A dipper dredger is a floating dipper excavator on water. Its dipper excavating machinery and equipment are basically the same as those of land-based dipper excavators.
Small and medium-sized ones are generally non-self-propelled, while large-sized ones are developing towards self-propulsion.
1. Basic Principle
1.1 Equipment Attributes: Mechanical dredger. According to the slewing device, it can be divided into full-slewing chamber type, semi-slewing chamber type and turntable type; according to the excavation direction, it can be divided into forward dipper and backhoe dredgers.
1.2 Core Structure: It mainly includes steel piles, steel pile trolleys, big arm, small arm, dipper, backhoe cab, slewing device, etc.
1.3 Working Flow: Although there are many types of dipper dredgers, their basic working principles are the same:
1.3.1 Full hydraulic drive rotary excavator (mostly backhoe type):
During operation, the dipper is extended into water by using the excavator boom and dipper handle, and the dipper stick is pushed by the thrust or tension of the hydraulic cylinder, so that the dipper is accurately placed at the excavation position and cuts into the soil for excavation.
After the excavation action is completed, the hoisting mechanism and the luffing mechanism work together —— the dipper is lifted by retracting the hydraulic cylinder, and at the same time, cooperating with the luffing action of the boom (in actual operation, especially when excavating hard soil layers, the method of "synchronous lifting and luffing" is usually adopted to obtain the maximum cutting force), so as to maximize the cutting force and lift the dipper full of sediment out of the water to an appropriate height. Subsequently, the slewing device is used to turn the excavator to the unloading place or above the adjacent barge, and the hydraulic mechanism is used to open the bottom door for unloading.
The empty dipper then slews back to the excavation position, and the cycle is repeated.
1.3.2 Old traditional steel cable forward dipper:
Dipper extends into water → push dipper handle to excavation position → tighten steel cable to make dipper cut into soil for excavation → tighten hoisting and luffing steel cables to lift dipper → slewing device turns to unloading place → open bottom door for unloading → dipper slews back to excavation position for cyclic operation.
2. Technical Performance
Core Parameters: The key parameters are bucket capacity (mainly 1～40m³) and digging depth; large ships have strong adaptability to sea conditions.
Performance Characteristics: Strong excavation capacity, high positioning accuracy of steel piles, small water area occupation; limited digging depth (forward dipper ≤15m, backhoe ≤25m, world's largest backhoe bucket capacity 40m³, maximum digging depth 32m).
3. Output Calculation
The Output Calculation of a dipper dredger is generally measured by its dipper capacity. The bucket capacity ranges from 1~30m³. Till today, the world's largest backhoe dredger has a maximum bucket capacity of 40m³ and a maximum dredging depth of 32m.
The operating hourly productivity of a dipper dredger can be calculated as follows:
Where:
W —— Operating hourly productivity of dipper dredger (m³/h)
n —— Number of buckets excavated per hour
c —— Dipper capacity (m³)
B —— Soil rock loosening coefficient, which can adopt the values:
The value of the loosening coefficient (also known as bulk factor / swell factor) depends on the type of rock and soil.
Common references are as follows:
Silt & clay: 1.1 – 1.3
Sand & silt: 1.1 – 1.2
Gravel & cobble: High due to large particles and abundant voids, generally 1.4 – 1.6
Rock or highly weathered rock: May reach 1.5 – 1.8 or higher
fm —— Dipper filling coefficient, which can adopt the values in Table below:
| Soil Type | Bucket Filling Coefficient fm |
|---|---|
| Sand | 0.90 |
| Medium Clay | 0.72 |
| Gravel | 0.60 |
| Stiff Clay / Hard Clay | 0.40 |
| Pre-treated / Fragile Rock | 0.33 |
| Weak Rock | 0.30 |
Note: When the thickness of the dredged soil layer is thin, the dipper filling coefficient can be appropriately reduced.
4. Main Dredging Instruments and Monitoring Systems
Core Instruments: Dredging depth indicator, Spud Carriage forward movement distance indicator, dredging slewing angle indicator, etc.
Monitoring System: Divided into basic type and extended type, the system composition is consistent with that of grab dredger.
5. Construction Technology
5.1 Overall Process
Dipper excavates dredged soil → load into barge → barge transports to designated area for mud dumping.
The dredged soil is loaded into the barge by the dipper of the dipper dredger, and then the barge dumps the dredged soil to the designated mud dumping area.
5.2 Commencement and Positioning
Mark the construction starting point on the navigation chart in advance or indicate it with leading marks and buoys;
After the dredger arrives at the starting point, lower the steel piles or dipper arm to fix the ship position, and then accurately position it through the steel piles and dipper arm.
5.3 Construction Method
Core Method: Longitudinal excavation method (forward dipper dredges forward along the dredging trench, backhoe dredges backward along the dredging trench);
Auxiliary Technology: Adopt strip and layered excavation according to construction conditions.
5.4 Technical Requirements
Strip dredging: Adopted when the width of the dredging trench exceeds the single excavation width. The strip width is determined by the slewing radius and slewing angle of the dipper (reduce the slewing angle for hard soil, increase for soft soil).
Layered dredging: Layered excavation shall be adopted when the soil layer is too thick. The layered thickness shall be determined according to the bucket height and excavated soil quality. The layered thickness for soft soil should be thick, and for hard soil can be thin. The layered thickness generally should not exceed 1.8~2.0 times the bucket height.
Special Soil dredging:
Hard soil and weathered rock: Adopt synchronous excavation and dipper lifting construction method;
Soft soil and projects with high flatness requirements: Adopt excavation braking and dipper lifting construction method.
Parameter Determination: Dredging parameters such as ship lifting height, slewing angle, dipper slewing angle advance and dipper forward movement distance for different soils shall be determined through trial excavation.
5.5 Dipper Selection
Small-capacity toothed dipper: Suitable for hard plastic clay, dense sand, medium dense gravel, weathered rock.
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