About 40-50 tankermen lose their lives every
year in tanker collision and explosion incidents. 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 latest collision
was early January 2018 when the Iranian M/T Sanchi
(double hull) was rammed and 32 tankermen were killed! As
the Sanchi collision was not perpendicular the structural
damages were all above waterline where the double hull
tanker outer/inner sides were ripped open by the flare of
the forecastle of the bulk carrier. The bulbous bow of the
bulk carrier never touched the tanker outer side below
waterline. The double hull ballast space was down flooded by
burning cargo oil from the breached cargo tanks making
things very nasty. Depending on the subdivision of the
tanker burning oil could have spread in the double hull
heating up undamaged cargo tanks from below, etc. Double
hull tankers are actually very unsafe in high energy
collisions.
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. The protection of the tankermen is
ignored!
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
FIREmay 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.
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.
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. 