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Marine & Station Keeping
Anchors and Chains
Anchors and Chains covers the drag embedment anchors and offshore mooring chain that form the seabed end of a mooring leg. It includes stud link and studless chain in grades R3 to R5, connecting links, anchor shackles, swivels, chasers and the fluke and shank assemblies of high holding power anchors used on semi-submersibles and moored drillships.
How anchors and chain hold a rig
A drag embedment anchor digs in as the leg is tensioned, and its holding capacity grows with penetration and soil strength. You set the fluke-to-shank angle for the soil, with a smaller angle for sand and a larger one for soft clay. The chain's weight forms a catenary. That catenary keeps the anchor shank close to horizontal and absorbs rig motion. Vryhof Stevpris and Bruce designs are common on mobile units. Vertical load anchors and suction piles appear mainly in pre-laid or long-term moorings.
Configurations
| Item | Variants | What they suit |
|---|---|---|
| Drag anchors | Fixed-fluke, adjustable-fluke | Mobile moorings, soft to medium soils |
| Chain | Stud link or studless | Studless is lighter per metre at the same diameter |
| Grade | R3, R3S, R4, R4S, R5 | Higher grades carry more load at the same diameter |
| Connectors | Kenter links, D and pear links, shackles | Joining shots and components |
Kenter connecting links have a poorer fatigue record than common links, and long-term moorings tend to avoid them.
Key specifications
| Parameter | Why it matters | Typical classes or units |
|---|---|---|
| Nominal diameter | Sets MBS and wildcat fit | mm (in) |
| Grade | Strength and toughness | R3 to R5 |
| Proof load and MBS | Certificate values for the analysis | kN (kips) |
| Shot length | Handling and joining | 27.5 m (15 fathoms) common |
| Anchor weight and type | Holding capacity in your soil | tonnes (kips), design name |
| Fluke angle | Penetration in sand or clay | degrees, per OEM table |
Standards
- IACS UR W22 and DNV-OS-E302 cover R-grade chain manufacture, including steel, heat treatment, proof load tests on every length and break load tests on samples. API Spec 2F covers mooring chain made to API grades.
- API RP 2I gives in-service inspection methods and discard guidance for chain, connectors and anchors.
- API RP 2SK and ISO 19901-7 set holding capacity and line tension checks for the mooring analysis.
- Class rules set approval and proof testing for high holding power anchors.
A used length of chain needs its original certificate, traceable marking and measured condition. Without the certificate, class will usually not accept it for a classed mooring.
Failure modes and inspection
- Interlink wear at the crown of each link reduces diameter. Measure it with calipers at the bearing points and compare with the limit set by your class society.
- General and pitting corrosion. Grit blast a sample and measure pit depth.
- Loose or missing studs in stud link chain. A loose stud lets the link deform and starts cracks.
- Fatigue cracks at the link crown and the flash butt weld. Use magnetic particle inspection on cleaned links.
- Bent or twisted links from handling over fairleads. Check visually and with gauges.
- Anchors bend at the shank and crack at the fluke welds and padeye. Use visual inspection and magnetic particle testing.
Documentation
- Maker's certificate for each length, with grade, diameter, proof load and break load test results.
- Class or third-party witness of manufacture.
- Marking that ties each length to its certificate.
- In-service inspection report with measured diameters.
- Anchor certificate with type, weight and proof test.
- Service history, including where and how long the chain worked.
Integration
The chain diameter and type must match your wildcats, fairleads and stoppers. The grade and MBS must match your mooring analysis. Connectors must pass over the wildcat and through the fairlead. The anchor weight and type must give the holding capacity your soil survey supports, and the anchor shackle must suit the chain and the anchor handling vessel's equipment.
Recertification and the used, surplus or refurbished choice
Recertifying used chain means cleaning it, inspecting every link, measuring diameters, testing with magnetic particles and, where class asks, proof loading it again on a test bed. Unused surplus chain from cancelled projects with full certificates is the simplest to approve. Anchors last for decades and are often reused after an inspection for bending and cracks. Connectors for chain and wire legs are in Mooring Lines and Towing Gear.
FAQ
Frequently asked questions
What is the difference between R3, R4 and R5 mooring chain?
R3, R4 and R5 are offshore mooring chain grades in rising order of strength. A higher grade carries a higher breaking load at the same diameter, which saves weight in deep water. The grade, with its certificate, must match your mooring analysis and class approval.
How is used anchor chain inspected?
Used anchor chain is cleaned, examined link by link and measured with calipers at the interlink wear points. Inspectors look for cracks, bent links, loose studs and corrosion pits, and use magnetic particle testing on suspect links. The readings are compared with the class limit for diameter loss.
Stud link or studless chain for a drilling rig?
Both are used, and the choice follows your windlass wildcats and mooring design. Studless chain is lighter at the same diameter and has no studs to work loose. Many older rigs run stud link chain, and their wildcats are cut for it.
Can used mooring chain be proof tested again?
Yes, used chain can be proof loaded again on a chain test bed, usually after cleaning and inspection. A new proof test shows the chain carries its rated proof load today. It does not restore lost diameter or replace the original manufacturing certificate.
What fluke angle should a drag anchor use?
Use a smaller fluke-to-shank angle in sand and a larger one in soft clay, following the anchor OEM's table. The wrong angle reduces penetration and holding capacity. Check the angle setting when buying a used anchor, since it may be set for its last location.
Looking for anchors and chains?
Tell us what you need and we will find out what is available. We reply within one business day.