RU2149304C1 - Method of pipe line transportation of gas by sea - Google Patents

Abstract

FIELD: laying marine pipe lines. SUBSTANCE: pipe line is laid between bottom of water basin and its surface attaining zero buoyancy to this pipe line and securing it by means of braces with ballast weights and floats. Pipe line is assembled of separate links and is connected by means of sleeves with limited angular mobility. EFFECT: simplified procedure of laying pipe lines. 10 dwg, 5 ex

Description

 The invention relates to the oil and gas industry and for laying a gas pipeline under water, in particular offshore, and transporting gas through this pipeline.

 There is a method of laying a pipeline on land bypassing sea coasts and transporting gas through this pipeline, which consists in the fact that the pipeline, consisting of separate rigidly interconnected metal pipes, is laid along the length of the highway along the coast line on a rigid base on the ground, and gas is passed through the pipeline. However, the known method has several disadvantages. In particular, a relatively long pipeline is required. The pipeline should bypass hard-to-reach, for example rocky, sections, bypass densely populated territories and territories of unfriendly states.

 The pipeline through the sea between two geographical points, for example A and B, directly (Fig. 1) will be much shorter than the land option and will be free from its other drawbacks inherent in the land option.

 Currently, there is a known method of laying a pipeline offshore and transporting gas through it using a pipeline made of individual metal pipes stacked together and laid along the bottom of the sea. This method is described in the Blue Stream project and in the article “Russia is ready to start laying a gas pipeline to Turkey along the bottom of the Black Sea” (newspaper RU, Nezavisimaya Gazeta, N 205 (1530), Thursday, 10/30/97, copy attached).

 The disadvantage of this method is that, as the designers indicate, "in world practice there is no experience in laying a pipeline at a depth of 2000 m with a length of this section of 215 km." This is due to the complexity of the diving operations carried out at such a depth. For example, at a depth of 2000 m, due to the extremely high water pressure of 200 technical atmospheres, it is necessary to use special mechanisms that can function stably under these extreme conditions. You should pay attention to the fact that the seabed is not smooth and even, like a football field, but has a rather complicated relief, with respect to which the pipeline should be exposed. These issues have not yet been resolved.

 The present invention is directed to solving the technical problem of laying a pipeline at a relatively shallow depth, ensuring its connection with the seabed with a balanced-suspended position, taking into account the buoyancy force. The technical result achieved in this case is to simplify the laying of the pipeline under water.

 The specified technical result is achieved by the fact that in the method of sea transportation of gas, in particular along the shortest path between two coastal points, which consists in laying under water a pipeline formed from hermetically joined separate metal pipes, and continuous supply of gas through this pipeline, the latter made of hermetically joined to each other and having a limited angular mobility of pipes at the junction, they are immersed in sea water to a depth exceeding the maximum draft of sea vessels and not subject to a significant influence of the storm, and provide a stable position of the pipeline in the water above the bottom due to its weightlessness or zero buoyancy, while weightlessness or zero buoyancy along the length of the pipeline is ensured by cable stretch marks fixed on separate sections of the pipeline and with ballast on the other the end, lowered to the seabed, and carrying floats on it to ensure zero-gravity tension of cable stretch marks by creating a lifting force.

 These features are significant and interconnected with the formation of a stable set of essential features sufficient to obtain the desired technical result.

 So, placing the pipeline at a fairly shallow depth in a balanced-hung position allows you to fix the pipeline relative to the seabed. In this case, the balanced position of the pipeline is ensured by buoyancy.

In FIG. 1 is a view of a pipeline at sea between two coastal points;
in FIG. 2 - external contour of pipes and its deepening in water;
in FIG. 3 is a diagram of the forces acting on a pipe segment immersed in water, section AA in FIG. 2;
in FIG. 4 is a diagram for accounting the weight of couplings;
in FIG. 5 is an example of a coupling design;
in FIG. 6 is a diagram of a cable tie mounting with a pipe and a bottom;
in FIG. 7 is an example of a float design;
in FIG. 8 is a diagram for determining the effect of stretch marks on the center of gravity of a pipe;
in FIG. 9 is a layout diagram of cable extensions along the pipeline;
in FIG. 10 - the location of the supports and the chain tension diagram for a two-link span;
in FIG. 11 is the same as in FIG. 9, for a three-link span;
in FIG. 12 - diagram of the weight load on one link of the chain inside the span;
in FIG. 13 is the same as in FIG. 12, plot of the moment.

 In the proposed method for laying and transporting gas through pipeline 1 (Fig. 1) along the shortest path between two coastal points A and B, it is proposed to lay the pipeline at a relatively shallow depth H, namely, at a depth greater than the sediment of large ships, and greater than the depth of the waves during a severe storm. For example, for the Black Sea, this depth is 30-40 m.

 Further, the proposed method determines the conditions under which the pipeline 1 will be, like spacecraft, in a state of complete weightlessness, that is, in a balanced-suspended position, which in marine practice is called zero buoyancy.

To do this, consider the balance of forces acting on the pipe segment 2 (Fig. 2, 3), lowered horizontally into the water. Denote:
l is the length of the pipe;
r is the outer radius of the pipe;
r ₁ is the inner radius of the pipe;
Δ is the wall thickness of the pipe;
G is the weight of the empty pipe;
s ₁ - the area of the end internal section of the pipe;
s ₀ - the area of the end outer section of the pipe;
γ is the specific gravity of the pipe material;
γ _in - the specific gravity of sea water at a depth of H;
γ _g is the specific gravity of the gas in the pipe;
P _in - the weight of water in the volume of the pipe at a depth of H;
F is the weight of the pipe segment at a depth of H;
P _g - the weight of the gas in the pipe.

According to Archimedes' law, a buoyant force acts on the body in the form of a hollow pipe 2 immersed in water, which is numerically equal to the weight of the water displaced by the pipe and applied to its center of gravity (see the Handbook of Physics, B.M. Yavorsky et al., Science, 1965, p. 300). The resulting weight of the considered pipe segment will be:
F = G - P _in , (1)
where G = (s ₀ -s ₁ ) lγ + P _g , s ₀ = πr ² , s ₁ = π (r-Δ) ² (2)
P _in = s ₀ lγ _in (3)
If the pipe is hollow, then assuming that in / 2 / P _r = 0, according to the formulas / 2 /, / 3 / we have:
G = πlΔ (2r-Δ) γ, (4)
P _in = πr ² lγ _in , (5)
and then using the equality / 1 / and / 4 /, / 5 /, we obtain the resulting force
F = πlQ, Q = Δ (2r-Δ) γ-r ² γ _in , (6)
For weightlessness of any length of the pipe segment, it is necessary to have F = 0 or according to / 6 /
Q = 0 (F = 0) (7)
With a fairly good approximation, assuming that Δ is small compared to 2r, i.e.
2r-Δ ≅ 2r, (8)
we can write the weightlessness condition of the pipe in sea water in the form of its diameter, which according to / 6 / - / 8 / will be:
D = 2r, r = 2Δγ / γ _in (9)
EXAMPLE 1 A steel pipe with a wall thickness Δ = 0,3 cm, and γ = 14 g / cm ³ in water with _a γ = g / cm ³ is weightless, i.e. balanced-hung out in seawater, with its diameter under / nine/
D = 2 • 2 • 0.3 • 14/1 = 16.8 cm.

If the pipe is filled with gas, air or another substance with a specific gravity of γ _g , then the force of its weight increases the weight of the pipe by
P _g = ls ₁ γ _r (10)
and according to / 1 /, / 6 /, / 10 /, / 2 / F = πlR, R = Q + (r-Δ) ² γ _g . (eleven)
When one section of the pipe is connected to another using the coupling 3, it is necessary to take into account the weight / in water / q of this coupling (Fig. 4), then we have
F = πlN, N = R + q / πl (12)
For approximate calculations, using, as indicated, for small pipe thicknesses the equality / 8 / and assuming that γ _g = 0, we obtain from / 6 /, / 11 /, / 12 /
F = πlQ + q, Q = 2rΔγ-r ² γ _in (Δ ≪ r). (thirteen)
Putting in / 6 / Δ = r, we obtain the force F for a continuous section of the pipeline in the form
F = πIr ² (γ-γ _in ). (Δ = r) (14)
Let us estimate approximately the effect of a change in the diameter of the pipe on the value of force F.

Example 2. Take the pipe diameter D ^F = 20 cm and the same values of Δ, γ, γ _in , which were indicated in example 1. Then, according to / 5 /, / 4 /, / 1 / we get for l = 1 cm
G = π [0.3 (20-0.3) 14] = 82.7πg,
P _in = 10 × 10 × 1πg,
F = 82.7π-10 × 10 × 1π = -17.3πg.
Comparing the results of two examples 1 and 2, we conclude that
(D ^F - D) / D ≈ -F / G (15)
The approximate equality / 15 / is easy to obtain in the general case, if we assume for two diameters that one of them is determined by the formula / 9 / and that the equality / 8 / holds.

 From the above examples and equality / 15 / it can be seen that an increase in the diameter of the pipe compared to the diameter ensuring its weightlessness creates a certain lifting force / F <0 /, and the pipe, if you do not hold it, will start to float from depth H to the free surface where H = 0.

 Therefore, if you want to have pipes of larger diameter, with their weightlessness, you need, according to / 9 /, to increase the thickness Δ of its walls or create a load opposite to the lifting force.

 Due to the fact that the water at sea and at depth is not perfectly calm, it is advisable to protect the pipeline from a dangerous bend. I think it is advisable to form the entire pipeline like a flexible chain (Fig. 4), consisting of joined links of a certain length l. Pipes are made, for example, metal. It is possible to use another, for example, polymeric material, if its strength and chemical-resistant characteristics allow it to be used in an aggressive environment, which is sea water.

In this case, the coupling is provided with a special coupling 3, which should: provide a small / 1-2 ^o / articulated rotation between the joined sections, ensure the strength of the joint in the longitudinal and transverse directions, ensure a tight seal of the joint, which does not allow water to leak into the pipe or gas leakage from the pipe into water, to ensure freedom of flow of gas through docked sections.

 A possible embodiment of a coupling design satisfying the above conditions is shown in FIG. 5.

This coupling 3 consists of two flanges 4, rigidly connected with the ends of the pipes to be joined 2. The rigid connection can be made in various ways, for example, in the form of welding or screw connections. The end surfaces of the flanges 4 tighten together an annular rubber gasket 5. The thickness of the gasket can be small 5-10 mm A stop ring 6 is located between the ends of the flanges, which is located with a small gap δ = 1-2 mm between its end and the adjacent end of one of the flanges. With a gap of δ = 2 mm, it is possible to rotate the connected sections of the ends of the pipes 2 indicated in Example 1 by an angle α = δ / r, that is, 1.3 ^o .

 The flanges are tightened together, for example, with bolts with nuts in an amount of 6 to 10. Moreover, the bolts have a sufficient free lateral fit in the flanges, which is provided by a side play of 0.01-0.02 mm.

 The strength of the connection is ensured by the fact that the cross-sectional area of all the bolts will be close to the cross-sectional area of the pipe and by the fact that the connection of the pipe to the flanges is ensured provided for their sufficient length, for example, thread 7 on the flange and the outer surface of the pipe.

 The tightness of the connection is ensured, in addition to compressing the rubber gasket with a thin, copper or plastic pipe 8, tightly inserted into the ends of the pipes 2.

 To ensure the stability of sections of the pipeline 1, consisting of several links (pipes 2), you can use (Fig. 6) thin cable ties 9 and their connection with the bottom. In order for the cable to have zero gravity (zero buoyancy), special floats 10 are attached to it in several places along the length. These floats 10 are, for example, a vacuum cylinder (Fig. 7) made of a pipe in which it is closed (welded) end sections. In the general case, the floats are a closed, hermetic barrel. The lifting force of the float (F <0) is determined based on its size using the approximate formulas / 13 /, / 9 / or / 12 /, and then the length of the cable is determined, the weight of which is balanced by the float. Fastening at the bottom of the cable 9 of its end can be carried out by ballast 11, for example, obtained from a float filled with water. The cable with the float can be fixed, for example, by winding several of its turns on the cylindrical surface of the float, as on a coil. Other methods of attaching the float to the cable and the cable to the pipe 2 are possible, for example, due to the implementation of clamps or brackets and latches covering the bracket.

Cable stretch capabilities can be determined from FIG. 8. In the triangle O ₁ CO _{2 the} peak C is the center of gravity (ts) of pipe 2, and the base O ₁ O ₂ is on the seabed. The cable O ₁ C and the cable O ₂ C can describe only tension arcs ca and cb with radii r ₁ and r _2, respectively. Therefore, any movement of the pipe in the area located outside these arcs is impossible. When the pipe and the cable are weightless to the area below the indicated arcs, the pipe movement, with perfect calm water, cannot also occur, because this increases the depth H, and with increasing depth, the density of water and its specific gravity γ _in increase, i.e. a force F <0 appears.

Naturally, for greater stability of the pipeline 1, some relatively small lifting force F <0, which, as can be seen from Example 2, can be easily created using the increased pipe diameter D ^F, can serve.

 Thus, even a small force F <0 will not allow the pipe to decrease, and cable stretch will not allow it to move upward or in any direction horizontally, that is, complete stabilization of the link with stretch marks. Placing them along the entire pipeline 1 (Fig. 9), we obtain stabilization of the entire pipeline.

 To give an idea of the possibility of creating rope-free stretch marks in sea water, we give Example 3.

Example 3. The lifting force F _{p of the} float shown in FIG. 7 is determined from the condition that r = 20 cm, Δ = 0.3 cm, l = 100 cm, γ = 14 g / cm ³ , γ _in = 1 g / cm ³ . Here we can use the formula / 13 /, assuming that q is the weight of the bottoms of the floats, that is, q = 2πr ² Δγ, then we have
Q = -232 g / cm, q = 10.5 g, F _p = -62.3 kg.

Допустимая сила натяжения определяется, исходя из допустимого напряжения σ = 1200 кг/см² и площади поперечного сечения троса, то есть

Из примеров 3 и 4 видно, что для невесомости троса на его длине 2000 м достаточно 5 поплавков, а для его натяжения можно использовать два дополнительных поплавка.Example 4. We determine the length l _{1 of the} cable, which, when connected to the float, will be weightless, and the permissible tension force N. Assuming that the cable has r = 0.2 cm, γ = 12 g / cm ³ , the force F is determined as for a solid rod, according to the formula / 14 / and for its weightlessness it is necessary to have the equality F + F _p = 0, that is, according to / 14 / and the data presented, we obtain

The allowable tension force is determined based on the allowable stress σ = 1200 kg / cm ² and the cross-sectional area of the cable, that is

From examples 3 and 4 it is seen that for the weightlessness of the cable along its length of 2000 m, 5 floats are sufficient, and for its tension, two additional floats can be used.

или T = nql/(n-1)α, R = nql, (16)
где T - натяжение звена, находящегося вблизи опоры;
R - нагрузка на опору от двух звеньев, расположенных с разных сторон опоры.Now we determine what loads will arise in the pipeline links when imperfect conditions arise at some depth at a depth of immersion: in particular, a small lifting force, a zone of turbulent water with undercurrents, etc. As a result, a certain distributed length will act on the links of the section of the chain chain load, which is denoted by the letter q [kg / cm] (see Fig. 10). Under the influence of such a load, the chain deforms (sags), and each link rotates relative to the neighboring link by an allowable angle α. And only at the locations of the stretch marks there should be no displacement. We call such cross-sections as reference, and the distance between them is the span L. When the span contains several links, for example, L = 2l (Fig. 10), L = 3l (Fig. 11), the chain has the appearance of a symmetrically broken line (Fig. 11). The static equilibrium of such a broken line can be written in the form (assuming that sin β = β, cosβ = 1),

or T = nql / (n-1) α, R = nql, (16)
where T is the tension of the link located near the support;
R is the load on the support from two links located on different sides of the support.

 In particular, for n = 2 we have T = 2ql / α, R = 2ql, for n = 3l we have T = 3ql / 2α, R = 3ql, and for L = 10l we get T = 10ql / 9α. It can be seen from this that an increase in the span length L changes the chain tension T relatively little, and the load on the support R is proportional to the span length.

 Let us now consider what bending moments M act on any link in the span. From the equilibrium condition of the link, for example a-b, from FIG. 11 it can be seen that it is a beam with supported ends, tensile loads T and a transverse load q, which must be balanced by the forces at the ends (Fig. 12).

Under the action of these transverse loads, bending moments arise, the plot of which is shown in FIG. 13. The ordinate axis is located in the middle of the beam and the maximum value of the bending moment is indicated by the letter M ₀ . Counting its value, we obtain
M ₀ = ql ^2/8. (17)
Note that in the case of location in the span L = 3l, not a three-tier chain, and the whole solid beams, then it in the middle of the span L, there would be a maximum torque M ₀ = qL ^2/8 = 9ql ^2/8, that is an order of magnitude greater . This proves the advantage of the chain system for the pipeline.

Example 5. We determine the total stress σ from bending σ _and and from stretching σ _t in one, extreme span of the span, as well as the stress σ _p in cable stretching, with lateral load q (Fig. 12). As the source data, we take:
l = 50 m, q = 1 kg / m, α = 2 × 10 ^-2 , L = 5l, r = 10 cm, Δ = 0.3 cm, r _mp = 0.4 cm, ζ = 60 ^° .
Then we obtain approximately the moment of inertia of the cross section J _x = πΔr ³ and, using for the M, T, R formulas / 16 /, / 17 /, we obtain:
σ _and = Mr / J _x = 332 kg / cm ² ,
σ _t = T / S ₀ -S ₁ = 166 kg / cm ² ,
σ _p = R / πr ² cosζ = 10 ³ kg / cm ² .
It can be seen here that the longitudinal tension of the beam has little effect on the total voltage of the beam, which confirms the advantage of the chain scheme over the continuous beam scheme in the span.

Claims (1)

 A method of sea transportation of gas by a pipeline, in particular along the shortest path between two coastal points, which consists in laying a pipeline formed from hermetically connected separate metal pipes, and continuously supplying gas through this pipeline, characterized in that the pipeline is made of a chain of hermetically joined together and having limited angular mobility of pipes at the junction with the help of couplings, they are immersed in sea water to a depth exceeding the draft of sea vessels and not subject to storm, and provide a stable position of the pipeline in the water horizontal and depth due to its weightlessness or the so-called zero buoyancy, while the weightlessness of the pipeline is ensured by the diametrical dimensions of the pipes and stretch marks weightless with floats, which are fixed on some pipes distributed over spans, and which contain at the free end ballast, lowered to the seabed, and the necessary tension of the stretch marks create the lifting force of the overhead additional floats.

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