Anders Björkman of Heiwa Co
proposes
new FPSO Design - easy as a Cake

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Most FPUs - Floating Production Units - are FPSOs - Floating Production Storage & Offloading units built similar to sea going oil tankers, i.e. enlarged pontoons. However an FPSO is not an oil tanker and the optimum design should be completely different. Like a life buoy - or a cake. And then it can be built on one or more 50x50 meters slipways. There is no need for a long building dock or similar. But you need a jetty and a free space of abt 200 meters outside the jetty to moor the complete, assembled unit for outfitting. It is also good to have 30 meters depth outside the jetty.

Heiwa Co has the pleasure to present a new arrangement of the Coulombi Egg 2.2 Mbls FPSO.

Particulars of the Coulombi Egg 2.2 MBls FPSO (Pat. pend.)

LOA 175.6 m

BOA 167 m

Depth 37 m

Draft 28 m

Cargo capacity 2.2 M barrels

Ballast capacity 0 m3

8 cargo tanks and 2 produced water tanks

(BLUE in General arrangement right)

Engine room (PINK in General arrangement right).

Cargo tanks and Engine room are protected by ten double bottom and side tank void spaces (YELLOW in General arrangement right) and by ten transverse voids with connection welding butts (GREEN in General arrangement right). All voids are 4 meters wide.

The Coulombi Egg 2.2 MBls FPSO consists of 10 off 50x50 meter tank/ER units each with steel weight 4 100 tons built on a slipway.

Coulombi Egg FPSO - General Arrangement

The transverse section of a Coulombi Egg 2.2 MBls FPSO base unit is shown right.

It is box shaped 37 meters deep and 41 meters wide. All plates are flat! All stiffeners are straight! Stiffeners are interrupted at watertight floors and bulkheads.

The cargo tank is 33x33 m surrounded by a 4 meters wide double side and bottom voids.

The deck and is longitudinally framed, s= 786 mm, supported by deck webs 5 meters apart. The deck webs are supported by a centre line girder. There shall be permanent means to inspect the deck webs.

The side voids are transversely framed (except the top part), s = 800 mm, supported by four horizontal girders for easy access and inspection.

The bottom void is longitudinally framed supported by transverse floors 5 meters apart.

All scantlings are based on local loads and assumed wave loads and different tank loadings. One severe load case is, e.g. all tanks fully loaded except the tank opposite the engine room that is empty, etc. The deck may be 12 mm, the bottom 20 mm and the sides average 16 mm.

The erection void between cargo tanks can evidently be eliminated to save weight (double bulkhead becomes single bulkhead between oil tanks) and to gain storage capacity (but the oil tank becomes more difficult to clean).

Coulombi Egg FPSO - Transverse section of unit

Each base unit is banana shaped at an angle of 36° explained as follows:

A typical base unit is shown right. It is about 50 meters long and 50 meters wide. A Coulombi Egg 2.2 Mbls FPSO consists of 10 such units.

Two meters from each end is an oil tight bulkhead and watertight floor in the double bottom. The oil tight bulkhead is vertically framed and supported by horizontal girders on the outside of the cargo tank and a centre line web in the erection voids.

In the middle of the base unit is a swash bulkhead and floor in double bottom fitted at an angle of 18°.

The other floors are fitted 5 meters apart perpendicular to the sides.

The base unit thus consists of a double bottom block 4 meters deep, side blocks 4 meters wide, two bulkheads, a swash bulkhead and a deck block. All surfaces are flat for easy construction in panel and block assembly lines. Total weight is about 4 100 tons, which can probably be reduced after detailed structural analysis. The blocks are painted at this stage. The bulkheads are also tested with air at completion.

When the base unit is launched it will float at a draft of about 3 meters and can easily be joined to other base units afloat.

Coulombi Egg FPSO - base unit


A typical base unit cargo tank is shown right. It is average (at its 'centre line') abt 35 meters long and 33 meters wide and 33 meters deep. A Coulombi Egg 2.2 Mbls FPSO consists of nine such units. As stated earlier there is a swash bulkhead fitted (at 18° angle) at the mid-length in the tank. The engine room unit is structurally similar with equipment modules stored in the 'tank'.

Cargo handling is one (or two) deepwell pumps in each tank. There is thus no piping in the bottom or penetrations of bulkheads (and no cargo pump room!).

As the complete FPSO will be spread moored, outside waves may in rare occasions cause transverse sloshing in some parts of some partly filled tanks that are lined up with the (big) waves. If there is a problem, it is easy to transfer cargo from one tank to another.

The structural area inside the tank is minimum and very little scale will form. Tank cleaning, gas freeing and access are therefore very simple. The flush bottom will be epoxy coated in the standard manner.

The cargo tank is protected by a U-shaped void, 4 meters wide/deep all around it.

Only structural elements inside a cargo tank to inspect are the deck webs and the swash bulkhead.

Coulombi Egg FPSO - typical cargo tank

At each end of a base unit is a 2 meters wide unit erection void.

When two units have been launched, they are moored together at the end of the jetty and can be assembled - welded together - afloat - from inside these voids. The units float at a draft of about three meters (on the double bottom void inside the watertight end floor), so the bottom joint has to be sealed from outside at the bottom. Then all longitudinals and plates can be welded together afloat. The final back welding of the outside, underwater butts take place, when all ten units have been joined and you can trim the complete unit and weld above waterline. Another method to complete the outside, underwater welding is of course to fit a temporary skirt around the bottom butt and weld from there.

The final erection void space is 4 meters vide and act as support of the transverse oil tight vertically framed bulkheads.

Coulombi Egg FPSO - base unit erection voids

A typical horizontal girder ring is shown in the picture right. There are four rings at various levels.

As the sides and bulkheads are transversely (vertically) framed, the loads on the sides and bulkheads are directed down into the horizontal girders and then via the girders to the bulkheads. In order to reduce the load in the outside girder the swash bulkhead is fitted and its girders transmit load to the inside girder, etc. The transverse bulkhead girder has a vertical web at mid-length (centre line), so the loads are there transmitted to deck and bottom.

The design load cases are evidently with one tank full and all other tanks empty at minimum draft and full draft with one cargo tank empty. In normal operations all tanks are filled at various levels at bigger draft, so the stresses in the girder ring will be much reduced.

Inspection of voids is very easily done from the horizontal girders. Cargo leaks are very easy to spot.

Topside Outfitting

When all ten base units have been assembled afloat and when the complete unit is moored at the end of a jetty, you can temporarily ballast the cargo tanks with sea water (also good for testing the tanks) and lift on top side modules (up to 60 000 tones) and the deck house/helipad on top of the engine room section, that has been outfitted previously.

 

Coulombi Egg FPSO - typical horizontal girder ring
Sea water lift pumps are installed in the moon pool. The modules are designed to line up with the deck webs and the deck girder. The flare tower is evidently fitted on the opposite side unit of the deck house unit and tank vent towers are fitted on extreme sides halfway between. The cargo vent system shall of course be the best system! As the unit is spread moored in any location, a turret is evidently not required. The risers are brought up from the moon pool.

The Coulombi Egg 2.2 MBls FPSO has no ballast tanks! Ballasting during transfer from yard to location, if required, is with water in the cargo tanks. At location the unit will always carry at least 500 000 bls of cargo after a typical 1 M bls offload, so no sea water ballast is required. UWILDS are not really required as all hull structure including erections butt joints can be inspected from the 20 off inside voids. Underwater outside may be, apart from coatings, protected by an ICCP system or anodes.

The Coulombi Egg 2.2 MBls FPSO could in fact be fitted with propulsion - a pod thruster of certain size - to be used in an emergency (mooring system damaged) and fitted in the engine room. You never know what USCG requires in GoM!

At Location

The unit is spread moored from some of the swash bulkhead corners. Risers are brought up from inside the pool. Offloading is best done via an adjacent buoy but it can also be done directly from the Coulombi Egg FPSO. One area of the unit is to be arranged to receive supply boats. The movement of the Coulombi Egg 2.2 M Bbl FPSO in seaways is that of a buoy but due to its large size it will hardly pitch or heave.

At end contract the unit can be towed to a jetty and the modules be replaced to suit another contract. At this time the underwater protection coatings + anodes) can be renewed by simply tipping the unit on one side.

It is just a piece of cake! Enjoy it.

 

Contact anders.bjorkman@wanadoo.fr 

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