Marine Construction (CE-503 (D)) - Unit 4 Short Notes

UNIT 4: MARINE CONSTRUCTION

I. HARBOR AND PORT PLANNING

Classification of Harbors (based on location)

Harbors are classified based on their orientation to the shoreline and natural protection offered.

Characteristics of a Good Harbor

A good harbor must be safe, accessible, and functional.

• Natural Protection: Sheltered from waves and winds by natural headlands or artificial breakwaters.

• Adequate Depth: Sufficient draft for expected vessels; requires regular dredging.

• Favorable Approach: Easy navigation with clear channels, minimal currents.

• Ample Space: For anchorage, maneuvering, and future expansion.

• Proximity to Hinterland: Good connectivity (rail, road, inland waterways) to commercial centers.

• Availability of Utilities: Fresh water, fuel, repair facilities, and cargo handling equipment.

Principles of Harbor Planning

1. Location & Orientation: Based on geotechnical, oceanographic, and economic studies.

2. Layout: Determined by dominant wind/wave direction (usually breakwaters parallel to shore).

3. Basin Design: Optimize size for wave dissipation and vessel maneuvering.

4. Approach Channel: Aligned with prevailing currents, marked with buoys.

5. Berthing Facilities: Positioned to minimize dredging and maximize operational efficiency.

6. Future Expansion: Plan for increased traffic and larger vessels.

Requirements of a Good Port

A port is a harbor plus commercial infrastructure. Requirements include:

• Efficient Cargo Handling: Modern gantry cranes, conveyors, ample stacking area.

• Storage Facilities: Warehouses, open storage, silos for bulk cargo.

• Transport Links: Direct rail/road connections to hinterland.

• Navigational Aids: Lighthouses, buoys, dredged channels.

• Support Services: Pilotage, towage, bunkering, repairs.

• Customs & Administrative Offices: For smooth clearance.

• Security & Safety: Patrol systems, firefighting, pollution control.

Case Study: Genoa Harbour (Italy)

• Type: Major artificial, commercial port on the Italian Riviera.

• Key Features:

  • Protected by extensive breakwater systems.

  • Multiple basins (e.g., Porto Antico, Porto Vecchio) for different functions.

  • Deep-water berths handling container ships, cruise liners, and bulk carriers.

  • Excellent hinterland connectivity via rail and road (Autostrada).

  • Integrated with the city, includes commercial, tourist, and residential zones.

• Significance: One of the busiest Mediterranean ports, exemplifies multi-purpose, urban-integrated port planning.

II. COASTAL PROCESSES AND PROTECTION WORKS

Sea Waves: Factors Affecting Length and Height

• Wave Height (H): Primarily determined by wind speed (V), fetch (F) (distance over which wind blows), and duration (t). Empirical formula: $H \propto \tanh(kd) \cdot \text{function of } V, F, t$.

• Wave Length (L): Depends on wave period (T) and water depth (d).

  • Deep water: $$\displaystyle L_0 = \frac{gT^2}{2\pi} $$

  • Shallow water: $$\displaystyle L = L_0 \sqrt{\tanh\left(\frac{2\pi d}{L_0}\right)} $$

• Wave Celerity (C): Speed of wave propagation.

  • Deep water: $$\displaystyle C_0 = \frac{gT}{2\pi} $$

  • Shallow water: $$\displaystyle C = \sqrt{gd} $$

Air Breakwater: Concept and Action

An air breakwater is a floating, perforated structure (like a pontoon with holes) placed offshore.

• Action: It does not block waves but dissipates their energy.

• Mechanism: Waves pass through the perforations, creating turbulence and air-water mixing inside the breakwater's chambers. This process absorbs wave energy, reducing wave height and force on the shore or main breakwater behind it.

• Use: Often used in combination with other breakwaters for energy dissipation.

Sand Dunes: Role in Coastal Geography

• Formation: Accumulation of wind-blown sand behind the high-tide line, stabilized by vegetation.

• Roles:

  1. Natural Barrier: Acts as the first line of defense against storm surges and high waves.

  2. Sand Reservoir: Supplies sand to the beach during erosion, maintaining beach profile.

  3. Ecological Habitat: Supports specialized flora and fauna.

  4. Landform Stability: Prevents inland migration of the shoreline.

• Threat: Destruction by human activity (trampling, construction) leads to coastal erosion.

Beaufort Scale: For Wind and Wave Measurement

A empirical scale (0-12) relating wind speed to observed sea conditions.

• Wind Speed: Measured in knots or m/s.

• Sea Description: From "calm" (0) to "phenomenal" (12) with specific wave heights, foam, and spray characteristics.

• Use: Quick visual estimation of wind force at sea, crucial for harbor operations and vessel safety.

Inland Water Transports and Coastal Protection Works: Integrated Overview

• Inland Water Transport (IWT): Navigation on rivers, canals, lakes. Requires dredging, locks, and bank protection.

• Coastal Protection Works: Structures like breakwaters, groynes, seawalls to combat erosion.

• Integration: IWT systems (e.g., National Waterways in India) often require coastal protection at their mouths/estuaries to maintain navigable depths and prevent siltation. Dredged material from IWT channels can be used for beach nourishment (a coastal protection method). Planning must consider sediment transport to avoid downdrift erosion.

Wall Type Breakwater: Construction Method with Sketches

A vertical-sided, gravity structure (concrete caissons or masonry). Construction Sequence:

1. Foundation Preparation: Dredge to level, place stone bedding or concrete mound.

2. Construction of Caissons: Prefabricated hollow concrete boxes in a dry dock or onshore.

3. Transport & Sinking: Float caissons to site, sink by controlled flooding onto prepared foundation.

4. Backfilling: Fill caisson interiors with rock or soil for stability.

5. Capping: Place concrete cap for crane access and wave overtopping resistance.

6. Rear Slope Protection: Place armor units (tetrapods, dolos) or revetment on the leeward side to prevent scouring.

DiagramCANVAS: Cross-section of a wall-type breakwater showing: (a) sunk caisson on rubble mound foundation, (b) backfilled interior, (c) concrete cap, (d) armor layer on seaward face, (e) rear slope protection.

III. HYDROGRAPHIC SURVEY

Equipment Used in Hydrographic Survey Operations

Hydrographic survey maps underwater topography (bathymetry).

IV. DESIGN OF MARINE STRUCTURES

Impact Pressure on Sea Structures

1. Derivation for Shallow Water Conditions

Assumes non-breaking waves hitting a vertical wall.

• Max. Pressure ($$\displaystyle P_{max} $$): Occurs at still water level (SWL).

$$ P_{max} = \gamma h \left(1 + \sqrt{\frac{\pi H}{2T \sqrt{gd}}}\right) \text{ or } P_{max} = \gamma h \left(1 + \sqrt{\frac{\pi H}{L}}\right) $$

Where $\gamma$ = unit weight of water, $h$ = water depth at SWL, $H$ = wave height, $T$ = wave period, $d$ = depth, $L$ = wave length.

• Pressure Distribution: Varies parabolically from $$\displaystyle P_{max} $$ at SWL to zero at wave trough and seabed.

• Total Force ($F$): $$\displaystyle F = \frac{1}{2} P_{max} \cdot h $$ (for triangular distribution approx.).

2. Derivation for Deep Water Conditions

For deep water ($$\displaystyle d > \frac{L}{2} $$), wave profile is circular.

• Max. Pressure ($$\displaystyle P_{max} $$): At SWL.

$$ P_{max} = \gamma H \left(1 + \sqrt{\frac{\pi H}{L}}\right) \text{ (simplified)} $$

More rigorous: $$\displaystyle P_{max} = \gamma \frac{H}{2} \left(1 + \frac{\pi H}{L}\right) $$

• Pressure Distribution: Hydrostatic type but with higher peak. Total force acts at $$\displaystyle \frac{1}{3}h $$ above seabed.

Key Difference: Shallow water pressure depends on depth (h), deep water on wave height (H).

Piers

Purpose and Necessity:

• Provide berthing facilities parallel to shoreline.

• Allow vessels to moor alongside for cargo/passenger handling without blocking navigation channels.

• Enable multiple berths in limited waterfront space.

• Access to deeper water away from shore.

Factors Determining Pier Dimensions:

1. Vessel Size: Length, beam, draft.

2. Maneuvering Space: Required berthing pocket (usually 1.2-1.5 x vessel length).

3. Fendering System: Protects pier and vessel; dictates stand-off distance.

4. Cargo Handling Equipment: Reach of cranes, length of apron.

5. Navigation Channel Width & Alignment.

6. Soil Conditions & Foundation Type.

7. Environmental Loads: Waves, currents, wind forces on moored vessel.

Dock Walls

Typical Cross-Sectional Details:

• Gravity Wall: Mass concrete or masonry, keyed into bedrock or on riprap foundation.

• Sheet Pile Wall: Interlocking steel/concrete sheets driven into soil, with anchor system (deadman, tie-rods) or strut bracing.

• Composite Wall: e.g., Sheet piles + concrete cap + backfill.

• Components: Front face, top deck (apron), backfill, drainage layer, fendering, bollards, crane rail (if any).

Key Aspects of Construction:

1. Foundation: Must be impermeable and stable (rock, dense sand, or piled).

2. Dewatering: Cofferdams or well-point systems for dry excavation.

3. Joint Waterproofing: Crucial for gravity walls.

4. Backfilling: Controlled, granular material with proper compaction.

5. Fendering Installation: To absorb berthing energy.

6. Drainage System: Weep holes or drainage layers to relieve hydrostatic pressure.

Lock Gates

Derivation: Show that compressive force on lock gate varies directly as width and inversely as rise of bill.

Assumptions: Gate is a vertical plate hinged at top/bottom. Water pressure acts horizontally. Gate is in equilibrium under water pressure, hinge reactions, and compressive force (C) at the heel (bottom on land side).

1. Water Pressure Distribution: Triangular, with max pressure $$\displaystyle P = \gamma H $$ at depth $H$ (water depth).

2. Total Water Force (F): $$\displaystyle F = \frac{1}{2} \gamma H^2 \cdot W $$, where $W$ = width of lock (gate width).

3. Taking Moments about Top Hinge (A):

   • Moment due to $F$: $$\displaystyle F \times \frac{H}{3} = \frac{1}{2} \gamma H^2 W \cdot \frac{H}{3} = \frac{1}{6} \gamma H^3 W $$

   • Moment due to Compressive Force (C) at heel (distance = rise of bill, $R$): $C \times R$

4. Equilibrium: $$\displaystyle C \times R = \frac{1}{6} \gamma H^3 W $$

5. Therefore:

$$ C = \frac{\gamma H^3 W}{6R} \quad \boxed{C \propto \frac{W}{R}} $$

**Conclusion:** Compressive force $C$ varies **directly with lock width (W)** and **inversely with rise of bill (R)**.

V. PORT FACILITIES AND SUPPORT SYSTEMS

Dolphins: Types and Functional Uses

Dolphins: Isolated, free-standing structures (piles, concrete) projecting above water, not connected to shore.

• Types:

  1. Single Dolphin: One cluster of piles.

  2. Dolphin Cluster/Group: Multiple dolphins tied together.

  3. Berthing Dolphin: For vessels to moor alongside.

  4. Mooring Dolphin: For bow/stern lines.

  5. Navigation Dolphin: Marks channel edges (unmanned).

• Functional Uses:

  • Mooring/Berthing Aid: Provide points for lines, reduce load on main berth.

  • Fendering: Protect structures from vessel impact.

  • Navigation Markers: Define channel limits.

  • Support for Pipelines/Utilities.

Repair Docks: Necessity and Explanation

Necessity: Vessels require periodic maintenance, inspection, and repair (hull cleaning, painting, propeller/ rudder repair, structural work) which is impossible in water. Types & Explanation:

1. Graving Dock (Dry Dock): Lock-gated basin from which water is pumped out. Vessel enters, gates close, water pumped out. Most common for major repairs.

2. Floating Dock: U-shaped, buoyant structure that submerges, vessel enters, then dewatered to lift vessel. Mobile, no civil works needed.

3. Marine Railway/Slipway: Vessel is hauled out of water on a cradle moving on inclined rails (traversing or fixed). Used for smaller vessels, routine maintenance.

Keel and Bilge Blocks: Purpose in Docking

• Keel Blocks: Central, tallest blocks placed under the keel (vessel's backbone). They support the primary weight of the vessel and prevent hogging/sagging.

• Bilge Blocks: Shorter blocks placed on either side of keel blocks, under the bilge (curve between hull side and bottom). They stabilize the vessel laterally and distribute load to the dock floor.

• Purpose Together: Provide stable, uniform support to the hull, prevent deformation during dry-docking, and allow safe access to the underwater hull.

Traversing Slipways: Mechanism and Use

• Mechanism: A slipway with a cradle mounted on wheeled trucks that run on parallel rails running perpendicular to the shoreline (traversing). The cradle can be moved laterally (traversed) into different berthing positions.

• Use: Allows multiple vessels to be docked/launched from a single slipway without requiring multiple parallel slipways. Increases throughput in repair yards with limited waterfront.

Overturning: Stability Consideration in Marine Structures

• Definition: Rotational failure where a structure (wall, gravity block) tips over about its toe or edge due to excessive moment from lateral loads (waves, earth pressure, ship impact).

• Stability Check: Factor of Safety against Overturning (F.S.) must be > 1.5-2.0.

$$ \text{F.S.} = \frac{\text{Resisting Moment (about toe)}}{\text{Overturning Moment (about toe)}} $$

• Resisting Moment: Due to self-weight and vertical earth pressure.

• Overturning Moment: Due to lateral earth pressure, wave pressure, ship berthing/mooring forces.

• Prevention: Increase base width, lower center of gravity, use keyways, ensure adequate foundation bearing pressure.

Staging System: Types and Application in Construction

Staging (Falsework): Temporary structure to support construction loads (materials, equipment, formwork) until permanent structure gains strength.

• Types:

  1. Frame Staging: Prefabricated steel frames (cup-lock, H-frame). Quick erection, high load capacity.

  2. Tube & Coupler Staging: Steel tubes connected by couplers. Highly flexible, adaptable to complex shapes.

  3. Sawhorse Staging: Simple timber/steel sawhorses with planks. For light loads.

  4. Truss Staging: For long spans, heavy loads (e.g., bridge construction).

• Application in Marine Construction:

  • Supporting formwork for deck/superstructure of piers, dolphins.

  • Providing working platforms for construction personnel and equipment.

  • Supporting pre-cast elements during installation.

  • Key Considerations: Stability against lateral loads (wind, waves), barge impact, soil bearing capacity, tidal variations. Must be independent of the permanent structure until it is self-supporting.