Marine Construction (CE-503 (D)) - Unit 2 Short Notes

UNIT 2: Marine Construction (Based on Nov 2022 Exam)

1. Harbours and Ports

Classification of Harbours (Based on Location)

Characteristics of a Good Harbour

1. Natural Protection: Sheltered from high waves and strong winds.

2. Adequate Depth: Sufficient draft for expected vessels; requires minimal dredging.

3. Favourable Approach: Safe, wide, and deep approach channels.

4. Good Anchorage: Safe, roomy, and well-sheltered anchorage area.

5. Availability of Space: Room for expansion (berths, yards, facilities).

6. Accessibility: Easy connection to hinterland (road, rail, inland waterways).

7. Soil Conditions: Suitable for construction of berths, breakwaters, and structures.

Principles of Harbour Planning

• Location Selection: Based on trade potential, natural protection, approach channels, hinterland connectivity.

• Layout Planning: Determine number & type of berths (general cargo, container, oil), breakwater alignment, basin configuration (single/double), turning circle.

• Orientation: Breakwaters aligned to offer maximum protection from dominant waves/winds.

• Water Area vs. Land Area: Balance between navigable basin and space for cargo handling/storage.

• Future Expansion: Plan for phased development.

Requirements of a Good Port

• Efficient Cargo Handling: Modern gear, sufficient berths, quick turnaround.

• Adequate Storage: Covered & open storage, container yards.

• Good Connectivity: Seamless multimodal transport links.

• Modern Infrastructure: Deep drafts, modern navigation aids, IT systems.

• Economic Viability: Competitive tariffs, efficient operations.

• Environmental Compliance: Pollution control, waste management.

• Security: ISPS compliance, safe working conditions.

[!TIP] Exam Focus: Distinguish between Harbour (sheltered water area for vessels) and Port (harbour + land-based cargo handling facilities). Genoa Harbour is a classic case of a natural, land-locked harbour significantly improved by breakwaters.

Case Study: Genoa Harbour (Italy)

• Type: Natural, land-locked harbour.

• Protection: Naturally protected by a headland (San Benigno) and enhanced by breakwaters.

• Layout: Complex with multiple basins (Porto Antico, Porto Vecchio, Porto Nuovo).

• Significance: One of the busiest Mediterranean ports; handles containers, cruises, and bulk cargo.

• Key Feature: Demonstrates how a superb natural harbour is augmented with modern engineering to become a major port.

2. Coastal Processes and Wave Mechanics

Factors Affecting Length & Height of Sea Waves

1. Wind Speed: Primary factor; higher wind speed → higher waves.

2. Fetch: Distance over which wind blows uninterrupted. Longer fetch → larger waves.

3. Wind Duration: Time wind blows over fetch. Sufficient duration needed for wave development.

4. Water Depth: Controls wave behaviour (shoaling, breaking). Deep water waves vs. shallow water waves.

5. Currents & Tides: Can modify wave height and direction.

Impact Pressure on Sea Structures

Concept: When a wave breaks against a vertical structure, the dynamic impact pressure can be several times the hydrostatic pressure of the same wave height. It's an impulsive, short-duration load critical for design.

i) Derivation in Shallow Water (Linear Wave Theory Assumption)

For a non-breaking wave in shallow water (depth d, wave height H):

• Maximum pressure at seabed: p_max ≈ ρgH (hydrostatic).

• Impact Pressure occurs during breaking. Simplified formula for breaking wave impact:

$$ p_{impact} = K \cdot \rho g H $$

where `K` is an **impact coefficient** (empirical, typically **2 to 5** for vertical walls). This accounts for momentum change.

ii) Derivation in Deep Water (Stoke's Higher Order Theory)

In deep water (d > L/2), wave particle motion is circular.

• Pressure at depth z (from MSL): p = ρg \frac{H}{2} \frac{1}{\cosh[k(d+z)]} \cos(kx - ωt)

• For impact, consider the velocity head of the wave particle just before impact.

• Simplified Dynamic Pressure: p_dyn ≈ \frac{1}{2} ρ v^2, where v is wave particle velocity at surface.

• Using linear theory, max surface orbital velocity v_max ≈ \frac{πH}{T}.

• Thus, p_impact ∝ ρ g H (similar form, but coefficient K may differ from shallow water case due to different kinematics).

[!TIP] Exam Focus: The shallow water impact pressure derivation is more commonly asked. Remember the key formula: p_impact = K * ρ * g * H. K is NOT a constant; it depends on wave steepness, structure geometry, and is determined experimentally.

Air Breakwater: Concept and Action

• Concept: A perforated vertical wall (or row of piles) with a compressed air supply system at the base.

• Action: Compressed air is bubbled through the perforations, creating an air-water mixture (foam) in front of the wall.

• Mechanism:

  1. The air bubbles dissipate wave energy through friction and turbulence.

  2. The bubbly mixture has lower density than seawater, reducing the effective mass impacting the structure.

  3. It prevents or reduces scouring at the structure's base by altering flow patterns.

• Use: Often used in combination with solid breakwaters or for protecting sensitive foundations.

Beaufort Scale for Wind and Wave Estimation

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

[!TIP] Exam Focus: You may be asked to estimate wave height from a given Beaufort number or vice-versa. Remember: Wave Height ≈ 0.024 * (Beaufort No.)^2 (rough approximation).

Sand Dunes: Formation and Relevance

• Formation: By aeolian (wind) deposition of sand in the backshore zone. Vegetation traps moving sand, initiating dune formation.

• Relevance to Marine Construction:

  1. Natural Coastal Defence: Act as a buffer against storm surges and high waves.

  2. Sand Source: Can be a source of beach fill material (but mining dunes is ecologically damaging).

  3. Construction Constraint: Dune areas are unstable for heavy structures; foundations require special design.

  4. Environmental Sensitivity: Dunes are ecologically vital (habitat, groundwater recharge). Construction must avoid destruction.

3. Hydrographic Surveying

Equipment Used in Hydrographic Survey Operations

Survey Process: Integrated system where GNSS provides position, Echo Sounder provides depth, Motion Sensor corrects for vessel motion, SVP corrects for sound speed variations. Data logged and processed in real-time.

4. Breakwaters and Coastal Protection

Wall Type Breakwater: Construction Method

1. Type: Vertical or near-vertical faced breakwater (e.g., caisson, composite, rubble mound with vertical wall).

2. Construction Sequence (Caisson Type - Common):

   • Foundation Preparation: Dredge to founding level; may use sand/gravel bed or piles.

   • Caisson Fabrication: Pre-cast concrete caissons (hollow, heavy) in a dock or onshore.

   • Transport & Sinking: Float caissons to site, ballast, and carefully sink onto prepared foundation.

   • Capping & Armour: Place concrete cap. Outer face may be protected with accropode, xbloc, or concrete armour units. Rubble mound type uses armour stones on a filter layer.

   • Backfilling: Fill caisson/core with rock or sand.

   • Crest & Superstructure: Construct roadway, parapet, and navigational aids on top.

3. Key Feature: Provides a vertical face for mooring, but requires robust foundation and armour against wave impact and scouring.

General Coastal Protection Works

• Hard Engineering: Physical structures.

  • Groynes/Jetties: Trap littoral drift, prevent erosion downdrift.

  • Seawalls/Revetments: Armour the shoreline directly.

  • Breakwaters: Create sheltered areas.

  • Offshore Breakwaters (Submerged): Induce sediment accretion on beach.

• Soft Engineering: Beach management.

  • Beach Nourishment: Adding sand to eroding beaches.

  • Dune Rehabilitation: Plant vegetation, fencing.

• Hybrid/Integrated: Combination (e.g., groyne field with periodic nourishment).

5. Piers and Docks

Piers

• Purpose & Necessity:

  • Provide deep-water berthing away from the shoreline.

  • Allow vessels to load/unload without interfering with each other.

  • Access to deep water in harbours with shallow approaches.

  • Dedicated use (e.g., container pier, oil pier, passenger pier).

• Factors Determining Dimensions:

  1. Vessel Size: Length, beam, draft, air draft (mast height).

  2. Mooring Arrangement: Number of mooring lines, breasting dolphins.

  3. Cargo Handling Equipment: Reach of cranes, storage area needed.

  4. Navigation Clearance: Distance between pier face and channel centreline.

  5. Water Depth & Approach: Required depth at berth and in approach channel.

  6. Wave Climate & Currents: Influence on vessel motions and berthing forces.

Docks

• Typical Cross-Section of Dock Wall:

  • Gravity Wall: Mass concrete or masonry, relies on weight for stability. Sloped or vertical face.

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

  • Reinforced Concrete Wall: Vertical or batter-faced, with relieving platforms.

  • Key Elements: Foundation (piles/raft), wall, relieving platform (to reduce earth pressure), coping, fenders, bollards.

  
  [DIAGRAM: CANVAS: Cross-section of a typical gravity-type dock wall showing: 1) Backfilled soil, 2) Relieving platform, 3) Main wall (mass concrete), 4) Foundation (piles or spread footing), 5) Fender system, 6) Coping, 7) Bollard. Label forces: Earth pressure (P), Water pressure (U), Surcharge load (q).]
  
  

• Construction Aspects of Dock Wall:

  • Sequential Construction: Build in panels/sections to allow for dewatering.

  • Dewatering: Use well points or deep wells to keep excavation dry.

  • Foundation: Piles for soft soils; raft for uniform bearing.

  • Waterproofing: Crucial to prevent leakage through joints/walls.

  • Joint Design: Waterstops in construction joints.

  • Backfilling: Controlled, granular material with proper compaction.

• Repair Docks: Why Necessary?

  • Essential Maintenance: Underwater hull cleaning, painting, propeller/shaft repair.

  • Damage Repair: Fix hull breaches, structural damage from collision/grounding.

  • Inspection: Regular dry-docking for surveys (classification society requirements).

  • Retrofitting: Install new equipment, modify hull.

  • Economic Lifespan: Extends vessel's service life cost-effectively.

6. Lock Gates and Hydraulic Structures

Lock Gates: Derivation of Compressive Force

Consider a miter gate (two leaves meeting at an angle θ). Water pressure acts on the upstream leaf.

• Total Hydrostatic Force (F) on one leaf:

$$ F = \frac{1}{2} \rho g H^2 \cdot B \cdot \cos\theta $$

where `ρ` = density, `g` = gravity, `H` = head (water depth), `B` = width of gate leaf, `θ` = miter angle.

• This force F is resolved along the gate's plane. The compressive force C at the miter post (hinge) is the component that the gate structure and its supports must resist.

• Resolution: C = F / \sin\theta (since F is perpendicular to leaf, C is along the mitre line).

• Substituting F:

$$ C = \frac{\frac{1}{2} \rho g H^2 B \cos\theta}{\sin\theta} = \frac{1}{2} \rho g H^2 B \cot\theta $$

• Bill Rise (h): The vertical distance from the miter post to the top of the gate. For a given B, θ is related to h and B by: \tan\theta = 2h / B → \cot\theta = B / (2h).

• Final Relationship:

$$ C = \frac{1}{2} \rho g H^2 B \cdot \frac{B}{2h} = \frac{\rho g H^2 B^2}{4h} $$

\boxed{C \propto B^2 / h} \quad \text{or} \quad \boxed{C \propto B \ \text{and} \ C \propto 1/h}

[!TIP] Exam Focus: The derivation shows compressive force C varies directly with the square of gate width B and inversely with bill rise h. This is critical for designing the miter post and anchorage.

Dolphins: Types and Uses

• Definition: Isolated, pile-supported structures located away from the main berth/wharf face. Not connected to land.

• Types:

  1. Suspension Dolphins: Mooring lines attach to the top; resist horizontal pull.

  2. Compression Dolphins: Vessel breasts against them; resist horizontal thrust.

  3. Combination Dolphins: Handle both tension and compression.

• Uses:

  • Mooring Points: For large vessels (tankers, bulk carriers) at offshore berths.

  • Breasting: To position vessel alongside the quay.

  • Fendering: Protect the main structure from vessel impact.

  • Navigation Aids: Mark channel edges or turning basins.

  • Support for Pipelines/Conveyors: (e.g., at oil terminals).

Overturning: Consideration in Design

• Definition: The tendency of a structure (e.g., breakwater, dock wall, lock gate) to rotate about its toe or a pivot point due to lateral forces (waves, wind, water pressure, seismic).

• Design Check: Ensure stabilizing moments (from weight, soil pressure) > overturning moments (from lateral loads) by a safety factor (typically 1.5 - 2.0).

• Calculation:

  1. Identify the critical overturning axis (usually the toe or a point on the foundation).

  2. Sum all vertical forces (W, V) and their lever arms (x) to get Stabilizing Moment (M_s).

  3. Sum all horizontal forces (P) and their lever arms (z) to get Overturning Moment (M_o).

  4. Factor of Safety Against Overturning: F.S. = M_s / M_o.

• Prevention: Increase base width, lower centre of gravity, use counterweights, improve foundation.

7. Ship Support Infrastructure

Keel and Bilge Blocks

• Keel Blocks: Central, continuous ridge of blocks running along the ship's keel (bottom centreline). Support the main weight of the vessel.

• Bilge Blocks: Placed on either side of the keel blocks, supporting the bilge (the curved part where hull side meets bottom). They take load when the ship is not perfectly level or has hog/sag.

• Purpose: Distribute the ship's weight uniformly on the dock floor or slipway to prevent hull deformation during construction, repair, or launching.

• Arrangement: Keel block spacing depends on hull strength (frames/transverse bulkheads). Bilge block spacing is closer.

Traversing Slipways

• Definition: An inclined ramp (slipway) with a travelling carriage or cradle that can move horizontally (traverse) along the top of the slipway.

• Purpose: To launch or haul out vessels.

• Action (Launching):

  1. Vessel is built on the cradle on the landward end of the slipway.

  2. Upon completion, the cradle (with vessel) is released and slides down the inclined slipway into water (gravity launch).

  3. Traversing Mechanism: After launch, the empty cradle is hauled back up the slipway using winches/rails on the traverse track at the top, ready for the next vessel.

• Advantage: Allows reuse of the slipway without building a new one for each vessel.

Staging Systems

• Definition: Temporary scaffolding/work platforms erected around a ship (in dock, on slipway, or afloat) to provide access for construction, repair, painting, or inspection.

• Types:

  • Tube & Clamp Staging: Modular steel tubes with clamps; versatile, quick to erect.

  • Frame Staging: Pre-fabricated frames with planks.

  • Suspended Staging: Hung from the ship's structure or dock cranes.

  • Barge-Mounted Staging: For afloat repairs.

• Key Considerations:

  • Load Capacity: Must support workers, tools, materials.

  • Safety: Guardrails, toe boards, secure anchoring to ship/structure.

  • Access: Safe means of access (ladders, gangways).

  • Stability: Must not impose excessive point loads on hull plating.

  • Adaptability: Conform to complex curved hull shapes.