In order to design a COULOMBI EGG tanker the midship section
and tank body layout must be correct from the
beginning. Generally speaking the single hull cargo
tank body, about 0.8 L long, should be divided by two
longitudinal bulkheads 0.2 B from the sides and by three
transverse bulkheads to form four cargo tanks, <0.2 L
long, across (the top side ballast tanks need only two
bulkheads) as per IMO Circ. 336. The centre mid-deck
is then located at 0.55 D from base and the wing mid-deck is
at 0.45 D from base. The wing mid-deck is sloping to connect
to a side cofferdam located at 0.25-0.35 D. You need the
side cofferdam to ensure that 0.75 D of the side is void or
ballast spaces. Slop tanks are arranged aft in the upper,
center part. The lower 0.25 D of the side is single hull.
The tank layout is suitable to carry three grades of cargo
handled by free flow. See the General Arrangement drawing
below as an example:
When the volumes of the individual cargo
tanks are known, you have to carry out an oil
outflow calculation as per Marpol I/13F(5)
guidelines (Heiwa
Co.will do this for you) to confirm
that the Environmental Protection Index E is
larger than one (E>1). This is always the case
if you follow the particular requirements of the
MEPC circular letter.
The COULOMBI EGG tanker spills oil
in collision only if (i) the longitudinal bulkhead
is damaged or if (ii) the damage extends into the
lower 0.25D above BL due to the topi side ballast
tank crush zone. In order to reduce oil outflow
from the lower side cargo oil tanks they may be
fitted with partial oil tight (upper part only)
bulkheads. In groundings oil outflow is minimal due
to the excess hydrostatic pressure, even if the
'probability of zero spill' is low by
definition. Outflow due to tide effects are always
nil.
The forward and aft lower side cargo tank spaces
(total very small cubic) may be very difficult to clean due
to structure, so they may become void spaces (or ballast
spaces) and this will increase E. An alternative is that you
make the longitudinal bulkhead inboard sloping at the ends,
as shown on the General Arrangement above. The forepeak will
not be used for ballast so it becomes a void space. To
prevent the fore peak to be flooded in groundings it is
suggested to fit it with a double bottom.
The scantlings of longitudinal structure is governed
by the Rule midship section modulus requirements. Thus the
deck, shell and bottom plates and stiffeners will be the
same as for single hull. The scantlings of the midheight
deck structure is governed by rules for local strength. To
maximize cargo capacity and to optimise the ballast capacity
the centre tank deck may be raised to form a trunk (as shown
below).
The transverse structure consists of a web frame
supported by the main deck, the mid-height deck and the
bottom. i.e. standard, single hull web frame design, albeit
with a midheight deck as support. All high, local stresses
are in the web face flats and are calculated by conventional
methods (2-D FEM). All web beams - deck centre, deck wings,
upper side shell, lower side shell, upper and lower
longitudinal bulkhead and bottom wing transverse webs are
only supported by the end structure and are connected to the
adjacent web with a triangular bracket. The lower centre
tank bottom web is also supported by two vertical crossties
to transfer the load on the centre bottom transverse web to
the mid-height deck transverse web.
COULOMBI EGG typical transverse web
frame is seen right.
The result in the standard loading
conditions - full cargo load and ballast - will be
a very lightly stressed transverse web frame with
the high stresses in the face flats. The local
loads on the transverse webs are then transferred
to the longitudinal members producing a reasonable
sagging moment in loaded condition and hogging
moment in ballast condition of the tank body
beam.
The forepeak will probably not be used for
ballast and will become a void space. It is
recommended to fit a double bottom in the
forepeak.
The residual strength of the tank body
structure is enormous; if an internal part is
damaged, the cargo (or ballast) will flow to the
adjacent tank and the local load is reduced, i.e.
the structure will not be further damaged. If an
external part is damaged in, e.g. loaded condition,
water will flow into the tanker, all loads are
balanced and the strength is maintained. No leaks,
no major damage.
In a Double Hull tanker internal or external
damage generally leads to flooded (by cargo or sea water)
double hull spaces, which in turn may affect the transverse
or longitudinal strength - the residual strength is reduced
as the tank boundary plates are main strength members of the
transverse forces and moments - the inner cargo tank plate
supporting the outer shell plate and vice versa (and no web
face flats).
Manholes in the web frame are provided in line with
longitudinal side stiffeners and transverse deck web frame
face flats that are arranged as permanent inspection
walkways of the upper deck and mid-deck structure and the
submerged tank washing machines in the lower tanks. Walkways
are also arranged on top of the bottom webs. In spite of the
fact that there is little natural light in the lower cargo
tanks, inspection is still easy by help of airdriven
intrinsic safe lights and good ventilation. Access in the
ballast tanks is very simple and repair of ballast tank
protective coating is thus very easy and can be carried out
while the ship is loaded at sea.
The transverse oil tight bulkheads are supported by
vertical web frames about 0.1 B apart and horizontal
stiffening. In order to provide access to the lower tanks
there are large access trunks forward and aft. Wash
bulkheads may be fitted at mid-length in the upper centre
tanks.
Access from deck to the lower cargo tanks are thus
via large trunks to the upper deck forward and aft of each
lower tank. In the centre tank these trunks are located in
the corners between a web and the bulkhead. Gas freeing and
ventilation of lower cargo tanks use the access trunks for
inlet and outlet. Gas freeing times are as for single
hull.
The tank body structure consists generally of flat
stiffened panels with webs made in a panel line and that are
later assembled into long volume blocks in the workshop. The
volume blocks are then brought to and welded together in the
building dock. The mid-height deck constitutes a good
platform for dimensional control during the block assembly
in dock.
Below follows extracts from TANKER TECHNOLOGY (The Naval
Architect, June 1993) about the COULOMBI EGG
tanker. Most of the observations are still valid. Note
that in the end it was the Swedish Maritime administration
that sponsored the approval at the IMO 1997.
Towards a safer Supertanker: the
Coulombi Egg
The French-based naval architect Anders Bjorkman
continues the campaign to promote his special mid-deck type
VLCC and argues its merits over double hulls. The concept is
said to be applicable to small tankers as well.
The era of the single-hull tanker comes to an end on 6
July 1993 when Marpol 73/78, regulation 13F enters into
force. At that point, all new tankers over 5 000dwt must
have double hulls, mid-decks with double sides, or an
alternative arrangement of equivalent environmental
protection. The 280 000dwt COULOMBI EGG tanker
is an alternative design, which uses well-known single hull
technology as much as possible to this end and introduces
some new features developed over the past three years.
Smaller tankers would have identical arrangements.
The structural drawings shown right andbelow, as
an example, were developed for a 280 000 DWT
COULOMBI EGG TANKER with following particulars
-
Table 1 - Particulars
Length pp 317.0m
Breadth 57.6m
Depth 28.8m/31.5m
Draught 21.0m
Block coefficient 0.824
Deadweight 280 000dwt
Cargo capacity (100%) 330
000m3:
Upper Centres including slops 4x32
650 m3, Lower Centres 4 x 34
900m3,
Lower Wings 2 x 7000m3, 2+2 x
8700m3, 2 x 5500m3
SBT (100%) 102
000m3:
Upper Wings 2 x 21
000m3, 2 x 6000m3, 2 x 20
000m3, Peak and trim tanks 8
000m3
Steel weight 36 000 tonnes: Outfit
+ margin 7856 tonnes, Lightship 43 856
tonnes.
Main engine MAN B&W 9S8OMC, Engine Output 41
040bhp,
Speed 16 knots
Midship section
The midship section longitudinal construction is
shown in Fig 1. This is a single-hull midship
section with a stepped mid-deck from side to side,
sloping at the side and connected to a 900mm wide
longitudinal cofferdam ending 0.25D above the base
line. The upper wing tanks are the Marpol
segregated ballast tanks. More than 80% of the
cargo is carried inboard of B/5 from the side. Less
than 10% of the cargo is carried in small wing
tanks adjacent to one side below 0.25D above the
base line. This basic layout introduces no greater
a risk of pollution than a two meters wide double
hull, as explained below.
There is approximately 20% more longitudinal
material than in a single hull tanker, i.e., the
added mid-deck. This adds some 8%-l0% to the total
steel weight.
Transverse webs
A typical transverse web is shown in Fig 2. This
is designed in mild steel for easy fabrication and
laid out for easy tank cleaning of the lower centre
tank. The side webs are supported by the mid-deck,
which reduces deflections considerably, and centre
bottom and mid-deck webs are connected by two
vertical struts to distribute the internal and
external loads between the webs. Most corner
brackets are of standard design and some are of
high-tensile steel.
Transverse strength
The number of loading permutations of the tank
body, with four upper and four lower centre tanks,
four pairs of lower cargo wing tanks, and three
pairs of upper wing ballast tanks, is increased
compared with other types of tanker. Twenty-four
different combinations are shown in Figs 3A, 3B and
3C.
In actual operation, only conditions numbers 1,
4, 10 and 12 (100% loaded, 80% loaded, 40% loaded
and segregated ballast) will probably be used, and
any intermediate combinations are achieved by slack
cargo tanks. It is possible to operate with an
upper centre tank full and the lower tank below
empty and vice versa at various draughts, but it is
not possible to operate with an upper centre tank
and a lower centre tank below simultaneously empty
at full draught or full at the small draught due to
longitudinal and local strength limitations.
The lower cargo tanks can be loaded only at
certain minimum operating draughts, according to
Marpol, to ensure that the outside hydrostatic
pressure exceeds the internal cargo/ vapour
pressure - then the pressure difference will retain
the cargo inside the tank body even if the shell
plating is damaged.
The cargo space is highly suitable for three
grades of cargo (upper centre, lower centre, lower
wings) handled by free flow. The cargo tank
arrangement evidently requires a new understanding
of the sequence of cargo loading and discharge,
also cargo distribution, but it does not differ
from a conventional single- or double-hull tanker
with full depth tanks, which also cannot be loaded
in certain combinations.
The web frame has been analysed by
finite-element techniques, and the stress in 27
face flats and the combined (von Mises) stress as a
percentage of the yield stress in 27 web panels
(shown in Fig 4) are given in Table2
and Table3.
The stresses particularly in the ordinary operating
conditions, are low as external and internal loads
balance fairly well, and the primary member
end-moments are balanced quite effectively.
The individual tank test conditions numbers
13-16 are not critical as then only one tank,
albeit with a test head, is loaded. The lower tanks
use a test head two-thirds the distance to the
upper deck as they are normally only loaded to the
mid-deck level; high stress conditions occur when
combinations of upper and lower tanks are full or
empty. The behaviour of the lower centre tank
struts has been investigated; in addition to
transmitting a compressive force, the struts are
subject to transverse bending as there is relative
displacement of the mid-deck and bottom webs.
However, the centre bottom tank structure is not
particularly different from a single-hull tanker
wing tank web frame with struts.
Transverse bulkheads
The transverse bulkheads are horizontally
stiffened and supported by vertical webs. The
vertical webs are, in turn, supported by the deck,
mid-deck and the bottom shell in a similar fashion
to the transverse side webs. The transverse
bulkheads are rarely laterally loaded in service
(compare Figs 3A, 3B and 3C), and the critical
condition (numbers 8 or 9) with upper and lower
bulkheads loaded in opposite directions is easy to
analyse. On the other hand it is the longitudinal
bulkhead, particularly the upper part, which is
always under lateral load and its supporting web
should be well designed - see, for example,
position numbers 18, 19 and 20 of Fig 4 and Tables
2 and 3.
Fabrication and erection
The tank body can be broken down in building blocks as
shown in Fig 5 (not included here but described as follows:
the bottom of the lower centre tank + long. bhds is the
first block. The bottom of the upper centre tank + long.
bhds is the second block . The lower side tank (three
panels) blocks P+S are the third and fourth blocks. The
upper centre deck + long.bhds is the fifth block. The top
side tank (two panels) blocks are the sixth and seventh
block).
Each block consists of flat plate panels with stiffeners
and rectangular open webs adapted for fully automatic
fabrication and welding in panel and web lines. Corner
brackets are fabricated in a separate line. The full width
mid-deck is a very good platform for erection welding and
dimensional control, and application of coatings to the
upper wing ballast tanks can take place during block
assembly or after assembling the blocks in the building
dock.
Safety aspects
Cracks and fractures of various types always occur in
a tanker structure, but operators must ensure that they do
not occur in the oil/ballast boundary structure, to avoid
cargo leaks into the ballast spaces. The COULOMBI EGG
tanker will perform very well here as the structure
concerned - the upper part of the longitudinal bulkhead - is
minimally stressed and is not subject to corrosion either
too difficult to control or too aggressive. If a leak
occurs, it is easily spotted, and oil will collect in the
outboard corner of the ballast tank from where it can be
transferred to another tank.
Environmental protection
The Egyptian maritime administration has agreed to
sponsor a submission to the 34th MEPC session from 5-9 July
1993 to the effect that the COULOMBI EGG tanker
be accepted according to regulation 13F as an alternative
design to a double bull. In a comparative study, it was
shown that three sizes - 50 000dwt, 150 000dwt and 280
000dwt - of COULOMBI EGG tanker have much lower
mean and extreme outflows in accidents than reference
double-hull tankers.
(Note
- the IMO delayed its approval of Guidelines to approve
alternative designs until 1995 and then Sweden accepted to
sponsor the submission. Approval by the IM0 was not until
September 1997 thus four years after this article was
published).
The reason for this is that grounding protection is
provided by the side-to-side mid-deck which effectively
reduces outflows to very low figures if the bottom shell is
breached. Collision protection is provided by the patented
COULOMBI EGG system, which takes into account
the non-uniform probability of side damage in collision. It
is a fact that deep penetration of the side is more frequent
above the waterline, where the COULOMBI EGG
tanker has B/5 wide wing tanks and more structural
protection than a 2 m wide double side shell. The B/5 wide
main deck can absorb a substantial amount of collision
energy.
The lower 0.25D part of the side is protected by an
automatic cargo transfer system. When there is a hole in the
lower side, the inflowing water pushes up cargo oil into the
access trunks of the lower wing tanks and from there, it
flows, through air, to an undamaged upper wing ballast tank
on the other side. This overall combination of grounding and
collision protection bas been shown to provide superior
protection than a 2 m wide reference double hull.
Cargo operations
The COULOMBI EGG tanker is very suitable for
handling three grades of cargo by free flow in the natural
segregations provided by the upper and lower cargo tanks.
Cargo suction piping is not necessary and three pumps can
take direct suction from the aft most tanks.
Another cost-effective situation is to use 10 deepwell
pumps: four in the upper centre, four in the lower centre
and one each in the lower wing tanks. Such an arrangement
saves energy during discharge as the cargo in the upper
centre tanks only need to be lifted half the tanker's
depth.
Tank cleaning of the lower tanks is accomplished by
submerged machines.
The structural arrangement in the lower centre tank with
its rather narrow side webs and two slender struts means
that only five or six machines are required for 100% direct
washing coverage of the whole structure. Tank cleaning is
similar to a single-hull tanker.
Operational draught limitations have been mentioned
earlier. Clearly, the arrangement with upper and lower tanks
requires a new understanding of cargo planning - the order
of loading and discharging cargo parcels and the
distribution of cargo differ from a conventional double or
single hull with full depth tanks.
Stability
Intact stability is always in order under normal
condition numbers 1-12 (Fig 3). Damage stability is also in
order. There are no ballast spaces in the tank body adjacent
to the bottom to be flooded in grounding, so the analysis is
very simple. In part-loaded conditions, an operator may
flood a lower cargo tank, but the COULOMBI EGG
tanker should always survive, according to the
regulations.
Beginning of a new era
The new Marpol 73/78, regulation 13F, which allows
alternative designs to double hulls, is the challenging
beginning of a new era. To develop a new tanker is not easy,
considering the total influence of any one modification on
the structure, fabrication, safety, environmental
protection, stability, ease of maintenance and operation of
tankers of various sizes.
The original COULOMBI EGG vessel was
conceived in 1989 and has since evolved with the side
cofferdam, the sloping mid-deck at the side, a mid-deck in
two levels, the trunked deck design to locate the main deck
at the side to act as a fender to absorb collision energy,
and the automatic wing tank cargo evacuation system which
handles holes in the side.
Production costs should not differ too much from a
single-hull tanker, although there is approximately 10% more
steel in the tank body, which might add 4~5% to the initial
cost.
Underwriters must appreciate the reduced risk that a
COULOMBI EGG tanker offers compared with
double-hull. It spills less oil in accidents and does not
have the safety hazards of a double-hull tanker. Double-hull
tankers have now become the standard reference type, thanks
to OPA 90. However, there have been major developments in
tanker design since OPA 90 was enacted in August 1990.
Therefore, the new Marpol rule providing for safer tankers
is a logical consequence of these developments. As a party
to the Marpol 73/78 convention, the USA has not objected to
regulation 13F. As a consequence, regulation 13F tankers
should be allowed to enter US waters. However, the US Coast
Guard has indicated that it will not accept regulation 13F
tankers and that OPA 90 remains the overriding law relating
to bull design, i.e., double-hull tankers only.
In effect, the US Administration is in the dubious
position of legislating one design and being a party to
another. It remains to be seen whether, at the same time, a
member of IMO can adopt and adhere to an international
convention and maintain that the same convention contradicts
national law.
The incoming Secretary of Transportation, Federico Pena,
faces a difficult decision during his first months. He can
maintain the outdated, double-hull only requirements of OPA
90, thereby denouncing Marpol, or he can uphold Marpol and
move to amend OPA 90. By opting to denounce Marpol, he
closes the door forever for the development of designs that
are, I believe, safer anti protect the US environment better
than double hulls. By moving to amend OPA 90, he leaves the
door open to develop the safest anti most effective means of
environmental protection for the entire world.
The COULOMBI EGG tanker is
one such safer design generally based on well-established
single-hull technology, and its mid-deck as grounding
protection is an old idea. The challenge has been to develop
and to introduce better collision protection in the
design without adding the safety hazards of a narrow double
side shell. It is my sincere wish that, all parties
responsible for the legislating and administering safe
transportation of oil by sea will now appreciate that,
uniform width double side is not the optimal collision
protection anti that the COULOMBI EGG system is
expected to better reduce oil spills and fires due to
collisions. JUNE 1993
Above article was as shown written in 1993. Very little
development work of alternative oil tanker design has since
taken place except that the COULOMBI EGG was
approved by the IMO in 1997, as OPA90 freezes all
innovation. Double hull tankers may reduce oil spills in
collisions and groundings but there is no guarantee tah
spills due to structural failures in the double hull will be
reduced.
The COULOMBI EGG tanker is a very good design
based on single hull technology. In a logical world the
development of technology and methods should of course
govern the regulatory frame work, but when it comes to oil
tankers the political double hull standard (2 meters wide
double hull regardless of size of tanker and actual accident
statistics) initiated by the OPA 90 still governs the IMO.
Nevertheless - as most VLCC's do no trade to US continental
ports, they are not governed by the OPA90 so there is
nothing to prevent COULOMBI EGG VLCC's to be
built and to trade worldwide.