Measurement and Analysis of Ocean Wave Fields in Four Dimensions

Ocean wave data comprises either instrumentally measured data or model-derived data, and the former type of data is preferred in the offshore industry. Instrumental data can be considered to be comprised of both directly measured sea surface displacement data and derived data, from the acceleration of buoys. It has been found that significant differences can occur between sea surface displacements, which are recorded in steep waves by fixed probes or lasers (Eulerian), or by free-floating buoys (Lagrangian). This has given rise to the situation where wave buoy data should not be used to estimate wave profiles in steep waves. Short crested and heaped waves, in moderate to high sea states, can also cause a problem when recording wave data at a fixed point, when it comes to determining the representivity of the results across a wave field. Recorded wave data is used as the basis for the development as well as the verification of all wave models and, given the above uncertainties, the authors propose a new wave measurement method, using the recently developed Automated Trinocular Stereo Imaging System (ATSIS), for the recording of three-dimensional surface wave displacements with respect to time. The ATSIS is a novel system, which measures the temporal evolution of three-dimensional wave characteristics for analysis. An oblique configuration for the system effectively increases spatial coverage, allowing observations of wave phenomena over a broad range of temporal and spatial scales. The details in the paper provide a solution of quantifying the behaviour of irregular, non-linear, and directionally spread (short crested waves), and provides an efficient method for developing better design criteria in the future.