Construction Technology of Trailing Suction Hopper Dredger - Zhenjiang Yan Yang Engineering Co., Ltd.

Dredging Operation Techniques of Trailing Suction Hopper Dredger

I. Basic Concepts of Dredging and Reclamation Works

Dredging works refer to underwater earth–rock excavation works conducted by hydraulic or mechanical means to widen and deepen water areas.

Capital Dredging: Dredging with new–construction, reconstruction or expansion nature for newly built waterways, ports, etc., or to increase their dimensions and improve navigation conditions.

Maintenance Dredging: Dredging to remove bottom sediments so as to maintain or restore the designed dimensions of a specified water area.

Reclamation works mean the operation of conveying sediment excavated by a dredger to a designated site for filling through a discharge pipeline.

II. Basic Performance and Classification of Trailing Suction Hopper Dredgers

Main technical parameters include hopper capacity, dredging depth, sailing speed, installed power, etc. The dredger features self–propulsion, self–dredging, self–loading, self–dumping and self–discharging. It operates under way, without occupying a large water area or closing the waterway, with little impact on navigation and convenient mobilization.

Classification by Hopper Capacity：

Small–sized: q < 4,000 m³

Medium–sized: 4,000 m³ ≤ q < 9,000 m³

Large–sized: 9,000 m³ ≤ q < 17,000 m³

Ultra–large–sized: q ≥ 17,000 m³

Main Dredging Instruments and Meters

Pressure gauge (vacuum gauge), velocity meter, densitometer, loading indicator, draghead depth–position indicator, etc. The dredging monitoring system is divided into basic type and extended type.

III. Standard Dredging Processes

1. Hopper Loading Process

Positioning and lowering draghead → Sailing on line in light condition → Dredging and loading → Full loading and hoisting draghead → Sailing in full load → Dumping sediment → Returning to construction area → Next cycle

2. Reclamation Process

Sand excavation and loading in borrow area → Sailing to reclamation area → Anchoring and positioning → Connecting bow–discharge pipeline → Pumping for reclamation → Reclamation completion → Disconnecting pipeline → Evacuating

3. Clay Bow–Discharge Process

Soil breaking and dredging → Layered and interval loading (sand–clay–sand–clay–sand) → High–pressure water mixing in hopper → Stable bow–discharge conveying → Reclamation ashore

4. Shallow–Spot Removal Process

Locating shallow areas → Calibrating sailing line → Fixed–depth dredging → Pressurized excavation → Inspection → Supplementary dredging → Acceptance

IV. Output Calculation Formulas

(1) Hourly productivity of dredging–transporting–dumping

W₁ = q₁ / (l₁/v₁ + l₂/v₂ + l₃/v₃ + t₁ + t₂)
q₁ = (G − γw × q) / (γ₀ − γw)

Symbols:

W₁: Hourly productivity (m³/h)

q: Hopper capacity (m³)

l₁: Loaded sailing distance (km); v₁: Loaded speed (km/h)

l₂: Light sailing distance (km); v₂: Light speed (km/h)

l₃: Dredging distance (km); v₃: Dredging speed (km/h)

t₁: Dumping time (h)

t₂: Turning and lining–up time (h)

(2) Hourly productivity of dredging–transporting–discharging

W₂ = q / (l₁/v₁ + l₂/v₂ + l₃/v₃ + t₂ + t₃)

Symbols:

W₂: Hourly productivity (m³/h)

t₃: Total discharging time (h)

(3) Hourly productivity of side–discharge / bypass

W₃ = Q × p × δ × η
p = (V₁ / V₂) × 100% = (γm − γw) / (γ₀ − γw) × 100%

Symbols:

Q: Slurry flow rate (m³/h)

p: Slurry concentration

δ: Effective trough–out coefficient

η: Time coefficient

V. Common Dredging Methods

Overflow Loading: Continue dredging and overflow thin upper slurry after full hopper; repeated loading to increase volume. Applicable to sufficient draft, long dumping distance, good overflow.
Non–Overflow Loading: Stop dredging immediately when full; sail to dump. Applicable to short haul, hard sedimentation, environmental ban, limited water depth.
Side Discharge: Slurry discharged directly from bow sides without hopper. Velocity ≥1.5 kn, cross diversion ≥1.0 kn. Applicable to fine sediment, permitted environment, shallow water or rush dredging.
Sectional Construction: For trench longer than one–load sailing; sectioned by soil, depth, curve, turning; reduce empty run.
Strip Construction: For wide trench, uneven depth, priority deepening; ensure flatness; used in basin and wide channel.
Layered Construction: Mandatory if layer >4.0 m. Soft soil: 3.0–4.0 m/layer; hard soil: 1.0–2.0 m/layer.
Shallow–to–Deep: Excavate shallow first for navigation depth; reduce shallow–removal work.
Upstream–to–Downstream: For unidirectional flow/tidal estuary; use current scouring.
Middle–First / Side–First: Shallow sides → excavate sides first; shallow middle → excavate middle first. ≤80 m wide: prefer sides.
Cross Construction: For complex soil/depth/tide/navigation; prioritize shallow/difficult zones.
Downstream / Upstream Dredging: Upstream: good rudder, safe; downstream: high efficiency, poor rudder. ≤2 kn: downstream; >2 kn: upstream.
S–Shape Dredging: For waterways; avoid longitudinal grooves, improve flatness.
Edge–Line Excavation: Start at designed slope crest; move inward layer by layer; form slope and trench together.
Center–Line Excavation: Widen from center to sides; use natural collapse for slope; reduce waste earth.
Shallow–Spot Removal: Precise positioning, increased pressure; S–route for fish–back spots; key for final stage.
Round–Trip Dredging: For short trench unable to load full; requires turning space.
Forward–Backward Dredging: For restricted water without turning; no reversing with draghead on bottom. Applicable to upstream 3.0–5.0 kn.
Positioning Dredging: Fixed line for whole section; precise positioning for shallow–spot stage.
Shallow–Water Construction: Bottom clearance ≥1.0 m; reduce oil/water, empty hopper; prioritize center deep channel.
Leveling Construction: Fixed–depth draghead; no deepening in pits; repeated passes on steep shallow; switch to normal after leveling.
Rapid–Current Construction:<2.0 kn:="">2.0 kn: upstream; prevent draghead from pressing under hull.

VI. Hopper Loading by Soil Type

Organic soil, peat, silt: Evacuate hopper; low–density discharge; high compensator pressure; wide teeth; away from overflow; stop when full.

Cohesive soil: Thin–layer fast work; evacuate hopper; medium–high pressure; single–row + double plow teeth; large–flow high–pressure water; 3–4 kn.

Silt: Evacuate hopper; low pressure; wide teeth; high–pressure small–flow water; 1.5–2.0 kn; reduce pump speed after overflow.

Silty fine sand: Evacuate hopper; low pressure; wide teeth; high–pressure low–flow water; 1.5–2.0 kn; raise draghead 5°–10° at high resistance.

Medium/coarse sand, gravel: No evacuation; medium pressure; wide teeth; low–pressure large–flow water; 1.5–2.0 kn; lower draghead 3°–5°.

Crushed stone: No evacuation; medium pressure; narrow teeth; optional cutter;<2.5 kn.

Coral reef: No evacuation; medium pressure; conical sharp teeth; with cutter;<2.5 kn; high–pressure water for dumping.

VII. Reclamation Dredging Technology

Three Reclamation Modes

Sand borrowing for land reclamation

Direct reclamation with dredged materials

Bow jet reclamation (distance: 50–150 m)

Borrow Area Requirements

Projects approved;
pure sand, less impurities; prefer fine sand; depth 15–25 m; follow select fine reject coarse, choose near avoid far.

Positioning Requirements

Width ≥2× ship length; sufficient loaded depth; emergency evacuation access.

Floating Pipeline Layout

Length 250–500 m; shorten if velocity >1 m/s; equipped with automatic air valves, pipe anchors and buoys.

Bow–Discharge Pipeline Connection

Bow discharge unit → universal joint → self–floating hose → floating pipeline → submerged pipeline → shore connection (unified diameter: 900 mm)

VIII. Large–Dredger Clay Bow–Discharge Technology

Principle

Increase draghead high–pressure water; use clay–special teeth.Layered interval loading: sand–clay–sand–clay–sand.Optimize hopper flushing to form vortex mixing; sand lubricates to prevent plugging and cavitation.

Innovations

Leverage high power and mobility.Layered loading solves viscosity and plugging.Flushing system improves mixing and discharge efficiency.Stable conveying, steady pump, no resonance or cavitation.

IX. Key Parameter Optimization

Dredging Parameters

Ground speed: low for soft soil, high for hard soil

Compensator pressure: 80% for hard soil, 20% for soft soil

Draghead high–pressure water: for compact sand liquefaction

Bow–Discharge Parameters

Hydraulic calculation and ship test based on soil, distance, diameter, layout, pump performance.

Data & Adjustment

Collect density, flow, pressure, vacuum, draft, drag–arm signals; plot loading, efficiency, fuel curves; form optimal parameter list.Adjust timely with changing conditions.

X. Quality & Safety Control

Contaminated soil: no overflow; overflow must comply with environmental rules.

Strictly control over–width and over–depth in sectional, strip, layered works.

Prevent grounding, draghead damage, collision in shallow water, rapid current, slope zones.

No sharp turn or reverse with draghead on seabed.

Reclamation shall not hinder navigation; pipelines firmly connected.

Precise operation and close navigation–operation coordination for shallow removal and positioning.

FAQ 1
Q: What are the main advantages of a trailing suction hopper dredger?

A: It is self-propelled, self-dredging, self-loading and self-discharging. It operates while sailing, has little impact on navigation, and is flexible for mobilization.

FAQ 2

Q: What is the standard construction process for hopper loading?

A: Positioning and lowering draghead → dredging and loading → full loading and lifting draghead → sailing to dump → returning to the construction area.

FAQ 3

Q: When is layered construction required?

A: Layered construction is mandatory when the dredging layer exceeds 4.0 meters. Soft soil: 3.0–4.0 m per layer; hard soil: 1.0–2.0 m per layer.