Class-Witnessed Fabrication: What DNV, ABS and Lloyd's Survey Means for a Marine Steel Order

When a marine structure has to be certified by a classification society, the fabrication is not finished when the welding is. It is finished when the surveyor signs. Class survey changes the sequence, the paperwork and the cost of a job, and a fabricator that has not done it before will underestimate all three.

This guide is for buyers ordering ship subassemblies, offshore module steel, davits, foundations or any structure that a class society will survey. It covers what the survey involves, who does what and how to check a fabricator’s claim to class experience. The marine and offshore structures page lists the related work.

Who the class society is and who appoints it

Classification societies such as DNV, ABS, Lloyd’s Register and Bureau Veritas set rules for the design and construction of ships and offshore structures and survey the work to those rules. The class contract is held by the owner or the yard, not by the fabricator. The owner or yard appoints the society and the society assigns a surveyor to the fabrication.

The fabricator’s job is to build to the rules and the approved drawings, give the surveyor access at the agreed hold points and produce the documentation the surveyor asks for. The fabricator does not hold a class contract and cannot “certify” the structure itself. A fabricator that claims to certify to class is describing something it does not do.

What the surveyor looks at

Survey of fabricated steel covers, in broad order:

1. Drawing approval. The design is approved by the society’s plan approval office before fabrication starts. Changes during fabrication go back for approval.
2. Material. Steel is either class-certified at the mill (with the society’s stamp on the certificate) or tested and witnessed at the fabricator. Ship grades A, AH36, DH36 and EH36 are the usual grades, with EN 10204 3.2 certificates where the society witnesses the tests.
3. Welding procedures and welders. WPS and PQR approved by the society, or approved by another body the society accepts. Welders qualified under the society’s scheme or to ISO 9606 as accepted.
4. Fit-up and welding. Hold points at fit-up on primary structure, with the surveyor checking bevel, gap and alignment before welding.
5. NDT. Extent set by the rules, often 100% UT on primary butt welds and MT or PT on fillets in defined areas. The NDT operator’s qualification is checked. The surveyor reviews the reports and may witness a sample.
6. Dimensional. Survey of principal dimensions against the approved drawing.
7. Coating. Where the society’s rules or the owner’s specification require it, coating inspection is witnessed or the reports reviewed.
8. Final. The surveyor issues a certificate or a report that goes with the structure to the yard.

Each hold point is a scheduled visit, and a missed hold point means either waiting for the surveyor or cutting the weld open to show what is underneath. The fabricator’s schedule has to include the surveyor’s availability, which is one of the reasons class work takes longer than the welding alone would suggest.

Material traceability

Class survey stands or falls on traceability. Every plate and section in the primary structure is identified by heat number, the heat number is on the mill certificate, the certificate carries the society’s stamp or the witnessed test results, and the plate carries the heat number in hard stamp or paint marking that survives cutting. When a plate is cut, the marking is transferred to every piece.

A fabricator without a traceability system cannot do class work, no matter how good its welding is. Ask to see how heat numbers are transferred on the shop floor before the order is placed.

Welding under class rules

Class welding requirements differ from structural codes in the details:

• The WPS has to be approved by the society or qualified to a standard the society accepts, with the PQR available.
• Consumables are class-approved products, and the approval covers the grade and the position.
• Welder qualifications are checked and often re-witnessed if they were issued under a scheme the society does not recognise.
• Preheat and interpass temperature control are recorded for higher grades and thicker plate.
• Repairs are recorded and re-inspected, and repeat repairs at the same location are limited.

Our weld standards guide covers how quality levels are set under the structural codes; class rules layer their own requirements on top.

NDT under class

The NDT extent is set by the rules for the structure’s category. The operator holds a qualification such as ISO 9712 Level 2 in the method, and the society may require the NDT firm itself to be approved. Reports name the weld, the method, the procedure, the equipment, the acceptance standard and the result, and they are cross-referenced to the weld map. The NDT guide explains what each method finds.

How to tell whether a fabricator has done class work

Ask for three things:

• A recent survey report or certificate from a class society, for a specific job, with the society and the surveyor’s name. A fabricator that has done class work has these and can show one with the client’s name removed.
• Class-approved WPS with the society’s stamp, in the grades and positions your job needs.
• The traceability procedure, on paper and on the floor.

A fabricator that offers a certificate of its own quality system instead of these has not understood the question. ISO 9001 is not class approval. Some societies operate approval schemes for workshops, and a workshop approval is a strong sign, but the job-level evidence is what matters.

What it does to the cost and the schedule

Class work costs more than the same structure built to a structural code without survey. The extra is in the material (class-certified or witness-tested), the consumables, the hold points, the NDT extent, the documentation and the surveyor’s fees, which are usually charged to the owner or yard but which affect the fabricator’s schedule. Buyers comparing quotes should check that every shop has priced the same scope, particularly the material certificate level and the NDT extent.

Specifying it

Before a class-surveyed structure goes out for quotes, state:

• The society and the rule set.
• Whether drawings are already approved, or whether approval is in the fabricator’s scope.
• Material grades and certificate level (3.1 or 3.2).
• The hold point schedule, or a request for the fabricator to propose one.
• NDT extent per the rules, and whether the NDT firm needs society approval.
• The documentation package required at delivery.
• Who pays the surveyor’s fees.

Getting it quoted

We only match class work to shops that can show a recent class-witnessed job and the traceability behind it, and we tell you which society it was. Send the drawing through the RFQ form with the society named, and within 24 business hours we tell you which shops in the network fit; up to three of them quote you directly. The marine and offshore structures page lists the rest of the marine work this covers.