Design method of trash screen based on water resistance

Abstract

The present invention provides a method for designing a trash-blocking net according to water resistance, which includes the following steps: selecting a trash-blocking mode according to interception objects, flow velocity, and wind and waves; estimating and allocating the scale and specifications of the trash-blocking net, and conducting mechanical calculation and analysis; and verifying whether the design performance meets the design requirements; designing the trash-blocking net according to the corresponding mode, the trash-blocking net is composed of a first plane net, a second net bag, and an Nth net bag; selecting an anchoring and fixing and a buoyancy and sinking force equipment system according to the corresponding mode for combined construction. Through theoretical calculation of the overall and local forces of the net, combined with regression curve analysis of the net model test results, the main parameters of the trash-blocking net and the performance indicators that can be achieved are determined under the condition of reasonably setting the safety factor, thereby ensuring that the trash-blocking net can operate reliably under extreme conditions.

Description

Technical Field

[0001] The invention relates to the technical field of hydropower generation, in particular to a method for designing a trash screen for hydropower generation based on water resistance. Background Art

[0002] The open water intake channel is a channel used by a water-intake hydropower station to divert water to the pressure forebay. The longitudinal slope of the channel is gentle, and the water flow velocity in the channel is low. Under certain conditions, it is a relatively economical non-pressure water diversion structure, mostly used in small hydropower stations. With the current human damage to the environment, many riverbed low-head hydropower stations have increased sewage. More and more hydropower stations have adopted floating trash barriers as a line of defense to prevent floating sewage from entering the water inlet of the power plant, but traditional floating trash barriers are all fixed metal fences, which are easily deformed under the impact of water flow, and the gaps between the metal fences are large, so the sewage interception effect is not ideal. Summary of the invention

[0003] The invention provides a scientific, reasonable and effective method for designing a trash screen based on on-site water resistance and other parameters.

[0004] To achieve the above purpose, the technical solution provided by the present invention is:

[0005] The method for designing a trash screen based on water resistance is characterized in that it includes the following steps:

[0006] 1) Select the pollution interception mode according to the interception object, flow velocity, wind and waves;

[0007] 2) Estimate and allocate the scale and specifications of the trash screen, and carry out mechanical calculation and analysis; and verify whether the design performance meets the design requirements;

[0008] 3) Design the trash net according to the corresponding mode. The trash net consists of the first plane net, the second net bag and the Nth net bag;

[0009] 4) Select the anchoring system according to the corresponding mode;

[0010] 5) Select the buoyancy and sinking force equipment system according to the corresponding mode;

[0011] 6) Carry out combined construction according to the anchoring fixing system and buoyancy and sinking force equipment system.

[0012] The anchoring system consists of an underwater pier, underwater anchor blocks and an onshore gravity pier. The onshore gravity pier adopts a pile foundation platform with four vertical pillars. Anchor rings are set on the sides of the pillars and the bottom anchor blocks to connect with the trash net.

[0013] The first plane net adopts a double-layer structure. All meshes are composed of ultra-high molecular weight polyethylene nets. The aperture size of the first-level mesh of the first plane net is 2500mm×2500mm, and the aperture size of the second-level mesh of the first plane net is 30mm×30mm. The second-level mesh is continuously tied to the first-level mesh by binding ropes to form a pollution-blocking net; the force calculation of the first-level mesh adopts a combination of mesh resistance theoretical calculation and net model test regression estimation, and the larger value of the two is taken. When calculating its force, the water resistance of the net itself and the water resistance after the interception object is attached are taken into account. The mesh resistance calculation formula is:

[0014] ;

[0015] The secondary mesh is calculated based on the total tension on both ends of the main cables and the total number of meshes assembled on the upper line. The formula is:

[0016] .

[0017] The second net bag and the Nth net bag are composed of buoys, flat pollution-blocking nets, ultra-high molecular weight polyethylene cables, and weighted anchor chains at the bottom of the net. The main cable on the water surface is set on the top of the net body and fixed by buoys. The underwater main cable is set at the bottom to fix the bottom of the net and the weighted anchor chain. Concrete anchor blocks are thrown at intervals of 10m and pulled by ultra-high molecular weight polyethylene cables. Each group of buoys is 10m apart horizontally, and the buoys and anchor blocks are pulled and fixed by cast steel anchor chains. Ultra-high molecular weight polyethylene cables are used to pull and fix the two sides near the diversion dike to fixed points on the shore;

[0018] When calculating the force on the second net bag, the water resistance of the net itself and the increased water resistance after the interception object is blocked are taken into account. The force on the garbage net when working is similar to that on the stretched net. It is calculated according to the theoretical formula for calculating the water resistance of the net or according to the empirical formula:

[0019] The theoretical formula is: ;

[0020] The empirical formula is: ;

[0021] The calculation formula for the maximum tension of the main cable on the water surface is: ;

[0022] The buoyancy and sinking force system ensures that the net can remain in a normal interception state under the influence of marine organisms and wind and waves. At the same time, since the plane interception net is in the shape of a catenary, the calculation formula according to the catenary and the net resistance coefficient is: .

[0023] The present invention carries out structural design of the trash net by means of main reference conditions and parameters of the trash net on site, and through theoretical calculation of the overall and local forces of the net and regression curve analysis of the net model test results, the main parameters and relevant performance indicators of the trash net are determined with a reasonably set safety factor, thereby ensuring the reliable operation of the trash net under extreme conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Trend chart of the impact of waves on the water resistance of the trash net.

[0025] Figure 2 Trend chart of increasing blocking resistance of trash net.

[0026] Figure 3 The relationship between the total tension curves on both ends of the main cable at different flow rates.

[0027] Figure 4 The relationship between the total tension curve of a single net bag on the second line of the trash net. DETAILED DESCRIPTION

[0028] The present invention will be further described below with reference to all the accompanying drawings. The preferred embodiments of the present invention are as follows: Figure 1 To Attachment Figure 4 The method for designing a trash screen according to water resistance of the present invention comprises the following steps:

[0029] 1) Select the pollution interception mode according to the interception object, flow velocity, wind and waves;

[0030] 2) Estimate and allocate the scale and specifications of the trash screen, and carry out mechanical calculation and analysis; and verify whether the design performance meets the design requirements;

[0031] 3) Design the trash net according to the corresponding mode. The trash net consists of the first plane net, the second net bag and the Nth net bag;

[0032] 4) Select the anchoring system according to the corresponding mode;

[0033] 5) Select the buoyancy and sinking force equipment system according to the corresponding mode;

[0034] 6) Carry out combined construction according to the anchoring fixing system and buoyancy and sinking force equipment system.

[0035] Take the above method as an example for a nuclear power plant site water intake open channel:

[0036] According to the on-site water intake flow of 166m3 / s, the total length is about 1.1km. The south water intake is located at the depth line of about -8.0m in the southeast of the plant area. The bottom width of the water intake gate is about 94m. The flow velocity at the long-term average low tide level is about 0.170m / s, the flow velocity at the low tide level of once in a hundred years is about 0.251m / s, and the flow velocity at the design base low water level (DBL) is about 0.004m / s. The south water intake faces the southwest side. The bottom width of the south water intake open channel is 80m, and the bottom elevation of the open channel is -7.0m. The warm water discharged from the 7 units shares a drainage open channel and is discharged on the east side of the plant site.

[0037] Wind force: The maximum wind speed on site was 28.3 m / s, and the extreme minimum temperature was -10.2℃.

[0038] Tide: The tides outside the plant site are controlled by the South Yellow Sea tidal wave system. The tidal nature of the plant site is 0.66, which is an irregular semi-diurnal tide. The phenomenon of two high and two low tides in one day is very obvious. Calculated from the average sea level of the Yellow Sea, the average high tide height for many years is 1.23m, the average low tide height for many years is -1.16m, and the average tidal range for many years is 2.39m. The tidal direction is northeast to southwest; the surface tidal current is large, the bottom tidal current is small, and the maximum flow rate is about 80cm / s.

[0039] 100-year high water level (P=1%): 1.72m.

[0040] Design base flood level = 10% exceedance probability astronomical high tide level + possible maximum storm surge + sea level anomaly = 1.08 + 2.07 + 0.21 = 3.36m;

[0041] Design base low water level = 10% exceedance probability astronomical low tide level + possible maximum storm surge reduction = -1.36-2.64=-4.00m.

[0042] Waves: The plant site is located on land to the west and the Yellow Sea to the east. It is an open sea area. The westward (NW-W-SW) waves are affected by the land, while the eastward waves can grow fully and spread directly to the engineering sea area. Therefore, the plant site sea area is mainly affected by the eastward (N-NE-E-SE-S) waves.

[0043] Swell is the main type of wave in the sea area of ​​the plant site, accounting for about 31.38% of the measured data; followed by mixed waves with wind waves and swell, accounting for about 28.99%. The common wave direction in the sea area of ​​the plant site is in the NE direction, with an annual frequency of 17.7%, and the second common wave direction is in the ENE direction, with an occurrence frequency of 16.6%. According to the observation data of the plant site dedicated station and the historical observation data of the nearby wave observation station, a comprehensive analysis determined that the strong wave direction in the sea area of ​​the plant site is NE. The largest waves appear in the WNW and NW directions. 91.04% of the one-tenth large waves (H1 / 10) in the engineering sea area have a wave height of less than 1.0m, 99.48% have a wave height of less than 2m, only 0.52% have a wave height greater than 2.0m, and only 0.14% have a wave height greater than 2.5m. 94.21% of the significant waves (H1 / 3) have a height less than 1.0m, 99.91% have a height less than 1.5m, and only 0.15% have a height greater than 2m.

[0044] Summer ocean currents: The direction of rising tide in the project area is between WS and WSE, and the direction of ebb tide is between NW and NE; during high tide, the maximum velocity of rising tide is between 19 and 172 cm / s, and the maximum velocity of ebb tide is between 15 and 114 cm / s; during mid tide, the maximum velocity of rising tide is between 11 and 123 cm / s, and the maximum velocity of ebb tide is between 12 and 112 cm / s; during low tide, the maximum velocity of rising tide is between 12 and 103 cm / s, and the maximum velocity of ebb tide is between 12 and 100 cm / s.

[0045] Interception purpose: The main targets of water interception are marine organisms such as enteromorpha, jellyfish, and copper algae.

[0046] Design and operation conditions of trash screen and its supporting equipment

[0047] The pollution-blocking net and its supporting equipment are installed in the open water channel sea area, and are exposed to the harsh environment of sunlight exposure, seawater corrosion, salt spray corrosion, and wind and wave impact for a long time.

[0048] Design water level

[0049] The elevation system adopts the 1985 National Elevation System.

[0050] Design high water level: 1.72m (100-year high water level)

[0051] Designed low water level: -2.41m (100-year low tide level)

[0052] DBF water level: 3.36m

[0053] Design Wave

[0054] The high tide level with a return period of 100 years is 1.72m, and the wave height with a return period of 100 years is less than 1m (at the sewage interception net inside the open channel) and 3.83m (at the mouth).

[0055] According to the "high tide period, the maximum flow velocity of the rising tide is 19 to 1.72 m / s", this flow velocity is taken as the maximum flow velocity that the first pollution-blocking net may encounter.

[0056] The average flow velocity at the third trash-blocking net at low tide for many years is about 0.34 m / s, and the flow velocity at the low tide once in a hundred years is about 0.61 m / s". This flow velocity is taken as the maximum flow velocity that the third trash-blocking net may encounter.

[0057] The maximum flow velocity that the second trash screen may encounter is 1.17m / s (the average value of the maximum flow velocity that the first and third trash screens may encounter).

[0058] The above-mentioned pollution interception nets must have sufficient ability to resist typhoon strong waves: the first pollution interception net is superimposed with waves with a return period of 100 years at 1% high tide level, and the second and third pollution interception nets are superimposed with waves with a height of 1m at 1% high tide level. Under severe working conditions such as marine biological disasters and the pollution interception net blockage rate reaching 50%, the structure of the pollution interception net should remain intact and be able to effectively intercept sinking Enteromorpha, jellyfish and copper algae, etc. The structural strength of the plane net should also be considered to ensure the safety of mooring facilities such as buoys under maximum water flow and ship collision.

[0059] According to the known conditions such as interception object, flow velocity, wind and waves, the type of pollution interception (interception) net is selected, the size and specifications of the pollution interception net are estimated and allocated, and mechanical calculation and analysis are carried out to verify whether the design performance meets the design requirements. Then the buoyancy and sinking force configuration of the pollution interception net, the type and weight of the fixing device, the selection of accessories and the construction process are determined.

[0060] The first pollution-blocking net:

[0061] The first pollution-control net is installed at the head of the breakwater, outside the entrance of the open water intake channel. It is floating and arranged in a herringbone shape.

[0062] The anchoring system of the herringbone net at the mouth is composed of underwater piers, underwater anchor blocks and onshore gravity piers, which jointly bear the external forces on the trash net. The pier adopts a pile foundation platform in the form of four vertical pillars. Anchor rings are set on the sides of the pillars and the bottom anchor blocks to connect with the trash net.

[0063] The top elevation of the trash net is designed according to the 100-year high tide level of 1.72m, extending to -8m from the bottom of the channel. The trash net uses a double-layer structure, made of ultra-high molecular weight polyethylene mesh, the aperture size of the first-level mesh is 2500mm×2500mm, and the aperture size of the second-level mesh is 30mm×30mm. The second-level mesh is continuously tied to the first-level mesh by tying ropes. When the local mesh needs to be replaced, only the second-level mesh needs to be replaced. The straight length of the trash net is about 260m.

[0064] A shore pier is set up on each side of the shore foundation on the water surface; 5 gravity piers are evenly arranged in the water. The gravity pier adopts a pile foundation platform with four vertical pillars, and anchor rings are reserved on the sides of the piers for hanging nets; the unit mesh lengths are divided into two types: 6 pieces of about 48m (between two piers) and 5 pieces of about 6.5m (between pier pillars). The mesh should be fastened to the anchor rings reserved on the sides of the pier pillars with buckles or ultra-high molecular weight polyethylene ropes, and a firm connection should be ensured.

[0065] Parameters of the first pollution-blocking net: 48m (length) × 10m (height), the aperture size of the secondary mesh is 30mm × 30mm, and the aperture size of the primary mesh is 2500mm × 2500mm.

[0066] The secondary mesh of the first trash-blocking net is composed of 6 pieces of 48m×10m flat mesh and 5 pieces of 6.5m×10m flat mesh to form the overall flat trash-blocking net.

[0067] The force calculation of the trash net mesh adopts a combination of mesh resistance theoretical calculation and regression estimation from the net model test, and the larger value of the two is taken.

[0068] Theoretical calculation of mesh resistance:

[0069] Each 48m trash net is arched in both horizontal and vertical directions after being stressed in water. When calculating its stress, it is necessary to consider the water resistance of the net itself and the water resistance after the intercepted objects are attached. According to the field standard, the net resistance is calculated as follows:

[0070] R=(1.8+2(d / a-0.01)β)SV2

[0071] Where:

[0072] R: The water resistance of the mesh when the angle of attack between the mesh and the water flow is 0.

[0073] d: network cable diameter (mm);

[0074] a: length of one leg of the network cable (mm);

[0075] β: angle of attack between the mesh and the water flow;

[0076] S: area of ​​mesh after shrinkage (m2);

[0077] V: flow velocity (m / s).

[0078] According to calculations: when the flow velocity is 1.72m / s, the water resistance of the entire net is about 125kN. When the wind and waves are affected (wave height 1~2.5m), the instantaneous load is 2.5 times that of the pure flow when the irregular wave with a wave height of 2.5m is tested by the model (reference is made to the results of the model test of the trash net, ( Figure 1) Trend of wave impact on water resistance of the trash net). When the trash net encounters more than 50% blockage caused by marine organisms, the increase in water resistance is 2.3 times the pure water resistance. The superimposed impact is estimated (reference Figure 2 The blockage resistance increase trend chart of the trash net is shown in the figure. It is estimated that the coefficient to prevent fatigue, wear and tear of ropes during long-term use is 2.6 times. This design uses a coefficient of 15 times and the above formula to calculate the maximum resistance of the trash net to be 1875kN. The flat net is mainly connected to the main cable at four corners. The force on the main cable and the fixed point is 1 / 4 of the total resistance of the net, which is 469kN.

[0079] In addition, according to the model test of the flat trash net, the total tension curve relationship between the two ends of the main cable of each flat trash net at different water flow speeds is shown in the attached Figure 3 .

[0080] analyze Figure 3 The test results show that when the secondary mesh of the first pollution-control net, which uses a 48 m*10 m flat mesh per piece, is subjected to an extreme flow rate of 1.72 m / s, the total tension at both ends of the upper main cable is 70 kN. That is, at this time, under extreme water flow and when no marine organisms touch the pollution-control net, the secondary mesh, which uses a 48 m*10 m flat mesh per piece, is subjected to a maximum tension of about 35 kN on the upper main cable.

[0081] In the first plane, the intercepting net encountered a large number of marine organisms, wave and current superposition, wind and other extreme conditions. Similarly, the 15-fold coefficient used in the theoretical calculation was used in the above text to estimate the force of the main cable and the consolidation point of each 48 m×10m plane net to be 525kN. Compared with the theoretical calculation, the results obtained by the model test are slightly larger, and due to the limitations of the use of theoretical calculation formulas, the results obtained by the model test have higher credibility. The model test results are used to estimate the water resistance of the intercepting net.

[0082] Single mesh stress and main mesh design method:

[0083] The force of the net bag and the flat net per mesh can be calculated based on the total tension on both ends of the main cable of the upper line and the total number of meshes assembled on the upper line, using the following formula:

[0084] F=R / T

[0085] Where:

[0086] F: force on each mesh; R: total tension on both ends of the main cable of the net bag or flat net; T: total number of meshes on each piece of trash net.

[0087] T = L / (E × 2a)

[0088] E: The shrinkage coefficient between the net bag and the upper line

[0089] 2a: Mesh length size, such as the mesh with an aperture size of 30mm×30mm, then a=30mm.

[0090] The first plane pollution-blocking net uses an ultra-high molecular weight polyethylene knotted mesh with an aperture size of 30mm*30mm and a mesh diameter of 2mm. When the flow rate is 1.72m / s, the total tension on both ends of the main cable of the upper line is 70kN. Each mesh connected to the upper line is evenly stressed. The length of the upper line is 48m, and the contraction coefficient between the mesh and the upper line is 0.60. The water resistance during pure flow is used to estimate the force of each mesh to be 0.052kN. When the first plane pollution-blocking net encounters a large number of marine organisms, wave and current superposition, wind force and other extreme conditions, it is estimated that the main cable of the upper line is subjected to a maximum tension of 1050 kN at both ends, and the force of each mesh is estimated to be 0.78kN. If a 2mm ultra-high molecular weight polyethylene knotted mesh is used, the knot strength is not less than 1.20kN, ensuring that the secondary mesh still has a safety factor of more than 1.5 times under the extreme maximum tension.

[0091] Calculation method of force on primary mesh:

[0092] The mesh size of the first-level mesh is 2500mm×2500mm, and it uses φ12mm ultra-high molecular weight polyethylene braided rope. According to the calculation of the resistance area ratio between the first-level mesh and the second-level mesh, it is estimated that the water resistance of the first-level mesh is 1 / 10 to 1 / 14 of the second-level mesh, and the extreme maximum tension is 37.5kN to 52.5kN. Safety factor: The knot strength of the 12mm ultra-high molecular weight polyethylene knotted mesh is not less than 119kN, ensuring that the first-level mesh still has a safety factor of more than 3 times under the extreme maximum tension. The first-level mesh can effectively increase the strength of the entire trash net and make it easy to replace the net when it is partially damaged.

[0093] The first-level mesh is reinforced with 18mm ultra-high molecular weight polyethylene rope on all sides, and the knot strength is not less than 225kN. A reasonable ratio of mesh is used when assembling the first-level mesh and the second-level mesh to ensure uniform force and ensure that the pollution control net can stably bear the force under various sea conditions.

[0094] Calculation method of main cable force:

[0095] The main cable needs to withstand the overall resistance of the net. The flat trash net uses upper and lower main cables to consolidate the vertical ends of the net. The net is deployed in the water after being impacted by water flow and intercepted objects. However, due to the uneven distribution of intercepted objects and the difference in the local impact of wind and waves on the net, the force on the upper and lower ends of the net cannot be guaranteed to be uniform. In order to ensure the safety of the cable under extreme conditions and consider factors such as wear and accidents, the upper and lower cables (main cables) can use ultra-high molecular weight polyethylene cables with a diameter of 54 mm. The breaking force of each cable is not less than 1960kN, ensuring that the upper main cable has a safety factor of 3.3 times when the primary and secondary meshes are subjected to extreme maximum tension.

[0096] Force calculation method for the weight anchor chain at the bottom of the trash net:

[0097] The buoyancy is to ensure that the net can still be in a normal interception state under the influence of marine organisms and wind and waves. Since the flat interception net is in the shape of a catenary, the calculation formula of the catenary and net resistance coefficient is:

[0098] T = (q × s2 / 8 × f) - (q × f / 2)

[0099] Where: T: horizontal component of cable tension (kgf);

[0100] q: load per meter of arc length (kg / m);

[0101] L: chord length of the cable (m);

[0102] f: sag (m);

[0103] S: length of the cable (m).

[0104] K=R / V2

[0105] Where: K: resistance coefficient;

[0106] R: water resistance of the net;

[0107] V: Flow rate.

[0108] According to the 50% blockage state of the trash net, the vertical force is calculated to be about 25.5kN. If the drag coefficient commonly used in nets is used for estimation, the buoyancy is generally between 0.1 and 0.2 of the drag coefficient, and the buoyancy needs to be about 52.5kN. According to the 50% force on the pier, the force on the float is about 26.25kN. Based on the above, the buoyancy is selected as 26kN, and the sinking force is selected as 26kN according to the same calculation of the buoyancy. Therefore, the pollution-blocking net uses an integrated large buoy (a large buoy with a rope, diameter: 800mm, length 1200mm, plane width 400mm, upper and lower main grooves 40mm; 5 grooves, groove width 40mm deep 25mm, LLDPE polyethylene material, filled with polyurethane inside, buoyancy 550KG, average wall thickness about 8mm. The service life of the buoy in seawater is more than 5 years.), one is installed at an interval of 10m, each with a static buoyancy of 5.39kN, a total length of 48m, a total of 4 buoys are installed, with a total buoyancy of 21.56kN, which meets the use requirements.

[0109] The weight is in the form of iron chain and counterweight block. Each section of flat mesh is 48m long, the counterweight chain has a diameter of 32mm, the unit weight is 20kg / m, the total weight is 960kg, and the counterweight anchor block is one every 10m, for a total of 4. The total weight of the iron chain and anchor block is greater than 26kN. It meets the use conditions.

[0110] Installation of the first pollution-blocking net and buoy anchor blocks

[0111] The flat net is composed of buoys, flat trash nets, ultra-high molecular weight polyethylene cables, and weighted anchor chains at the bottom of the net. A surface main cable is set on the top of the net body and fixed by a buoy. An underwater main cable is set at the bottom to fix the bottom of the net and the weighted anchor chain. Concrete anchor blocks are set at intervals of 10m and pulled by ultra-high molecular weight polyethylene cables. Each group of buoys is 10m apart horizontally, and the buoys and anchor blocks are pulled and fixed by cast steel anchor chains. Ultra-high molecular weight polyethylene cables are used to pull and fix the two sides near the diversion dike to fixed points on the shore.

[0112] The main cable on the water surface is made of ultra-high molecular weight polyethylene cable, which is used to fix all buoys in sequence; the main cable on the bottom is made of ultra-high molecular weight polyethylene cable and fixed with cast steel anchor chain weight. Both ends of the main cable are fixed to the reserved anchor rings at the top and bottom of the side of the pier pillar.

[0113] Other related parts:

[0114] Main cable: The surface of the cable is painted with anti-marine growth paint.

[0115] Float: It adopts one-piece horizontal rotational molding float, the float shell is 8mm thick and filled with polyurethane.

[0116] The service life of the flat mesh is 1 year. The flat mesh is treated with anti-marine biological attachment, wear-resistant and anti-aging coating, which will not affect the normal use of the flat mesh. The effectiveness of the agent is not less than 6 months.

[0117] The mesh structure should be easy to install and disassemble.

[0118] Based on the above force calculations and related parameters (auxiliary materials), the designs of mesh, cables, buoys and anchor chains are guaranteed to fully meet the use requirements.

[0119] Design method of the second and third pollution-blocking plane net:

[0120] Two series of spliced ​​nets and flat nets are set in the open channel, and the design water level is designed according to 1% high tide level + 1m effective wave height. Nets are set in the middle of the open channel, and flat nets are spliced ​​on both sides. The second pollution interception net is arranged in the open channel after passing through the arc section along the direction of water flow, and the third pollution interception net is arranged 50m behind the dry construction cofferdam. The straight length of the second pollution interception net is about 152m, and the straight length of the third pollution interception net is about 165m. The third pollution interception net takes into account the requirements of the ship section.

[0121] The second trash-blocking net is installed in the open channel after passing through the arc section along the direction of water flow. After the wave is dissipated, the wave height here is less than 1.0m. It adopts full-section floating net bags and is arranged in a straight line.

[0122] The second trash-blocking net is installed in the open channel after passing through the arc section along the direction of water flow, and is fixed by buoys, anchor blocks and gravity piers on the shore.

[0123] The top elevation of the second and third trash-blocking nets is designed to be 2.72m, extending to -7.00m from the bottom of the channel. The nets are made of ultra-high molecular weight polyethylene nets. Each net is 6m long horizontally, 10m vertically, and 30m long longitudinally, including an 8m net bag. The guide net mesh is 10mm×10mm, and the length is 22m (tentative); the collection net mesh is 5mm×5mm, and the length is 8m (tentative). Because the depth between the two ends and the shore is shallow and varies greatly, the shore-side wave-breaking blocks will wear the nets, so ultra-high molecular weight polyethylene flat nets are used.

[0124] Flat trash nets on both sides of the second and third trash nets: 15m (length) × 10m (height) × 5m (height).

[0125] The third trash-blocking net is located in the South Intake Open Channel, about 50m downstream of the main construction cofferdam. It adopts a full-section floating net bag and is arranged in a straight line. It is fixed by buoys, anchor blocks, caissons and gravity piers on the shore. Two ship piers are set in the open channel for ships to pass through. The ship piers adopt a gravity caisson structure.

[0126] The average flow velocity at the third trash-blocking net at low tide for many years is about 0.34m / s, and the flow velocity at low tide once in a hundred years is about 0.61m / s. The trash-blocking nets between the two ends and the shore are made of ultra-high molecular weight polyethylene flat nets. Two piers are set at an interval of 30m (tentative) in the middle of the open channel for ships to pass. The piers divide the trash-blocking net into three sections, and a manual winch and matching pulley block are set on the piers.

[0127] Second and third line of pollution-blocking nets: Mechanical analysis of the net bag mesh

[0128] When calculating the force of the net bag type trash net, it is necessary to take into account the water resistance of the net itself and the increased water resistance after the interception object is blocked. The force of the trash net when working is similar to that of the stretched net. It is calculated according to the theoretical formula of the net water resistance or the empirical formula:

[0129] Theoretical formula: R = 1 / 2ρ × K × S × V2

[0130] Empirical formula: R = K × (1416 + 2.54 × d / a × c2) × v1.62

[0131] Where:

[0132] R: water resistance of the net; S: area of ​​the net line; K: resistance coefficient; Ρ: water density;

[0133] V: flow velocity; d: mesh wire diameter; a: mesh length.

[0134] Using the above two formulas, when the flow rate is 1.17m / s, the water resistance of a single net bag type trash screen is calculated to be 23.6kN and 21.5kN respectively.

[0135] According to the results of the trash net model test, the total tension curve relationship of the main cable at both ends of the net bag type trash net at different water flow speeds is shown in the figure. Figure 4 .

[0136] According to the resistance variation curve obtained from the model test, when the second net bag trash can is subjected to an extreme flow velocity of 1.17 m / s, the total tension on both ends of the main cable of each net bag is 12 kN, that is, the water resistance of the single top block net is 24 kN.

[0137] In the second line, the net was affected by a large number of marine organisms, wave and current superposition, wind and other extreme conditions. It is estimated that the resistance transmitted to the upper line by a single net is 180kN (according to the wave mentioned above, the net was blocked by more than 50% of marine organisms, fatigue and wear caused by long-term use, etc., and the combined impact is estimated to be 15 times). The total resistance transmitted to the upper line by the 22 nets in the second line is 3960kN, part of which is transmitted to the underwater anchor block through the underwater traction cable, bearing 50% of the total resistance (calculated based on 22 anchor blocks, each bearing an average of 90kN tension), and the remaining 1980 kN resistance is transmitted to the anchor piers on both sides through the main cable of the upper line. The connection point between the main cable and the anchor piers on both sides is the stress concentration point, and each end is subjected to a force of 990kN.

[0138] The third trash-blocking net: force calculation of the net bag part

[0139] The force analysis of the net bag of the third line of pollution-blocking net is similar to that of the second line, but the maximum flow velocity that the third line of pollution-blocking net may encounter is 0.61m / s, which is relatively small. The theoretical calculation formula is the same as that of the second line. For example, according to the regression curve of the net model test, referring to the "relationship of the total tension curve of the single net bag of the second line of pollution-blocking net", the total tension on both ends of the main cable of each net bag is 3kN, that is, at this time, under extreme water flow, when no marine organisms touch the pollution-blocking net, the connection point between the main cable of the upper line and the anchor piers on both sides is subjected to a force of 1.5kN.

[0140] In the third net bag, the trash intercepting net encountered a large number of marine organisms, wave and current superposition, wind and other extreme conditions. It is estimated that the total tension on both ends of the main cable of each net bag is 30kN (based on the wave model test and the superposition of the impact of more than 50% blockage caused by marine organisms, with a coefficient of 10). The total tension on both ends of the main cable of the four parallel net bags between the two central piers is 120kN.

[0141] There are 8 parallel net bags configured on both sides of the two piers. The total tension at both ends of the upper main cable is 240kN, and the tension at the connection points with the anchor piers on both sides (the force concentration points at each end) is 120kN.

[0142] Second and third line of pollution control nets: force calculation of the plane net part

[0143] According to the estimation based on the fishing gear model test, the aperture size of the 10mm×10mm flat pollution-blocking net is 15m (length)×10m (height)×5m (height).

[0144] The resistance variation trend obtained from the model test results shows that when the plane nets on both sides of the second trash interception net are subjected to an extreme flow rate of 1.17m / s, the total tension at both ends of the main cables on a 15m (length) × 10m (height) × 5m (height) plane net is 16kN, that is, under extreme water flow at this time, when no marine organisms touch the trash interception net, the main cables on a plane net are subjected to a maximum tension of about 8kN. The second trash interception net (plane net part) encounters a large number of marine organisms, wave and current superposition, wind force and other extreme conditions. It is estimated that the main cables on each plane net part are subjected to a maximum tension of about 80kN (based on the wave model test and the superposition of the impact of the trash interception net encountering more than 50% blockage caused by marine organisms).

[0145] Based on the above calculations, the maximum tension on the main rope of the second trash interception net is:

[0146] 990kN+80kN×2=1150kN.

[0147] In order to ensure the safety of the cables under extreme conditions and take into account factors such as wear, ultra-high molecular weight polyethylene cables with a diameter of 54 mm can be used for the upper and lower cables (main cables). The breaking force of each cable is not less than 1960kN, and the safety factor is about 2 times.

[0148] The third trash-blocking net: Force calculation of the plane net part:

[0149] When the plane nets on both sides of the third trash interception net were subjected to an extreme flow rate of 0.61m / s, the total tension on both ends of the main cables of a 15m (length) × 10m (height) × 5m (height) plane net was 16kN, that is, under extreme water flow at this time, when no marine organisms touched the trash interception net, the main cables of a plane net were subjected to a maximum tension of about 4.8kN. The third trash interception net (plane net part) encountered a large number of marine organisms, wave and current superposition, wind force and other extreme conditions. It is estimated that the main cables of each plane net part were subjected to a maximum tension of about 48kN (based on the wave model test and the superposition of the impact of the trash interception net encountering more than 50% blockage caused by marine organisms, with a coefficient of 10).

[0150] Single mesh stress analysis and network cable design:

[0151] The force of the net bag and the flat net per mesh can be calculated based on the total tension on both ends of the main cable of the upper line and the total number of meshes assembled on the upper line, using the following formula:

[0152] F=R / T

[0153] Where:

[0154] F: force on each mesh; R: total tension on both ends of the main cable of the net bag or flat net; T: total number of meshes on each piece of trash net.

[0155] T = L / (E × 2a)

[0156] E: The shrinkage coefficient between the net bag and the upper line

[0157] 2a: Mesh length size, such as the mesh with an aperture size of 10mm×10mm, then a=10mm.

[0158] The second net bag trash interception net (net bag part) uses an aperture size of 10mm×10mm. When the flow rate is 1.17m / s, the total tension on both ends of the main cable of the net bag upper line is 12kN. According to the uniform force on each mesh connected to the upper line, the contraction coefficient between the net bag mesh and the upper line is 0.55, and the length of the upper line is 6m. According to the above formula, the force on each mesh is 0.022kN. When the second net bag trash interception net encounters a large number of marine organisms, wave and current superposition, wind force and other extreme conditions (based on the wave model test and the superposition of the impact of the pollution interception net encountering more than 50% blockage caused by marine organisms, 10 times the coefficient), it is estimated that the main cable of each net bag upper line is subjected to a maximum tension of 60kN, and the force on each mesh is estimated to be 0.22kN.

[0159] According to the estimation of the stress on a single mesh, the second-line trash-blocking net uses an ultra-high molecular weight polyethylene knotted mesh with a mesh diameter of 0.6mm and a knot strength of 0.288kN, which meets the design requirements.

[0160] The second trash interception net (plane net part), when the flow rate is 1.17m / s, each mesh connected to the upper line is evenly stressed, and the contraction coefficient between the net bag mesh and the upper line is 0.60. According to the above formula, the force of each mesh is 0.0065kN. When the second trash interception net (plane net part) encounters a large number of marine organisms, wave and current superposition, wind force and other extreme conditions, a piece of plane net estimates that the main cable of the upper line is subjected to a maximum tension of 80kN, and the force of each mesh is estimated to be 0.065kN.

[0161] According to the estimation of the stress on a single mesh, the second trash-control net (flat net part) uses an ultra-high molecular weight polyethylene knotted mesh with a wire diameter of 0.6mm and a knot strength of 0.288kN, which meets the strength requirements.

[0162] Design of the third trash screen:

[0163] The maximum tension on the main rope of a trapezoidal flat trash net and 8 net bags between the shore foundation on one side of the third trash net and a pier in the water is: 240kN+48kN=288kN.

[0164] The mesh cable specifications, apertures and dimensions are made of the same materials as the second trash-blocking net. When the third trash-blocking net is subjected to external conditions with a smaller flow rate (maximum 0.61m / s) and smaller waves, and when the force analysis of the second trash-blocking net can meet the stability requirements, the third trash-blocking net has a higher safety factor.

[0165] Design of main cable, buoyancy device and weighted anchor chain:

[0166] Main cable: The main cable needs to bear the overall resistance of the net, and the upper and lower main cables are used to consolidate the vertical ends of the pollution control net respectively.

[0167] According to the above calculated values, the maximum tension on the main rope of the second trash net is: 1150kN.

[0168] The upper and lower cables (main cables) are made of ultra-high molecular weight polyethylene cables with a diameter of 54mm, and a breaking force of 1770kN. The total load reaches 3540kN, which can meet the use conditions.

[0169] Buoyancy equipment:

[0170] Calculation of buoyancy and sinking force of net

[0171] The buoyancy of the net needs to ensure that the net mouth still floats on the water surface under the maximum force state. According to the buoyancy calculation formula:

[0172] Q=H×R / N

[0173] According to calculation, the buoyancy is about 108kN. An integrated horizontal roto-molded float is installed in the combined part of the two net bag type trash nets (a large integrated float with rope, diameter: 800mm, length 1200mm, plane width 400mm, upper and lower main grooves 40mm; 5 grooves, groove width 40mm deep 25mm, LLDPE polyethylene material, filled with polyurethane inside, buoyancy 550KG, average wall thickness about 8mm.), each static buoyancy is 5.39kN, and 4 EVA floats with a diameter of 285mm are evenly installed between every two integrated floats, each static buoyancy is 98N. 34 similar floats are installed along the two sides of the back net, with an installation spacing of 1200mm, and 10 collection net bags are installed. The total buoyancy of a single net bag is 10.1kN.

[0174] The sinking force is the same as the buoyancy of the net mouth, which is 10kN. A 32mm diameter iron chain is used, and the counterweight is 165kg. After the lower line is connected to the iron chain and next to the anchor block, the weight of the anchor block in the water plus the counterweight iron chain is greater than the sinking force requirement of the net bag type trash net.

[0175] It is proposed to use iron anchors as the net fixing device. The formula for calculating the gripping force of anchors or anchor blocks is:

[0176] P=C×W

[0177] Where: W is the anchor weight; C is the grip coefficient (usually around 10 for a high grip anchor, generally around 4 to 8).

[0178] According to the maximum total resistance of a single net bag trash net of 180kN, double anchors are used to fix both ends, and the grip coefficient is calculated as 8. Assuming that the anchor bears 50% of the force of a single trash net, the weight of the iron anchor is about 1200kg. If cement anchor blocks are used, the grip is weaker than that of iron anchors, so the weight is several times that of iron anchors. The fixed anchor chain can be a 22mm diameter iron chain with a breaking strength of more than 60kN. It can meet the use conditions.

[0179] 3.2.9 Installation plan for the second and third trash screens

[0180] The third trash-blocking net is composed of 24 bag-shaped trash-blocking nets and two flat trash-blocking nets. The two central piers are 21m apart and are equipped with four parallel net bags. Ten parallel net bags are arranged on each side of the two piers. A trapezoidal flat trash-blocking net is arranged on each side of the third trash-blocking net.

[0181] The net bag is provided with an independent mooring and traction fixing structure.

[0182] The net bag is composed of the front row anchor blocks (only the second pollution-blocking net), the rear row anchor blocks, the surface main cable, the underwater main cable, the vertical main cable, the rotomolded buoy, the anchor piers on both sides and the lashing and fixing accessories, etc. The net bag mooring fixed frame structure is composed of the front row anchor blocks (only the second pollution-blocking net), the rear row anchor blocks, the surface main cable, the underwater main cable, the vertical main cable, the rotomolded buoy, the anchor piers on both sides and the lashing and fixing accessories. The distance between the front and rear row anchor blocks is 10m, and the distance between the horizontal anchor blocks is 6m. The final distance between the anchor blocks is determined according to the net bag layout.

[0183] The upper end of the flat net is connected to the surface main cable, the bottom is connected to the underwater main cable, and the side is connected to the vertical main cable.

[0184] Nets are arranged at the front and rear openings of the net bag, and reinforcing ribs are arranged on the net surface. The mesh, nets and reinforcing ribs are all made of ultra-high molecular weight polyethylene cables.

[0185] Winch design:

[0186] According to the force analysis of the net bags of the third line of pollution control net, and referring to the "total tension curve relationship of a single net bag on the second line of pollution control net", the total tension on both ends of the main cable of each net bag is 3kN, that is, the maximum flow rate that may be encountered is 0.61m / s. When no marine organisms touch the pollution control net, the connection point between the main cable of a single net bag and the anchor piers on both sides is subjected to a force of 1.5kN, and the total force of the four net bags at the connection points of the anchor piers is 6kN.

[0187] The maximum force of the winch cable is 6kN. According to the rated maximum load of a single winch of 50kN, the two winches can reach a total of 100kN, with a safety factor of more than 16 times. Due to the need for ship passing, the winch raises and lowers the four net bags in the middle section of the third trash net, which is safer.

[0188] The above method determines the main parameters and achievable performance indicators of the trash net by theoretical calculation of the overall and local forces of the net, combined with regression curve analysis of the net model test results, while reasonably setting the safety factor, thereby ensuring the reliable operation of the trash net under extreme conditions.

Claims

1. A method for designing a trash screen based on water resistance, characterized in that: It includes the following steps: 1) Select the pollution interception mode according to the interception object, flow velocity, wind and waves; 2) Estimate and allocate the scale and specifications of the trash screen, and carry out mechanical calculation and analysis; and verify whether the design performance meets the design requirements; 3) The trash net is designed according to the corresponding mode. The trash net consists of the first plane net, the second net bag and the Nth net bag. The first plane net adopts a double-layer structure. All meshes are composed of ultra-high molecular weight polyethylene nets. The aperture size of the first-level mesh of the first plane net is 2500mm×2500mm, and the aperture size of the second-level mesh of the first plane net is 30mm×30mm. The second-level mesh is continuously tied to the first-level mesh by binding ropes to form a trash net. The force calculation of the first-level mesh adopts a combination of mesh resistance theoretical calculation and net model test regression estimation, and the larger value of the two is taken. When calculating its force, the water resistance of the net itself and the water resistance after the interception object is attached are taken into account. The mesh resistance calculation formula is: R=(1.8+2×(d / a-0.01)×β)×S×V 2 ; Where: R is the water resistance of the mesh; d is the diameter of the mesh wire; a is the length of the mesh foot; S is the area of ​​the mesh after contraction; V is the flow velocity; β is the angle between the mesh and the water flow; The secondary mesh is calculated based on the total tension on both ends of the main cables and the total number of meshes assembled on the upper line. The formula is: F = R / T; Where: F: force on each mesh; R: total tension on both ends of the main cable of the net bag or flat net; T: total number of meshes on each piece of trash screen; The second net bag and the Nth net bag are composed of buoys, flat pollution-blocking nets, ultra-high molecular weight polyethylene cables, and weighted anchor chains at the bottom of the net. The main cable on the water surface is set on the top of the net body and fixed by buoys. The underwater main cable is set at the bottom to fix the bottom of the net and the weighted anchor chain. Concrete anchor blocks are thrown at intervals of 10m and pulled by ultra-high molecular weight polyethylene cables. Each group of buoys is 10m apart horizontally, and the buoys and anchor blocks are pulled and fixed by cast steel anchor chains. Ultra-high molecular weight polyethylene cables are used to pull and fix the two sides near the diversion dike to fixed points on the shore; When calculating the force on the second net bag, the water resistance of the net itself and the increased water resistance after the interception object is blocked are taken into account. The force on the garbage net when working is similar to that on the stretched net. It is calculated according to the theoretical formula for calculating the water resistance of the net: The theoretical formula is: R = 1 / 2ρ × K × S × V2; Where: R is the water resistance of the net; ρ is the water density; K is the resistance coefficient; S is the area of ​​the net line; V is the flow velocity; The formula for calculating the maximum tension of the main cable is: F0 = F1 + F2 + F3... Where: F0 is the maximum tension of the main cable, F1 is the force between the main cable and the anchor pier, and F2 to countless are the resistance of each plane net or net bag connected to the main cable; 4) Select the anchoring system according to the corresponding mode; the anchoring system consists of an underwater pier, an underwater anchor block and an onshore gravity pier. The onshore gravity pier adopts a pile foundation platform in the form of four vertical pillars. Anchor rings are set on the side of the pillar and the bottom anchor block to connect with the pollution control net; 5) Select the buoyancy and sinking force equipment system according to the corresponding mode; the buoyancy and sinking force equipment system is to ensure that the net can still be in a normal interception state under the influence of marine organisms and wind and waves. At the same time, since the plane pollution interception net is in the shape of a catenary, the calculation formula according to the catenary and the net resistance coefficient is: T=(q×s 2 / (8×f))-(q×f / 2) Where: T: horizontal component of cable tension; q: load per meter of arc length; L: chord length of the cable; f: drape; S: length of the cable; K=R / V 2 Where: K: resistance coefficient; R: water resistance of the net; V: flow rate; 6) Carry out combined construction according to the anchoring system and buoyancy and sinking force equipment system.

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