Section
6 Structural design – Tanker conversions
6.1 General
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.
6.1.3 The assessment of environmental loads may be based on the results of
model tests and/or by suitable direct calculation methods of the actual loads on the
hull at the site-specific location, taking into account the following
service-related factors:
- Site-specific environmental loads including relevant nonlinear
effects.
- Mooring system and riser loads.
- Unit orientation and wave loading directions.
- Long-term service effects at a fixed location.
- Range of tank loading conditions, including empty tanks required
for on-station surveys.
- Potential relocations if applicable.
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.
6.1.5 Where a unit is intended to operate in Moderate Environments, the
existing scantlings of the hull need not be re-assessed, subject to the following
conditions:
- the vessel was built under the survey of a member of IACS before
conversion;
- the vessel has been maintained in Class by a member of IACS
before conversion;
- CAP assessment 1 or 2 assigned;
- all necessary repairs are made to delete any Conditions of
Class;
- the in-service corrosion margins applied after conversion are
the same as those applicable as a trading tanker;
- LR Transfer of Class (TOC) procedures are complied with if the
vessel is transferring Class to LR;
- a Special Survey is conducted during the conversion;
- the loading on the structure is not increased;
- the structure is not changed;
- the vessel was originally approved for worldwide service.
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.
6.1.6 For Moderate service further to the reassessment criteria specified in
Pt 10, Ch 1, 6.1 General 6.1.5, the hull scantlings are to be reassessed where the ShipRight
RBA notation is assigned. If the structure is modified or the loading
changed then the hull scantlings affected by these changes should be reassessed.
Hull scantlings of a conversion may need to be reassessed for the following reasons:
- integration of the mooring system of an internal turret;
- loads from topsides equipment on the upper deck;
- redefinition of loading limitations assigned as a tanker (for
example, changes to permissible still water bending moments and shear
forces) where required for unit-specific loading conditions;
- measured corrosion found to be in excess of that permitted for a
trading tanker.
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.
6.1.11 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.
6.1.18 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 Hull scantlings
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:
- The unit is to be deployed in harsh service;
- 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
- 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 site-specific wave bending moments and shear forces
for on-site operation and transit to be taken as less than 50 per cent of the value
for unrestricted worldwide transit.
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 Fatigue analysis
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 Strength analysis
6.4.1 Requirements for direct calculations are given in the LR ShipRight
Procedure for Ship Units.
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