Sea spray concentrations and the icing of fixed offshore structures

We compile measurements of sea spray droplet concentrations near the ocean surface for wind speeds from 0 to 28.8 m s-1. We plot each concentration distribution with the Andreas/Fairall spray generation function for that wind speed to display the production velocity distribution that is required for them to be compatible. The comparison shows that the equilibrium assumption is not consistent with this spray generation function. As an alternative, we try a spray concentration function from the literature and find that it represents the data well for moderate to high wind speeds. Using the compiled data, we then extend this concentration function to the very high wind speeds that generate spume droplets. This function has a stronger dependence on wind speed and a longer tail than the concentration function for jet and film droplets produced by bursting bubbles in whitecaps. To validate these concentration functions, we use a simple ice accretion model with weather data and icing observations from two offshore platforms. The results show that moderate-to-high wind speeds that generate film and jet droplets result in small ice accumulations. However, larger spume droplets created at very high wind speeds produce high icing rates. The spray generation function that is consistent with the equilibrium assumption has a median volume droplet radius characteristic of jet droplets for moderate and high wind speeds and a radius that is characteristic of spume droplets at very high speeds. Copyright (C) 2011 Royal Meteorological Society