Section 2 Structural categories
2.1 General
2.1.1 For the determination of steel grades in accordance with Pt 4, Ch 2, 4.1 General, 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.
2.2 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:
- 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.
- The shell plating in way of the intersections of vertical
columns with platform decks and upper and lower hulls.
- End connections and major intersections of primary bracing
members.
- 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.
- External brackets, portions of bulkheads, flats, and frames
which are designed to receive concentrated loads at intersections of
major structural members.
- Structure supporting concentrated mooring loads.
- Deck cantilevers
- Towing brackets
- Primary structure:
- 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.
- The shell plating of vertical columns, lower and upper
hulls, and diagonal and horizontal braces.
- 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.
- Main support structure to cantilevered helicopter decks and
lifeboat platforms.
- Heavy substructures and equipment supports, e.g.,
drillfloor substructure, crane pedestals, anchor line fairleads and their
supporting structure, see also
Pt 4, Ch 2, 2.1 General 2.1.3
- Riser support structure.
- Secondary structure:
- Upper platform decks or decks of upper hulls, except areas
where the structure is considered as primary or special application.
- 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.
- Helicopter platforms and deckhouses.
- Lifeboat platforms.
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:
- Vertical columns in way of intersections with the mat
structure.
- Intersections of lattice type leg structures which
incorporate novel construction, including the use of steel castings.
- Leg to spudcan connections.
- Jack-house and/or bulkheads supporting locking.
- Primary structure:
- 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.
- External plating of cylindrical legs.
- Plating of all components of lattice type legs.
- Jack-house supporting structure.
- External shell plating of footings and mats and structural
components which receive initial transfer of loads from the leg
structures.
- Internal bulkheads and girders of supporting structure of
footings and mats which are designed to distribute major concentrated or
uniform loads into the structure.
- Main support structure to cantilevered helicopter decks and
lifeboat platforms.
- 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.
- Towing brackets.
- Secondary structure:
- Deck and shell boundaries of the main hull or platform,
except where the structure is considered as primary application.
- Internal bulkheads, decks stiffeners and girders of the
main hull structure, except where the structure is considered as primary
structure.
- Internal diaphragms, girders or stiffeners in cylindrical
legs.
- Internal bulkheads, stiffeners and girders of footings and
bottom mat supporting structures, except where the structure is
considered primary or special application.
- Helicopter platforms and deckhouses.
- 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 withPt 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 Pt 4, Ch 2, 4.1 General 4.1.6.
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:
- 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.
- Sheerstrake or rounded gunwale.
- Stringer plate at strength deck.
- Deck strake at longitudinal bulkheads.
- Bilge strake.
- Continuous longitudinal hatch coamings.
- Primary structure:
- Strength deck and raised quarter deck plating except where
categorised ‘special’.
- Bottom shell plating of the main hull except where
categorised ‘special’.
- Bulkhead plating in way of moonpools, drilling wells and
circumturret.
- Upper strake in longitudinal bulkheads.
- Continuous longitudinal members above strength deck except
where categorised ‘special’.
- Vertical strake (hatch side girder) and upper sloped strake
in top wing tanks.
- Main support structure to cantilevered helicopter decks and
lifeboat platforms.
- Heavy substructures and equipment supports, e.g. integrated
support stools to process plant, drill floor substructure, crane
pedestals, 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.
- Riser support structures.
- Turret bearing support structure.
- Swivel stack support structure.
- Supporting structures to external turret.
- Deck cantilevers
- Towing brackets
- Secondary structure:
- 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
- Helicopter platforms and deckhouses.
- 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:
- 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.
- Intersections of structures which incorporate novel
construction including the use of steel castings.
- Complex padeyes.
- Highly stressed structural elements of anchor-line
attachments.
- 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:
- External shell plating of buoys, deep draught caissons,
turrets and subsea modules.
- Strength decks of buoys and deep draught caissons.
- Truss structure supporting decks on deep draught
caissons.
- Miscellaneous structures:
- Support stools to process plant.
- Bearing support structure.
- Swivel stack support structure.
- Turntable construction.
- Chain tables.
- Riser support structure.
- Hawser support structure.
- Yoke and mooring arm construction.
- Mooring towers.
- Main support structures to cantilevered helideck and
lifeboat platforms.
- Heavy substructures and equipment supports, e.g., crane
pedestals, anchor line fairleads for positional moorings, chain stoppers
and their supporting structures.
- Boundary bulkheads of moonpools.
- Towing brackets
- Secondary structure:
- 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.
- Helicopter platforms and deckhouses.
- Lifeboat platforms, walkways, guard rails and minor
fittings and attachments, etc.
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