Section 2 Structure

2.2.1 The general hull strength is to comply with the requirements ofPt 4 STEEL UNIT STRUCTURES taking into account the drilling structures and applied equipment weights and forces introduced by the drilling operations. Attention should be paid to loads resulting from hull flexural effects at support points. For surface type units, see also Pt 3, Ch 1, 1 Rule Application.

2.2.2 The design loadings for the strength of the drill floor and substructure are to be defined by the designers/Builders and calculations are to be submitted.

2.2.3 Strength calculations are to be submitted for moveable drilling cantilevers, skid beams and their supports. The clearances between the cantilever support claw and the skidding guides is the responsibility of the designers/Builders.

2.2.4 The maximum reaction forces from the drilling derrick are to be determined from an acceptable National Code or Standard and should take into account the load effects from vessel motions, the drillpipe setback, hook load, rotary table and tensioning equipment, see Pt 3, Ch 7 Drilling Plant Facility.

2.2.5 When the unit is to operate in an area which could result in the build-up of ice on the drilling derrick and other structures, the effects of ice loading is to be included in the calculations, see Pt 4, Ch 3, 4 Structural design loads.

2.2.6 The local structure should be reinforced for the component forces from drilling equipment and tensioner forces, and the design loadings are to be determined in accordance with Pt 3, Ch 7 Drilling Plant Facility.

2.2.7 The supporting structure and attachments under large equipment items are also to be designed for the emergency condition as defined in Pt 3, Ch 8, 1.4 Plant design characteristics

2.2.8 Attention should be paid to the capability of support structures to withstand buckling, see Pt 4, Ch 5, 4 Buckling strength of primary members.

2.2.9 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 Pt 4, Ch 5, 2.1 General 2.1.1.

2.3.1 The primary hull strength of the unit is to be maintained in way of drilling wells and other large deck openings and suitable compensation is to be fitted as necessary. For surface type units the minimum hull modulus in way of the drilling well is to satisfy the Rule requirements for longitudinal strength.

2.3.2 Arrangements are to be made to ensure continuity of strength at the ends of longitudinal 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.

2.3.3 The boundary bulkheads of drilling wells are to be designed for the maximum forces imposed by the drilling operations. The minimum scantlings of well bulkheads are to comply with the requirements for tank bulkheads in Pt 4, Ch 6, 7 Bulkheads using the load head measured to the top of the well, but in no case is the well plating to have a thickness less than 9,0 mm.

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 surface type units, see also Pt 4, Ch 4, 4 Surface type units.

2.4.2 Permissible stresses for lattice type structures may be determined from an acceptable code, see Pt 3, Ch 17 Appendix A Codes, Standards and Equipment Categories.

2.5.1 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 otherwise agreed with LR.

2.5.2 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

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

2.6.2 The strength of containerised modules which do not form part of the main hull structure will be specially considered in association with the design loadings.

2.6.3 When containerised modules can be subjected to wave loading the scantlings are not to be less than required by 2.6.1.

2.7.2 The pipe rack supports are also to be designed for an emergency condition as defined in Pt 3, Ch 8, 1 General.

2.7.3 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

2.7.4 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.

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

2.8.2 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.

2.8.3 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.

2.9.1 The general requirements for watertight and weathertight integrity are to be in accordance with Pt 4, Ch 7.

2.9.2 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.

2.9.3 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.

2.9.4 Where free-standing bulk storage vessels penetrate watertight decks or flats the arrangements to ensure watertight integrity will be specially considered, see Pt 3, Ch 2, 2.8 Bulk storage vessels 2.8.3.