INTRODUCTION

The need to measure wave height and wave direction with no radio range limitation led the French Ocean Agency, l'Institut Francais de Recherche pou l'Exploitation de la Mer, to design and test new products.

The first SPEAR buoy, a wave height spectrum buoy transmitting via ARGOS, was used to evaluate the wave energy resource along the French Atlantic coast (Ollitrault and Coudeville, 1982).

The Wadibuoy (Néreides, France) uses the heave, pitch and roll Hippy sensor (Datawell, the Netherlands) and a Thomson flux gate compass, processes the data in-board and transmits energy and mean direction in 19 selected period bands via the ARGOS system. This wave directional spectrum buoy in high sea-states. (Ezraty and Cavanie, 1981; Cavanie et al., 1982; Ezraty et al., 00).

In both cases, the use of the ARGOS system to transmit the processed data and to locate the buoys has been very satisfactory. On the other hand, the hard-wire technology to build the SPEAR buoy in the later 1970s led to complex equipment which has to be set and maintained by skilled people.

This paper described these buoys; emphasis is put on the new SPEAR-F buoy.

The SPEAR-F buoy can be easily used and maintained; its acquisition and processing sub-system is Waverider-compatible. Intercomparisons between the SPEAR-F spectrum and the standard 27 MHz Waverider buoy data have been made. At-sea test results are presented. The data collected from the two wave height buoys agree very well in different sea-state conditions.

Besides this work, a new static wave directional sensor has been designed by IFREMER to eliminate the drawbacks of non-static sensors which are more fragile. It is based on three accelerometers and three flux-gate sensors. The data collected with the static sensor are very promising. An intercomparison between this sensor and the Hippy sensor (Datawell) was made during the winter of 1985/6.

The Wadibuoy

This 2.5 m diameter, flat-bottom fiberglass toroid buoy is equipped with a Datawell Hippy 40 sensor and Thomson flux-gate compass. The total weight, including the 1 year life battery is about 800 kg (fig 1). Furthermore, to prevent the buoy from capsizing a cone-shaped structure was mounted above the main body.

Fig. 1. The Wadibuoy - mooring operation (available in full paper)

The buoy was first fitted with a radio transmitter to send the raw data to shore. This configuration was successfully tested during two 3-month campaigns off Brittany. The received data were processed in real time, using fast Fourier transforms according to the method proposed by Longuet-Higgins et al. (1963). The heave and slope information which are redundant were used to compute slope transfer function of the anchored buoy: results showed that it was flat and close to unity for periods from 2.5 to 16 s (Fig 2). (Ezraty et al., 00,1983)

To extend the potential use of the buoy to the open ocean, the ARGOS satellite data transmission and positioning system was used, this having already proved its ability for collecting non-directional wave spectra. Since an ARGOS message is 256 bits long.

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