Section 2 Structure

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.

2.2.1 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.

2.2.7 Critical 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. Where unavoidable, plate material with suitable through thickness properties will be required, see Ch 3, 8 Plates with specified through thickness properties of the Rules for Materials and Pt 4, Ch 2, 4.1 General 4.1.3.

2.2.8 When blast walls are fitted on the unit, the primary supporting structure in way of the blast walls is to be designed for the maximum design blast force with the permissible stress levels in accordance with Table 5.2.1 Factors of safety for the combined load cases – load case (d) in Pt 4, Ch 5, 2.1 General.

2.2.9 Turret structures, swivel stacks, mooring arms and yoke structures, etc., are to comply with the requirements of Pt 3, Ch 13 Buoys, Deep Draught Caissons, Turrets and Special 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.

2.4.1 In general, the permissible stresses in the structure in operating, transit and survival conditions are to comply with Pt 4, Ch 5, 2 Permissible stresses but the minimum scantlings of the local structure are to comply with Pt 4, Ch 6 Local Strength. For ship units, see Pt 10 Ship Units. For other surface type units, see Pt 4, Ch 4 Structural Unit Types.

2.4.2 Permissible stresses for lattice type structures may be determined for an acceptable Code, see Pt 12, Ch 1 Recognised Codes and Standards

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.

2.5.3 Circumturret bulkheads and the boundary bulkheads of moonpools and drilling wells are to be designed for the maximum forces imposed on the structure. For ship units, see Pt 10 Ship Units. For other surface type units, see Pt 4, Ch 4, 4 Surface type units. For other unit types, see Pt 4, Ch 6 Local Strength .

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.

2.7.1 The scantlings of structural deckhouses are to comply with Pt 4, Ch 6, 9 Superstructures and deck-houses. Where deckhouses support equipment loads, they are to be suitably reinforced.

2.7.2 Offshore containers are to comply with Pt 3, Ch 4, 2.4 Offshore containers.

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.

2.9.1 Free-standing bulk storage vessels are to comply with the requirements of Pt 3, Ch 8, 4 Pressure vessels and bulk storage.

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.

2.10.1 The general requirements for watertight and weathertight integrity are to be in accordance with Pt 4, Ch 7 Watertight and Weathertight Integrity and Load Lines.

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.

2.10.4 Where bulk storage vessels penetrate watertight decks or flats, the arrangements to ensure watertight integrity will be specially considered, see Pt 3, Ch 3, 2.9 Bulk storage vessels 2.9.3.

2.11.1 For requirements in respect of coamings and closing of deck openings, see Pt 4, Ch 7, 6 Miscellaneous openings.

2.11.2 The access arrangements on ship units and other surface type units are to comply with Pt 3, Ch 3, 2.12 Access to spaces in oil storage areas. For other unit types, the general requirements of Pt 3, Ch 3, 2.12 Access to spaces in oil storage areas are to be complied with, as applicable.

2.11.3 Ladders and platforms in tanks, pump-rooms, cofferdams, access trunks and void spaces are to be securely fastened to the structure.

2.12.1 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.

2.13.1 The general requirements of Pt 4, Ch 7, 6 Miscellaneous openings are to be complied with.

2.14.2 Where the stored oil is to be loaded or heated to higher temperatures than those specified in Pt 3, Ch 3, 2.14 Loading of hot oil in storage tanks 2.14.1 before unloading, temperature distribution investigations and thermal stress calculations may be required. For ship units and other surface type units, see Pt 4, Ch 9, 12 Cargo temperatures of the Rules for Ships.

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.

2.15.2 Pump-rooms and other adjacent compartments are also to comply with Pt 3, Ch 3, 2.15 Compartment minimum thickness 2.15.1.

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.