4.11
General
This section defines the design loads to be considered with regard to the
requirements in 4.16, 4.17 and 4.18. This includes:
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.1 load categories (permanent, functional,
environmental and accidental) and the description of the loads;
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.2 the extent to which these loads shall be
considered depending on the type of tank, and is more fully detailed in the
following paragraphs; and
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.3 tanks, together with their supporting
structure and other fixtures, that shall be designed taking into account
relevant combinations of the loads described below.
4.12
Permanent loads
4.12.1
Gravity loads
The weight of tank, thermal insulation, loads caused by towers and other
attachments shall be considered.
4.12.2
Permanent external loads
Gravity loads of structures and equipment acting externally on the tank
shall be considered.
4.13
Functional loads
4.13.1 Loads arising from the operational use of the
tank system shall be classified as functional loads. All functional loads that are
essential for ensuring the integrity of the tank system, during all design
conditions, shall be considered. As a minimum, the effects from the following
criteria, as applicable, shall be considered when establishing functional loads:
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.1 internal pressure;
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.2 external pressure;
-
.3 thermally induced loads;
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.4 vibration;
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.5 interaction loads;
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.6 loads associated with construction and
installation;
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.7 test loads;
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.8 static heel loads; and
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.9 weight of cargo.
4.13.2
Internal pressure
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.1 In all cases, including 4.13.2.2,
Po
shall not be less than MARVS.
-
.2 For cargo tanks, where there is no
temperature control and where the pressure of the cargo is dictated only by
the ambient temperature, Po
shall not be less than the gauge vapour pressure of the cargo at a
temperature of 45°C except as follows:
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.1 lower values of ambient temperature
may be accepted by the Administration or recognized organization
acting on its behalf for ships operating in restricted areas.
Conversely, higher values of ambient temperature may be required;
and
-
.2 for ships on voyages of restricted
duration, Po
may be calculated based on the actual pressure rise during the
voyage, and account may be taken of any thermal insulation of the
tank.
-
.3 Subject to special consideration by the
Administration and to the limitations given in 4.21 to 4.26, for the various
tank types, a vapour pressure Ph
higher than Po
may be accepted for site specific conditions (harbour or other
locations), where dynamic loads are reduced. Any relief valve setting
resulting from this paragraph shall be recorded in the International
Certificate of Fitness for the Carriage of Liquefied Gases in Bulk.
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.4 The internal pressure Peq
results from the vapour pressure Po
or Ph
plus the maximum associated dynamic liquid pressure Pgd
, , but not including the effects of liquid sloshing loads. Guidance
formulae for associated dynamic liquid pressure Pgd
are given in 4.28.1.
LR 4.13-01 Consideration will be given to the use of a higher or
lower ambient temperature where appropriate. In such cases, the temperature which
has been used will be included in the class notation, see also LR III.
LR 4.13-02 Where a vapour pressure, Ph, higher
than Po, is accepted in accordance with 4.13.2.3, such conditions
are to be clearly indicated in the ship’s Loading Manual.
4.13.3
External pressure
External design pressure loads shall be based on the difference between
the minimum internal pressure and the maximum external pressure to which any portion
of the tank may be simultaneously subjected.
4.13.4
Thermally induced loads
4.13.4.1 Transient thermally induced loads during
cooling down periods shall be considered for tanks intended for cargo temperatures
below -55°C.
4.13.4.2 Stationary thermally induced loads shall be
considered for cargo containment systems where the design supporting arrangements or
attachments and operating temperature may give rise to significant thermal stresses
(see 7.2).
4.13.5
Vibration
The potentially damaging effects of vibration on the cargo containment
system shall be considered.
LR 4.13-03 Vibration analysis of the Pump Tower is to be carried
out in accordance with LR’s ShipRight Procedure Additional Design Procedures,
Procedure for Analysis of Pump Tower and Pump Tower Base.
4.13.6
Interaction loads
The static component of loads resulting from interaction between cargo
containment system and the hull structure, as well as loads from associated
structure and equipment, shall be considered.
4.13.7
Loads associated with construction and installation
Loads or conditions associated with construction and installation, e.g.
lifting, shall be considered.
4.13.8
Test loads
Account shall be taken of the loads corresponding to the testing of the
cargo containment system referred to in 4.21 to 4.26.
4.13.9
Static heel loads
Loads corresponding to the most unfavourable static heel angle within
the range 0° to 30° shall be considered.
4.13.10
Other loads
Any other loads not specifically addressed, which could have an effect
on the cargo containment system, shall be taken into account.
4.14
Environmental loads
Environmental loads are defined as those loads on the cargo containment
system that are caused by the surrounding environment and that are not otherwise
classified as a permanent, functional or accidental load.
4.14.1
Loads due to ship motion
4.14.1.1 The determination of dynamic loads shall take
into account the long-term distribution of ship motion in irregular seas, which the
ship will experience during its operating life. Account may be taken of the
reduction in dynamic loads due to necessary speed reduction and variation of
heading.
4.14.1.2 The ship's motion shall include surge, sway,
heave, roll, pitch and yaw. The accelerations acting on tanks shall be estimated at
their centre of gravity and include the following components:
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.1 vertical acceleration: motion accelerations
of heave, pitch and, possibly, roll (normal to the ship base);
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.2 transverse acceleration: motion
accelerations of sway, yaw and roll and gravity component of roll; and
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.3 longitudinal acceleration: motion
accelerations of surge and pitch and gravity component of pitch.
4.14.1.3 Methods to predict accelerations due to ship
motion shall be proposed and approved by the Administration or recognized
organization acting on its behalf.
LR 4.14-01 Direct calculation procedures capable of deriving the
dynamic loads due to ship motions, are to take into account the ship’s actual form
and weight distribution. LR’s direct calculation method involves derivation of
response to regular waves by appropriate sea-keeping software, short-term response
to irregular waves using the sea spectrum concept, and long-term response
predictions using statistical distributions of sea states. Other direct calculation
methods submitted for approval are expected to contain these three elements and
produce similar and consistent results when compared with LR’s method. Simplified
dynamic loading spectra, where proposed, are to be submitted for consideration.
4.14.1.4 Guidance formulae for acceleration components
are given in 4.28.2.
4.14.1.5 Ships for restricted service may be given
special consideration.
4.14.2
Dynamic interaction loads
Account shall be taken of the dynamic component of loads resulting from
interaction between cargo containment systems and the hull structure, including
loads from associated structures and equipment.
4.14.3
Sloshing loads
4.14.3.1 The sloshing loads on a cargo containment
system and internal components shall be evaluated based on allowable filling
levels.
4.14.3.2 When significant sloshing-induced loads are
expected to be present, special tests and calculations shall be required covering
the full range of intended filling levels.
LR 4.14-02 Where loading conditions are proposed including one or
more partially filled tanks, calculations or model tests will be required to show
that the resulting loads and pressure are within acceptable limits for the
scantlings of the tanks. In general, calculations are to be carried out in
accordance with LR’s ShipRight Procedure Design and Construction Procedure,
Structural Design Assessment, Sloshing Loads and Scantling Assessment.
Alternative procedures may be specially considered.
LR 4.14-03 Investigations to ensure that the internal structure,
equipment and pipework exposed to fluid motion are of adequate strength are also to
be carried out. The assessment of Pump Tower and Pump Tower Base due to fluid motion
is in general to be carried out in accordance with LR’s ShipRight Procedure
Additional Design Procedures, Procedure for Analysis of Pump Tower and Pump
Tower Base.
4.14.4
Snow and ice loads
Snow and icing shall be considered, if relevant.
4.14.5
Loads due to navigation in ice
Loads due to navigation in ice shall be considered for vessels intended
for such service.
LR 4.14-05 Where a vessel is intended to navigate through ice,
the vessel's interaction with ice is to be considered. See Pt 8 of the Rules
for Ships.
4.15
Accidental loads
Accidental loads are defined as loads that are imposed on a cargo
containment system and its supporting arrangements under abnormal and unplanned
conditions.
4.15.1
Collision loads
The collision load shall be determined based on the cargo containment
system under fully loaded condition with an inertial force corresponding to 0.5 g in
the forward direction and 0.25 g in the aft direction, where "g" is gravitational
acceleration.
4.15.2
Loads due to flooding on ship
For independent tanks, loads caused by the buoyancy of an empty tank in
a hold space flooded to the summer load draught shall be considered in the design of
the anti-flotation chocks and the supporting hull structure.