WO1998030437A1

WO1998030437A1 - Floating storage vessel

Info

Publication number: WO1998030437A1
Application number: PCT/NO1997/000345
Authority: WO
Inventors: Kåre G. Breivik; Tor E. Hilden
Original assignee: Den Norske Stats Oljeselskap A.S
Priority date: 1996-12-18
Filing date: 1997-12-17
Publication date: 1998-07-16
Also published as: NO965464D0; AU5350698A

Abstract

Floating storing unit for liquid cargo, such as oil, condensate etc. The storing unit should during normal operations, be unmanned, and is moored in a free, rotatable way relative to a cargo transferring construction mounted close to the bow of the unit. The unit comprises an elongated hull having narrow centre tanks stretching in the longitudinal direction of the unit and side tanks extending on each side of the centre tanks, and being self-draining toward the centre tank. The centre tank is drained toward cargo transferring pumps in the forward and/or aftward end of each centre tank. At the stern of the unit loading/unloading equipment is mounted. The centre and side tanks are filled with gas and liquid so that the volatile liquids in a small extent liberate gases into the atmosphere.

Description

FLOATING STORAGE VESSEL

This invention relates to a floating storing unit for liquid cargos, such as oil, condensate or similar. The invention also relates to methods for loading and unloading of a liquid cargo in the floating storing unit . Such units are already known, serving as intermediate storage for hydrocarbons being produced from a subsea oil field. The known storage units are anchored in the vicinity of the oil producing installation (s) and the anchoring will usually be of a kind allowing for rotation of the storage unit relative to the anchoring, depending on wind, current and waves .

When the produced oil is to be taken away from a storing unit of a known type a tanker will approach the storing unit and connect to it so that the cargo may be transferred through one or more hoses . During the approach of the cargo-relieving tanker, the coupling of it to the storing unit, as well as the controlling of the cargo transfer from the storing vessel to the tanker, a considerable amount of work is done by the crew of the storing unit. During the cargo transfer it is especially important for the crew to provide for a draining of the tanks to maintain the stability of the storing unit, and also make sure the tanks are drained so that the hull does not loose or damage its integrity because of changes in the load distribution.

The present invention relates to a storing unit normally being unmanned. The lacking necessity for manning is obtained largely in that the different processes during loading/unloading are controlled to elapse in a pre- determined ways through the construction of the storing unit. The storing unit is also constructed to provide a possibility for handling/steering/controlling, both when loading oil into the storing unit and further when transferring the cargo into another vessel, from remote positions (the production installation or the tanker coming to load the cargo) .

The storing unit should be ruggedly built and in such a way that filling or emptying may be performed as required. Especially the tanks should be self draining toward the centre line of the ship, or possibly toward the stern, where loading/unloading equipment is mounted. To be able to adjust the orientation in relation to the tanker coming to load cargo the unit will usually be provided with one or more thrusters mounted in the rear which may be remote controlled from the loading tanker.

As already mentioned the storing unit (vessel/floater) is anchored in the vicinity of an oil producing installation in order to receive and intermediately store the product . The storing vessel may be anchored freely rotatable, preferably using STL equipment (Submerged Turret Loading) near the bow, thus enabling the unit to orient itself according to wind, waves and current. Of course other anchoring systems than STL may be considered. The storing unit has to be anchored sufficiently far from the oil producing installation to allow rotation around the anchor point .

The invention is defined in the accompanying claims. The invention is explained in detail below referring to the drawings, where: Figure 1 shows the storing unit as seen from one side. Figure 2 shows the storing unit as seen from above . Figure 3 shows a cross section of the rear end of the storing unit. Figure 4 shows a cross section through the storing tanks of the storing unit.

Figure 5 also shows the storing unit as seen from above. Figure 6 shows the storing unit as seen from one side with the stern trimmed down in the sea. Figure 7 shows a cross section through the storing tanks of the storing unit.

Figure 8 shows a perspective view of the storing tank construction.

Figure 9 shows the stern of the storing unit with equipment for transferring of cargo. Figure 10 shows how gas and oil fill the tanks during different loading operations. The storing unit is constructed from a ship hull 1. The hull will normally be built in steel, but concrete may be considered as an alternative. In the hull one or more centre tanks 2 stretches in the longitudinal direction. The tank may typically be 8 metres wide.

If one centre tank is used the tank may be "open" in the longitudinal direction, but there may be baffle plates 3 to avoid unwanted movements in the liquid. In the baffle plates closing valves may be provided, so as to enable closing of parts of the centre tank for e.g. cleaning.

On each side of the centre tanks a number of side tanks 4 extends, each centre tank preferably being connected to two side tanks on each side, as at least two bulk valves 11 are mounted toward each side tank. In the front and rear room is provided for trim tanks 5. The side tanks are self draining toward the centre tank, and the centre tank is drained toward cargo transferring pumps 6 in the front and/or rear part of each tank, or possibly in the stern. Pipes in the tanks are thus avoided. Transversely draining is obtained by slanting the bottom of each tank toward the centre and by mounting the bulk valves of the respective side tanks so that they are in the lowest, possibly lowered, part of the tank. Longitudinally draining may be obtained in different ways, either by forward or aftward trim. The storing unit thus has the flexibility as it may be divided into as many autonomous sones as there are centre tanks .

Tanks 19 secures the necessary buoyancy capacity, even with 100% full cargo tanks. These buoyancy tanks are preferably positioned as wing tanks or double bottom tanks in the storing unit .

As mentioned previously the rotatably anchoring is preferably of the STL type. This equipment 7 is positioned close to the bow part of the storing unit . The STL construction, which allows for transferring of load to the storing unit, consists of a submerged, moored buoy carrying a cargo transferring hose. The buoy, in which the outer part is capable of rotating in relation to a moored, central buoy part, is admitted into a submerged space in the bottom part of the storing unit. Even if the preferred mooring system is of the STL type, other systems may of course also be considered. The space and the shaft for the STL buoy will typically be cylindrical, with a diameter normally being in the range of 10 metres. The room may be built for an overpressure, typically 8 bar. The is access to the room and the shaft from the deck level.

The hull and the hull beam should endure all possible combinations of load, i.e. loading and unloading of the side tanks may be performed in any order without any resulting damage .

The load transferring pumps 6 will preferably be of a submerged type, mounted in the front or rear part of each centre tank.

A trolley on longitudinal tracks secure transportation of equipment on deck rear where the main crane is mounted. Pumps may thus be pulled using cranes on deck, possibly using a crane mounted on the trolley.

The machine room 18 and the living quarters 19 of the storing unit, for maintenance crews at the necessary periodical maintenance, is in the aft, preferably at the deck level and in a superstructure. The machine room is preferably on one side and the crew quarters etc on the other side . In the rear one or more thrusters 8 are positioned. The thrusters will be used for controlling the rear end of the storing unit in relation to the tanker approaching to receive cargo. The thrusters may be remote controlled from the tanker an/or from a nearby production platform. The thrusters may of course also be controlled locally, if it is necessary. The thrusters may be lifted into a room in the hull for maintenance. To this aim a special lifting arrangement under deck or a passage for the main crane in the form of a shaft down to the thruster(s) is provided. Room for bunkers etc is also positioned in the aft. Required minimum ballast may be arranged e.g. by casting concrete in the construction. Permanent fresh water, mixed with corrosion inhibitors, in special ballast tanks may also be an alternative. As an alternative to "permanent ballast" extra load capacity may possibly be built. I.e. the storing tanks are not more than that the requirement for minimum ballast is maintained.

The storing unit is built for a minimum VOC (Volatile Organic Components) outlet. This is obtained both in the way the centre tanks are constructed, and in the way they are filled. The narrow centre tanks and each of the side tanks are interconnected with an arrangement of pipes and Pressure/vacuum regulating valves 9, the latter of which may provide an opening to the atmosphere. The valves are made to open at a certain gas overpressure in the tanks, typically 5000 mm VS (water pressure) . The overpressure in the tanks is kept at a level being higher than usual for similar tankers. This higher gas pressure will also have an advantageous effect on the VOC evaporation.

The method for reducing the VOC outlet is shown in figure 10. The centre tank as well as the side tanks usually contains a mixture of neutral gas 22 and hydrocarbon gas (HC) 23, the neutral gas being lighter than the HC gas. The bulkhead valves between the centre tank and the different side tanks are closed, cargo is loaded from the cargo transferring construction in the bow of the unit to drop lines extending down and along the bulkhead in the ends of each of the centre tanks, the pressure/vacuum regulating valves 9 in the pipes 10 connecting the tanks being set to open against the atmosphere at a certain overpressure. The pressure of HC gas is allowed to build up in the centre tanks and side tanks, where in the tanks the largest concentration of HC-gas builds up lowest in the side tanks, where it will make a border layer against the inflowing oil from the centre tank. This will positively reduce the VOC evaporation. The neutral gas have the highest concentration in the top of each tank. The bulkhead valves 11 are opened to allow the cargo to flow from the centre tanks to the side tanks, where the gas is compressed because of the loading. When the set pressure for the valves 9 is reached the valves are opened for venting primarily neutral gas into the atmosphere. The set pressure of the valves 9 is chosen so that under loading a pressure builds up of HC gases in the tanks which will lead to an increased reduction in the evaporation of volatile organic components (VOC) in the tank. The loading is continued until the chosen filling if the tank is obtained.

The narrow centre tanks gives little circulation in the oil, which also reduces the VOC evaporation. As the tank is small, the critical sone is quickly passed during the loading in which the resonant frequency of the liquid content of the centre tank and the roll period of the ship. The strengthening element 16 in the centre tank (fig.7) will also typically reduce/dampen movements in the liquid during rolling. The pipe system for equalizing pressure between the gas volumes in the tanks ( Hydrocarbon gas HC from the cargo in the tanks, as well as neutral gas being introduced into the tanks) , may also be used for equalizing in a self-adjusting way of the fluid levels in the tanks. In this case the pipe system is formed so as to extend down into the cargo tanks, and not just from the top of the tank, as the situation usually will be.

In the stern of the storing unit a loading/unloading system provided with a hose 12 for transferring of load to another ship. The hose is adapted to hang down into the ocean and away from the stern of the unit . The necessary distance to the stern is obtained by hanging the hose from a jib 13. The loading hose is connected to pipes being connected to the cargo pumps in each centre tank, or in the stern. The free, submerged end of the hose is connected to a line 24 being in its other end normally connected to a self-winding winch mounted on the stern deck of close to the unloading jib. The tanker intending to load may for example fish up the forerunning line which, immediately before the fishing has been winded by the self-winding winch, hauling in the hawser and the loading hose 12, and coupling them to the respective positions before starting the transferring of the cargo.

Different other systems may of course be used to transfer cargo from the storing unit to another vessel. The hose may for example be fastened to a buoy, which the other vessel picks up and uses to pull the end of the hose in. Other solutions may also be contemplated in which the hose in exchanged with a pipe hanging from the storing unit, or that the storing unit simply comprises coupling means for different loading systems on different other vessels for taking over cargo. Combinations of these solutions are also possible.

A transfer pipe for stretches from the cargo transferring construction (STL) in the bow part of the unit to the centre tank, so that oil, under normal operations flows from the STL-area in front and directly into one or more centre tanks at the time and thereafter is distributed to the side tanks. When the cargo is to be transferred, it will normally be pumped out from the tanks and to a tanker. Situations may, however, be contemplated in which the storage tanks are not used, e.g. because of repairs or maintenance. In such a case a by pass pipeline 21 may be installed (with manually operated valves) from the STL room, over deck and to the unloading system mounted in the stern of the storing unit .

When the tanker is to be loaded it may connect to the free end of the cargo transferring hose of the storing vessel, the bulkhead valves are opened between the side tanks and the centre tank in the storing unit are opened and the cargo transferring pumps in the storing unit are started for pumping of load from the storing unit to the loading ship. The cargo is thus brought to flow from the side tanks til the centre tank and the centre tank is drained toward the cargo pumps.

Normally the storing unit will be rotatably moored in relation to a point close to its bow part, and the coupling to another vessel by the stern of the storing vessel. Other solutions, such as an exchange of these positions, are however technically possible, and is also covered by this invention.

Claims

P a t e n t k r a v

1. Floating storing unit for liquid cargo, such as oil, condensate etc, the storing unit during normal operations being adapted to be unmanned, and where the unit is anchored in a rotatable way in relation to a cargo transferring construction (7) mounted in the vicinity of one end of the unit, preferably the bow, c h a r a c t e r i z e d in that there in the direction along the unit stretches one or more narrow centre tanks

(2) , at each side of which side tanks (4) extends being self draining toward the centre tank, and where the centre tank is drained toward the cargo transferring pumps (6) positioned in the front and/or rear ends of each centre tank, or in the stern of the unit, a loading/unloading system being mounted in the other ends, preferably the stern end, of the unit, for transferring cargo to or from another vessel.

2. Floating storing unit according to claim 1, c h a r a c t e r i z e d in that the loading/unloading system comprises a hose, and that the hose is adapted to hang into the sea and away from the stern end of the unit, so as to allow the end of the hose to be picked up from the other vessel.

3. Floating storing unit according to claim 1 or 2 , c h a r a c t e r i z e d in that the hull has tanks (9) for securing the necessary buoyancy capacity, even at 100% full cargo tanks, the buoyancy tanks preferably being positioned as wing tanks or double bottom tanks .

4. Floating storing unit according to claim 1, 2 or 3, c h a r a c t e r i z e d in that each centre tank is connected to side tanks on each side of the centre tank, at least one pair of bulkhead valves being mounted against each side tank, and the bottom of each side tank being inclined toward the centre tank and the bulkhead valves of the respective side tanks being mounted so that they are lowest, possibly in a recess, in the tank.

5. Floating storing unit according to one of the preceding claims, c h a r a c t e r i z e d in that pipes for transferring cargo stretches from the cargo transferring construction in the bow part of the unit an, possibly via drop lines, down and close to bulkheads in the centre tanks .

6. Floating storing unit according to one of the preceding claims, c h a r a c t e r i z e d in that the side tanks and the centre tanks in size, shape and location in the hull is adapted to, during unloading, provide centre tank draining by lowering the stern of the storing unit in relation to the bow, so that the cargo in the centre tank flows toward the stern and the load transferring pumps (6) positioned therein.

7. Floating storing unit according to one of the preceding claims, c h a r a c t e r i z e d in that trim tanks (5) are positioned in the stern of the hull, and possibly in the bow part, where the stern during filling of the trim tanks in the stern sets deeper in the water than the bow, so that the load in the centre tank(s) flow toward the stern and the load transferring pumps (6) positioned therein.

8. Floating storing unit according to one of the preceding claims, c h a r a c t e r i z e d in that the bottom of the centre tank is inclined from its front end to the rear end, thus obtaining a draining of the cargo toward the stern.

9. Floating storing unit according to one of the claims 1- 5, c h a r a c t e r i z e d in submerged pumps mounted in each end of the narrow centre tanks so as to enable unloading independently of for or aft trimming.

10. Floating storing unit according to one of the preceding claims, c h a r a c t e r i z e d in that the centre tank and the individual side tanks are interconnected with an arrangement of pipes and pressure/vacuum regulating valves, where the latter may provide an opening to the atmosphere, where the pressure equalization between the gas volumes in the tanks (hydrocarbon gas from the cargo in the tanks as well as neutral gas introduced into the tanks) is provided in a self regulating way that the liquid levels in the tanks are equalized.

11. Floating storing unit according to one of the preceding claims, c h a r a c t e r i z e d in that a jib (13) is arranged by the stern of the unit for a load transferring hose (12) , the hose thus hanging vertically down into the sea at a distance from the stern of the unit, and where the submerged, free end of the hose is connected to a line (14) being connected to a self winding winch at its other end for controlling the line.

12. Floating storing unit according to one of the preceding claims, c h a r a c t e r i z e d in that thrusters (8) are mounted in the stern of the unit .

13. Method for loading of a floating storing unit for liquid cargo, such as oil, condensate etc, the storing unit during normal operations being adapted to be unmanned, and the unit being anchored in a rotatable way relative to a cargo transferring construction (7) preferably mounted in the vicinity of the bow of the unit, and where the unit is designed as given in claims 1-12, c h a r a c t e r i z e d in that the centre tank(s) (12) , as well as the side tanks (4) usually initially contains a mixture of neutral gas and hydrocarbon gas (HC) , the neutral gas being lighter than the HC gas, and that the bulkhead valves (11) between the centre tanks and each side tank are closed, that cargo is transferred from the cargo -l rt Ω 01 rt P rt o Ω Ω Pi < rr tr P Ω H <J rr ^►d Ω H Hi P P •d rr < Ω rt Ω Ω rt P ii rr

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16. Method according to claim 15, c h a r a c t e r i z e d in that the trim tanks (5) in the stern of the storing unit are filled so that the stern sets deeper into the water than the bow, and the cargo in the centre tank flows toward the stern and the cargo transferring pumps positioned therein.