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Passage to the High North – when spray matters


Why bother about a bit of spray? In mild climate latitudes rain and spray water is hardly a concern; it will run off and the ship happily carries on. But going to the High North it’s different. Albert Aalbers, a.aalbers@marin.nl


W


ith the economic development of the High North and Arctic areas, partly driven by the reduced


ice conditions and partly by opportunities to develop oil & gas and scarce mineral resources, maritime operations in these areas are booming. But safety of the environment, crew and their ships is of prime concern. In the Arctic, global warming doesn’t mean milder conditions...


One of the operational aspects to be con- sidered is icing, formed by precipitation and through seawater spraying onto the ship in cold weather. A major hazard leading to intense icing is a so-called ‘Polar Low’, which are small, low-pressure systems developing into storms with sharp temperature rises – typically from -20 to -1 ˚C – and this is combined with precipitation. A ship in these conditions will be cold, plus the seawater is cold and spray and precipitation will stick.


Wave to wall showing drop clouds (upper from impact, lower from rapid immersion jet)


Time domain simulation In the SALTO JIP icing is one of the parameters considered in the time domain simulation for operations in Arctic conditions. In the metocean model- ling, provided by the Danish Meteorological Institute, precipitation icing is computed,


while MARIN has formed a co-operation with Delft University to develop a computational model for seawater spray. This will predict spray volumes and drop sizes and then the thermodynamic and ballistic process in which the spray water will develop an ice cover on the ship.


The generation of spray on a ship in waves is a hydrodynamic No Man’s Land. In the past year a probabilistic model was developed, taking into account the prime elements of spray generation comprising: - Ship motions - Above water hull shape - Bow wave (at speed) - Wave non-linearity


A thorough examination of high-speed video of large scale waves impacting on a wall, thanks to the Sloshel JIP, found that two mechanisms have to be considered: jet forming due to rapid immersion of a bow section and jet forming due to impacts.


Rapid immersion spray leads to a jet breaking up due to vorticity formed by friction along the hull, according to the well-known Kelvin- Helmholz instability process. The break-up


22 report


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