ISO 23113:2020

Overview

ISO 23113:2020 - Ships and marine technology - Seats for closed chocks defines requirements for the seats that carry closed chocks used to lead mooring and towing ropes from a ship’s inboard to outboard. The standard specifies types, nominal sizes, dimensions, materials, and rules for construction, manufacturing, inspection and marking. It is intended to ensure safe, durable seats for closed chocks that transmit loads reliably to the hull without plastic deformation or cracking.

Key topics and technical requirements

• Classification and types
  • Type A: deck‑mounted closed chock seats
  • Type B: deck and side shell‑mounted seats
  • Type C: brackets for bulwark‑mounted closed chocks
• Nominal sizes and dimensions
  • Nominal sizes correspond to closed chock sizes (see ISO 13729). Examples in the standard include sizes such as 250 × 200 × 214 and 500 × 400 × 525B. Detailed tables and figures specify dimensional and welding requirements for each type.
• Materials
  • Seats shall be manufactured from weldable steel plates with a yield point of not less than 235 N/mm².
• Construction and strength
  • Foundations and welds must be designed for the actual load directions so that maximum loads are transmitted to hull structure without plastic deformation or cracks.
  • Annex A provides the basis for strength assessment using finite element analysis and defines horizontal and vertical load cases relevant to mooring/towing operations.
  • Stress criterion: combined stress under the SWL is limited (the standard sets allowable stress margins relative to material yield).
• Manufacturing and inspection
  • All surfaces, including welds, must be free of visible flaws.
  • Rope‑contact surfaces must be smooth to avoid abrasion damage.
  • External anti‑corrosion protective coating is required.
• Marking and documentation
  • Safe Working Load (SWL) must be recorded on the ship’s towing and mooring plan per IMO Circular MSC/Circ.1175 and clearly marked on the seat or deck (in tonnes, “t”).

Applications and users

• Practical uses: specifying and verifying seats for closed chocks during ship design, outfitting, refit and maintenance; ensuring compatibility with mooring/towing plans; guiding manufacturers in producing compliant fittings.
• Typical users:
  • Shipbuilders and shipyards
  • Naval architects and marine engineers
  • Manufacturers of mooring and towing fittings
  • Classification societies and surveyors
  • Ship operators and crew responsible for mooring/towing arrangements

Related standards

• ISO 13729 (closed chocks - nominal sizes/thickness)
• IMO Circular MSC/Circ.1175 (guidance on shipboard towing and mooring equipment)

Keywords: ISO 23113:2020, seats for closed chocks, ship mooring fittings, SWL, closed chock seats, marine technology, mooring and towing.