ANNEX M SPECIAL DESIGN PROVISIONS FOR COLUMN STABILIZED UNITS
M.2 STRUCTURAL CLASSIFICATION AND MATERIAL SELE CTION
M.2.1 Structural classification
Selection of steel quality, and requirements for inspection of welds, shall be based on a systematic classification of welded joints according to the structural significance and the complexity of the joints/connections as documented in Chapter 5.
In addition to in-service operational phases, consideration shall be given to structural members and details utilised for temporary conditions, e.g. fabrication, lifting arrangements, towing
arrangements, etc.
Basic considerations
Structural connections in column stabilised units designed in accordance with this Annex should normally not fall within the categorisation criteria relevant for Design Class DC1 or DC2. In particular, relevant failure of a single weld, or element, should not lead to a situation where the accidental limit state condition is not satisfied. Structural connections will therefore be categorised within classification groups DC3, DC4 or DC5.
Consideration shall be given to address areas where through thickness tensile properties may be required.
Special consideration shall be given to ensure the appropriate inspection category for welds with high utilisation in fatigue if the coverage with standard local area allocation is insufficient.
Examples of typical design classes applicable to column stabilised units are stated below. These examples provide minimum requirements and are not intended to restrict the designer in applying more stringent requirements should such requirements be desirable.
Typical locations - Design class : DC3
Locations in way of connections at major structural interfaces, e.g. pontoon/pontoon,
pontoon/column, column/deck, and, major brace connections, should generally be classified as DC3 due to the complexity of the connections and the general difficulty with respect to inspection and repair.
Local areas in way of highly stressed complex structural connections, e.g. in way of fairleads, riser supports, topside structures, or other similar locations with high complexity should also be
classified as DC3.
DC3 areas may be limited to local, highly stressed areas if the stress gradient at such connections is large.
Typical locations - Design class : DC4
Except as provided for in the description for DC3 and DC5 structural categorisation, connections appropriate to general stiffened plate fields, including connections of structural elements supporting the plate fields, e.g. girders and stiffeners, should normally be classified in Design Class DC4.
General brace structural connections, (e.g. butt welds in brace plate fields) should be designated DC4, as should connections of the main supporting structure for topside components and
equipment.
Annex M Rev. 1, December 1998 Typical locations - Design class : DC5
All welds for non-main loadbearing structural elements, including ; non-watertight bulkheads, mezzanine('tween) decks, and, deck superstructures may be classified as DC5.
The same applies for welds for internal outfitting structures in general.
M.2.2 Material selection
Material specifications shall be established for all structural materials utilised in a column stabilised unit. Such materials shall be suitable for their intended purpose and have adequate properties in all relevant design conditions. Material selection shall be undertaken in accordance with the principles given in Chapter 5.
When considering criteria appropriate to material grade selection, adequate consideration shall be given to all relevant phases in the life cycle of the unit. In this connection there may be conditions and criteria, other than those from the in-service, operational phase, that provide the design requirements in respect to the selection of material. (Such criteria may, for example, be design temperature and/or stress levels during marine operations.)
In the selection of material grades adequate consideration shall be given to the appropriateness of the design temperature including the definition of such. When considering design temperature related to material selection the applicability that standard NORSOK requirements to fabrication of steel structures (NORSOK, M-CR-101) are based upon a minimum design temperature of -14 degrees Celsius shall be evaluated.
Selection of steel quality for structural components shall normally be based on the most stringent Design Class of the joints involving the component.
Consideration shall be given to the presence of tensile stress in the direction of the thickness of the plate when determining the appropriate steel quality.
M.2.3 Inspection categories
Welding, and the extent of non-destructive examination during fabrication, shall be in accordance with the requirements stipulated for the appropriate 'inspection category' as defined in NORSOK, M-101. Determination of inspection category should be in accordance with the criteria given in table 5.3 and 5.4.
Inspection categories determined in accordance with NORSOK N-004 provide requirements to the minimum extent of required inspection. When considering the economic consequence that repair may entail, for example, in way of complex connections with limited or difficult access, it may be considered prudent engineering practice to require more demanding requirements to inspection than the required minimum.
When determining the extent of inspection, and the locations of required NDE, in additional to evaluating design parameters (for example fatigue utilisation) consideration should be given to relevant fabrication parameters including; positioning of block (section) connections, the possibility that manual welded connections may not achieve the quality that automatic weld locations achieve, etc.
The lower the extent of NDT selected, the more important is the representativeness of the NDT selected and performed. The designer should therefore exercise good engineering judgement in indicating madatory locations for NDT, where variation of utility along welds is significant.
M.2.4 Guidance to minimum requirements
Figure M.2-1 to Figure M.2-3 illustrate minimum requirements to selection of Design Class, and Inspection Category for typical column stabilised unit, structural configurations.
Figure M.2-1 Typical 'ring' pontoon design classes and inspection categories
Key:
1. [DCn, ..] Design Class n, N 2. [….., N] Inspection Category, N Notes:
1. In way of the pontoon/column connection (except in way of brackets), the pontoon deckplate fields will normally be the continuous material. These plate fields should normally be material with documented through-thickness properties (i.e. Steel Quality Level I material).
2. Shaded areas indicated in the figures are intended to be three-dimensional in extent. This implies that, in way of these locations, the shaded area logic is not only to apply to the outer surface of the connection but is also to extend into the structure.
3. ‘Major brackets’ are considered, within the context of this figure, to be primary, load-bearing brackets supporting primary girders. (‘Minor brackets’ are all other brackets).
4. The inspection categories for general pontoon, plate butt welds and girder welds to the pontoon shell are determined based upon, amongst others: accessibility, fatigue utilisation, and dominating stress direction. (See NORSOK N-004, Tables 5.3 and 5.4). (e.g. Variations in dynamic stress levels across the section of the pontoon and also along the length of the pontoon may alter a general inspection category designation.)
5. Major bracket toes will normally be designated as locations with a mandatory requirement to MPI.
Annex M Rev. 1, December 1998
Figure M.2-2 Typical brace/column design classes and inspection categories
Key:
1. [DCn, ..] Design Class n, N 2. [….., N] Inspection Category, N Notes:
1. In way of the column/brace connection the brace, and brace bracket plate fields, will normally be the continuous material. These plate fields should normally be material with documented through-thickness properties (i.e. Steel Quality Level I material).
2. Figure M.2-2 illustrates a relatively small brace connected to a column. For large brace connections the extent of the DC3 area may be limited somewhat to that illustrated..
Figure M.2-3 Typical column/upper hull (deckbox) design classes and inspection categories
Key:
1. [DCn, ..] Design Class n, N 2. [….., N] Inspection Category, N Notes:
1. In way of the column/upper hull connection ( except in way of brackets) the upper hull deckplate fields will normally be the continuous material. These plate fields should normally be material with documented through- thickness properties (i.e. Steel Quality Level I material).
2. Shaded areas indicated in the figures are intended to be three-dimensional in extent. This implies that, in way of these locations, the shaded area logic is not only to apply to the outer surface of the connection but is also to extend into the structure.
3. ‘Major brackets’ are considered, within the context of this figure, to be primary, load-bearing brackets supporting primary girders. (‘Minor brackets’ are all other brackets).
4. The inspection categories stated for general upper hull are determined based upon, the assumption that the fatigue utilisation may be categorised as being 'low' (i.e. that NORSOK N-004, Table 5.3 is relevant). This assumption shall be verified.
Annex M Rev. 1, December 1998