CN1646362A - Floating semi-submersible oil production and storage arrangement - Google Patents

Abstract

An arrangement for the storage of marketable quantities of crude oil at a semi-submersible floating production vessel. The storage is achieved by hanging a segmented reinforced concrete tank (2) to the underside of the semi-submersible vessel. The semi-submersible vessel can be an existing semi-submersible drilling rig. By maintaining the mass of the tank and contents slightly greater than the displacement of the tank and by arranging the centre of gravity of the tank below its centre of buoyancy, the metacentric height of the semi-submersible vessel is approved. The storage arrangement for the oil provides the necessary maintenance of mass by either storing approximately 4/5 of the oil in oil-over-water chambers and approximately 1/5 in gas-over-oil chambers or by using a gas-over-oil-over-water arrangement in all the chambers. The piping arrangements minimize the free surface of liquids in the tank. The design ensures the internal pressure in the tank can be less than the external pressure which minimizes required reinforcement. The design provides a net positive suction head to oil export pumps located above the storage tank in a location which allows easy maintenance. The design allows all pumps, valves and instrumentation necessary for handling ballast water to be maintained within the hull of the semi-submersible where they can be easily maintained. The design allows all the valves, pumps and instrumentation necessary for handling oil to be in locations where they can be easily maintained.

Description

Floating semi-submersible oil production and storage arrangements

Background technique

The oil industry often employs floating production and storage systems to develop small, remote fields. These are usually converted crude oil tankers utilizing mooring buoys tied to purpose built vessels. In order to minimize wave, tidal and wind forces acting on the tanker, the mooring is designed to allow the tanker to swing with the wind around the buoys under the influence of the resulting environmental forces. The processing equipment is installed on the deck of the drilling platform.

Crude oil production from the reservoir occurs through one or more subsea wellheads through flexible flowlines from each wellhead to mooring buoys and from the buoys to the drilling platform. The system is also fitted with pipelines to carry gas and/or water from the tanker to the wellhead. Also installed are hydraulic and electrical lines from the rig to allow control of the subsea wellhead. Because the drilling platform must freely rotate around the buoys, the many fluid flow paths through the buoys result in the need for complex and expensive devices called hawse swivels. The unit is a precision engineered piece of equipment to withstand high pressure, high temperature corrosive fluids from the reservoir and therefore must be manufactured flawlessly if high maintenance costs are to be avoided.

A further disadvantage of float generation systems employing tankers is that tankers are susceptible to pitch, roll and heave. Since the separation of oil, water and gas, including liquid flow, from the reservoir is performed by gravity separation in relatively high pressure rigs, liquid sloshing caused by ship motion can severely impact the separation process.

An alternative type of floating production system that eliminates these problems is the use of semi-submersible drilling platforms. Semi-submersible drilling rigs have been used in the offshore industry for many years as mobile drill ships, crane ships, pipelay vessels and specialized floating production rigs. As shown in Figure 1, the semi-submersible drilling platform comprises a deck 1 supported on a plurality of columns above the water line (OWL). The columns extend from the deck to two floating pontoons 31 located (usually) some distance below the waterline. Compared with the ship-shaped drilling platform, the advantages of the semi-submersible drilling platform are the following two points. First, the area of a semi-submersible rig exposed to waves at the waterline is smaller than that of a ship-shaped rig, thus reducing the forces of horizontal waves. Second, since the pontoons providing buoyancy are farther below the waterline than the underside of the ship, vertical forces are very small. (This is due to the rapid decrease in wave influence as one moves deeper into the water.)

The result of these advantages is that a semi-submersible production rig can be moored in the ocean without the need to provide a weather vane and with reduced sloshing of liquids in the rig to the deck.

Semi-submersible floating production systems (SSFP systems) however have two disadvantages. First, there is no capacity to store the crude oil produced in significant quantities. This means they can only be used where there are pipelines to transport produced crude to onshore storage/processing facilities or where dedicated moored tankers are available adjacent to SSFP rigs.

A second disadvantage is the limited amount of processing equipment that can be mounted on the deck, since the center of gravity of the SSFP rig is raised due to the weight being added to the deck. This reduces resistance to rig capsizing. This resistance to capsizing is quantified by a property of the drilling rig called the pitch center height (commonly denoted GM). A higher GM means a higher resistance to overturning.

Many fields have been developed using SSFP rigs utilizing converted used semi-submersible drillships. If the crude being produced is viscous and requires large pressure vessels for separation or if gas injection or water injection equipment is required, new larger semi-submersible rigs are required to accommodate this equipment.

A number of attempts have been made to provide storage of crude oil in semi-submersible drilling platforms (eg UK patent applications GB22116849, GB2207892, GB2188291). However, these attempts have only allowed the storage of relatively small quantities of crude oil. These systems still require specially moored tankers to store salable quantities of crude oil. Storage provided in semi-submersible rigs provides storage for only a few days of production to allow storage tankers to sail to nearby refineries for offloading.

Contents of the invention

According to an aspect of the present invention, there is provided an oil storage assembly for a semi-submersible oil production drilling platform, the oil storage assembly for a semi-submersible oil production drilling platform includes a deck structure, at least two for an underwater pontoon providing buoyancy to said deck structure, and a plurality of columns connecting said deck to said pontoon, wherein a concrete box is attached below said pontoon, said concrete box is subdivided into Multiple chambers for storing fluids.

The advantage of the arrangement for storing oil according to the invention is that it provides a system for storing large volumes of oil which are not susceptible to extreme environmental conditions, are not degraded, and in fact it can be constructed to increase the Drilling equipment resists overturning forces and is easy to maintain in situ.

More particularly, the present invention preferably provides a semi-submersible, floating production, storage and offloading system for developing offshore oil and gas fields, said system comprising a drill ship, an oil tank according to the present invention connected to the bottom of the drill ship. Storage assembly, means for adding water to or draining water from the bottom of each chamber of the concrete tank using the ballast water pump of the drill ship and directing produced oil to the top of each chamber or from the bottom of each chamber The top boots out.

The present invention also provides a method of storing oil in an offshore floating oil production facility, said method comprising the steps of: connecting a concrete tank divided into a plurality of chambers to the bottom of the buoyant tank structure, filling said chambers with at least one to adjust the buoyancy of the production facility, and to drain the fluid from the chamber by pumping the produced oil into the chamber in a controlled manner so that the mass of the concrete tank and its contents remains approximately constant.

Preferably, said concrete tank is divided into a number of chambers, in particular separated by a plurality of fluid-tight partitions, at least one of said chambers being located substantially in the center of the concrete tank and open at the top and bottom, so that the concrete tank Through-holes are provided. In this way the drilling rig is able to carry out drilling or well servicing operations with the attached concrete box.

Preferably, each chamber of the concrete tank is always kept filled with at least one fluid, thereby controlling the ballast of the arrangement. The fluid used in this way may for example be sea water, natural gas or a mixture of two or more of these.

Preferably at least some of the chambers comprise a first inlet/outlet tube terminating substantially at the bottom of the chamber and a second inlet/outlet tube terminating substantially at the top of the chamber. In particular, it is preferred that at least some of the chambers comprise water inlet/outlet pipes and oil inlet/outlet pipes. Each of such chambers is at least partially filled with water prior to commencing oil production so that water is drained from the chamber through the water pipe when oil is added to the chamber through the oil pipe. In this way, the drained oil by using water injected through the water pipe can be drained from the concrete tank to, for example, a tanker for transportation to the coast. The water pipe preferably terminates near the bottom of the chamber since seawater is generally denser than oil, and the water pipe preferably has a diffuser tube at its end to minimize mixing of the oil and water as water is drawn into the chamber. The oil pipe preferably terminates on the inner surface of the upper surface of the chamber, an advantage of this arrangement is that it avoids the possibility of air pockets forming in the concrete box.

It has been found that when producing oil or unloading oil, if the first plurality of chambers operates the oil above water storage arrangement and the second plurality of chambers operates with oil, natural gas or a mixture thereof stored therein, particularly It is advantageous to keep the mass of the concrete box and contents substantially constant. In particular, each of the second plurality of chambers is preferably provided with a first pipe terminating near the bottom of the chamber and a second pipe terminating near the top of the chamber through which oil is pumped as it is produced. into the chamber while exhausting the gas already in the chamber through the second tube. Preferably, said second plurality of chambers are arranged in a series arrangement, and each second tube extends in series from near the top of one chamber to the bottom of the next chamber, thereby forming a One chamber for the first tube. An advantage of this arrangement is that the chambers in the series fill or evacuate sequentially rather than simultaneously, ie once the first chamber in the series is filled it overflows into the second chamber. As a result, only one of the second plurality of chambers contains a mixture of oil and gas at any one time - the rest are filled with gas or oil, thereby reducing the free liquid surface within the arrangement.

Preferably, the first plurality of chambers constitutes substantially 80% of the chambers and the second plurality of chambers constitutes substantially 20% of the chambers, whereby oil is drawn from said first and second chambers simultaneously. Supply or discharge, preferably substantially 80% of the flow is directed to said first plurality of chambers and substantially 20% of the flow is directed to said second plurality of chambers. Since the density of crude oil is roughly 80% of that of seawater, this arrangement has the advantage of maintaining a constant mass in the concrete tank when loading with or unloading oil from the arrangement, while at the same time minimizing the free flow of fluid in the chamber. surface, which contributes to the stability of the arrangement.

In an alternative embodiment, in addition to said first and second inlet/outlet pipes, each chamber also includes a third inlet/outlet pipe, the third inlet/outlet pipe being part of the distance down the chamber. Terminate at a distance from the top of the chamber that is substantially 20% of the height of the chamber. In this embodiment, the first pipe is provided with inlet/outlet for seawater, the second pipe is provided with inlet/outlet for gas and the third pipe is provided with inlet/outlet for oil, all chambers of this arrangement are equipped with have the same piping arrangement and are supplied simultaneously. At this point a mixture of water and gas is used to ballast each chamber, without oil each chamber is 80% filled with water with gas on top, the volume of gas in each chamber changes as oil is added/expelled from it, thus Make sure that the volume of water displaced by the oil when the oil is pumped in is only 80% of the volume of the oil. In this way the overall mass of the arrangement is kept constant.

Description of drawings

Some embodiments of the invention will be described below by way of example with reference to the accompanying drawings, in which:

Figure 1 shows a typical semi-submersible floating production system;

Figure 2 shows in perspective view a semi-submersible drilling rig connected to a concrete tank according to the invention;

Figure 3 shows a cross-sectional view of the drilling rig in Figure 2;

Figure 4 is a schematic diagram of the arrangement of the connections between the chambers of the concrete tank in Figures 2 and 3;

Figures 5 and 6 are further schematic diagrams illustrating how the chambers are connected;

Figure 7 shows additional details of the drilling rig in Figures 2 and 3; and

Figure 8 shows an alternative gas/oil/water loading/unloading arrangement.

Detailed ways

Referring to FIG. 2 , the drilling rig 1 has a concrete tank 2 connected under a column 30 and a buoy 31 .

As shown in FIG. 3 , the concrete boxes are separated by fluid-tight partitions in the form of inner concrete walls 3 . At least one cell in the center of the concrete box is configured to be open at the top and bottom to create a hole through the center of the concrete box whereby drilling equipment can perform drilling or well servicing operations with the box attached.

Figures 3 and 5 also show the water inlet/outlet pipe 4 and the oil inlet/outlet pipe 5 of one chamber. The water pipe terminates in a diffuser tube 6 near the bottom of the chamber which minimizes mixing of oil and water as water is pumped in. The tubing terminates on the inside face of the upper surface to avoid the possibility of air bubble formation. In order to keep the mass of the concrete tank and contents constant when producing or unloading oil, only 4/5 of the chamber operates on the oil over water principle shown in FIG. 3 .

Figure 5 shows the piping arrangement to ensure the longitudinal stability of the concrete tank by ensuring that the center of mass remains stationary during the loading/unloading process of the oil above water chamber. The remaining 1/5 of the chamber utilizes a gas over oil scheme using piping as shown in FIG. 6 . The oil enters the first chamber A through the oil pipe 9 . When chamber A is filled with oil, natural gas is delivered to chamber B through pipe 10 . When chamber A is full, the oil will then flow through the pipe 10 to chamber B, which in turn moves the gas into chamber C through the wire, and so on until all the chambers are filled with oil and the gas has been expelled from the final gas outlet pipe 11 . This process takes place while 4/5 of the oil is being directed to the oil above water chamber. Since the density of crude oil is about 4/5 that of sea water, this arrangement maintains a constant mass in the concrete box when loading or unloading. These arrangements minimize the free surface of the liquid in the chamber, which is beneficial for the stability of the offshore drilling rig. Figure 4 shows an example layout for the chambers where the chambers running the gas over oil scheme are arranged in a straight line along the center of the concrete tank and the chambers running the water over oil scheme are arranged symmetrically on either side, To ensure lateral stability during loading/unloading handling.

Figure 8 shows an alternative arrangement to achieve this mass balance. In this arrangement the loading system is the same for the chambers. Seawater inlet/outlet pipes are placed at the bottom of the chamber, oil inlet/outlet pipes are arranged at 4/5 of the height of the chamber and natural gas inlet pipes are arranged in the upper surface of the chamber. When there is no oil in it, 4/5 of each chamber is filled with sea water and natural gas on it. When oil is produced, oil enters through the oil pipe 26 and seawater exits through the water pipe 25 . At the same time, the natural gas is released through the natural gas inlet/outlet pipe 27 to ensure that the volume of water expelled by the oil is only 4/5 of the volume of the oil entered.

The invention is also characterized in that the mass of the concrete box and contents is slightly greater than the buoyancy of the concrete box. This means that to bring the combined semi-submersible rig/concrete box structure to the same draft that the semi-submersible rig operates in the unincorporated state, some ballast water must be drained from the semi-submersible rig, thereby Leading to further improvement of the pitch center height of the merged rig. This arrangement creates tension between the concrete box and the drilling rig.

A further feature of the invention is that by eliminating the gap between the underside of the semi-submersible buoyancy tank and the upper surface of the concrete tank this tension can be changed to a pressure which is beneficial to the fatigue life of the drilling platform.

By constructing the bottom of the concrete box from a material that is denser than the top of the concrete box, the center of gravity of the concrete box and contents is slightly below the center of buoyancy of the concrete box. This increases the center of inclination height of the offshore rig, allowing increased payload to be added to the deck of the semi-submersible rig.

The volume of the concrete box is sufficiently large that when the concrete box is empty and the drilling equipment is attached thereto, the upper surface of the concrete box is at a considerable distance above the waterline. Thus, each chamber can be at least partially filled with a gas, such as air, thereby reducing the overall mass of the concrete box including its contents and thereby reducing its draft for easier maintenance. This allows access to pipes above the surface of the upper concrete tank, manholes in the concrete tank for internal inspections, the connection of the concrete tank to the semi-submersible drilling platform and all external parts of the semi-submersible drilling platform. This allows inspections required by the surveyor society to be carried out without the rig having to be run to dry dock. The only parts of the rig that are not inspectable in dry dock are the underside and lower walls of the concrete box. Since concrete boxes are constructed of concrete, periodic visual inspection by the operator is considered adequate.

Concrete as a structural material performs best when subjected to compressive loads. The tension must be resisted by steel bars embedded in the concrete. The amount of reinforcement required is minimized by keeping the external pressure on the concrete box greater than the internal pressure. To achieve this, the water outlet lines from each oil-over-water chamber are connected to a break tank 12 located within the column of the semi-submersible offshore drilling rig shown in FIG. 7 . The break tank is located below the level of the operational waterline (OWL). The break tank is vented to atmosphere 13 and the water level in the break tank is maintained by water level switches 14 and 15 and control valve 17 acting on the seawater ballast pump 16 of the semi-submersible offshore drilling platform to add and drain water as required . To ensure that the oil system cannot overpressurize the concrete tank, the oil inlet/outlet is connected to atmosphere at a safe location 18.

Large centrifugal pumps 19 are required to offload the stored oil to the shuttle tanker. Such pumps require a net positive suction head (NPSH) on the suction side in order to operate efficiently. Usually this is achieved by locating the pump at a lower height than the bottom of the storage tank. The arrangement of the present invention allows the pump to be located on top of the concrete tank in the docking chamber connected to one of the semi-submersible columns which can be easily removed by the rig crane for maintenance but still provide a net positive suction head .

An oil output meter 20 , a ballast water cleaning device 21 , an oil in water alarm 22 and a gas/oil/water separator 23 are shown in FIG. 7 .

It is also a feature of the invention that all necessary valves, pumps and instruments for the seawater system can be located within the column of the semi-submersible offshore drilling rig where they are dry, benign and easily accessible for maintenance.

The present invention does not require piping or equipment extending from the bottom of the concrete tank. This allows the concrete box to be easily built onshore, slid into the ocean, setting the concrete box on a suitable seabed to fit to a semi-submersible drilling platform.

The invention is also characterized by the ability to construct the connection between the concrete tank and the semi-submersible drilling rig in a dry environment even if it is submerged during operation. Once constructed, the concrete box will be set on the sea floor with a few meters of water above it. A semi-submersible offshore drilling rig at minimum draft will float above the concrete tank and discharge ballast water down onto the top of the concrete tank. The concrete tank is then ballasted enough to lift the semi-submersible offshore rig without water to allow permanent connections to be built in the dry.

Of course, it will be appreciated that the ratio of 80% used above relates to the relative density of seawater to crude oil and is therefore only an approximation. Furthermore, where the arrangement is used with a liquid other than oil and/or water, the relative displacement and/or relative numbers of the first and second plurality of chambers will depend on the relative density of the liquid actually used.

Claims (31)

1. one kind is used for the oily memory module that semi-immersed oil is produced drilling platform, comprise that deck construction, at least two are used for providing the buoyancy tank under water of buoyancy and a plurality of columns that described deck are connected to described buoyancy tank to described deck construction, it is characterized in that, concrete box is connected under the described buoyancy tank, and described concrete box is subdivided into a plurality of chambers that are used for storing fluid.

2. oily memory module according to claim 1, wherein said concrete box is subdivided into a plurality of vertical chambers, described vertical chamber forms the array that passes described concrete box, in the described chamber at least one is at top and bottom opening, thereby through hole is provided in concrete box, carries out oily production operation by this through hole.

3. oily memory module according to claim 2, wherein said concrete box is divided by a plurality of fluid-tight next door again.

4. according to each described oily memory module in the claim 1 to 3, at least some chambers in the chamber of wherein said concrete box comprise the first inlet/outlet pipeline that nestles up cavity bottom and are positioned at the second inlet/outlet pipeline on the top 20% of chamber substantially.

5. oily memory module according to claim 4, more than first chamber in wherein said at least some chambers arranged in the mode of install in series, and the second inlet/outlet plumbing connection of a chamber of described a plurality of chambers is to the first inlet/outlet pipeline of the next chamber of described install in series.

6. oily memory module according to claim 5, wherein the first inlet/outlet plumbing connection of first chamber of install in series is to oil pipe, and the second inlet/outlet plumbing connection of last chamber of install in series is to tracheae.

7. according to each the described oily memory module in the claim 4 to 6, wherein more than second described chamber has their first inlet/outlet pipeline that is connected to fluid supply tube and their the second inlet/outlet pipeline that is connected to oil pipe, and the scheme of fluid utilization oil on suitable fluid is stored in described more than second chamber.

8. oily memory module according to claim 7, the part that wherein forms described at least some chambers of described more than second chamber equals the density of oil and the ratio of fluid density substantially, oil is stored on described fluid in described more than second chamber, and the part that forms described at least some chambers of described more than first chamber equals 1 substantially and deducts the ratio that forms described more than second chamber.

9. according to claim 7 or 8 described oily memory modules, wherein oil is stored on the water in described more than second chamber, and 20% and described more than second chamber that described more than first chamber constitutes described at least some chambers constitute 80% of described at least some chambers.

10. according to each the described oily memory module in the claim 4 to 9, the wherein said second inlet/outlet pipeline is positioned at the top of each described chamber substantially.

11. oily memory module according to claim 4, the described second inlet/outlet pipeline is positioned at the position that equals chamber height 1/5th apart from the distance of chamber roof substantially.

12. oily memory module according to claim 11, each in wherein said at least some chambers comprise the 3rd inlet/outlet pipeline that is positioned at chamber roof substantially.

13. oily memory module according to claim 12, the wherein said first inlet/outlet plumbing connection be to water pipe, the described second inlet/outlet plumbing connection to oil pipe and described the 3rd inlet/outlet plumbing connection to tracheae.

14. according to each the described oily memory module in the claim 4 to 13, the wherein said first inlet/outlet pipeline has the diffuser assembly that is arranged on its end, to minimize the mixing of the fluid in each chamber.

15. according to each the described oily memory module in the aforesaid right requirement, wherein concrete box is by the material structure of different densities, thereby the center of gravity of concrete box and its contents always is lower than the center of its buoyancy, and the metacentric height of this oil memory module is increased.

16. according to each the described oily memory module in the aforesaid right requirement, wherein the volume of concrete box is: when it when being empty to small part, its buoyancy is enough to that oil is produced drilling platform and remains on such height with respect to the water surface, promptly this height concrete box upper surface on floating line.

17. according to each the described oily memory module in the aforesaid right requirement, also comprise the break tank that is communicated with atmosphere, described break tank is positioned at the height under the operation floating line in the inside of described drilling platform, the internal pressure in the concrete box remains by break tank and is lower than external pressure.

18. oily memory module according to claim 17, one of them break tank is positioned at the inside of the column of described drilling platform.

19. according to claim 17 or 18 described oily memory modules, wherein the fluid level in the break tank can be by the control of fluid level controller, described fluid level controller actuating seawater ballasting water pump adds from the ocean to allow seawater with discharge water and actuation control valve.

20. each the described oily memory module in requiring according to aforesaid right wherein provides pump, so that oil is discharged to oil tanker, described pump is arranged in of described column on the described concrete box height.

21. according to each the described oily memory module in the aforesaid right requirement, wherein said concrete box is by ferro-concrete and/or prestressed concrete structure.

22. one kind be used to develop offshore oil and nature-gas field can half submergence, float production, storage and uninstalling system, the ballasting water pump that comprise drilling ship, be connected to each described oily memory module in the requiring according to aforesaid right of drilling ship bottom, is used to utilize drilling ship is with to increasing water or being directed to the top of each chamber or the device that guides from the top of each chamber from the device of the bottom discharge water of each chamber of concrete box and the oil that is used for producing.

23. the oily method of storage in the oily production facility that at sea floats, the concrete box that comprises the steps: to be divided into again a plurality of chambers is connected to the bottom of floating box structure, with the described chamber of at least a fluid filled to regulate the buoyancy of described production facility, and by in the described chamber of oily suction that will produce in a controlled manner and described fluid is discharged from described chamber, constant substantially thereby the quality of concrete box and its contents keeps.

24. method according to claim 23, also comprise the steps: by more than first described chambers being filled first fluids and more than second described chambers being filled the buoyancy of the described equipment of second fluid regulation, thereby described more than first chamber of the oily suction of the first ratiometric production therefrom discharged described first fluid, and thereby described more than second chamber of the oily suction of the described production of second ratiometric therefrom discharged described second fluid, described first and described second ratio constant substantially based on the quality that the relative density of the oil of producing and the described first fluid and second fluid calculates to keep the fluid in the described concrete box.

25. method according to claim 24, thereby also comprise the steps: to move the described first ratiometric oil of empty oil from described concrete box and discharge and the described second ratiometric oil is discharged from described more than second chamber from described more than first chamber by with relative quantity described first fluid being drawn back in described more than first chamber and described second fluid being drawn back in described more than second chamber, wherein the quality of concrete box and its contents keeps constant substantially during the oil unloading of producing.

26. according to claim 24 or 25 described methods, wherein said first fluid is a water and described second fluid is a gas, described more than first chamber constitute substantially the concrete box volume 80% and hold substantially production oil 80%, and described more than second chamber constitute substantially the concrete box volume 20% and hold substantially production oil 20%.

27. according to each the described method in the claim 24 to 26, also comprise the steps: to connect described more than second chamber, thereby the fluid of discharging from a chamber enters the series connected next chamber with series system.

28. method according to claim 23, also comprising the steps: provides water inlet/outlet conduit in the bottom of each chamber substantially, provide gas access/outlet conduit at the top of each chamber substantially, and 1/5 place for chamber height provides oil-in/outlet conduit at the described top of distance substantially, each chamber of described concrete box is filled the buoyancy of the gentle compound of water with conditioning equipment, it is gentle to discharge described water to be extracted in each chamber to the oil that will produce by described oil pipe, thereby keep each chamber to be full of fluid and minimize wherein free fluid surface thus, and control is constant substantially with the total mass that keeps concrete box and its contents from the gentle relative scale of described chamber drain water by described oil.

29. method according to claim 28 wherein by drawing back in each chamber from concrete box unloading oil water is gentle with relative scale, thereby keeps the total mass of concrete box and its contents constant substantially.

30. according to claim 28 or 29 described methods, wherein said water and described gas from each chamber discharge and/or each chamber of suction described ratio be 80% water and 20% gas.

31., thereby also comprise the steps: at least in part each chamber blanketing gas of described concrete box is reduced the total mass of the described concrete box that comprises its contents and reduces its draft thus so that safeguard according to each the described method in the claim 23 to 30.

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