Overview
ISO 13799:2020 - Ships and marine technology - Ship's mooring and towing fittings - Recessed bitts (casting type) specifies design, materials, manufacturing, inspection and marking requirements for casting-type recessed bitts used for normal towing and mooring. The standard covers nominal sizes (850 mm and 920 mm), six classification types (Type 75, 110, 135, 100, 150, 180) and the associated safe working loads (SWL), material yield strengths and dimensional requirements. Annex A defines the strength assessment methodology based on finite element model (FEM) analysis.
Key topics and requirements
- Scope and classification
- Two nominal sizes: 850 mm and 920 mm.
- Six types by nominal size and casting yield point:
- 850: Type 75 (235 N/mm², SWL 75 t), Type 110 (350 N/mm², SWL 110 t), Type 135 (430 N/mm², SWL 135 t).
- 920: Type 100 (235 N/mm², SWL 100 t), Type 150 (350 N/mm², SWL 150 t), Type 180 (430 N/mm², SWL 180 t).
- Dimensions and construction
- Detailed dimensional tables and figure define geometry, mounting and welding interfaces.
- Recessed bitts are considered part of the hull side shell and require reinforcement (carlings, stiffeners).
- Welding method to hull must be approved by a Classification Society.
- Materials
- Weldable steel castings with specified minimum yield strengths according to type.
- Strength assessment (Annex A)
- FEM-based analysis conditions, mesh sizing, boundary conditions and load cases.
- Stress limit under SWL: 85% of material yield stress.
- Wear allowance: 2 mm deducted where rope rubs.
- Manufacturing, inspection and marking
- Surfaces and rope-contact areas free from flaws and abrasive irregularities.
- External anti-corrosion protective finish required.
- SWL must be recorded in the ship’s towing and mooring plan (IMO MSC/Circ.1175) and marked on the fitting in tonnes (e.g., “SWL XXX t”).
Applications and users
ISO 13799:2020 is essential for:
- Shipbuilders and shipyards fitting recessed bitts during construction or retrofit.
- Marine equipment manufacturers designing and casting recessed bitts.
- Naval architects and structural engineers specifying towing fittings and hull reinforcement.
- Classification societies, shipowners, shipmasters and fleet operators for compliance, inspection and operational planning.
- Offshore and towing contractors selecting appropriate mooring/towing points.
Use cases include specifying recessed bitt geometry and SWL for new vessels, validating strength by FEM, ensuring correct welding and hull reinforcement, and marking and documenting SWL in mooring and towing plans.
Related standards
- ISO 4990 - Steel castings - General technical delivery requirements
- ISO 9477 - High strength cast steels for general engineering and structural purposes
- IACS UR A2 - Shipboard fittings & supporting hull structures for towing and mooring
- OCIMF Mooring Equipment Guidelines (MEG4)
- IMO MSC/Circ.1175 - Guidance on shipboard towing and mooring equipment
Keywords: ISO 13799:2020, recessed bitts, ship's mooring and towing fittings, casting type, SWL, FEM, dimensions, welding, marking.