The Coulombi Egg Oil Tanker - General Arrangement and Structure
Better protection, safer and more economical than Double Hull

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COULOMBI EGG Oil Tanker Design - Structure

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 Table 2 and Table 3. 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.  

Contact anders.bjorkman@wanadoo.fr