1. Why this matters on offshore projects
Most Norwegian and European design offices produce structural drawings to ISO 2553. Most South Korean and American fabrication yards — where the majority of offshore topsides, jackets, and modules are built — work to AWS A2.4. When a drawing package crosses that boundary, a weld symbol that looks identical in both systems can specify a completely different weld configuration.
The welder sees an ISO symbol. He reads it as AWS. He places the fillet on the wrong side of the joint. The weld passes visual inspection. It passes magnetic particle testing. The NCR doesn't surface until radiography or TOFD during pre-installation inspection — at which point the structure may be half-assembled, the joint is inaccessible, and the repair cost is entirely disproportionate to the original error.
This problem is not hypothetical. It is a well-documented pattern on NCS projects where Norwegian engineering contractors issue drawings to Korean or Singaporean fabrication yards without explicitly declaring the weld symbol standard. The risk is compounded by the fact that most welders and many QC inspectors do not spontaneously question which standard applies — they default to whatever standard they trained on.
Understanding exactly where ISO 2553 and AWS A2.4 diverge — and where they appear identical but are not — is essential knowledge for any engineer who issues or reviews offshore structural drawings.
2. The two standard families
ISO 2553:2019 (Welding and allied processes — Symbolic representation on drawings — Welded joints) is published by the International Organization for Standardization. It is the dominant standard in Europe, most of Africa, and much of Asia outside Korea and Japan. Norway, the UK, the Netherlands, France, Germany, and Australia all work to ISO 2553 by default. DNV-ST-0378, NORSOK M-120, and EN 1090 all reference ISO 2553.
AWS A2.4:2020 (Standard Symbols for Welding, Brazing, and Nondestructive Examination) is published by the American Welding Society. It is the standard in the United States, Canada, South Korea, and Japan. Most major fabrication yards in Ulsan, Geoje, and Mokpo work to AWS A2.4 as their internal reference, regardless of what the design drawings state — unless the project contract explicitly requires otherwise.
Key structural differences at a glance
The most significant systemic differences between the two standards are:
- Arrow-side / other-side convention — both use above/below the reference line, but with opposite meaning in certain symbol orientations. This is the single most common source of misinterpretation.
- Throat vs. leg size notation — ISO distinguishes throat (prefix "a") from leg (no prefix) explicitly; AWS uses leg size as the default with a separate convention for throat.
- Intermittent weld notation format — completely different syntax for specifying weld length and pitch.
- Supplementary symbol placement — where finish, contour, and backing symbols are positioned relative to the reference line differs between the two systems.
3. Arrow line and reference line
Both ISO 2553 and AWS A2.4 use the same basic drawing element: an arrow line pointing to the joint, connected to a horizontal reference line. Weld symbols are placed above or below this reference line to indicate which side of the joint is being specified. Here the similarity ends.
ISO 2553 convention
In ISO 2553, symbols placed below the reference line apply to the arrow side of the joint — the side the arrow physically points to. Symbols placed above the reference line apply to the other side (the side opposite the arrow). The standard also provides for a dashed identification line, placed parallel to the reference line, which can be drawn above or below it to further clarify which side is intended. In ISO 2553, the dashed line is on the same side as the symbol when specifying the other side.
AWS A2.4 convention
In AWS A2.4, symbols placed below the reference line also apply to the arrow side. Symbols placed above the reference line apply to the other side. So far, this matches ISO.
The critical difference emerges with the broken arrow. In AWS A2.4, when the arrow breaks (has a kink or jog in it before reaching the joint), this signals that the arrow points specifically to the member that receives the bevel or groove preparation — not merely to the joint in general. ISO 2553 has no exact equivalent of the AWS broken-arrow convention, though it uses arrow-side / other-side designation for the same purpose. An ISO drawing engineer who adds a bend in the arrow for clarity (a common informal practice) may unintentionally trigger the AWS broken-arrow interpretation at a Korean yard.
4. Fillet weld notation
The fillet weld is the most common weld type in offshore structural work — joining stiffeners to plates, braces to chords, gussets to beams. The symbol is a right triangle in both standards. But the size specification differs significantly.
ISO 2553 fillet notation
ISO 2553 uses two possible size designations for a fillet weld:
- Leg length — specified as a plain number to the left of the triangle symbol, without prefix. Example: 6 means a fillet with 6 mm leg length.
- Throat thickness — specified with the lowercase prefix "a" immediately before the number, to the left of the symbol. Example: a5 means a 5 mm throat. For a standard 45° fillet, a5 corresponds approximately to leg = 7 mm (since throat = leg × 0.707).
ISO: a5 ◁ → throat = 5 mm fillet (leg ≈ 7 mm)
ISO: 6 ◁ → leg = 6 mm fillet (throat ≈ 4.2 mm)
The "a" prefix is mandatory in ISO 2553 when throat thickness is the design basis. This distinction matters: European structural codes (EN 1993-1-8) and NORSOK design checks are throat-based — the design strength is calculated on the effective throat. When a Norwegian design engineer writes a5, he means the weld must achieve a 5 mm throat regardless of leg geometry. A Korean QC inspector reading this as a leg measurement will accept an undersized weld.
AWS A2.4 fillet notation
AWS A2.4 specifies fillet weld size as leg length by default, written to the left of the weld symbol. There is no "a" or "S" prefix in the standard symbol position for a standard fillet; size to the left of the symbol is always leg. Effective throat can be specified, but it requires an additional notation — it is not the default.
AWS: 6 ◁ → leg = 6 mm fillet (throat ≈ 4.2 mm for standard fillet)
AWS: E6 ◁ → effective throat = 6 mm (non-standard; requires explicit callout)
Symbol orientation
The fillet triangle symbol itself is visually the same in both standards — a right triangle with the vertical leg on the left and the hypotenuse on the right. ISO places the symbol so the right angle points toward the reference line; AWS uses the same orientation. In practice the symbols are drawn identically by most CAD systems, which means the standard ambiguity cannot be resolved by looking at the symbol shape alone — the title block and drawing notes are the only indication of which system applies.
5. Butt weld and groove weld notation
Groove welds (butt welds) cover a wide range of joint preparations: square butt, V-groove, double-V, U-groove, J-groove, bevel groove, and combinations thereof. Both standards define symbols for all major groove types, but there are important differences in how certain joint geometries are represented and how the prepared side is designated.
V-groove and bevel-groove
For a single-V groove weld, both ISO 2553 and AWS A2.4 use a V-shape symbol on the reference line. The differences emerge in the details:
- Root opening — ISO places the root opening dimension inside parentheses to the right of the symbol: (3) for a 3 mm root gap. AWS places it inside the symbol itself, conventionally written as a number at the root of the V.
- Groove angle — ISO indicates the included angle in degrees to the right of the symbol. AWS places the groove angle inside the symbol above or below the V, depending on configuration.
- Groove depth / weld size — AWS uses the convention where groove depth (S) and weld size (E) are both shown to the left of the symbol as S(E). ISO uses a different notation where the depth of preparation is shown without a letter prefix, and the distinction between a partial-penetration and full-penetration weld is handled by the weld-all-around symbol and text notes.
Flare-bevel and flare-V groove
AWS A2.4 explicitly defines flare-bevel and flare-V groove symbols for joints between a curved surface (pipe or round bar) and a flat plate, or between two curved surfaces. These are distinct symbols with their own graphic representation in AWS.
ISO 2553 does not have dedicated symbols for flare-bevel and flare-V geometries. ISO handles these either through a compound edge weld symbol or by using standard groove symbols with an accompanying note specifying the joint geometry. An AWS drawing with a flare-bevel symbol arriving at an ISO-trained checker may produce confusion, or the symbol may be misinterpreted as a standard bevel groove.
Partial penetration welds
AWS specifies partial joint penetration (PJP) welds using the S(E) notation: S is the required depth of groove preparation, E is the weld size (effective throat for PJP). ISO 2553 handles partial penetration differently — the design throat "a" is specified, and the drawing note or WPS states whether full or partial penetration is required. This creates an interpretation gap when a partial penetration groove is specified: an ISO-trained engineer reads "a" as throat regardless; an AWS-trained inspector looks for the S(E) format and may not recognise the ISO partial penetration designation.
6. Finish and surface contour symbols
Both standards provide symbols to specify the required surface contour of the finished weld face — whether it should be flush (dressed), convex, or concave — and to indicate the method of finishing (grinding, machining, chipping, or unspecified).
Contour symbols
Both ISO 2553 and AWS A2.4 define three surface contour conditions, shown above the weld symbol:
| Contour | ISO 2553 symbol | AWS A2.4 symbol | Notes |
|---|---|---|---|
| Flush | Straight horizontal line above symbol | Straight horizontal line above symbol | Identical in both standards |
| Convex | Arc bowing upward — convex curve line | Same arc bowing upward; letter R indicates grinding to convex profile in some notations | AWS explicitly uses letter suffixes (G, M, C, R) for finish method; ISO places the method letter separately |
| Concave | Arc bowing downward — concave curve; letter C placed above | Arc bowing downward; letter C placed above symbol | Both use C, but AWS also uses the arc as the primary indicator |
Finish method letters
AWS A2.4 uses a specific set of letter codes placed above the contour symbol to indicate how the weld surface should be finished: G (grinding), M (machining), C (chipping), R (rolling), or no letter if the method is unspecified. These letters are mandatory for critical joints where the finishing method affects fatigue performance.
ISO 2553 also uses finish method letters, but they are placed differently — above the symbol in some editions, and the set differs slightly. ISO does not always require a finish symbol where the weld is expected to be left in the as-welded condition, even on critical structural joints. AWS, by contrast, requires a contour symbol on any weld where surface quality is specified, and omitting the symbol is taken to mean the contour is at the welder's discretion.
Weld-all-around
Both standards use the same symbol for weld-all-around: a circle at the intersection of the arrow line and the reference line. This is one of the few areas where ISO and AWS are genuinely identical in both symbol shape and meaning. The same is true for field weld — both use a filled flag at the same location (though AWS uses a different flag orientation in some editions).
7. Supplementary symbols — staggered intermittent welds
Intermittent fillet welds — where the weld is not continuous but deposited in short runs at regular intervals — are extremely common in offshore structural work. They reduce distortion, save weld metal on lightly loaded connections, and are specified on gratings, secondary stiffeners, and non-structural attachments. The notation for intermittent welds is completely different between the two standards, and the numbers used look superficially similar while meaning different things.
ISO 2553 intermittent notation
ISO 2553 uses the format: n × l / e, placed to the right of the weld symbol, where:
- n = number of weld runs (segments)
- l = length of each individual weld run (mm)
- e = pitch — centre-to-centre distance between adjacent weld runs (mm)
ISO: 6 × 50 / 150 → 6 runs, each 50 mm long, at 150 mm pitch (centre-to-centre)
Gap between runs = 150 − 50 = 100 mm
AWS A2.4 intermittent notation
AWS A2.4 uses the format: length – pitch, placed to the right of the symbol (and sometimes broken across both sides of the reference line for chain or staggered welds), where:
- length = length of each individual weld run (inches or mm)
- pitch = centre-to-centre spacing (inches or mm)
- No run count is specified — the total joint length on the drawing determines how many runs result
AWS: 50 – 150 → weld runs of 50 mm, at 150 mm centre-to-centre spacing
(number of runs is implicit from joint length)
Gap between runs = 150 − 50 = 100 mm
Staggered intermittent welds
For staggered intermittent welds — where the welds on the arrow side are offset by half a pitch from the welds on the other side — both standards use symbols on both sides of the reference line. ISO places the symbols in a staggered arrangement relative to each other. AWS uses the same staggered arrangement but adds the explicit pitch dimension on both sides.
8. How to avoid mismatches on offshore projects
The fundamental control measure is simple and costs nothing: declare the weld symbol standard in the drawing title block. Every drawing should carry a note in the title block or the general note block stating: "All weld symbols per ISO 2553:2019" or "All weld symbols per AWS A2.4:2020." Without this declaration, the fabricator has no contractual basis for choosing between interpretations.
For projects crossing standards boundaries
When the design is done to ISO 2553 but the fabrication yard works to AWS A2.4 — the standard situation for Norwegian design / Korean fabrication — the following practices are effective:
Standards references on Norwegian/offshore projects
For clarity, the primary references applicable to NCS and offshore projects:
- DNV-ST-0378 (Offshore and onshore mechanical completion and commissioning) references ISO 2553 for weld symbols on structural drawings.
- NORSOK M-120 (Material data sheets for structural steel) references ISO 2553 implicitly through its reference structure to ISO and EN standards.
- EN 1090-2 (Execution of steel structures) references ISO 2553 as the mandatory weld symbol standard for CE-marked structural steel execution.
- Where a Korean yard is contracted, the project contract should explicitly require: "All weld symbols on supplier and sub-supplier drawings shall be per ISO 2553:2019 unless otherwise agreed in writing."
9. Quick reference table and NCR checklist
The table below summarises the most significant differences between ISO 2553 and AWS A2.4 across the symbol elements most commonly encountered on offshore structural drawings.
| Symbol element | ISO 2553:2019 | AWS A2.4:2020 | Risk if misread |
|---|---|---|---|
| Fillet leg size | Number to the left of symbol, no prefix — e.g. 6 | Number to the left of symbol, no prefix — e.g. 6 | Low — notation identical; dimension units must still be confirmed |
| Fillet throat size | Lowercase "a" prefix before the number — e.g. a5 | No equivalent default prefix; effective throat requires explicit callout (e.g. E5 or separate note) | High — ISO a5 read as leg 5 at AWS yard; throat becomes ~3.5 mm instead of 5 mm |
| Intermittent weld notation | n × l / e (runs × length / pitch) — e.g. 6 × 50 / 150 | length – pitch — e.g. 50 – 150; run count implicit | Medium — numbers superficially similar; total weld length diverges when joint length doesn't match the ISO run count |
| Arrow side / other side | Below ref. line = arrow side; above = other side. Dashed identification line may be above or below. | Below ref. line = arrow side; above = other side. Broken arrow indicates specific prepared member. | Medium — base rule matches; broken-arrow and dashed-line conventions differ and can cause side reversal on complex groove joints |
| Weld-all-around | Circle at the arrow/reference line junction | Circle at the arrow/reference line junction | Low — identical in both standards |
| Flush contour / dressing | Horizontal line above symbol; method letter (G, M, C) placed separately | Horizontal line above symbol; method letter (G, M, C, R) placed above contour line | Low — visually similar; letter position differs slightly but interpretation is generally clear |
| Backing bar / backing strip | Rectangle symbol below the groove symbol; "MR" indicates removable backing | Rectangle symbol below the groove symbol on the other-side of reference line; (M) in parentheses indicates removable | Medium — notation for removable vs. permanent backing differs; a permanent backing left in place due to misreading creates a stress concentration in fatigue-sensitive joints |
| Field weld | Filled flag at arrow/reference junction (flag points away from arrow) | Filled flag at arrow/reference junction (flag orientation may vary by edition) | Low — both standards convey the same meaning; minor flag orientation differences do not affect interpretation |
NCR prevention checklist — before releasing drawings to fabrication
Use this checklist before a drawing package is issued for construction (IFC) to any yard where standard ambiguity exists:
- Weld symbol standard explicitly declared in the drawing title block (ISO 2553:2019 or AWS A2.4:2020) on every drawing in the package
- All fillet welds using the ISO "a" prefix for throat-based design have been reviewed; a note or WPS cross-reference confirms the throat dimension independently of the symbol
- A weld symbol legend sheet is included as the first sheet of the drawing package, covering all symbol types used and their interpretation under the declared standard
- Intermittent weld callouts on primary structural members have been reviewed for correct format (ISO: n × l / e; AWS: l – e) and the format matches the declared standard
- Backing bar callouts on groove welds specify clearly whether backing is permanent or removable, with the applicable notation under the declared standard confirmed with the yard QC team
For projects with high cross-standard risk, add a weld callout table to the general notes sheet that lists every weld type used on the project, the symbol as drawn, and the explicit dimensional requirements (throat in mm, leg in mm, total weld length for intermittent welds). This removes interpretation ambiguity entirely. The extra sheet costs one drafter half a day. The NCR it prevents can cost a week of production time at the yard.
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