A Comparison of Wave Directional Spreading Measurements Made With a Spotter Buoy and a Directional Waverider Buoy in Parallel

The directional spreading is an important characteristic of a sea state. For example, the probability of extreme wave crests occurring within the sea state and the loading on offshore structures are both dependent on the amount of spreading. Accordingly, field measurements of waves that allow quantification of directional spreading provide valuable insight into the directional characteristics of sea states and consequently into wave crest and wave load modelling. Wave buoys that measure three orthogonal component of surface displacement permit directional spreading to be estimated; the Datawell Directional Waverider buoy and the Sofar Spotter buoy are two examples. The Directional Waverider is a 90 cm diameter buoy and has seen widespread use since its introduction some 35 years ago. The Spotter is a recent development, and due its small size, with a diameter of 38 cm, and relatively low-cost, it has become popular solution for wave measurements, particularly in remote regions of the ocean. The Directional Waverider wave measurements are based on its own acceleration measured with onboard accelerometers, whereas the Spotter wave measurements are based on its own velocity measured with GPS. In this paper we compare the relative performance of the Directional Waverider and the Spotter buoys, with particular focus on the directional spreading, using parallel data sets from measurements made at a near shore, 17 m water depth, location in the US Army Corps of Engineer's field research facility at DUCK, on the east coast of USA.