Offshore Mooring Chain — Grades R3 to R5

Not the same product as anchor chain

This is the distinction that costs money when it is got wrong. Ship anchor chain cable is governed by one set of class requirements; offshore mooring chain by another. They share the same load equation and the same link proportions, and nothing else. A works approval for Grade U3 ship chain does not entitle a maker to supply Grade R4 mooring chain — the grades are approved individually, and even an R3 approval does not automatically confer R3S.

Anchoring equipment on a ship is explicitly for temporary mooring in harbour or sheltered water. An offshore mooring is a permanent or long-term station-keeping system designed for the environment it sits in. Different job, different rules.

The grade ladder

Minimum tensile strength climbs steeply and the metallurgical requirements climb with it:

• R3 — 690 N/mm², yield 410, elongation 17 %, reduction of area 50 %.
• R3S — 770 N/mm², yield 490.
• R4 — 860 N/mm², yield 580. Impact testing moves to −20 °C, and the steel must carry at least 0.20 % molybdenum.
• R4S — 960 N/mm², yield 700. Vacuum-degassed steel; hardness capped.
• R5 — 1000 N/mm², yield 760. Vacuum-degassed, hardness capped, and additional qualification data — fatigue, strain-age and temper and hydrogen embrittlement studies.

From R3S upward, every heat is hydrogen-embrittlement tested. That is not a formality: these are the grades where a delayed failure mechanism, not an overload, is the thing that ends a mooring leg.

Stud and studless

Breaking load is identical for stud and studless chain of the same grade and diameter; proof load is not, and the table below gives both. Studless chain is used for single-deployment permanent moorings, where the absence of studs removes a fatigue initiation site and a maintenance liability that a permanently submerged mooring cannot service anyway.

Inspection

The whole of the chain is proof-loaded. Break samples of at least three links are taken to a sampling table with every cast represented, held for 30 seconds. Bar is 100 % magnetic-particle or eddy-current inspected and forgings 100 % ultrasonically inspected, with dimensional checks on at least 5 % of links plus a continuous five-link length check under a light load. The chain is shot- or sand-blasted before non-destructive examination so the inspection sees metal, not scale.