CN111910581A - Cement sand bag dam body suitable for protecting offshore segment submarine cable and evaluation method - Google Patents
Cement sand bag dam body suitable for protecting offshore segment submarine cable and evaluation method Download PDFInfo
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- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02B—HYDRAULIC ENGINEERING
- E02B3/00—Engineering works in connection with control or use of streams, rivers, coasts, or other marine sites; Sealings or joints for engineering works in general
- E02B3/04—Structures or apparatus for, or methods of, protecting banks, coasts, or harbours
- E02B3/12—Revetment of banks, dams, watercourses, or the like, e.g. the sea-floor
- E02B3/121—Devices for applying linings on banks or the water bottom
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
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- G06F30/28—Design optimisation, verification or simulation using fluid dynamics, e.g. using Navier-Stokes equations or computational fluid dynamics [CFD]
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Abstract
The invention discloses a cement sand bag dam body suitable for protecting the coverage of an offshore submarine cable and an evaluation method, wherein the cement sand bag dam body is positioned on a submarine cable (1); the cement sand bag comprises a cement sand bag bottom layer (2), a cement sand bag middle layer (3), a cement sand bag top layer (4) and a seabed (5); the cement sand bag bottom layer (2), the cement sand bag middle layer (3) and the cement sand bag top layer (4) are piled up from top to bottom to form a cement sand bag dam body (6), the cement sand bag dam body (6) is positioned on the seabed (5), and the whole cement sand bag dam body (6) is of a trapezoidal structure piled up from left to right; the method overcomes the defects that the traditional protection scheme of the gabion cover plate or the concrete interlocking row in the prior art needs to be prefabricated on the shore and transported to the site for construction, the cost is relatively high, and the construction period is long.
Description
Technical Field
The invention relates to the field of submarine cable protection, in particular to a cement sand bag dam body suitable for protecting submarine cables at offshore sections and an evaluation method.
Background
With the rapid development of economy in China and the progress of offshore industry, submarine power cable engineering is gradually increased. Because of the importance of submarine cable assets, which must be protected from external hazards, the cable protection costs tend to be a large proportion of the total investment. It has become widely recognized to design good cable protection measures to improve cable reliability and system effectiveness, and to reduce the operating costs required for cable system repair and maintenance. However, in recent years, the problem that the submarine cable is damaged by ship breakdown, ocean current impact, and the like during operation is becoming more prominent, and therefore, it is very necessary to develop a protection measure for the submarine cable and a method for evaluating the protection against anchor damage.
Protection of submarine cables should employ comprehensive protection measures including selection of appropriate cable routing, design of appropriate cable armor, adoption of reliable external protection methods, and protection during maintenance.
The protection measures for submarine cables can be divided into two categories, namely protection of submarine cables and protection of submarine cables buried in seabed during laying to prevent anchor damage and other accidents.
The first category is the protection of the submarine cable itself, mainly the protection measures taken during the manufacture of the submarine cable. The outer layer of the submarine cable is provided with an anticorrosive layer, a mothproof layer and an armor layer, so that the submarine cable can prevent seawater erosion and marine microbial decay, and has certain mechanical strength to resist damage of external force.
The second type is effective in resisting accidents such as anchor damage, and additional external protection measures can be adopted when the submarine cable is laid, so that the submarine cable is buried in the seabed or covered with a protection piece to ensure the safe operation of the submarine cable.
According to different construction methods, the conventional submarine cable protection schemes at home and abroad mainly comprise a cable plough, hydraulic flushing, pre-trenching, stone throwing, a cover plate and the like. The offshore shallow water area can also adopt protection means such as an iron sheath and an embedded steel pipe, but only the anchor with smaller tonnage can be resisted by the protection means, and the protection and the pre-trenching are carried out simultaneously to improve the protection effect.
a) Cable plough
Ditching and burying the cable directly under the sea floor are the most common protection methods at present. The use history of several decades of ditching with a cable plough is that when a narrow cable trench is cut by a plough share, the cable can be directly guided to be placed behind the plough share or buried in the soil through a hollow groove of the plough share.
The cable plough is limited by the lack of power of the cable plough, the cable plough is dragged by means of a sea surface ship and the cable plough needs to exert great dragging force on the cable, so that the position control of the plough is extremely difficult, particularly when rock and large round stones are encountered, the plough faces abnormally, and the cable has potential risks of extrusion and damage.
b) Hydraulic flushing and burying device
The other direct burial protection mode is burial protection, namely, high-pressure water jet is adopted to impact the seabed to form a cable trench, and the submarine cable is buried in the cable trench. The flush protection scheme is the most widely applied protection scheme at present, has good cable protection effect, short construction period and low cost, and is particularly suitable for protection construction of newly laid cables. However, the hydraulic buried equipment needs to be powered by a large construction ship, and the water depth of a shallow water area is often limited, so that the large buried equipment is often only suitable for the deep water area.
c) Pre-trenching
Before the cables are laid, trenching is carried out on the cable routes through pre-trenching equipment, and the cables are laid and then backfilled by using backfill soil, so that the submarine cables are protected. Obviously, this kind of protection mode only is applicable to the shallow water region in intertidal zone that the machine of digging can excavate the construction, and geological conditions can guarantee the channel excavation shaping.
d) Protection against riprap
The protection against stone is a protection method commonly used in an external protection scheme, namely a protection method for forming a stone dam by covering stone with a specific grading on a cable to resist anchor damage. Large riprap ships generally can not enter shallow water areas for construction, and the riprap dikes and dams located in the shallow water areas are greatly influenced by wind, storm and tide, and the problem of stability of the dikes and dams exists in long-term operation, so that the large riprap ships are generally suitable for deep water areas. In addition, the stone-throwing protection scheme needs to perform simulation calculation and theoretical analysis aiming at the marine environment so as to determine the protection capabilities of the stone dam such as anchoring resistance and the like. The stone throwing protection scheme has large stone consumption, high requirements on construction ships and teams and high manufacturing cost.
d) Cover plate protection
The cover plate protection can also be used for protecting cables in deep sea areas, but the construction process is complex, the construction period of laying the cover plate in the deep sea area is long, the cost is high, and the cover plate protection method is generally applied to shallow sea areas on the side of the near shore. The cover plate operation is mainly used for a cross spanning protection scheme adopted when a submarine cable or a pipeline has a cross condition, the construction efficiency is extremely low, and the cover plate operation is not suitable for large-scale submarine cable protection. According to the research result of DNV gabion cover plate protection analysis, the stability of the gabion cover plate on the seabed is not a problem, but the longest service life of the wrapping steel wire under seawater corrosion is only 20 years, and the gabion cover plate basically has no protective capacity for anchors of 1t or above. The gabion cover plate scheme is only suitable for submarine cable protection in the offshore shallow water area which is easy to construct and only has small ship movement.
In conclusion, the ditching protection (cable plowing, punching and pre-ditching) scheme is widely suitable for protecting the cable on the soil layer with the geological condition from shallow sea soft soil to medium hardness soil, and has good protection effect, short construction period and low cost;
the stone throwing scheme has mature experience, and the construction period, the cost and the anchoring resistance are all superior to those of the cover plate scheme, so that the method is suitable for protecting submarine cables in deep sea areas; the gabion cover solution has no advantages in terms of construction period and economy and is only suitable for submarine cable protection in shallow water areas near shore.
Therefore, when the ocean substrate at the near shore section is a rock layer or a coral reef, the ocean substrate generally cannot be dug and buried by a cable plough, a hydraulic jet machine or a digging machine because of high strength, and the traditional gabion cover plate or concrete interlocking row protection scheme needs to be prefabricated on the shore and transported to the site for construction, so that the cost is relatively high and the construction period is relatively long. Therefore, it is necessary to find a technical solution for protecting offshore submarine cables, which is economical and convenient for construction.
Meanwhile, the current analytical evaluation method for the submarine cable protection technical scheme is not mature, and related results are rarely seen in the literature.
For the flush protection, Norwegian classification in 2009 researches the relation between the non-drainage shear strength of the viscous soil and the penetration depth of a ship anchor, so as to determine the burying depth of a submarine cable in the viscous soil, however, for the sandy seabed and the silty seabed, due to the opacity of the soil body, the instantaneity of the flush process, the harshness of the seabed environment and other factors, the intrinsic mechanism of water jet flushing of the soil is still unclear, and a corresponding quantitative evaluation analysis method is lacked. For the protection against riprap, the existing research results are often focused on the resistance effect of the form of the riprap dam body on the anchoring damage, and the stability of the ocean current of the dam body is neglected.
In the research process aiming at anchor damage protection, relevant research results often focus on statistical analysis of the penetration depth of various fishing net accessories and the penetration depth of various ship anchor falling anchors or give a suggested value of the submarine cable protection penetration depth by combining the capabilities of flush equipment. And the analysis and evaluation method for the near bank segment protection technical scheme is almost blank. Therefore, the method has engineering practice significance for developing deep research on the submarine cable protection and anchor damage prevention evaluation method at the offshore section of the submarine cable.
Therefore, a structure and an evaluation method are urgently required to solve the above problems.
Disclosure of Invention
A first object of the present invention is to overcome the above-mentioned drawbacks of the background art and to propose a cement sandbag dam for the protection of submarine cables covering the offshore segment.
The first purpose of the invention is implemented by the following technical scheme: the cement sand bag dam body is suitable for the coverage protection of the submarine cable at the offshore section and is positioned on the submarine cable; the device comprises a cement sand bag bottom layer, a cement sand bag middle layer, a cement sand bag top layer and a seabed;
the cement sand bag dam body is formed by piling the cement sand bag bottom layer, the cement sand bag middle layer and the cement sand bag top layer from top to bottom, is positioned on the seabed and is integrally in a trapezoidal structure;
the long sides of the cement sandbags in the bottom layer of the cement sandbag, the middle layer of the cement sandbag and the top layer of the cement sandbag on each layer are arranged perpendicular to the cable path, and the short sides of the cement sandbags are mutually butted.
In the above technical scheme: the angle formed between the cement sand bag dam body in the trapezoid structure and the horizontal plane of the seabed is 20-30 degrees.
In the above technical scheme: the submarine cable is positioned right below the cement sand bag dam body.
In the above technical scheme: the deviation value of the center of the submarine cable relative to the center of the cement sandbag dam body is +/-0.2 m, and the two ends of the cement sandbag dam body exceed the rock or coral reef section by at least 1 m.
In the above technical scheme: the cement sand bag dam bodies are distributed in a strip shape along the cable route.
In the above technical scheme: and a gabion cover plate is additionally arranged above the cement sand bag dam body and covered with 6 covers.
In the above technical scheme: the whole bottom width E of the cement sand bag dam body is 1.5-3.5 m; the top width F is 0.6-1.3 m, and the height H is 0.6-1.0 m; the length A of each cement sand bag is 0.66 meter, the width is 0.3 meter, the height C is 0.2 meter, and the ratio of cement to sand of each cement sand bag is 1:4-1: 5.
A second object of the present invention is to overcome the above-mentioned drawbacks of the background art and to propose a method for quantitative evaluation of a cement sandbag dam suitable for offshore submarine cable coverage protection
The second purpose of the invention is implemented by the following technical scheme: the quantitative evaluation method of the cement sand bag dam body suitable for the offshore segment submarine cable coverage protection comprises the following steps;
firstly, evaluating the anchoring damage resistance of the dam body of the cement sand bag:
the method comprises the following steps: taking the anchor damage condition of the sea area, namely the main anchor weight, the geological condition of the sea bed and the seawater hydrological condition as boundary constraint conditions;
step two: calculating the impact energy when the ship anchor falls to the dam body; the speed of the object falling at a constant speed in the fluid can be calculated by the following formula:
in the formula:
m-mass of falling object (kg);
g-acceleration of gravity (m/s)2)；
V-volume of object (volume of water displaced) (m)3)；
ρwaterDensity of water (kg/m)3)；
CD-drag coefficient of the object;
a-the vertical projected area of the object in the falling direction;
vT-the final speed of the object (m/s);
after the ship anchor falls onto the cement sand bag dam body, the energy of the ship anchor is absorbed by the cement sand bag dam body, and according to an energy formula, the energy of the falling ship anchor is as follows:
step three: calculating the invasion depth of the ship anchor; the energy of the falling ship anchor is totally absorbed by the dam body, and the energy absorbed by the cement sandbag dam body can be expressed as:
in formula (2) and formula (3):
Ep-kinetic energy of the falling object;
gamma' -the effective severity of the soil;
d, the size of the outer contour of the falling object;
a-the projected area of the falling object in the falling direction;
z-the depth of intrusion of the falling object;
Nγ、Nq-a load factor;
the invasion depth z of the falling object can be calculated according to the formula;
step four: verifying the relative size of the dam height H and the invasion depth z, and judging whether the height of the cement sand bag dam meets the requirement of anchoring damage resistance;
when the height H of the cement sand bag dam body is larger than or equal to the invasion depth z, the cement sand bag dam body can be considered to meet the requirement of anchoring damage resistance,
when the height H of the cement sand bag dam body is less than the invasion depth z, the cement sand bag dam body can not meet the requirement of anchoring damage resistance,
the number of layers or the height of the cement sandbags in the dam body of the cement sandbag is increased, and the steps from the first step to the fourth step are repeated for checking calculation again;
secondly, evaluating the stability of the ocean current of the dam body of the cement sand bag:
the method comprises the following steps: calculating the critical shear stress of the water-bearing sediment bag material:
τcr＝(ρr-ρw)·g·D50·ψcr (4)
in the above formula:
τcrcritical shear stress (N/m)2)；
ρrCement sandbag density (kg/m)3)；
ρwDensity of water (kg/m)3)；
g-acceleration of gravity (m/s)2)；
D50-medium particle size;
ψcr-Shields parameters.
Step two: the combined shear stress caused by the current and wave action is calculated:
the equation (5) can be modified:
τw＝0.5ρw·fw·(kw·Ub)2
wherein:
τcwcombined shear stress (N/m) caused by current and wave action2)；
fw-wave coefficient of friction exp (-6+5.2 (a)b/Ks)-0.19) Not greater than 0.3;
Ab-a bottom horizontal displacement amplitude (m);
Ks-a bottom roughness factor (m);
Vavg-water depth average constant flow rate (m/s);
Ub-bottom horizontal displacement speed (m/s);
C-Chey parameter (m)1/2/s)；
kw,kc-turbulence factor of waves and currents;
Step three: verifying the stability of the ocean current of the dam body; when the combined shear stress caused by water flow and waves is less than or equal to the critical shear stress of the cement sand bag material, the stability of the cement sand bag dam body meets the requirement,
when the combined shear stress caused by water flow and waves is larger than the critical shear stress of the cement sand bag material, the cement sand bag dam body does not meet the stability requirement,
the size of the cement sand bag is required to be increased, and the steps one to three are repeated for re-checking calculation;
impact safety evaluation on the cable in the cement sand bag construction process:
the method comprises the following steps: calculating the final speed of the cement sandbag when falling in water; calculating the final speed v of the cement sandbag by using the formula (1)T；
Step two: calculating the energy of the cement sandbag when impacting the submarine cable;
step three: comparing the relative magnitude of the impact energy with the maximum allowable impact energy of the submarine cable (1), if the energy when the cement sandbag impacts the submarine cable is less than the allowable impact energy which can be borne by the submarine cable, the cable can be considered to be safe,
if the energy of the cement sandbag when impacting the submarine cable is larger than the allowable impact energy which can be borne by the submarine cable, the size of the cement sandbag is adjusted to be reduced.
In the above technical scheme: the cement sand bag is prepared by mixing cement and sand, and the mixing ratio of the cement to the sand is 1:4-1: 5.
The invention has the following advantages: 1. the invention can effectively protect the submarine cable from being damaged by external force factors such as anchor dropping, anchor dragging, fishing gear and the like.
2. The invention does not need to cut and flush the original seabed, is suitable for the substrate of the hard seabed and has wide application range.
3. The invention can effectively resist the action of ocean currents on the seabed, and the service life can reach 8-10 years
4. The invention can effectively protect the cable from being damaged during construction, and has convenient and safe construction.
5. The cement sandbag is simple in material obtaining, simple, easy and feasible, can be prepared on site on a construction ship, does not need prefabrication, and can save about 30 percent of the construction period.
6. The invention has relatively low manufacturing cost and can reduce the construction cost by about 50 percent.
Drawings
FIG. 1 is a cross-sectional view of a cement sand bag dam
FIG. 2 is a top view of a cement sand bag dam
FIG. 3 is a three-dimensional view of a single cement sandbag
In the figure: the system comprises a submarine cable 1, a cement sand bag bottom layer 2, a cement sand bag middle layer 3, a cement sand bag top layer 4, a seabed 5 and a cement sand bag dam body 6.
Detailed Description
The embodiments of the present invention will be described in detail with reference to the accompanying drawings, but they are not to be construed as limiting the invention, and are merely illustrative, and the advantages of the invention will be more clearly understood and appreciated by those skilled in the art.
Referring to FIGS. 1-3: the cement sand bag dam body is suitable for the coverage protection of the submarine cable at the offshore section and is positioned on the submarine cable 1; the device comprises a cement sand bag bottom layer 2, a cement sand bag middle layer 3, a cement sand bag top layer 4 and a seabed 5;
the cement sand bag bottom layer 2, the cement sand bag middle layer 3 and the cement sand bag top layer 4 are piled up from top to bottom to form a cement sand bag dam body 6, the cement sand bag dam body 6 is positioned on the seabed 5, and the whole cement sand bag dam body 6 is of a trapezoidal structure;
the long sides of the cement sandbags in the bottom layer 2, the middle layer 3 and the top layer 4 of the cement sandbag on each layer are arranged perpendicular to the cable path, and the short sides of the cement sandbags are mutually butted.
As shown in fig. 2: the angle formed between the cement sand bag dam body 6 in the trapezoid structure and the horizontal plane of the seabed 2 is 20-30 degrees; the submarine cable 1 is positioned right below the cement sand bag dam body 6; the deviation value of the center of the submarine cable 1 relative to the center of the cement sandbag dam body 6 is +/-0.2 m, and the two ends of the cement sandbag dam body 6 exceed the rock or coral reef section by at least 1 m.
The cement sand bag dam bodies 6 are distributed in a belt shape along the cable route; a gabion cover plate is covered on the upper portion 6 of the cement sand bag dam body 6.
As shown in fig. 3: the whole bottom width E of the cement sand bag dam body 6 is 1.5-3.5 m; the top width F is 0.6-1.3 m, and the height H is 0.6-1.0 m; the length A of each cement sand bag is 0.66 meter, the width is 0.3 meter, the height C is 0.2 meter, and the ratio of cement to sand of each cement sand bag is 1:4-1: 5.
The invention also provides a quantitative evaluation method related to the stability of the cement sandbag and the like, which comprises the following steps: the quantitative evaluation method of the cement sand bag dam body suitable for the offshore segment submarine cable coverage protection comprises the following steps;
firstly, evaluating the anchoring damage resistance of the dam body of the cement sand bag:
the method comprises the following steps: taking the anchor damage condition of the sea area, namely the main anchor weight, the geological condition of the sea bed and the seawater hydrological condition as boundary constraint conditions;
step two: calculating the impact energy when the ship anchor falls to the dam body; the speed of the object falling at a constant speed in the fluid can be calculated by the following formula:
in the formula:
m-mass of falling object (kg);
g-acceleration of gravity (m/s)2)；
V-volume of object (volume of water displaced) (m)3)；
ρwaterDensity of water (kg/m)3)；
CD-drag coefficient of the object;
a-the vertical projected area of the object in the falling direction;
vT-the final speed of the object (m/s);
after the anchor falls onto the cement sand bag dam body 6, the energy of the anchor is absorbed by the cement sand bag dam body 6, and according to an energy formula, the energy of the falling anchor is as follows:
step three: calculating the invasion depth of the ship anchor; the energy of the falling anchor is totally absorbed by the dam, and the energy absorbed by the cement sandbag dam (6) can be expressed as:
in formula (2) and formula (3):
Ep-kinetic energy of the falling object;
gamma' -the effective severity of the soil;
d, the size of the outer contour of the falling object;
a-the projected area of the falling object in the falling direction;
z-the depth of intrusion of the falling object;
Nγ、Nq-a load factor;
the invasion depth z of the falling object can be calculated according to the formula;
step four: verifying the relative size of the dam height H and the invasion depth z, and judging whether the height of the cement sand bag dam 6 meets the requirement of anchoring damage resistance;
when the height H of the cement sand bag dam body is larger than or equal to the invasion depth z, the cement sand bag dam body 6 can be considered to meet the requirement of anchoring damage resistance,
when the height H of the cement sand bag dam body is less than the invasion depth z, the cement sand bag dam body 6 can not meet the requirement of anchoring damage resistance,
the number of layers or the height of the cement sand bags in the dam body 6 of the cement sand bag is increased, and the steps from the first step to the fourth step are repeated for checking calculation again;
secondly, evaluating the stability of the ocean current of the dam body 6 of the cement sand bag:
the method comprises the following steps: calculating the critical shear stress of the water-bearing sediment bag material:
τcr＝(ρr-ρw)·g·D50·ψcr (4)
in the above formula:
τcrcritical shear stress (N/m)2)；
ρrCement sandbag density (kg/m)3)；
ρwDensity of water (kg/m)3)；
g-acceleration of gravity (m/s)2)；
D50-medium particle size;
ψcr-Shields parameters.
Step two: the combined shear stress caused by the current and wave action is calculated:
the equation (5) can be modified:
τw＝0.5ρw·fw·(kw·Ub)2
wherein:
τcwcombined shear stress (N/m) caused by current and wave action2)；
fw-wave coefficient of friction exp (-6+5.2 (a)b/Ks)-0.19) Not greater than 0.3;
Ab-a bottom horizontal displacement amplitude (m);
Ks-a bottom roughness factor (m);
Vavg-water depth average constant flow rate (m/s);
Ub-bottom horizontal displacement speed (m/s);
C-Chey parameter (m)1/2/s)；
kw,kc-turbulence factor of waves and currents;
Step three: verifying the stability of the ocean current of the dam body; when the combined shear stress caused by water flow and waves is less than or equal to the critical shear stress of the cement sand bag material, the stability of the cement sand bag dam body 6 meets the requirement,
when the combined shear stress caused by water flow and waves is larger than the critical shear stress of the cement sand bag material, the cement sand bag dam body 6 does not meet the stability requirement,
the size of the cement sand bag is required to be increased, and the steps one to three are repeated for re-checking calculation;
impact safety evaluation on the cable in the cement sand bag construction process:
the method comprises the following steps: calculating the final speed of the cement sandbag when falling in water; calculating the final speed v of the cement sandbag by using the formula 1T；
Step two: calculating the energy of the cement sandbag when impacting the submarine cable 1;
step three: comparing the relative magnitude of the impact energy with the maximum allowable impact energy of the submarine cable 1, if the energy when the cement sandbag impacts the submarine cable 1 is less than the allowable impact energy that the submarine cable 1 can bear, the cable can be considered to be safe,
if the energy of the cement sandbag when impacting the submarine cable 1 is larger than the allowable impact energy which can be borne by the submarine cable 1, the size of the cement sandbag should be adjusted to be reduced.
The cement sand bag is prepared by mixing cement and sand, and the mixing ratio of the cement to the sand is 1:4-1: 5. When the size of the cement sand bag is too small, the resistance to the interference of tide and waves in the falling process of the sand bag is weakened, the construction precision is difficult to guarantee, and the overall protection effect of the dam body of the cement sand bag is influenced. However, when the size of the cement sandbag is too large, the requirement of construction on the capability of a ship is increased, and the impact energy when the cement sandbag falls to the upper part of the cable is possibly too large, so that the risk of damaging the cable exists; therefore, in order to quantitatively evaluate the size or quality of the sandbag and the construction safety thereof,
example 1: the invention relates to a quantitative evaluation method of a cement sand bag dam body for protecting the coverage of submarine cables at the offshore segment, which comprises the following steps:
the first step is as follows: calculating the anchoring resistance of the cement sand bag dam body, and checking the calculation steps as follows:
1. the anchoring damage weight of a certain sea area is investigated to be 1 ton, and the flow velocity of the sea wall is investigated to be 2 m/s.
2. And calculating the impact energy when the ship anchor falls. The velocity of an object falling at a constant velocity in a fluid can be calculated by:
the energy of the falling anchor was calculated to be 14813J.
3. The depth of penetration of the ship anchor is calculated,
from the above formula, the invasion depth z of the falling object can be calculated to be 0.3m, the height of the cement sand bag dam body is 0.6m larger than the invasion depth z, and the cable is safe.
The second step is that: calculating the ocean current stability of the dam body 6 of the cement sand bag, and checking the calculation steps as follows:
calculated, the volume of a single cement sand bag is more than 125cm3The ocean current stability of 2m/s can be satisfied, so that the single volume of the cement sand bag is 39600cm3Meets the requirement of ocean current stability.
The third step: calculating the impact safety of the submarine cable 1 in the cement sand bag construction process, and the steps thereof
The maximum generated impact energy 314J of the cement sandbag is calculated to be smaller than the allowed impact energy 1000J of the submarine cable 1, so that the invention meets the requirements of anchoring resistance, stability and construction safety of the sea area.
The above-mentioned parts not described in detail are prior art.
Claims (9)
1. The cement sand bag dam body is suitable for the coverage protection of the submarine cable at the offshore section and is positioned on the submarine cable (1); the method is characterized in that: the cement sand bag comprises a cement sand bag bottom layer (2), a cement sand bag middle layer (3), a cement sand bag top layer (4) and a seabed (5);
the cement sand bag bottom layer (2), the cement sand bag middle layer (3) and the cement sand bag top layer (4) are piled up from top to bottom to form a cement sand bag dam body (6), the cement sand bag dam body (6) is positioned on the seabed (5), and the whole cement sand bag dam body (6) is of a trapezoidal structure;
the long sides of the cement sandbags in the cement sandbag bottom layer (2), the cement sandbag middle layer (3) and the cement sandbag top layer (4) on each layer are arranged perpendicular to the cable path, and the short sides of the cement sandbags are mutually butted.
2. A cement sandbag dam adapted for offshore segment submarine cable coverage protection according to claim 1, wherein: the angle formed between the cement sand bag dam body (6) in the trapezoid structure and the horizontal plane of the seabed (2) is 20-30 degrees.
3. A cement sandbag dam adapted for offshore segment submarine cable coverage protection according to claim 1 or 2, wherein: the submarine cable (1) is positioned right below the cement sand bag dam body (6).
4. A cement sandbag dam adapted for offshore segment submarine cable coverage protection according to claim 3, wherein: the deviation value of the center of the submarine cable (1) relative to the center of the cement sand bag dam body (6) is +/-0.2 m, and the two ends of the cement sand bag dam body (6) exceed the rock or coral reef section by at least 1 m.
5. A cement sandbag dam adapted for offshore segment submarine cable coverage protection according to claim 4, wherein: the cement sand bag dam bodies (6) are distributed in a strip shape along the cable route.
6. A cement sandbag dam adapted for offshore segment submarine cable coverage protection according to claim 5, wherein: a gabion cover plate is covered on the upper part of the cement sand bag dam body (6) by a cover 6.
7. A cement sandbag dam adapted for offshore segment submarine cable coverage protection according to claim 6, wherein: the whole bottom width E of the cement sand bag dam body (6) is 1.5-3.5 m; the top width F is 0.6-1.3 m, and the height H is 0.6-1.0 m; the length A of each cement sand bag is 0.66 meter, the width is 0.3 meter, the height C is 0.2 meter, and the ratio of cement to sand of each cement sand bag is 1:4-1: 5.
8. Method for the quantitative evaluation of a cement sandbag dam suitable for offshore segment submarine cable coverage protection according to any one of claims 1-7, wherein: it comprises the following steps;
firstly, evaluating the anchoring damage resistance of the dam body of the cement sand bag:
the method comprises the following steps: taking the anchor damage condition of the sea area, namely the main anchor weight, the geological condition of the sea bed and the seawater hydrological condition as boundary constraint conditions;
step two: calculating the impact energy when the ship anchor falls to the dam body; the speed of the object falling at a constant speed in the fluid can be calculated by the following formula:
in the formula:
m-mass of falling object (kg);
g-acceleration of gravity (m/s)2)；
V-volume of object (volume of water displaced) (m)3)；
ρwaterDensity of water (kg/m)3)；
CD-drag coefficient of the object;
a-the vertical projected area of the object in the falling direction;
vT-the final speed of the object (m/s);
after the anchor falls onto the cement sand bag dam body (6), the energy of the anchor is absorbed by the cement sand bag dam body (6), and according to an energy formula, the energy of the falling anchor is as follows:
step three: calculating the invasion depth of the ship anchor; the energy of the falling anchor is totally absorbed by the dam, and the energy absorbed by the cement sandbag dam (6) can be expressed as:
in formula (2) and formula (3):
Ep-kinetic energy of the falling object;
gamma' -the effective severity of the soil;
d, the size of the outer contour of the falling object;
a-the projected area of the falling object in the falling direction;
z-the depth of intrusion of the falling object;
Nγ、Nq-a load factor;
the invasion depth z of the falling object can be calculated according to the formula;
step four: verifying the relative size of the dam height H and the invasion depth z, and judging whether the height of the cement sand bag dam (6) meets the requirement of anchoring damage resistance;
when the height H of the cement sand bag dam body is larger than or equal to the invasion depth z, the cement sand bag dam body (6) can be considered to meet the requirement of anchoring damage resistance,
when the height H of the cement sand bag dam body is less than the invasion depth z, the cement sand bag dam body (6) can not meet the requirement of anchoring damage resistance,
the number of layers or the height of the cement sandbags in the dam body (6) of the cement sandbag is increased, and the steps from the first step to the fourth step are repeated for checking calculation again;
secondly, evaluating the stability of the ocean current of the dam body (6) of the cement sand bag:
the method comprises the following steps: calculating the critical shear stress of the water-bearing sediment bag material:
τcr＝(ρr-ρw)·g·D50·ψcr (4)
in the above formula:
τcrcritical shear stress (N/m)2)；
ρrCement sandbag density (kg/m)3)；
ρwDensity of water (kg/m)3)；
g-acceleration of gravity (m/s)2)；
D50-medium particle size;
ψcr-Shields parameters.
Step two: the combined shear stress caused by the current and wave action is calculated:
the equation (5) can be modified:
τw＝0.5ρw·fw·(kw·Ub)2
wherein:
τcwcombined shear stress (N/m) caused by current and wave action2)；
fw-wave coefficient of friction exp (-6+5.2 (a)b/Ks)-0.19) Not greater than 0.3;
Ab-a bottom horizontal displacement amplitude (m);
Ks-a bottom roughness factor (m);
Vavg-water depth average constant flow rate (m/s);
Ub-bottom horizontal displacement speed (m/s);
C-Chey parameter (m)1/2/s)；
kw,kc-turbulence factor of waves and currents;
Step three: verifying the stability of the ocean current of the dam body; when the combined shear stress caused by water flow and waves is less than or equal to the critical shear stress of the cement sand bag material, the stability of the cement sand bag dam body (6) meets the requirement,
when the combined shear stress caused by water flow and waves is larger than the critical shear stress of the cement sand bag material, the cement sand bag dam body (6) does not meet the stability requirement,
the size of the cement sand bag is required to be increased, and the steps one to three are repeated for re-checking calculation;
impact safety evaluation on the cable in the cement sand bag construction process:
the method comprises the following steps: calculating the final speed of the cement sandbag when falling in water; calculating the final speed v of the cement sandbag by using the formula (1)T；
Step two: calculating the energy of the cement sandbag when impacting the submarine cable (1);
step three: comparing the relative magnitude of the impact energy with the maximum allowable impact energy of the submarine cable (1), if the energy when the cement sandbag impacts the submarine cable (1) is less than the allowable impact energy which can be borne by the submarine cable (1), the cable can be considered to be safe,
if the energy of the cement sandbag when impacting the submarine cable (1) is larger than the allowable impact energy born by the submarine cable (1), the size of the cement sandbag is adjusted to be reduced.
9. The method for the quantitative evaluation of a cement sandbag dam adapted for offshore segment submarine cable coverage protection according to claim 8, wherein: the cement sand bag is prepared by mixing cement and sand, and the mixing ratio of the cement to the sand is 1:4-1: 5.
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