Section 2 Structure

2.2.1 The hull strength is to take into account the applied weights and forces due to the accommodation, deck cargo, cranes and, if applicable, mooring forces and the local structure is to be suitably reinforced. Appendices A2, A3, A4 and A5 of the Guidance Note referred to in Pt 3, Ch 16, 1.3 Guidance note include summary Tables indicating the relevant Parts and Chapters of these Rules and the Rules for Ships, which are to be applied to the individual unit types for hull strength requirements.

2.2.2 For the unit types identified in Pt 3, Ch 16, 1.2 General definitions, 1.2.3, 1.2.4 and 1.2.5, the hull scantlings for each unit type are to be calculated in accordance with the relevant parts of the Rules identified in Appendices A2, A3, A4 and A5 of the Guidance Note Mobile Offshore Units – Wind Turbine Installation Vessels.

2.2.3 For the unit type identified in Pt 3, Ch 16, 1.2 General definitions, the hull scantlings for each unit type are to be calculated in accordance with the relevant parts of the Rules identified in the Guidance Note Mobile Offshore Units – Liftboats.

2.2.4 The design loadings for all purpose built cargo stacking arrangements, support frames and trusses are to be defined by the designers/Builders and calculations are to be submitted in accordance with an internationally recognised Code or Standard as defined in Pt 12, Ch 1 Recognised Codes and Standards. The supporting structure and attachments below the purpose built cargo stacking arrangements, support frames and trusses are to be designed for all operating conditions and for the emergency condition as defined in Pt 3, Ch 8, 1.4 Plant design characteristics. For a surface type self-elevating unit in the afloat condition, the angle of inclination in the emergency static condition is to be considered in accordance with the requirements for a self-elevating unit.

2.2.5 The supporting structure and attachments below any other mission equipment items are to be designed for all operating conditions and for the emergency condition as defined in Pt 3, Ch 8, 1.4 Plant design characteristics. For a surface type self-elevating unit in the afloat condition, the angle of inclination in the emergency static condition is to be considered in accordance with the requirements for a self-elevating unit.

2.2.6 When the unit is intended to operate in an area which could result in the build-up of ice on the crane, leg and any other structure, the effects of ice loading are to be included in the calculations. See Pt 4, Ch 3, 4 Structural design loads.

2.2.7 For column-stabilised and self-elevating units, the decks and other under-deck structure supporting the mission equipment and deck cargo are to be suitable for the local loads at the mission equipment and deck cargo support points and an agreed uniformly distributed load acting on the deck. See Pt 4, Ch 6, 2 Design heads.

2.2.8 For surface type and surface type self-elevating units, the decks and other under-deck structure supporting the mission equipment and deck cargo are to be suitable for the local loads at the mission equipment and deck cargo support points and an agreed uniformly distributed load acting on the deck. See Pt 3, Ch 3, 5 Design loading of the Rules for Ships.

2.2.9 In general, all seatings, platform decks, girders and pillars supporting mission equipment and deck cargo are to be arranged to align with the main hull structure, which is to be suitably reinforced, where necessary, to carry the appropriate loads.

2.2.10 Attention should be paid to the capability of support structures to withstand buckling. For column-stabilised and self-elevating units, see Pt 4, Ch 5, 3 Buckling strength of plates and stiffeners. Surface type and surface type self-elevating units are to comply with Pt 3, Ch 4, 7 Hull buckling strengthof the Rules for Ships, but aspects which relate to the specialised offshore function of the unit will be considered on the basis of Pt 4, Ch 5, 3 Buckling strength of plates and stiffeners.

2.2.11 Crane pedestals are classification items and are to comply with the requirements of Chapter 11.

2.2.12 For liftboats, a fatigue life assessment of all relevant structural elements in accordance with Pt 4, Ch 5, 5 Fatigue design is required. Structural elements to be assessed include lattice legs and connections to mats and footings and leg support structure. The fatigue loading spectrum may be based on the transit environmental criteria.

2.2.13 The minimum fatigue life of a liftboat is to be specified by the Owners, but is generally not to be less than 20 years, unless agreed otherwise with LR.

2.2.14 For liftboats, when considering the overturning moment, in no case is the variable load to be taken greater than 10 per cent of the maximum variable load. The percentage of variable load used when considering the overturning moment is to be stated in the Operations Manual.

2.2.15 For liftboats, when calculating the overturning moment, the unit should be considered supported through the centre line of the legs about which the unit is considered rotating. However, for hard foundation bases, the maximum stressed edge of the mat may be taken as an appropriate support position. In this instance, a safety factor of at least 1,2 against overturning is considered acceptable.

2.2.16 For liftboats, the Owner is to specify the minimum design environmental criteria and return periods for which the unit is to be approved. In general, a return period of not less than 1 year should be used for operational conditions and 100 years for survival conditions.

2.2.17 For liftboats, restricted to seasonal operations in order to avoid extremes of wind and wave, such seasonal limitations must be specified. The unit's actual minimum design environmental criteria and return periods used in the design of the liftboat are to be stated in the Operations Manual.

2.2.18 The thickness of marine growth to be taken into account during the design of submerged members on lift boats is not to be less than 50 mm. The actual thickness of marine growth used in the design of the liftboat is to be stated in the Operations Manual and the design limit is not to be exceeded in service.

2.2.19 For liftboats, the minimum design deck loads are to be specified by the Owner and are not to be less than the minimum design deck loads required by Pt 4, Ch 6,2.

2.2.20 For liftboats, the foundation fixity need not be considered for the in-place strength analysis.

2.3.1 For column-stabilised and self-elevating units, the scantlings of structural deckhouses are to comply with Pt 4, Ch 6, 9 Superstructures and deck-houses. Where deck-houses support equipment loads, they are to be suitably reinforced.

2.3.2 For surface type and surface type self-elevating units, the scantlings of structural deckhouses are to comply with Pt 3, Ch 8, 2 Scantlings of erections other than forecastles of the Rules and Regulations for the Classification of Ships, July 2022. Where deck-houses support equipment loads, they are to be suitably reinforced.

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

2.4.1 In general, for column-stabilised and self-elevating units 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.

2.4.2 In general, for surface type and surface type self-elevating units the primary hull strength and the minimum scantling requirements for the local structure can be considered under Pt 3, Ch 4 Longitudinal Strengthand Pt 4, Ch 1 General Cargo Shipsof the Rules for Ships. However, aspects which relate to the specialised offshore function of the unit will be considered under the basis of Pt 4, Ch 5, 2 Permissible stresses.

2.4.3 Permissible stresses for lattice type structures may be determined from an acceptable code, seePt 12, Ch 1 Recognised Codes and Standards .

2.5.1 For column-stabilised and self-elevating units, 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.

2.5.2 For surface type and surface type self-elevating units, the general requirements for watertight and weathertight integrity are to be in accordance with Pt 3, Ch 11 Closing Arrangements for Shell, Deck and Bulkheadsand Pt 3, Ch 12 Ventilators, Air Pipes and Discharges of the Rules for Ships.

2.5.3 The integrity of the weather deck is to be maintained. Where mission equipment penetrates the weather deck and is intended to constitute the structural barrier to prevent the ingress of water to spaces below the deck, its structural strength is to be equivalent to the Rule requirements for this purpose. Otherwise, such items are to be enclosed in superstructures or deck-houses fully complying with the Rules. Full details are to be submitted for approval.

2.5.4 Where items of mission equipment penetrate watertight boundaries, the watertight integrity is to be maintained and full details are to be submitted for approval.

2.6.1 For column-stabilised and self-elevating units, the general requirements for materials are to be in accordance with Pt 3, Ch 1, 4 Materials and Pt 4, Ch 2 Materials.

2.6.2 For surface type and surface type self-elevating units, the general requirements for materials are to be in accordance with Pt 3, Ch 2 Materials and Pt 4, Ch 1, 2 Materials and protection of the Rules for Ships. Aspects which relate to the specialised offshore function of the unit will be considered under the basis of Pt 3, Ch 1, 4 Materialsand Pt 4, Ch 2 Materials.