Section
2 Structure
2.1 Plans and data submission
2.1.1 In addition to the structural plans and information as required by Pt 3, Ch 1, 2 Information required and Pt 4, Ch 1 General the following additional plans and information are to
be submitted:
- General arrangement.
- General arrangement plans of the production plant and process
equipment layout.
- Structural supports below plant equipment.
- Structural plans of crude oil tanks, ballast tanks, cofferdams,
void spaces, pump-rooms and machinery spaces.
- Deckhouses and modules.
2.1.2 When the unit is fitted with drilling equipment, the additional plans
required by Pt 3, Ch 2, 2 Structure are to be submitted as applicable.
2.2 General
2.2.1 The general hull strength is to comply with the requirements of Pt 4 Steel Unit Structures, taking into account the type of unit, the imposed
equipment weights and forces from the production and process plant, mooring forces
and drilling plant, when fitted. Attention should be paid to loads resulting from
hull flexural effects at support points.
2.2.2 The supporting structure below equipment is to be designed for all
operating conditions and the maximum design loadings from the production and process
plant imposed on the structure are to be determined in accordance with Pt 3, Ch 8 Process Plant Facility.
2.2.3 Decks and other under-deck structure supporting the plant are to be
suitable for the local loads at plant support points and an agreed uniformly
distributed load acting on the deck, see
Pt 4, Ch 6, 2 Design heads. The structure in way of marine risers
is to be suitably reinforced for the imposed loads.
2.2.4 In general, all seatings, platform decks, girders and pillars supporting
plant items are to be arranged to align with the main hull structure, which is to be
suitably reinforced, where necessary, to carry the appropriate loads. Attention
should be paid to the capability of support structures to withstand buckling,
see
Pt 4, Ch 5, 4 Buckling strength of primary members.
2.2.5 The strength of the unit in way of openings is to be maintained.
Structure in way of openings of unusual size, configuration and/or shape may require
investigation by structural analysis when requested by LR.
2.2.6 Insert plates of adequate thickness and steel grade, appropriate to the
stress concentrations and locations, may be required in way of openings and
structural discontinuities in primary structure.
2.3 Drilling structures
2.3.1 When a unit is fitted with a drilling derrick, the requirements of Pt 3, Ch 3, 2 Structure are to be complied with, as applicable.
2.3.2 The design loadings for the strength of the drill floor and substructure
are to be defined by the designer/Builders and calculations are to be submitted.
2.4 Permissible stresses
2.5 Well structure
2.5.1 The primary hull strength of the unit is to be maintained in way of
moonpools, turret openings, drilling wells and other large deck openings and
suitable compensation is to be fitted, as necessary. For ship units and other
surface type units, the continuity of longitudinal material is to be maintained, as
far as is practicable, in way of turret openings and wells and the minimum hull
modulus is to satisfy the Rule requirements for longitudinal strength.
2.5.2 Arrangements are to be made to ensure continuity of strength at the ends
of moonpools and well side bulkheads. In general, the design should be such that the
bulkheads are connected to bottom and deck girders by means of large, suitably
shaped brackets arranged to give a good stress flow at their junctions with both the
girders and bulkheads.
2.6 Mud tanks
2.6.1 The scantlings of structural mud tanks are not to be less than those
required for tanks in Pt 4, Ch 6, 7 Bulkheads using the design density of the mud.
In no case is the relative density of wet mud to be taken less than 2,2 unless
agreed otherwise with LR.
2.6.2 Divisions in mud tanks or pits are to be designed for one-sided loading
and the scantlings are to comply with the requirements for tanks in Pt 4, Ch 6, 7 Bulkheads.
2.7 Deckhouses and modules
2.7.2 Offshore containers are to comply with Pt 3, Ch 4, 2.4 Offshore
containers.
2.8 Pipe racks
2.8.1 The pipe rack is to be designed for the following normal operating loads
as applicable:
- Gravity loads.
- Maximum dynamic loads due to wave induced unit motions.
- Direct wind loads.
- Ice and snow loads.
- Hull flexure due to hull girder bending
2.8.2 The pipe rack supports are also to be designed for an emergency
condition, as defined in Pt 3, Ch 8, 1 General.
2.8.3 In general, the pipe rack supports are to be aligned with the primary
under-deck structure. Where this is not practicable, additional under-deck supports
are to be fitted. Deck girders and under-deck supports are to comply with Pt 4, Ch 6, 4 Decks.
2.8.4 In the emergency condition, arrangements are to be made to restrain the
pipes in their stowed position and details are to be submitted for approval.
2.9 Bulk storage vessels
2.9.2 The deck supports under free-standing bulk storage vessels are to comply
with the requirements for local structure in Pt 4, Ch 6 Local Strength taking into account the maximum design reaction
forces.
2.9.3 Where bulk storage vessels penetrate watertight decks and can be
subjected to external hydrostatic pressure due to progressive flooding in hull
damage conditions, the bulk storage vessel is to be suitably reinforced and the
permissible stress is not to exceed the Code stress in accordance with Pt 3, Ch 8, 4 Pressure vessels and bulk storage.
2.10 Watertight and weathertight
integrity
2.10.2 The integrity of the weather deck is to be maintained. Where items of
plant equipment penetrate the weather deck and are intended to constitute the
structural barrier to prevent the ingress of water to spaces below the deck, their
structural strength is to be equivalent to the Rule requirements for this purpose.
Otherwise such items are to be enclosed in superstructures or deckhouses fully
complying with the Rules. Full details are to be submitted for approval.
2.10.3 Where items of plant equipment or pipes penetrate watertight boundaries,
the watertight integrity is to be maintained and full details are to be submitted
for approval. Free flooding pipes, which penetrate shell boundaries, are to have a
wall thickness not less than the adjacent shell plating.
2.11 Access arrangements and closing
appliances
2.11.3 Ladders and platforms in tanks, pump-rooms, cofferdams, access trunks and
void spaces are to be securely fastened to the structure.
2.12 Access to spaces in oil storage
areas
2.12.1 Access arrangements to tanks for the storage of oil in bulk and adjacent
spaces, including cofferdams, voids, vertical wing and double bottom ballast tanks,
is to be direct from the open deck and such as to ensure their complete
inspection.
2.12.2 In column-stabilised units where access from the open deck is not
practicable, access to oil storage tanks and adjacent spaces is to be from trunks
which are mechanically ventilated in accordance with Pt 3, Ch 3, 3 Hazardous areas and ventilation. Every space is to be provided with a separate access without
passing through adjacent spaces.
2.12.3 Access to double bottom tanks in way of oil storage tanks, where wing
ballast tanks are omitted, is to be provided by trunks from the exposed deck led
down the bulkhead. Alternative proposals will, however, be considered, provided the
integrity of the inner bottom is maintained.
2.12.4 Access to double bottom spaces may also be through a cargo pump-room,
pump-room, deep cofferdam, pipe tunnel or similar compartments, subject to
consideration of ventilation aspects.
2.12.5 Where a duct keel or pipe tunnel is fitted, and access is normally
required for operational purposes, access is to be provided at each end and at least
one other location at approximately mid-length. Access is to be directly from the
exposed deck. Where an after access is to be provided from the pump-room to the duct
keel, the access manhole from the pump-room to the duct keel is to be provided with
an oiltight cover plate. Mechanical ventilation is to be provided and such spaces
are to be adequately ventilated prior to entry. A notice board is to be fitted at
each entrance to the pipe tunnel stating that before any attempt is made to enter,
the ventilating fan must have been in operation for an adequate period. In addition,
the atmosphere in the tunnel is to be sampled by a reliable gas monitor, and where
an inert gas system is fitted in cargo tanks, an oxygen monitor is to be
provided.
2.12.6 Every double bottom space is to be provided with separate access without
passing through other neighbouring double bottom spaces.
2.12.7 Where the tanks are of confined or cellular construction, and have a
length of 35 metres or more, two separate means of access from the weather deck are
to be provided, one to be provided at either end of the space. Tanks less than 35
metres in length shall be served by at least one access hatchway and ladder. When a
tank is subdivided by one or more swash bulkheads or similar obstructions which do
not allow ready means of access to the other parts of the tank, at least two
hatchways and ladders shall be fitted. All tanks are to be provided with clear
access for inspection without any obstruction by permanent and temporary provisions
arrangement for ventilation or others.
2.12.8 For access through horizontal openings, hatches or manholes, the
dimensions are to be sufficient to allow a person wearing a self-contained
air-breathing apparatus and protective equipment to ascend or descend any ladder
without obstruction and also to provide a clear opening to facilitate the hoisting
of an injured person from the bottom of the space. The minimum clear opening is to
be not less than 600 mm x 600 mm.
2.12.9 Where practicable, at least one horizontal access opening of 600 mm x
800 mm clear opening is to be fitted in each horizontal girder in all spaces and
weather deck to assist in rescue operations.
2.12.10 For access through vertical openings, or manholes providing passage
through the length and breadth of the space, the minimum clear opening is to be not
less than 600 mm x 800 mm at a height of not more than 600 mm from the bottom shell
plating, unless gratings or other footholds are provided.
2.12.11 In double hull construction where the wing ballast tanks have restricted
access through the vertical transverse webs, permanent arrangements are to be
provided within the space to permit access for inspection at all heights in each
bay. These arrangements, which should comprise fixed platforms, or other means, are
to provide sufficiently close access to carry out Close-Up Surveys, as defined in
Pt 1, Ch 3 Periodical Survey Regulations, using limited portable equipment where appropriate.
Details of these arrangements are to be submitted for approval.
2.12.12 On units with very large oil storage tanks, it is recommended that
consideration be given to providing permanent facilities for staging the interior of
tanks situated within the oil storage region and of large tanks elsewhere. Suitable
provisions would be:
- Staging which can be carried on board and utilised in any tank,
including power-operated lift or platform systems.
- Enlargement of structural members to form permanent, safe
platforms, e.g., bulkhead longitudinals widened to form stringers (in
association with manholes through primary members).
- Provision of inspection/rest platforms at intervals down the
length of access ladders.
- Provision of manholes in upper deck for access to staging in
cargo tanks.
2.13 Access hatchways to oil storage
tanks
2.14 Loading of hot oil in storage
tanks
2.14.1 Hot oil may be loaded in oil storage tanks at the temperatures given
below, without the need for temperature distribution and thermal stress
calculations, provided the following temperatures are not exceeded during
operations:
- 65°C for sea temperatures of 0°C and below;
- 75°C for sea temperatures of 5°C and above; and
- by linear interpolation between (a) and (b) above, for sea
temperatures between 0°C and 5°C.
2.15 Compartment minimum thickness
2.15.1 On semi-submersible units, within the oil storage tank region in oil
storage units including wing ballast tanks and cofferdams at the ends of or between
oil storage tanks, the thickness of primary member webs and face-plates, hull
envelope and bulkhead plating is to be not less than 7,5 mm.
2.15.3 The minimum compartment thickness in deep draught caisson units and
buoys will be specially considered but is not to be less than 7,5 mm.
2.15.4 The compartment minimum thickness is to comply with:
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