Stephen F. Barstow

ASSESSING THE GLOBAL WAVE ENERGY POTENTIAL

In this paper the evaluation of the global wave energy potential is presented based on data from a global wind-wave model (validated and calibrated against satellite altimeter data) and buoy data (the WorldWaves database). The theoretical potential was computed first using all the available wave data and, in a second step, areas in which the power level is very low (P≤5kW/m) were excluded. Finally, in the third step, areas impacted by sea ice were removed. Annual and seasonal power distributions are presented both in tables and maps. The technical resource was also assessed for the west coast of Iberian peninsula showing a significant power decrease from north to south within only 500 km.Copyright

Satellite wave measurements for coastal engineering applications

Measurements from the GEOSAT, ERS-1 and 2 and Topex/Poseidon satellites have now accumulated to over 15 years of global ocean wave and wind data. Extraction of wave height, wind speed and wave period from the satellite altimeters and directional wave spectra from the synthetic aperture radars are reviewed along with recent validation and calibration efforts. Applications of the data to a variety of problems illustrate the potential of satellite wave measurements.

The Wave Energy Resource

On an average day, about 1TWh of wave energy enters the coastal waters of the British Isles. It is tempting to call this amount huge - it is about the same as the to-tal energy of the terrible Indian Ocean tsunami of the 26th of December 2004. It brings home the scale of human energy demands to realise that this is also about the same amount of energy as is used in electricity in the British Isles on an aver-age day. The same approximate equivalence holds at a world scale: the total wave energy resource is of the same order of magnitude as world electricity consumption (~ 2TW). The exploitable limit is probably at most about 10-25% of the resource; thus ocean wave energy is potentially a significant contributor to human energy demands, not a panacea. Its key advantages are that it comes in a high quality form - mechanical energy of oscillation - and that it travels very long distances with little loss, so that small inputs over a large ocean can accumulate and be harvested at or near the ocean’s edge. Other advantages include the point absorber effect, whereby devices can extract energy from a fraction of a wavelength on either side; this makes small devices, with capacities of the order of 1MW, relatively attractive.

Wave Crest Sensor Intercomparison Study: An Overview of WACSIS

The Wave Crest Sensor Intercomparison Study (WACSIS) was designed as a thorough investigation of the statistical distribution of crest heights. Measurements were made in the southern North Sea during the winter of 1997-1998 from the Meetpost Noordwijk in 18 m water depth. The platform was outfitted with several popular wave sensors, including Saab and Marex radars, an EMI laser, a Baylor wave staff and a Vlissingen step gauge. Buoys were moored nearby to obtain directional spectra. Two video cameras viewed the ocean under the wave sensors and their signals were recorded digitally. The data analysis focused on comparisons of the crest height measurements from the various sensors and comparisons of the crest height distributions derived from the sensors and from theories. Some of the sensors had greater than expected energy at high frequencies. Once the measurements were filtered at 0.64 Hz, the Baylor, EMI and Vlissingen crest height distributions match quite closely, while those from the other sensors were a few percent higher. The Baylor and EMI crest distributions agreed very well with the statistics from second order simulations, while previous parameterizations of the crest height distribution were generally too high. We conclude that crest height distributions derived from second order simulations can be used with confidence in engineering calculations. The data were also used in investigations of crest and trough shapes and the joint height/period distribution INTRODUCTION Knowledge of the statistical distribution of crest heights given the wave spectrum is central to the problem of setting deck heights. Unfortunately, there is still considerable uncertainty about the form of this distribution. The empirical evidence is confusing, since different types of instruments have tended to give different results. The theoretical problem is difficult since it is essential to account for the non-linearity of the waves. The participants at the E&P Forum (1995) Workshop on Uncertainties in the Design Process felt that there was a need to

Directional wave spectra by inversion of ERS-1 synthetic aperture radar ocean imagery

An algorithm that extracts the directional ocean wave spectrum from synthetic aperture radar (SAR) ocean image spectra is implemented and applied to spaceborne C-band SAR data obtained from the ERS-1 satellite. The nonlinear iterative algorithm is based on the Hasselmanns forward spectral transform extended to include the range bunching effect. An analytic expression for the wave spectral increment is derived based on the exact gradient of the quasilinear ocean-to-SAR transform. Enhanced wave spectra have been obtained using first-guess wave spectra either from the numerical wave model WINCH operated by the Norwegian Meteorological Institute or synthesized from nondirectional wave data and meteorological conditions. The inverted spectra are compared to in situ directional wave data. It is concluded that the wave imagery from ERS-1 appears to be of excellent quality, and as soon as the backscatter modulation transfer functions are properly understood, satellite SAR data will be an important tool for enhancing and extending conventional wave measurements and results from numerical wave models. >

The Wavescan second generation directional wave buoy

The authors describe Wavescan, a multipurpose data buoy specially designed for directional wave measurements and meteorological data collection. Their objective was to produce a second-generation high-capability metocean data buoy, with full in situ processing, real-time telemetry, and onshore result presentation. Emphasis is on the design of a buoy hull with the wave-following capability needed to accurately measure wave slope while at the same time retaining the stability to operate and collect meteorological data under the extreme conditions the buoy is likely to meet. The authors briefly review the advantages and disadvantages of the various buoy hulls that have been employed for collection of metocean data. The stability and dynamic response of the final design are then discussed, and results from a field test intercomparison during which a prototype buoy was deployed for several weeks off the mid-Norway shore are examined. The Wavescan system functions and the directional wave analysis are summarized. It is concluded that Wavescan has reached its design goals. >

Some recent developments in wave buoy measurement technology

Two new wave sensors, the Motion Reference Unit (MRU) and the differential Global Positioning System (GPS) technology used in the new Smart-800 buoy are described and the results of a series of sea trials and intercomparisons with conventional wave measuring buoys using Datawell sensors are given. The results have shown that the directional spectra derived from the MRU are practically indistinguishable from the Datawell Heave/Pitch/Roll (Hippy) sensor. The Smart-800 directional wave data are also shown to be of good quality. Both sensors have no moving parts and are, therefore, more robust than the conventional accelerometer based wave sensors which have dominated the market for the last 30 years. In addition, of importance in particular to coastal applications and transportation in the high latitudes, neither sensor is sensitive to extremes of temperature. Both sensors have also low mass making for lighter buoys, easier deployment and transportation.

Observations of the High-Frequency Range of the Wave Spectrum

This paper takes a new look at the high frequency range of the wave spectrum. The analysis is based on data sets from two recent field campaigns offshore Portugal and Crete carried out in the MAST II WAVEMOD project, data from the WADIC experiment in the North Sea, and deep sea data from Haltenbanken and Voeringplataaet offshore Norway. In addition, the authors also show spectra obtained by spectral inversion of ERS-1 SAR imagery. The influence and calibration of wave measuring instrumentation and the use of wavenumber spectra when comparing spectra from shallow water is emphasized.

A field validation of a directional waverider in a SEAWATCH buoy

A directional waverider has been tested in the multi-sensor SEAWATCH buoy in a field trial near to the island Froya on the exposed western coast of Norway in which the SEAWATCH buoy was moored close to a standard directional waverider. In this paper a statistical intercomparison of simultaneous directional wave spectra from the two buoys shows that the SEAWATCH buoy hull-mooring system does not have any significant influence on the directional wave measuring capabilities of the directional waverider. It is therefore verified that the directional waverider can be added to the wide range of sensors already routinely used in the SEAWATCH buoy.<<ETX>>

Intercomparison of Seastate and Zerocrossing Parameters From the WACSIS Field Experiment and Interpretation Using Video Evidence

During the WACSIS field experiment, wave elevation time series data were collected over the period December 1997 to May 1998 on and near the Meetpost Nordwijk platform off the coast of the Netherlands from an EMI laser, a Saab radar, a Baylor Wave Staff, a Vlissingen step gauge, a Marex radar and a Directional Waverider. This paper reports and interprets, with the help of simultaneous dual video recordings of the ocean surface, an intercomparison of both single wave and sea state wave parameters.Copyright