Characteristics of the Infragravity Wave Climate

Abstract

Infragravity waves are very long period waves below the frequency of typical wind waves. They are most significant in shallow water locations and therefore have a high impact on the response of moored vessels. For the Oil and Gas business this can be an important consideration for tanker on/offloading operations (including LNG vessels) — these larger vessels, with longer natural periods, are particularly susceptible. There are implications for both the design and operation including the calculation of extreme loading on the mooring system, extreme vessel motions, fatigue of mooring systems and the availability of on/offloading operations. There are currently limited design practices to account for the effect of infragravity waves. This may be attributed to two main factors: The development of infragravity waves is difficult to model and is sensitive to many factors, including the magnitude and shape of the incident wind and swell spectra, local bathymetry, directionality and near shore wave breaking. Secondly, very little measured data exist since the infragravity wave frequencies lie below the conventional range of commonly deployed wave measurement devices. The present paper will provide a description of the infragravity waves acting on the US coast at two locations, Duck, North Carolina, and Baja, California. The results will characterize parameters including the significant wave height, peak period, and comparison of infragravity waves through time. In addition, the relationships between the spectral shape will be examined including directionality. This type of information is needed to set design criteria for infragravity waves, and in the longer term to develop and enhance infragravity wave models e.g. Reniers 2002 (1) and ultimately contribute to establishing design practices.