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Normalized hydrostatic restoring force in heave and water plane (WP) area for different mean drafts.
Source publication
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 alte...
Contexts in source publication
Context 1
... respect to the draft, Figure 6 shows how the normalized hydrostatic restoring force in heave and water plane area vary with it; Figure 7 Copyright © 2012 by ASME ...
Context 2
... to this, even small changes in the model floating position (dr) or trim position had a significant influence on the water plane area, A w . As dr was increased to the medium or high level, for which the water free surface was close to or above the top of the buoyant element, A w dramatically reduced indeed ( Figure 6); the same happened when trim occurred (Figure 7), even for small values barely noticeable to the naked eye (e.g. around a 30% decrease in A w for just 2° trim difference).These variations in A w in turn affected the hydrostatic stiffness (as c 33 = ρ · g · A w , being c 33 the heave hydrostatic stiffness) and, in cases where respectively the added mass and the potential damping were almost constant, also the natural frequency of oscillation ω 0 and the damping ratio, ζ. ...
Citations
... For a point observer, single or multi-line catenary moorings [5][6][7][8][9][10] or taut mooring [11] are used. Large floating WEC systems of Pelamis [12] and Wave Dragon [13,14] are moored with a CALM type spread configuration. A Single Anchor Leg Mooring (SALM) is also implemented in the WEPTOS system [15,16]. ...
This paper presents the design and analysis of a mooring buoy and its mooring systems to moor a floating platform mounting an arrayed Wave Energy Converters (WECs). The mooring buoy allows the WEC platform to weathervane around the mooring buoy freely by the prevailing environment directions, which enables consistent power generation. The WEC platform is connected to the buoy with synthetic hawsers, while station-keeping of the buoy is maintained with catenary mooring lines of chains tied to the buoy keel. The buoy also accommodates a power cable to transfer the electricity from the WEC platform to the shore. The WEC platform is designed to produce a total of 1.0 MW with multiple WECs installed in an array. Fully coupled time-domain analyses are conducted under the site sea states, including extreme 50 y and survival 100 y conditions. The buoy motions, mooring tensions and other design parameters are evaluated. Strength and fatigue designs of the mooring systems are validated with requirements according to industry standards. Global and local structural designs of the mooring buoy are carried out and confirm the design compliances.
... 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.
... Commercial: -AQWA [61] x [173] x [5,38,153,174,175] * SIMA [176] x [18,38] * SIMO [177] [153,175,178,179] -FLEXCOM [180] x x x x x [39] -OrcaFlex [91] x x x [10,11,16,18,35,41,81,170,[181][182][183][184][185][186][187][188]] -Proteus DS [189] x x [83,90,190] Open-source: -MAP [191] x [95,100] -MoorDyn [192] x [19] In-house: -AQUA-FE x [62,193] -MooDy [194] x [7,17,195] -MoDEX [196] x [22,32,123] -WHOI Cable [197] x x [65,80] The commercial packages include AQWA, DNV SESAM, Flexcom, OrcaFlex and Proteus-DS. AQWA was developed by ANSYS and provides a toolset for investigating the effects of environmental loads on floating and fixed offshore structures. ...
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.
... Relatively few physical modeling studies of commercial WEC devices are reported in literature due to the business incentives experienced by WEC developers (i.e., protection of intellectual property and investor relations). Parmeggiani et al. performed a series of tests at a scale λ ≈ 1/50 to assess the effectiveness of a special survival mode (see subsequent Survival Configurations section for more on this topic) [43,44]. Forces along the device's main mooring line were measured in irregular waves representative of 10, 50 and 100-year return periods at the target deployment site. ...
... The study achieved a 50% reduction in device response with a need for only a 10% change in mass. Terminator devices often change their angle to incoming waves or increase operational mode survival mode their draft to reduce energy absorption, and therefore loading, in large waves [34,43,44,59]. In other devices, the PCC can be "locked" to prevent motion and problems with component end stops [45,6,35]. ...
Survivability is by no means a new concept to ocean engi- neering; ships must remain stable and structurally intact in vio- lent sea states; the same is true for offshore oil and gas structures. While knowledge from the ship and offshore sectors can be valu- able for designing wave energy converters (WECs) for survival in rough seas, the unique scale, siting and operational charac- teristics of WECs pose a distinct set of engineering challenges. This paper seeks to provide a review of methods for modeling the loading and dynamic response of WECs and analogue ma- rine structures, such as ships and offshore structures, in large nonlinear waves. We identify current knowledge gaps in our un- derstanding of WEC survivability and provide recommendations for future research to close these gaps.
... In autumn 2011 this numerical model has been used to perform a hydrodynamic characterization of Wave Dragon, using the experimental response data about motions and tension in the main mooring line obtained in the above mentioned tank tests to estimate the surge drag coefficient of the device. The study has been conducted in cooperation with the Centre of Ships and Oceanic Structures (CeSOS) at the Norwegian University of Science and Technology [10]. The hydrodynamic parameters determined will be used in future to characterize an updated panel model more similar to the envisaged WD-DanWEC unit, allowing to assess alternative configurations of the external mooring system through time-domain analysis.. ...
... The model mass is 52.32 kg, while M a,0 varies with the draft of the device. Its value has been derived through the hydrodynamic analysis in frequency domain of the device [10], being very similar in the cases of R c low and R c survival (respectively 49.5 kg and 48.3 kg at model scale) while for R c high it is much lower (18.7 kg). T s depends therefore both on the mooring stiffness, k, and on the floating level of the device, through M a,0 . ...
... A third factor of interest is the mean pitch position, or trim. As previously found the ability of the device to naturally adopt a negative trim (i.e., ramp lower than the rear) is a desirable behavior as it helps reducing the extreme mooring tension [10]. In operational conditions, when power production would benefit from having zero trim instead, this tendency can be actively counteracted through the air pressure system. ...
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 differences between the task of designing safe
mooring systems for large floating platforms of the oil and gas
offshore industry, that have severe accident consequences, and
the task of designing moorings for ocean energy devices, with
reduced accident consequences, are discussed. None of the
existing guidelines satisfy the needs of wave energy devices.
These guidelines do not consider the working principles of the
devices, are too demanding as to safety or too simple and lead to
expensive solutions. Wave energy devices will require a new set
of design guidelines containing a new low consequence class,
since mooring failure will not lead to “unacceptable
consequences such as loss of life and uncontrolled outflow of oil
or gas”. It should be up to the owner, insurance company and
investors to balance their economic risks above a certain
minimum safety level. A deeper analysis of mooring systems for
ocean energy devices, compared to what is demanded for
offshore oil and gas platforms, is required to properly assess the
effect on the power take-off and survivability, the latter leading
to economical benefits from a possible low consequence class.
