Section 5 Structural design – New-build units

5.1.1 This Section outlines the hull structural calculation and analysis requirements for new-build ship units engaged in production and/or oil storage/offloading moored at offshore locations. Requirements are given for permanently moored units and disconnectable units.

5.1.2 The hull structure is to be designed to withstand the static and dynamic loads imposed on the structure in all operating conditions and all anticipated pre-service conditions. All relevant loads as defined in Pt 4, Ch 3 Structural Design are to be considered, including the effects of partial and/or non-homogeneous loading in cargo bulk storage tanks. When considering the design loading conditions, the Owner/ designer is to take account of the requirements for on-station tank inspection/maintenance. Loads during construction, installation and decommissioning, towing/ transportation should be considered, as applicable. Reference is also made to the LR ShipRight Procedure for Ship Units.

5.1.4 For Moderate service, the site-specific loads can be used. The loads for unrestricted worldwide transit from Pt 10, Ch 2 Loads and Load Combinations may be used at the Owner's discretion. For Harsh service the site-specific loads must be used. Where the unit is intended for operation at more than one location, the most severe design criteria are to be applied. Where the ShipRight RBA notation is assigned, the site-specific loads must be used.

5.1.5 On-site tank inspections/maintenance are to be restricted to reasonable weather as defined in Pt 1, Ch 2 Classification Regulations. For design purposes, the permissible still water bending moments and shear forces for tank inspection/maintenance conditions may be based on 100-year return period seasonal site criteria. Tank inspection/maintenance conditions are to be included in the unit's loading manual and the limiting environmental criteria are to be defined in the Operations Manual.

5.1.6 Where it is intended to dry-dock a unit during its service life, this is to be taken into account at the design stage and the docking condition is to be submitted to LR for approval. The bottom structure should be suitably strengthened to withstand the bearing pressures and loads imposed by dry-docking.

5.1.7 Disconnectable units, as defined in Pt 10, Ch 1, 1.1 Application 1.1.2, will remain in class in the sail-away condition and the loading conditions are to be submitted for approval.

5.1.8 The hull structure of is to be assessed for applicable transit conditions in accordance with Pt 10, Ch 1, 1.3 Application of transit conditions.

5.1.9 The general requirements for investigating accidental loads are defined in Pt 4, Ch 3, 4.16 Accidental loads. Collision loads against the hull structure will normally cause only local damage to the hull structure and consequently need not be investigated from the overall strength aspects.

5.1.10 Structural strength and fatigue analyses are generally required to verify that hull structure and critical structural connections, when subjected to the site-specific load combinations and other relevant load combinations, are suitable for the required service life on location.

5.1.11 Hull integration structure in way of moorings, topsides and other concentrated loads is to be verified by direct calculations. Permissible stress levels are to be in accordance with Pt 10, Ch 3 Scantling Requirements or Pt 4, Ch 5 Primary Hull Strength, .

5.1.12 Where permitted by the relevant National Authority, single hulled units may be accepted.

5.1.13 Sufficiently robust underdeck reinforcement should be provided in the way of the welded connections of the topsides support structure to the main hull. The support structures should be aligned with the primary members of the hull structure.

5.1.14 Hull structure and mooring integration structures: for disconnectable units at locations exposed to cyclones, the environmental loads when disconnected are not to be taken less than required by Pt 10, Ch 2 Loads and Load Combinations for unrestricted worldwide transit.

5.2.1 The longitudinal strength of the unit is to comply with the requirements of Pt 10, Ch 3 Scantling Requirements. The total stresses from the combined effects of site-specific wave loads, still water loads, mooring loads, etc. are not to exceed permissible values.

5.2.2 When the site-specific wave bending moments and shear forces are below the values for unrestricted worldwide transit, the site-specific values may generally be used for design, see Pt 10, Ch 1, 5.1 General 5.1.4. However, in no case are the sitespecific wave bending moments and shear forces to be taken as less than 50 per cent of the value for unrestricted worldwide transit.

5.2.3 The requirement for hull girder inertia given in Pt 10, Ch 3 Scantling Requirements is to be complied with.

5.2.4 The strength of the unit in the transit condition and in the site-specific installation condition is to be investigated and submitted to LR for approval.

5.2.5 For initial design purposes, site-specific environmental factors are given in Pt 10, Ch 2, 3.3 Environmental factors with the associated Dynamic Load Combination factors (DLCF) given in Pt 10, Ch 2, 7 Environmental loads for unrestricted worldwide transit condition for the unrestricted worldwide transit condition and Pt 10, Ch 2, 8 Environmental loads for site-specific load scenarios for the On-site Operational condition.

5.2.6 For the final design, the loads derived in accordance with the LR ShipRight Procedure for Ship Units must be used.

5.3.1 The scantlings of the primary structure of the cargo bulk storage tank area are to be verified by direct calculations based on a three-dimensional finite plate element analysis carried out in accordance with the LR ShipRight Procedure for Ship Units.

5.3.2 The corrosion additions are to be determined as described in Pt 10, Ch 1, 12 Corrosion additions.

5.4.1 The fatigue assessment of the hull structure of ship units is to be verified in accordance with the LR ShipRight Procedure for Ship Units.

5.4.2 In all cases, the fatigue assessment should address the primary hull structure connections, primary topside support structure and hull integration, together with other primary structure connections subject to significant dynamic loading. Account should be taken of all important sources of cyclic loading, see also Pt 4, Ch 5, 5.2 Fatigue life assessment.

5.4.3 Fatigue calculations for the mooring structures and integration of the mooring system within the unit’s hull structure are also to be carried out, see Pt 3, Ch 10 Positional Mooring Systems.

5.4.4 The turret-bearing support structures are to be assessed for fatigue damage due to cyclic loading in accordance with Pt 4, Ch 5, 5 Fatigue design.

5.4.5 The general requirements for fatigue design and factors of safety on fatigue life for supporting structures to drilling and process plant, flare towers, derricks, cranes and crane pedestals and mooring structures are to comply with Pt 4, Ch 5, 5 Fatigue design.

5.4.6 The minimum design fatigue life for structural elements should not be less than the intended field life, but in general should not be less than 25 years. The cumulative damage ratio for individual components should take account of the degree of redundancy and accessibility of the structure and also the consequence of failure, see also Pt 4, Ch 5, 5 Fatigue design.