Part B - Design Loads
Clasification Society 2024 - Version 9.40
Statutory Documents - IMO Publications and Documents - International Codes - 2014 IGC Code - International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk - Chapter 4 – Cargo Containment - Chapter 4 – Cargo Containment - Part B - Design Loads

Part B - Design Loads

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:

  • .1 load categories (permanent, functional, environmental and accidental) and the description of the loads;

  • .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

  • .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:

  • .1 internal pressure;

  • .2 external pressure;

  • .3 thermally induced loads;

  • .4 vibration;

  • .5 interaction loads;

  • .6 loads associated with construction and installation;

  • .7 test loads;

  • .8 static heel loads; and

  • .9 weight of cargo.

  4.13.2 Internal pressure

  • .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:

    • .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.

  • .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.

  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.

  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:

  • .1 vertical acceleration: motion accelerations of heave, pitch and, possibly, roll (normal to the ship base);

  • .2 transverse acceleration: motion accelerations of sway, yaw and roll and gravity component of roll; and

  • .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.

  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.

  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.

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.

Parent topic: Chapter 4 – Cargo Containment
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