Section 6 Structural design – Tanker conversions

6.1.1 This Section outlines the hull structural calculations and analysis requirements for tanker conversions engaged in production and/or cargo storage/offloading moored at offshore locations. Requirements are given for permanently moored units and disconnectable units. At the Owner’s request, the requirements given in Pt 10, Ch 1, 5 Structural design – New-build units may be applied instead of the requirements given in this Section.

6.1.2 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 and the effects of partial and/or non-homogeneous loading in cargo bulk storage tanks are to be considered. 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, and towing/ transportation should be considered, as applicable. Reference is also made to the LR ShipRight Procedure for Ship Units.

6.1.4 For Moderate service, the site-specific loads can be used. The loads for unrestricted worldwide transit service from Pt 10, Ch 2 Loads and Load Combinations may be used at the Owner's discretion. For Harsh service, the unit is to be reassessed as for a new build. 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 sitespecific loads must be used.

Where these conditions are not met (for example, turret integration structure, supporting structure under topsides and crane pedestals), the structure is to be re-assessed in accordance with Pt 10, Ch 1, 5 Structural design – New-build units.

6.1.7 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 wave bending moments and shear forces for tank inspection/maintenance conditions may be based on the existing assigned permissible still water values. Where the existing assigned permissible still water values are insufficient, wave bending moments and shear forces may be based on 100-year return period seasonal site criteria and still water permissible values adjusted accordingly. 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.

6.1.8 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.

6.1.9 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.

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

6.1.11 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.

6.1.12 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.

6.1.13 Hull integration structure in way of moorings, topsides, crane pedestals, flare towers and other concentrated loads is to be verified by direct calculations. Permissible stress levels are to be in accordance with Pt 4, Ch 5 Primary Hull Strength.

6.1.14 The detailed scope of analysis required for hull structural assessments of tanker conversions will be considered on a case-by-case basis, see Pt 10, Ch 1, 6.2 Hull scantlings.

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

6.1.16 Sufficiently robust underdeck reinforcement should be provided in way of the welded connections of the topsides support structure to the main hull. Special attention should be given to alignment of primary members.

6.1.17 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 service for the assessment of structures required by Pt 10, Ch 1, 6.1 General 6.1.6.

6.1.18 For units permanently moored by the stern, the structural arrangements and scantlings of all exposed structure located in the aft end of the unit are to be specially considered. The strengthening of the bottom structure is to be specially considered.

6.2.1 Hull scantlings are to be re-assessed in accordance with the requirements for new-build units, see Pt 10, Ch 1, 5.3 Strength analysis, whenever any of the following apply:

1. The unit is to be deployed in harsh service;
2. The total hull girder bending moments (hogging and sagging) approved prior to conversion, i.e. vertical wave bending moment + permissible still water vertical bending moment, are exceeded; or
3. The total hull girder shear forces (positive and negative) approved prior to conversion, i.e. vertical wave shear force + permissible still water vertical shear force, are exceeded.

6.2.2 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, 6.1 General 6.1.4. 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.

6.2.3 If the environmental factors, as defined in Pt 10, Ch 1, 2.3 Plans and information to be supplied on board the unit, calculated for the hull girder bending moments ( or ) or shear force () exceed 1,0, then the hull scantlings are to be re-assessed in accordance with the requirements for new-build units, see Pt 10, Ch 1, 5.3 Strength analysis.

6.2.4 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.

6.2.5 Where the conversion includes provision for an internal turret mooring system, the effects of such openings and reinforcement structure on hull girder strength should be evaluated.

6.2.6 It is recommended that, in general, corrosion additions are to be determined based on Pt 10, Ch 1, 12 Corrosion additions; however, consideration will be given to alternative proposals submitted by the Owner.

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

6.3.2 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.

6.3.3 Fatigue calculations for the mooring structure 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.

6.3.4 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.

6.3.5 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.

6.3.6 Fatigue calculations for installations based on tanker conversions should take into account the fatigue damage accumulated as a trading tanker prior to conversion.

6.3.7 The design corrosion additions are to be deducted from the scantlings, measured at the time of conversion, as described in the LR ShipRight Procedure for Ship Units. This is to ensure the calculation of fatigue damage after conversion accounts for any reduction in the as-built scantlings. The analysis is required to verify that the remaining fatigue life of the converted hull structure is compatible with the required service life on location, see also Pt 10, Ch 1, 6.3 Fatigue analysis 6.3.8.

6.3.8 The minimum design fatigue life (after accounting for fatigue damage accumulated as a trading tanker prior to conversion) for structural elements should not be less than the intended field life, but should not be less than 5 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.

6.3.9 The in-service Class survey reports for the vessel from build until conversion are to be submitted to LR for review. All critical locations in the existing structure which may be prone to fatigue cracking are to be examined by MPI or other suitable NDE methods at the time of conversion. The critical locations are to be selected based on the previous service history of the vessel and the recommendations in the LR ShipRight Procedure for Ship Units. A detailed NDE plan is to be submitted for approval.

6.4.1 Requirements for direct calculations are given in the LR ShipRight Procedure for Ship Units.