Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
API Bull 2S-1995 (2001): Technical Bulletin on Offshore Mooring Chain Application
Guidelines for Design, Testing, and Inspection of Mooring Chain Systems
Scope and Purpose
API Bull 2S-1995 (2001) is a technical bulletin issued by the American Petroleum Institute (API) that provides supplementary guidance on the application, testing, and inspection of offshore mooring chain. Although categorized as a bulletin rather than a full specification or recommended practice, it serves as an essential reference for engineers, fabricators, and operators involved in mooring system design and integrity management. The document complements API Spec 2S (Specification for Offshore Mooring Chain) and offers clarification on critical issues that emerged during early implementation of the specification.
The bulletin addresses common misinterpretations of material requirements, nondestructive testing (NDT) acceptance criteria, and chain-handling procedures. It also provides recommendations for proof-load testing, statistical sampling of production chain, and performance verification under simulated service conditions. While the document is not mandatory for regulatory compliance, many classification societies and project specifications incorporate it as a baseline for quality assurance.
Technical Requirements and Material Specifications
API Bull 2S prescribes minimum requirements for chain grade, chemical composition, heat treatment, and mechanical properties. Table 1 summarizes the key technical parameters that fabricators must verify during production and prototype testing.
| Chain Grade | Diameter Range (mm) | Proof Load (kN) | Minimum Breaking Load (kN) | Elongation at Break (%) |
|---|---|---|---|---|
| R3 | 32 – 100 | 1.6 × MBL* | Variable by diameter | ≥ 12 |
| R3S | 32 – 120 | 1.7 × MBL | Variable by diameter | ≥ 12 |
| R4 | 32 – 130 | 1.8 × MBL | Variable by diameter | ≥ 10 |
| R5 | 32 – 157 | 1.9 × MBL | Variable by diameter | ≥ 10 |
* MBL = Minimum Breaking Load as specified by the manufacturer and verified by prototype testing.
In addition to mechanical values, the bulletin mandates strict limits on sulfur and phosphorus content (≤ 0.025% each) and requires Charpy V-notch impact testing at −20 °C for all grades R3S and above. Heat treatment must be fully documented and traceable for each production lot.
Caution: API Bull 2S emphasizes that proof loading beyond the specified factor may introduce residual stresses and reduce fatigue life. Always verify proof load values against the manufacturer’s certified curve.
Testing and Inspection Criteria
The bulletin outlines three levels of inspection: manufacturing (in-process), final acceptance, and periodic in-service inspection. For new chain, each link must undergo magnetic particle inspection (MPI) or liquid penetrant testing (PT) after heat treatment and proof loading. Production lots are subject to random destructive samples to confirm tensile and impact properties.
For in-service chain, API Bull 2S recommends eddy current or ultrasonic testing for internal flaws and dimensional checks for link elongation and wear. Table 2 provides the rejection thresholds for common wear patterns.
| Parameter | Acceptance Limit | Critical Threshold |
|---|---|---|
| Link diameter reduction (wear) | < 5% of original | ≥ 10% of original |
| Link length increase | < 3% of original | ≥ 5% of original |
| Crack depth (surface NDT) | < 1 mm | ≥ 2 mm |
| Nicks & gouges | Depth ≤ 0.5 mm | Depth > 0.5 mm |
Critical Note: The bulletin warns that crack-like indications at the link crown or shoulder—even if within the acceptance limit—shall be evaluated by an engineer experienced in mooring chain fracture mechanics. Ignoring such flaws has led to sudden failures in dynamic positioning systems.
Best Practice: Schedule in-service inspection at intervals not exceeding 3 years for chains operating in deep-water or harsh environments. For mooring legs with a safety factor below 3.0, consider annual inspection.
Compliance and Implementation Notes
While API Bull 2S itself is not a mandatory code, it is widely adopted by classification societies (e.g., ABS, DNV, Lloyd’s) as a reference for mooring system certification. Bridge documents between the bulletin and class rules are common. Operators should ensure that all purchased chain meets the bulletin’s traceability requirements, including casting lot records, heat treatment logs, and tensile test certificates.
Implementation highlights include:
- Adoption of the bulletin’s sampling plan (5% of links for NDT, with escalation to 100% if any defect is found).
- Use of the prescribed proof-load factors for acceptance testing.
- Incorporation of maximum allowable wear curves in the mooring line integrity monitoring program.
- Training of NDT personnel to the bulletin’s sensitivity levels (≥ 0.5 mm notch for MPI).
Compliance Advantage: Vessels and floating units that align their mooring chain procurement and inspection with API Bull 2S often experience lower life-cycle costs, fewer emergency repairs, and smoother port‑state or flag‑state approvals.
Note that API Bull 2S was reaffirmed in 2001 without technical changes. However, fabricators should always confirm the latest edition when ordering new chain, as many class societies now reference the subsequent editions of API Spec 2S for grades higher than R5.
Q: How does API Bull 2S differ from API Spec 2S?
A: API Spec 2S is a formal specification that sets minimum requirements for manufacturing, chemical composition, and mechanical properties of offshore mooring chain. API Bull 2S is a supplementary bulletin that provides guidance on the application of that specification, clarifying test procedures, acceptance criteria, and operational limits that had been ambiguous in earlier editions of the spec. The bulletin does not supersede the spec but is often referenced alongside it in project documents.
A: API Spec 2S is a formal specification that sets minimum requirements for manufacturing, chemical composition, and mechanical properties of offshore mooring chain. API Bull 2S is a supplementary bulletin that provides guidance on the application of that specification, clarifying test procedures, acceptance criteria, and operational limits that had been ambiguous in earlier editions of the spec. The bulletin does not supersede the spec but is often referenced alongside it in project documents.
Q: Is compliance with API Bull 2S mandatory for mooring system certification?
A: No, it is not mandatory by the API itself. However, many classification societies and oil company specifications require that mooring systems meet or exceed the requirements of both API Spec 2S and API Bull 2S. In practice, most chain manufacturers incorporate the bulletin’s recommendations into their quality plans to satisfy third‑party certification.
A: No, it is not mandatory by the API itself. However, many classification societies and oil company specifications require that mooring systems meet or exceed the requirements of both API Spec 2S and API Bull 2S. In practice, most chain manufacturers incorporate the bulletin’s recommendations into their quality plans to satisfy third‑party certification.
Q: Does the bulletin address accessories such as shackles and connectors?
A: API Bull 2S focuses on chain links only. Accessories are covered by other API documents, such as API Spec 2C (for cranes) and API Spec 8C (for hoisting equipment). The bulletin does provide recommendations for connecting links and an approximate method for adjusting proof loads when chain is tested in assemblies with attached hardware.
A: API Bull 2S focuses on chain links only. Accessories are covered by other API documents, such as API Spec 2C (for cranes) and API Spec 8C (for hoisting equipment). The bulletin does provide recommendations for connecting links and an approximate method for adjusting proof loads when chain is tested in assemblies with attached hardware.
Q: What is the significance of the 1995 edition with 2001 reaffirmation?
A: The 1995 edition introduced the current NDT acceptance levels and proof‑load factors. The 2001 reaffirmation confirmed that no changes were necessary at that time, indicating the bulletin’s provisions remained industry‑accepted. Since then, the offshore industry has seen the release of higher‑strength chain grades (R5S, R6) that may require additional qualification beyond the original bulletin scope.
A: The 1995 edition introduced the current NDT acceptance levels and proof‑load factors. The 2001 reaffirmation confirmed that no changes were necessary at that time, indicating the bulletin’s provisions remained industry‑accepted. Since then, the offshore industry has seen the release of higher‑strength chain grades (R5S, R6) that may require additional qualification beyond the original bulletin scope.
Copyright © 2026. This article is published for informational purposes and does not replace the official API document. Always refer to the latest version of API Bull 2S for authoritative guidance.