U1, U2 and U3 are three grades of stud link chain and fittings for ship anchor cables, named in Lloyd's Register's materials rules [3]. They are strength bands for the chain steel, not a quality ranking. IACS and ISO use numbers for the same three-step idea: the IACS unified requirements say Grades 1, 2 and 3, and ISO 1704:2022 defines grades 1 to 4 [1][2][5].
On one point the correspondence is checkable. The proof-load and breaking-load expressions LR publishes for U1, U2 and U3 are the same ones IACS publishes for Grades 1, 2 and 3, so both naming systems sit on a single load basis [2][3]. That does not make the labels interchangeable: the strength bands and the test exemptions behind them do not line up exactly, so a grade has to be read against the rule that issued it [1][3].
What a grade controls is wider than the label. At a given Equipment Number it sets the minimum chain diameter; at a given diameter it sets the proof load the cable is tested to; and it sets the test and certification scope behind the certificate. Change the grade and all three move.
What a grade designates
A chain-cable grade is a band of chain steel, defined in the document that names it. IACS UR A1 Table 3 gives each of its three grades a material designation — mild, special quality and extra special quality steel — together with a range of ultimate tensile strength, and it allows a society to split Grade 1 into 1a and 1b [1]. UR W18 states the same subdivision in a single sentence: stud link chain cables are divided into Grades 1, 2 and 3 depending on the nominal tensile strength of the steel used to make them [2]. LR's materials rules set out U1, U2 and U3 as a range of grades of stud link chain and fittings intended for anchor cables for ships, and require the design to follow a standard LR recognises, such as ISO 1704 [3].
Three points follow from that wording.
First, a grade names a property of the steel, stated as a band rather than a single figure. Second, the label belongs to the document that defines it, which is why the same three-step idea appears under two different naming habits. Third, LR separates the two questions: it supplies the U labels, and it delegates the geometry to a recognised standard. Naming and standard are separate choices, and a chain specification needs both.
The grade set is also not fixed for all time. ISO 1704:2022 defines grades 1 to 4 of stud link anchor chain and records grade 4 as newly defined in that edition [5]. The three-step set is therefore a habit of the IACS and LR documents, not a property of every standard in use.
A third scheme works at a different level again. CCS names rolled bar material for anchor chain AM1, AM2 and AM3 — bar grades for the steel the chain is made from, not a fourth label for the finished chain [6].
A grade is a field of the product, on the same footing as nominal diameter and shot length. It is not a score for the maker, and a higher grade does not by itself describe a better-made chain: it describes a different strength band with its own test obligations.
How the grade enters equipment selection
The sizing rule and the grade rule are separate documents, and UR A1 says so directly: manufacture of anchors and anchor cables is to follow UR W29 and UR W18 [1]. UR A1 also states its own working scope. The anchoring equipment it covers is intended for temporary mooring of a ship in a harbour or sheltered area while awaiting berth or tide, and it is not described there as designed to hold a ship off an exposed coast in rough weather [1]. Grade questions sit inside that scope.
Within it, the grade is an input to the equipment table, not an accessory to it. UR A1 Table 1 tabulates the required minimum chain diameter separately for Grade 1, Grade 2 and Grade 3 at the same Equipment Number, and the Grade 1 value is the largest of the three [1]. The table is not offering three equivalent options at three prices. It is stating that the diameter the rule requires depends on which grade the cable is.
The load basis moves in the other direction at a fixed size. At the same nominal diameter, the proof and breaking loads required rise with grade, and UR A1 expresses the Grade 2 and Grade 3 values as multiples of the Grade 1 value [1]. Read the two statements together and the grade is doing two jobs at once: at a given Equipment Number it changes the diameter, and at a given diameter it changes the load the cable has to withstand.
Neither statement is a route to a lighter cable on an existing ship. A smaller tabulated diameter at a higher grade is what the table requires for a new selection at that Equipment Number; it is not permission to fit a lighter chain to an arrangement that was approved on a different grade. Changing the grade moves the required diameter, the proof load and the test scope together, and whether a particular vessel can be changed is a question for the society that approved the equipment, not something the grade label answers.
Grade, diameter, test and certificate are four different things
Four fields tend to collapse into the single word "grade". They are worth keeping apart, because each one is where a quotation goes wrong.
Diameter. UR A1 takes the cable from Table 1 for the calculated Equipment Number and requires it to be tested to the proof loads corresponding to that cable [1]. Diameter and grade are read together from one table; once the other is fixed, neither is a free parameter.
Proof test. Testing runs by length of cable, not by shipment. Under UR W18 each length of cable is proof loaded on an approved testing machine, and breaking-load samples are drawn from every fourth length, with the surveyor present when the sample links are separated from the cable [2]. The unit behind the record is the length.
Test scope. The tests required depend on the grade and on the condition in which the chain is supplied. In UR W18 Table 6, flush-butt welded Grade 1 in the as-welded condition carries no tensile or Charpy requirement, while Grade 2 and Grade 3 carry specified specimens [2]. LR's rules say the same thing in their own terms: mechanical property testing is not required for U1 cable, while U2 in the as-welded condition and U3 in all conditions require tensile and Charpy specimens [3]. The same physical chain, certified to a different grade, carries a different test file.
Approval coverage. Grade approval is conditional rather than blanket. UR W18 allows a procedure test carried out on a high-strength chain cable to cover approval of lesser grades only where the material type, the method of manufacture and the nature of the heat treatment are the same [2]. The grade on a certificate therefore describes the range that was actually approved, not every grade that shares the name.
Where the labels come from
| Where the label appears | How the grades are labelled | What that label does and does not tell you |
|---|---|---|
| IACS unified requirements (UR A1, UR W18) | Grades 1, 2 and 3 | Identifies the strength band the cable is made and tested to. Does not give a diameter; that comes from the equipment table for the ship's Equipment Number. |
| Lloyd's Register materials rules, ship anchor cables | U1, U2, U3 | The same three-step idea under LR's own naming, with the geometry taken from a standard LR recognises. Does not make the labels interchangeable with another document's numbering. |
| ISO 1704 | Grades 1 to 4 | Fixes the grade set in that standard, with grade 4 added in the 2022 edition. Does not mean every rule in use has four grades. |
| CCS rolled-bar guideline | AM1, AM2, AM3 | Names bar steel for anchor chain cable and accessories. Does not label the finished chain. |
| Offshore mooring chain series (LR) | R3, R3S, R4, R4S, R5 | A separate series for offshore mooring cable in the same rules. Does not sit above U3 for a ship anchor cable. |
| Other societies | Check the rule the order is placed against | The label is defined only inside that rule. Do not infer the set from another society's document. |
Four labels that are not chain grades
An equipment number that looks like a U-grade. One manufacturer's catalogue shows the collision plainly. In its equipment table, U1 to U24 run down the page as equipment-number labels alongside anchor mass and chain length, and the chain grade columns — Grade 1, Grade 2, Grade 3 — sit beside them [7]. That is how one catalogue lays out its own table, cited here as an illustration rather than as a pattern across the industry. The usable lesson is narrow: read the column heading before reading a U-number as a grade.
A shackle or lifting-chain grade. A shackle and a lifting chain are each specified by their own product standard, and a grade quoted for one of those products is that product's designation. It does not become an anchor-cable grade because the number looks familiar.
The offshore series. LR's materials rules list U1, U2 and U3 for ship anchor cables and, in the same rules, a separate offshore mooring chain series [4]. That series is not a further step above U3 for a ship anchor cable; it belongs to mooring cable, and a ship's bower cable is not ordered on it.
"A higher grade is a straight upgrade." It is a different strength band with its own tabulated diameter, its own proof load and its own test scope. Whether it is the right choice for a given ship is settled by the Equipment Number, the approved arrangement and the rule the order is placed against — not by the letter or the number in the label.
What to send before a comparison
If the question is whether a quoted chain matches the one on the drawing, the labels alone will not settle it. What settles it is the standard and edition the order is placed against, the grade exactly as printed on the specification or certificate, the nominal diameter and shot length, the connecting parts at each end, and the record of which tests the cable has passed.
For the fields that together make up a chain specification — construction, diameter, standard, lengths, connecting configuration — the overview of ship anchor chains sets out the terminology and the part names. The anchor chain range covers stud link cable, and connecting parts covers the shackles and swivels at the ends. If you already hold a specification or a certificate, send it to us together with the standard it cites, so the grade can be read against the rule that issued it.
References
[1] International Association of Classification Societies, UR A1 — Anchoring Equipment (Rev.7 2020, Corr.1 Sep 2021), A1.1.1–A1.1.2 (scope), A1.1.6, A1.5.1.1, A1.5.2 and Table 3 (grades), Table 1 (minimum diameters by grade), A1.5.3 and Table 4 (load basis by grade) (https://ww2.eagle.org/content/dam/eagle/regulatory-news/2022/IACS-UR-A1.pdf). Cited for the working scope of the anchoring equipment, the three-grade structure, the grade columns of the equipment table and the change of load basis with grade. No load or diameter value is reproduced from it here, and a later revision of UR A1 exists that was not read for this article.
[2] International Association of Classification Societies, UR W18 — Anchor Chain Cables (Rev.6 Sep 2021), 1.1–1.2 (scope and grades), 1.3.2 (conditional approval), Table 6 (test specimens by grade and supply condition), 4.1.2–4.1.3 (proof loading by length, breaking-load samples) (https://www.caspianlloyd.az/wp-content/uploads/senedler/Material%20and%20Welding%20IACS.pdf). Cited for the material/manufacture rule, the test scope that varies with grade, and the conditional coverage of a grade approval. The text was read in a third-party mirror of the IACS Materials and Welding book, not on iacs.org.uk; the load formulas in it are not reproduced here.
[3] Lloyd's Register, Rules for the Manufacture, Testing and Certification of Materials (July 2022), Chapter 10 Section 2, 2.1.1–2.1.3, 2.3.2 and Table 10.2.4 (https://imorules.com/LRMAT_CH10_2.html). Cited for the U1/U2/U3 naming of ship anchor-cable grades, the reference of chain design to a recognised standard such as ISO 1704, the absence of a mechanical property test requirement for U1, and the identity of the load expressions with the IACS ones. A licensed HTML reproduction of the rules, not the LR PDF; read only for the clauses listed. Not used to state values, and not used to claim that the labels are interchangeable.
[4] Lloyd's Register, Rules for the Manufacture, Testing and Certification of Materials (July 2022), Chapter 3 Section 9, 9.1.1–9.1.2 (https://imorules.com/LRMAT_CH3_9.html). Cited only for the statement that the same rules provide for three ship grades of stud link chain cable and a separate five-grade offshore mooring cable series. Not used to describe offshore mooring design, and not as evidence that the offshore series extends the ship grade sequence.
[5] International Organization for Standardization, ISO 1704:2022 — Ships and marine technology — Stud-link anchor chains, foreword (main changes) and Table 1, preview copy (https://cdn.standards.iteh.ai/samples/77746/bf35a8e6871a4fcaabbef575f43d1dab/ISO-1704-2022.pdf). Cited only for the grade set of that edition and the statement that grade 4 was newly defined in it. A 13-page preview, not the complete standard; its designation, marking and documentation clauses were not available and are not cited.
[6] China Classification Society, Guideline W-08 — Rolled steel bars for anchor chain cables and accessories (201510), 3.1 (https://www.ccs.org.cn/ccswzen//file/download?fileid=201950000000001321). Cited only for the AM1/AM2/AM3 designation of rolled bar material for marine anchor chain cables. These are bar-steel grades, not finished chain grade labels, and no bar property value is reproduced.
[7] SOTRA Anchor & Chain AS, Catalogue, 8th edition 2021, equipment table (https://www.sotra.net/wp-content/uploads/2021/08/Catalogue-8.pdf). Cited as one published example of a table in which U1 to U24 are equipment-number labels sitting beside chain grade columns. That catalogue's layout only; not evidence of an industry-wide practice, and not evidence of any supplier's certification.
