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

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About 40-50 tankermen lose their lives every year in tanker collision and explosion accidents. Even if most of these accidents occur on conventional single hull tankers it is thought that Double Hull will not reduce these accidents, as the particular risks behind these accidents (severe collisions and oil leaks into ballast spaces) were not properly addressed when Double Hull was mandated for large crude oil carriers.

The COULOMBI EGG arrangement on the other hand was developed based on accident statistics and an attempt was done to eliminate those risks that cause loss of life (and not only oil pollution) in accidents. It is thus easy to demonstrate that the COULOMBI EGG provides better safety for seamen than Double Hull in collisions, even if this aspect has never been properly addressed by the IMO after Double Hull and alternative designs were mandated from 1 July 1993. Then only oil spills due to collisions were considered.

Double Hull prevents oil spill in 75% of all collisions according to the IMO/MEPC Marpol I/13F(5) accident statistics (MEPC66(33)). In 25% of the collisions Double Hull spills from one or more cargo centre tanks. With seven cargo tanks the average spill in all collisions is about 4.8% of total cargo, or average 19.2% of total cargo, when you actually spill. In 16% of all collisions you may breach the upper deck above the cargo tank and a FIRE may start. Unless the side is damaged below waterline, oil may flow down in the empty double side, i.e. the fire is spread into the Double Hull.


COULOMBI EGG tanker protection - a ballast top side tank crush zone - is also estimated to prevent oil spill in about 75% of all collisions. However, the mean spill volume is reduced considerably, as COULOMBI EGG protection permits different size cargo tanks to be located in different risk areas, e.g. in 20% of the collisions you spill only from small lower side tank(s) below waterline (but look here)and in only 5% of the collisions from cargo centre tanks above waterline. The mean spill will not exceed 1.2% of total cargo, i.e. four times less than Double Hull. And in only 4% of the collisions you might breach the upper deck above the cargo tank, when a FIRE may start, but then always the side tank is flooded and water is cooling the cargo tank. Thus, the collision risk picture changes as seen to the left.


The logic of the collision protection is easy to explain. Only when the collision into the tanker is perpendicular, the lower side of the tanker (the hit ship) is damaged below waterline as a function of the forward draught of the striking ship. The longitudinal extent of the damage is short. As collisions between similar size ships (except VLCCs) are most frequent, the main deck of the tanker should absorb the focsle structure ot the striking ship, and the cofferdam and the sloping mid-deck should absorb the stem and bulb of the striking ship. Horizontal deck structure is much stronger than vertical plates - double hull - to absorb collision forces (compare car design).

In non-perpendicular collisions, the flare of the striking ship is assumed to rip off the top side tank structure of the tanker and this is fine for a COULOMBI EGG tanker. The bulb of the striking ship may then not even touch the lower side: it is kept away from the side of the stricken ship by the flare of the striking ship. As isolated collision damage never occurs in the lower 0.25 D of the side, single hull is the best protection there. The 0.25D lower double side of a Double Hull tanker does not add to any protection at all - so why fit it? There are of course many other ways to better protect oil tankers in collisions., e.g. outboard sloping top sides of the tanker extending the crush zone in perpendicular collision and acting as a structural zone that can be ripped off in non-perpendicular collisions. However, such an arrangement is very impractial, when the tanker is loading/discharging and moored against a jetty, when a vertical side is required against jetty fenders, etc. as the tanker draught changes all the time. Another idea is that all ships should have transversly framed bows and flares! The reason is that such structure is very strong vertically (against sea loads, when pitching into waves) but is weaker horizontally, i.e. in collisions. Luckily most ships have transversly framed bows, and most tankers have longitudinal tankbody construction, i.e. the bow of the striking ships deforms and breaks first and absorbs most of the kinetic energy that has to be dissipated in the collision.

Regardless - the IMO Marpol 92/OPA 90 rules only mandate an arbitrary two meters crush zone of new tankers in the upper side - COULOMBI EGG has a B/5 wide crush zone, which is always much, much better - at least four times. It will save many lives of tanker seamen, when fully implemented.

The IMO damage statistics (MEPC66 (33)) have been converted into non-dimensional form and are therefore not fully representative for very large tankers - VLCC's. VLCC's extremely seldom collide with each other and therefore the vertical extent of damage below the water line is never more than about 12 m - the maximum forward draught of 99.9% of all ships. Therefore the actual spill risk from a lower wing cargo tank of a COULOMBI EGG VLCC is not 0.2 (as shown in the above figure) but much less. A realistic spill risk in collision of a COULOMBI EGG VLCC is thus only 0.05 while it remains 0.25 for a Double Hull VLCC.

Furthermore - Double Hull has very large areas inside the double hull to protect against corrosion and between the cargo and ballast spaces, where leaks may occur. COULOMBI EGG protection reduces these areas to less than one third i.e. the safety of the tanker is increased and maintenance costs are reduced.  

 Contact anders.bjorkman@wanadoo.fr