Section 1 Column-stabilised units

1.1.1 This Section outlines the structural design requirements of column-stabilised (semi-submersible) units as defined in Pt 1, Ch 2, 2 Definitions, character of classification and class notations. Additional requirements for particular unit types related to the design function of the unit are given in Pt 3 FUNCTIONAL UNIT TYPES AND SPECIAL FEATURES.

1.1.2 Units which are required to operate while resting on the sea bed are also to comply with the requirements of Pt 4, Ch 4, 2 Sea bed-stabilised units.

1.1.3 Production and oil storage units are to comply with the requirements ofPt 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.

1.2.1 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 the highest predicted design wave crest. Reasonable clearance is to be maintained at all times, taking into account 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.

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.

1.3.1 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.

1.3.3 All modes of operation are to be investigated and the relevant design load combinations defined in Pt 4, Ch 5, 1.2 Structural analysis are to be complied with. The loading conditions applicable to a column-stabilised unit are shown in Pt 4, Ch 4, 1.3 Structural design 1.3.3.

1.3.4 The overall strength of the unit is to be analysed by a three-dimensional finite element method in accordance with Pt 4, Ch 3, 3 Structural idealisation.

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 Pt 4, Ch 4, 1.3 Structural design 1.3.3 need not be investigated from the overall strength aspects. The requirements for very slender columns will be specially considered.

1.3.7 The permissible stress levels after credible failures or accidents are to be in accordance with Pt 4, Ch 5 Primary Hull Strength.

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.

1.5.1 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.

1.5.2 In general, internal spaces within the columns are to be designed for the pressure heads defined in Pt 4, Ch 3, 4.14 Hydrostatic 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.

1.6.1 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.

1.7.1 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 inPt 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.

1.7.5 Leak detection and drainage arrangements of watertight bracings are to be in accordance with Pt 5, Ch 13, 3 Drainage of compartments, other than machinery spaces for column-stabilised units.

1.7.6 The scantlings and arrangements of free-flooding bracings will be specially considered.

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.

1.9.1 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.

1.9.4 In the upright condition and in the inclined condition the permissible stresses are to comply with Pt 4, Ch 5, 2.1 General 2.1.1, loadcase (a) and (b) respectively.

1.9.5 After installation of the lifeboats, testing is to be carried out to the satisfaction of LR’s Surveyors.

1.10.1 The minimum scantlings of superstructures and deckhouses are to comply with the requirements of Pt 4, Ch 6, 9 Superstructures and deckhouses. Bulwarks and guard rails are to comply with Pt 4, Ch 6, 10 Bulwarks and other means for the protection of crew and other personnel.

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.

1.10.3 The boundary bulkheads of accommodation spaces which may be subjected to blast loading are to be designed in accordance with Pt 4, Ch 3 Structural Design, Pt 4, Ch 4 Structural Unit Types and permissible stress levels are to satisfy the factors of safety given in Pt 4, Ch 5, 2.1 General 2.1.1.

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.