Section 7 Deep draught caisson units

7.1.1 This Section outlines the structural design requirements of deep draught caisson units and similar floating installations as defined in Pt 1, Ch 2, 2 Definitions, character of classification and class notations, but excluding other unit types defined in this Chapter.

7.1.2 Additional requirements for particular unit types related to the design function of the unit are also given in Pt 3 FUNCTIONAL UNIT TYPES AND SPECIAL FEATURES.

7.1.3 The hull of caisson units are to be divided into watertight compartments and have adequate buoyancy and floating stability in all conditions defined in Pt 4, Ch 4, 7.5 Structural design 7.5.2.

7.1.4 Watertight compartments which are to be temporarily flooded during site installation or in upending conditions are to have tank bulkhead scantlings as required by Pt 4, Ch 6, 7 Bulkheads.

7.1.5 Venting arrangements are to be fitted to all floodable spaces to ensure that air is not trapped in any operating mode or temporary condition.

7.1.6 Any spaces filled with permanent ballast are to be specially considered with regard to the material and its attachment to the structure.

7.1.7 Production and oil storage units are to comply with the requirements of Pt 3, Ch 3 Production and Storage Units. Caissons designed for the storage of oil in bulk storage tanks are to comply with the relevant requirements of the National Authority.

7.2.1 In all floating modes of operation, the unit is to be designed to have a clearance air gap between the underside of the top side deck structure and the highest predicted design wave crest. Model test results are to be submitted for consideration.

7.3.1 The Owner or designer is to specify the environmental criteria for which the installation is to be approved. The extreme environmental conditions applicable to the location are to be defined, together with all relevant operating environmental limits. Full particulars are to be submitted with sufficient supporting information to demonstrate the validity of the environmental parameters, see Pt 4, Ch 3, 4 Structural design loads.

7.3.2 Although a deep draught caisson unit will not be classed during transit and during the installation procedure at the operating location, the specified limiting design environmental criteria for transit/loadout, upending, and mating conditions for which LR structural approval is required are to be clearly defined and submitted.

7.3.3 Environmental loads and motions are to be established for each mode of operation, including the upending condition, by suitable analysis. Model tests will normally be required.

7.3.4 In determining environmental loads, account is to be taken of the effect of marine growth, see Pt 4, Ch 3, 4.13 Marine growth.

7.4.1 The test programme and the model test facilities are to be to LR’s satisfaction, see also Pt 4, Ch 3, 4 Structural design loads.

7.4.2 The relative directions of wind, wave and current are to be varied as required to ensure that the most critical loadings and motions are determined. The tests are to be of sufficient duration to establish low frequency motion behaviour.

7.4.3 Model tests are to demonstrate clearly that the air gap as required by Pt 4, Ch 4, 7.2 Air gap 7.2.1 is maintained in all operating modes.

7.5.1 The general requirements for structural design are given in Pt 4, Ch 3 Structural Design, but the additional requirements of this Chapter are to be complied with.

7.5.2 The structure is to be designed to withstand the static and dynamic loads imposed on the unit and the structural analysis and determination of primary scantlings are to be on the basis of the distribution of loadings expected in all modes of operation and temporary conditions, including loadout, transportation, upending, lifting and mating, as applicable.

7.5.3 All relevant loads as defined in Pt 4, Ch 3 Structural Design are to be considered and special attention is to be made in determining vortex-induced action effects due to wind and sea currents. The arrangement and scantlings of helical plate attachments on the hull, where fitted to keep vortex-induced responses at acceptable levels, are to be specially considered. The shell plating in way of attachments is to be increased.

7.5.4 Local forces from mooring lines and risers are to be included in the analyses for normal operating conditions.

7.5.5 Where units have combined crude oil bulk storage and ballast tanks which are intended to remain full in operating conditions, consideration is to be given to taking the design hydrostatic loading as the difference between external and internal pressures subject to adequate safeguards against accidental loading and agreed survey requirements. The corrosion wastage allowance in such tanks is to be specially considered, see Pt 4, Ch 4, 7.10 Corrosion protection.

7.5.6 Permissible stresses due to the overall and local 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.