ISO 13729:2020

Overview

ISO 13729:2020 specifies requirements for closed chocks used as ship’s mooring and towing fittings. It covers types, nominal sizes, dimensions, materials, construction, manufacturing, inspection and marking of closed chocks installed to lead mooring and towing rope. The standard applies to both deck-mounted (Type A) and bulwark-mounted (Type B) closed chocks and includes normative guidance for strength assessment using finite element analysis (FEM).

Key topics and technical requirements

• Classification and types: Type A (deck-mounted) and Type B (bulwark-mounted) with defined nominal sizes (e.g. 250 × 200 × 214 up to 500 × 400 × 525B).
• Dimensions: Detailed dimensional tables and figures define openings, depth and radii for each nominal size.
• Materials: Steel casting with a minimum yield point of 235 N/mm² (or equivalent); carbon content ≤ 0.23% to maintain weldability.
• Construction and foundation: Foundations and weld connections must transmit maximum loads to hull structure without plastic deformation or cracking.
• Rope interaction (D/d ratio): Guidance to select fittings and ropes considering the diameter ratio (surface diameter D to rope diameter d) to reduce strength loss. A bending ratio of 12× rope diameter is indicated as the design basis for suggested wire rope diameters.
• Strength assessment: Annex A details FEM-based assessment and loading cases - horizontal (rope deflected 180°) and vertical load cases (specified outboard/inboard angles). Horizontal and vertical loads are evaluated individually.
• Manufacturing & inspection: Surfaces (including welds) must be free of visible flaws; rope-contact surfaces must be smooth to avoid abrasion; external anti-corrosion coating required.
• Marking & SWL: Closed chocks must be clearly marked with the Safe Working Load (SWL) in tonnes by weld bead or equivalent and referenced in the ship’s towing and mooring plan (see IMO Circular MSC/Circ.1175). Actual on-board SWL must consider foundation and reinforcement and must not exceed values in the standard.

Practical applications

• Ensures consistent selection, design and procurement of closed chocks for merchant and naval vessels
• Guides shipbuilders, naval architects and deck outfitting suppliers on dimensional, material and welding requirements
• Supports classification societies and ship operators during inspection, installation and maintenance
• Minimizes rope abrasion and strength loss by specifying D/d considerations and smooth contact surfaces

Who uses this standard

Shipyards, mooring-fittings manufacturers, OEMs, naval architects, ship operators, port engineers and classification societies.

Related standards

• IMO Circular MSC/Circ.1175 (guidance on shipboard towing and mooring equipment)
• ISO/TC 8 (technical committee context for ships and marine technology)

Keywords: ISO 13729:2020, closed chocks, ship mooring fittings, SWL, D/d ratio, finite element analysis, deck-mounted, bulwark-mounted, wire rope.