Marine Hydrokinetic Devices Research Articles

Numerical study on micropile group behavior supporting fixed bottom marine hydrokinetic devices in sandy seabed

This study aimed to investigate the behavior of micropile groups supporting fixed bottom marine hydrokinetic (MHK) devices in sandy seabed. A parametric study using PLAXIS 3D was conducted to examine the influence of key design parameters on the lateral load and moment capacity of the micropile group. The parameters considered were micropile diameter, spacing, micropile cap size, number of micropiles, micropile-soil interface properties, and sand packing state. Using the parametric study results, fitting regression techniques were employed to derive equations for predicting the MR (moment resistance in the absence of lateral load) and HR (Lateral resistance in the absence of moment loading), while meeting ultimate limit state (ULS). The equations were formulated based on soil relative density (DR), micropile group polar moment of inertia (J), and an interface reduction factor (Rint). A linear failure envelope is proposed in the Moment-Horizontal (M-H) load space by connecting MR and HR. The proposed framework was then applied to assess the load utilization ratio for various configurations, focusing on a load example of anchoring a 1 MW marine tidal turbine. The results of the study provide insights into the behavior and design of micropile groups supporting fixed bottom MHK devices in sandy seabed.

Modeling Morphodynamic Impacts and Optimization of Marine Hydrokinetic Arrays in Shallow Offshore Environments

Marine hydrokinetic (MHK) devices hold the promise of expanding renewable energy production by tapping into the power of waves and currents for electricity generation. However, these devices remain in the developmental stage, necessitating research to understand their environmental impacts, lower operational costs, and prevent equipment failures. In this study, we investigate various MHK array configurations to gain insights into their effects on wave patterns, water flow, and sediment conditions, considering both short-term and long-term morphodynamic changes under average and extreme conditions in shallow offshore environments. Our objectives encompass understanding the influence of mean and extreme environmental conditions on MHK devices, evaluating their impact on the bathymetry of the ocean floor, and exploring the role of different array configurations in morphodynamic evolution. Our findings, based on modeling these devices as static lumps, reveal that sediment erosion downstream of MHKs increases by 50% after one year of average conditions. When accounting for the absorption of 30% of the energy by MHK devices, downstream sediment deposition surges by almost 125%. Moreover, alterations in MHK arrays, such as spacing, size, and number, result in noticeable changes in sedimentation magnitudes during storm conditions. While long-term mean wave conditions have minimal effects on sedimentation, extreme wave conditions, akin to large storm events, bring about significant alterations in ocean floor bathymetry, potentially leading to costly maintenance of the MHK arrays. Our research provides a valuable framework for site analysis, enabling the estimation of maintenance needs and the optimization of array configurations to minimize sedimentation-related issues.

Modeling the acoustic noise from a wave energy converter farm and its impact on marine mammals at the PacWave South site, offshore Newport Oregon

Marine hydrokinetic devices, such as wave energy converters (WECs), can unlock untapped energy from the ocean's currents and waves. Acoustic impact assessments are required to ensure that the noise these devices generate will not negatively impact marine life, and accurate modeling of noise provides an a priori means to viably perform this assessment. We present a case study of the PacWave South site, a WEC testing site off the coast of Newport, Oregon, demonstrating the use of ParAcousti, an open-source hydroacoustic propagator tool, to model noise from an array of 28 WECs in a 3-dimensional (3-D) realistic marine environment. Sound pressure levels are computed from the modeled 3-D grid of pressure over time, which we use to predict marine mammal acoustic impact metrics (AIMs). We combine two AIMs, signal to noise ratio and sensation level, into a new metric, the effective signal level (ESL), which is a function of propagated sound, background noise levels, and hearing thresholds for marine species and is evaluated across 1/3 octave frequency intervals. The ESL model can be used to predict and quantify the potential impact of an anthropogenic signal on the health and behavior of a marine mammal species throughout the 3-D simulation area.

Flow effects of finite-sized tidal turbine arrays in the Chacao Channel, Southern Chile

To characterize energy resources and study of hydrodynamic effects induced by marine hydrokinetic devices in tidal channels, numerical models need to provide reliable representations of turbine arrays. In regions disconnected from the grid, near coastal protected areas and other relevant economic activities, there is a pressing need to evaluate the impacts of limited-size arrays. Here, we use the emblematic Chacao Channel in Southern Chile to understand the effects of bathymetry and array placement on energy extraction in strongly tidal channels. We implement in FVCOM a parameterization from a previously derived high-resolution model to represent a group of turbines in different locations. We first analyze the complexity of the bathymetry to define the appropriate grid size and obtain a correct representation of the interaction of turbines with the bed morphology. We simulate a base case to identify three suitable locations in the channel where we analyze the effects of the turbines: From simulations we compute the changes in the mean velocity, turbulent kinetic energy (TKE), and bed shear stress. The results show that baseline velocities and TKE are the main factors on the momentum extraction despite the bed complexity. However, in flatter bathymetries, changes on TKE and bottom shear are significantly larger compared to complex morphologies, since turbine arrays modify considerably the original flow conditions. Simulations also provide additional insights that are critical to evaluate the local impacts, showing the directionally-dependent flow resistance of tidal channels, in which the interactions with bathymetry change the downstream effects of turbine arrays in flood or ebb regimes.

Flow Field Measurement of Laboratory-Scaled Cross-Flow Hydrokinetic Turbines: Part I—The Near-Wake of a Single Turbine

Recent developments in marine hydrokinetic (MHK) technology have put the cross-flow (often vertical-axis) turbines at the forefront. MHK devices offer alternative solutions for clean marine energy generation as a replacement for traditional hydraulic turbines such as the Francis, Kaplan, and Pelton. Following previous power measurements of laboratory-scaled cross-flow hydrokinetic turbines in different configurations, this article presents studies of the water flow field immediately behind the turbines. Two independent turbines, which operated at an average diameter-based Reynolds number of approximately 0.2×105, were driven by a stepper motor at various speeds in a closed circuit water tunnel with a constant freestream velocity of 0.316 m/s. The wakes produced by the three NACA0012 blades of each turbine were recorded with a monoscopic particle image velocimetry technique and analyzed. The flow structures with velocity, vorticity, and kinetic energy fields were correlated with the turbine power production and are discussed herein. Each flow field was decomposed into the time averaged, periodic, and random components for all the cases. The results indicate the key to refining the existed turbine design for enhancement of its power production and serve as a baseline for future comparison with twin turbines in counter-rotating configurations.

An uncertainty analysis of propagated sound from an array of marine hydrokinetic devices

Marine hydrokinetic (MHK) devices provide an alternate energy source from tidal, current, and wave motion; however, these devices introduce anthropogenic noise in the marine ecosystem and must meet regulatory guidelines. Paracousti is a 3D finite-difference, time-domain solution to the governing velocity-pressure equations and is used to predict the propagation of sound from an array of MHK sources in any environment. This solution allows for multiple sources, each with unique sound profiles, and for spatially varying sound speeds, bathymetry, and soil composition. However, fluctuations in the hydrodynamic field will introduce operational uncertainties for the sound profiles of the MHK devices, which may not be captured through idealized sound profiles. Preliminary results for a single device in a Pekeris waveguide indicate differences in broadband sound pressure levels of 25 and 60 dB re 1μPa for peak amplitudes and frequencies of 1-20 m, 150 Hz and 1 m, 50-300 Hz, respectively. A Monte Carlo approach is presented for an array of sources to further characterize the range of uncertainties associated with variations in source amplitude and frequency. It is demonstrated that the idealized, deterministic solution, vastly underestimates the compounding uncertainty on the final sound field.

Stochasticism in noise generated by an array of marine hydrokinetic devices

Marine hydrokinetic (MHK) devices generate electricity from the motion of tidal and ocean currents and ocean waves and provide another source of renewable energy. Additionally, MHK devices are also a new source of anthropogenic noise in the marine ecosystem and must meet regulatory guidelines that mandate a maximum amount of noise that may be generated. In the absence of measured levels from in-situ deployments, a model for predicting the propagation of sound from an array of MHK sources in a real environment needs to be established. A 3D finite-difference, time-domain solution to the governing velocity-pressure equations is used, which permits a finite number of complex sources and spatially varying sound speeds, bathymetry, and bed composition. However, deterministic solutions to these types of problems cannot capture uncertainties in the source profiles that may result from operational changes. This work presents the broadband sound pressure levels from an array of MHK sources as the amplitude and frequency from each source are allowed to vary. This Monte Carlo approach demonstrates that the idealized, deterministic solution, vastly underestimates the compounding uncertainty on the final sound field and that these small variations in the source profile cannot be ignored.

Numerical performance analysis of acoustic Doppler velocity profilers in the wake of an axial-flow marine hydrokinetic turbine

The use of acoustic Doppler current profilers (ADCPs) for the characterization of flow conditions in the vicinity of both experimental and full scale marine hydrokinetic (MHK) turbines is becoming increasingly prevalent. The computation of a three dimensional velocity measurement from divergent acoustic beams requires the assumption that the flow conditions are homogeneous among all beams at a particular axial distance from the instrument.In the near wake of MHK devices, the mean fluid motion is observed to be highly spatially variable as a result of torque generation and energy extraction. We examine the performance of ADCP measurements through modeling of a virtual ADCP (VADCP) instrument in the wake of an MHK turbine resolved using unsteady computational fluid dynamics (CFD). This is achieved by sampling the CFD velocity field at equivalent locations to the sample bins of an ADCP, and performing the coordinate transformation from beam coordinates to instrument coordinates, and finally to global coordinates.The stream-wise velocity deficit and tangential swirl velocity caused by the moving rotor lead to significant misrepresentation of the true flow velocity profiles by the VADCP. The most significant errors were observed in the transverse (cross-flow) velocity direction, with a relative error of 17% of the incident velocity. The relative RMS error over the rotor plane was calculate to be less than 2% of the incident velocity by a distance of x/D⩾2,x/D⩾1 and x/D⩾6 for the stream-wise, transverse and vertical directions, respectively. For the default instrument orientation, A maximum error in the V-velocity occurs when the instrument is located directly behind the turbine, and a maximum W-velocity error is found at a lateral offset y/D=0.20. For an instrument directly behind the MHK device, the maximum error in the U-velocity and V-velocity occurs at a yaw angle of ϕ≈30° and ϕ≈60°.

Anchor selection study for ocean current turbines

This paper compares anchoring systems suitable for ocean current turbines, specifically those proposed to be installed off the coast of Southeast Florida near latitude 26° 5’ N. This location boasts one of the most energy dense ocean current resources, with a mean kinetic energy flux of approximately 2.34 kW/m2. Seafloor types ranging from unconstrained sediment to high relief ledges were observed during regional benthic surveys for which applicable anchor types would include deadweight, plate, pile, and drag embedment. Numerical simulations of single point moored marine hydrokinetic devices were used to extract anchor loading at a likely deployment location for mooring scopes from 1.25 to 2.0 and turbine rotor diameters between 3–50m. These anchor loading data were used for preliminary sizing of deadweight and driven plate anchors on both cohesionless and cohesive soils. Finally, the capabilities of drag embedment and pile anchors relevant to ocean current turbines are discussed. Multiple types of drag embedment anchors can support all of the predicted loads if adequate sediment exists and the loading is horizontal, while pile and H-type anchors can support all of the evaluated loading scenarios and chain-in-hole anchors can support turbines with rotor diameters up to 30m.