6.1.3.1 The purpose of the structural categorisation is to assure adequate material and suitable inspection to avoid brittle fracture. The purpose of inspection is also to remove defects that may grow into fatigue cracks during service life.
Guidance note:
Conditions that may result in brittle fracture are to be avoided. Brittle fracture may occur under a combination of: - presence of sharp defects such as cracks
- high tensile stress in direction normal to planar defect(s) - material with low fracture toughness.
Sharp cracks resulting from fabrication may be found by inspection and repaired. Fatigue cracks may also be discovered during service life by inspection.
High stresses in a component may occur due to welding. A complex connection is likely to provide more restraint and larger residual stress than a simple one. This residual stress may be partly removed by post weld heat treatment if necessary. Also a complex connection shows a more three-dimensional stress state due to external loading than simple connections. This stress state may provide basis for a cleavage fracture.
The fracture toughness is dependent on temperature and material thickness. These parameters are accounted for separately in selection of material. The resulting fracture toughness in the weld and the heat affected zone is also dependent on the fabrication method.
Thus, to avoid brittle fracture, first a material with suitable fracture toughness for the actual design temperature and thickness is selected. Then a proper fabrication method is used. In special cases post weld heat treatment may be performed to reduce crack driving stresses, see also DNV-OS-C401. A suitable amount of inspection is carried out to remove planar defects larger than those considered acceptable. In this standard selection of material with appropriate fracture toughness and avoidance of unacceptable defects are achieved by linking different types of connections to different structural categories and inspection categories.
6.1.3.2 Components are classified into structural categories according to the following criteria: — significance of component in terms of consequence of failure
— stress condition at the considered detail that together with possible weld defects or fatigue cracks may provoke brittle fracture.
Guidance note:
The consequence of failure may be quantified in terms of residual strength of the structure when considering failure of the actual component.
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6.1.3.3 The structural category for selection of materials shall be determined according to principles given in
Table 6-1.
Guidance note:
Monopile structures are categorised as “Primary”, because they are non-redundant structures whose stress pattern is primarily uniaxial and whose risk of brittle fracture is negligible.
Likewise, towers are also categorised as “Primary”.
Tubular joints are categorised as “Special” due to their biaxial or triaxial stress patterns and risk of brittle fracture. This will influence the thickness limitations as specified in Table 6-7.
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6.1.3.4 Requirements and guidance for manufacturing of steel materials are given in DNV-OS-C401. For supplementary guidance, reference is made to EN 1090-1 and ENV 1090-5. Steel materials and products shall be delivered with inspection documents as defined in EN 10204 or in an equivalent standard. Unless otherwise specified, material certificates according to Table 6-2 shall be presented.
6.1.3.5 Requirements for type and extent of inspection of welds are given in DNV-OS-C401 depending on assigned inspection category for the welds. The requirements are based on the consideration of fatigue damage and assessment of general fabrication quality.
6.1.3.6 The inspection category is by default related to the structural category according to Table 6-3. Table 6-1 Structural categories for selection of materials
Structural category Principles for determination of structural category
Special Structural parts where failure will have substantial consequences and are subject to a stress condition that may increase the probability of a brittle fracture.1)
Primary Structural parts where failure will have substantial consequences. Secondary Structural parts where failure will be without significant consequence.
1) In complex joints a triaxial or biaxial stress pattern will be present. This may give conditions for brittle fracture where tensile stresses are present in addition to presence of defects and material with low fracture toughness.
Table 6-2 Material certificates
Certification process Material
certificate (EN10204)
Structural category
Test certificate
As work certificate, inspection and tests witnessed and signed by an independent third party body
3.2 Special
Work certificate
Test results of all specified tests from samples taken from the products supplied. Inspection and tests witnessed and signed by QA department
3.1 Primary
Test report
Confirmation by the manufacturer that the supplied products fulfil the purchase specification, and test data from regular production, not necessarily from products supplied
2.2 Secondary
Table 6-3 Inspection categories
Inspection category Structural category
I Special
II Primary
6.1.3.7 The weld connection between two components shall be assigned an inspection category according to the highest category of the joined components. For stiffened plates, the weld connection between stiffener and stringer and girder web to the plate may be inspected according to inspection Category III.
6.1.3.8 If the fabrication quality is assessed by testing, or if it is of a well known quality based on previous experience, the extent of inspection required for elements within structural category primary may be reduced, but the extent must not be less than that for inspection Category III.
6.1.3.9 Fatigue-critical details within structural category primary and secondary shall be inspected according to requirements given for Category I. This requirement applies to fatigue-critical details in the support structure and the foundation, but not in the tower.
6.1.3.10 Welds in fatigue-critical areas not accessible for inspection and repair during operation shall be inspected according to requirements in Category I during construction.
6.1.3.11 For monopile type structures, the longitudinal welds in the monopile and in the transition piece to the grouted connection shall be inspected according to requirements given for Category I.