Overview
ISO 20438:2017 - Ships and marine technology - Offshore mooring chains - specifies requirements for the design, manufacture and dimensional control of offshore mooring chains. The standard defines grades, materials, shapes, proportions, dimensions and tolerances for mooring chain components used to position and moor offshore structures (chain links, enlarged links, end links and shackles). ISO 20438:2017 is aimed at ensuring consistent mechanical performance and interchangeability for chains used in demanding marine environments.
Key Topics and Technical Requirements
- Grade classification: Mooring chains and base steels are classified into five tensile grades - R3, R3S, R4, R4S and R5 - with corresponding mechanical property requirements (yield strength, tensile strength, elongation, reduction of area and impact energy at specified temperatures).
- Material and steelmaking:
- Specified manufacturing processes (electric process, basic oxygen or secondary refining); R4S and R5 require vacuum degassing.
- Minimum 0.20% molybdenum for R4, R4S and R5.
- Mill test requirements for higher grades: non‑metallic inclusion inspection (ISO 4967), macroetch testing (ASTM E381), and Jominy hardenability data (ASTM A255).
- Dimensions and shapes: Standardized geometries and nominal dimensions for:
- Common studless and stud links
- Enlarged links and end links
- Kenter and LTM shackles
Dimension tables (Tables 3–12) provide nominal sizes and derived parameters (pitch, link length, width).
- Tolerances and measurement:
- Diameter measured at the crown; no negative average tolerance, plus tolerance not to exceed 5% of nominal diameter.
- Specified negative crown tolerances by nominal diameter (e.g., −2 mm for 50–84 mm, −3 mm for 85–122 mm, up to −8.5 mm for >210 mm).
- Weld plus tolerance limited to 15% of nominal chain diameter.
- Five‑link length and other dimensional tolerances explicitly specified.
- Proof and breaking loads: Proof load and breaking load values are given per grade and depend on nominal diameter (see Table 2 in the standard).
Practical Applications and Users
ISO 20438:2017 is used by:
- Mooring chain manufacturers and foundries to specify production and inspection criteria.
- Offshore engineers and designers (FPSO, floating platforms, floating wind turbines) to select chains meeting target strength and fatigue requirements.
- Shipyards, rig owners and asset operators for procurement and regulatory compliance.
- Classification societies and testing laboratories for conformity assessment, mill-test verification and acceptance testing.
- Maintenance and inspection teams referencing dimensional tolerances and material requirements for replacement planning.
Keywords: ISO 20438:2017, offshore mooring chains, mooring chain grades, chain dimensions, proof load, breaking load, marine technology, LTM shackle.
Related Standards (referenced)
- ISO 19901‑7 (terms/definitions)
- ISO 4967 (non‑metallic inclusions)
- ASTM A255 (hardenability / Jominy)
- ASTM E112 (grain size)
- ASTM E381 (macroetch testing)
