Sea contamination of fuel.
As all the bunker tanks are seawatercompensated so in essence you could say the fuel was always in contact (or contaminated) by sea water. Due to a substantial difference in the specific gravity between the sea water and dieselfuel, approximately 0.87, the seawater is settled in the lower part of the tanks, the fuel to the consumption tanks (daytank/settlingtanks) in the engineroom is taken from the upper part of the bunkertanks. If by accident some of the fuel should be seawater contaminated, the engineers would notice this by the spyglasses on the settling tanks which are fitted with drainage facilities to the dirty fuel oiltank below the floorplating. As Simon are quoting, the fuelling hose was filled with air floating both the hose and phonecable. This air would eventually be forced into the bunkertanks where it collects on top of the fuel. As you do not want this air in your system you have venting possibilities inside the submarine. As previously mention I guess the fuelling hose was emptied by blowing same by air which eventually was vented as mention above.
Tore