Section
1 Column-stabilised units
1.1 General
1.1.3 Production and oil storage units are to comply with the requirements of
Pt 3, Ch 3 Production and Storage Units. Columns and pontoons designed for the
storage of oil in bulk storage tanks are to be of double hull construction. If
pontoon oil storage tanks are always kept empty in transit conditions, a double
bottom need not be fitted, except where a double bottom is required by a National
Administration and/or Coastal State Authority.
1.1.4 If it is intended to dry-dock the unit, the bottom structure is to be
suitably strengthened to withstand the loadings involved. The proposed docking
arrangement plan and maximum bearing pressures are to be submitted.
1.2 Air gap
1.2.1 In all floating modes of operation, column stabilised units are normally
to be designed to have a clearance ‘air gap’ between the underside of the upper hull
deck structure (and any underdeck structure or critical equipment as applicable to
the specific design) and the highest predicted design wave crest. The minimum
clearance is not to be less than 1,5m, taking into account the most onerous
conditions and the predicted motion of the unit relative to the surface of the sea.
Calculations, model test results or prototype reports are to be submitted for
consideration. Where necessary for operational purposes, any platforms or structures
below the air gap are to be designed for the hydrodynamic forces encountered in
waves present in these areas.
1.2.2 In cases where the unit is designed without a clearance air gap, the
scantlings of the upper hull deck structure are to be designed for wave impact
forces, see also
Pt 4, Ch 4, 1.4 Upper hull structure 1.4.4.
1.3 Structural design
1.3.1 The general requirements for structural design are given in Pt 4, Ch 3 Structural Design, but the additional requirements of this Section are to
be complied with.
1.3.2 The structure is to be designed to withstand the static and dynamic loads
imposed on the unit in transit and semi-submerged conditions. All relevant loads as
defined in Pt 4, Ch 3 Structural Design are to be considered and the permissible
stresses due to the overall and local load effects are to be in accordance with
Pt 4, Ch 5 Primary Hull Strength. The minimum local scantlings of the unit
are to comply with Pt 4, Ch 6 Local Strength.
1.3.4 A strength and fatigue assessment of the special and primary structure
is to be carried out by a three-dimensional finite element method in accordance with
the LR ShipRight Procedure for Semi-submersibles.
1.3.5 In order to ensure adequate structural redundancy after credible failure
or accidents, the structure is to be investigated for loading condition (d) in Table 4.1.1 Design loading
conditions. The environmental loads
for this load case are to be taken as the same as determined for loading condition
(b). The structure is to be able to withstand the following failures without causing
the overall collapse of the unit’s structure:
- The failure of any main primary bracing member.
- When the upper hull structure consists of heavy or box girder
construction, the failure of any primary slender member.
1.3.6 The general requirements for investigating accidental loads are defined
in Pt 4, Ch 3, 4.16 Accidental loads, but in the case of a column-stabilised
unit, collision loads against a column or pontoon will normally only cause local
damage to the structure and consequently loading condition (c) in Table 4.1.1 Design loading
conditions need not be investigated
from the overall strength aspects. The requirements for very slender columns will be
specially considered.
1.4 Upper hull structure
1.4.1 Decks and supporting grillage structures forming part of the primary
structure are to be designed to resist both the overall and local loadings.
1.4.2 Openings in primary bulkheads and decks are normally to be represented
in the structural model. Bulkhead openings in ‘tween decks are not, in general, to
be fitted in the same vertical line. When large bulkhead openings are cut in the
structure which were not included in the structural model, the bulkhead thickness is
to be increased in way of the opening to compensate for the loss of shear area and
stiffness.
1.4.3 When the primary deck structure consists of heavy or box girder
construction and the infill deck plating is considered to be secondary structure,
only the main deck girders and the secondary deck plating stiffeners need satisfy
the buckling strength requirements given in Pt 4, Ch 5 Primary Hull Strength. The infill deck plating thickness and its contribution
to the overall strength of the structure will be specially considered, see
also
Pt 4, Ch 6, 4 Decks.
1.4.4 When the upper hull structure is designed to be waterborne for
operational purposes the upper hull scantlings are not to be less than those
specified for shell boundaries of self-elevating units as defined in Pt 4, Ch 6, 3 Watertight shell boundaries.
1.4.5 Columns should be aligned and integrated with the bulkheads in the upper
hull structure. Particular attention should be given to the detail design at the
intersection of columns with the upper hull structure to minimise stress
concentrations.
1.5 Columns
1.5.1 Columns are to be designed to withstand the forces and moments resulting
from the overall loadings, together with forces and moments due to wave loadings and
internal tank pressures.
1.5.3 High local loads are also to be taken into account in the overall design
strength of the columns.
1.5.4 Internal column structure supporting main bracings is in general not to
be of a lesser strength than the bracing itself.
1.5.5 When bracing forces are designed to be transmitted to the column shell,
the resulting column shell stresses are to be combined with the stresses due to the
hydrostatic pressure and overall forces.
1.6 Lower hulls
1.6.1 Lower hulls or pontoons are to be designed for overall bending, shear
forces, and axial forces due to end pressure when combined with the local
hydrostatic pressure as defined in Pt 4, Ch 3, 4.14 Hydrostatic pressures.
1.6.2 Irrespective of the tank loading arrangement, the scantlings of tanks
are to be verified in both full and empty conditions.
1.6.3 Columns are, as far as practicable, to be continuous through the plating
of the lower hull deck structure and be aligned and integrated with the internal
bulkheads and/or side shell.
1.6.4 Where the column shell plating is intercostal with the lower hull deck,
the deck plating below the columns is to be suitably increased and is to have steel
grades with suitable through thickness properties, see
Pt 4, Ch 2, 4.1 General 4.1.3.
1.6.5 Particular attention should be given to the design of the local
structure at the intersection of columns with lower hulls and due account should be
given to penetrations and stress concentrations.
1.7 Main primary bracings
1.7.1 Bracing members are to be designed to withstand the stresses imposed by
the overall loading, together with local stresses due to wave, current and buoyancy
forces and, when applicable, hydrostatic pressure.
1.7.2 Bracings are in general to be made watertight and provided with adequate
means of access to enable internal inspection to be carried out when the unit is
afloat.
1.7.3 Watertight bracings are to be designed for the hydrostatic pressure loads
defined in Pt 4, Ch 3, 4.14 Hydrostatic pressures, and the scantlings are to be verified
against buckling due to combined axial stresses and hoop stresses caused by external
hydrostatic pressure. Ring stiffeners are to be fitted where necessary.
1.7.4 Attachments and penetrations to the shell of bracings are to be avoided
as far as practicable. If attachments are unavoidable they are generally to be
welded to suitable doubler plates having well rounded corners. Special consideration
will be given to alternative proposals. In all cases the attachment is to be
designed to minimise the resulting stress concentration in the brace and the fatigue
life is to be checked.
1.7.6 The scantlings and arrangements of free-flooding bracings will be
specially considered.
1.8 Bracing joints
1.8.1 Joints at the intersection of bracings or between bracings and columns
are to be designed to transmit the bending, direct and shear forces involved in such
a manner as to reduce, so far as possible, the risk of fatigue failure. Stress
concentrations are to be minimised by good detail design and, in general, nominal
stress levels are to be made lower than in the adjacent structure by increasing
plate thickness or suitably flaring the member ends, or both. Ring stiffeners or
other welded attachments across the principal stress direction are to be avoided
wherever possible in all regions of high stress. It this is not possible (e.g.,
where required to support bracket ends on otherwise unstiffened plating), the weld
is to have a smooth profile without undercutting. Continuity of strength is to be
maintained through the joint, and shear web plates and other axial stiffening
members are to be made continuous.
1.8.2 Special attention is also to be given to the qualities of bracing
details, e.g., openings, penetrations, stiffener ends, brackets and other
attachments. The welding procedure is to be such as to minimise the risk of cracks,
lack of penetration and lamellar tearing of the parent steel.
1.8.3 Joints depending upon transmission of tensile stresses through the
thickness of the plating of one of the members (which may result in lamellar
tearing) are to be avoided wherever possible. Plate steel used in such locations
shall have suitable through thickness properties.
1.9 Lifeboat platforms
1.9.1 The strength of lifeboat platforms is to be verified with the unit in the
upright condition and in the inclined condition at an angle corresponding to the
worst damage waterline, and at an inclined angle of 15° in any direction.
1.9.2 For calculation purposes, the weight of the lifeboat is to be taken as
the weight when fully manned and equipped. The platform weight is to be taken as the
steel weight plus the weight of davits and equipment. Symmetrical and unsymmetrical
load cases are to be considered as appropriate, e.g., one lifeboat launched and the
other lowering. The design calculations are to be submitted for information.
1.9.3 The following dynamic load factors are to be included in the
calculations:
Item:
|
Factor:
|
Platform weight
|
0,3 g
|
Lifeboat weight when
stowed
|
0,3 g
|
Lifeboat weight when
lowering
|
0,5 g
|
1.9.5 After installation of the lifeboats, testing is to be carried out to the
satisfaction of LR’s Surveyors.
1.10 Topside structure
1.10.2 For units fitted with a process plant facility and/or drilling equipment,
the support stools and integrated hull support structure to the process plant and
other equipment supporting structures including derricks and flare structures are
considered to be classification items, regardless of whether or not the
process/drilling plant facility is classed, and the loadings are to be determined in
accordance with Pt 3, Ch 8, 2 Structure. Permissible stress levels are to
comply with Pt 4, Ch 5 Primary Hull Strength.
1.10.4 Units with a process plant facility which comply with the requirements
of Pt 3, Ch 8 Process Plant Facility will be eligible for the assignment of
the special features class notation PPF.
1.10.5 Units with a drilling plant facility which comply with the requirements
of Pt 3, Ch 7 Drilling Plant Facility will be eligible for the assignment of
the special features class notation DRILL.
1.10.6 Units with a pipe-laying system which comply with the class requirements of Pt 3, Ch 17 Pipe-laying Units will be eligible for
the assignment of the special features class notation PLS.
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