Marine Construction (CE-503 (D)) - Unit 1 Short Notes

UNIT 1: MARINE CONSTRUCTION (CE-503(D))

Based on D MARINE CONSTRUCTION - NOV 2022 Past Paper

1.0 HARBOUR PLANNING, CLASSIFICATION & REQUIREMENTS

1.1 Characteristics of a Good Harbour

A good harbour must provide safe anchorage and efficient cargo handling.

• Natural Protection: Sheltered from prevailing waves, winds, and currents (e.g., by headlands, breakwaters).

• Adequate Depth: Sufficient water depth for vessels at all tides; approach channels must be dredged if necessary.

• Favorable Location: Proximity to hinterland (market/industrial areas) and major sea trade routes.

• Land Availability: Sufficient, affordable land for port facilities (yards, warehouses, roads, rail).

• Good Soil Conditions: Geologically stable foundation for structures; minimal siltation.

• Accessibility: Easy connection to road/rail networks.

1.2 Principles of Harbour Planning

• Entrance Orientation: Minimize wave transmission and siltation. Consider dominant wind/wave direction and longshore drift.

• Basin Layout: Design basins, breakwaters, and wharves to allow smooth vessel movement and minimize dredging.

• Tidal & Current Considerations: Account for tidal range in lock/dock design; currents affect siltation and berthing.

• Future Expansion: Plan layout for phased development and adaptability to larger vessels.

1.3 Classification of Harbours (Based on Location)

1.4 Requirements of a Good Port

(Distinct from harbour; focuses on operational & commercial efficiency)

• Adequate Berthing Facilities: Sufficient number & length of berths for expected vessel traffic.

• Modern Cargo Handling Equipment: Gantry cranes, conveyors, forklifts for quick turnaround.

• Efficient Intermodal Connectivity: Seamless transfer to rail/road.

• Adequate Storage Space: Open yards, covered sheds, warehouses, cold storage.

• Navigational Aids: Lighthouses, buoys, dredged channels, pilotage services.

• Commercial & Administrative Services: Customs, banking, shipping agents.

• Future Expansion Scope: Land and design flexibility.

• Environmental & Social Compliance: Pollution control, community impact mitigation.

[!TIP] Exam Focus: Be prepared to differentiate between Harbour (sheltered water area) and Port (commercial facility with infrastructure). Questions often ask for both characteristics and requirements separately.

2.0 COASTAL PROTECTION & BREAKWATER STRUCTURES

2.1 Types and Functions of Breakwaters

• Primary Function: Break/attenuate waves to create calm conditions in the harbour/port.

• Types:

  1. Mound/Rubble Mound Breakwater:

     • Structure: Sloping mound of rubble stones (core) protected by armour units (large rocks or concrete shapes like tetrapods, dolosse).

     • Use: Most common; flexible, can settle, good for deep water/soft soil.

     • DiagramSEARCH: "rubble mound breakwater cross-section armour units"

  2. Vertical/Wall-Type Breakwater:

     • Structure: Vertical or near-vertical concrete wall (caissons, sheet piles) founded on a rubble mound toe.

     • Use: Where space is limited; allows quay wall on seaward side.

     • Construction: Detailed in 2.2.

  3. Composite Breakwater:

     • Structure: Combines mound (seaward) and vertical wall (landward) sections.

     • Use: Economical for varying water depths; mound absorbs wave energy, wall provides vertical face.

2.2 Construction Method for Wall-Type Breakwater

1. Foundation Preparation: Dredge to remove soft soil; place/level rubble mound foundation (toe).

2. Construction of Rubble Mound Toe/Under-layer: Provides stable base and filters.

3. Placement of Caissons/Precast Units:

   • Caissons: Large, watertight concrete structures built onshore, floated to site, sunk on prepared bed, and filled with sand/mortar.

   • Precast Concrete Units: Placed by crane/grabbing.

4. Superstructure Construction: Cast concrete cap on top of caissons/units for stability and to support parapet.

5. Underwater Works: Use tremie concrete (poured through a pipe to prevent segregation) for foundations and base slabs.

[!TIP] Common Pitfall: Confusing construction sequences. Remember: Foundation → Toe → Vertical Units → Cap. Tremie concrete is key for underwater concreting.

2.3 Air Breakwater (Pneumatic Breakwater)

• Principle: Uses a curtain of compressed air bubbles released from a perforated pipe on the seabed.

• Mechanism:

  1. Air bubbles rise, creating an air-water mixture.

  2. This mixture has lower density than seawater, causing waves to break and dissipate energy over the bubbly region.

  3. Reduces wave height and force behind the curtain.

• Applications: Protection of small craft harbours, temporary works, environmentally sensitive areas (minimal visual impact).

• Limitations: High operating cost (continuous air supply), effectiveness depends on wave height, requires reliable air compression system.

3.0 WAVES, HYDRODYNAMICS & IMPACT PRESSURE

3.1 Characteristics of Sea Waves

• Wave Length (L): Distance between successive crests. Depends on: Fetch (unobstructed wind distance), wind speed, wind duration.

• Wave Height (H): Vertical distance from trough to crest. Depends on: Same as above + water depth (shoaling effect).

• Wave Classification:

  • Wind Sea (or Sea): Locally generated, chaotic, short-period waves.

  • Swell: Long-period, well-organized waves that have propagated out of their generation area.

  • Tsunami: Long wavelength, high-speed waves caused by seismic activity; behave as shallow water waves in deep ocean.

3.2 Impact Pressure on Sea Structures

General Concept: Dynamic pressure from breaking/non-breaking waves is much higher than static (hydrostatic) pressure due to wave momentum and velocity.

i) Derivation in Shallow Water (Wave Breaking)

• Assumption: Wave breaks on structure, water comes to rest (velocity ≈ 0).

• Apply impulse-momentum principle to a water column of height h (still water depth) striking the structure.

• Dynamic Pressure (P_dyn): $$\displaystyle P_{dyn} = \rho g h + \frac{1}{2} \rho v^2 $$

  where $\rho$ = water density, $g$ = gravity, $v$ = wave particle velocity at impact.

• For a breaking wave, the velocity head term $$\displaystyle \frac{1}{2} \rho v^2 $$ is significant. Maximum pressure can be 2-3 times hydrostatic pressure.

• \boxed{P_{impact} \approx k \cdot \rho g h} \quad (k \text{ is impact coefficient, } k>1)

ii) Derivation in Deep Water (Non-Breaking Waves)

• Use wave theory (e.g., Stokes 2nd Order or Airy Linear).

• Pressure at a point below mean water level z (positive downward) for a progressive wave:

$$ p = \rho g \eta + \frac{\rho g H}{2} \frac{\cosh[k(z+h)]}{\cosh(kh)} \cos(kx - \omega t) $$

where $H$ = wave height, $$\displaystyle k=2\pi/L $$, $$\displaystyle \omega=2\pi/T $$, $h$ = depth, $\eta$ = surface elevation.

• Maximum Pressure occurs when wave crest is at the structure ($$\displaystyle \cos(kx-\omega t)=1 $$).

• For deep water ($$\displaystyle h/L > 0.5 $$), $$\displaystyle \cosh[k(z+h)]/\cosh(kh) \approx e^{kz} $$ (pressure decays exponentially with depth).

• Impact is less severe than shallow water breaking waves; pressure distribution is oscillatory.

[!TIP] Exam Tips: Shallow water derivation focuses on momentum change (impulse). Deep water uses wave theory equations. Always state assumptions (breaking vs. non-breaking).

4.0 DOCKS, LOCK GATES & ASSOCIATED STRUCTURES

4.1 Dock Walls

• Purpose: Retain soil/water, provide berthing face.

• Typical Cross-Section Types:

  • Gravity Wall: Mass concrete/masonry, relies on self-weight.

  • Cellular Wall: Interlocking steel sheet piles filled with granular material.

  • Anchored Wall: Vertical sheet piles anchored by tie-rods to deadman anchors.

• Construction Aspects:

  • Foundation: Must be on stable strata; often require pile foundation.

  • Structural Elements: Coping (top), wall stem, base slab.

  • Fendering: Rubber/wood fenders on seaward face to absorb berthing energy.

  • Drainage: Weep holes, drainage layer behind wall to relieve hydrostatic pressure.

4.2 Lock Gates

• Types: Mitre (V-shaped, common), single-leaf, double-leaf, floating gates.

• Forces: Hydrostatic pressure from differential water levels between chamber and upstream/downstream sides.

• Derivation of Compressive Force on Mitre Gate:

  1. Consider a gate leaf of width W, height H (from sill to water level), meeting at an angle 2θ (mitre angle, typically 60°-75°).

  2. Hydrostatic pressure distribution on leaf is triangular: $$\displaystyle p = \rho g y $$, where y is depth from water surface.

  3. Total force on one leaf: $$\displaystyle F = \frac{1}{2} \rho g H^2 \cdot W $$ (per unit width into page).

  4. This force acts at centroid of triangle ($H/3$ from bottom).

  5. Resolve F along (compressive) and perpendicular (opening) to gate leaf.

  6. Compressive Force (C) along the leaf: $$\displaystyle C = F \cos\theta = \frac{1}{2} \rho g H^2 W \cos\theta $$.

  7. Rise of Bill (R): Vertical distance from sill to point where leaf meets mitre line. $$\displaystyle R = H \sin\theta $$.

  8. Substitute $$\displaystyle \cos\theta = \sqrt{1 - \sin^2\theta} \approx \text{constant for small } \theta $$ variation.

  9. Result: $$\displaystyle C \propto \frac{W}{R} $$.

  \boxed{C \propto \frac{\text{Width of Lock (W)}}{\text{Rise of Bill (R)}}}

  • Interpretation: For a given water head, wider locks require stronger (higher R) gates to resist compression.

4.3 Repair Docks (Dry Docks)

• Necessity: Provide dry, stable environment for ship inspection, maintenance, repair, and construction (hull cleaning, painting, propeller/ rudder work).

• Types:

  1. Graving Dock: Fixed, land-based, basin-type. Vessel enters, water pumped out.

  2. Floating Dock: Pontoon structure, submerged to receive vessel, de-ballasted to lift.

  3. Marine Railway (Syncrolift): Vessel on cradle moved on inclined rails out of water.

• Key Components:

  • Keel Blocks: Support vessel's keel (longitudinal strength).

  • Bilge Blocks: Support bilges (transverse support), adjustable.

  • Pumping System: For graving docks (pumps out water).

  • Pontoon: Watertight hull of floating dock.

  • Cradle & Winches: For marine railway.

5.0 PIERS, DOLPHINS & MOORING STRUCTURES

5.1 Piers

• Purpose: Extend from shore into water for berthing vessels and cargo/passenger handling.

• Determination of Dimensions:

  • Vessel Size: LOA (Length Overall), beam, draft.

  • Mooring Forces: Wind, current, wave-induced loads on vessel.

  • Berthing Velocities & Energy: Design fendering system.

  • Soil Bearing Capacity: Dictates foundation type (piles vs. shallow).

  • Water Depth & Tidal Range: Dictates pier height/deck level.

  • Operational Requirements: Space for cargo gear, storage, access roads.

• Types:

  • Open Pile Pier: Pile-supported deck, minimal obstruction to flow.

  • Solid Fill (Revetment) Pier: Rubble mound or earth fill with protective armour.

  • Composite Pier: Solid base for stability + open pile top for reduced load.

5.2 Dolphins

• Definition: Isolated, free-standing structures (not connected to shore) used for mooring or berthing.

• Purpose: Provide mooring points away from main pier/wharf; act as breasting dolphins to prevent vessel contact with main structure.

• Types:

  • Clump Dolphin: Cluster of piles (usually 4-6) tied together with a cap.

  • Breasting Dolphin: Positioned on berth line to receive vessel's side.

  • Single/Multi-Pile Dolphin: Single large pile or group.

• Design Considerations: Withstand mooring line tensions (from ship's bollards) and berthing impact forces.

6.0 HYDROGRAPHIC SURVEY & SITE INVESTIGATION

6.1 Equipment for Hydrographic Survey Operations

[!TIP] Key Link: Hydrographic survey data (depths, seabed type) directly informs dredging volumes, breakwater foundation design, and navigational charting.

7.0 CONSTRUCTION METHODS & AUXILIARY SYSTEMS

7.1 Staging System

• Purpose: Provide dry, stable working platform for construction of marine structures (breakwaters, piers) in water.

• Types:

  • Pile Staging: Temporary piles driven, braced, with decking.

  • Trestle Staging: Braced trestle frames on piles, often with rail tracks.

  • Pontoon Staging: Floating pontoons moored/anchored; used where piling difficult.

• Design Considerations: Load capacity (cranes, materials), stability against waves/currents, access for vessels, ease of dismantling.

7.2 Traversing Slipways

• Purpose: Launching or retrieving vessels/prefabricated units (caissons) from water onto land or vice-versa.

• Design & Operation:

  • Inplane Plane: Railway track on a sloping bed (1:15 to 1:30).

  • Carriage/Way: Wheeled cradle or platform that travels on rails.

  • Winching System: Powerful winches with steel wire ropes pull carriage up/down.

  • Greased Ways: Longitudinal timber/steel ways greased to reduce friction during launch.

• Process: Vessel built on carriage, launched by gravity (release brakes) or winched down.

7.3 Keel and Bilge Blocks

• Purpose in Dry Docks: Support the ship's hull evenly during docking to prevent strain.

• Keel Blocks: Placed along the keel (centerline); provide primary longitudinal support. Arranged in lines.

• Bilge Blocks: Placed on each side of keel blocks, under bilges (curvature of hull); provide transverse stability and support.

• Arrangement: Blocks are adjustable (height, position) to fit different hull shapes. Load distribution must be even to avoid overstressing hull plating.

• Materials: Hardwood or steel with rubber/wood packing.

7.4 Overturning (Stability Concept)

• Definition: Tendency of a structure (gravity breakwater, retaining wall) to rotate about its toe/base edge due to lateral forces (waves, earth pressure, water).

• Factor of Safety against Overturning (FSO):

$$ FSO = \frac{\text{Stabilizing Moment (M_s)}}{\text{Overturning Moment (M_o)}} $$

*   **Stabilizing Moment:** Due to **self-weight** (acts through centroid).

*   **Overturning Moment:** Due to **lateral forces** (wave pressure, soil pressure).

• Requirement: FSO > 1.5 (typical code requirement). Check that resultant force falls within middle third of base to avoid tension.

• Calculation: Sum moments about the toe (most critical point).

8.0 COASTAL & ENVIRONMENTAL FEATURES (SHORT NOTE TOPICS)

8.1 Sand Dunes

• Formation: Accumulations of wind-blown sand behind the high-tide line, stabilized by vegetation (marram grass, etc.).

• Role in Coastal Protection:

  • Act as a sand reservoir for beaches during erosion.

  • Form a natural barrier against storm surges and high waves.

  • Protect low-lying inland areas from flooding.

• Threats: Erosion (storms, human trampling), sand mining, coastal development.

• Stabilization Methods: Planting vegetation (grasses, shrubs), fencing to restrict access, sand traps.

8.2 Genoa Harbour (Case Study)

• Location: Northwestern Italy, Ligurian Sea.

• Type: Natural harbour significantly enhanced by extensive artificial breakwaters.

• Key Features:

  • One of the busiest ports in the Mediterranean.

  • Features a long outer breakwater (Molo Vecchio) providing protection.

  • Includes multiple basins (Porto Antico, newer terminals).

  • Historical significance as a major maritime republic port.

• Significance: Example of transforming a natural harbour into a major modern container port through massive breakwater and land reclamation.

8.3 Beaufort Scale

• Purpose: Empirical scale to estimate wind force and corresponding sea state based on visual observations of wind effects on land/sea.

• Scale: 0 (Calm) to 12 (Hurricane).

• Key Parameters per Force:

  • Wind Speed (knots or m/s).

  • Sea Conditions (wave height, foam).

  • Land Indicators (e.g., smoke drift, leaves rustle, trees break).

• Relevance to Marine Construction:

  • Preliminary assessment of design wave conditions.

  • Operational decisions: Safe working limits for marine construction, vessel operations.

  • Historical data analysis for long-term wave climate.

9.0 INLAND WATER TRANSPORT & COASTAL PROTECTION WORKS

9.1 Inland Water Transport (IWT) Systems

• Definition: Transport of goods/passengers on navigable rivers, canals, lakes.

• Advantages: Low energy consumption, low pollution, high cargo capacity per vessel, reduces road/rail congestion.

• Components:

  • Navigable Waterways: Natural rivers (with dredging) or man-made canals.

  • Terminals: Jetties, warehouses for loading/unloading.

  • Vessels: Barges, push-tugs, passenger boats.

• Challenges:

  • Siltation: Requires regular dredging.

  • Depth Maintenance: Seasonal variations, low water levels.

  • Fixed Obstructions: Bridges with limited vertical clearance.

  • Lock Operations: Time-consuming on canalized rivers.

  • Slow Speed: Not suitable for time-sensitive cargo.

9.2 Coastal Protection Works

• Objectives: Prevent coastal erosion, protect infrastructure & property, reclaim land, maintain beaches.

• Hard Engineering (Structural):

  • Seawalls: Vertical/sloping concrete walls; reflect waves, cause scouring.

  • Groynes/Jetties: Traverse to trap sand, interrupt longshore drift.

  • Breakwaters: Create calm areas (see Section 2).

  • Revetments: Sloping, permeable armour (riprap) to absorb wave energy.

• Soft Engineering (Non-Structural/Natural):

  • Beach Nourishment: Add sand from offshore to eroding beaches.

  • Dune Stabilization: Plant vegetation, fencing.

  • Managed Retreat: Allow natural processes by moving assets inland.

• Integrated Coastal Zone Management (ICZM): Holistic approach considering ecological, economic, and social factors; combines hard & soft measures, involves stakeholders.