RU2558442C1 - Method of consolidation of soil base of hydraulic engineering structure - Google Patents

Abstract

FIELD: construction.

SUBSTANCE: invention relates to the field of hydraulic engineering. The method provides for submersion of the structure by acceptance of the ballast by the structure and placement of the structure base onto sea bottom. For this purpose on the sea bottom outside the area of structure placement they install electrodes, which are connected to a negative pole of a DC source. The structure is submerged by ballasting of the platform body to provision of the rated contact of the platform body foundation foot contact with the sea bottom. Afterwards the element of the facility structure is connected to the positive pole of the DC source, and electric current is sent, providing for generation of lower water saturation of soil in the area of foundation interaction with the soil base, providing for consolidation of the soil base of the platform, upon completion of which the electric current is switched off. As a result of physical and chemical processes taking place under action of DC, around the anode under the structure body there is an area of lower water saturation, and near the surface of the cathode moisture increases, and the area of higher water saturation of the soil is created outside the area of the gravity structure. Free water accumulated around the cathode may be removed with the help of a needle filter.

EFFECT: substantial acceleration of soil consolidation process, which may take place just within a few days, and increased soil strength, which provides for increased stability of a structure with smaller weight and dimensions of the structure and smaller amount of ballast.

7 cl, 3 dwg

Description

The invention relates to the field of hydraulic engineering and can be used in the operation of marine hydraulic structures after installation on the seabed for the development of hydrocarbon resources of the continental shelf.

The preferred field of application of the invention is the operation after setting on the seabed of marine hydraulic structures of gravity or gravity-pile types in the period when the consolidation of the soil base.

From the patent of the Russian Federation No. 2337205, an earthen building on weak natural foundations is known, which proposes a method of consolidating a soil foundation by arranging sand piles with a sand cushion along which ground water is discharged.

The known structure contains a support base in the form of a sand bed under the bottom of the foundation with sand piles, deepened to a certain depth. Each pile is a sand cylinder of well-drained material. Sand piles are installed under the soil bed with a certain step, depending on the properties of the soil base. On the top, the sand piles are interconnected by a sand layer (bed) laid on a natural base for organized drainage of pore water from the ground base.

The disadvantages of the known solutions in terms of the method are:

- the complexity and high complexity of the device of sand piles associated with the drilling of cylindrical wells, and filling them with sandy soil. The probability of successful implementation of such operations in a real environment, especially in difficult natural northern conditions (low temperature of water and air), is extremely small. All this also leads to significant costs and lengthy implementation of these operations;

- low efficiency in accelerating and passing consolidation of the soil base. All the same, consolidation takes a long time, given that bound water is not removed and remains in the pores of the soil.

These shortcomings are partially eliminated in the technical solution, which is described in the work Shibakin S.I., Levachev S.N., Kurillo S.V. Foundations of gravity oil and gas platforms. M .: VNIIIS, Building structures, No. 7, 1988.

In a solution known from the indicated source, in order to accelerate the consolidation process, under the bottom of the foundation of the marine object, needle filters are installed, which are connected to a single hydraulic system in which constant vacuum is maintained, due to which pore water is removed and pore pressure is reduced in the soil base. However, this method cannot quickly remove pore pressure at the base of the structure and even more so remove bound water from the soil. This reduces the effectiveness of this method. In addition, the consolidation process is delayed for a long time and does not have time to end before the application of significant loads from waves and ice, when the soil foundation does not have increased strength, which can lead to a construction accident.

Incomplete consolidation of the base leads to significant costs in the manufacture of the platform due to the increased mass of the structure to ensure the stability of the structure.

The objective of the invention is to reduce the material consumption of a hydraulic structure by speeding up the consolidation process, as well as the ballast consumption to ensure a given gravity in order to ensure the stability of the structure under the action of a combination of external loads while improving operational reliability and reducing the risks of loss of stability of a hydraulic structure after installing it on the seabed due to the accelerated and more complete due to the removal of bound water in the process of consolidation of the soil foundation.

The technical results that are provided by the implementation of the invention are as follows:

- reduction of material consumption and laboriousness when creating a hydraulic structure due to more complete and faster consolidation of the soil foundation before the main loads on the structure are applied;

- reducing the volume of ballast to ensure the given stability of the hydraulic structure by reducing pore pressure in the soil, which is formed during installation of the structure on the seabed;

- increasing the stability and bearing capacity of the soil base of the hydraulic structure due to a more complete removal of pore pressure in the soil base;

- improving the reliability and reducing the risk of loss of stability of the hydraulic structure during the operation of the entire complex of offshore engineering facilities and oil and gas equipment by increasing the stability of the hydraulic structure;

- reduction of the risk of emergencies, environmental pollution due to the loss of stability by the hydraulic structure and violation of the conditions of normal operation of downhole equipment and pipeline oil and gas field systems.

The problem is solved in that in the method of operating a hydraulic structure after setting on the seabed, which immerses the structure by taking the ballast with the structure and installing it on the soil base of the sea bottom, according to the invention, electrodes are placed in the soil of the sea floor outside the zone of setting the structure, which are connected to the negative pole direct current source, install the structure on the seabed, after which the structural element of the structure (body and / or insulated buried elements) are connected to the positive pole of a direct current source and pass an electric current that provides osmotic movement of pore water in the soil in the direction from the soil base of the structure and the formation of a region of lowered water saturation (consolidation) of the soil in the installation zone of the structure. The electric current is turned off after the consolidation of the soil foundation is completed, when the pore pressure in the soil foundation becomes equal to or close to zero.

The completion of the formation of the low water saturation region according to the invention is judged by the readings of the pore pressure sensors installed in the soil base.

The proposed method is based on the use of electroosmosis. Its essence is as follows.

With the usual method of consolidating a soil foundation, especially installed structures on cohesive soils, consolidation takes place over several years, and sometimes tens of years, which leads to a decrease in the bearing capacity of the soil base, since a significant part of the gravity is transferred to the ground water, creating pore pressure, and only part of it is transmitted to the skeleton of the soil, which resists the shear of the structure. According to the invention, in order to increase the strength of the soil under the foundations of the gravity structure, to increase the friction forces at the base of the foundation, it is proposed to provide an accelerated consolidation process with the removal of not only free water, but bound, it is proposed to use electroosmosis.

For this, electrodes connected to the negative pole of a direct current source (generator) are driven into the soil of the seabed (or placed on the seabed), and the building body and / or buried elements are connected to the positive pole of the same generator.

1. As a result of physicochemical processes occurring under the influence of direct electric current, a zone of reduced water saturation is formed around the anode under the building’s body, and humidity increases near the surface of the cathode and a zone of increased water saturation of the soil is created outside the zone of the gravity structure. Free water that has accumulated near the cathode can be removed using a needle filter. As a result, there is a significant acceleration of the process of soil consolidation, which can occur in just a few days, and an increase in the strength of the soil, which provides increased stability of the structure with smaller overall dimensions of the structure and less ballast. Moreover, electric current is passed in the direction coinciding with the direction of movement of the pore water under the action of the gravitational component from the structure, which leads to an accelerated squeezing of water from the soil base, accelerating the consolidation process.

To increase the efficiency of the consolidation process, direct electric current is passed by pulses and with a change in the direction of electric current, creating dynamic loads on pore water, which allows you to "tear off" water from soil particles.

Thanks to this combination of features that are in functional unity, a new effect is achieved that has not been previously achieved either in the prototype or in other known solutions, which consists in reducing the complexity and energy consumption, mass of the structure, as well as ballast consumption while improving the reliability of the operational properties of the hydraulic structure at seabed in difficult environmental conditions.

The invention is illustrated by drawings, where figure 1 shows the main nodes of the hydraulic structures and elements that ensure the implementation of the proposed method. The situation depicted in FIG. 1 corresponds to the moment the hydraulic structure arrived at the point before it was placed on the seabed.

Figure 2 shows the layout of the electrodes for electroosmosis; figure 3 shows the main nodes of the hydraulic structure and the organization of electroosmosis before operation, shows a diagram of the formation of the zone of low water saturation under the base of the hydraulic structure using electroosmosis.

The hydraulic structure contains (Fig. 1) a support housing 1, which is made (in whole or in part with an electrode installed) of an electrically conductive material. The upper building 2 with a direct current source 3 is installed on the support building. In the lower part of the housing are buried elements-electrodes-anodes 8. The buried elements, like the building’s body, are also made completely or partially from electrically conductive material. The housing 1, as well as the buried elements 8, are the anode. A direct current source (generator) is placed on the upper structure of the hydraulic structure. 3. Electrodes-cathodes of types 4 or 5 are placed on the seafloor. The cathodes can be gravitational 4 or pile 5. The type of anode is selected depending on the complex of hydrogeological conditions in the zone where the structure is placed on the sea bottom. The cathodes are placed (figure 2) evenly around the hydraulic structure outside the zone of its setting. The distance between the anodes is determined depending on the selected conditions of the process of electroosmosis, taking into account the complex of hydrogeological conditions in the zone of placing the structure on the seabed. The cathodes are interconnected (Fig. 3) by an electric cable 6, which, in turn, is connected to the negative pole of the direct current source 3. With the positive pole of the direct current source 3, the building 1 or the anode electrodes are connected using the electric cable 7.

The invention is implemented as follows.

In stationary factory conditions (for example, a dry dock), the hydraulic structure is prepared for transportation to the setting point.

To do this, in stationary conditions, a direct current source (generator) is placed on the upper structure of the hydraulic structure and an electric cable is installed to establish the electrical connection of the positive pole of the direct current source with structural elements of the structure (the case and / or anode electrodes buried in the seabed). A set of cathodes and an additional supply of electric cable for their connection with each other, as well as with a direct current source, are also prepared and placed in accordance with established requirements.

After the platform arrives at the point, preparations are made for its installation on the seabed.

For this, a set of cathodes is placed on the seabed outside the zone of setting the structure. Depending on the type of cathodes and the complex of hydrogeological conditions in the zone where the structure is placed on the seabed, they are installed using piling equipment (pile cathodes) or directly lowered to the seabed (gravity cathodes).

The distance between the cathodes is determined depending on the selected conditions of the process of electroosmosis, taking into account the complex of hydrogeological conditions in the area where the structure is placed on the seabed.

The cathodes are interconnected using an electric cable, which is connected to the negative pole of the DC source.

After that, with the help of ballast, the structure is gradually immersed until the contact of the buried elements with the seabed is established, and then, continuing the ballasting, the anodes are crushed into the soil base to a predetermined depth until the base of the foundation is completely in contact.

The establishment of full contact is judged, for example, using special contact pressure sensors installed in the lower part of the base of the foundation. It is advisable at this moment to carry out work to align the position of the structure in order to ensure a more dense and uniform contact of the foundation plate with the seabed. The tight contact of the foundation plate with the seabed provides a more efficient course of electroosmosis.

Maintaining the structure in the required position is necessary due to the action of underwater currents, which tend to displace the structure from the point of setting. Such support can be provided by any known method, for example, by means of the structures installed on the bottom of the building’s hull protruding below the buried elements and plunged into the soil of the seabed by several locking pins.

After the contact of the foundation plate with the seabed is established, the direct current source is switched on and the process of electroosmosis is started.

In this case, two schemes are possible for organizing such a process.

In the first case, the building body can be connected to the positive pole of the current source. This scheme is the simplest, because for its implementation it is possible to use the standard system of electrochemical protection (ECP) of the structure against corrosion, which is equipped with every marine hydraulic structure. However, with such a scheme, the efficiency of electroosmosis will be low due to large leakage currents on the way from the casing to the electrodes installed at a certain distance from the structure.

Therefore, to increase the efficiency of electroosmosis, reduce the time and energy consumption for forming, under the base of the construction, a zone of lowered water saturation of the soil, it is advisable to use buried elements, electrode-anodes, and connect the positive pole of the current source with them.

As a result of passing electric current between the electrodes, there is a directed movement of water in the seabed soil, both free and connected from the anode to the cathode, with the formation of a region of increased water saturation (increased pore pressure) in the cathode zone.

Pore pressure reduced as a result of a decrease in water saturation significantly increases the strength of the soil. Increasing the strength of the soil leads to a decrease in the mass of the structure and ballast to ensure the stability of the structure. Water entering the cathode can be pumped out using a needle filter.

Thus, by passing a constant electric current between the electrodes, the strength of the soil base increases significantly and the entire load is transferred to the soil skeleton at zero pore pressure.

To obtain the maximum effect of increasing the bearing capacity of the soil base, the final ballasting of the structure and its installation on the seabed should be carried out after the formation of a region of reduced water saturation under the base of the structure. This can be judged, for example, using pore pressure sensors (by reducing and subsequently stabilizing their readings).

Increasing the bearing capacity of the soil (increasing the strength of the soil base) during the installation of the structure on the seabed allows you to abandon the additional ballasting of the support block, resulting in reduced volumes of ballast tanks and the size of the support block.

In addition, increasing the strength of the soil in the described way allows us to ensure reliable operation of the structure on the continental shelf with unfavorable soil characteristics of environmental factors (ice, hard waves). This, in contrast to the known solutions, allows ensuring the design stability of the offshore structure to the effects of a set of environmental factors in the shelf areas of freezing seas.

Thus, the application of the proposed method allows you to expand the range of depths (in the direction of a significant increase in their value) at which the effective use of gravity structures is possible.

The analysis of the prior art showed that the claimed combination of essential features set forth in the claims is unknown. This allows us to conclude that it meets the criterion of "novelty."

To verify the conformity of the claimed invention with the criterion of "inventive step", an additional search was carried out for known technical solutions in order to identify features that match the distinctive features of the claimed technical solution from the prototype. It is established that the claimed technical solution does not follow explicitly from the prior art. Therefore, the claimed invention meets the criterion of "inventive step".

The new, industrially applicable technical solution described above represents a single inventive concept, which, in our opinion, meets the criterion of an inventive step, in connection with which a patent for an invention is proposed for legal protection.

Claims (7)

1. The method of consolidation of the soil base of a hydraulic structure installed on the seabed, involving immersion of the structure by adopting the ballast structure and setting the foundation of the structure on the seabed, characterized in that electrodes are placed on the seabed outside the setting zone of the structure, which are connected to the negative pole of the constant source current, immerse the structure with ballasting of the platform body until the design contact of the base sole of the platform body with the seabed is ensured then the structural element of the structure is connected to the positive pole of the direct current source and electric current is passed, which ensures the formation of a region of reduced soil water saturation in the zone of interaction of the foundation with the soil base, ensuring the consolidation of the soil base of the platform, after which the electric current is turned off. 2. The method of consolidation of the soil base of a hydraulic structure according to claim 1, characterized in that the body of the structure is connected to the positive pole of the direct current source. 3. The method of consolidation of the soil base of a hydraulic structure according to claim 1, characterized in that the buried electrodes located under the building body are connected to the positive pole of the direct current source. 4. The method of consolidation of the soil base of a hydraulic structure according to claim 1, characterized in that electrodes located at a certain distance from the structure body are connected to the negative pole of the direct current source. 5. The method of consolidation of the soil foundation of a hydraulic structure according to claim 1, characterized in that the electric current is passed in the direction coinciding with the direction of movement of the pore water under the action of the gravitational component from the structure. 6. The method of consolidation of the soil base of a hydraulic structure according to claim 1, characterized in that the electric current is transmitted by pulses. 7. The method of consolidation of the soil base of a hydraulic structure according to claim 1, characterized in that the completion of the formation of a region of reduced water saturation of the soil base is judged by the readings of the pore pressure sensors.

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