Grab dredgers are classified into self-propelled and non-self-propelled types.
Self-propelled dredgers are suitable for short-distance self-mobilization. Some models are equipped with a hopper to store dredged material, which is then discharged at a designated disposal site after the hopper is full (rarely used in China).
Non-self-propelled dredgers rely on barges for loading and unloading mud, and are the most commonly used in engineering projects.
By rated bucket capacity:
Small-sized: < 8 m³
Medium-sized: 8–20 m³
Large-sized: > 20 m³
At present, the world’s largest grab capacity reaches 200 m³, and the maximum dredging depth is 30 m.
2. Structural Components
A grab dredger mainly consists of a crane system, grab, boom, moving winch, and anchor cables.
Crane system: Main power unit, with three core functions: lifting/closing the grab, rotating, and luffing the boom.
Grab: Classified by shape into clam-shell and orange-peel types; by weight into light, medium, heavy, and extra-heavy types to suit different soils.
Boom: Normally truss or section steel structure, supporting grab wire ropes (opening, closing, and lifting ropes). It rotates 360° with the crane platform and can luff; equipped with an inclinometer, the safe pitching angle is 45°–57°.
Moving winch and anchor cables: Used for vessel positioning, moving, and barge handling. Installed on the middle deck, sides, or fore/aft decks, equipped with wire rope drums and chain drums.
3. Basic Working Principle
Grab dredgers are mechanical dredgers that operate using the gravity of the free-falling grab.
Operation flow:Positioning → Setting dredging depth → Lowering and digging → Lifting and rotating → Loading into barge → Moving to next position after reaching target depth.
Working sequence:The grab is lowered by wire rope to the preset underwater depth, penetrates the mud layer by gravity, and closes to grab sediment. The crane lifts the full grab above water, swings it over the barge, and opens to discharge mud. The empty grab returns to the digging point and repeats the cycle.
4. Technical Performance
Strong adaptability: Dredges silt, common clay, loose sandy soil (N < 15), as well as stiff clay, sandy soil with stones, and gravel (N = 25–40).
Adjustable dredging depth by changing the length of lifting and closing wire ropes.
Simple structure, few wearing parts, and relatively low vessel cost.
Key parameters: bucket capacity, dredging depth, grab lifting speed; production capacity is mainly rated by bucket capacity.
5. Productivity Calculation
Hourly productivity formula:W = ncf / B
Where:
W: Hourly productivity of grab dredger (m³/h)
n: Number of grabs per hour
c: Grab volume (m³)
f: Soil swell factor:
Silt: 1.2–1.5
Sand or sandy clay: 0.9–1.1
Stony soil: 0.3–0.6
B: Coefficient
Details pls refer to : Grab Dredger Production Calculation
https://www.yanyangmarine.com/news/grab-dredgers-operation-procedure-and-production-calculations.html
6. Dredging Instruments and Monitoring System
6.1 Main Dredging Instruments
Automatic grab depth control, depth indicator, grab opening meter, lift meter, load meter, crane rotation angle indicator, etc.
6.2 Dredging Monitoring System
Basic type: Dredging track and profile display, equipment control and monitoring, alarm system, instruments and meters.
Extended type: Includes all basic-type functions, plus automatic dredging control and data recording system.
7. Vessel Positioning and Moving Methods
7.1 Anchor Cable Positioning
Standard layout: 4 sets of anchor cables (2 bow, 2 stern), arranged in a figure eight pattern symmetric to the dredging trench.
Bow cables: 200–300 m
Stern cables: 150–200 m
All anchors are laid about 100 m outside the trench edge.
Moving by winching anchor cables; each shift: 30–50 m. Keep the hull straight and control movement by guides or onboard navigation.
Lengthen cables in strong current or hard bottom; avoid the grab catching cables.
7.2 Pile Positioning
Used in restricted waters, high precision jobs, or where anchors are unsuitable.Move by alternately lowering/lifting bow and stern piles, assisted by grab swinging. Control position using onboard navigation and heading data.
8. Full Construction Procedure & Key Points
8.1 Construction Preparation
Check main and auxiliary engines; test dredging to verify grab condition. Calibrate positioning instruments and depth indicators. Import construction documents.
8.2 Mobilization and Positioning
Tow the dredger to the site, verify water depth against drawings, then position. Lay anchors or lower piles according to current and wind.In deep, fast, or windy conditions: first position in calm shallow water, then winch into position using tugs. For berth dredging, dock first then position.
8.3 Typical Trial Construction
Conduct trial dredging in a representative section to determine: grab arrangement, overlapping, forward movement, layer thickness, etc. Optimize the scheme, check efficiency and barge allocation.
8.4 Construction Method
Normally longitudinal digging. Use trenching, staging, layering, downstream or upstream dredging as needed.
Prefer downstream dredging: hull parallel to trench axis, bow toward digging direction.
Upstream dredging allowed in weak or tidal currents.
8.5 Trenching, Staging, and Layering
Trenching: When trench width exceeds maximum working width.
Max width ≤ 2 × effective crane radius.
In strong, deep current: trench width ≤ vessel width.
Overlap between strips ≥ 15 × layer thickness; increase for loose/soft soil or long intervals.
Staging: When trench length exceeds one anchor layout.
Downstream: 75% of initial bow cable length.
Upstream: 60% of initial bow cable length.
Normal stage length: 60–70 m.
Layering: When mud thickness exceeds single-pass capacity or for tide-dependent work. Follow thick upper layers, thin lower layers.
2 m³ grab: 1.0–1.3 m
8 m³ grab: 1.5–2.0 m
Reduce for hard soil; use layering for steep or complex slopes.
8.6 Dredging Control
Forward movement: 0.5–0.8 × fully opened grab width. Keep hull centerline parallel to trench.
Grab spacing: wider for soft soil/thin layers; narrower for hard soil/thick layers.
Overlap: reduce for clay/dense sand (under filling); increase for thick soft soil (over filling).
Correct grab position in strong current; use heavier/larger grabs if needed.
Keep anchor cables tight to avoid shifting, missing, or overlapping.
Use depth markers or automatic depth control; use precision depth control for the final layer in high quality projects.
8.7 Slope Dredging
If not specified, excavate in steps following over-dig lower, under-dig upper, balance over under; over-under ratio = 1.0–1.5. No shallow spots allowed.
8.8 Grab Selection
Silt, soft clay, loose sand: light large-capacity flat-mouth grab
Medium plastic clay, medium-dense sand: medium grab
Stiff clay, dense sand, medium-dense gravel: heavy full-tooth grab
Weathered rock, dense gravel: extra-heavy grab
9. Advantages
Strong adaptability to various water and soil conditions, high flexibility.
Simple construction technology and easy operation.
Compatible with multiple grab types for different soil hardness.
Simple equipment, few wearing parts, low cost.
Adjustable dredging depth via wire rope length.
10. Disadvantages
Low efficiency: limited volume per grab; silt removal rate ≈ 30%. Repeated passes needed for large areas, time-consuming.
Poor sensitivity to soft surface mud: tends to dig hard subsoil while leaving surface mud, affecting stability.
Disturbs sediment, causes secondary pollution and floating mud; hard to avoid missed areas, impairing flood discharge and water quality.
Complicated mud treatment (dewatering, solidification) and high transportation cost.
FAQs
Q:What are the main positioning methods of a grab dredger?
A: The main positioning methods are anchor cable positioning and steel pile positioning. Anchor cable positioning uses four sets of figure-eight anchor cables; steel pile positioning is suitable for restricted waters and high-precision construction.
Q:How to select the appropriate grab for different soil conditions?
A: Light large-capacity flat-mouth grabs are used for silt and loose sand; medium grabs for medium plastic clay; heavy full-tooth grabs for stiff clay and dense sand; extra-heavy grabs for weathered rock and dense gravel.
Q:What are the main advantages and disadvantages of grab dredgers?
A:Advantages include strong soil adaptability, simple equipment, low cost and flexible construction. Disadvantages include low dredging efficiency, easy disturbance of sediment, secondary pollution and high sludge treatment cost.
Copyright © Zhenjiang Yanyang Engineering Co., Ltd. sitemap
This website uses cookies to ensure you get the best experience on our website.
Inquire
Phone
Comment
（0）