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The type of dam is selected on the basis of a technical and economic comparison of layout options for the structure as a whole, taking into account the purpose of the dam, engineering-geological, climatic and other conditions.
Depending on the type of building material, dams are built from
· concrete and reinforced concrete,
· wood,
· soils.
Dams, being built from soils, are called ground. Widespread soil dams is explained by the following advantages: the material for the construction of dams is local, the cost of extracting the material is minimal, it can be used in most geographic areas; the soil placed in the body of the dam does not lose its properties over time. Soil dams can be built to almost any height; all processes during their construction are highly mechanized.
Along with the advantages, earth dams have flaws: limited possibilities for releasing maximum flows through the dam crest; the presence of a filtration flow in the dam body, potentially creating conditions for filtration deformations; the possibility of large losses of water due to filtration if the body of the dam is made of soils with increased water permeability; the difficulty of laying the embankment at significant and prolonged subzero temperatures; uneven settlement along the transverse profile of the dam; restrictions on the use of certain types of soils for the dam body and foundations.
Based on the design of the body and anti-filtration devices, they are distinguished the following types earth dams:
from homogeneous and heterogeneous soil,
· with a screen made of ground and non-ground material,
with a core made of soil material,
· with a diaphragm made of non-priming material.
According to the anti-filtration measures at the base, dense structures are distinguished:
with a tooth, a lock, a diaphragm, with a tongue-and-groove wall, with a combination of a tongue-and-groove wall and a tooth, with an injection curtain (brought to a waterproof point or hanging), with a droop.
Soil dams are classified according to their height:
· low – with a pressure of up to 15 m;
medium height – with a pressure of 15–50 m,
· high – with a pressure of more than 50 m.
For the main part of the dam profile, all types of soils are used, with the exception of: those containing water-soluble inclusions of chloride or sulfate-chloride salts in an amount of more than 5% or sulfate salts of more than 2% of the mass; containing incompletely decomposed organic substances in an amorphous state in an amount of more than 8% of the mass.
The best soils for a homogeneous soil dam are considered loams and sandy loams. Sandy and sandy-gravel soils are quite suitable, however, due to their water permeability, it is necessary to provide anti-filtration devices. For the dam's anti-filtration elements, cohesive, plastic, low-permeability soils are used: clays, loams, and peat with a degree of decomposition of at least 50%.
Silty soils, as well as those that move easily when saturated with water, are unsuitable for laying in the dam body. Important quality soil for the dam body - its easy compaction during rolling. The choice of soil for the dam body is justified by technical and economic calculations.
If there is a sufficient amount of relatively waterproof soil (loam, loess) in the construction area, a dam is built from homogeneous soil. The advantages of homogeneous dams are simplicity and speed of construction, the possibility of using complex mechanization, which significantly reduces the cost of work compared to other types of earth dams.
At insufficient quantities low-permeability soils, the dam can be built from locally available sandy soils, sandy loams or other permeable materials. In this case, strong filtration of water through the body of the dam will occur. To prevent this phenomenon, anti-filtration devices are used in the form of a core, screen, or diaphragm. In our work, we provide a kernel device to prevent filtration processes.
The plastic core is made of clay or heavy loam and placed vertically under the crest of the dam, preferably closer to the upstream slope, in order to reduce the volume of water-saturated soil of the upstream prism facing the upstream, and to make the downstream part of the dam, i.e., located from the downstream side.
The same requirements apply to foundation soils as to dam body soils. Soils at the base of the dam body with an undecomposed root system and humified soils, as well as those with passages for digging animals, are usually removed.
According to the method of carrying out work, earth dams are divided into dams:
· with dry backfilling using the pioneer method and mechanical soil compaction,
· with soil poured into water, alluvial,
· constructed using directed explosions.
The bulk method is considered the most accessible and cheapest. With this method, the soil delivered from the quarry is leveled into a layer 20–25 cm thick in a loose state. The soil is compacted with self-propelled or trailed rollers - smooth or spiked, sometimes with crawler tractors or self-propelled scrapers. Heavy-duty pneumatic trucks (weighing up to 26 tons) are also used, compacting soil layers up to 60 cm thick, and vibratory rollers, compacting soil layers up to 0.8–1.0 m. The degree of soil compaction is controlled in the laboratory and using density meters. To achieve the required degree of soil compaction, it is sometimes necessary to wet it with water, since the best soil compaction occurs at optimal humidity. The latter depends on the nature of the soil and the mass of the skating rink. For heavier rollers, the optimal humidity decreases, and for lighter ones, it increases. Soil moisture is determined experimentally in laboratory and field conditions. After compacting the layer, its surface is harrowed for better adhesion to the next layer.
If there is low-permeable soil (clay or loam) at the base of the dam with a thickness of at least 2 m, before laying the dam body, only the plant layer is removed to a depth of 30 cm from the surface.
When the low-permeable layer is located no deeper than 4 m, in addition to removing the plant layer, a lock is installed at the base of the dam. When the aquiclude lies at a depth of 4 to 6 m, a castle 2–3 m deep is built and a sheet pile row is driven into its bottom, cutting through the entire water-permeable layer and entering the aquifer at 1 m. The sheet pile row is constructed from beams or thick boards and the upper part is included in lock 0.5 m.
The interface of the dam body with the banks should be done in the form of inclined planes with short ledges for ease of work. Treatment of slopes with vertical ledges is not allowed, since sudden changes in the height of the embankment result in the formation of dangerous transverse cracks along the ledges. Their presence will contribute to increased filtration of water and destruction of the dam.
We are designing an earth dam made of sand, which will be erected by backfilling using the pioneer method. To reduce filtration, we will arrange a core and a lock.
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3. CONSTRUCTION OF EMBARKS BY THE METHOD OF PILLING SOILS INTO WATER
3.1. The method of dumping soil into water is used for the construction of dams, dikes, anti-filtration elements, pressure structures in the form of screens, cores, depressions and backfilling at the junction of earthen structures with concrete ones. For the construction of an embankment by dumping soil into water and preparing the foundation and interfaces with the banks for it, the design organization must develop technical specifications, including requirements for organizing geotechnical supervision.
3.2. Filling of soil into water should be carried out using the pioneer method, both in artificial, formed by embankment, and in natural reservoirs. Filling soil into natural reservoirs without installing bridges is allowed only in the absence of current speeds capable of eroding and carrying away small fractions of soil.
3.3. Soil filling should be carried out in separate dumps (ponds), the dimensions of which are determined by the work plan. The axes of the maps of the laid layer, located perpendicular to the axis of the structures, should be shifted relative to the axes of the previously laid layer by an amount equal to the width of the base of the embankment dams. Permission to create ponds for filling the next layer is issued by the construction laboratory and technical supervision of the customer.
3.4. When pouring embankments into natural reservoirs and ponds with a depth from the water's edge to 4 m, the preliminary thickness of the layer should be assigned based on the physical and mechanical properties of the soil and the availability of a supply of dry soil above the water horizon to ensure the passage of vehicles according to Table. 2.
Table 2
Spoy thickness | Transport capacity | Layer of dry soil, cm, above the horizon water in the pond when filling |
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fill, m | funds, t | sands and sandy loams | loams | |
The thickness of the fill layer is adjusted during the construction of embankments.
At depths of natural reservoirs from the water's edge of more than 4 m, the possibility of filling soils should be determined experimentally in production conditions,
3.5. Embankment dams within the structure being constructed should be made from soil placed into the structure. Longitudinal embankment dams can serve as transition layers or filters with screens on the internal slope made of waterproof soils or artificial materials.
The height of the embankment dams must be equal to the thickness of the backfill layer.
3.6. When dumping soil, the water horizon in the pond must be constant. Excess water is drained into an adjacent map through pipes or trays or pumped to the overlying map by pumps.
Filling should be carried out continuously until the pond is completely filled with soil.
In case of a forced break in work for more than 8 hours, the water from the pond must be removed.
3.7. Compaction of the dumped soil is achieved under the influence of its own mass and under the dynamic influence of vehicles and moving mechanisms. During the filling process it is necessary to ensure uniform motion transport over the entire area of the map being poured.
3.8. When transporting soil using scrapers, dumping soil directly into the water is not allowed. In this case, dumping the soil into the water should be carried out by bulldozers.
3.9. When the average daily air temperature is down to minus 5 °C, work on dumping soil into water is carried out using summer technology without special measures.
When the outside air temperature is from minus 5 °C to minus 20 °C, soil filling should be done using winter technology, taking additional measures to maintain a positive soil temperature. Water must be supplied to the pond at a temperature above 50 °C (with an appropriate feasibility study)
3.10. The sizes of maps when working using winter technology should be determined based on the conditions for preventing interruption in work; soil filling on the map must be completed during one continuous cycle.
Before filling the cards with water, the surface of the previously laid layer must be cleared of snow and the upper crust of frozen soil must be thawed to a depth of at least 3 cm.
3.11. When dumping soil into water, you should control:
meeting project requirements and technical specifications for the construction of structures by dumping soil into water;
compliance with the design thickness of the filling layer;
uniform compaction of the surface soil layer by moving vehicles and mechanisms;
compliance with the design depth of water in the pond;
temperature of the surface of the base of the fill map and the water in the pond.
3.12. To determine the characteristics of soils, samples should be taken one for every 500 m2 area of the backfilled layer (underwater) with a thickness of more than 1 m - from a depth of at least 1 m, with a layer thickness of 1 m - from a depth of 0.5 m (from water horizon in the pond).
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During the construction of water supply and sewerage systems, leveling embankments in the form of dams and earthen dams are installed as part of regulatory and reserve reservoirs, sludge reservoirs, river water intakes and other structures. All leveling embankments, regardless of their purpose, are erected from homogeneous soils with leveling of the poured soil in horizontal or slightly inclined layers and their subsequent compaction.
To fill the soil, the embankment section is divided into maps of equal area, on each of which the following operations are performed sequentially: unloading, leveling, moistening or drying and compacting the soil (Fig. 4.27, a). The choice of the type of machine for constructing an embankment depends on the general scheme of its construction, i.e. from lateral reserves, excavations or quarries, as well as from the distance of soil transportation.
To fill embankments from side reserves or excavations, the following machines are used: bulldozers - with an embankment height of up to 1 m and a movement range of up to 50 m, scrapers - with an embankment height of up to 1 ... 2 m and a delivery range of 50 ... 100 m; dragline excavators - for laying soil in embankments with a height of 2.5 ... 3 m. In the case of filling the embankment from special reserves (quarries), from which the soil is moved in the longitudinal direction, the following are used: for a movement range of up to 100 m - powerful bulldozers, from 100 to 300 m - self-propelled scrapers with a capacity of 9 .. 15 m 3 and excavators (single-bucket or multi-bucket) with loading soil into vehicles. Embankments constructed from soil delivered by dump trucks are divided into sections of 100 m; on one of them the soil is unloaded, and on the other it is leveled with bulldozers and compacted (Fig. 4.27, b). In this case, the unloaded soil is leveled with a bulldozer across the entire width of the embankment in layers 0.3 ... 0.4 m thick. The thickness of the leveled layers must correspond to the capabilities of soil compaction machines. When laying soil with scrapers, it is leveled with a scraper knife during the filling process.
Rice. 4.27 – Technological diagrams for the installation of leveling embankments
1 – dump truck, 2 – bulldozer, 3 – direction of movement of dump trucks, 4 – sequence of movement of the roller, 5 – roller
When delivering soil by cars or wheeled tractor-tractors in earth-carts, the thickness of the poured and compacted layer can reach: from clay and loamy soil 0.5 m, from sandy loam 0.8 and from sandy 1.2 m. If the embankment is poured in layers of 0.3 m using dump trucks, tractors with trailers and scrapers, then it is not necessary to compact the soil layers, since in the process of filling the embankment with machines it will be compacted so much that its settlement will be insignificant. The movement of vehicles (dump trucks, scrapers) should be regulated across the entire width of the embankment. You can proceed to filling the next layer only after leveling and compacting the underlying soil layer to the required density. The required soil compaction can be achieved with optimal soil moisture. Therefore, it should be compacted immediately after filling to prevent it from drying out.
Embankments are erected in horizontal layers followed by compaction. The lower layers can be filled from dense clays, and the upper ones only from draining sandy soils. When constructing the entire base of an embankment from waterproof clay soils, the installation of thin drainage layers 10...15 cm thick is required, but it is unacceptable to lay both layers mixed and in inclined layers. Filling should be carried out from the edges of the embankment to the middle for better compaction of the soil limited to the edge areas of the embankment. For filling the embankment, it is not recommended to use sandy loam, fatty clays, peat, or soils with organic inclusions.
The compaction criterion is the required soil density, expressed by the volumetric mass of the soil skeleton, or the standard compaction coefficient (K y), equal to the ratio of the required density of the soil skeleton to its maximum standard density. A soil compaction coefficient of 0.95 ... 0.98 is optimal and ensures sufficient strength of the entire structure, while possible soil settlement over time will be insignificant. In dry, hot weather, it is advisable to water the soil before compaction.
Mechanical methods Compactions, depending on the nature of the impact of the working bodies on the ground and the design solution of the mechanization means, are mainly divided into the following types: rolling, vibrating, compacting and a combined method.
When compacting soil by rolling, pneumatic, cam, lattice and smooth rollers are used. They can be of different weights, self-propelled, semi-trailer or trailed.
Pneumatic rollers, depending on their type and soil characteristics, can compact cohesive soils with a layer thickness (in a loose state) of 15 ... 75 cm and non-cohesive soils with a layer thickness of 25 ... 90 cm; the number of roller passes along one track during experimental compaction is respectively 5 ... 12 and 4 ... 10 times.
Cam rollers compact only cohesive soils with a layer thickness of 20 ... 85 cm and a number of passes of 6 ... 14 times.
Rollers with smooth rollers are used to compact cohesive and non-cohesive soils with a layer thickness of 10 ... 15 cm.
When compacting soil by rolling, there are two motion patterns for rollers: shuttle and circular.
When soil compacts vibrating Vibratory rollers (vibratory rollers), vibratory plates, vibratory rammers and deep-well vibratory compactors are used. This method is rational mainly for non-cohesive and poorly cohesive soils.
Vibratory rollers with smooth rollers are used to compact cohesive soils with a thickness of 15 ... 50 cm and non-cohesive soils with a thickness of 15 ... 70 cm. Of particular interest are single-drum small-sized self-propelled vibratory rollers with a weight of up to 0.7 tons, providing a width of the compacted strip of 66 cm. carry out compaction in cramped conditions, including narrow trenches, near pipelines, foundations and walls, where the use of other machines is difficult.
Vibrating plates are also used to compact non-cohesive and poorly cohesive soils. By design, they consist of a compacting plate with a vibration exciter and a sub-motor frame with an engine, on which a control handle or crane suspension is mounted. Self-moving light and heavy vibrating plates of type D and S vp are used for backfilling of sinuses and trenches to compact a layer of non-cohesive soil 20 ... 60 cm thick. Suspended (to the crane) vibrating plates of the VPP type (with a mass of 1 ... 2.7 tons) used for compacting cohesive and non-cohesive soils with a layer thickness of 50 ... 80 cm.
Deep compaction using a VUPP-type vibroimpact installation is effective for water-saturated medium- and fine-grained sands at a depth of 2.5...6 m. The installation is immersed and removed from the ground using a vibratory driver and a crane. Sand compaction is ensured over an area with a diameter of 4 - 5 m.
Soil compaction by compaction is carried out using tamping machines, mounted slabs and mechanical tampers. This method gives a good effect in compacting cohesive and non-cohesive soils, including coarse soils, as well as dry lumpy clays.
Using tamping machines of the DU-12 type, soils are compacted at the base with a layer thickness of up to 1.2 m. Compaction is carried out by penetrations 2.6 m wide by alternating impacts with two slabs weighing 1.3 tons by free falling onto the ground.
When using mounted tamping plates, the depth of soil compaction depends on the diameter and weight of the tamping element. Freely suspended slabs are raised to a height of 1 - 2 m and, when they fall, the soil is compacted several times.
Compaction with heavy slabs with a diameter of 1 - 1.6 m and a mass of 2.5 - 4.5 tons ensures compaction of a layer with a thickness of 1.2 - 1.6 m for cohesive soil and 1.4 - 1.8 m for non-cohesive soil. The soil is compacted with strips 0.9 times the diameter of the tamping body, overlapping adjacent tracks by 0.5 times the diameter.
To compact soils in cramped conditions, it is advisable to use attachments such as hydraulic and pneumatic hammers with compaction plates. The thickness of the compacted layer, depending on the type of hammer, will be 0.25 - 0.7 m and 0.25 - 0.4 m for cohesive soils, 0.3 - 0.8 m and 0.3 - 0.5 m for non-cohesive soils In such cases, pneumatic punches and percussion rope drilling machines are also effective. The wells formed during compaction should be filled with local soil in layers of 1 m with compaction. As a result, a zone of compacted soil measuring 2.5 - 3 times the diameter of the well is formed around the well.
In cramped and inconvenient places when backfilling, for example, trenches, holes and pits, mechanical rammers with manual control are used, including self-propelled electric rammers of the IE type and pneumatic rammers TR and N. Electric rammers weighing from 18 to 180 kg compact non-cohesive soil with a layer thickness of 0 .15 - 0.5 m, weighing 80 and 180 kg - cohesive soil with a layer thickness of 0.3 and 0.4 m, respectively.
The most common type of purely gravity platforms are reinforced concrete structures or steel foundations ballasted with large weights. Reinforced concrete platforms can be a monocone, a structure with columns, or a structure with almost vertical walls. Steel structures, as a rule, have a large number of ballast tanks to receive water or weighted material. A common feature is the presence of voluminous cavities for receiving ballast, which provides greater downforce. Gravity foundations are installed in areas where ice conditions exist.
Figure 5 – Steel base on a support mat
Figure 6 – Steel base
Figure 7 – Reinforced concrete base
Overpasses. Stationary platforms with a through support block
The most interesting from the point of view of developing the resources of the Black and Azov Seas are overpasses and stationary platforms with a through base.
The structures under consideration are united, first of all, by the permeability to waves and currents of their load-bearing structures that support the deck with the upper structure. The main structural element of these structures are steel pipes. In addition, the trestles and the vast majority of support platforms have pile foundations, which provide stability to the entire structure on the seabed.
Overpasses. Overpasses are extended structures that provide a continuous surface connection between drilling sites and the shore. Drilling rigs and other technological equipment typical for oil and gas fields are located on overpass sites. The width of the roadway of overpasses (usually 3.5 m) allows one-way traffic movement, therefore, in addition to drilling sites, traveling areas are arranged along overpasses. In terms of functionality, overpasses are similar to dams with widening for drilling sites, but they are erected at relatively large depths - about 6-15 m in some cases in water areas 20 m or more deep.
The main load-bearing element of an overpass is piles - usually metal pipes with a diameter of 0.3-0.5 m. Reinforced concrete prismatic piles or shell piles are used much less frequently. The supporting element of the overpass consists of two inclined piles connected by a crossbar at a level exceeding the crest of the design wave. The piles are also connected by braces to give the structure greater rigidity. Bridge structures made of rolled profiles are laid on top of the crossbars of the supporting elements.
As the sea depth increases at the overpass construction site, the difficulties of installing flat support blocks increase due to their insufficient rigidity in the direction of the axis of the structures. Therefore, at depths of about 20 m, spatial support blocks are used from two pairs of inclined piles connected by braces in the longitudinal and transverse directions. At the same time, the pitch of the supports increases, and the spans, instead of a beam structure, take the form of spatial trusses.
The first overpasses were built in the oil fields of the Caspian Sea in the 30s. By the beginning of the 70s. the total length of overpasses in this area reached 360 km. A large number of overpasses were built in the United States during the development of shallow offshore areas in the California region and in the Gulf of Mexico. At shallow depths, the installation of trestles is carried out in a pioneering way: the next supporting element is installed into the water from a ready-made area using a crane. Prismatic or pyramidal support blocks are placed on the bottom using crane vessels, secured by spans to the already constructed portion of the trestle, and secured to the bottom by driving piles.
Platforms on pile foundations. This is the largest group of hydraulic structures on the sea shelf. The first platform was built in 1936 in the Caspian Sea, in 1947 the first platform appeared abroad - in the Gulf of Mexico, at a depth of 6 m. The total number of platforms built throughout the world since that time is estimated by different sources from three to ten thousand.
In the Caspian Sea alone, the number of platforms built (they are called “steel islands”) is approaching 1000. Most of the platforms are installed at shallow depths, but about 2000 are operated at depths from 30 to 300 m. And in the future, metal platforms on pile foundations are considered as the main ones structures intended for shelf development.
Since the construction of the first platforms, the capabilities of conducting installation work at different depths in the open sea, the tasks solved on the shelf have changed, and, as a result, the structural forms of the platforms have changed. As the depths of the sea at which the platforms are installed increase, the proportions, structure of the supporting blocks and methods of their construction change.
However, all these changes do not manifest themselves in the form of any qualitative leaps associated with certain depth values or other factors, so the division of platforms into any groups is conditional.
Platforms on several supporting blocks are built mainly at depths of up to 100 m. The first platforms, built in the 50s. at depths of up to 30 m, they consisted of four to six prismatic or pyramidal blocks, rectangular in plan, with a common upper structure. Such structures are still used today at depths of up to 40 m. . Depending on the depth of the sea, the blocks receive plan sizes from 8x16 to 20 x 20 m. Living quarters are arranged, as a rule, on a separate support block, spaced 30-50 m from the platform for fire safety reasons and connected to it by a transition bridge. Transportation and installation of blocks is carried out using crane vessels. At depths greater than 40 m, the stability of loose prismatic blocks during installation is insufficient. Therefore, the blocks are given a pronounced pyramidal shape, and their total number is reduced to two. As the depth increases and the number of blocks decreases, the dimensions and masses of the individual support blocks increase. Thus, at sea depths of 60-80 m, the mass of one block is 1.2-2.0 thousand tons, and at depths of 100-120 m it reaches 4 thousand tons.
Platforms with monoblock support. Platforms with a supporting monoblock on a pile foundation are built in the entire range of sea depths at which stationary platforms are operated, i.e. from a few meters to 300 m or more) Starting from a depth of about 100 m, designs with two or more supporting blocks are almost never used . Variants of support monoblocks are shown in Figure 9. With access to greater depths of the sea, the functions of the support block and pile foundation also changed. In overpasses and multi-block platforms, piles play the main role - they directly absorb loads from the superstructure and carry horizontal loads from waves, currents and ice. Support blocks in such structures only add rigidity to the entire spatial system. In deepwater platforms on a monoblock, the piles and the spatial truss work together. Measures are taken to rigidly connect the supporting block with the piles (cementation of the interpipe space, welding connection), and as a result, the loads from the upper structure are absorbed by both the piles and the supporting block. In late-built platforms, the piles end at the bottom of the block, and the block racks transfer part of the load directly to the ground.
Support blocks are made on the shore in full or from several sections (tiers). They are transported either on special barges or afloat. During the installation period (before fastening with piles), a monoblock placed on the bottom is more stable than individual blocks of a multi-block support structure.
The supporting monoblock of a deep-sea platform consists of panels - side flat trusses - and diaphragms connecting them - flat trusses, which impart rigidity to the entire spatial structure. The main element of the panels and the entire supporting block are racks - metal pipes with a diameter of 1.2-3.0 m (in some cases up to 10 m), with walls 15-50 mm thick. The total number of racks in a block can be different - from 4 to 15. According to the height of the block, the racks can have unequal diameters, and different racks of the same block can differ in diameter. To impart buoyancy to the supporting block, the posts of one of the panels are made significantly larger in diameter than all the others. The panel braces and diaphragms are made of tubular elements of smaller diameters than the racks. With an increase in the diameter of the racks, the difficulties of ensuring the stability of the shape of the shells, which are subject to significant external hydrostatic pressure, sharply increase. How difficult it is to ensure the rigidity of the structure is shown in Figure 11, which shows fragmentary diaphragms, bulkheads and stiffeners inside a rack with a diameter of 8 m.
Increasing the diameter of the posts in order to achieve the necessary buoyancy of the support block leads to a significant increase in the metal consumption of the structure. Therefore, in the designs of high support blocks it is necessary to resort to stepwise changes in the diameter and thickness of the pipes that make up the racks.
An example of this design approach is a drilling platform designed to be installed at a depth of 395 m (Figure 12). The relatively light upper structure of the platform (its mass is 1.5 thousand tons) is supported by a support block with a mass 40 times greater (60 thousand tons). In addition, 30 thousand tons of steel should be spent on the piles securing the block, and 3 thousand tons on the riser columns for a cluster of 24 wells.
The upper structure (modules with process and power equipment, drilling rig, storage and living quarters, helipad) is located on the deck - a metal deck laid on beams, which* in turn rest on a frame that transfers loads to the support block. Top modules
Figure 11 - Design of a large diameter support column
buildings are installed in 2-3 tiers. The total mass of the superstructure can be reduced if it is constructed as a single structure. At the same time, due to the inherent rigidity of the superstructure, the supporting block can also be lightened. However, installation work in this case requires cranes with a very large lifting capacity. Typically, the deck is made separately from the supporting block and installed on it already in the water area after the block is secured with piles. In the case when the deck is connected to the support block on shore, towing the structure afloat is difficult, but installation work at sea is simplified. The deck flooring must prevent contamination of the water area with drilling fluid, oil and other substances, and therefore has a flange.
The piles securing the support block to the ground are steel pipes with a diameter of 0.92 - 2.13 m and walls thickness of 3 8 - 64 mm, they are driven into the bottom soil to a depth of up to 150 m (in some cases, even deeper). The main piles are driven inside the pillars of the support block, their upper end is at deck level. Piles driven by blows at the upper end have an open lower end. If the hammer is placed inside the pile (this solution is more effective, especially if the pile is long), its lower end is plugged. As the pile sinks into the ground, it is built up from above by welding. After the pile is immersed to a given depth, the part of it protruding above the support block is cut off. Along the top, the pile and the block stand are connected by welding, and the space between them is cemented. In some cases, to strengthen the structure in the most vulnerable places - at the level of exposure to ice and the entrance to the ground - one or more additional pipes are immersed inside the pile and the entire space between them is cemented.
The holding force of piles driven through the legs of the support block may not be sufficient to ensure the stability of a deepwater platform from capsizing. In this case, the border piles are additionally driven. They can be placed along the contour of the block or concentrated near the racks. It is possible to widen the lower part of the support block in the form of a lattice grillage, fastening it with piles along the entire contour. This solution is especially interesting because it allows you to do without the main piles (inside the racks), and drive the border piles vertically. Additional (edging) piles are attached to the support block under water directly at the bottom using couplings - guides of short pieces of pipe welded at several levels to the support block. After driving the piles to a given depth, the space between them and the couplings is filled with cement mortar (expanding cements are used for this). Large-diameter support posts have a plug at the bottom and rest on the ground, transferring some of the loads from the support block to it. In this case, the piles are placed around the racks.
In support blocks with racks that change in steps in diameter, only bordering piles can be used, the heads of which are located near the ground surface. Specifically, the support block must be secured with 56 piles, of which 16 are driven through couplings located between the block posts, and the remaining 40 in groups of four around all nine posts.
The layout of the pile foundation is shown in Figure 13 . Through couplings - pipes with a diameter of 1.72 m - "short" piles are first driven to a depth of 75 m (they ensure the stability of the block during the initial period of installation work at sea). These piles are made from pipes with a diameter of 1.52 m and walls 25 mm thick. Then, holes are drilled inside the “short” piles and pipes with a diameter of 1.22 m are immersed into them to a depth of 135 m below the bottom surface. All pipes (couplings and piles) are completed at a level of 45 m above the bottom surface. The space between all pipes is cemented. Note that at the entrance to the ground, all pipes have 15-meter-long inserts with thicker walls.
The mass of the support blocks of deep-sea platforms significantly exceeds the lifting capacity of floating cranes and crane vessels. Therefore, regardless of the method of delivering the block to the installation site, the operation of placing it on the seabed is always preceded by the position of the block afloat. The buoyancy of the block is achieved not only due to
a significant increase in the diameter of some of the racks, which subsequently leads to large loads on the structure from waves and currents, but also the use of temporary buoyancy - cylindrical tanks or pontoons attached to the block before launching.
The deepest platforms installed after 1975 are operated in oil fields in the Santa Barbara Channel (California) and in the Gulf of Mexico: Hondo (sea depth 260 m), Gervaise (285 m), Cognac (312 M). (162m), "Magnus" (186m). Some information about these platforms will be given below. It should be noted that the more severe conditions of the North Sea determined the significantly higher material consumption of steel platforms installed there. For comparison: the mass values of the Gervaise and Brent A platforms, installed at depths of 285 and 140 m, are approximately the same - 39.7 and 33.0 thousand tons. This ratio is also typical for other platforms in these two shelf areas.
/Platforms on a submerged pontoon or shoes. The sharp increase in the cost and labor intensity of constructing a pile foundation with increasing water depth forces us to look for design solutions in which piles are not used at all or their role in ensuring the stability of the structure turns out to be secondary. The French company "Sitank" proposed a platform design with a through support block on a reinforced concrete pontoon, which combines structural elements the main types of deepwater platforms discussed in this and the previous paragraphs.
A through metal support block is fixed to a reinforced concrete pontoon. The pontoon has the same cellular structure as that of the Cormoran A and Brent C platforms. The cellular pontoon imparts buoyancy to the structure when transported from the shore to the installation site on the bottom, then used for ballasting and, finally, for oil storage. In the platform version intended for production drilling and production at a sea depth of 200 m, the capacity of the oil storage facility is determined to be 150 thousand m 3. The supporting block must support the upper structure weighing about 25 thousand tons and has an area of 5 thousand m 2. Eight (or other number) cylinders at the corners of the pontoon are used for ballasting and then storing oil.
The reinforced concrete pontoon rests directly on the seabed; its area and mass are determined taking into account the requirements for the stability of the structure from shear and overturning. To increase the resistance to shear along the ground, it is possible to immerse metal shells into the ground through special holes in the pontoon. In general, such structures can be classified as gravitational.
The advantage of the considered design (it is called composite or combined) is that it can be used in cases where driving piles is impossible (the presence of rock under a relatively thin layer of soft soil). At the same time, it has less resistance to wave propagation and flow (like all end-to-end support blocks) and makes it possible to successfully resolve the issue of storing produced oil.
Figure 14 - Teknomare platforms installed in the Loango fields (near Congo) at a depth of 86 m (a), in the North Sea at a depth of 95 m (b) and designed for depths up to 200 m (c)
1 - steel truss of the support block; 2 - ballast tanks with a support shoe (oil storage facilities); 3 - riser columns; 4 - ballast tanks
Another solution to the problem of ensuring the stability of a through-support block without the use of a pile foundation is embodied in the design of the Teknomare platform. The support block is attached to three cylindrical ballast tanks, supported by widened and weighted shoes, installed directly on the seabed. Support block configuration, tank and deck dimensions selected from the conditions of the operating area, the purpose of the platform and the depth of the sea.
The first four Teknomare platforms (Figure 14 a) were installed in 1976 at a depth of 86 m in the Congo region. They are designed for waves of Amy height and are intended for drilling 15 wells (each) and oil production without storage. The platform was built in 1983 . in the North Sea at a depth of 95 m (Figure 14 b), it is designed for drilling 24 wells and oil production. It has large-volume ballast tanks; during operation, they are used to store 100 thousand m 3 of oil. The diameter of the tanks is 25.7 m. Three shoes with a diameter of 47 m are loaded with solid ballast with a total mass of 51 thousand tons. The tanks of the shoes form a triangle in plan with sides equal to 90 m. The entire structure is made of steel, the total consumption of which is 41.7 thousand tons. This structure is designed for waves 27 m high. The platform shown in Figure 14 c is intended for installation in the Mediterranean Sea at a depth of 200 m.
The advantages of steel gravity support blocks of this type compared to reinforced concrete ones include the fact that they can be completely manufactured in a pit, since they have a small draft before receiving liquid and solid ballast. The block is towed in a vertical position, in an area with a sufficiently large depth it is sunk and takes over the fully assembled upper structure from the barge, then it is guided to the landing site and ballasted. It is assumed that such structures will find application at sea depths of up to 300 - 400 mi in areas with heavy wind conditions.
The design of the Mandrill platform (Figure 15) resembles a sliding tripod used for installing film or photographic equipment. It is believed that such designs could find application in shelf areas with heavy wind and wave conditions, such as the North Sea, and in areas with a depth of 200-500 m. The design option shown in Figure 15 was developed for a depth of 350 m.
Figure - 15. “Mandrill” platform (a) and options for resting the “legs” of the platform on the ground (b-d)
1 - legs" forming an A-shaped frame; 2 - folding "leg"; 3 - screed; 4 - riser columns; 5 - piles; 6 - couplings for securing piles; 7 - support shoe
The platform is intended for drilling 56 production wells and oil production; its upper structure weighing 55 thousand tons has dimensions of 70 x 120 m in plan and rises above the water by 26 m (the estimated wave height is taken to be 31 m). The spatial support structure is mounted underwater from a flat system of articulated lattice elements assembled on shore and transported afloat. This system includes: an A-shaped rigid connection of two “legs” and a spacer, a third folding “leg” and two more spacers. Three options are proposed for resting the “legs” of the platform on the ground: with driving inclined piles (Figure 15b) - steel pipes with a diameter of 2.44, a length of up to 130 m and a weight of up to 450 tons through conductors mounted on the inclined “legs”; with driving vertical piles (Figure 15 c), driven through holes in the support shoes; without driving piles (Figure 15 15 d) - with rigid or hinged fastening to widened shoes. Last option support is suitable in the presence of sufficiently strong soils.
Platforms with a through support block in the form of a mast with guys. The designs of such platforms are similar to ground-based structures used as supports for radio, radio relay and television antennas (Figure 16). It is believed that the design can be used in the depth range of 200 - 700 m. The fundamental difference platform in the form of a mast from other deep-sea stationary structures is that it does not transmit bending moment to the soil base.
The support block (underwater mast trunk) is made in the form of a steel pipe truss, cross section it is formed by a square. Conductors for lowering drill strings are located inside the block. The trunk is held in a vertical position with the help of guy ropes attached to the garlands of arrays lying on the bottom. From the arrays, the guys continue to the pile anchors. Under normal loads on the structure, the strings of arrays lie on the bottom. Under extreme loads (during a severe storm), the garlands come off the bottom and thereby absorb the shocks transmitted to the guys from the swaying trunk. Calculations and experiments on a large-scale model showed that the adopted scheme for damping the oscillatory movements of the system ensures small (no more than 2%) deviations of the trunk from the vertical.
Two options have been developed for resting the barrel on the ground. In the first, the trunk has a pile foundation. In this case, part of the piles transfers to the ground all the loads from the upper structure of the platform, i.e. these piles are immersed in the ground through the racks of the support block and are connected at the upper end to the deck structure. This solution is typical for most other structures with a through support block on a pile foundation. The other part of the piles secures the trunk from twisting, and their heads are fixed at the lower end of the trunk. In another option, a pile foundation is not used: the lower end of the trunk is given a cone-shaped shape, thanks to which it is immersed 2-15 m into the ground under the weight of the block itself, ballast and due to the vertical component of the tension force of each guy.
The upper ends of the guys are attached to the shaft through a special belt slightly below the water surface (so as not to complicate the approach of service vessels) and approximately at the level of the resultant horizontal loads on the structure. In relation to the vertical axis of the barrel, the guys deviate by approximately 60°.
The first platform "Lena" in the form of an underwater mast with guys was installed at a depth of 305 m. The supporting block (trunk) with a total height of 330 and a width of 36 m has a mass of 27 thousand tons (including piles), which is half that of the platform " Cognac", installed at a depth of 312 m. Piles made of pipes with a diameter of 1.37 m, supporting the upper structure, are driven into the ground to a depth of 170 m, i.e. the total length of each of them is about 500 m. The same pipes, but respectively of shorter length, used as piles to secure the trunk from twisting. To secure the trunk, 20 guy ropes were installed - cables with a diameter of 137 mm and a length of 550 m - with the inclusion in each of them of a garland of arrays with a total mass of 200 tons. The design strength in the guy rope was determined to be 5-6 MN, and the breaking force was 15 MN.
A more daring design solution was adopted for the platform intended for installation in the Gulf of Mexico at a depth of 700 m. The 40 m wide shaft is secured by 16 guys with a diameter of 100 mm with garlands of arrays weighing 165 tons. Anchor piles - pipes with a diameter of 1.5 m - are loaded from drilling ships into pre-drilled wells to a depth of 15 m and cemented. The lower cone-shaped end of the trunk is buried in the ground and does not have a pile foundation.
To install the support block of a deep-sea platform, it is proposed to use the method first used in the construction of the Hondo platform. The support block is manufactured at the shore base in the form of two parts equipped with ballast tanks. Both parts are delivered to the installation site of the platform on barges, launched into the water and connected in one whole afloat. After receiving "ballast ( sea water) into the tanks of that part of the block that should be facing downwards, the block gradually turns and moves into a vertical position without the help of crane equipment. After driving the anchor piles and securing the block with guys (first four in two mutually perpendicular directions, and then the rest), all ballast tanks are filled with water, and the piles are driven in (if provided) or the block is immersed in the ground due to its own weight.
The operation of connecting the parts of the block afloat is very complex, especially since it has to be performed directly above the installation site of the platform, i.e. in the open sea. Therefore, it is recommended, if possible, to assemble the entire block on shore. This is exactly what was done during the construction of the Lena platform. The support block was launched from a barge and immediately assumed a vertical position due to the fact that in the lower part it had ballast in the form of iron ore, and in the upper part - inside the block - 12 ballast tanks -buoyancy vessels with a diameter of 6 and a length of 36 m.
It is noteworthy that the block was lowered from the barge not through the stern, as usual, but through the side. Inside the block, on the shore, the main piles (those that should support the upper structure) were placed. They were built up and slaughtered using equipment mounted on a barge. The deck of the platform's upper structure was also installed from the barge.
The depth of 700 m is not the maximum for this type of fixed platforms.
Installation and piling works. A variety of crane and piling equipment are used in the construction of overpasses and platforms in areas with shallow depths. Choose technological processes, least dependent on weather conditions.
Initially, floating pile drivers were used for driving piles. Piling work and installation of flooring could only be carried out in calm weather. The pioneering construction method significantly expanded the range of weather conditions for installation and piling work. Numerous modifications of the pioneer method are associated with various technological characteristics of the crane equipment used. Let us consider, as an example, the technology of installing an overpass.
An element of the upper structure - a truss with a transom beam attached to it, as well as piles (Figure 17a) - is suspended from the boom of a special trestle-building crane. After turning the crane 180°, the entire block is hung above the installation site (b), with one edge of the truss resting on the crossbar of the already finished section of the overpass and attached to it with clamps or temporary welding. Following this, the piles, held in the guides of the pile driver frame, are passed through the forks of the crossbar (c) and driven in. Once the design driving depth (or failure) has been reached, a hole is made in the pile directly below the crossbar into which a stop for the crossbar is inserted.
The parts of the piles located above the crossbar are cut off, all mounting assemblies are welded, the deck is installed (d), and then the crane moves forward to the length of the new section. Overpass construction cranes are designed for the construction of overpasses in sections of up to 20 m at depths of about 30 m. Overpass platforms are erected in the same pioneering way when work is carried out in a direction perpendicular to the axis of the overpass.
The installation of blocks for the supporting structure of platforms weighing up to 3 thousand tons is carried out, as a rule, from crane ships, on which the blocks are delivered to a given area. The most important operation is turning - moving the block to a vertical position. Apply various ways tilting: on the water with the block posts resting on the ground; through the side of the vessel resting on a special console beam; with fastening the upper part of the block to the deck bollard; blocks that have their own buoyancy in water when controlling the intake of ballast into the racks.
After landing on the bottom, the block is leveled by various means. Bottom unevenness can be eliminated directly under the posts by washing with water supplied through pipes attached to the posts. The leveled block is secured using metal tubular piles driven through the posts. If failure during pile driving occurs before reaching the calculated immersion depth, it is necessary to drill out a soil plug to reduce the resistance to pile driving. The pipe cavity is then filled with concrete to a level of 5-8 m above the bottom surface. A combination of driven piles with anchoring is possible: the pile is driven to the roof of rocky or semi-rocky soil, then a well is drilled into which the anchor is lowered, and then the well and the pile cavity with the anchor rod passed through it are filled with concrete. To increase bearing capacity piles are sometimes used to inject cement mortar into the surrounding soil. To do this, the soil plug is completely drilled out of the pile and the solution is fed through the lower end of the pile and holes specially provided for this along its length. Such an operation leads to an increase in the load-bearing capacity of the pile on the ground by 2-2.5 times. Another way to increase the load-bearing capacity of a pile is as follows: a well is drilled through a driven pile, which is then expanded using a sliding device into the resulting expansion and bottom part the piles are driven into a reinforced frame and the entire space is filled with concrete.
The technology of manufacturing and installation of deep-sea platforms differs from that used for overpasses and platforms with several support blocks due to the higher degree of industrialization of work and the complexity of individual operations caused by the large dimensions and weight of the support block.
The production of monoblocks is carried out at specialized enterprises and shipbuilding complexes and includes the following main operations: preparation of individual parts, pipes and beams; assembly of units; intermediate processing of nodes; assembly of modules; final assembly of the support block; loading or removal from the dock.
Pipes of small and medium diameters, as well as rolled profiles, are supplied to the enterprise in finished form. Pipes of large diameters (2-Yum) and beams of large deck height (up to 3 m) are manufactured directly at the enterprise, equipped for this with semi-automatic production lines.
Assembly of units - connections of the supporting parts of the platform and the surface area, buoyancy tanks, tubular units, stiffeners,
floors of intermediate decks, ladders - is carried out in assembly shops equipped with special welding machines and devices, lifting and transport mechanisms, assembly devices for various purposes. Manual welding used only for making seams that are not available automatically. The largest mass of units is determined by the lifting capacity of the crane equipment of assembly shops and usually does not exceed 100 tons. The use of self-elevating trolleys and hydraulically driven platforms allows the mass of units to be increased to 200 tons.
Intermediate processing of assemblies before sending them to the place of final assembly of the support block consists primarily of relieving stresses in the material that arise during the welding process. For this purpose, annealing is used in special chambers - furnaces. Intermediate processing also includes shot blasting of components, degreasing, etching, application of protective coatings, and galvanization.
The final assembly of the support block is carried out on a slipway, in a dock or in a pit. First, the flat panels are assembled. The entire support block is assembled from panels and diaphragms in a horizontal position. The panels are lifted and installed in a vertical position using several cranes (up to 6-10) on caterpillar tracks with a total lifting capacity of 200-400 tons. Guys are used to temporarily secure the panels in a vertical position.
Transportation and installation of support blocks of deep-sea platforms on the bottom are carried out using their own buoyancy (when the tubular elements of the block are sealed) and ballast tanks or pontoons attached to the racks. Blocks assembled in a pit or dry dock float up after the pit is flooded and are towed to the installation site afloat. Blocks assembled on slipways are lowered into the water or moved to special barges. These barges must have large decks and provide the necessary stability when loaded, taking into account the high position of the block's center of gravity. In particular, to transport a block with a length of 435 m and a mass of 50 thousand tons, intended for the construction of the Balwinkle platform in the Gulf of Mexico at a depth of 411 m, a barge with dimensions of 250 x 62 x 15 m is being built. Large blocks are rolled onto barges using traction winches and hydraulic jacks.
Transportation of blocks on barges is more common, despite the fact that during the process of lowering from the barge, special loading conditions for the block arise, requiring the introduction of an additional lattice into the structure of the block. Assembling the block in a pit on pontoons simplifies transport operations and, in some cases, eliminates the need to deepen the pit and approach channel. However, blocks transported on pontoons must be designed to withstand waves during the transit period.
The masses and dimensions of the support blocks of deep-sea platforms are such that the use of crane vessels or floating cranes during transportation and installation on the bottom is excluded. Several methods of lowering blocks into the water and placing them in a vertical position are shown in Figure 19. The easiest way to install a block on the bottom is when it is towed afloat. By ballasting tanks, internal compartments in racks or pontoons (a), the block gradually turns in the water and acquires a vertical position. After this, it is aimed more accurately over the design installation point, ballasted and placed on the bottom. The pontoons can then be detached from the block and removed. In another method (b), the block is transported on two pontoons installed across the block. After pulling out one pontoon, the block rotates around the other pontoon and lowers down. A method for transporting a block on a barge and pontoon (c) is proposed. Ballasting the pontoon causes the block to rotate around the stern of the barge and slide down at the same time.
The method of lowering and installing the block, shown in Figure d, was used during the construction of the Hondo platform (water depth 260 m). Two sections 140 m long were transported over a distance of 480 km to the installation area on barges, launched by scuttling the barges and connected afloat with the help of specially designed cone-shaped grips mounted on four corner posts. The docking operation was carried out in a protected harbor near the installation site of the platform. The alignment of the sections on the water was achieved by ballasting the buoyancy in the posts. The docking units with their spring clamps and pneumatic couplings are close to hinged ones. therefore, after blowing out the rack compartments, welders were lowered into them and welded the joints from the inside.
Lowering long blocks from a barge is dangerous due to overstresses when the block rests only on the rotating frame on the edge of the barge. To avoid damage to the block, an additional grid is created in it - trusses. At the stern of the barge, intended for lowering long blocks, a double rotating frame (d) is mounted. The loads on the block when leaving the barge are also reduced in the case when the launching is not accompanied by a simultaneous lowering of the block to the bottom (e).
This is exactly how the entire supporting block of the Gervaise platform, 290 m high and weighing 24 thousand tons, was lowered. The block was transported on a barge 200 m long, and almost the entire overhang of the block fell on its narrower (upper) part. To lower the block, the barge was assigned the slope was 3° by ballasting the aft part, and the block was given an initial shear force of 14 MN (the coefficient of static friction was 0.11). After leaving the barge, the block equipped with ballast tanks assumed a horizontal position afloat. Then, by ballasting the tanks attached to the bottom of the block. , the latter was transferred to a vertical position and placed on the bottom.
Descent to the bottom from a horizontal position afloat (Figure 20) is considered the most controllable. The block is transferred to a vertical position by ballasting the rack compartments as shown (positions IV and U).
In world practice, there are examples of assembling the support block of a deep-sea platform from three tiers under water. We are talking about the Cognac platform (Figure 22), a block which was divided in height into tiers measuring 47, 97 and 184 m (total height of the block 328 m, sea depth 312 m). The lower tier with guides - couplings for the bordering piles - They were assembled in a pit in a vertical position and towed in the same position to the installation site at a distance of 200 km. The second and third tiers were assembled in a horizontal position and transported on barges. The successive building of the block from the tiers was carried out using two crane vessels and a hydroacoustic guidance system under water. The dimensions of the block at the bottom were 116 x 122 m.
Figure 21 - Stages of assembling the support block of the Cognac platform
Work continues on the transverse descent of the block from the barge (over the side). This method of descent allows you to do without reinforcing the block with srengels and save up to 10% of metal. However, it is difficult to ensure that the entire block goes overboard at the same time, and the barge’s roll at this moment reaches 3.0°. Nevertheless, a support block with a length of 330 m and a mass of 27 thousand tons (the Lena platform, which will be discussed later) was lowered entirely over the side of a barge having a length of 176 and a width of 49 m. The descent was controlled remotely, with the entire team was removed from the barge.
Driving piles is the most labor-intensive stage of installing blocks at the site of operation. Until a certain part of the piles is driven in, the structure is not stable, which is especially dangerous during a storm. There are known cases when an unsecured block lost stability even in a calm situation - due to erosion of the soil by bottom currents.
How labor-intensive pile driving is is demonstrated by the example of securing a support block in the North Sea at a depth of 108 m, when it took three weeks to drive 24 piles with a diameter of 1.52 m to a depth of 45 m under quite favorable weather conditions. In view of these difficulties, on another platform in the North Sea, a gradual increase in the holding force of the piles was applied: first, 1.82 m diameter piles were driven to a depth of 30 m, and then piles with a diameter of 1.22 m were driven through them to a depth of 60 m.
One of the circumstances that makes driving piles difficult is that the mass of the piles is comparable to the mass of the hammer, and the elasticity of a long pile can absorb all the impact energy. In this regard, for driving long piles, hammers are used, placed inside the pile - in its lower part. Due to the complexity of pile work, the advantages of the method of assembling the support block used in the construction of the Cognac platform are revealed. There, the main and border piles were driven while only the lower section of the support block was on the ground. The piles are 190 m long and 2.13 m in diameter with a thickness walls of 57 mm and weighing 465 tons were delivered afloat. After receiving the ballast, they were transferred to a vertical position, guided into the guides of the support block and immersed under the influence of gravity into the ground to 45 m. Further immersion to a depth of 150 m was carried out by an underwater hammer. It was cemented with piles and guide couplings. Piling work lasted 21 days.
A different technology of piling work was used in the construction of the Hondo platform. The supporting block was strengthened with eight piles with a diameter of 1.22 and a length of up to 380 m, driven through racks, and twelve border piles with a diameter of 1.37 and a length of up to 115 m. The piles were delivered on barges in sections 20-70 m each and were connected by welding as they lowered inside the racks. To reduce the load on the floating crane that held the pile during its raising and lowering, the sections of the pile were equipped with waterproof partitions. After welding the tenth of the thirteen sections, the pile reached the ground surface and waterproof partitions. The work on driving one pile took 3.5 days. The bordering piles were driven using extensions.
Installation of the topside is the final stage of construction of a deepwater platform. Most of the built platforms have a modular upper structure. Modules weighing 700-1600 tons or more are delivered on transport barges and installed using crane vessels. The use of a modular assembly method allows not only to reduce the overall duration of work, but also to reduce their cost. Please note that similar installation work drilling equipment, produced at sea are 8-10 times more expensive than on shore. The high cost of operating crane vessels, transport barges and indispensable rescue vessels, their downtime under unfavorable hydrometeorological conditions can bring the cost of installation of the top structure to 30% of the cost of installation of the support block. This explains the trend towards larger modules of the top structure.
Stationary ice-resistant platforms
Ice-resistant structures must be designed for year-round operation on the shelf of Arctic and freezing seas, as well as in large areas of non-freezing seas, where they can be exposed to drifting ice fields and impacts from individual ice floes. Generally speaking, ice-resistant structures should be considered those whose structural shape and dimensions load-bearing elements determined primarily by the ice regime. The special approach to the design of ice-resistant platforms is explained not only by the specifics of the main impact from environment, but also the conditions in which construction must take place. This is a very short summer season (2-3 months), when a free sea surface or covered with floating ice allows the construction of the structure afloat or on barges to the site of operation. These are low air temperatures, which contribute to freezing of the structure and the appearance of brittle cracks in the material, low water temperatures, which complicate underwater technical work.
Global experience in the construction and operation of ice-resistant platforms is still limited. The development of the Arctic shelf areas is carried out mostly from artificial islands. However, the need to reach depths at which the construction of islands becomes economically unfeasible prompts the search for designs for ice-resistant platforms. The first ice-resistant platforms were built in the 60s. Currently, they are exploited in several areas of the World Ocean: in Cook Inlet (off the southern coast of Alaska, USA) at depths of 20-40 m, in the Beaufort Sea (at Canadian site shelf) at depths of up to 30 m, in the frozen Azov Sea at depths of up to 8 m. In the future, areas with more severe climatic conditions will have to be developed, in hard to reach places and with a wider range of depths. This task has a special important for our country, since more than half of the USSR shelf is covered with ice for a long time of the year. In particular, on the shelf of the marginal seas of the Arctic Ocean, only a very small part of the sea surface (the Barents Sea in the Kola Peninsula area) is almost always free of ice. Ice covers large areas of the Baltic, Black, Caspian and Azov seas. One-
However, the problem of ice resistance of structures in these areas is not paramount; the design and dimensions of the elements are determined by storm conditions. In the Arctic regions, the forceful impact of ice fields usually 1.5-2 m thick significantly exceeds what is possible during the most severe storms.
The implemented and proposed designs for the support foundations of ice-resistant platforms are varied in configuration and methods of construction and at the same time differ markedly from those designed mainly for the perception of wind and wave influences. The specificity of ice-resistant platforms is also manifested in the layout of the top structure, since such structures must have greater autonomy, i.e., allow the placement of a sufficient amount of reserves to carry out drilling and other work for 3-6 months (instead of 1 month in areas with a temperate climate), when transport connections by water are impossible. Prolonged low air temperatures (temperatures below 0 °C last from 7 to 10 months, and minimum temperatures reach -46 °C), frequent storm winds in winter and snow charges in summer force one to resort to protecting all work sites. The drainage pipes through which wells are drilled must also be protected from the effects of ice.
When designing ice-resistant platforms, several basic techniques are used to reduce the impact of ice on a structure:
Reducing the number of supporting elements in the waterline area or narrowing the structure supporting the topside;
Device protective covers around supports to prevent damage from the abrasive action of ice;
Giving the outer surface of the support a conical or other shape that facilitates the transition of the ice cover from compression to bending.
Ice-resistant platforms with a through support block on a pile foundation. They differ from conventional platforms in the absence of braces in the waterline area and the presence of an ice-protective casing on the supporting columns. Such platforms ( total number 14) are installed and operated in Cook Inlet, where severe ice conditions are aggravated by semi-diurnal tides up to 12 m high and strong tidal currents with speeds up to 4 m/s. The platforms are installed at depths from 19 to 40 m.
A typical design of an ice-resistant platform is shown in Figure 22. The platform support block is made of four columns with a diameter of 4.6 m, connected by braces and horizontal tubular connections only in the underwater part - below the ice-exposed area. The columns are connected at the top by a superstructure. Eight piles with a diameter of 0.75 m are immersed through the columns into the ground to a depth of 27 m. The piles absorb loads from the superstructure, as well as shearing and overturning forces from the influence of ice on the columns. The interpipe space in the columns is filled with concrete, and the columns themselves have a protective casing about 15 m high. The structures of the platform in Cook Inlet are made of high-quality steel with a yield strength of at least 350 MPa. Due to the large diameter of the columns, the support block has its own buoyancy and is delivered to the installation site from the coastal base using tugs.
IN metal structure ice-resistant support block of a small platform installed on a gas field in the Sea of Azov (figure), there are also no horizontal and inclined connections in the area exposed to ice. This helps reduce the overall shearing and overturning forces caused by ice on the columns. Unlike the design described above, the piles are driven not inside the support columns, but through guides mounted on a lattice grillage, which has larger dimensions in plan than the platform deck. The columns are made of three coaxial pipes with diameters of 1420, 1020 and 630 mm, the interpipe space is filled with concrete. The platform is designed for a cluster of four wells drilled through columns. Thus, the columns not only support the equipment deck, but also protect the drill pipes from the effects of ice.
A large number of columns and their too close arrangement in the support block lead to a delay broken ice and the formation of hummocks directly below the deck. In connection with this, the design of the support block in the area of the windline should be as permeable as possible to ice fields.
There is experience in operating a drilling platform with one support column (Fig. 23). It is installed at a depth of 22 m in Cook Inlet and is designed for ice pressure up to 1.8 m thick. The 8.7 m diameter column has at its base
a lattice structure formed by pipes with a diameter of 4.6 m and two cylindrical pontoons, used as buoyancy when towing the structure and as containers (volume about 4 thousand m3) during operation. The stability of the platform from shifting and capsizing is ensured by liquid ballast (water and oil in the pontoons) and piles immersed through pipes in the pontoons at 15-20 m. 16 wells are drilled through the column, and then oil and gas are produced. Such designs of ice-resistant platforms are considered appropriate at depths of up to 30 m.
Gravity ice-resistant platforms. Such platforms are held in place mainly by their own weight and ballast. Ice-resistant platforms, with all the variety of structural forms, always have a developed support base, usually round in plan. The platform body can be reinforced concrete or metal. To reduce the force impact of ice on a structure, various techniques are used: narrowing the hull in the area of the waterline, giving a cone-shaped shape to the hull and support column supporting the top structure in the zone of ice influence, and the use of movable (floating) cone nozzles on cylindrical columns. Several design options for ice-resistant gravity platforms are shown in Figure 25. Search optimal solutions continues because every design decision in different conditions exhibits positive or negative properties.
The cylindrical shape of the support column is convenient from the point of view of work, reduces the material consumption of the structure, and has a small area over which freezing with the ice cover is possible. On the other hand, the cylindrical shape of the barrier does not contribute to the bending of the ice cover, and ice destruction occurs when it reaches its compressive strength in contact with the support.
The conical shape of the support helps to reduce the horizontal component of the pressure of the ice field on the structure. Ice, creeping onto the support, undergoes bending and collapses when the tensile strength is reached at some distance from the support (the mechanism of destruction of the ice field is shown in paragraph 6.6). The vertical component of ice pressure on the support helps, when it is directed downward, to increase the stability of the structure against shear. The disadvantage of the conical shape is the possibility of formation of hummocks and their freezing when the ice field stops, which is especially likely in shallow water. Freezing of a conical surface with a flat field is also dangerous, since it occurs over a significantly larger area than in the case of a cylindrical support, and at the beginning of the movement of the ice field can lead to a strong increase in the load on the structure. In addition, the conical shape of the support complicates the work, increases the cost of materials, and complicates the approach of vessels servicing the platform.
Gravity ice-resistant platforms are developed for operation at relatively shallow depths. The platform's own mass together with ballast is not always enough to ensure the stability of the structure from shifting under ice pressure. In such cases, you have to resort to the help of piles. The use of local materials as ballast brings gravity platforms closer to artificial islands. Sometimes it is difficult to determine what type of ice-resistant structure is. You can be guided by the following feature of the platform - after deballasting and removing the piles, it can be completely (or divided into a hull and a superstructure) moved to another location and reused. The submersible fencing blocks of the artificial island can also be deballasted and moved to another area, but the soil body of the island remains on the seabed. Gravity platforms, unlike islands, have a bottom resting on the bottom or bed over the entire area.
An ice-resistant platform, often called an “ice island,” is shown in Figure 25, d. This platform is designed for drilling operations on the Arctic shelf of Canada at sea depths of up to 22 m. The platform body weighing 16 thousand tons is towed to the site of operation at the beginning of freeze-up. After receiving ballast - sea water - into cellular compartments formed by pipes with a diameter of 12 m, the platform sits on the bottom. Using a refrigeration unit, the ballast is frozen and gives the structure rigidity and the ability to resist the effects of ice fields up to 1.8 m thick. In four pipes with a diameter of 2. 4 m each, 8 conductors are placed for drilling wells. If it is necessary to change the location of the platform, the ballast is melted and pumped out.
Blocking the river bed during the construction of a river hydroelectric complex is one of the difficult stages of work in the overall scheme of skipping construction costs. The essence of the damming process is to switch water flows in the river to a drainage tract prepared in advance at stage I (various holes, tunnels, channels) by gradually or instantly blocking the riverbed with various types of materials (sand-gravel mixture, rock mass, sorting stone, special concrete elements (cubes, tetranuclei, etc.), (Fig. 2.13).
Rice. 2.13. General scheme channel closures
1-stone banquet; 2-proran; 3 - preliminary constriction of the channel; 4-circuit of earthen dam; 5 - supply channel; 6- slot in the top jumper; 7-earth dam; 8 spillway openings from the construction period; 9-slot in the bottom jumper
The channel is blocked in the following ways (Fig. 2.14): by frontal filling of a stone banquet into flowing water (frontal method); pioneer pouring a stone banquet into flowing water (pioneer method); alluvium of sand and gravel soil by means of hydromechanization (alluvial method); instantaneous collapse of earthen or rock masses into the channel (directed explosion method); other special methods (dropping large concrete masses or overturning them, flooding floating structures, driving sheet piling rows, immersing wicker or straw mattresses, etc.).
The most common methods of blocking a river bed are the frontal and pioneer methods of pouring a stone banquet into the water. The complexity of overlap when using these methods depends mainly on two factors: the maximum flow speed in the hole Umax and the maximum specific flow power, as well as the total flow power N.
(2.1)
,
where Q is the total flow through the hole; q - specific flow rate in the hole; - density of liquid (water); -difference in water level in the hole.
It is the difference in hydraulic conditions and the corresponding maximum speeds that distinguish these methods.
Rice. 2.14. Blocking the river bed (a-frontal method, b-pioneer method; c - alluvial method, d - directed explosion method, e - concrete masses)
1-banquet of preliminary restriction of the channel; 2- proran; 3 - river flow; 4- dumped material; 5 - dump truck; b-bridge; 7 - ryazhevye abutments; 8- soil supply by hydraulic transport; 9 - washable layers; 10 - blasted rocky slope of the river; 11-directional material spread; 12-platform for the production of concrete mass; 13 concrete mass before capsizing, 14 concrete mass after capsizing
With frontal overlap Four characteristic configurations of a stone banquet were identified as the difference across the banquet increased and the flow velocity increased (Fig. 2.15). In this case, one should distinguish between three characteristic drops at a banquet: a critical drop, a drop when the cast comes out of the water, and a final drop.
Fig.2.15. Stages of banquet formation and hydraulic conditions for the frontal method of overlap
I - stone banquet; 2 - anti-filtration screen
The critical difference corresponds to the achievement of maximum power and flow rate. Approximately for the front overlap we can take:
; (2.2)
Changes in differences, speeds, flow rates and flow power during frontal overlap can be clearly presented in the form of an integral graph (Fig. 2.16).
With pioneer overlap Two stages are distinguished: overflow and rapid flow, or plume formation.
Rice. 2.16. Graph of changes in the hydraulic characteristics of the flow in the hole with the frontal method of blocking
The maximum speed and maximum specific power during pioneer closure are observed when the banquet slopes are closed along the bottom. In this case, a critical difference is achieved, and it is close to the final difference (Fig. 2.17), i.e. for pioneer overlap it can be taken
Rice. 2.17. Changes in hydraulic characteristics when blocking a river using the pioneer method
Critical difference with frontal overlap; critical difference during pioneer overlap; critical speed for frontal overlap; critical speed during pioneer overlap; flow through the drainage tract; flow through the hole; final drop
Thus, maximum speeds with frontal overlap it is significantly lower than with pioneer overlap (with the same final differences). Therefore, it has an advantage for use in blocking rivers that have easily eroded soils in their beds. But its use is complicated by the need to build a bridge across the hole for filling the banquet. When using the pioneer method of covering, on the contrary, the hydraulic conditions in the riverbed become more difficult, but the organization and execution of work is simplified, and no bridge is required.
The choice of overlapping method should, in principle, be made on the basis of a technical and economic comparison of options.
The greatest influence on the choice of slab method is exerted by the natural geological and hydrological conditions in the slab area. The choice of the estimated flow rate of the dam and the timing of the damming of the channel also depend on hydrological conditions.
The timing of channel closures coincides with low-water periods and is usually set at the end of the shipping period in the autumn-winter months.