Anchor Chain Selection Guide: Stud-Link vs Open-Link, Grades and Sizing
Selecting the right anchor chain for a vessel or marine installation is not a one-size-fits-all decision. The choice involves understanding the difference between stud-link and open-link chain, selecting the appropriate grade (U2 or U3), determining the correct diameter, and ensuring the chain meets the classification requirements of the relevant classification society — ABS, Lloyd’s Register, BV, or others.
Stud-Link vs Open-Link Chain
The fundamental structural difference lies in whether each link has a transverse bar passing through its centre.
| Feature | Stud-Link Chain | Open-Link Chain |
|---|---|---|
| Centre bar in each link | Yes | No |
| Prevents twisting | Yes — primary function of the stud | No — chain can rotate |
| Strength | Higher — stud adds stiffness | Lower for same diameter |
| Stowage on windlass | Clean stacking, less tangling | Prone to twisting on the barrel |
| Weight | Slightly heavier | Lighter |
| Typical use | Mooring chain (standard) | Cargo hook chain, towing, dredging |
Stud-link is the universal standard for mooring chains on ships and offshore platforms. The centre stud prevents individual links from spinning under load, keeping the chain neat in the chain locker and reducing wear from link-to-link friction. The stud also adds about 10% more tensile strength compared to an equivalent open-link chain.
Open-link chain is used for cargo handling (cargo hooks), towing operations, anchor drogue lines, and dredging. In a mooring context, open-link is rarely specified by classification societies for new builds.
Quick Guide: If it connects an anchor to a ship permanently, use stud-link. If it’s used for lifting cargo, towing, or temporary securing, open-link is acceptable.
Chain Grades: U2 vs U3 (R3 vs R4)
Marine anchor chain is graded by minimum tensile strength, expressed as proof coil load in tonnes for a reference diameter:
| Grade designation | ISO / DIN equivalent | Min. proof load basis | Typical tensile strength |
|---|---|---|---|
| U2 | R3 | ~4.0 tonnes (66mm dia. reference) | ~36.7 tonnes (66mm) |
| U3 (R3S, R4) | R3 Stretch / R4 | Higher — ~R4 basis | ~42.5 tonnes (66mm) |
U2 (R3) is the standard grade for most commercial vessels, tugs, and workboats. It meets the classification requirements of ABS, Lloyd’s Register, DNV, and other major societies for ships up to a certain gross tonnage and mooring load.
U3 (R4) is specified for larger vessels, offshore platforms, and deepwater anchor handling where the breaking load at standard diameters exceeds what U2 can provide. Newbuild tankers, bulk carriers, and offshore support vessels typically require U3 for their mooring chain.
Diameter Selection and Breaking Load
Selecting the correct chain diameter depends on the vessel’s size, the type and weight of anchor, and the classification society’s mooring requirements for that specific vessel class. Reference table for U2 (R3) stud-link chain:
| Chain diameter (mm) | Min. breaking load (tonnes) | Approx. weight (kg/m) |
|---|---|---|
| 18 | 5.6 | 2.0 |
| 22 | 8.5 | 3.0 |
| 24 | 10.1 | 3.6 |
| 28 | 13.8 | 5.0 |
| 30 | 15.6 | 5.8 |
| 34 | 20.0 | 7.5 |
| 38 | 24.9 | 9.3 |
| 44 | 33.5 | 12.5 |
| 54 | 50.4 | 18.8 |
These are reference values. Actual specifications must be verified against the latest classification society rules and manufacturer data sheets.
ABS and Lloyd’s Register (LRS) Classification Requirements
Both ABS and Lloyd’s Register require anchor chain to be manufactured, heat-treated, and tested in accordance with their respective specifications. Key requirements include:
- ✓ Chain must be manufactured by an approved forge with classification society surveyor oversight
- ✓ Each length (typically 27.5m) must be individually tested and tagged
- ✓ Proof coil (sample link) testing is mandatory — one link per length is pulled to yield and inspected
- ✓ Chain must be furnished with joining shells, swivels, and anchor shackle as ordered
- ✓ Heat treatment process (quenched and tempered) must be documented and traceable
Sealinkers supplies chain forged to both ABS and Lloyd’s Register specifications, with all required certificates and traceability documentation.
Summary
Anchor chain selection involves three interdependent choices: stud-link (almost always for mooring), the appropriate grade (U2 for commercial vessels, U3 for large ships and offshore work), and diameter based on the vessel’s classification requirements. Never undersize — mooring chain failure is one of the most dangerous failures a vessel can experience.
