Horizontal Spar Wave Direction Monitoring Buoy

ABSTRACT The equilibrium position of a center tethered buoyant spar buoy on the surface of the ocean is parallel to the crests of the wave train. This phenomenon has been used in the design of a wave direction monitoring buoy which permits reliable and inexpensive measurement of wave direction by an unattended buoy recording system when placed in a coastal or open ocean wave environment. The system design is discussed, with emphasis on the conceptual and operational aspect. Field tests are presented and discussed which indicate close correlation between measured and observed data. INTRODUCTION The measurement of surface wave amplitude and period provides only limited understanding of the wave forces on an offshore structure, vessel, or shore-based construction project. With an ever increasing emphasis being directed towards the ocean in search of petroleum and minerals, and with the construction of billion dollar structures, such as nuclear power plants, the need has developed for an instrument to measure wave amplitude and period, and also wave direction. A number of oceanographic instruments have been devised to measure wave direction, but the art has remained a relatively complex task. A multiple float surface buoy1, multiple bottom mounted pressure sensors, multiple wave staffs, the use of a 2 axis horizontally oriented solid state current meter2, or airborne radar techniques have been employed but have disadvantages in baseline environmental surveys due to the need for special platforms, lack of reliability, or the need for complex and costly computer processing. The method described in this paper measures wave direction directly from a center tethered lightly ballasted horizontal spar. The design minimizes the effects of current and wind and can be used with moorings in any depth of water. The recorded directional data is the direction of the wave train in cases of waves originating from a common origin or a measurement of the direction of the maximum wave energy in sea conditions of multiple wave train origins. BACKGROUND The concept of the horizontal spar wave direction monitoring buoy evolved from work carried out by Braincon Corporation (now Environmental Devices Corporation) on the P1ank-On-Edge (P.O.E.) buoy. It was observed during model tank tests and at sea that the buoy, with the absence of current, would orient itself parallel to the wave crests. It appeared that the buoy would orient to experience minimum forces from the waves. A number of requests were received from customers for a buoy which would provide wave direction data for such projects as beach erosion studies, pipeline surveys, docking facility investigations and O.C.S. environmental baseline studies. The requests also stressed the need for a simple and reliable device, which was easy to maintain, and cost effective. The general requirement was for a device which provided a good first order measurement, not one which detailed the fine structure of the waves. The basic question was "Would the proposed structure survive?".