The Coulombi Egg Oil Tanker - Grounding Protection
Better protection, safer and more economical than Double Hull


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Double Hull prevents oil spill in about 75% of all groundings according to the IMO/MEPC Marpol I/13F accident statistics, but, in every grounding considered, it is assumed that the outer hull is breached and water enters the fore peak and/or double hull ballast spaces. This means that the displacement increases and that the tanker will be stuck on the ground. In 25% of the groundings Double Hull spills from one or more cargo tanks, i.e. the displacement is reduced, but it is probable that the tanker remains stuck on the ground. Even if some oil collects inside the Double Hull, the discharge into the environment is important and expensive to clean up. According to the statistics Double Hull will be associated with major oil spills also in the future.

A Marpol I/13F(4) Mid-deck tanker (figure below) is expected to reduce the total discharge in all groundings considerably, because the cargo oil in the lower tanks is in hydrostatic balance with the outside sea water.



The GROUNDING FORCE is expected to breach the cargo tank bottom (and retard the tanker) in almost all groundings of a Mid-deck tanker, but the total discharge or pollution is reduced considerably compared with Double Hull, as the cargo outflow is restricted by an excessive external sea water pressure pushing up the cargo against the tank top. The only discharge is the INITIAL EXCHANGE LOSSES i.e. when the Mid-deck tanker runs aground at high speed and is slowed down and the single bottom is breached and water (dark blue in the figure) enters the bottom cargo tank(s), then the cargo itself forms a 'bubble', which floats inside the breached tank. However, this 'bubble' is not slowed down, but it is pushed against the forward bulkhead during the retardation phase, and some oil spills out below the bulkhead as can be seen on the figure above. These initial exchange losses are very small - average 1% of the capacity of the breached tank volume (depending on the speed of the tanker before the accident) - but they are subject to much controversy, even if it has been shown that the Marpol13F Mid-deck tanker reduces total pollution in all accidents very much compared with Double Hull. Note that, if the Mid-deck tanker does not have a double bottom in the fore peak tank, the fore peak is flooded in grounding and the tanker trims on the bow (it might act as a break!). It should also be remembered that the Marpol Mid-deck tanker has ballast side tanks down to the bottom, which will also fill up in grounding and may list the ship. The safest grounding protection has no or minimal void spaces in the bottom to prevent any water inflow anywhere, so you lose buoyancy and get stuck. This is the COULOMBI EGG logic.

The COULOMBI EGG tanker (below) grounding protection is expected to reduce the Initial Exchange Losses to virtually nil. When a COULOMBI EGG tanker runs aground and is slowed by the grounding force and the bottom is breached and water flows in (dark blue in the figure), the 'oil bubble' formed in a lower tank is of course pushed forward, but it is also pressed up into a big access trunk forward and aft of each tank, from where the cargo later can escape by gravity under controlled conditions to the top side ballast tanks, which acts as evacuation tanks (if there is a tide). By permitting the 'oil bubble' to expand upwards through air, which offers much less resistance than water and to allow an equivalent amount of water to enter the breached tank, the risk that cargo oil spills out below the forward bulkhead as initial exchange losses is reduced. Of course some oil may spill out, if the side is ripped away, but the amounts are small. 

In all event, the COULOMBI EGG arrangement is the only tanker grounding protection which eliminates all further grounding pollution due to falling tides - all cargo at risk is transferred by gravity (actually pushed up) to the top side ballast tanks before the tide falls.

Finally, as the COULOMBI EGG tanker virtually does not increase its displacment in any grounding - it floats on the cargo - it is expected that you can quickly get off the ground and move to a sheltered location (without assistance of tugs). You then surround the tanker with booms, transfer the oil in the breached tanks to the ballast tanks or to a lightering tanker, and then, in principle, you should be able to proceed to the discharge port (subject to many other restrictions, e.g. the residual strength of the vessel which is always good - the upper deck and the mid-deck constitute a lot of strength). The best solution is of course to to off-load all cargo, trim the vessel on the stern (assuming the damage is forward) and to clean the damaged lower cargo tanks and to seal the tanks, and then proceed to a repair yard.

If you go aground and spill oil, the first GOLDEN RULE is to spill small amounts that can be mopped up quickly by interested parties at a reasonable cost using existing technology. There is no technology available that can mop up a big oil spill and the only permitted Marpol tanker design today associated with big spills is Double Hull. The second GOLDEN RULE is to get off the ground as quickly as possible and to move to a sheltered place. The IMO/Marpol grounding statistics do not include consequential damages, e.g. that the ship, that was initially grounded, becomes later a total loss due to changing weather. Thus it is possible that future accident statistics of DoubleHull tankers will have a fair number of groundings, where the tanker initially gets stuck and does not spill any oil but later becomes a total loss because it could not get off the ground due to breached and water filled ballast tanks.