Directional Spreading in Ocean Swell

Abstract

Directional wave spectra derived from a data set measured off the west coast of New Zealand are used to investigate the directional spreading within swell. The location where the measurements were made is particularly useful for the study as a more or less constant swell component originating from the Southern Ocean is observed in the sea states. The spectra are partitioned into wind-sea and swell components, and estimates of the directional spreading of the swell component is made. A function for the directional distribution of the swell is proposed. INTRODUCTION The wave directional distribution is an important quantity in wave forecasting and in the design and operation of offshore engineering facilities. Considerable effort has focused on the directional distribution of active wind-seas, resulting in significant improvement in the understanding of directionality during wave growth. In most regions around the world, the extreme sea states for which offshore facilities must be engineered are also associated with active wind-seas. However, there are locations, such as offshore West Africa, and offshore operations for which swell is equally if not more important. For example, swell governs the wave design criteria offshore Nigeria, and the persistent swell from the Southern Ocean has an important influence on the workability of vessels off the west coast of New Zealand. But the directionality of swell, and particularly the spreading in swell, has received far less attention than the wind-sea component. The paper reports an evaluation of a wave directional data set, recorded off the west coast of New Zealand. The spectra are partitioned into wind-sea and swell components, characteristics of the swell spreading are established, and a swell spreading function is developed. 1 K. C. Ewans Metocean Engineer, Offshore Technology, Upstream Sector, Shell Global Solutions International B.V., P.O. Box 60, 2280 AB Rijswijk, The Netherlands, k.ewans@siep.shell.com Ewans 1 THE LOCATION The wave data were recorded with a Datawell WAVEC buoy moored adjacent to the Maui A platform, in 110m of water, approximately 30 km off the west coast of the North Island of New Zealand. The location is dominated by a more or less constant swell component that arrives from the southwest. These components arrive through a narrow window defined by a sector with bearings 219° and 249° at the Maui location. These two bearings coincide with the northwestern tip of the South Island of New Zealand and the southern tip of Tasmania respectively. The window defined by this sector has great circle paths that extend all the way to Madagascar, some 11,000 km away. The measurement location and the Southern Ocean swell window are given in Figure 1. 249o 219o 45° S 30° E