Grab Dredgers: Operation Procedure and Production Calculations
Author: Yanyang Engineering Technical Team
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1. Basic Principles
The grab dredger is a type of mechanical dredger that utilizes a crane mounted on the vessel to operate a grab bucket as the underwater excavation tool. Grab dredgers come in diverse configurations and have a wide range of applications, with the majority being non-self-propelled vessels. Despite the variety of designs, the fundamental working principle remains the same. A mechanical device for underwater excavation and grabbing of sediment is installed on the hull of the dredger. The grab bucket, suspended by steel cables, relies on the gravitational force of its free-fall to plunge to a certain depth underwater. By inserting the bucket into the sediment layer and closing it, the dredger excavates and grabs the mud and sand. Subsequently, the crane rotates, lifting the bucket filled with sediment above the water surface and swinging it to a predetermined position above a mud barge moored alongside the dredger. The bucket is then opened to discharge the sediment directly into the barge. Once emptied, the crane rotates the bucket back to the excavation point to begin the next cycle of dredging operations. This process is repeated continuously. A schematic diagram of the basic structure of a grab dredger is shown in the figure.
2. Technical Specifications
Grab dredgers are widely used, with their main technical parameters including bucket capacity, dredging depth, and grab bucket hoisting speed. By adjusting the length of the hoisting and opening/closing steel cables, grab dredgers can meet the requirements of different excavation depths during construction. Furthermore, compared to other types of dredgers, grab dredgers have simpler equipment, fewer wearing parts for the dredging machinery, and a lower construction cost for the vessel. The production capacity of a grab dredger is generally measured by the bucket capacity. Currently, the largest grab dredgers in the world have a bucket capacity of up to 200 m³ and a maximum dredging depth of 30 m. Based on the designed bucket capacity, grab dredgers are classified into three grades: small grab dredgers (designed capacity < 8 m³), medium grab dredgers (designed capacity 8–20 m³), and large grab dredgers (designed capacity > 20 m³).
3. Productivity Calculation
The excavation volume of a grab dredger does not solely rely on the power of the vessel but primarily depends on the impact force of the falling bucket and the depth of penetration. Therefore, grabs of different types and weight coefficients should be selected according to the soil type.
The productivity of a grab dredger is calculated using the following formula:
Basic Output Calculation Formula (Hourly Productivity)
Where W — Hourly productivity of the grab dredger (m³/h);
n — Bucket cycle rate per hour (buckets/hour), also known as “bucket frequency”;
C — Grab bucket capacity (m³);
fm — Soil loosening coefficient (for silt, take 1.2–1.5; for sand or sandy clay, take 0.9–1.1; for rocky soil, take 0.3–0.6).
B —Soil loosening coefficient (also known as the loosening factor, typically taken as 1.1 to 1.3; indicates the expansion ratio of soil volume after excavation)
In Yanyang Marine actual engineering practice, W typically refers to the natural volume of excavated material (calculated based on the original soil surface volume), hence it must be divided by the loosening factor B.
Daily Production Volume Calculation Formula
Qd =W×T×η
▶ Qd : Daily output (m³/day)
▶ W: Hourly productivity (m³/h)
▶ T: Daily effective operating hours (e.g., 20 hours)
▶ η: Time utilization rate (accounting for downtime, vessel relocation, slurry discharge waiting, etc.; typically ranges from 0.55 to 0.85)
Example :
Bucket volume C=13 m³
Buckets per hour n=12
Filling coefficient fm=1.0
Loosening coefficient B=1.1
Then W=12×13×1.0/ 1.1≈141.8 m³/h
Assuming 20 hours of daily operation with 60% time utilization → Daily output ≈ 1702 m³
Practical calculation: 140 m³/h × 20h X 0.6 = 1700 m³/day (rounded for simplicity)
Comprehensive Production Capacity (More Realistic)
When the daily output is constrained by the barge round-trip cycle, it must be verified using the "Split Hopper Barges Turnover Method":
▶ q : Loading capacity of a single split hopper barge
▶ N : Number of split hopper barges
▶ K: Working hours per day (e.g., 24 or 20)
▶ tcycle: Round-trip cycle time** (including sailing time and unloading time)
Note: If the dredger's production capacity exceeds the transport capacity of the barges, the actual output is determined by the barges.
4. Main Dredging Instrument Configuration and Monitoring System
The main dredging instruments on a grab dredger include:
--automatic grab depth control devices,
--grab depth indicators,
--grab opening meters,
--grab hoisting meters, grab load meters,
--grab crane rotation angle indicators.
Additionally, with the development of information technology, cutter suction dredgers are also equipped with dredging monitoring systems. Depending on the complexity of functions and technical level, the grab dredger dredging monitoring system can be divided into a basic type and an extended type.
-- Composition of the Basic Dredging Monitoring System
The basic dredging monitoring system consists of the following subsystems:
(1) Dredging trajectory and profile display system.
(2) Equipment control and monitoring system.
(3) Monitoring and alarm system.
(4) Dredging instruments and meters.
-- Composition of the Extended Dredging Monitoring System
The extended dredging monitoring system includes all the subsystems of the basic type, as well as the following additional subsystems:
(1) Automatic dredging control system.
(2) Dredging data recording system.
5. Working Procedure of Grab Dredger
The grab dredger loads the dredged material into self-propelled mud barges using its grab bucket. The barges then transport the dredged material to the designated dumping area. The main technological process of the grab dredger is shown in the figure：
▶ Grab Dredger Excavation
↓（Loading）
▶ Mud Barge Loaded
↓（Self-propelled or Towed）
▶ Disposal Area – Mud Disposal
↓（Return Voyage）
▶ Empty Mud Barge
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