Cutter Suction & Bucket Ladder Dredger: Output Calculation Formulas & Construction Parameters

2026-04-06 15:39:23     Category：Technical Resources     Browse number：370     Release time：2026-04-06 15:39:23

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Cutter Suction & Bucket Ladder Dredger: Output Calculation Formulas & Parameters

1.  Cutter suction dredgers are classified into four levels according to their total installed power.
   
   - Small cutter suction dredger:              total installed power   N < 5000kW;
   - Medium cutter suction dredger:          total installed power   5000kW≤ N <10000kW；  
   - Large cutter suction dredger:              total installed power  10000kW≤ N <20000kW； 
   - Ultralarge cutter suction dredger:     total installed power   N ≥20000kW .
   
   

   
   

2. The Output Calculation of a cutter suction dredger is defined in two ways:   excavation output and pumping/pipeline transport output.

The smaller value of the two represents the actual output.This is because the dredger operates with simultaneous cutting and discharge, which are mutually restrictive.

①Cutting output is mainly related to soil properties, cutter power,winch traversing power, etc.:

 W=60K×D×T×v

Where:

•    W: cutter cutting productivity (m3/h);

• ·  D: cutter advance distance (m);

• ·  T: cutting thickness (m);

• ·  v: cutter traversing speed (m/min);

• ·  K: cutting coefficient (related to actual cutting area), be typically taken as 0.8∼0.9.

   

②Pumping/Pipeline Transport Output
 This is primarily related to soil properties, pump characteristics, and pipeline characteristics. 

  W=Q·p

Where:

W: pump‑pipeline discharge productivity m3 /h ;

p: slurry concentration (calculated by volume concentration of in‑situ soil);

Q: operating flow rate of pump‑pipeline (m3 /h).

The slurry concentration can be calculated according to the following formula:

where:
Ym – density of slurry (t/m³);

ys – in-situ density of soil (t/m³);

Yw – density of local water (t/m³).

When a dredger operates at a new site, the optimum output shall be obtained through trial dredging, and the optimized operating parameters shall be determined, including dredge pump speed, cutter advance distance, cutting thickness, cutter rotational speed and traversing speed.

3.Dredging Flowchart of Cutter Suction Dredger

Cutter Suction Dredger Excavation → Slurry Pipeline Transport → To Reclamation Area

4. Operation Methods of Cutter Suction Dredger

The dredging methods of cutter suction dredgers include symmetric spud side-casting method, spud carriage side-casting method, three-wire positioning side-casting method, single spud with double anchors & four wires method, anchor-wire side-casting method, etc. The method shall be selected according to equipment performance and construction conditions. Dredging shall be carried out by segmental, layered and lane-by-lane processes.

For cutter suction dredgers equipped with carriage, double spuds and three-wire positioning system:

Three-wire positioning side-casting method shall be adopted in wide waters with heavy waves, soft soil, and low positioning accuracy requirements.

Spud carriage side-casting method shall be adopted in narrow waters, for rock/hard soil excavation, and dredging of foundation trenches with high accuracy requirements.

For cutter suction dredgers equipped with carriage, single spud and three-wire positioning system:

Three-wire positioning side-casting method shall be adopted in wide waters with heavy waves, soft soil, and low positioning accuracy requirements.

Single spud with double anchors & four wires method shall be adopted in narrow waters, for rock/hard soil excavation, and dredging of foundation trenches with high accuracy requirements.

5. Lane-by-Lane Dredging

① For spud side-casting and three-wire side-casting with anchor rod deployment, lane width shall be determined as follows:a. Under normal conditions, lane width equals the horizontal projection length from the spud/three-wire column center to the cutter front.b. Narrow the lane width appropriately in hard soil or high-velocity areas.c. Widen the lane width appropriately in soft soil or downstream construction.

② For spud side-casting and anchor-wire side-casting with anchor boat deployment, lane width shall be determined as follows:a. Under normal conditions, lane width is preferably 1.1 times the horizontal projection length from the spud/three-wire column center to the cutter front.b. Narrow the lane width appropriately in hard soil or high-velocity areas.c. Widen the lane width appropriately in soft soil or downstream construction.

③ For anchor-wire positioning side-casting, lane width shall not exceed 50% of the main anchor-wire length; it shall be reduced appropriately in mountain rivers with rapid current.

④ Minimum lane width shall be greater than the dredger's minimum dredging width, determined as follows:

a. If pre-dredging water depth < dredger draft: minimum width = maximum width avoiding bow corner collision with bank slope at dredge edge.

b. If pre-dredging water depth > dredger draft: minimum width = maximum width avoiding cutter ladder & traversing pulley collision with bank slope at dredge edge.c.

For symmetric spud side-casting with pre-dredging depth > draft: minimum width = swing width required for spud replacement during advance.d. For dredgers with rear wires, cross side-casting method is used to reduce minimum dredge width where water depth permits.

6. Segmental Dredging

① Segmentation shall be made based on effective length of dredger and floating pipeline when dredge length exceeds it.
② Segmentation shall follow turning points if dredge edge is polygonal.

③ Segmentation shall follow dimensional changes and schedule requirements for different dredge specifications.
④ Segmentation shall follow soil changes where construction method/parameters differ significantly.
⑤ Segmentation shall follow agreed avoidance measures to reduce navigation/construction interference.

7. Layered Dredging

① When dredge thickness exceeds single-pass suitable thickness, layer thickness is determined by dredger performance, soil and operation method:

Silt & loose sand: 1.5–2.0 × cutter diameter

Soft clay & dense sand: 1.0–2.0 × cutter diameter

Hard clay: 0.75–1.0 × cutter diameter

Soft rock: 0.3–0.75 × cutter diameter

② If pre-completion mud surface is too high, first layer thickness (excavated at high tide) is determined by draft, excavation capacity and tide height.

③ For stepped excavation on stable slopes steeper than dredge slope, layer thickness = integer multiple of step height.

④ Layer by contract staged depths if required.

⑤ If max dredging depth is insufficient (only reachable at low tide), layer by high-tide depth or suitable low-tide thickness.

⑥ For long-duration projects with thick mud & back-siltation, bottom layer thickness shall maintain high productivity and clearance quality.

8. Dredging Direction

① Favorable for pipeline layout and shortening floating pipeline; floating/land/underwater pipeline joints behind the dredger.
② Upstream in slow current; downstream in high current with hard soil.
③ Upstream for anchor-wire side-casting.④ Avoid cross-current construction.

9. Positioning & Anchoring

① Mark starting point on navigation chart or by beacons/buoys; prepare discharge pipeline before mobilization.
② Vessel entry direction preferably upstream.

③ For spud positioning:Tug slows/stopping near starting point; zero ground speed before lowering one spud and setting traversing anchor.Spud lowering while moving is prohibited.

④ For anchor-wire side-casting:Set windward/upstream anchor first, or lower cutter ladder to mud bed to fix position, then deploy other wires.Inland upstream construction: set upstream bow anchor first.

⑤ For three-wire positioning:Lower ladder or deploy boat anchor to control position; stern anchor on dredge centerline.For five-anchor five-wire: side anchors symmetric on both sides, wire angle 90°–150°.

⑥ For spud positioning:Anchor position & moving distance based on soil & current;traversing wire angle with centerline ≥ 45° under normal conditions.

⑦ Connect and adjust discharge pipeline; no sharp bends in floating pipeline.

⑧ Verify position before dredging: positioning center on dredge axis, cutter at starting point.

10. Applicable Soil Types for Various Cutter Heads of Cutter Suction Dredgers

 

(1) For soft soil types such as silt, soft plastic clay, and loose sand, select a crown-shaped flat-blade cutter with a larger front-end diameter.

(2) For soft plastic clay, plastic clay, and peat, select a crown-shaped square-tooth cutter with a larger diameter.

(3) For hard plastic clay, medium-dense and dense sand, crushed stone, and cobblestone, select a crown-shaped cutter with a smaller diameter and replaceable teeth, equipped with chisel teeth.

(4) For rock, select a conical rock-excavating cutter with replaceable teeth, equipped with pointed teeth.

(5) A bucket wheel can also be selected for hard plastic clay.


11. Bucket-ladder dredgers are classified into two categories based on their hourly production rate: small and large. Those with a production rate ≤ 500 m³/h are small bucket-ladder dredgers; those with a production rate > 500 m³/h are large bucket-ladder dredgers.


12. The hourly production rate during the operation time of a bucket-ladder dredger can be calculated using the following formula:

Where: 

W – hourly output of bucket ladder dredger during operation (m³/h);

n – bucket chain speed (buckets/min), values in Table 2.9-5 may be used;

c – bucket capacity (m³);

fm – bucket filling factor, values in Table 2.9-6 may be used;

f₀ – ladder inclination factor, may be obtained from Figure 2.9-6 according to actual dredging depth;

B – soil loosening factor, values in Table 2.9-7 may be used.

13.When dredging with bucket dredgers and slurry discharge vessels, mud barges should be selected according to the dredging conditions. When disposing of mud overboard (water disposal), bottom-dump barges should be used; for cohesive soils, bottom-dump or split-hull barges with steeper bulkheads are preferred; when discharging slurry, full-bottom (hopper) barges are preferred; for offshore disposal, self-propelled bottom-dump or split-hull barges are recommended.

Calculation of the required number of mud barges:

Where:
N – number of barges equipped;
l₁ – distance from dredging area to disposal area (km);
v₁ – speed under load (km/h);l₂ – distance from disposal area to dredging area (km);
v₂ – speed in ballast (km/h);t₀ – total time for loading, dumping, turning, and approaching/leaving the dredger (h);
W – hourly output of the dredger during operation (m³/h);
q₁ – carrying capacity of barge (m³);
n_B – number of spare barges;
B – soil loosening factor, to be determined in accordance with Table 2.9-7.

Calculation of required number of tugs:

T —— Required number of tugboats;

D₀ —— Number of mud barges that can be towed by a tugboat in a single trip.

14. Main Process Flow Chart of Bucket-ladder Dredger

[Bucket Ladder Dredger Dredging] ──▶Loading──▶ [Hopper Barge] (Self-propelled / Towed) ──▶ [Dumping at Disposal Area] │ ──▶ [Empty Hopper Barge] │ ──▶│ Return Voyage │


15. The main dredging methods for bucket-ladder dredgers include: oblique transverse excavation, fan-shaped transverse excavation, cross-shaped transverse excavation, and parallel transverse excavation, etc. Dredging adopts construction techniques such as strip division, sectioning, and layering.

The dredging methods of bucket-ladder dredgers shall comply with the following provisions:

(1) When the water conditions in the dredging area are good, and the dredger is not restricted by the width of the excavation trench or the edge water depth, the oblique transverse excavation method should be adopted.

(2) When the excavation trench is narrow and the water depth at the edge of the trench is less than the draft of the dredger, the fan-shaped transverse excavation method is preferred.

(3) When the water depth at the edge of the trench is less than the draft of the dredger, and the width of the trench is less than the length of the dredger, the cross-shaped transverse excavation method is preferred.

(4) When the flow velocity in the dredging area is relatively high, the parallel transverse excavation method can be adopted.

16. Strip division dredging of bucket-ladder dredgers:

(1) When the width of the excavation trench exceeds the maximum excavation width of the dredger or the thickness of the mud layer in the trench varies significantly, strip division should be carried out. The width of the strip depends on the laying length of the main anchor cable, generally taken as 100m. When dredging in shallow water areas, the minimum width of the strip must meet the operational needs of the dredger and the berthing needs of the mud barge.

17. Sectioning dredging of bucket-ladder dredgers:

(1) When the length of the excavation trench is greater than the length that the dredger can excavate in a single main anchor laying, the trench should be sectioned according to the excavatable length.

(2) When the boundary of the excavation trench is a polyline, sectioning should be carried out according to the inflection points of the boundary.

(3) When the specifications of the excavation trench are inconsistent or the construction period requirements are different, sectioning should be carried out according to the changes in trench specifications and construction period requirements.

(4) When sectioning construction can avoid or reduce navigation and other construction interferences, sectioning should be carried out according to the agreed avoidance methods.

18. Layering dredging of bucket-ladder dredgers:

(1) When the thickness of the mud layer is greater than the suitable thickness for one-time excavation, layering should be carried out. The layer thickness is determined according to the soil type and bucket height, generally not exceeding 2 times the bucket height.

19. Bucket-ladder dredgers generally lay out 6 groups of anchor cables during construction, including 1 bow anchor, 1 stern anchor, and 1 left/right bow and stern side anchor each; when constructing against the current with high and stable flow velocity, the stern anchor may not be set. The laying of anchors shall meet the following requirements:

① The bow anchor should be laid on the dredging centerline. The length of the bow anchor cable should be determined according to the cable capacity and site conditions, and should not be less than 500m. When the area through which the bow anchor cable passes is water, a cable-supporting barge should be set; when it is a beach, a cable-supporting roller should be set.

② The stern anchor should be laid on the dredging centerline. The cable length is determined by the flow direction: 100-200m for upstream construction, and appropriately lengthened for downstream construction.

③ The left and right side anchors should be laid out symmetrically.

20. The dredging process parameters of bucket-ladder dredgers should be selected according to the following principles:

① Determine the excavation thickness per pass according to the bucket height and soil type, generally 1 to 2 times the bucket height. Increase appropriately for soft soil layers, and decrease appropriately for hard clay, sand, and stone.

② Determine the advance distance (swing) according to the bucket height and soil type, and refer to the layer thickness and transverse speed. The advance distance is larger for soft soil and smaller for hard soil.

③ Determine the transverse speed according to the cutting characteristics of the dredged soil, cutting thickness, advance distance, and bucket chain running speed, generally controlled at 6-8m/min.

④ Determine the bucket chain running speed according to the excavated soil type and water flow. Appropriately increase the bucket speed when digging silty soil and sandy soil; appropriately reduce the bucket speed when the flow velocity is high, digging loose sand, when there are obstacles such as garbage, debris, and large stones in the trench, or when digging hard clay, dense sand, gravel, and other hard soils, and when mud does not easily fall off the bucket walls.