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ASME Conference Presenter Attendance Policy and Archival Proceedings

2015;():V009T00A001. doi:10.1115/OMAE2015-NS9.
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This online compilation of papers from the ASME 2015 34th International Conference on Ocean, Offshore and Arctic Engineering (OMAE2015) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Ocean Renewable Energy: Competition on Hydrodynamic Modeling of a Rigid Body

2015;():V009T09A001. doi:10.1115/OMAE2015-41448.

The objective of this work is to understand and evaluate the hydrodynamics modeling of a floating rigid body in regular and irregular ocean surface waves. Direct time-domain numerical simulation, based on the potential-flow formulation with the use of a quadratic boundary element method, is employed to compute the response of the body under the action of surface waves including fully-nonlinear wave-body interaction effects associated with steep waves and large-amplitude body motions. The viscous effect due to flow separation and turbulence is included by empirical modeling. The simulation results of body motions are compared with laboratory experimental measurements. The nonlinear effects due to body motion and wave motion are quantified and compared to the viscous effect. Their relative importance in the prediction and modeling of a rigid body motion under various wave conditions is investigated. This study may provide essential information pertaining to develop effective modeling of nonlinear wave-body interactions which is needed in design of offshore structures and wave energy conversion devices.

Commentary by Dr. Valentin Fuster
2015;():V009T09A002. doi:10.1115/OMAE2015-41732.

In the present paper, a hybrid Computational Fluid Dynamics (CFD) and Boundary Integral Element Method (BIEM) framework is developed in order to study the response of a moored Multibody wave Energy Device (MED) to a panchromatic sea state. The relevant results are the surge and heave responses of the MED. The Numerical Analysis Framework (NAF) includes two different models; the first model uses Navier-Stokes equations to describe the flow field and is solved with an in-house CFD code to quantify the viscous damping effect, while the second model uses boundary-integral equation method and is solved with the tool WAMIT\SIMO\RIFLEX. By studying the free decay tests with the Navier-Stokes based model, the uncoupled linear and quadratic damping coefficients of the MED in surge and heave directions are calculated. These coefficients are given as input to the WAMIT\SIMO\RIFLEX model and the responses of the MED to different wave conditions are determined. These responses are compared with the experimental data and very good agreement is obtained. The MED responses calculated by the presented NAF have been obtained in connection with a hydrodynamic modeling competition and selected as one of the numerical models, which well predict the blind experimental data that were unknown to the authors.

Commentary by Dr. Valentin Fuster
2015;():V009T09A003. doi:10.1115/OMAE2015-41752.

The hydrodynamic behavior of a submerged horizontal cylinder moored in waves was studied through numerical methods as a participant of the COER Hydrodynamic Modeling Competition. The potential theory with additional viscous damping has been used to model the problem. Three different simulation cases have been constructed to identify (i) the general behaviors of the cylinder subjected to the input wave, (ii) the effect of the second-order hydrodynamic forces, and (iii) the effect of exact wetted surface issue. Computational results were compared to the experiment, and a reasonable degree of accuracy has been confirmed.

Commentary by Dr. Valentin Fuster
2015;():V009T09A004. doi:10.1115/OMAE2015-42182.

This work describes the overall aspects of the Competition on Hydrodynamic Modelling of a Rigid Body, a competition outlined to evaluate different ways to model and simulate the motions of a rigid body in waves. The main objective is to determine a hydrodynamic model for a submerged horizontal cylinder which best predicts a recorded motion to a specific excitation in panchromatic waves. A blind study was performed by the competition participants, i.e., the simulation results were obtained without knowledge of the actual recorded motion of the cylinder. Only the geometry of the cylinder in solid model and the time series of the incoming waves were issued to the participants. The proposed approaches by the participants for modelling the rigid body and the fluid motions are based on the boundary-integral equation methods (potential flow theory) with additional viscous damping forces, where the drag terms are calculated either empirically or via the Navier-Stokes equation method. This paper describes the details about rationale for choice of the rigid body, the experimental tests, the competition criteria and an overview of all modelling approaches proposed by the competition participants.

Topics: Modeling
Commentary by Dr. Valentin Fuster
2015;():V009T09A005. doi:10.1115/OMAE2015-42288.

The Center for Ocean Energy Research (COER) at the University of Maynooth in Ireland organized a hydrodynamic modeling competition in conjunction with OMAE2015. Researchers were challenged to predict the dynamic response of a floating rigid-body device that was experimentally tested in a series of wave-tank tests. Specifically, COER set up a blind competition, where the device specifications and test conditions were released, but the experimental results were kept private until all competition participants submitted their numerical simulation results.

The National Renewable Energy Laboratory and Sandia National Laboratories entered the competition and modeled the experimental device using both the WEC-Sim and FAST numerical modeling tools. This paper describes the numerical methods used to model the device and presents the numerical modeling results. The numerical results are also compared to the experimental results provided by COER at the completion of the competition.

Commentary by Dr. Valentin Fuster
2015;():V009T09A006. doi:10.1115/OMAE2015-42325.

Ocean industries such as oil and gas, defence, and marine renewables, face the challenge of costly and risky deployments and operations due to their complex and capital intensive nature. Numerical simulation tools are valuable assets that can be used to anticipate motions and stresses and therefore inform structural and operational design before deployment. Simulation tools that can capture all pertinent hydrodynamic phenomena increase their value by reducing design time, uncertainty, risk and capital associated with a deployment.

Validation of numerical tools is critical to ensure accuracy and reliability. The following paper reviews a framework for simulation of moored, multi-body, floating systems, including the component models employed, the results of a model verification study, and the challenges encountered in the project. Tank test data of a moored horizontal cylinder was provided for the purposes of numerical tool validation.

Commentary by Dr. Valentin Fuster

Ocean Renewable Energy: Current Energy: Analysis Concepts and Developments

2015;():V009T09A007. doi:10.1115/OMAE2015-41024.

The horizontal axis tidal stream turbine with a mono-pile foundation is one of the most used devices for the exploitation of tidal stream energy, because of its convenience and low expense in construction and high efficiency in extracting energy. A 3D hydrodynamic model is developed to investigate flow field around the tidal stream turbine subjected to a steady current. After verified with the previous experimental works, the numerical model is used to study the complex flow motion around the horizontal axis tidal stream turbine with a mono-pile foundation. The numerical results show that the existence of tidal stream turbine may largely change the flow dynamics. A wake region with low velocity is formed behind the device. Water surface in front of the turbine fluctuates periodically. It is also found that the installing elevation of the turbine and the incoming flow speed have significant impacts on the mean kinetic energy and the resulting fluid force of the rotating turbine.

Commentary by Dr. Valentin Fuster
2015;():V009T09A008. doi:10.1115/OMAE2015-41035.

High resolution RANS CFD analysis is performed to support the design and development of the Ocean Renewable Power Company (ORPC) TidGen™ multi-directional tidal turbine. Two-dimensional and three-dimensional unsteady, moving-mesh CFD is utilized to parameterize the device performance and to provide guidance for device efficiency improvements.

The unsteady CFD analysis was performed using a well validated, naval hydrodynamic CFD solver and implementing dynamic overset meshes to perform the relative motion between geometric components. This dynamic capability along with the turbulence model for the expected massively separated flows was validated against published data of a high angle of attack pitching airfoil.

Two-dimensional analyses were performed to assess both blade shape and operating conditions. The blade shape performance was parameterized on both blade camber and trailing edge thickness. The blades shapes were found to produce nearly the same power generation at the peak efficiency tip speed ratio (TSR), however off-design conditions were found to exhibit a strong dependency on blade shape. Turbine blades with the camber pointing outward radially were found to perform best over the widest range of TSR’s. In addition, a thickened blade trailing edge was found to be superior at the highest TSR’s with little performance degradation at low TSR’s.

Three-dimensional moving mesh analyses were performed on the rotating portion of the full TidGen™ geometry and on a turbine blade stack-up. Partitioning the 3D blades axially showed that no sections reached the idealized 2D performance. The 3D efficiency dropped by approximately 12 percentage points at the peak efficiency TSR. A blade stack-up analysis was performed on the complex 3D/barreled/twisted turbine blade. The analysis first assessed the infinite length blade performance, next end effects were introduced by extruding the 2D foil to the nominal 5.6m length, next barreling was added to the straight foils, and finally twist was added to the foils to reproduce the TidGen™ geometry. The study showed that making the blades a finite length had a large negative impact on the performance, whereas barreling and twisting the foils had only minor impacts. Based on the 3D simulations the largest factor impacting performance in the 3D turbine was a reduction in mass flow through the turbine due to the streamlines being forces outward in the horizontal plane due to the turbine flow resistance. Strategies to mitigate these 3D losses were investigated, including adding flow deflectors on the turbine support structure and stacking multiple turbines in-line.

Commentary by Dr. Valentin Fuster
2015;():V009T09A009. doi:10.1115/OMAE2015-41231.

Placed in a fluid flow, a cantilevered flexible plate flaps spontaneously above a critical flow velocity. The resulting self-sustained vibrations of such a flag may be used to produce electrical energy and power an output circuit using piezoelectric patches covering the flag that deforms with the flapping motion. Previous work showed only moderate harvesting efficiency with a resistive output circuit, but proposed numerous directions for improvement. We propose a numerical and experimental investigation of the coupled dynamics of such a fluid-solid-electric system, and analyze the influence of the output circuit on the dynamics and harvesting efficiency. In particular, inductive-resistive circuits are considered. Our numerical results show that such resonant circuits lead to a destabilization of the system and a spontaneous flapping at lower fluid velocities. Also they significantly increase the energy harvesting efficiency of the piezoelectric flags as a result of a frequency lock-in between the flag and the electrical circuit. Wind tunnel tests are performed with prototypes of piezoelectric flags and basic resonant circuits. We show that such circuits effectively enhance the energy harvesting performance when they are in resonance with the piezoelectric flag. Meanwhile, the internal resistance of the inductive elements also shows an important influence on the harvested electrical power. In both numerical and experimental studies, the importance of piezoelectric coupling strength is observed. Our results show that promising efficiency enhancements of such flow energy harvesters would be achieved through the optimization of the output circuit as well as development of new piezoelectric materials.

Commentary by Dr. Valentin Fuster
2015;():V009T09A010. doi:10.1115/OMAE2015-41713.

This paper presents a numerical study on the self-induced flapping dynamics of an inverted flexible foil in the context of energy harvesting using piezoelectric elements. The inverted foil considered in this study is clamped at the trailing edge and the leading edge is free to oscillate. To simulate the nonlinear flapping dynamics of an inverted flexible foil, a high-order coupled fluid-structure solver based on the combined field with explicit interface (CFEI) has been developed. Additionally, a simplified piezoelectric model has been presented to determine the electric energy that can be harvested through flapping. The coupled solver is validated over a flexible foil fixed at the leading edge and trailing edge free to oscillate. A systematic study on the flapping response of an inverted flexible foil has been performed for a wide range of non-dimensional bending rigidity for a fixed Reynolds number of 1000 and mass ratio of 0.1. As a function of decreasing bending rigidity, four flapping regimes have been observed: (i) fixed-point stable, (ii) inverted limit-cycle oscillations, (iii) deformed flapping and (iv) flipped flapping. The inverted limit-cycle oscillations are characterized by low-frequency large amplitude oscillations which generate O(103) times greater strain energy than a flexible foil fixed at the leading edge, which has a profound impact on the development of ocean current based energy harvesting devices.

Commentary by Dr. Valentin Fuster
2015;():V009T09A011. doi:10.1115/OMAE2015-41780.

Recently, focus has been placed on ocean energy resources because environmental concerns regarding the exploitation of hydrocarbons are increasing. Among the various ocean energy sources, tidal current power (TCP) is recognized as the most promising energy source in terms of predictability and reliability. The enormous energy potential in TCP fields has been exploited by installing TCP systems. The flow speed is the most important factor for power estimation of a tidal current power system. The kinetic energy of the flow is proportional to the cube of the flow’s velocity, and velocity is a critical variable in the performance of the system. Since the duct can accelerate the flow speed, its use could expand the applicable areas of tidal devices to relatively low velocity sites. The inclined angle of the duct and the shapes of inlet and outlet affect the acceleration rates of the flow inside the duct. To investigate the effects of parameters that increase the flow speed, a series of simulations are performed using the commercial computational fluid dynamics (CFD) code ANSYS-CFX. Experimental investigations were conducted using a circulation water channel (CWC). Also, mooring system concepts are investigated using the commercial mooring analysis software WADAM and OrcaFlex. Due to other floating structures operating within a limited area, station-keeping is needed in order to keep the motions of the floating duct structures within permissible limits. In this study, methods for optimizing the mooring system of a floating duct-type tidal current power system in shallow water are investigated. Based on the performance and mooring analysis results of the 10 kW floating duct-type TCP system, a new design for a small capacity floating TCP system is introduced.

Commentary by Dr. Valentin Fuster
2015;():V009T09A012. doi:10.1115/OMAE2015-41820.

Tidal turbines are emerging technologies offering a great potential by the harnessing of a renewable and predictable resource. However, exploitation at sea comes with significant design, installation, grid connection, and maintenance operations challenges. Consequently, guidelines and standards are required to ensure safety, reliability and quality for these innovative technologies, to support designers and to accelerate the development and commercialisation of the tidal technology. As tidal energy concepts are only at the demonstration stage, only few standards have been published about tidal and current turbines and no dedicated certification procedures have been developed so far.

The aim of this paper is to present a risk-based certification process developed by Bureau Veritas for tidal turbines and published in the Guidance Note NI603 Current and Tidal Turbines. Based on experience accumulated over the past years with tidal turbines technology developers, typical challenges related to the design and installation of tidal devices at sea will be highlighted in this paper. To support tidal turbines designers to take up these challenges, Bureau Veritas provides a decision-making guide gathering 1) recommendations from the existing experience at sea of tidal devices, 2) best practices from related sectors, such as shipping, wind energy or offshore oil & gas, 3) a risk-based approach to consider for the particular requirements of each tidal turbine installation.

In particular for tidal turbines, projects are highly site specific with huge impacts on farm layouts and foundation designs, to name but a few of the issues to be addressed. Paradoxically, the aim of certification societies is to develop rules and tools that can be applicable to a wide range of designs. Consequently, trying not to be design-specific, a proposal of a generic certification process is made in this paper. Existing certification principles from more mature sectors such as wind energy, offshore oil & gas or shipping have been adapted to the specificities of tidal turbines. This paper addresses different certification procedures such as prototype certification, component certification, type certification and project certification. Their respective application and interactions are developed, with a focus on prototype and type certification. In addition, particular attention is paid to the novelty induced by tidal turbines. Consequently, a risk-based guidance is provided to use qualification of new technology for the most innovative parts of the tidal device.

Topics: Design , Tidal turbines , Risk
Commentary by Dr. Valentin Fuster
2015;():V009T09A013. doi:10.1115/OMAE2015-41985.

This paper describes the methodology behind the partial safety factors and design fatigue factors of the DNV GL standard for certification of tidal turbines. The standard follows a risk based approach that allows the adjustment of the design requirements to the identified risk level of the system.

The work undertaken in this study has involved the identification of the uncertainties during the design process. Tidal turbines are located in highly energetic sites which are very difficult to characterise, hence site conditions are one of the largest sources of uncertainty. Key parameters like turbine inflow conditions or predictions of extreme values are still grey areas. Load simulation tools are still quite uncertain and are often dependent on the experience of the people running them.

Both partial safety factors and design fatigue factors have been calibrated to different target safety levels with due account for the uncertainties introduced by load models as well as site characterisation. The different target safety levels have been selected in accordance to the risk associated to structural failures of tidal turbine support structures and aim for a more streamlined design from the safety requirements point of view.

Commentary by Dr. Valentin Fuster
2015;():V009T09A014. doi:10.1115/OMAE2015-42095.

This paper addresses the innovative turbine for hydrokinetic energy extraction proposed by (Fernandes and Armandei, 2014). This turbine harvests energy via torsional galloping. The turbine consists of a rectangular flat plate located vertically initially aligned with the water current and connected with a torsional spring. By changing the axis position, some experimental tests are conducted to assess the improvements on the performance of the turbine. It is observed that the optimal position for the elastic axis happens when the axis is located at 0.75 of the chord length from the leading edge. This is in accordance with the ideal axis position obtained from the Theodorsen theory reported in a PhD thesis (Armandei, 2013). Also, in order to have estimate of the performance of the turbine in the scaled-up level, a similarity analysis is made. Then the results are modified to find the estimate for the performance of the array of turbines. The comparison between the estimated power of the torsional galloping turbine and some conventional turbine types gives a general idea about how this turbine would operate in full scale.

Topics: Turbines
Commentary by Dr. Valentin Fuster
2015;():V009T09A015. doi:10.1115/OMAE2015-42133.

This paper presents the results of the study on the wavecurrent interactions of an idealized full scale marine current turbine (MCT). A multi-phase flow model is used for simulation of three cases: still water and two different wave heights. The Standard k-ω turbulence model is chosen based on the stability of the pressure and velocity plots upstream and downstream the turbine rotor plane. The three cases are used in the present study to compare the effects of wave height and current velocity on the turbine rotor. The velocity, and pressures on the turbine blades are computed for each case using ANSYS FLUENT CFD Software. The thrust, torque, and power in the MCT are calculated using the results obtained from the CFD simulation.

The turbine rotor blades are drafted in 3D using SolidWorks by extruding cross sections of a 43.2 m diameter turbine blade published by the National Renewable Energy Laboratory (NREL). Tetrahedral mesh elements are used to represent the multiphase fluid domain and rotor blades in ANSYS ICEM CFD due to its simplicity and speed of computation. The ANSYS FLUENT simulation is set up to run air and water phases in the domain, while the rotor blade is suspended in the fluid domain, such that there is 20 m of water in front and 100 m behind the plane of rotation. The effects of varying wave heights on the thrust, torque, and power are presented based on the tip speed ratios. The power generated by the turbine rotor from the wave cases is found to be higher than those for the still water case, at lower current velocities. However, at current velocities higher than 2.00 m/s, the power generated from the still water case is higher than the wave cases. At lower tip speed ratios, the thrust on the turbine, subjected to wave conditions, is lower than that for the still water condition. At higher tip speed ratios, the thrust on the turbine, under wave conditions, is higher than that for the still water condition. The torque decreases exponentially with increases in the tip speed ratio for all three cases, but the torque remains nearly constant with increases in wave height. The results provide detailed information which would be valuable in the design and operation of marine current turbines in wave environments.

Commentary by Dr. Valentin Fuster
2015;():V009T09A016. doi:10.1115/OMAE2015-42333.

Flow Induced Motions (FIM) of a single-cylinder VIVACE Converter is investigated using two-dimensional Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with the Spalart-Allmaras turbulence model at Reynolds numbers (30,000 ≤Re≤120,000, 5.50≤U*≤9.85) in the TrSL3 flow regime. Computational results compare very well with experimental data. With implementation of Passive Turbulence Control (PTC), the VIVACE Converter can harness hydrokinetic energy from currents or tides over an expanded range of FIM synchronization, including Vortex Induced Vibrations (VIV) and galloping. The General Grid Interface (GGI) with topological mesh changes is proved to be an effective method for handling high-amplitude FIM response. Within the test Reynolds number range, five regions are clearly observed, including the no-FIM range, the VIV initial branch, the VIV upper branch, transition from VIV to galloping, and galloping. The power envelope calculated based on the CDF simulations for FIM agrees very well with the corresponding power envelope generated based on experiments. The range between VIV and galloping can be eliminated by adjusting the spring-stiffness and the harnessing damping-ratio. This is verified by both experiments and numerical simulation.

Topics: Cylinders
Commentary by Dr. Valentin Fuster

Ocean Renewable Energy: Wave Energy: Analysis Concepts and Development

2015;():V009T09A017. doi:10.1115/OMAE2015-41087.

Wave energy converters (WECs) face many technical challenges before becoming a cost-competitive source of renewable energy. The levelized cost of electricity could be decreased by implementing real-time control strategies to increase average power produced by a WEC. These control strategies typically require knowledge of the immediate future excitation force, caused by the waves. This paper presents a disturbance prediction methodology that is independent of the local wave climate and can be implemented on a wide range of devices.

A time-domain model of a generic heaving WEC is developed with the Cummins equations. The model is simulated with measured water surface elevation data collected off the Oregon Coast. A simplified linear frequency-invariant state-space model is used in conjunction with a Kalman filter to estimate the current excitation force with measurements of the WEC’s motion. Future excitation forces are then predicted multiple steps in the future with a recursive least squares filter. The results show this approach makes accurate predictions of excitation force over short time horizons (up to 15 seconds), but accurate predictions become infeasible for longer horizons.

Topics: Waves , Excitation
Commentary by Dr. Valentin Fuster
2015;():V009T09A018. doi:10.1115/OMAE2015-41140.

A new wave energy converter (WEC) design and some test results are discussed in this work. Among a variety of WEC technologies being explored to date, a huge majority employs wave-driven reciprocating motion (e.g., heave, pitch, sway, reciprocating bending or curving, etc.) to harness energy. It is well known that reciprocating WECs only work well at or near a predefined wave frequency, in a preferred alignment angle with the wave direction (except for the heave type), and in organized waves. But real ocean waves are chaotic and have daily changing frequencies and propagation directions. To circumvent those issues of the reciprocating WECs, a new unidirectional WEC concept — a vertical axis wave turbine — is explored in this research. The key component of the wave turbine is a rotor, which has a number of uniquely arranged hemispherical shells as blades. When the rotor is exposed in waves with its shaft vertically oriented, local water motion in any spatial directions (due to waves) can always drive the rotor for unidirectional rotation regardless of the wave type and propagation direction. In other words, the rotor can rely on omnidirectional water motion to realize its unidirectional rotation. A model wave turbine employing this rotor design has been tested in a wave flume. Upon a successful demonstration in simulated irregular waves, the rotor’s unidirectional performance was systematically characterized under various experimental conditions in simple waves.

Topics: Waves , Turbines , Blades
Commentary by Dr. Valentin Fuster
2015;():V009T09A019. doi:10.1115/OMAE2015-41523.

The hydrodynamic analysis and estimation of the performance of wave energy converters (WECs) is generally performed using semi-analytical/numerical models. Commercial boundary element codes are widely used in analyzing the interactions in arrays comprising of wave energy conversion devices. However, the analysis of an array of such converters becomes computationally expensive, and the computational time increases as the number of devices in the system is increased. As such determination of optimal layouts of WECs in arrays becomes extremely difficult. In this study, an innovative active experimental approach is presented to predict the behaviour of theWECs in arrays. The input variables are the coordinates of the center of the wave energy converters. Simulations for training examples and validation are performed for an array of OscillatingWave Surge Converters, using the mathematical model of Sarkar et. al. (Proc. R. Soc. A, 2014). As a part of the initial findings, results will be presented on the performance of wave energy converters located well inside an array. The broader scope/aim of this research would be to predict the behaviour of the individual devices and overall performance of the array for arbitrary layouts of the system and then identify optimal layouts subject to various constraints.

Commentary by Dr. Valentin Fuster
2015;():V009T09A020. doi:10.1115/OMAE2015-41529.

While linear and nonlinear system identification is a well established field in the control system sciences, it is rarely used in wave energy applications. System identification allows the dynamics of the system to be quantified from measurements of the system inputs and outputs, without significant recourse to first principles modelling. One significant obstacle in using system identification for wave energy devices is the difficulty in accurately quantifying the exact incident wave excitation, in both open ocean and wave tank scenarios. However, the use of numerical wave tanks (NWTs) allow all system variables to be accurately quantified and present some novel system tests not normally available for experimental devices. Considered from a system identification perspective, this paper examines the range of tests available in a NWT from which linear and nonlinear dynamic models can be derived. Recommendations are given as to the optimal configuration of such system identification tests.

Topics: Waves , Wave energy
Commentary by Dr. Valentin Fuster
2015;():V009T09A021. doi:10.1115/OMAE2015-41775.

The purpose of this study is to determine the optimal parameters for a control strategy of an oscillating body type wave energy converter (WEC) for a WEC development and demonstration project in Japan. We plan to use a reactive control strategy for calm sea states, because wave heights in the seas near around Japan is about 1 m in summer. We also developed a combined and tuned control strategy with both reactive and resistive controls, in which the control parameters are changed to sea states. The proposed control system achieves both better performance and safer operation. In this numerical study, we determine the control parameters for the sea states by the time-domain simulation using the Newmark–β method.

Commentary by Dr. Valentin Fuster
2015;():V009T09A022. doi:10.1115/OMAE2015-41971.

In continuation of [1], this paper presents the progress made towards the development of a new modeling tool based on the Weak-Scatterer approaches. Recent developments are the coupling of the fluid and body solver in order to predict the free motion response of the body. Pressure field over the wetted area is obtained by solving an additional boundary value problem for the time derivative of the velocity potential. Tanizawa’s [2] and Cointe’s [3] formulations for the acceleration condition on the body are revisited. Numerical prediction with the present method for a submerged body in vertical free motion is presented and energy conservation is verified. In order to adapt the mesh to the moving body geometry, advanced mesh moving schemes have been integrated based on radial basis functions [4] and spring analogy methods. In this way it is possible to solve the problem with an Arbitrary Euler Lagrangian formalism and preserve the order of the numerical scheme. However moving mesh methods are limited in time and automatic remeshing generation algorithms have been integrated in order to enable simulating longer durations. Finally, comparisons of wave diffraction and radiation predicted by linear theory, a fully nonlinear BEM solver and the present method are shown.

Topics: Waves , Approximation
Commentary by Dr. Valentin Fuster
2015;():V009T09A023. doi:10.1115/OMAE2015-42103.

In this paper, we present a concept of near/off-shore Oscillating Water Column (OWC) Wave Energy Converter (WEC) that is equipped with a Power Take Off (PTO) unit based on Dielectric Elastomer Generators (DEGs). DEGs are soft/deformable generators with variable capacitance able to directly convert the mechanical energy that is employed for their deformation into electrostatic energy.

The proposed WEC is based on an existing tubular collector chamber of an OWC system designed by the company Sendekia, that is combined with an Inflatable Circular Diaphragm (ICD) DEG. This simplified design presents a very reduced number of moving parts showing potentially high efficiency, reliability and noise-free operation.

A multi-physics dynamic model of the system is built using time domain linear hydrodynamics coupled with an analytical non-linear electro-hyperelastic model for the DEG-based PTO. The power matrix of the system is calculated for both regular and irregular waves. Some design issues are introduced showing that the electro-elastic response of the DEG provides the system with an additional stiffness that adds up to the hydrostatic stiffness and affects the resonance of the WEC. As a consequence, the geometric shape/dimensions of the OWC chamber and the layout of the DEG diaphragm should be chosen using an integrated procedure aimed at tuning the overall response of the WEC to the spectra a reference wave climate.

Commentary by Dr. Valentin Fuster
2015;():V009T09A024. doi:10.1115/OMAE2015-42370.

This paper begins with a brief review of the equation of motion for a generic floating body with modification to incorporate the influence of a power-take-off (PTO) unit. Since the damping coefficient is considered the dominant contribution to the PTO reaction force, the optimum non time-varying values are presented for all frequencies, recovering the well-known impedance-matching principle at the resonance condition of the coupled system. The construction of a laboratory-scale permanent magnet linear generator (PMLG), developed at the University of California at Berkeley, is discussed along with the basic electromagnetic equations used to model its performance. Modeling of the PMLG begins with a lumped magnetic circuit analysis, which provides an analytical solution to predict the magnetic flux available for power conversion. The voltage generated across each phase of the stator, induced by the motion of the armature, provides an estimate for the electromagnetic damping as a function of the applied resistive load. The performance of the PMLG and the validation of the proposed analytical model is completed by a set of dry-bench tests. Results from the bench test showed good agreement with the described electromechanical model, thus providing an analytical solution that can assist in further optimization of the PMLG.

Commentary by Dr. Valentin Fuster

Ocean Renewable Energy: Wave Energy: Operations and Applications

2015;():V009T09A025. doi:10.1115/OMAE2015-41076.

When looking for a location for a wave energy converter (WEC) installation, developers usually look for sites with high or very high wave energy resource. From this perspective, countries like Scotland or Ireland have made great effort to include this energy source in their energy mix due to their expected high untapped potential. However, higher resource carries marine operation restrictions. Because of that, the selection of a site for a WEC deployment, the installation, operation and maintenance factors have to be considered from the beginning.

In this work an analysis of the suitable locations for the development of wave energy is performed based on the operation and maintenance (O&M) parameters. This study is performed across the globe coastlines taking the met-ocean climate data from Reguero et al (2011) global reanalysis database (GOW) developed at IH Cantabria.

Firstly, an analysis of the global availability and accessibility levels is performed all around the globe taking different wave height thresholds into account. Seven specific locations (North-West Denmark, West of Ireland, Chile, North of Spain, West Portugal, South-West Australia and North of Scotland) with high interest on wave energy have been further analyzed and compared.

Secondly, the O&M access limits are quantified in terms of the weather windows and the waiting period between available weather windows. A statistical analysis of these parameters is performed within different weather windows lengths (6 h, 12 h and 24 h). The seasonality of these parameters is also analyzed. Finally, a failure analysis will be carried out, simulating the repair operation along the lifecycle of the device for different failure rates and waiting times. The affection of this failure and repair scheme over the power production of a device analyzed previously in Andres et al (2014) will be presented.

In this study, some locations with high resource (Spain, Nova Scotia) lead to medium to high accessibilities/availabilities due to the balance between resource and persistence of the weather conditions. Some locations with high resource such as Chile or Australia resulted inaccessible during very long periods of time due to the persistence of severe conditions and then not very recommended for novel converters with uncertain failure rates.

Commentary by Dr. Valentin Fuster
2015;():V009T09A026. doi:10.1115/OMAE2015-41532.

Wave energy converter (WEC) devices are designed to sustain the wave-induced loads that they experience during both operational and survival sea states. The extreme values of these forces are often a key cost driver for WEC designs. These extreme loads must be carefully examined during the device design process, and the development of a specific extreme condition modeling method is essential. In this paper, the key findings and recommendations from the extreme conditions modeling workshop hosted by Sandia National Laboratories and the National Renewable Energy Laboratory are reviewed. Next, a study on the development and application of a modeling approach for predicting WEC extreme design load is described. The approach includes midfidelity Monte-Carlo-type time-domain simulations to determine the sea state in which extreme loads occur. In addition, computational fluid dynamics simulations are employed to examine the nonlinear wave and floating-device-interaction-induced extreme loads. Finally, a discussion on the key areas that need further investigation to improve the extreme condition modeling methodology for WECs is presented.

Commentary by Dr. Valentin Fuster
2015;():V009T09A027. doi:10.1115/OMAE2015-41581.

The Rio floating point absorber (FPA) is designed for a reference site located near an island offshore Rio de Janeiro. According to the reference site characteristics, a two-body floating point absorber concept design is chosen to convert ocean wave energy into electrical power. An innovative procedure aiming at finding an optimal shape adapted to predefined wave climate conditions, using the Design of Experiments (DOE) method, is applied. A simple linear damper model is used to represent the Power Take-Off (PTO) mechanism. The optimization procedure is divided into Buoy and support (spar/plate) steps, so the optimized buoy is determined first and then a proper support is determined to reach a satisfactory two-body FPA system. The nonlinearities are not considered in this study and linear Numerical models are developed using AQWA/ANSYS and Minitab software in frequency domain. Finally, a preliminary optimized model of the two-body FPA is determined in accordance with the particular sea site information of the Rio de Janeiro.

Commentary by Dr. Valentin Fuster
2015;():V009T09A028. doi:10.1115/OMAE2015-41908.

An index to estimate the cost of electricity (COE) generated by a wave farm from the design parameters of a wave energy converter (WEC), such as the body size and the generator capacity, was examined to show the validity of index value in this study. The validation tests are performed for three different wave farm settings at three different locations.

The result displays the potential of index to capture the trend of COE value especially when the wave farm size is small. The calculation result of COE reveals that the parameter combination to give better profitability is determined by the balance between WEC construction fee and installation fee. So, it would be different from the optimum size to have the best energy conversion efficiency. It also explains the shift of parameter combination to give the better profitability when the size of wave farm is changed. However, the index contains certain level of error because of the lack of this feature. Therefore, the error becomes larger when the size of wave farm becomes larger. As a result, it was found that the modification of the index is needed to improve the accuracy by including the cost related to the number of buoys in the wave farm.

Topics: Waves , Design , Optimization
Commentary by Dr. Valentin Fuster
2015;():V009T09A029. doi:10.1115/OMAE2015-41975.

This paper presents a numerical study on a floating coaxial ducted OWC wave energy converter equipped with a biradial air turbine to meet the requirements of an oceanographic sensor-buoy. The study used representative sea states of the Monterey Bay, California, USA. The geometry of the coaxial ducted OWC was hydrodynamically optimized using a frequency domain approach considering a linear air turbine. Afterwards, a time domain analysis was carried out for the system equipped with a biradial turbine. The turbine rotor diameter and the optimum generator’s control curves were determined, based on results for representative sea states. Results show that mean power output fulfills the requirement for oceanographic applications (300–500W) using a turbine rotor diameter of 0.25 m. Furthermore, the system’s performance is strongly influenced by the inertia of the turbine and the generator rated power. These results confirmed the suitability of using the coaxial ducted OWC as a self-sustainable oceanographic sensor-buoy.

Commentary by Dr. Valentin Fuster
2015;():V009T09A030. doi:10.1115/OMAE2015-41993.

A three dimensional time-domain model, based on Cummins equation, has been developed for an axisymmetric point absorbing wave energy converter (WEC) with an irregular cross section. This model incorporates a number of nonlinearities to accurately account for the dynamics of the device: hydrostatic restoring, motion constraints, saturation of the power-take-off force, and kinematic nonlinearities. Here, an interpolation model of the hydrostatic restoring reaction is developed and compared with a surface integral based method. The effects of these nonlinear hydrostatic models on device dynamics are explored by comparing predictions against those of a linear model. For the studied WEC, the interpolation model offers a large improvement over a linear model and is roughly two orders-of-magnitude less computationally expensive than the surface integral based method.

Commentary by Dr. Valentin Fuster
2015;():V009T09A031. doi:10.1115/OMAE2015-42056.

The Oscillating Water Column (OWC) is one of the simplest and most studied concepts for wave energy conversion. The commercial scale diffusion of the OWC technology is, however, strongly dependent upon the device optimization. Research at a fundamental level is therefore still required. Analytical, numerical and experimental models are necessary tools for advancing in the knowledge of the system and thus promoting its passage at the commercial level. In this work, a simplified frequency domain rigid piston model has been applied to preliminary select expected ranges of air pressures and air velocities for the instrumental set up of laboratory experiments. The set up of a Computational Fluid Dynamic (CFD) model implemented in the open source OpenFOAM®1 environment is then presented. The multiphase model solves incompressible 3D Navier-Stokes equations, using Large Eddy Simulation (LES) for turbulence modelling, and adopts a Volume of Fluid method (VOF) to track the air-water interface. A preliminary validation of the model with physical tests data is conducted. The numerical approach seems to be promising for an accurate simulation of the OWC device energy conversion process. Hence, the validated model can be a useful research tool for different problems, particularly for systematic parameter studies to extend the range of conditions tested in the laboratory.

Commentary by Dr. Valentin Fuster
2015;():V009T09A032. doi:10.1115/OMAE2015-42074.

WEC-Sim (Wave Energy Converter-SIMulator) is an open-source wave energy converter (WEC) code capable of simulating WECs of arbitrary device geometry subject to operational waves. The code is developed in MATLAB/Simulink using the multi-body dynamics solver SimMechanics, and relies on Boundary Element Method (BEM) codes to obtain hydrodynamic coefficients such as added mass, radiation damping, and wave excitation. WEC-Sim Version 1.0, released in Summer 2014, models WECs as a combination of rigid bodies, joints, linear power take-offs (PTOs), and mooring systems. This paper outlines the development of PTO-Sim (Power Take Off-SIMulator), the WEC-Sim module responsible for accurately modeling a WEC’s conversion of mechanical power to electrical power through its PTO system. PTO-Sim consists of a Simulink library of PTO component blocks that can be linked together to model different PTO systems. Two different applications of PTO-Sim will be given in this paper: a hydraulic power take-off system model, and a direct drive power take-off system model.

Commentary by Dr. Valentin Fuster
2015;():V009T09A033. doi:10.1115/OMAE2015-42178.

The design of an energy harvester involves achieving the two following objectives: to install a safe structure with a reasonable safety margin; and to install an effective device which is able to capture energy in a variety of environmental conditions. In this context, the long-term modelling of the environmental variables plays a crucial role.

In the context of wave energy harvesters, the occurrence of sea storms is a critical element in the design process. Indeed, its identification is required for determining extreme loads as well as controlled de-activations of the device for preserving the mechanical components into the device.

Considering these issues, the paper proposes an analysis of the wave climate oriented to the determination of the downtime and of the energy losses. Specifically, the paper provides expressions: for calculating the average deactivation time of a wave energy device, given that it must be deactivated if the significant wave height is larger than a certain threshold; and for calculating the energy “lost” (as it is not absorbed by the device) during a storm in which the maximum wave height is larger than the mentioned threshold.

The paper shows that closed-form expressions can be obtained by relying on the Equivalent Triangular Storm (ETS) model and that the adequacy of the estimations improves for larger values of the significant wave height threshold.

Commentary by Dr. Valentin Fuster
2015;():V009T09A034. doi:10.1115/OMAE2015-42198.

This article describes and presents results from research focused on appraising the new technical specification (TS) for the assessment of wave energy resources developed by technical committee 114 of the International Electro-technical Commission (IEC-TC-114). The new IEC TS is appraised through an extensive pilot application to the waters off the west coast of Vancouver Island, British Columbia, Canada. A series of wave models are developed and used to simulate the wave conditions and estimate the wave energy resource over the study area. The accuracy of the various resource estimates derived from the model outputs is assessed through comparison with measurements from a directional wave buoy. Furthermore, sensitivity analyses are conducted to determine the main sources of error and uncertainty impacting the precision of resource assessments obtained following the IEC methodology. Preliminary results indicate that the IEC TS can be applied to the estimation of wave energy resources with a reasonable level of effort and accuracy.

Topics: Wave energy
Commentary by Dr. Valentin Fuster
2015;():V009T09A035. doi:10.1115/OMAE2015-42265.

WEC-Sim is a midfidelity numerical tool for modeling wave energy conversion devices. The code uses the MATLAB SimMechanics package to solve multibody dynamics and models wave interactions using hydrodynamic coefficients derived from frequency domain boundary element methods. This paper presents the new modeling features introduced in the latest release of WEC-Sim. The first feature discussed is the conversion of the fluid memory kernel to a state-space approximation that provides significant gains in computational speed. The benefit of the state-space calculation becomes even greater after the hydrodynamic body-to-body coefficients are introduced as the number of interactions increases exponentially with the number of floating bodies. The final feature discussed is the capability to add Morison elements to provide additional hydrodynamic damping and inertia. This is generally used as a tuning feature, because performance is highly dependent on the chosen coefficients. In this paper, a review of the hydrodynamic theory for each of the features is provided and successful implementation is verified using test cases.

Commentary by Dr. Valentin Fuster
2015;():V009T09A036. doi:10.1115/OMAE2015-42365.

Real time hybrid modeling as a structured approach of implementing a real time control system has been proven as an efficient strategy to assess and optimize wave energy converter. In this paper an existing real time hybrid modeling framework for wave energy converter is reviewed, in which the main problem is divided into multiple sub-domains. Each sub-domain uses a preferred method, e.g. experimentally and/or computationally, which contributes to solve the main initial problem as a whole. An interface including actuators and sensors enables the simultaneously running sub-domains to communicate in a closed control loop in “real time”. Specifically, the entire power takeoff of a novel WEC called the “Wave Carpet”, which is classified as a submerged pressure differential device, is shifted into the computational domain. The interaction of the WEC’s absorber unit with incident waves is left in the experiment due to its highly nonlinear characteristics. An extended setup allows to reveal further optimization potential of the novel converter design as a case study. Results of the converter behavior under variable wave states, and for different characteristics of the simulated PTO units are presented. In particular, the presented results show the expected broad band absorption capability of the Wave Carpet by closer examination of the influence of variable PTO unit resistance coefficients on the total, and also on the individual units’ performance.

Commentary by Dr. Valentin Fuster

Ocean Renewable Energy: Wind Energy: Analysis Concepts and Development

2015;():V009T09A037. doi:10.1115/OMAE2015-41045.

Computational fluid dynamics (CFD) simulations were carried out on the OC4-DeepCwind semisubmersible to obtain a better understanding of how to set hydrodynamic coefficients for the structure when using an engineering tool such as FAST to model the system. This study focussed on the drag behavior and the effects of the free surface, free ends and multimember arrangement of the semisubmersible structure. These effects are investigated through code-to-code comparisons and flow visualizations. The implications on mean load predictions from engineering tools are addressed. This study suggests that a variety of geometric factors should be considered when selecting drag coefficients. Furthermore, CFD simulations demonstrate large time-varying loads caused by vortex shedding that FAST’s hydrodynamic module, HydroDyn, does not model. The implications of these oscillatory loads on the fatigue life needs to be addressed.

Commentary by Dr. Valentin Fuster
2015;():V009T09A038. doi:10.1115/OMAE2015-41053.

A hydrodynamics computer module was developed to evaluate the linear and nonlinear loads on floating wind turbines using a new fluid-impulse formulation for coupling with the FAST program. The new formulation allows linear and nonlinear loads on floating bodies to be computed in the time domain. It also avoids the computationally intensive evaluation of temporal and spatial gradients of the velocity potential in the Bernoulli equation and the discretization of the nonlinear free surface. The new hydrodynamics module computes linear and nonlinear loads — including hydrostatic, Froude-Krylov, radiation and diffraction, as well as nonlinear effects known to cause ringing, springing, and slow-drift loads — directly in the time domain.

The time-domain Green function is used to solve the linear and nonlinear free-surface problems and efficient methods are derived for its computation. The body instantaneous wetted surface is approximated by a panel mesh and the discretization of the free surface is circumvented by using the Green function. The evaluation of the nonlinear loads is based on explicit expressions derived by the fluid-impulse theory, which can be computed efficiently.

Computations are presented of the linear and nonlinear loads on the MIT/NREL tension-leg platform. Comparisons were carried out with frequency-domain linear and second-order methods. Emphasis was placed on modeling accuracy of the magnitude of nonlinear low- and high-frequency wave loads in a sea state. Although fluid-impulse theory is applied to floating wind turbines in this paper, the theory is applicable to other offshore platforms as well.

Commentary by Dr. Valentin Fuster
2015;():V009T09A039. doi:10.1115/OMAE2015-41240.

There are potential offshore applications where renewable energy and more distributed power sources could supplement or replace costly equipment upgrades for additional power supply, or costly fuel operating costs. Renewable energy technologies can also be employed in lieu of expensive power umbilicals to provide power to subsea pumps for long distance tiebacks in deepwater. For example, power umbilicals alone required to provide 69kV to subsea pumps in deepwater could be upwards of $300MM for 100 mile long distance tie-backs. A renewable energy source, with storage, integrated into that system could significantly reduce both the CAPEX and OPEX costs.

In 2013, Chevron performed an in-depth evaluation of a Renewable Energy plus Compressed Air Energy Storage and Regeneration system for a 2.6MW application. For the purpose of that study, a floating wind turbine in 365m water depth off the coast of Oregon was evaluated as the energy source as the base case. The system was found to be feasible with initial CAPEX costs replaced within 12 years of operations as compared to installation of a diesel power generation system and the requisite fuel required to run the equipment.

This paper provides a description of the OCAES system, and discusses potential applications in support of the offshore oil and gas industry.

Commentary by Dr. Valentin Fuster
2015;():V009T09A040. doi:10.1115/OMAE2015-41301.

This paper aims at comparing different implementations of the Morison equation for seakeeping analysis in frequency domain. For more consistency, different wave models are considered and the total wave field (incoming wave, the diffracted and the radiated wave field) is included in the Morison equation.

A state-of-the-art of theMorison equation and the drag force linearized forms are presented. The implementation procedure, based on an iterative frequency domain scheme, is developed for the regular and the irregular wave cases.

Seakeeping analysis of an offshore wind turbine is considered as an application case. A comparison between numerical simulations and measured responses is presented.

For the floater’s numerical model, skirts damping effect and hydrodynamic loads applied on cylindrical bracings are modeled using the Morison equation. The drag and inertia coefficients are considered constant for all sea states and calibrated using the experimental results.

Response amplitude operators (RAOs) and short-termstatistics of motions show a good agreement between experimental and numerical results. The influence of different calculation parameters including the wave model (regular/irregular) and the wave fields (incident/total) are investigated.

Commentary by Dr. Valentin Fuster
2015;():V009T09A041. doi:10.1115/OMAE2015-41364.

Tuned Liquid Dampers (TLDs) have proved their efficiency to mitigate the vibratory response of slender buildings under wind or earthquake excitation. Simple semi-analytical models are proposed here to derive the hydrodynamic coefficients (added mass and damping) of axisymmetric TLDs fitted with circular or radial perforated screens. Comparisons are made with experimental values obtained with an Hexapode test bench. Good agreement is observed.

Topics: Dampers
Commentary by Dr. Valentin Fuster
2015;():V009T09A042. doi:10.1115/OMAE2015-41369.

This paper presents a preliminary analysis of Offshore Floating Wind Turbine system subjected to coupled wind and wave loads in time domain. Floating Wind Turbine system in deep water has the great potential in exploiting renewable wind energy in the near future due to the energy and environmental issues. Distinct structural arrangements determine the complexity of motion behaviors and loading characteristics of OFWTs. The aerodynamic loads play a dominant part in the loading pattern, which is distinguished from traditional floating offshore oil and gas structures. To simulate the response induced by wind and wave, a calculation scheme is proposed containing the coupling between aerodynamics and hydrodynamics. Accordingly, a set of time-domain numerical codes is developed and presented. With the integrated aero and hydro dynamic analysis codes, simulations are conducted to obtain performance of Hywind system. Cases for decay, white noise, wind only and the coexistence of wind and wave are investigated. The results are compared to the test statistics collected from a model test, which was carried out in Deepwater Offshore Basin in Shanghai Jiao Tong University.

Commentary by Dr. Valentin Fuster
2015;():V009T09A043. doi:10.1115/OMAE2015-41388.

The viability of offshore wind turbines is presently affected by a number of technical issues pertaining to the gearbox and power electronic components. Current work is considering the possibility of replacing the generator, gearbox and electrical transmission with a hydraulic system. Efficiency of the hydraulic transmission is around 90% for the selected geometries, which is comparable to the 94% expected for conventional wind turbines. A rotor-driven pump pressurises seawater that is transmitted across a large pipeline to a centralised generator platform. Hydroelectric energy conversion takes place in Pelton turbine. However, unlike conventional hydro-energy plants, the head available at the nozzle entry is highly unsteady. Adequate active control at the nozzle is therefore crucial in maintaining a fixed line pressure and an optimum Pelton turbine operation at synchronous speed. This paper presents a novel control scheme that is based on the combination of proportional feedback control and feed forward compensation on a variable area nozzle. Transient domain simulation results are presented for a Pelton wheel supplied by sea water from an offshore wind turbine-driven pump across a 10 km pipeline.

Commentary by Dr. Valentin Fuster
2015;():V009T09A044. doi:10.1115/OMAE2015-41391.

Wind power has great potential because of its clean and renewable production compared to the traditional power. Most of the present researches for floating wind turbine rely on the hydro-aero-elastic-servo simulation codes and have not been exhaustively validated yet. Thus, model tests are needed and make sense for its high credibility to master the kinetic characters of floating offshore structures.

The characters of kinetic responses of the spar-type wind turbine are investigated through model test research technique. This paper describes the methodology for wind/wave model test that carried out at Deepwater Offshore Basin in Shanghai Jiao Tong University at a scale of 1:50. A Spar-type floater was selected to support the wind turbine in this test and the model blade was geometrically scaled down from the original NREL 5 MW reference wind turbine blade. The detail of the scaled model of wind turbine and the floating supporter, the test set-up configuration, the mooring system, the high-quality wind generator that can create required homogeneous and low turbulence wind, and the instrumentations to capture loads, accelerations and 6 DOF motions are described in detail, respectively. The isolated wind/wave effects and the integrated wind-wave effects on the floating wind turbine are analyzed, according to the test results.

Commentary by Dr. Valentin Fuster
2015;():V009T09A045. doi:10.1115/OMAE2015-41599.

Two scaling methodologies are presented to address the dissimilitude normally experienced when attempting to measure global aerodynamic loads on a small scale wind turbine rotor from a full scale reference. The first, termed direct aerofoil replacement (DAR), redesigns the profile of the blade using a multipoint aerofoil optimisation algorithm, which couples a genetic algorithm (GA) and XFOIL, such that the local non-dimensional lift force is similar to the full scale. Correcting for the reduced Reynolds number in this manner allows for the non-dimensional chord and twist distributions to be maintained at small scale increasing the similitude of the unsteady aerodynamic response; an inherent consideration in the study of the aerodynamic response of floating wind turbine rotors. The second, the geometrically free rotor design (GFRD) methodology, which utilises the Python based multi-objective GA DEAP and blade-element momentum (BEM) code CCBlade, results in a more simplistic but less accurate design.

Numerical simulations of two rotors, produced using the defined scaling methodologies, show an excellent level of similarity of the thrust and reasonably good torque matching for the DAR rotor to the full scale reference. The GFRD rotor design is more simplistic, and hence more readily manufacturable, than the DAR, however the aerodynamic performance match to the full scale turbine is relatively poor.

Commentary by Dr. Valentin Fuster
2015;():V009T09A046. doi:10.1115/OMAE2015-41604.

The aerodynamic characteristics of floating wind turbines in yaw are more complex than those of turbines with fixed foundations as a result of the floating platform dynamics under wave action. This paper applies numerical simulation tools to investigate the time varying rotor thrust and shaft power characteristics of offshore floating wind turbines (OFWTs) under different rotor yaw angles and regular sea wave conditions. The study is based on the NREL1 5 MW baseline OFWT installed on the MIT2 tension-leg platform (TLP). Both the wind speed and rotor speed are maintained constant throughout the analysis, though different sea wave heights and periods are considered. The predictions from three different aerodynamic models are compared. These include the Blade-Element-Momentum (BEM) and the General Dynamic Wake (GDW) methods and a higher fidelity Free-Wake Vortex model (FWVM) that is capable of modelling the unsteady skewed helical wake development of the yawed rotor. Initially the motions of the OFWT under both axial and yawed rotor conditions are estimated in a time domain using FAST, an open source software developed by NREL. These motions are then prescribed to WInDS, a FWVM developed by the University of Massachusetts Amherst, to determine the aerodynamic rotor thrust and power as a function of time. The three models have consistently shown that the TLP motion under the modelled wave states exhibits a negligible impact on the time-averaged rotor shaft thrust and power of the yawed rotor. On the other hand, the cyclic component of rotor thrust and power are found to be significantly influenced by the wave state at all yaw angles. Furthermore, significant discrepancies between the predictions for this cyclic component from the three models observed.

Commentary by Dr. Valentin Fuster
2015;():V009T09A047. doi:10.1115/OMAE2015-41874.

The analysis of a FWT is a complex problem, which requires advanced tools. Several numerical solutions have been used to couple hydrodynamics and aerodynamics and some of the available numerical tools have been compared in code-to-code comparisons. However the code validation for analysis of FWTs is limited by the number of available experimental data.

In the present article, DNV GL and Glosten present a code comparison of four numerical tools against model test results. The design used for the analysis is the Pelastar Tension Leg Platform (TLP) by Glosten. A 1/50 downscaled model of the platform and NREL-5 MW wind turbine was tested in MARIN ocean basin. The results from the model tests are used to verify the results from the numerical codes. The FWT is modelled using four different codes: HAWC2 (by DTU and used by DNV GL), BLADED (by DNV GL and used by DNV GL), SIMA (by Marintek and used by DNV GL) and ORCAFLEX (by Orcina and used by Glosten). Although differences exist among these codes, comparable approaches have been used.

Results from the numerical codes are compared against the experimental data, in terms of:

- Natural periods

- Response in regular waves

- Response in irregular waves

- Response in irregular waves with aerodynamic loads.

In general, the results show a good agreement between the different numerical models and all the codes are capable to reproduce the main dynamics of the system. Some deviations were found and should be solved, in order to use these models for a detailed analysis. However these differences do not seem to be due to limitations of the codes and they might be solvable with a more accurate model of the system.

Commentary by Dr. Valentin Fuster
2015;():V009T09A048. doi:10.1115/OMAE2015-41979.

In this work a wind tunnel with an open jet configuration is investigated for use in offshore wind turbine testing. This study characterizes the open-jet wind-tunnel using measurements of the velocity field detailing mean velocities and turbulence intensities with and without a scaled wind turbine. Measurements have been taken downstream to evaluate the expected area of turbine operation and the shear zone. The effects on the flow due to the wake and turbine blockage have also been identified. Additionally, the combination of honeycomb and screens necessary to produce a homogeneous flow at the desired velocity with low turbulence intensity has been identified.

This work provides a useful data set that will be used as a benchmark to evaluate the benefits of recirculating wind tunnels. The data set has resulted in identifying conditions that would prevent producing the desired flows. The data set has also resulted in recommendations concerning the shape of the wind tunnel sections at the University of Maine’s wind-wave (W2) facility to minimize its interactions with the turbine wake expansion, turbine blockage, and the turbine associated wake shear zone.

Commentary by Dr. Valentin Fuster
2015;():V009T09A049. doi:10.1115/OMAE2015-41989.

The effects of operational wave loads and wind loads on offshore mono pile wind turbines are well understood. For most sites, however, the water depth is such that breaking or near-breaking waves will occur causing impulsive excitation of the mono pile and consequently considerable stresses, displacements and accelerations in the mono pile, tower and turbine.

Model tests with a flexible mono pile wind turbine were carried out to investigate the effect of breaking waves. In these model tests the flexibility of the turbine was realistically modelled. These model tests were used for validation of a numerical model for the flexible response of wind turbines due to breaking waves. A focusing wave group has been selected which breaks just aft of the wind turbine.

The numerical model consists of a one-way coupling between a CFD model for breaking wave loads and a simplified structural model based on mode shapes. An iterative wave calibration technique has been developed in the CFD method to ensure a good match between the measured and simulated incoming wave profile. This makes a deterministic comparison between simulations and measurements possible. This iteration is carried out in a 2D CFD domain (long-crested wave restriction) and is therefore relatively cheap. The calibrated CFD wave is then simulated in a (shorter) 3D CFD domain including a (fixed) wind turbine. The resulting wave pressures on the turbine have been used to compute the modal excitation and subsequently the modal response of the wind turbine. The horizontal accelerations resulting from this one-way coupling are in good agreement with the measured accelerations.

Commentary by Dr. Valentin Fuster
2015;():V009T09A050. doi:10.1115/OMAE2015-42015.

Floating offshore wind turbines (FOWTs) contribute to an emerging green energy technology, by exploiting higher and consistent wind speeds above the ocean. There are several challenges facing the design of mooring system of FOWTs, including installation costs, stability of light-weight minimalistic platforms, and shallow depths (50–300m). The extreme tension in mooring lines of a light displacement platform in shallow-water is dominated by snap loads. This is because light pre-tension requirements in the line may be insufficient to prevent the mooring lines from being exposed to wave motion induced slack and shock events.

In this paper, we present a comparative analysis of a semi-submersible based FOWT exposed to a 100-year storm condition, based on model test data and numerical simulations of well-known industry standard software. The data was obtained from a 1/50th-scale FOWT with the wind turbine modeled after the NREL 5MW wind turbine. The software, OrcaFlex, was used for numerical simulations of the mooring system. NREL’s FAST software was coupled to OrcaFlex to obtain aerodynamic loads along with hydrodynamic load for FOWT analyses. The numerical simulation of the moored FOWT in a 3-hour storm was executed in both the frequency-domain and the time-domain to determine the dynamic behavior of the platform and mooring system, respectively. Snap–type impact events were observed in both test data and numerical simulation. Tension maxima were fitted into extreme value distributions and comparisons are made between simulated and measured data. It is seen that snap events follow a different exceedance probability distribution compared to the cycle-to-cycle tension maxima.

Commentary by Dr. Valentin Fuster
2015;():V009T09A051. doi:10.1115/OMAE2015-42028.

For shallow and intermediate water depths, large monopile foundations are considered to be promising with respect to the levelized cost of energy (LCOE) of offshore wind turbines. In order to reduce the LCOE by structural optimization and de-risk the resulting designs, the hydrodynamic loads must be computed efficiently and accurately. Three efficient methods for computing hydrodynamic loads are considered here: Morison’s equation with 1) undisturbed linear wave kinematics or 2) undisturbed second order Stokes wave kinematics, or 3) the MacCamy-Fuchs model, which is able to account for diffraction in short waves. Two reference turbines are considered in a simplified range of environmental conditions.

For fatigue limit state calculations, accounting for diffraction effects was found to generally increase the estimated lifetime of the structure, particularly the tower. The importance of diffraction depends on the environmental conditions and the structure. For the case study of the NREL 5 MW design, the effect could be up to 10 % for the tower base and 2 % for the monopile under the mudline.

The inclusion of second order wave kinematics did not have a large effect on the fatigue calculations, but had a significant impact on the structural loads in ultimate limit state conditions. For the NREL 5 MW design, a 30 % increase in the maximum bending moment under the mudline could be attributed to the second order wave kinematics; a 7 % increase was seen for the DTU 10 MW design.

Commentary by Dr. Valentin Fuster
2015;():V009T09A052. doi:10.1115/OMAE2015-42086.

Scaling effects caused by applying Froude-scaling to both wind and waves during model-testing of floating offshore wind turbines (FOWTs) results in poor model-scale aerodynamic performance of geometrically scaled turbines. This led to the “performance-scaled” MARIN Stock Wind Turbine (MSWT) which showed to be successful in obtaining the correct thrust loads at model-scale conditions. Additionally it was found that conventional blade-element-momentum-theory-based (BEMT) modelling tools are not suitable for model-scale conditions.

Recent research in which these problems have been addressed are presented in this paper. First a 3D CFD study was performed in which the behaviour of the flow over the commonly studied NREL 5MW baseline turbine and the MSWT geometries was performed. Both model and full-scale conditions were studied for a fixed non-moving platform and rotor-only turbine. It was found that scaling effects are indeed significant and a highly three-dimensional and additionally separated flow was observed. Based on these findings two methods were proposed to expand the applicability of BEMT-based tools to off-design and modelscale conditions.

First, instead of using commonly used 2D XFOIL data, 2D CFD RANS data was used. The use of purely 2D data from 2D CFD RANS computations did however not result in the desired improvements when compared to XFOIL-based results. The second proposed method is based on the use of 2D airfoil data obtained by post-processing of 3D flow data coming from 3D CFD computations. This new approach was shown to be successful and can therefore be extremely useful for future model-scale FOWT testing campaigns to do preliminary performance predictions. All BEMT-based and CFD results presented in this paper were compared to model-scale experimental data of the NREL 5MW turbine and the MSWT over the full range of TSR.

Commentary by Dr. Valentin Fuster
2015;():V009T09A053. doi:10.1115/OMAE2015-42154.

It is useful to complement model tests of a floating wind turbine with simulations mimicking the scaled-down turbine. Standard engineering tools have some short-comings to model a rotor at the very low Reynolds that Froude scaled wind and rotor’s rotation speed impose. The flow around an airfoil at the scale of a wave basin brings new distinct challenges than at full scale. The capacity of standard engineering tools for the design of wind turbines to capture this complexity may be questioned. Therefore, work-around solutions need to be proposed. This paper looks at a common solution that consists of optimizing the load coefficients of the rotor to reproduce the measured rotor loads. 3 variants of optimizations are applied to a semisubmersible floating wind turbine at scale 1/50th, the DeepCwind semisubmersible platform. The effects of the differences between these 3 methods on the motions of the floater in waves and wind are analyzed. In the absence of a controller for the rotor, no significant differences related to the induced aerodynamic damping was noticed, but an offset in the motion related to a thrust deficit was observed.

Commentary by Dr. Valentin Fuster

Ocean Renewable Energy: Wind Energy: Operations and Applications of Analyses

2015;():V009T09A054. doi:10.1115/OMAE2015-41018.

This paper will give a short overview of the path of development of the so called GICON® - Tension Leg Platform (TLP) for offshore wind turbines. The main part of the paper will provide a summary as well as insights from three different model basin tests. Furthermore, the comparison of a truss like structure (first concept) with a shell type structure (third concept) deduced from the measured results and also by comparison of the natural frequencies will be presented. Both structures were tested in wave tanks in a scale of 1:25. The results also include a focus on the overall dynamic behavior of the structure. In addition to the two 1:25 models, a 1:37 model was also tested at MARIN, utilizing the MARIN stock wind turbine. This model is also included in the comparison. Therefore the different scales are considered but the comparison is presented exclusively for wave loads as only the 1:37 model was tested under wind and wave conditions.

Commentary by Dr. Valentin Fuster
2015;():V009T09A055. doi:10.1115/OMAE2015-41065.

This paper presents an overview of the successful conclusion of 18 months of testing the first grid-connected floating offshore wind turbine prototype in the Americas. The prototype, called VolturnUS 1:8, was installed off Castine, Maine, USA. The prototype is a 1:8 scale prototype and serves to de-risk the deployment of a full-scale 6MW turbine. VolturnUS utilizes innovations in materials, construction, and deployment technologies such as a concrete semi-submersible hull and an advanced composite tower to reduce the costs of offshore wind. The prototype unit was designed following the American Bureau of Shipping (ABS) “Guide for Building and Classing Floating Offshore Wind Turbine Installations”. Froude scaling was used in designing the 1:8-scale VolturnUS prototype so that the motions of the prototype in the relatively protected site represent those of the full-scale unit in an open site farther offshore. During the past year, a comprehensive instrumentation package monitored key performance characteristics of the platform during operational, extreme, and survival storm conditions. Data collected include: wind speed, turbine power, rotor angular frequency, blade pitch, torque, acceleration; tower bending moment, 6 DOF accelerations at tower top and base, mooring line tensions, and wave elevation at the platform. During the past year the prototype has experienced many environments representative of scaled ABS design conditions including operational wind and sea-states, 50-year sea states and 500-year survival sea states.

This large data set provides a unique view of a near full-scale floating wind turbine subjected to its prescribed environmental conditions. Inspections of the concrete hull following removal provided confirmation of material durability. Marine growth measurements provide data for future design efforts.

Commentary by Dr. Valentin Fuster
2015;():V009T09A056. doi:10.1115/OMAE2015-41078.

The operation of a floating wind turbine may be severely affected by met-ocean conditions. In harsh climates, platform motions might exceed their safety limits and thus force the machine shutdown. It is here proposed a methodology for evaluating the effect of met-ocean conditions on the long-term energy production and dynamic response of such machines.

Given a sample wind turbine, located off the coast of Santander, Spain, met-ocean data are extracted from reanalysis databases for a twenty years lifespan.

The behavior of the wind turbine is simulated in the time domain for a subset of 500 hourly conditions, selected using a maximum dissimilarity algorithm (MDA), to reduce the computational effort. Results regarding floating platform motions are then interpolated for the whole set of data using radial basis functions (RBF).

Tower inclination and hub acceleration are selected as relevant operating parameters. When one of them exceeds its safety threshold, the machine is supposed to be stopped. If no stops are considered, the capacity factor is 39%, while imposing more restrictive tolerances results in a non-linear decrease of the energy yield.

This approach can be helpful in determining a good tradeoff between energy production and reliable operation, bridging the design and operational phases of the project.

Commentary by Dr. Valentin Fuster
2015;():V009T09A057. doi:10.1115/OMAE2015-41126.

There is a large financial incentive to minimise operations and maintenance (O&M) costs for offshore wind power by optimising the maintenance plan. The integration of condition monitoring (CM) and structural health monitoring (SHM) may help realise this. There is limited work on the integration of both CM and SHM for offshore wind power or the use of imperfectly operating monitoring equipment. In order to investigate this, a dynamic Bayesian network and limit state equations are coupled with Monte Carlo simulations to deteriorate components in a wind farm.

The CM system has a ‘deterioration window’ allowing for the possible detection of faults up to 6 months in advance. The SHM system model uses a reduction in the probability of failure factor to account for lower modelling uncertainties.

A case study is produced that shows a reduction in operating costs and also a reduction in risk. The lifetime levelised costs are reduced by approximately 6%.

Commentary by Dr. Valentin Fuster
2015;():V009T09A058. doi:10.1115/OMAE2015-41130.

Offshore wind energy turbines are being deployed massively in the North Sea. Most of the latest developments are monopile based due to the local bathymetry. However, future offshore wind farms will be located at larger water depths. Mainly because the nearest sites to the shoreline will be already occupied, future wind farms will be in 60 m water depth at least. This is, approximately, the limit for fixed support structures, such as monopiles, tripods and jackets. Some developers have already identified this need and some prototypes are under testing, such as WindFloat and Hywind.

Floating wind technology will face some challenges. One of the most important ones is how to moderate the cost of the platform and the mooring system. Consequently, it is necessary to reduce the uncertainty during design steps.

In this paper, new extreme mixed model will be applied to mooring system design. This extreme model combines instrumental and reanalysis data in order to obtain more accurate design parameters, reducing the uncertainty and improving the input that is required for the structural design of these concepts.

Topics: Stress , Design , Mooring
Commentary by Dr. Valentin Fuster
2015;():V009T09A059. doi:10.1115/OMAE2015-41157.

The development of renewable energy sources is a critical global need. The Atlantic coast and Gulf of Mexico of the U.S., with large wind resources and proximity to major population centers, are natural places for such development; however, these regions are also at considerable risk from severe hurricanes or tropical cyclones. Current international guidelines for the design of offshore wind turbines (OWTs) do not explicitly consider loading under hurricane conditions, however subsequent editions are anticipated to include language specific to hurricanes. Variability in extreme loads is greater in areas where hurricanes are likely and the design loads and risk profile of offshore structures installed in such areas are expected to be strongly influenced by hurricanes. For many offshore structures, environmental conditions at design recurrence periods and beyond are often estimated through extrapolation of long-term (i.e. multiple decades) wind and wave measurements from buoys, however, for offshore structures located at areas exposed to hurricanes, it is accepted practice to use physics-based models to augment the historical record of Atlantic hurricane activity and generate a stochastic catalog of synthetic hurricanes that provides tens of thousands of realizations for one year of potential hurricane activity. Once a stochastic catalog has been established, appropriate hazard intensity measures (e.g. the one-minute sustained wind speed, the significant wave height, and the peak spectral wave period) can be estimated for each storm at any site using well-known wind and wave parametric models. In this study, we consider several sites along the Atlantic coast and quantify the impact of estimating hazard for design recurrence periods and beyond for three different methods. The first is based on an extrapolation of wind and wave measurements from buoys, and the second and third are based on a stochastic catalog of synthetic hurricanes with wind and wave intensities estimated based on deterministic and probabilistic relationships.

Commentary by Dr. Valentin Fuster
2015;():V009T09A060. doi:10.1115/OMAE2015-41312.

Approximately 75% of installed offshore wind turbines (OWTs) are supported by monopiles, a foundation whose design is dominated by lateral loading. Monopiles are typically designed using the p-y method which models soil-pile resistance using decoupled, nonlinear elastic Winkler springs. Because cyclic soil behavior is difficult to predict, the cyclic p-y method accounts for cyclic soil-pile interaction using a quasistatic analysis with cyclic p-y curves representing lower-bound soil resistance. This paper compares the Matlock (1970) and Dunnavant & O’Neill (1989) p-y curve methods, and the p-y degradation models from Rajashree & Sundaravadivelu (1996) and Dunnavant & O’Neill (1989) for a 6 m diameter monopile in stiff clay subjected to storm loading. Because the Matlock (1970) cyclic p-y curves are independent of the number of load cycles, the static p-y curves were used in conjunction with the Rajashree & Sundaravadivelu (1996) p-y degradation method in order to take number of cycles into account. All of the p-y methods were developed for small diameter piles, therefore it should be noted that the extrapolation of these methods for large diameter OWT monopiles may not be physically accurate; however, the Matlock (1970) curves are still the curves predominantly recommended in OWT design guidelines. The National Renewable Energy Laboratory wind turbine analysis program FAST was used to produce mudline design loads representative of extreme storm loading. These design loads were used as the load input to cyclic p-y analysis. Deformed pile shapes as a result of the design load are compared for each of the cyclic p-y methods as well as pile head displacement and rotation and degradation of soil-pile resistance with increasing number of cycles.

Commentary by Dr. Valentin Fuster
2015;():V009T09A061. doi:10.1115/OMAE2015-41437.

In an effort to harness the abundant offshore wind resource over deepwater, the development of numerical design tools for floating offshore wind turbines (FOWTs) has progressed steadily in recent years. However, at present, a validated model capable of completely coupling the full elastodynamic response between the mooring system, floating support structure, turbine tower and the wind turbine is not commercially available.

The University of Maine has developed a new FOWT design, VolturnUS, which utilizes a concrete semi-submersible hull. For the VolturnUS design effort a number of numerical models were developed to analyze the system’s global performance. This paper presents the results of a validation study conducted to quantify the accuracy and suitability of a subset of these models for use in the design of the VolturnUS FOWT. Validation was conducted via comparisons of numerical model results to test data obtained from a 1:50 scale model testing campaign conducted by the University of Maine at the Maritime Research Institute, Netherlands offshore basin.

The validation study evaluated the performance and capabilities of the numerical models over a range of design conditions. Emphasis was placed on design load cases (DLCs), which were found to govern the design of the FOWT. The DLCs follow the American Bureau of Shipping’s (ABS) Guide for Building and Classing Floating Offshore Wind Turbines.

Through this method of model validation this work sought to quantify the numerical models’ accuracy, highlight their limitations, justify design assumptions, and identify areas requiring further development in the field of FOWT numerical modeling.

Commentary by Dr. Valentin Fuster
2015;():V009T09A062. doi:10.1115/OMAE2015-41512.

Offshore floating wind turbines (OFWT) are supported by the flexible mooring systems subjected to nonlinear hydrodynamic wave and current forces. Depending on the floater type and environmental condition, the mooring responses can have a significant impact on the overall dynamic performance of OFWT. To evaluate the dynamic responses of OFWT, both uncoupled (quasi-static) and coupled (dynamic) mooring models have been proposed in the literature and in practice based on the use of the well-known FAST software and the FAST-Orcaflex package, respectively. This paper will investigate and compare the dynamics of the OFWT and the mooring lines using uncoupled vs coupled models, based on the OC3-Hywind Spar platform supporting the 5MW wind turbines developed by the National Renewable Energy Laboratory. Preliminary numerical studies in several load cases reveal substantial differences in the OFWT and mooring dynamics obtained by the two approaches, e.g. under regular and irregular waves. The levels of differences are reported, and the comparisons with available experimental results are also made to validate the model analyses and outcomes. The importance of mooring line dynamics and their contributions to the overall 6-DOF responses of OFWT are highlighted which should be recognised in the analysis and optimization design.

Commentary by Dr. Valentin Fuster
2015;():V009T09A063. doi:10.1115/OMAE2015-41544.

A floating offshore wind turbine platform supporting a 2MW downwind-type turbine was successfully installed offshore of Kabashima Island, Goto city, Nagasaki prefecture, Japan on October 18, 2013. It has been operating since October 28, 2013 as the first grid-connected multi-megawatt floating wind turbine in Japan. The spar platform has a unique feature, that is, the upper part of the spar is made of steel (as usual) but the lower part is made of precast prestressed concrete (PC). Such a configuration is referred to as hybrid-spar. In this paper, the design methodology of the hybrid spar is presented — including environmental design conditions, DLCs (Design Load Cases), dynamic analysis, fatigue analysis, etc. Also, the installation procedure is presented briefly.

Commentary by Dr. Valentin Fuster
2015;():V009T09A064. doi:10.1115/OMAE2015-41588.

The present study uses Quikscat data to assess the offshore wind energy potential at the Eastern part of Indonesia, around the Sulawesi and Maluku Islands. Weibull distribution with two parameters is used to represent the characteristics and distribution of the wind model. In order to confirm the accuracy of computations, statistical and numerical methods are used to compute the Weibull parameters which are regression analysis, maximum likelihood method and moment method. The obtained power density is used to draw monthly wind power maps for the sea areas around the islands. The maps are used to determine the most suitable location of a mobile conversion system every month. Moreover, by assuming that a NM72/2000 NEG Micon is installed, Monte Carlo Simulation is performed to determine the expected power percentage produced by the system every month.

Commentary by Dr. Valentin Fuster
2015;():V009T09A065. doi:10.1115/OMAE2015-41666.

With the demand of renewable energy due to the pressure from environmental pollution and global warming, the wind industry has been growing rapidly over the past decades during which the offshore wind innovation is becoming more and more attractive because of vast offshore wind resources.

In this paper, a novel tapered-column semi-submersible floating foundation with economical mooring system is developed to support 6MW wind turbine in South China Sea. The coupled aero-hydro-servo-structural analysis is done in GH-Bladed software to obtain the global dynamic response of the whole floating wind turbine. By combining the derived wind loads with the wave and current induced loads, the global performances of the foundation and attached mooring system under both extreme conditions and normal operation conditions are analyzed in AQWA software. The result reveals that the floating foundation complies with the design standard and meets requirements of wind turbine.

This novel patent-pending floating foundation with tapered columns is proved as a successful design with high material efficiency and good seakeeping performance. Also a reliable and efficient design methodology of floating foundation based on optimal cost is provided in this paper which can be used as design reference of floating foundations.

Commentary by Dr. Valentin Fuster
2015;():V009T09A066. doi:10.1115/OMAE2015-41734.

This study examines the long-term reliability analysis of a floating vertical axis wind turbine (VAWT) situated off the Portuguese coast in the Atlantic Ocean. The VAWT, which consists of a 5-MW 3-bladed H-type rotor developed as part of the EU-FP7 H2OCEAN project, is assumed to be mounted on the OC4 semi-submersible floating platform. Given metocean conditions characterizing the selected turbine site, a number of sea states are identified for which coupled dynamics simulations are carried out using the FloVAWT design tool. Short-term turbine load and platform motion statistics are established for individual sea states that are analysed. The long-term reliability yields estimates of 50-year loads and platform motions that takes into consideration response statistics from the simulations as well as the metocean (wind-wave) data and distributions. Results can be used to guide future floating VAWT designs.

Commentary by Dr. Valentin Fuster
2015;():V009T09A067. doi:10.1115/OMAE2015-41947.

One of the main aspects of a floating offshore wind turbine design is its mooring system, which can strongly influence the floater stability and motions. This is illustrated by considering two catenary mooring systems for the same semi-submersible. The main difference between the two systems is the position of the connection points of the mooring lines on the floater, the so-called fairleads. The philosophy is that the design can be improved by shifting the fairleads to the highest feasible level.

For both mooring systems, the floater motions and stability are assessed. Stability curves are derived, taking both the effect of hydrostatics and the mooring system into account. Floater motions are analyzed using both uncoupled frequency domain calculations and coupled aero-hydro-servo-elastic time domain simulations.

The mooring system is found to have a considerable effect on the floating stability. The effect on the motions is less profound for the considered mooring systems and limited to the low frequency range. Mooring line tensions are however significantly affected by the fairlead position.

It is concluded that, with a well-designed mooring system, a smaller and thus less expensive floater can be used while still meeting the requirements in terms of stability and maximum motions. In addition, the mooring lines may be lighter as well.

Commentary by Dr. Valentin Fuster
2015;():V009T09A068. doi:10.1115/OMAE2015-42000.

With an increasing demand for renewable energy, offshore wind farms become more and more important. Within the next 15 years the German government intends to realize offshore wind farms with a capacity of 15 GW of electrical energy. This corresponds to approximately 3000 to 4000 new turbines.

The grouted joint is a common structural detail for the connection between substructure and foundation piles in offshore wind turbine structures. For lattice substructures such as jackets, the connection is located just above the seabed and is permanently surrounded by water.

Prior investigations by Schaumann et al. showed that the surrounding water may have an impact on the fatigue performance of grouted joint specimens. Thus far, very few results of submerged fatigue tests on grouted joint specimens are published and their statistical reliability is insecure.

Within this paper, 24 individual test results are presented. Regarding test parameters, the focus is set on two different applied load levels, two different loading frequencies and two different grout materials. All parameters are varied in a factorial experiment and are statistically evaluated.

The evaluation shows that load level and loading frequency have a significant effect on the fatigue performance of the connection. Moreover, both effects are significantly correlated. For the used grout materials no significant impact is visible, which can be explained by their similarity regarding mechanical properties and micro structure. Furthermore, the mean displacement and the stiffness degradation of the specimens during fatigue tests are discussed in detail in the paper.

In conclusion, previously published results on the fatigue performance of submerged small scale grouted joint specimens can be confirmed. Load level as well as loading frequency can be stated as most relevant parameters for the fatigue performance.

Topics: Fatigue , Testing
Commentary by Dr. Valentin Fuster
2015;():V009T09A069. doi:10.1115/OMAE2015-42008.

As an alternative to the conventional intact stability criterion for floating offshore structures, known as the area-ratio-based criterion, the dynamic-response-based intact stability criteria was initially developed in the 1980s for column-stabilized drilling units and later extended to the design of floating production installations (FPIs). Both the area-ratio-based and dynamic-response-based intact stability criteria have recently been adopted for floating offshore wind turbines (FOWTs).

In the traditional area-ratio-based criterion, the stability calculation is quasi-static in nature, with the contribution from external forces other than steady wind loads and FOWT dynamic responses captured through a safety factor. Furthermore, the peak wind overturning moment of FOWTs may not coincide with the extreme storm wind speed normally prescribed in the area-ratio-based criterion, but rather at the much smaller rated wind speed in the power production mode. With these two factors considered, the dynamic-response-based intact stability criterion is desirable for FOWTs to account for their unique dynamic responses and the impact of various operating conditions.

This paper demonstrates the implementation of a FOWT intact stability assessment using the dynamic-response-based criterion. Performance-based criteria require observed behavior or quantifiable metrics as input for the method to be applied. This is demonstrated by defining the governing load cases for two conceptual FOWT semisubmersible designs at two sites. This work introduces benchmarks comparing the area-ratio-based and dynamic-response-based criteria, gaps with current methodologies, and frontier areas related to the wind overturning moment definition.

Commentary by Dr. Valentin Fuster
2015;():V009T09A070. doi:10.1115/OMAE2015-42062.

Numerical tools for a single floating offshore wind turbine (FOWT) have been developed by a number of researchers, while the investigation of multi-unit floating offshore wind turbines (MUFOWT) has rarely been performed. Recently, a numerical simulator was developed by TAMU to analyze the coupled dynamics of MUFOWT including multi-rotor-floater-mooring coupled effects.

In the present study, the behavior of MUFOWT in time domain is described through the comparison of two load cases in maximum operational and survival conditions. A semi-submersible floater with four 2MW wind turbines, moored by eight mooring lines is selected as an example. The combination of irregular random waves, steady currents and dynamic turbulent winds are applied as environmental loads. As a result, the global motion and kinetic responses of the system are assessed in time domain. Kane’s dynamic theory is employed to formulate the global coupled dynamic equation of the whole system. The coupling terms are carefully considered to address the interactions among multiple turbines. This newly developed tool will be helpful in the future to evaluate the performance of MUFOWT under diverse environmental scenarios.

In the present study, the aerodynamic interactions among multiple turbines including wake/array effect are not considered due to the complexity and uncertainty.

Commentary by Dr. Valentin Fuster
2015;():V009T09A071. doi:10.1115/OMAE2015-42267.

The presented research has the objective of supporting the integrated conceptual design of floating offshore wind turbines (FOWT). The dynamics of the multidisciplinary coupled system with the aerodynamics, hydrodynamics, structural dynamics, the catenary mooring lines and the controller shall be represented in simulation models adapted to the current design stage. Here, a linear model-predictive controller (MPC) as an optimal multiple input-multiple output (MIMO) controller is designed for a novel concept of the floating foundation for a 10MW wind turbine. The performance of this controller is easily adjustable by a cost function with multiple objectives. Therefore, the MPC can be seen as a benchmark controller in the concept phase, based on a simplified coupled simulation model with only approximate model information. The linear model is verified against its nonlinear counterpart and the performance of the MPC compared to a SISO PI-controller, which is also designed in this work. The developed models show to be well suited and the linear MPC shows a reduction of the rotor speed overshoot and tower bending from a deterministic gust.

Commentary by Dr. Valentin Fuster
2015;():V009T09A072. doi:10.1115/OMAE2015-42320.

The effect of pile-soil interaction on structural dynamics is investigated for a large offshore wind turbine in the hurricane-prone Western Gulf of Mexico (GOM) shallow water. The offshore wind turbine has a rotor with three 100-meter blades and a mono-tower structure. Loads on the turbine rotor and the support structure subject to a 100-year return hurricane are determined. Several types of soil are considered and modeled with a distributed spring system. The results reveal that pile-soil interaction affects dynamics of the turbine support structure significantly, but not the wind rotor dynamics. Designed with proper pile lengths, natural frequencies of the turbine structure in different soils stay outside dominant frequencies of wave energy spectra in both normal operating and hurricane sea states, but stay between blade passing frequency intervals. Hence potential resonance of the turbine support structure is not of concern. A comprehensive Campbell diagram is constructed for safe operation of the offshore turbine in different soils.

Commentary by Dr. Valentin Fuster

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