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 byPt 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, 4.13 Marine growth.

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.

7.5.8 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.

7.5.9 Where the hull form incorporates a space frame or truss system of braces, the requirements of Pt 4, Ch 4, 1.7 Main primary bracings and Pt 4, Ch 4, 1.8 Bracing joints are to be complied with.

7.6.1 Caissons are to be designed to withstand the forces and moments resulting from the overall loadings together with the forces and moments due to local loadings, including internal and external pressures.

7.6.2 In general, internal spaces within the caisson are to be designed for the pressure heads defined in Pt 4, Ch 3, 4.14 Hydrostatic pressures. The minimum head on shell boundaries is not to be less than 6 m, see also Pt 4, Ch 4, 7.5 Structural design.

7.6.3 The minimum scantlings of shell boundaries including moon pools are to comply with Pt 4, Ch 6, 3.4 Buoys and deep draught caissons.

7.6.4 The general requirements for watertight and tank bulkheads are to comply with Pt 4, Ch 6, 7 Bulkheads. The scantlings of the boundaries of internal watertight compartments adjacent to the sea which are required for buoyancy and stability to support the structure are to comply with the requirements for tank bulkheads, see also Pt 4, Ch 4, 7.10 Corrosion protection.

7.6.5 Internal caisson structure supporting main bracings is in general not to be of a lesser strength than the bracing itself.

7.6.6 The supports for riser systems and mooring systems are to comply with Pt 4, Ch 6 Local Strength.

7.7.1 The scantlings of deck support structures which are designed as a trussed space frame structure are to be determined by analysis. The requirements of Pt 4, Ch 4, 7.5 Structural design 7.5.9 are to be complied with.

7.7.2 The minimum scantlings of decks are to comply with Pt 4, Ch 6, 4 Decks.

7.7.3 The scantlings of superstructures and deckhouses are to comply with Pt 4, Ch 6, 9 Superstructures and deck-houses.

7.8.1 The strength of lifeboat platforms is to be determined in accordance with the requirements of Pt 4, Ch 4, 1.9 Lifeboat platforms.

7.9.1 The structure of deep draught caissons and highly stressed structural elements of mooring line attachments, chain stoppers and supporting structures is to be assessed for fatigue damage due to cyclic loading.

7.9.2 The general requirements for fatigue design and the factors of safety on fatigue life are to comply with Pt 4, Ch 5, 5 Fatigue design.

7.10.1 The general requirements for corrosion protection are to comply with Pt 8 Corrosion Control.

7.10.2 In tanks referred to in Pt 4, Ch 4, 7.5 Structural design 7.5.5, due to design operating procedures or in areas where it is not considered practicable to inspect internal spaces or replace corrosion protection systems, the structure is to be designed with adequate corrosion margins and protection for the service life of the caisson. The corrosion wastage allowance and protection of all structural components are to be to the satisfaction of LR and agreed at the design stage.

7.10.3 Where practicable, suitable inspection coupons or other inspection aids are to be incorporated into the structure so that the degree of corrosion in inaccessible spaces can be monitored during Periodical Surveys required by Pt 1 Regulations.