The “virtual” double bottom

Stena argues that switching all single-hull VLCCs to hydrostatic balanced loading would improve environmental protection.

It is decision time for the world`s VLCC fleet. Of over 500 ships sailing, or on order, 200 are pre-Marpol single-hulled vessels which will reach 25 years of age by 2003. If owners of all the older tonnage decided to scrap their ships there would be a shortage of transport for the 40 million barrels of oil which are moved around the world each day. This would create big problems for the world economy.

Oil tankers of 20,000 dwt and above, and product carriers of 30,000 dwt and above, can continue trading beyond 25 years by adopting one of two strategies: installing tanks protecting 30 per cent of the ship`s side or bottom (SBT) or, alternatively, move to hydrostatic balanced loading (HBL). For the majority of vessels, creating the required protective tanks would be expensive and reduce load volume by between 10 and 20 per cent. HBL only requires changes to the loading and inert gas operations, while imposing a minimal loss of cargo volume.

With conventional loading to say 98 per cent, the oil exerts a pressure on the bottom of the tank which is greater than that exerted by the surrounding seawater. So if the bottom is holed oil will flow out. With HBL, the cargo level is reduced slightly (while ballasting to maintaining the draught) and the inert gas pressure in the ullage space is kept to a minimum. This ensures that the water pressure under the hull is greater than that exerted by the cargo – so if the tank bottom is holed, water will flow in rather than oil escaping.