Section 2 Structural categories

2.1 General

2.1.1 For the determination of steel grades in accordance with Pt 4, Ch 2, 4 Steel grades, all structural components of the unit may be grouped into structural categories taking into account the following aspects:

Applied loading, stress level and the associated stress pattern.
Critical load transfer points and stress concentrations.
Consequence of failure.

2.1.2 The structural categories can be summarised as follows:

Special structure:
Primary structural elements which are in way of critical load transfer points and stress concentrations.
Primary structure:
Structural elements essential to the overall integrity of the unit.
Secondary structure:
Structural elements of less importance than primary structure, failure of which would be unlikely to affect the overall integrity of the unit.

2.1.3 For the structural categories of supporting structures of drilling plant and production and process plant, see Pt 3, Ch 7, 2.2 Materials and Pt 4, Ch 8, 2.2 Impact test requirements respectively.

2.2 Column-stabilised and tension-leg units

2.2.1 In general, the structural members of column-stabilised and tension-leg units are to be grouped into the following structural categories:

Special structure:
1. The plating of decks, heavy flanges, shell boundaries and bulkheads of the upper hull or platform which form 'box' or 'I' type supporting structure in way of critical load transfer points and which receive major concentrated loads.
2. The shell plating in way of the intersections of vertical columns with platform decks and upper and lower hulls.
3. End connections and major intersections of primary bracing members.
4. Critical load transfer by 'through' material used at connections of vertical columns, upper platform decks and upper or lower hulls which are designed to provide proper alignment and adequate load transfer.
5. External brackets, portions of bulkheads, flats, and frames which are designed to receive concentrated loads at intersections of major structural members.
6. Structure supporting concentrated mooring loads.
7. Deck cantilevers
8. Towing brackets
Primary structure:
1. The plating of decks, heavy flanges, shell boundaries and bulkheads of the upper hull or platform which form 'box' or 'I' type supporting structure except where the structure is considered as special application.
2. The shell plating of vertical columns, lower and upper hulls, and diagonal and horizontal braces.
3. Bulkheads, flats or decks, stiffeners and girders which provide local reinforcement or continuity of structure in way of intersections, except areas where the structure is considered as special application.
4. Main support structure to cantilevered helicopter decks and lifeboat platforms.
5. Heavy substructures and equipment supports, e.g. drillfloor substructure, crane pedestals and boom rests, anchor line fairleads and their supporting structure, see also Pt 4, Ch 2, 2.1 General 2.1.3.
6. Riser support structure.
7. Bulkhead plating in way of moonpools and drilling wells.
8. Jacket restraint structures for crane units.
Secondary structure:
1. Upper platform decks or decks of upper hulls, except areas where the structure is considered as primary or special application.
2. Bulkheads, stiffeners, flats, decks, girders and web frames in vertical columns, upper and lower hulls, diagonal and horizontal bracing, which are not considered as primary or special application.
3. Helicopter platforms and deckhouses.
4. Lifeboat platforms, walkways, guard rails and minor fittings and attachments, etc.

2.3 Self-elevating units

2.3.1 In general, the structural members of self-elevating units are to be grouped into the following categories:

Special structure:
1. Vertical columns in way of intersections with the mat structure.
2. Intersections of lattice type leg structures which incorporate novel construction, including the use of steel castings.
3. Leg to spudcan connections.
4. Jack-house and/or bulkheads supporting locking.
Primary structure:
1. The plating of bulkheads, decks and shell boundaries of the main hull or platform which in combination form 'box' or 'I' type main supporting structure.
2. External plating of cylindrical legs.
3. Plating of all components of lattice type legs.
4. Jack-house supporting structure.
5. External shell plating of footings and mats and structural components which receive initial transfer of loads from the leg structures.
6. Internal bulkheads and girders of supporting structure of footings and mats which are designed to distribute major concentrated or uniform loads into the structure.
7. Main support structure to cantilevered helicopter decks and lifeboat platforms.
8. Heavy substructures and equipment supports, e.g., drillfloor substructure, drilling cantilevers, supports for raw water towers and crane pedestals, see also Pt 4, Ch 2, 2.1 General 2.1.3.
9. Towing brackets.
10. Bulkhead plating in way of moonpools and drilling wells.
Secondary structure:
1. Deck and shell boundaries of the main hull or platform, except where the structure is considered as primary application.
2. Internal bulkheads, decks stiffeners and girders of the main hull structure, except where the structure is considered as primary structure.
3. Internal diaphragms, girders or stiffeners in cylindrical legs.
4. Internal bulkheads, stiffeners and girders of footings and bottom mat supporting structures, except where the structure is considered primary or special application.
5. Helicopter platforms and deckhouses.
6. Lifeboat platforms and walkways.

2.4 Ship units and other surface type units

2.4.1 Material classes and steel grades for individual areas of the hull structure of ship and barge type units are to comply with Pt 3, Ch 2, 2 Fracture control of the Rules for Ships.

2.4.2 Where the minimum design temperature, see Pt 4, Ch 2, 3.1 General, for exposed structure is –5°C or below, or for structural components not addressed by Pt 4, Ch 2, 2.4 Ship units and other surface type units 2.4.1, the requirement of Pt 4, Ch 2, 2.4 Ship units and other surface type units 2.4.3 should be complied with and the steel grades should be assigned in accordance with Table 2.4.1 Thickness limitations for structural steels for various application categories and design temperatures for use in welded construction including plates, bars, sections forgings, and castings.

2.4.3 In general, the structure of ship units and other surfaces type units is to be grouped into the following structural categories:

Special structure:
1. Structure in way of critical load transfer points which are designed to receive major concentrated loads in way of mooring systems, including yokes and similar structures, and supports to hawsers to mooring installations including external hinges, complex padeyes, brackets and supporting structures.
2. Sheerstrake or rounded gunwale.
3. Stringer plate at strength deck.
4. Deck strake at longitudinal bulkheads.
5. Bilge strake.
6. Continuous longitudinal hatch coamings.
Primary structure:
1. Strength deck and raised quarter deck plating except where categorised ‘special’.
2. Bottom shell plating of the main hull except where categorised ‘special’.
3. Bulkhead plating in way of moonpools, drilling wells and circumturret.
4. Upper strake in longitudinal bulkheads.
5. Continuous longitudinal members above strength deck except where categorised ‘special’.
6. Vertical strake (hatch side girder) and upper sloped strake in top wing tanks.
7. Main support structure to cantilevered helicopter decks and lifeboat platforms.
8. Heavy substructures and equipment supports, e.g. integrated support stools to process plant, drill floor substructure, crane pedestals and boom rests, anchor line fairleads and chain stoppers for positional mooring systems and their supporting structures, see also Pt 4, Ch 2, 2.1 General 2.1.3.
9. Riser support structures.
10. Turret bearing support structure.
11. Swivel stack support structure.
12. Supporting structures to external turret.
13. Deck cantilevers
14. Towing brackets
15. Jacket restraint structures for crane units.
Secondary structure:
1. Bulkhead plating, side shell, longitudinals, stiffeners, deck plating including poop deck and forecastle deck, flats, girders and web frames, etc., except where the structure is categorised as special or primary structure. For topside plant supporting structures, see also Pt 4, Ch 2, 2.1 General 2.1.3.
2. Helicopter platforms and deckhouses.
3. Lifeboat platforms, walkways, guard rails, minor fittings and attachments, etc.

2.5 Buoys, deep draught caissons, turrets and miscellaneous structures

2.5.1 In general, the structure of buoys, deep draught caissons, turrets, and other miscellaneous structures included in Pt 3, Ch 13 Buoys, Deep Draught Caissons, Turrets and Special Structures is to be grouped into the following structural categories:

Special structure:
1. Structure in way of critical load transfer points which are designed to receive major concentrated loads including external brackets, portions of bulkheads, flats and frames.
2. Intersections of structures which incorporate novel construction including the use of steel castings.
3. Complex padeyes.
4. Highly stressed structural elements of anchor-line attachments.
5. Bearings and structure at the base of mooring towers.
Primary structure: The following structural members are categorised as primary, except when the structure is considered as special application:
1. External shell plating of buoys, deep draught caissons, turrets and subsea modules.
2. Strength decks of buoys and deep draught caissons.
3. Truss structure supporting decks on deep draught caissons.
4. Miscellaneous structures:
  - Support stools to process plant.
  - Bearing support structure.
  - Mating cone structure for turret buoys.
  - Swivel stack support structure.
  - Turntable construction.
  - Chain tables.
  - Riser support structure.
  - Hawser support structure.
  - Yoke and mooring arm construction.
  - Mooring towers.
5. Main support structures to cantilevered helideck and lifeboat platforms.
6. Heavy substructures and equipment supports, e.g., crane pedestals, anchor line fairleads for positional moorings, chain stoppers and their supporting structures.
7. Boundary bulkheads of moonpools.
8. Towing brackets
Secondary structure:
1. Bulkheads, stiffeners, decks, flats, etc., except where the structure is categorised as Special or Primary structure. For topside structures, see also Pt 4, Ch 2, 2.1 General 2.1.3.
2. Helicopter platforms and deckhouses.
3. Lifeboat platforms, walkways, guard rails and minor fittings and attachments, etc.