Section
5 Structural design – New-build units
5.1 General
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.
5.1.3 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 sitespecific 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.
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.
5.1.9 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.
5.1.12 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 Hull scantlings
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 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.
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 design purposes, site-specific environmental factors and Dynamic
Load Combination factors (DLCFs) derived in accordance with the LR ShipRight
Procedure for Ship Units should be used for hull scantlings.
5.2.7 For DLCFs, the given factors in Pt 10, Ch 4 Dynamic Load Combination Factors can be applicable for design purposes under class agreement,
considering the site-specific condition, mooring method and size of the units. It is
recommended that the designer consults with LR on the conditions of application,
early on in the design stage.
5.3 Strength analysis
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.
5.4 Fatigue analysis
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.
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