WO2015053634A1

WO2015053634A1 - Method and device for control of rising force from a submerged ballast tank device

Info

Publication number: WO2015053634A1
Application number: PCT/NO2014/050185
Authority: WO
Inventors: Knut Ove Steinhovden; Jostein H. REINSNOS; Bjørn JETMUNDSEN
Original assignee: Ikm Technique As
Priority date: 2013-10-08
Filing date: 2014-10-02
Publication date: 2015-04-16
Also published as: NO20131345A1; NO336524B1

Abstract

Method for control of rising force in fluid (18) where a submerged ballast tank (1) comprising a fillable cavity (2), is sufficiently stiffened to withstand an existing ambient pressure, the method comprising to generate rising force by removing medium (4) from the ballast tank (1) during pressure reduction in the ballast tank (1).

Description

METHOD AND DEVICE FOR CONTROL OF RISING FORCE FROM A SUBMERGED

BALLAST TANK DEVICE

The present invention concerns a method for control of rising force from a submerged body. More specifically, the invention relates to a method for control of rising force in a fluid wherein a submerged ballast tank comprising a fillable cavity is sufficiently stiffened to be able to withstand the existing ambient pressure. The invention furthermore relates to a ballast tank device.

During lifting operations on or close to the seabed which is common for instance during petroleum recovery operations offshore, it is common to use so-called WROVs (Work-Class Remotely Operated Vehicles) for lifting or controlling items.

When a body is lowered in a fluid, displaced fluid exerts a force called buoyancy on the body. Buoyancy works in the opposite direction of local gravitation and seeks to lift the body out of the fluid. The size of the buoyancy force is determined by the mass of the displaced fluid and the local gravitation (the acceleration of gravity in the actual location).

Gravitation also works on the mass of the body. Since buoyancy and gravitation work in opposite directions, the net effect on the body will be zero if the two forces, buoyancy and gravitation, are of the same size. The body will then be neutral in the fluid . If the buoyancy exceeds the gravitation working on the body, the body will rise in the fluid. If the buoyancy is smaller than the gravitation, the body will sink in the fluid.

The difference between buoyancy and gravitation is in the following referred to as rising force. The body rises in the fluid if the rising force is greater than zero, and the body sinks if the rising force is smaller than zero.

The rising force is important for all types of vessels, as the largest available rising force reflects the carrying capacity of the vessel. The invention is particularly directed to submersible vehicles and other constructions which are used under submerged conditions. A remotely operated submersible vehicle WROV, ROV, of the kind which inter alia is used in connection with oil activity offshore, is prepared and operated on board a surface vessel. The vehicle is typically provided with buoyancy bodies to make it neutral or close to neutral in submerged condition. If the vehicle is going to pick up an item from the seabed and move the item or bring it to the surface, the vehicle has to be provided with sufficient rising force to lift the item. This can be done in various ways.

The most practical would be to use the rising propellers of the vehicle, vertical thrust- ers, but unfortunately the available power from these is limited. Moreover, the water current from the thrusters whirls up mud from the seabed and makes navigation by means of underwater cameras difficult or impossible.

Another solution is to provide the vehicle with ballast, for instance in the form of a weight with a similar negative rising force to that of the item which is to be picked up. A gripper on the vehicle grabs the item and disconnects the weight which is left on the seabed.

So-called VBS-systems (Variable Buoyancy System) are often based on water-filled ballast tanks. When an item is going to be lifted, the ballast tank is completely or partly emptied by means of pressurized air from an air accumulator on board the vehicle, as known also from manned submergible vehicles. The method is inconvenient and partly unsuitable when a remotely operated submersible vehicle is to work at great depths.

The pressure in the air accumulator must be higher than the ambient pressure in order to be able to blow water out of the tank. At the same time the air volume at the actual pressure has to be the same as the water volume which is to be displaced from the ballast tank. Filling of air before submersion involves compression of air which takes up a very big volume at atmospheric pressure. It is time consuming and costly, and it is necessary with extra safety measures, at least as long as a filled air accumulator is above the water surface.

A variant of doing this is to use pressurized air to blow up a balloon in order to increase the buoyancy and thereby the rising force. This solution has a disadvantage in that the air in the balloon expands as the balloon rises in the water body, due to the decreasing ambient pressure. The buoyancy increases correspondingly. In order to keep the buoyancy constant, air has to be bled off the balloon as the vehicle rises.

Similar problems apply in connection with operations where equipment, for instance equipment related to petroleum activity offshore, is going to be placed on the seabed or picked up from the seabed. It is common to use a crane ship on the surface in order to lower heavy underwater constructions into place or in order to lift such a construction from the seabed in order to move the construction or to lift it to the surface. It is costly to use large crane ships, but as important is the fact that crane ships can only operate within limited wind speed and wave height.

It is known to remedy some of the disadvantages during lifting operations from the surface by using buoyancy bodies and ballast on the construction. When the construction is to be lowered into place, a buoyancy body may contribute to reducing the necessary lifting force. When the construction has landed, it is desired to remove buoyancy bodies or add ballast such that the construction stands safely on the seabed without being moved by water currents. When the construction is to be lifted from the seabed in order to be moved or in order to be lifted to the surface, it is also desirable to be able to do this without making use of a large crane ship. Required lifting force may be reduced by adding rising force to the construction, i.e. reducing the weight, increasing the buoyancy or using vertically directed propulsion equipment like lift thrusters.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art.

The object is achieved in accordance with the invention through the features which are specified in the description below and in the claims that follow.

In accordance with a first aspect of the invention, a method for control of rising force in a fluid is provided, wherein a submerged ballast tank comprising a fillable cavity is sufficiently stiffened to be able to withstand the current ambient pressure, and wherein the method is distinguished by comprising the step of generating rising force by removing medium from the ballast tank during pressure reduction in the ballast tank.

By filling medium, usually in the form of a liquid, in the ballast tank, the total weight is increased . In one embodiment a pump is used to empty the ballast tank.

The cavity or cavities in the ballast tank has/have a lower pressure than the ambient pressure at the work site. It is therefore possible to fill the cavity or cavities by means of the ambient pressure without supply of external energy.

The method thus includes controlling the rising force by leading liquid into or out of the ballast tank. In a first embodiment the ballast tank is arranged to be able to be filled with liquid from the surroundings, the surrounding liquid constituting a reservoir. If the ballast tank is submerged in sea water, said liquid is sea water.

The method may comprise:

- coupling a ballast tank to an item; and

- controlling the rising force in the ballast tank.

A relatively big and controllable rising force may thereby be applied to the item.

The method may further comprise:

- arranging a plurality of ballast tanks on an item; and

- controlling the rising force in each ballast tank individually.

This solution is particularly useful when an item is going to be trimmed and stabilized in a position in the water. The solution may also be used on the WROV itself, for instance in order to compensate for load added in one end of the WROV.

In accordance with a second aspect of the invention, a submerged ballast tank device is provided, the ballast tank comprising a fillable cavity, wherein the ballast tank is sufficiently stiffened in order to be able to withstand an existing ambient pressure, and wherein rising force is generated by removal of medium from the ballast tank during pressure reduction in the ballast tank.

The cavity typically communicates at its lower portion with a pump which delivers liquid to a reservoir. The pump, which is arranged to be able to pump medium out from the cavity, is operated in a manner known per se, for instance by means of electricity or hydraulics from the WROV. The pump may be provided with or be allocated a check valve preventing medium to flow to the cavity through the pump.

The cavity communicates with the reservoir through a valve. The valve is controllable and is used for opening and closing of the backflow of medium to the cavity.

Several ballast tanks may communicate with the same pump through a pump valve each. It is thereby possible to control from which cavity medium shall be pumped, which is particularly relevant when it comes to trimming and stabilizing an item in the water. Alternatively, each ballast tank has to be provided with its own pump.

As the ballast tank is being emptied and the pressure decreases, medium in the form of liquid which is in the ballast tank, may boil. In order to avoid that the pump has to work only with steam, the suction side of the pump is connected to the lowest point of the ballast tank, in terms of height. If the liquid boils, the steam bubbles will rise while the liquid is collected at the lower portion of the ballast tank.

If the liquid in the ballast tank is boiling, the temperature in the liquid will drop because the phase transition from liquid to steam requires heat, and the heat is taken from the liquid which is thereby cooled. There may be a risk of the liquid becoming so cold that it freezes and forms solid medium in the form of ice. Ice formation may result in the invention not working as intended, and the ice may damage the pump. The risk of ice formation may be reduced by adding heat. This can be done in a number of ways. A simple solution is to arrange the ballast tank for heat transfer from the surroundings to the fluid in the ballast tank. This may be done passively with a contact surface provided in a material having good thermal conductivity. By means of appropriate measures, see below, the wall may be very thin and thus contribute to efficient transmission of heat through the tank wall.

Heat may also be added more actively by heat exchanging through a circulating fluid or with a heating coil which is activated as the temperature of the fluid drops towards the freezing point. The pump may be arranged inside the cavity. A purpose with this is to make use of heat from the pump for heating the fluid to eliminate or reduce the risk of freezing .

As the ballast tank is being emptied, the pressure in the ballast tank will, as mentioned, drop. Required strength may be obtained in a manner known per se by a suitable form and sufficient wall thickness of the ballast tank. The ballast tank may alternatively be provided with an interior support structure or stiffenings, such as frame work, or it may be filled with a stiff, porous material through which the fluid may easily flow.

Interior support structure reduces available tank volume and increases the weight, but the wall thickness may be considerably reduced . In sum, an interior structure will provide a light weight ballast tank which can withstand a large overpressure without collapsing.

The ballast tank may comprise small, open bodies, for example balls, which are surrounded by a liquid seal, wherein the liquid seal constitutes the tank wall. The liquid seal may be formed in a suitable material and may at moderate pressures be made of a preferably reinforced synthetic material.

If so desired, the liquid seal may enclose a support structure. In the first embodiment the reservoir is constituted by the surrounding water. In this case the pump empties the medium to the surroundings. The valve opens for inflow to the cavity from the surroundings.

In a second embodiment the invention comprises at least two receptacles which are arranged to be able to be used in a submerged condition, and where the receptacles are connected to each other to a closed system and such that a medium may be transmitted between them. The first receptacle is a ballast tank as explained above. The second receptacle is flexible such that the volume of the receptacle is increased when the medium is brought in and such that the volume of the receptacle is reduced when the medium is removed. In order to simplify the description the second and flexible receptacle, and possibly further flexible receptacles, is/are referred to as bal- loon(s) in the following. If it is relevant to connect further receptacles to the ballast tank, such further receptacles will also be of a flexible type and referred to as balloons.

The second embodiment of the invention is suitable for use of a medium having lower density than the surrounding fluid. When a submerged balloon is being filled with such medium, the buoyancy of the balloon will exceed the gravitation force working on the balloon and its content. The balloon will thus get a positive rising force. By transferring such a medium from the ballast tank to a balloon, the total weight of the ballast tank may be reduced such that the ballast tank gets a positive rising force at the same time as the balloon gets a positive rising force. By fastening the ballast tank and the balloon to a vehicle or other equipment which is used submerged, the rising force may be adjusted by moving medium between the ballast tank and the balloon. By allocating two or more balloons to the ballast tank, the vehicle may be trimmed.

Liquids having lower density than sea water are known to the skilled person.

The medium may in some cases be formed by small, solid bodies, for example in a possibly porous metal or synthetic material. The density of the medium may be lower than the density of sea water.

In some cases it may be desired to increase the backflow capacity of the ballast tank by means of a filling pump.

The method and the device in accordance with the invention make possible a considerable improvement of the lifting capacity of a WROV. The invention is applicable in combination with most types of ROV's. It is obvious that the ballast tank, which may be parked on the seabed, facilitates a considerable simplification of submerged lifting and transport operations.

In the following is described an example of a preferred method and embodiment illustrated in the accompanying drawings, wherein :

Fig. 1 shows a ballast tank in accordance with the invention;

Fig. 2 shows the ballast tank from figure 1, emptied and with full rising force;

Fig. 3 shows the ballast tank in a second embodiment;

Fig. 4 shows the ballast tank in a further embodiment;

Fig. 5 shows a design of the ballast tank;

Fig. 6 shows another design of the ballast tank;

Fig. 7 shows a ballast tank on the seabed;

Fig. 8 shows the ballast tank during transport to an item which is to be lifted;

Fig. 9 shows the ballast tank after connection to the item and emptied sufficiently of medium for the item to be lifted;

Fig. 10 shows the same as in figure 8, but where the ballast tank constitutes part of a WROV;

Fig. 11 shows the same as in figure 9, but with the same design as in figure 10;

Fig. 12 shows a planar elevation where an item is provided with a plurality of ballast tanks; and

Fig. 13 shows a connection diagram for the device in figure 12.

On the drawings the reference numeral 1 denotes a ballast tank comprising a cavity 2 which initially is filled with medium 4 in the form of a liquid. The ballast tank 1 is designed such that it can withstand the existing pressure on the working place even when the pressure in the ballast tank 1 is very low.

At its lower portion 6, normally at its lowest position with regards to height, the ballast tank 1 is via a connection 8 coupled to a pump 10. The cavity 2 thus communicates with the pump 10. The pump 10 may for instance be a piston pump with not shown ceramic seals which is designed for dry running conditions. The pump 10 delivers medium 4 through a check valve 12 and a tubing 14 to a reservoir 16, here in the form of surrounding water 18. A valve 20 communicates with the tubing 14 and the connection 8.

In figure 1 the ballast tank 1 is being emptied of medium 4 by means of the pump 10 pumping the medium 4 out of the reservoir 16 at the same time as the pressure in the cavity 2 drops. The valve 20 is closed. Thereby the rising force from the ballast tank 1 increases. In figure 2 the cavity 2 is shown in emptied state with a considerable internal underpressure.

If the rising force is to be reduced, the valve 20 is opened whereby fluid is sucked from the reservoir 16 and into the cavity 2 and, if desired, fills it completely.

In an alternative embodiment, see figure 3, the pump 10 delivers the medium 4 to a reservoir 16 in the form of a balloon 22 in the surrounding water 18. This example embodiment is appropriate if another medium 4 than water, possibly water with an additive and/or a medium with lower density than the surrounding water, is used to fill the ballast tank 1.

When the medium 4 in the form of liquid is pumped out and the pressure in the cavity 2 drops, a boiling of the liquid will occur. The cavity 2 will thereby not completely reach a depressurized state, but have a considerable underpressure. The boiling requires energy which mainly comes from the medium 4 in the cavity 2. It is reasonable to expect that the last part of the medium 4 which is to be pumped out, may freeze. This phenomenon may be counteracted by arranging the pump 10 in the lower portion 6 of the cavity 2, as shown in figure 4. Heat which develops during operation of the pump 10 counteracts the freezing of the medium 4. If this is not sufficient, or as an alternative, a not shown, heat element may be arranged in or near a lower portion 6.

In figure 4, the ambient fluid pressure has led the medium 4 back from the balloon 22 and to the cavity 2.

The ballast tank 1 may be designed as a pressure tank as shown in figures 1-4. This means that the ballast tank 1 has sufficient strength to withstand the prevailing pressure. However, it may be necessary to arrange a stiffening 24 in the ballast tank 1 as indicated in figure 5.

In figure 6 the ballast tank 1 is formed by a multitude of hollow and open ball-shaped bodies 26 which on the outside is sealingly enclosed by a, possibly reinforced, liquid seal 28, here in the form of a fabric of synthetic material. The bodies 26 take up the surrounding pressure and the liquid seal 28 constitutes the tank wall. In figures 5 and 6, arrows indicate the external pressure acting on the ballast tank 1.

In figure 7 a ballast tank 1 is shown arranged on a seabed 30. A WROV 32 is about to connect to the ballast tank 1 by means of a so-called hot stab 34, through which hydraulic pressure and possibly electrical signals are transmitted. The pump 10 and the valve 20 are operated and controlled in a way known per se from the WROV 32.

In figure 8 the ballast tank 1 is shown just before it is coupled to an item 36. In figure 9 the ballast tank 1 is connected to the item 36 and sufficient medium 4 is pumped out of the cavity 2 to allow the item 36 to be lifted in the surrounding water 18.

The embodiment of figures 10 and 11 shows a ballast tank 1 mounted on the WROV 32, but operated in the same way as described above.

In figure 12 an embodiment is shown where four ballast tanks 1 are mounted on an item 36. By controlling the liquid flow individually to and from the ballast tanks 1, the item 36 may be trimmed and stabilized in the desired position in the surrounding water 18. Figure 13 shows a connection diagram for this embodiment where a pump valve 38 is arranged between each ballast tank 1 and the pump 10 in order to be able to control from which one of the ballast tanks 1 medium is to be pumped. Each ballast tank 1 is provided with a valve 20 for filling.

The ballast tanks 1 are in figure 13 shown with different liquid levels, the item 36 being in a trimmed and stabilized position.

Claims

P a t e n t c l a i m s

1. Method for control of rising force in liquid (18) where a submerged ballast tank (1) comprising a fillable cavity (2), is sufficiently stiffened to withstand an existing ambient pressure, c h a r a c t e r i s e d i n that the method comprises to generate rising force by removing medium (4) from the ballast tank (1) while reducing pressure in the ballast tank (1).

2. Method in accordance with claim 1, c h a r a c t e r i s e d i n that the method comprises to reduce the rising force by filling medium (4) to the ballast tank (1) while increasing the pressure in the ballast tank (1).

3. Method in accordance with claim 1, c h a r a c t e r i s e d i n that the method comprises to control the rising force by removing or feeding desired amount of medium from or to the ballast tank (1).

4. Method in accordance with claim l to3, c h a r a c t e r i s e d i n that the method comprises:

- to couple the ballast tank (1) to an item (36); and

- to control the rising force of the ballast tank (1).

5. Method in accordance with claim l to3, c h a r a c t e r i s e d i n that the method comprises:

- to arrange several ballast tanks (1) on an item (36); and

- to control the rising force of each ballast tank (1) individually.

6. Submerged ballast tank device (1) comprising a fillable cavity (2), the ballast tank device (1) being sufficiently stiffened to be able to withstand an existing ambient pressure, c h a r a c t e r i s e d i n that the rising force is provided by removal of medium (4) from the ballast tank (1) during pressure reduction in the ballast tank (1).

7. Device in accordance with claim 5, c h a r a c t e r i s e d i n that the cavity (2) at its lower portion (6) communicates with at least one pump (10) which delivers medium (4) to a reservoir (16).

8. Device in accordance with claim 6, c h a r a c t e r i s e d i n that the cavity communicates with the reservoir (16) through a valve (20).

9. Device in accordance with claim 6, c h a r a c t e r i s e d i n that multiple ballast tanks (1) communicates with the same pump (10) through a pump valve (38) each.

10. Device in accordance with claim 6, c h a r a c t e r i s e d i n that the pump (10) is arranged inside the cavity (2).

11. Device in accordance with claim 5, c h a r a c t e r i s e d i n that the ballast tank (1) is designed as a pressure tank.

12. Device in accordance with claim 5, c h a r a c t e r i s e d i n that the ballast tank (1) comprises an interior stiffening (24).

13. Device in accordance with claim 5, c h a r a c t e r i s e d i n that the ballast tank (1) comprises small, open bodies (26) enclosed by a liquid seal (28).

14. Device in accordance with claim 7, c h a r a c t e r i s e d i n that the surrounding water (18) constitutes the reservoir (16).

15. Device in accordance with claim 7, c h a r a c t e r i s e d i n that a balloon (22) located in the surrounding water (18), constitutes the reservoir (16).