Article

# Comparative wave measurements at a wave energy site with a recently developed low-cost wave buoy (Spotter), ADCP and pressure loggers

Authors:
• Bermello Ajamil & Partners Inc.
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## Abstract

Wave measurements from a new, low-cost, real-time wave buoy (Spotter) are investigated in a comparative study as part of a site characterization study at a wave energy candidate site at King Island, Tasmania, Australia. Measurements from the Sofar Ocean Spotter buoy are compared with concurrent measurements from a Teledyne RD Instrument (RDI) 1200 kHz Work Horse ADCP and two RBRsolo ³ D wave16 pressure loggers. The comparison period between 8 th August – 12 th October 2019 provides both the shallowest and longest continuous published comparison undertaken with the Spotter buoy. Strong agreement was evident between the Spotter buoy and RDI ADCP of key wave parameters including the significant wave height, peak wave period, and mean wave direction, with the mean values of those parameters across the full deployment period agreeing within 3%. Surface wave spectra and directional spectra are also analyzed with good agreement observed over the majority of the frequency domain, although the Spotter buoy records approximately 17% less energy within a narrow frequency band near the peak frequency when compared to the RDI ADCP. Measurements derived from the pressure loggers routinely underestimated the significant wave height and overestimated the mean wave period over the deployment period. The comparison highlights the suitability of the Spotter buoy for low-cost wave resource studies, with accurate measurements of key parameters and spectra observed.

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... The site location has a micro tidal range [17]. The relatively small tidal range means large currents are not expected in the area, however due to the relatively shallow deployments (particularly at Location 2), the tides are likely to affect sediment transport due to the different wave transformations and orbital wave velocities at the seabed, at high or low tide. ...
... October 2019 by Lancaster, et al. [17], where the two instruments were found to agree on key wave parameters within 3%, illustrating the high accuracy of the Spotter buoy. The Lancaster, et al. [17] study also provides more information regarding typical frequency and directional spectrums at the project site. ...
... October 2019 by Lancaster, et al. [17], where the two instruments were found to agree on key wave parameters within 3%, illustrating the high accuracy of the Spotter buoy. The Lancaster, et al. [17] study also provides more information regarding typical frequency and directional spectrums at the project site. The location of the wave buoy throughout the OWC WEC deployment at King Island referred to in this manuscript is given in Figure 1b, with a mean water depth of approximately 9.7 m, latitude of −40.066692 • and longitude of 144.058334 • . ...
Article
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This study provides the first ever published measurements of scour and morphological change around an Oscillating Water Column (OWC) Wave Energy Converter (WEC) device at a real-world site, with the intention of informing future designs to reduce costs of the technology. A 200-kW prototype OWC WEC was deployed at King Island, Tasmania, Australia in January 2021, providing a unique opportunity to monitor the device using a combination of dive footage, multi-beam surveys and bedrock surveys. Settlement of the device was observed and monitored before ceasing once the foundation made contact with the underlying bedrock at the site. It is hypothesized that the settlement is caused by scour undermining the gravity structure’s foundations. The processes causing this scour are explored and possible future design modifications are suggested to reduce the risk of scour and settlement.
... Recent advances in accurate and low-cost motion sensors and GPS technology have led to the development of a low-cost easy-to-handle wave measurement platform called Spotter (Raghukumar et al. 2019;Lancaster et al. 2021). It is a sea state detector with unprecedented coverage (Smit et al. 2021;Houghton et al. 2021) and thanks to its small size, it might turn out to be a practical tool for estimating wind speed from wave spectra (Voermans et al. 2020). ...
... The scatter of the peak parameters is explained more by the ill-defined nature of the parameter than any actual differences between the devices. Otherwise, the validation statistics are similar to those determined for other small buoys when validated against established technologies (Raghukumar et al. 2019;Lancaster et al. 2021). ...
Article
Wave buoys are a popular choice for measuring sea surface waves, and there is also an increasing interest for wave information from ice-covered water bodies. Such measurements require cost-effective, easily deployable, and robust devices. We have developed LainePoiss (LP)—an ice-resistant and lightweight wave buoy. It calculates the surface elevation by double integrating the data from the inertial sensors of the microelectromechanical system (MEMS), and transmits wave parameters and spectra in real time over cellular or satellite networks. LP was validated through 1) sensor tests, 2) wave tank experiments, 3) a field validation against a Directional Waverider, 4) an intercomparison of several buoys in the field, and 5) field measurements in the Baltic Sea marginal ice zone. These extensive field and laboratory tests confirmed that LP performed well (e.g., the bias of H m 0 in the field was 0.01 m, with a correlation of 0.99 and a scatter index of 8%; the mean absolute deviation of mean wave direction was 7°). LP was also deployed with an unmanned aerial vehicle and we present our experience of such operations. One issue that requires further development is the presence of low-frequency artifacts caused by the dynamic noise of the gyroscope. For now, a correction method is presented to deal with the noise. Significance Statement Operational wave buoys are large and therefore expensive and inconvenient to deploy. Many commercially available devices cannot measure short waves and are not tested in ice. Our purpose was to develop an affordable wave buoy that is lightweight, ice resistant, capable of measuring short waves, and also has a longer operating life than existing research buoys. The buoy is easily deployed with a small boat or even an industrial drone, thus reducing operating costs. The buoy is accurate, and captures waves that are too short for operational wave buoys. This is relevant for coastal planning in, e.g., archipelagos and narrow fjords. We measured waves in ice in the Baltic Sea, and are planning to extend these measurements to Antarctica.
... The wave climate has been monitored at the location of the OWC WEC prior to the deployment and measurements of wave parameters by a Sofar Ocean Spotter buoy and Nortek AWAC are presented in Fig. 3. The performance of the Spotter buoy has previously been favourably assessed with an inter-comparison of wave measurements at the King Island site along with an RDI ADCP and two pressure loggers, confirming it's high measurement fidelity [22]. ...
... A time series showing the variation of the significant wave height (Hm0) and peak wave period (Tp), along with a scatter diagram showing the distribution of the significant wave height and mean wave direction is shown in Fig. 3. The device is situated in a location with a mean water depth of approximately 6 m, and a micro tidal range is present with a maximum tidal range of 1.56 m recorded at the site over a two month measured period by [22]. ...
Conference Paper
Full-text available
This manuscript reviews experimental scour investigations with reference to a recent Oscillating Water Column (OWC) Wave Energy Converter (WEC) deployment, in addition to presenting preliminary numerical results for scour potential around large caisson-like structures, similar to an OWC WEC. Previous research on OWC WEC’s has focused mainly on the energy efficiency or internal flow patterns, with limited understanding of potential scour behaviour. Despite this, large scale OWC WECs have significant scour potential due to the complex interactions of highly energetic wave climates with the bluff body structure and alternating inflow/ outflow of water within the device. The wave-seabed-structure interactions contribute to increased sediment motion due to the potential formation of lee-wake vortices and wave reflection/ diffraction effects. Besides environmental impacts that need to be quantified, scour around an OWC WEC has the potential to directly affect the structures stability, and create settlement issues which in turn can lead to compromised device performance and further deterioration of structure stability. It is likely that for future investigations of scour potential around OWC WEC’s, project-specific numerical or experimental investigations will be required to enable optimisation of the device geometry and test design innovations for scour reduction. Preliminary results using the open-source CFD REEF3D model are presented showing scour development around a caisson-like box structure. Utilising the Smagorinsky Sub-grid Scale (SGS) turbulence model, results are shown to compare well with previous experimental measurements, however fundamentally different scour patterns form when the Reynolds-Averaged Navier-Stokes (RANS) k-ω turbulence model is used. Understanding the (in)ability of CFD models to represent the relevant physical processes is therefore critical to the development of the OWC WEC industry.
... The natural frequency is 1.2 Hz (Raghukumar et al., 2019). Spotters have been increasingly used for ocean surface wave observations worldwide (Kodaira et al., 2021;Lancaster et al., 2021;Smit et al., 2021). Spotter utilizes GPS-based ocean-wave measurements (Joodaki et al., 2013). ...
Article
Full-text available
Ocean surface wind and wave information is important in a wide variety of areas, such as coastal disaster reduction, offshore structure design, and atmosphere-ocean flux estimation. This study proposed a new method for ocean surface wind estimation from surface wave spectrum information measured by small GPS buoys. The concept of this method relies on the assumption that the high-frequency part of the ocean-wave spectrum is proportional to u∗f^−4 where u∗ is the friction velocity and f is the frequency. The determination algorithm for the coefficient of f^−4 was optimized in this study. The wind direction was determined by the wave cross-spectrum, assuming that the wind direction aligns the propagation direction of the high-frequency part of the wave. The proposed wind estimation method was applied to bay and open ocean observations, and the performance of the proposed wind estimation method was similar between the bay and the open ocean. The proposed method improves the wind estimation especially in coastal areas and at high wind speeds in the open ocean compared with the previous method. The performance of the method of the previous study differs between the bay and open ocean due to their spectral shape differences. High-quality wind and wave information can be obtained using the proposed method. If the mass deployment of small drifting buoys covered the global ocean, the information based on the proposed method could be considerably powerful, and could compensate for the weakness of satellite-based wind and wave estimations.
... (a) The time sequence is transformed to its frequency components using Fourier analysis, the pressure correction range is 0.05∼0.33 Hz (Lancaster et al., 2021). (b) Each frequency component is multiplied by the inverse of the attenuation suffered by that particular frequency due to the depth of the logger. ...
Article
Full-text available
Waves, currents, and related sediment transport are important factors driving the development of saltmarshes. Separating the effects of waves and currents accurately from simple field observations is a technical challenge in shallow water environments with limited inundation such as saltmarshes. In this paper, the estimation method of both wave and current information was studied mainly through the data obtained by a solely used field ADV (Acoustic Doppler Velocimeter, Vector). Phase and pressure inversion wave methods were both used to estimate wave parameters, which were then compared to synchronous observation by wave loggers at the front of a saltmarsh. Our results show that ADV is able to achieve simultaneous observations of tidal currents and waves independently. The pressure inversion wave method (r2 ∼ 0.996) is more effective than the phase method (r2 ∼ 0.876) in estimating wave energy when comparing with wave logger results. However, the former is more restricted by submergence depth and duration in field, while the latter provides cut-off frequencies for the pressure inversion wave method. Both methods can be combined to best estimate wave parameters from field ADV data. Further, hydrodynamic observation on a Scirpus mariqueter patch at the front of a saltmarsh was used as an application to indicate the importance of obtaining both wave and current information from field data. The patch was found to mainly reduce the advection of tidal currents, but it slightly increases wave energy. Therefore, it is necessary to consider the different effects of plants on tidal currents and waves simultaneously in field observations. This study confirms that ADV and associated analysis can detect waves and currents at reliable accuracy at the marsh edge, which is vital in assessing the long-term resilience of marshes to sea level rise and increased storm severity.
... The short two-day hydrodynamic deployment in favourable weather conditions provided only a snapshot of the hydrodynamic forcing due to waves on the seabed at 10 m water depth. However, a long-term deployment is required to monitor the wave regime and local seabed conditions for energy production feasibility and stability of the WEC as done on the east side of King Island [28]. ...
Article
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Marine renewable energy is still in its infancy and poses serious challenges due to the harsh marine conditions encountered for wave or tidal installations and the survivability of devices. Geophysical and hydrodynamic initial site surveys need to be able to provide repeatable, reliable, and economical solutions. An oscillating water column wave energy converter is to be installed on the west coast of King Island, Tasmania. The location is in a high-energy nearshore environment to take advantage of sustained shoaling non-breaking waves of the Southern Ocean and required site-specific information for the deployment. We provide insight into scalable geophysical site surveys capable of capturing large amounts of data within a short time frame. This data was incorporated into a site suitability model, utilising seabed slope, sediment depth, and water depth to provide the terrain analysis needed to match deployment-specific characteristics. In addition, short-term hydrology and geotechnical work found a highly energetic seabed (near seafloor water velocities <1 m/s) with sufficient bearing capacity (6 MPa). In a highly energetic environment, care was taken to collect the relevant data needed for an assessment of critical information to an emerging technology companies primary project. This is in addition to the malleable methodology for a site suitability model that can incorporate various weighted parameters to prioritise the location for shallow wave energy sites in general.
Article
The interaction between offshore waves and complex reef bottom profiles determines the hydrodynamic processes within the reef beaches, for instance, Sea-Swell (SS) wave dissipation, Infragravity (IG) wave generation as well as wave-induced currents and setups. These hydrodynamics phenomena are important to several ecological processes and control the associated shoreline erosion and flooding. Hence, a good understanding of the dynamics is needed that wave observations are challenging and expensive otherwise. Here we present a short term (12 hours) investigation of non-directional spectral waves using a series of HOBO pressure sensors. Six pressure sensors were deployed at 3 locations along the fringing reefs of Tunda Island, Banten Province - Indonesia. The wave spectra were estimated from the surface water fluctuations using linear wave theory for each hourly burst data. During observation, the Tunda waters were calm with significant wave heights were less than 10 cm with peak wave periods of ∼12 s at all sites. Despite the short period measurement due to memory issue, the result demonstrated the ability of HOBO pressure sensors to measure non-directional wave spectra at shallow water.
Article
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A yearlong wind and wave dataset derived from a cost-effective GPS wave buoy (Spotter) is compared to waves obtained from an accelerometer wave buoy (Datawell) and winds obtained from a collocated meteorological buoy. Overall, there was a strong correlation in bulk wave parameters obtained from the Spotter buoy and the Datawell buoy (RMSE of 0.06 m, 0.27 s, and 21.89°for significant wave height, mean wave period, and peak wave direction, respectively). Similarly, the bulk characteristics compared well when separating the directional wave spectra into sea and swell components with more scatter in mean wave periods for lower frequencies. Measurements of mean directional spread from the Spotter buoy were 7.51°larger than the Datawell, spreading more wave energy across different directions outside the peak direction. Additional analysis indicates that the increased directional spreading arises from a bias in the a1 and b1 spectral moments. Using the equilibrium frequency range of the wave energy spectrum, the Spotter buoy can derive wind speed and direction. This study found the Spotter buoy can report wind speed and direction with reasonable accuracy but underestimates wind speeds less than 4 m/s and greater than 8 m/s and was less accurate reporting wind directions oriented offshore. A comparison with a Spotter buoy in deeper water revealed the Spotter buoy further offshore was most accurate in reporting wind directions indicating this proxy-based measurement of wind speed and wind direction may be significantly influenced by fetch and local geography (e.g., large-scale cuspate coastlines).
Article
Experimental measurements of wave scour are reported for a 1:20 scaled model of an operating 200 kW Oscillating Water Column (OWC) Wave Energy Converter (WEC) in King Island, Tasmania, Australia. Test cases include different wave conditions (Keulegan-Carpenter (KC) number range of 0.45-1.02) and multiple geometric changes to the device, which are compared to scour measurements taken in the field around the King Island OWC WEC device. The measurements therefore provide the first ever comparison of field data with a scaled laboratory model of scour around an OWC WEC. Results highlight that scour forms at the back corners of the device before undermining the structure with depths up to 64 mm (scour depth to structure width ratio, S/B, of 0.094), although the magnitude of this scour is shown to be reduced by 23% by curving the back corners. Scour at the front of the device is highly sensitive to the configuration of the front chamber door, with very different scour scenarios depending on whether the chamber door is open, closed, or removed entirely. Comparison with field data shows that undermining scour at the back of the device is reproduced in both settings, and this leads to settlement of the structure in both the laboratory and field. This suggests that the physical modelling forms an accurate representation of a real-world site, and therefore demonstrates scour processes with applicability to OWC WECs and other gravity-based bluff body structures such as caissons or gravity-based foundations.
Article
Full-text available
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The use of Triaxys directional wave buoy and acoustic Doppler current profiler (ADCP) for wave measurements are relatively recent. The US National Oceanic and Atmospheric Administration’s (NOAA) National Ocean Service (NOS) acquired these instruments in 2001 and systematic laboratory and field tests were conducted during 2001–2002. This paper describes the field tests conducted near the US Army Corps of Engineers’ Field Research Facility (FRF) ocean pier and near the Barren Islands in the Chesapeake Bay. At the FRF site, Triaxys buoy wave measurements were compared with FRF’s field standards of pressure sensor arrays and Datawell Waverider buoy. For the Bay test, ADCP was compared with the Triaxys buoy. There are significant numbers of outlier in the Triaxys peak periods at both test sites. In the Chesapeake Bay, which is dominated by high frequency and low energy waves, there is much scatter in the Triaxys data for significant wave heights below 0.2 m. Detailed analyses were performed after these outliers and noisy data were removed. Statistics of differences in significant wave heights, peak periods and directions between each comparative pair were computed and characteristics of frequency and frequency-direction spectra were examined. Overall, correlations between each instrument pair are very good in significant wave heights, fair in wave peak periods (except the ADCP/Triaxys pair), and marginal in wave directions. Triaxys buoy compared better with Waverider buoy than with others. Both ADCP and FRF pressure sensor array exhibit higher resolution in detecting multi-modal and multi-frequency waves. In most cases, energy distribution of spectral peaks in Triaxys buoy data differs significantly from those obtained from FRF pressure sensor array and ADCP.
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This paper considers the comparative performance of a new pressure sensor wave recorder with that of two acoustic wave recorders. This new recorder is small (265 mm times 38 mm OD), and can be mounted on the sea or harbour floor, fastened to an existing structure, or attached to a mooring line. Recorders of this new type have been deployed in an open bay alongside both Nortek AWAC and RD Instruments ADCP acoustic instruments. The output of interest of any wave recorder is the significant wave height. It was demonstrated that as the sea-state became rougher, the new recorder correlated well with both acoustic systems. For calmer seas, the new recorder showed some discrepancies with the AWAC system, however, the AWAC was able to detect much higher wave frequencies during these episodes. This is a consequence of the deployment depth and the different measuring techniques of the AWAC system and the pressure-based recorder which is disadvantaged because the pressure signal of higher frequency waves experiences rapid attenuation with depth. These deployments demonstrated that the new recorder is a viable low cost method of measuring tide and sea-state in moored applications provided care is taken with the mooring details. To optimize the deployment, a new method for computing attenuation and other parameters is presented
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A simple, computationally efficient method is proposed as a standard procedure for the routine analysis of pitch-and-roll buoy wave data. The method yields four directional model-free parameters per frequency: the mean direction, the directional width, the skewness, and the kurtosis of the directional energy distribution. For most applications these parameters provide sufficient directional information. The estimation procedure and error characteristics of the parameter estimates are discussed and illustrated with computer simulated data. An optional interpretation of the combination of skewness and kurtosis as an indicator of uni-modality of the directional energy distribution is suggested and illustrated with field observations.
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Shadows of individual islands are observed in directional spectra sampled with a high resolution linear array at Torrey Pines Beach, California. A detailed investigation of the spectra indicates that the Channel Islands restrict the wave energy density to certain narrow directional sectors. A deep spectral trough, associated with San Clemente Island, is a predominant feature in the well resolved spectra (wave frequencies ~0.06-0.15 Hz). Negligible values of energy density in the center of this directional gap'' were consistently observed in the range 0.082-0.114 Hz. Measurable but low gap energy density values are seen in the high and low frequency regimes. Generation of high frequency waves (f>0.13 Hz) by local winds generally smears the island windowing effects and even creates a spectral peak in a directional sector which is blocked from deep ocean exposure. Several estimation techniques are used in the directional spectrum analysis. These include the Maximum Likelihood Method (MLM) and two methods developed in this work. The two new techniques show significant improvement over the MLM in the definition of gaps in the spectrum. Although none of these methods is considered an optimal'' estimator for general use, each displays some superior merit in particular directional spectrum estimation problems.
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Surface waves can be recorded in two kinds of ways, either with a fixed (Eulerian) probe or with a free-floating (Lagrangian) buoy. In steep waves, the differences between corresponding properties can be very marked. By a simple physical model and by accurate calculation it is shown that the Lagrangian wave period may differ from the Eulerian wave period by as much as 38 %. The Lagrangian mean level is also higher than the Eulerian mean, leading to possible discrepancies in remote sensing of the ocean from satellites. Surface accelerations are of interest in relation to the incidence of breaking waves, and for interactions between short (gravity or capillary) waves and longer gravity waves. Eulerian accelerations tend to be very non-sinusoidal, with large downwards peaks, sometimes exceeding - g in magnitude, near to sharp wave crests. Lagrangian accelerations are much smoother; for uniform gravity waves they lie between −0.388 g and +0.315 g . These values are verified by laboratory experiments. In wind-generated waves the limits are probably wider. In progressive gravity waves in deep water the horizontal accelerations generally exceed the vertical accelerations. In steep waves, the subsurface accelerations can slightly exceed those at the free surface. A novel application is made to the rolling motion of ships. In very steep, irrotational waves it is shown theoretically that the flow near the wave crest can lead to the rotation of the hull through angles up to 120° by a single wave, even if the wave is not breaking. This is confirmed by simple experiments. The efficiency of the keel appears to promote capsizing.
Article
Although the measurement of waves with pressure transducers has been practised since around 1947, there still remains a considerable difference in findings on the adequacy of linear wave theory to compensate the pressure records. Measurements of surface elevation in gravity waves are compared with corresponding estimates from simultaneous subsurface pressure measurements. A review of previous work precedes a description of laboratory tests in a 103-m-long wave flume at Canada's National Water Research Institute. Pressure transducers and surface-piercing capacitance wave probes are used to collect data in water depths of 0.9 and 1.2 m with monochromatic and irregular waves. Results indicate that linear theory is adequate to compensate pressure records to give surface wave heights to within five percent. Reasons for greater discrepancies in previous studies are discussed.
Conference Paper
Directional wave measurements from an RDI Workhorse with Waves are compared with those from the traditional Datawell Directional Waverider buoy. The measurements were conducted simultaneously at the same location over several months, and show good agreement. The principles of two measurement techniques are contrasted.
Wave monitoring equipment comparison: An evaluation of current and emerging in-situ ocean wave monitoring technology. Queensland Dept. of Environment and Science Coastal Impacts Unit Publ
• E Andrews
• L Peach
Andrews, E., and L. Peach, 2019: Wave monitoring equipment comparison: An evaluation of current and emerging in-situ ocean wave monitoring technology. Queensland Dept. of Environment and Science Coastal Impacts Unit Publ., 63 pp.
2020: Data validation
• T Janssen
Janssen, T., 2020: Data validation: Statistics at Mavericks. Sofar Ocean, https://www.sofarocean.com/posts/data-validation-statisticsat-mavericks.
Waves primer: Wave Measurements and the TRDI ADCP Waves Array Technique
• R D Teledyne
• Instruments
Teledyne RD Instruments, 2011: WavesMon v3.08 user's guide. Teledyne RD Instruments Doc., 72 pp. --, 2017: Waves primer: Wave Measurements and the TRDI ADCP Waves Array Technique. Teledyne RD Instruments Doc., 32 pp., http://www.teledynemarine.com/Documents/Brand_Support/ RD_INSTRUMENTS/Technical_Resources/Manuals_and_Guides/ General_Interest/Waves_Primer_Jul17.pdf.
Wave Swell Energy's UniWave200 is installed at King Island
Wave Swell Energy, 2021: Wave Swell Energy's UniWave200 is installed at King Island. Wave Swell Energy, https://www. waveswell.com/king-island/wave-swell-energys-uniwave200-isinstalled-at-king-island/.