular motion of this hinge above the middle float. The machine performance was previously assessed for various locations in the eastern North Atlantic including the European Marine Energy Centre (EMEC) site west of the Orkney Islands, Scotland, for wave power output (Santo et al., 2016a) and extreme response (Santo et al., 2017). In this study, we apply the analysis to a location off Albany on the south coast of western Australia, an area well-known for almost continuous exposure to long period swells. We use Australian Department of Transport (DOT) wave buoy data measured in 60 m of water over the period 2009−2017. The hourly data is close to continuous but contains some gaps corresponding to ∼ 13% of the total duration, these are patched to form a continuous wave record.
Having sized the machine based on mean wave period, extreme wave height statistical analysis is performed using storm-based identification and a peaks-over-threshold technique, following Santo et al. (2016b), providing information relevant for any wave energy converter at the location. From operability and power scheme economics, we then compare the optimal size of machine, practical power output and the associated variability in power produced by an M4 machine at Albany to the open North Atlantic location off the Orkneys. This is performed with the methodology outlined in Santo et al. (2016a). For survivability, it is important to identify extremes of machine motion. Hence, extreme responses are also compared for the central hinge angle of the machine in survival mode with the power take-off turned off. We find that a much larger machine is required at Albany, because of the longer waves compared to Orkney. However, at the two very different locations the power/cost ratios are similar.
Experimental investigation on the power performance of a bottom hinged oscillating wave surge converters (OWSC) with different power take-off (PTO) damping strategies (provided by a generic PTO simulation platform) are conducted in regular and irregular waves. The hydrodynamic performance of the OWSC under different damping modes, in regular waves and irregular waves, is observed. For regular waves, the effects of the main influential parameters (including the incident wave height, wave frequency, phase difference between the buoy velocity and wave elevation) on the output power were quantitatively studied. Six damping coefficients of the linear PTO damping is examined under constant incident wave height, and increasing wave frequencies and an output power curve along wave frequency are presented for each input gain of the PTO simulation platform in both linear damping mode and nonlinear damping mode. Additionally, the best coefficient or input gain is obtained for both linear or nonlinear PTO damping mode in different wave conditions. The phase difference between the buoy velocity and wave elevation of the OWSC model in irregular waves has the same trend as that in regular waves. The output electricity in the JONSWAP spectrum is found to be (approximately 300%) higher than that in a user-defined spectrum for the same wave parameters. However, nonlinear PTO strategies have no distinct advantage in the amount of electricity output but have better stability and broader damping range.
Introduction to coastal engineering and management, World Scientific, Advanced series on Ocean Engineering
- J W Kamphuis
J. W. Kamphuis, Introduction to coastal engineering and management, World Scientific,
Advanced series on Ocean Engineering, Volume 16, 2000
Wave Dragon 1:4.5: up-scaling of mooring system, EU contract ENK5-CT- 2002-00603, Work package 2.6, deliverable 33
NIRAS, Wave Dragon 1:4.5: up-scaling of mooring system, EU contract ENK5-CT-
2002-00603, Work package 2.6, deliverable 33, 36& 43, September 2006.
Hydraulic Model Tests on Modified Wave Dragon
- T Hald
- J Lynggaard
T. Hald, J. Lynggaard, Hydraulic Model Tests on Modified Wave Dragon, Technical
Report, Project No. ENS-51191/00-0067, Aalborg University, Department of Civil
Engineering, 2001.
Wave Dragon 1:4.5: up-scaling of mooring system
NIRAS, Wave Dragon 1:4.5: up-scaling of mooring system, EU contract ENK5-CT-2002-00603, Work package 2.6, deliverable 33, 36& 43, September 2006.