I. Overview
Construction of river pipe, river core water head and shore well connection pipe, sewage discharge pipe to the water system, long-distance river bottom or submarine water pipe, etc., should be based on the length of the underwater pipeline, the depth of the water system, the flow rate of the water system, the soil quality of the bottom, and the shipping requirements. Factors such as the age of the pipeline, tides and wind and waves, etc., choose the appropriate construction method. Generally, the method of laying underwater pipelines is the cofferdam method and the underwater pipe laying method.
According to the technical and economic comparison, the water system is shallow, the flow rate is small, the shipping is infrequent, and the building materials can be taken locally. When the water pollution can be controlled within the allowable range, the cofferdam method is generally adopted. However, when the above conditions are not met, the underwater pipe laying method is often used.
Underwater pipelines are generally available in two ways:
1. Laying on the bottom of the water;
2. Buried in the groove under the water;
When buried in the groove under the water, the depth of the top of the pipe is generally 3-4 times of the pipe diameter to avoid the impact of ship anchoring and riverbed erosion. Buried laying of submarine pipelines should also prevent the pipeline from floating or sinking during storms. For this reason, the pipeline should be buried deep enough under the seabed. In addition, if the waterway is deep, the pipelaying above the bottom of the water will not affect the shipping, the bottom of the water is flat, there are no obstacles and suspended ground along the pipeline, the pipeline will not be anchored by the vessel, fluid power, soil liquefaction, bed bottom soil movement, riverbed erosion or other If the cause causes damage, the pipe can be laid directly on a stable riverbed or seabed.
The seismic performance of pipelines directly laid on the bottom of the water is better than that of underwater trenches.
Second, underwater trench excavation
There are generally three cases of pipe underwater trenching:
1. First bury the pipe after grooving;
2. The trenching and the buried pipe are carried out simultaneously;
3, first put the pipe and sink into the soil.
The choice of these methods depends on factors such as bottom soil quality, water system width and depth.
The advantage of the buried pipe after the trenching is that the construction equipment is simple; the disadvantage is that the pipeline positioning is not easy to be accurate, the flatness of the groove bottom is poor, the groove collimation is low, and the siltation is easy. Therefore, when using this method, the groove bottom width and the excavation depth should be appropriately selected. The bottom width is generally 0.8-1.0 m of the outer diameter of the tube. The excavation depth is determined according to the back-sludge condition, and the slope coefficient is 1:2—1:4. The back siltation of the clay river bed is not serious, and the sand is quickly decanted. Commonly used underwater trench excavation methods and equipment include blasting method, shore shovel, dredger, high pressure pump and the like.
Blasting excavation: suitable for rocky riverbeds;
Shore rope shovel: suitable for narrow water systems. The bucket is hoisted by the shore hoist. The bucket is pulled up the road and the slide is lowered as the depth of the digging increases. This method can control the planar position and collimation of the groove more accurately.
Dredgers and high-pressure pumping vessels: a wide-scale water system generally using a grab-type or multi-dump dredger to excavate underwater trenches. The earthwork is unloaded on the downstream side of the trench flow or transported by barge to a distant place; The soil is loose, and the high-pressure pump can be used to blast the bottom soil layer for excavation.
When excavating by ship or other floating equipment, dredgers should be temporarily anchored to ensure accurate center position of the groove. The underwater groove centerline is displayed with a shore mark or buoy and measured with a theodolite or laser collimator. When conditions permit, the centerline of the pipe can be drawn between the two shores. The position of the underwater groove and the elevation of the bottom of the groove can be measured with a ruler or a hammer ball.
In order to prevent the effect of back silting, a construction method of segmental excavation pipe laying can be adopted to shorten the underwater drying time.
Third, the underwater laying of the pipeline
1. Floating and towing pipe laying:
The method of floating and towing the pipe is to first connect the pipe to a pipe length of a certain length at the shore, block the plate at both ends of the pipe section, float to the pipe laying position, fill the pipe into the pipe, sink to the bottom or the groove, and remove the block. The plate is then interfaced between the various pipe sections under water.
If the water system is shallow and there is a deep shore, the height difference between the shore and the water surface is not large. The pipe section can be prepared on the original ground of the extension line of the river crossing center; or the height difference between the shore and the water surface is large, the bank must be excavated. Side, reduce the height difference with the water surface, and reduce the groundwater level in the excavation area before preparing the pipe section. The prefabricated pipe section is floated by a vessel or by a traction device (winding machine, tractor, etc.) placed on the opposite side.
However, in most cases, the prefabricated pipe sections on the shore are parallel to the water system. After the pipe sections are prepared, they are loaded with buoys and pushed into the water, and are towed by the ship float on the water surface.
Both ends of the tube are blocked by flange bolts. A venting hole and a water inlet hole having a diameter of 1/2 〞 - 1 设有 are provided on the blocking plate.
The pipe section floats from the surface of the water to the top of the groove, and is placed in the groove by the positioning crane, and the pipe section is placed in the groove.
The underwater positioning and interface of the pipe section are operated by the diver. The diver's communication tool is in contact with the positioning crane vessel, and the positioning vessel anchoring position and the ship's boom operation are adjusted to make the sinking pipe section and the laid pipe section opposite.
2, underwater towing pipe
When the pipe section with a large length is difficult to float, it can be dragged under the water. When hauling, it is less affected by wind, waves and tides. The operation is safe and there is no need to divert the boat, but the hauling horsepower is large. Applicable to long-distance deep-water pipelaying, such as laying sewage pipes in the sea.
If the pipeline is prefabricated, the pipeline should be interfaced one by one during the towing process to increase the drag length. Pipes can be towed up to tens of meters in length.
3, pipe laying ship pipe
The pipe is transported to the pipe-laying vessel by means of a transport pipe, and after the pipe joint is placed on the pipe-laying vessel, the water slide is taken along the slideway, pipe bracket and the like on the pipe-laying vessel. This method is suitable for pipe laying work of long distance pipe sections away from the shore.
4, rushing through the soil layer pipe
The quality of the pipe sections laid on the bottom of the water, such as the method of pre-grooving, the positioning of the pipeline, the collimation of the grooves, and the smoothing of the bottom of the ditch are not easy to guarantee. In order to avoid the disadvantages caused by pre-grooving, the pipe laying method can be used.
The pipe-laying method is to first place the pipe on the bottom of the water, and then use the mud to shoot the high-pressure water to the bottom layer of the pipe, so that the bottom of the pipe is liquefied, the bearing capacity is lost, and the pipe is buried in the bottom. The thickness of the liquefied soil layer is generally 3-4 times the diameter of the pipe.
A prerequisite for this method is that the subsoil layer can be liquefied.
5, integrated work ship laying pipe
The disadvantage of the method of pre-drilling underwater trench pipelaying is that the groove may be back-sludged, and the work of spoil and backfill is large. In order to overcome this shortcoming, a comprehensive work boat can be used.
The groove is pumped by high-pressure water from the pump, and the excavated soil is pumped up by the sand pump to the backfilling groove, which minimizes the groove drying time and eliminates the long-distance handling of the backfill.
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