Marine classification societies set the welding rules for commercial vessels and offshore structures. ABS, DNV, and Lloyd’s Register each publish their own welding requirements covering welder qualification, base material grades, filler metal approvals, NDT requirements, and in-service repair restrictions. If the vessel carries a class certificate, every structural weld must meet the applicable society’s standards.
What Classification Societies Do
Classification societies are the technical authorities for ship and offshore structure quality. They operate independently from flag states (the governments that register vessels) but flag states rely on classification society rules as the baseline for structural safety.
A classification society’s role in welding covers:
Rule development. Each society publishes rules for materials, design, construction, and survey of hull structures. These rules include specific welding requirements for every structural joint on the vessel.
Plan review. Before construction begins, the society reviews structural drawings, welding procedure specifications, and quality plans. Welding details that don’t meet the rules get flagged for revision.
Construction survey. During construction, classification surveyors inspect welding operations. They witness welder qualification tests, review WPS documents, monitor NDT results, and inspect completed welds. Their approval is required at defined hold points before construction can proceed.
Periodic survey. After delivery, classed vessels undergo periodic surveys (annual, intermediate, and special surveys on a 5-year cycle) that include inspection of structural welds for cracking, corrosion, and other deterioration.
ABS (American Bureau of Shipping)
ABS is the primary classification society for vessels built and operating in the Americas. ABS rules govern most commercial and military vessels in the United States.
ABS Welding Rules
ABS welding requirements are published in the “Rules for Building and Classing Marine Vessels” (previously “Rules for Building and Classing Steel Vessels” and separate aluminum rules). Key provisions:
| Requirement | ABS Specification |
|---|---|
| Welder Qualification | ABS-administered test or recognized equivalent (AWS D1.1, D1.2 for aluminum). Tests include plate and pipe in 3G, 4G, and 6G positions. |
| WPS Qualification | Procedure qualification per ABS rules. Each combination of process, base metal, and joint configuration must be qualified by testing. |
| Base Metal | ABS-grade steels (A, B, D, E for hull structural; AH, DH, EH for higher strength) or approved equivalents. Aluminum per ABS-approved alloys (5083, 5086, 5383, 6061). |
| Filler Metal | ABS-approved or AWS-classified filler metals matching the base metal strength group. |
| NDT | Radiography or UT on butt welds per ABS-specified percentage. Visual on all welds. MT or PT on critical fillet welds. |
| Surveyor Witness | ABS surveyor witnesses welder qualification tests and inspects structural welds during construction at defined hold points. |
ABS Steel Grades for Hull Structures
ABS classifies hull structural steel by strength and toughness:
| Grade | Yield Strength (ksi min) | CVN Test Temp | Application |
|---|---|---|---|
| Grade A | 34 | No impact test required | General hull structure, mild service |
| Grade B | 34 | No impact test required | General hull structure, improved quality |
| Grade D | 34 | 32F (0C) | Hull structure in cold service |
| Grade E | 34 | -4F (-20C) | Hull structure in severe cold service |
| Grade AH36 | 51 | No impact test required | Higher-strength hull structure |
| Grade DH36 | 51 | -4F (-20C) | Higher-strength, cold service |
| Grade EH36 | 51 | -40F (-40C) | Higher-strength, severe cold service |
The filler metal must match or exceed the strength of the base metal grade. AH36 hull plate requires a minimum 70 ksi filler (E70xx or equivalent), not 60 ksi.
DNV (Det Norske Veritas)
DNV is a Norwegian classification society with strong presence in European and offshore markets. DNV rules are widely applied on offshore platforms, subsea structures, and vessels in the North Sea and global offshore operations.
DNV Welding Rules
DNV publishes welding requirements in “Rules for Classification of Ships” and the DNV-OS series of offshore standards. Key provisions:
Welder qualification: DNV requires testing per DNV or ISO 9606 (for steel) and ISO 10042 (for aluminum). Tests must be administered and witnessed by a DNV surveyor or approved testing body. Qualifications are valid for 2 years with continuous employment and satisfactory work record.
WPS qualification: Procedures qualified per ISO 15614 series or DNV-approved equivalent. DNV acceptance criteria for qualification coupons are specific to the application (hull, offshore, piping).
Material grades: DNV grades (NV-A, NV-B, NV-D, NV-E for normal strength; NV-AH, NV-DH, NV-EH, NV-FH for high strength) parallel ABS grades but with DNV-specific chemistry and testing requirements. DNV requires Charpy impact testing on all higher-strength grades and specifies test temperatures based on the vessel’s intended service.
Filler metal approval: DNV maintains its own filler metal approval system. Welding consumables must carry DNV type approval for the specific strength group and toughness grade. The approval number appears on the filler metal packaging.
DNV-Specific Offshore Requirements
DNV offshore standards (DNV-OS-C401 for fabrication and testing, DNV-RP-C203 for fatigue design) add requirements for welded connections on offshore structures:
- Fatigue-rated weld details with specific weld profile and toe grinding requirements
- Tighter acceptance criteria for welds in fatigue-sensitive locations
- Enhanced NDT requirements for critical nodes and tubular joints
- Weld improvement techniques (toe grinding, hammer peening, TIG dressing) to extend fatigue life
Lloyd’s Register
Lloyd’s Register (LR) is one of the oldest classification societies, founded in London in 1760. LR rules are widely applied on vessels registered under UK and Commonwealth flag states and on many international trade vessels.
Lloyd’s Welding Rules
Lloyd’s welding requirements are published in “Rules and Regulations for the Classification of Ships” and various ShipRight guidance documents.
Welder qualification: LR accepts qualification testing per ISO 9606, EN 287 (superseded by ISO 9606), or LR-specific test requirements. The LR surveyor witnesses or reviews all qualification tests for class-related welding.
Procedure qualification: WPS qualification per ISO 15614 or LR-approved equivalent, with destructive testing witnessed by the LR surveyor.
Material requirements: LR-grade materials (LR-A, LR-B, LR-D, LR-E and corresponding higher-strength grades) with chemistry and mechanical properties per LR rules. Material certificates must be endorsed by the LR surveyor at the producing mill.
Quality assurance: LR requires fabricators to maintain a documented quality management system covering welding operations. This includes welder identification, material traceability, NDT procedures, and corrective action processes.
Welder Qualification Across Societies
Each classification society administers its own welder qualification tests, but the test formats share common elements:
| Test Element | ABS | DNV | Lloyd's Register |
|---|---|---|---|
| Plate Butt Test | 3G and/or 4G | Per ISO 9606 positions | Per ISO 9606 positions |
| Pipe Butt Test | 6G (fixed 45 degrees) | Per ISO 9606 positions | Per ISO 9606 positions |
| Fillet Weld Test | T-joint in applicable positions | Per ISO 9606 if fillet only | Per ISO 9606 if fillet only |
| Destructive Testing | Bend tests, macro examination | Per ISO 9606 requirements | Per ISO 9606 requirements |
| Validity Period | Indefinite with continuous work | 2 years with 6-month endorsement | 2 years with periodic endorsement |
| Witness | ABS surveyor | DNV surveyor or approved body | LR surveyor or approved body |
A welder qualified under one society is not automatically qualified under another. Cross-acceptance agreements exist between some societies for certain test types, but full mutual recognition is not standard practice. A shipyard working on vessels for multiple classification societies may need to maintain separate qualification records for each.
Material Grade Requirements
Marine-Grade Steel
All structural steel used in classed vessel construction must meet the classification society’s material grade requirements. The steel mill must be approved by the society, and each heat of steel is tested and certified under the society’s surveyor’s witness.
Key material requirements common across societies:
- Chemical composition limits (carbon, manganese, silicon, sulfur, phosphorus)
- Mechanical properties (yield, tensile, elongation)
- Impact testing at specified temperatures for grades requiring notch toughness
- Through-thickness (Z-direction) testing for plates subject to lamellar tearing risk
- Surface quality and dimensional tolerances
Marine-Grade Aluminum
Marine-grade aluminum for classed vessels must be from approved alloys (5083, 5086, 5383, 5456, 6061, 6082) with temper designations approved for marine service. The key requirement is exfoliation corrosion resistance testing per ASTM G66 (ASSET test) for 5xxx-series alloys exposed to seawater.
Non-marine-grade aluminum alloys (standard 5052, 3003, or 6xxx alloys without marine temper designations) are not acceptable for hull structures on classed vessels.
In-Service Repair Restrictions
Repairing structural welds on a classed vessel in service is strictly controlled. The classification society maintains authority over all structural modifications and repairs throughout the vessel’s service life.
Repair Planning
Before any structural weld repair on a classed vessel:
- Notify the classification society. Provide drawings showing the damaged area, proposed repair method, and repair welding procedure.
- Obtain approval. The society reviews the repair plan and approves, modifies, or rejects it. This approval must be in hand before repair welding begins.
- Use qualified personnel. The repair welder must be qualified under the society’s rules for the specific process and material.
- Follow approved procedures. The WPS for the repair must be qualified per the society’s rules.
- Inspection and documentation. NDT of the completed repair, documented results, and surveyor approval of the final product.
Emergency Repairs
Emergency repairs at sea (to prevent sinking, to maintain watertight integrity, or to reach port safely) may be performed without prior society approval. However:
- The master must document the circumstances and repair details
- The repair must be reported to the classification society at the earliest opportunity
- A surveyor will inspect the emergency repair at the next port of call
- The emergency repair may need to be replaced with a permanent repair under society supervision
Common Repair Scenarios
Crack repair: The cracked area must be fully excavated (ground or air-arc gouged to sound metal), inspected by NDE to confirm complete removal of the crack, and re-welded per an approved WPS. A crack that’s only partially removed will re-initiate under service loading.
Corrosion wastage: When hull plating has wasted below the minimum thickness required by the society’s rules, an insert plate (crop and renew) or doubler plate repair is required. The welding must match the original construction standard.
Collision or grounding damage: Structural damage from impact events requires engineering assessment, damage survey by the classification surveyor, and a repair plan developed by a naval architect or structural engineer approved by the society.
Classification requirements add time and cost to marine welding projects, but they exist because structural failures at sea are uniquely dangerous. A structural weld failure on land is a problem. A structural weld failure at sea, miles from shore, in heavy weather, is a potential catastrophe. The classification system’s rigor reflects that reality.
Back to marine and shipbuilding welding for more marine welding topics. See also welding aluminum boat hulls for the practical side of marine aluminum welding.