pode estar a referir-se a overside discharge valves que seriam vávulas de descarga exteriores

não me ocorre outra maneira de o dizer em Português de momento...são vávulas que estão do lado de fora do casco do navio...

veja algumas referencias:

[PDF] www.portvancouver.com/frames/safenav/harbour_manual.pdf
... of any liquids from a vessel other than ballast water. (Ballast water ... Operations Manual
2.2 SEALING OF OVERSIDE DISCHARGE VALVES It is an offence to ...


Glossary of Terms-ea-b
... spaces, which are normally discharged overside at sea. It is an ... and are also known
as isolation valves. ... Taking material out of a vessel or tower to reduce the ...
www.tisd.net/~wcjcptac/glossary of terms-e a-b.html



Espero que ajude um pouco...