Technical Specifications

Introduction

Technical specifications are critical in defining the engineering parameters and structural requirements of cargo holds in dry bulk and general cargo ships. These specifications ensure that the cargo holds are designed and built to withstand the stresses of loading, transport, and unloading of various cargo types while complying with international regulations and safety standards. This chapter provides detailed technical information on the design parameters, structural requirements, and volume calculations for different types of cargo holds.

1. Box-Shaped Cargo Holds

1.1 Technical Parameters

Box-shaped cargo holds are designed with specific ratios and structural elements to maximize efficiency and safety.

Design Specifications:

• Aspect Ratio (Length/Width): 1.2 – 1.8

  • Ensures optimal space utilization and structural stability.

• Hold Index (Depth/Width): 0.45 – 0.65

  • Balances the hold's depth for efficient stowage and structural integrity.

• Structural Efficiency Factor: 0.82 – 0.88

  • Measures the hold's weight efficiency relative to its cargo capacity.

• Corner Radius: 250 – 450 mm (Grade A steel)

  • Rounded corners reduce stress concentrations and facilitate cleaning.

• Stiffener Spacing (Longitudinal): 600 – 850 mm

  • Adequate spacing ensures structural support and minimizes weight.

1.2 Structural Requirements

Tank Top Loading Capacities:

• Heavy Cargo: 25.0 t/m²

• Steel Coils: 35.0 t/m²

• Container Stacks: 90.0 t per stack (point load)

• Structural Safety Factor: 1.67 (as per IACS requirements)

  • Ensures the structure can withstand loads beyond the maximum expected.

Frame Specifications:

• Web Frame Spacing: 2,400 – 3,600 mm

• Web Depth: 600 – 900 mm (dependent on vessel size)

• Flange Width: 150 – 300 mm

• Material Grade: AH32/AH36 (high tensile steel)

  • High-strength steel improves structural integrity while reducing weight.

1.3 Volume Calculations

Accurate volume calculations are essential for cargo planning and stability assessments.

Net Hold Volume Calculation:

Net Hold Volume = L × W × H × CF

Where:

• L = Internal length between bulkheads

• W = Internal width between side shells

• H = Height from tank top to hatch coaming

• CF = Capacity factor (0.93 – 0.96 for box-shaped holds)

• Capacity Factor (CF): Accounts for structural intrusions and non-cargo spaces within the hold.

2. Hopper-Shaped Cargo Holds

2.1 Geometry Specifications

Hopper-shaped holds are engineered to facilitate the self-trimming of bulk cargoes.

Standard Angles:

• Lower Hopper Angle: 45° ±2°

  • Optimized for most bulk materials to promote flow during loading and unloading.

• Upper Hopper Angle: 30° ±2°

  • Interfaces with topside tanks and contributes to overall hold shape.

• Bilge Radius: 350 – 500 mm

  • Smooth transitions reduce cargo hang-up and ease cleaning.

Critical Dimensions:

• Hopper Plate Thickness: 12 – 16 mm (Grade AH36)

  • Thicker plates resist wear from abrasive cargoes.

• Stiffener Spacing: 700 – 900 mm

• Corner Radius: 350 – 450 mm

2.2 Self-Trimming Calculations

Self-trimming efficiency is crucial for operational efficiency in bulk cargo handling.

Self-Trimming Efficiency Calculation:

Self-Trimming Efficiency = (Vs / Vt) × 100%

Where:

• Vs = Volume of self-trimmed cargo

• Vt = Total hold volume

• Typical Efficiency Range: 85% – 92%

Angle of Repose Requirements:

• Grain Cargoes: 20° – 25°

• Coal: 30° – 35°

• Iron Ore: 35° – 40°

• Cement: 25° – 30°

Note: The hopper angle must be steeper than the cargo's angle of repose to ensure proper flow.

3. Double-Hull Cargo Holds

3.1 Technical Requirements

Double-hull cargo holds enhance safety and environmental protection.

Double Hull Spacing:

• Minimum Width: 1,000 mm (as per SOLAS requirements)

• Optimal Range: 1,200 – 1,800 mm

  • Provides sufficient space for inspection, maintenance, and structural integrity.

• Access Trunk Diameter: 600 – 800 mm

  • Allows safe entry for personnel during inspections.

Structural Integration:

• Longitudinal Stiffener Spacing: 600 – 800 mm

• Web Frame Spacing: 2,400 – 3,200 mm

• Plate Thickness: 13 – 17 mm (Grade AH36)

• Material Selection: High tensile steel is used for strength and weight efficiency.

General Structural Considerations

Material Grades

• Grade A Steel: Used for standard structural components.

• High Tensile Steel (AH32/AH36): Employed where higher strength is required without increasing weight.

Safety Factors

• Structural Safety Factor: 1.67

  • Provides a margin of safety against unexpected loads or material defects.

Stiffeners and Frames

• Stiffeners: Support the plating and prevent buckling under load.

• Frames: Provide overall structural support to the hull and cargo holds.

Compliance with Regulations

• IACS (International Association of Classification Societies): Sets unified requirements for structural safety factors and material standards.

• SOLAS (International Convention for the Safety of Life at Sea): Specifies minimum double hull spacing and other safety features.

• Classification Societies: Such as ABS, DNV, and Lloyd's Register, provide detailed rules for design and construction.

Conclusion

Technical specifications are fundamental to the design and operation of cargo holds. They ensure that the holds are capable of safely carrying the intended cargoes, withstand the stresses of maritime operations, and comply with international safety and environmental regulations. Understanding these specifications allows ship designers and operators to optimize cargo capacity, ensure structural integrity, and maintain high safety standards.