Section 3 Design Loads
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Offshore Units, July 2022 - Part 11 Production, Storage and Offloading of Liquefied Gases in Bulk - Chapter 4 Cargo Containment - Section 3 Design Loads

Section 3 Design Loads

Parent topic: Chapter 4 Cargo Containment

3.1 General

3.1.1 This Section defines the design loads to be considered with regard to the requirements in Pt 11, Ch 4, 4.1 General, Pt 11, Ch 4, 4.2 Structural analyses and Pt 11, Ch 4, 4.3 Design conditions. This includes:
  • Load categories (permanent, functional, environmental and accidental) and the description of the loads.

The extent to which these loads shall be considered depends on the type of tank, and is more fully detailed in the following paragraphs.

Tanks, together with their supporting structure and other fixtures, that shall be designed taking into account relevant combinations of the loads described below.

3.2 Permanent loads

3.2.1  Gravity loads

The weight of tank, thermal insulation, loads caused by towers and other attachments.

3.2.2  Permanent external loads

Gravity loads of structures and equipment acting externally on the tank.

3.3 Functional loads

3.3.1 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:
  • Internal pressure.
  • External pressure.
  • Thermally induced loads.
  • Vibration.
  • Interaction loads.
  • Loads associated with construction and installation.
  • Test loads.
  • Static heel loads.
  • Weight of cargo.
3.3.2  Internal pressure
  1. In all cases, including (b) below, P o 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, P o shall not be less than the gauge vapour pressure of the cargo at a temperature equal to the maximum daily mean ambient air temperature for the unit’s proposed area of operation based on the 100 year average return period. The ambient temperature is to be rounded up to the nearest degree Celsius, and not to be taken as less than 45°C unless agreed by LR.
  3. Subject to special consideration by the Administration and to the limitations given in Pt 11, Ch 4, 6.1 Type A independent tanks to Pt 11, Ch 4, 6.6 Semi-membrane tanks, for the various tank types, a vapour pressure P h higher than P o may be accepted for site-specific conditions where dynamic loads are reduced.
  4. The internal pressure P eq results from the vapour pressure P o or P h plus the maximum associated dynamic liquid pressure P gd, but not including the effects of liquid sloshing loads. Guidance formulae for associated dynamic liquid pressure P gd are given in Pt 11, Ch 4, 7.1 Guidance Notes for Chapter 4 7.1.1.

3.3.3  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.

3.3.4  Thermally induced loads

Transient thermally induced loads during cooling down periods shall be considered for tanks intended for cargo temperatures below –55°C.

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 Pt 11, Ch 7, 1.2 Design of systems.

3.3.5  Vibration

The potentially damaging effects of vibration on the cargo containment system shall be considered.

3.3.6  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.

3.3.7  Loads associated with construction and installation

Loads or conditions associated with construction and installation shall be considered, e.g. lifting.

3.3.8  Test loads

Account shall be taken of the loads corresponding to the testing of the cargo containment system referred to in Pt 11, Ch 4, 6.1 Type A independent tanks to Pt 11, Ch 4, 6.6 Semi-membrane tanks.

3.3.9  Static heel loads

Loads corresponding to the most unfavourable static heel angle within the range 0° to 30° shall be considered.

3.3.10  Other loads

Any other loads not specifically addressed, which could have an effect on the cargo containment system, shall be taken into account.

3.4 Environmental loads

3.4.1 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.

3.4.2  Loads due to the motions of the ship unit

The determination of dynamic loads shall take into account the long-term distribution of the motions of the ship unit in irregular seas, which the ship unit will experience during its operating life. Account may be taken of the reduction in dynamic loads due to heading control.
  1. The motions of the ship unit shall include surge, sway, heave, roll, pitch and yaw. The accelerations, derived from site-specific wave data and the heading analysis, acting on tanks, shall be estimated at their centre of gravity and include the following components:
    • vertical acceleration: motion accelerations of heave, pitch and possibly roll (normal to the base of the ship unit);
    • transverse acceleration: motion accelerations of sway, yaw and roll and gravity component of roll;
    • longitudinal acceleration: motion accelerations of surge and pitch and gravity component of pitch.
  2. Methods to predict accelerations due to ship motion shall be proposed to LR and approved by LR.
  3. Guidance formulae for acceleration components are given in Pt 11, Ch 4, 7.1 Guidance Notes for Chapter 4 .
  4. The determination of the dynamic loads may be based on the results of model tests and/or by suitable direct calculation methods of the actual loads on the cargo containment system at the site-specific location, taking into account the following service-related factors:
    • site-specific environmental loads including relevant non-linear 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.

    The actual form and weight distribution of the unit and the longitudinal and transverse locations of the tanks are to be taken into account.

3.4.3  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.

3.4.4  Sloshing loads

The sloshing loads on a cargo containment system and internal components, induced by any of the site-specific motions referred to in Pt 11, Ch 4, 3.4 Environmental loads 3.4.2, shall be evaluated based on allowable filling levels.

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.

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. Additionally, investigations should be made to ensure that the internal structure, equipment and pipework exposed to fluid motion are of adequate strength.

If the liquefied gas storage tanks are to have no filling restrictions, the capacity of the cargo containment system to resist the greatest predicted sloshing pressures is to be assessed for fill heights representative of all filling levels in accordance with this Section.

If filling restrictions are contemplated, the capacity of the cargo containment system to resist sloshing predicted pressures needs to be assessed only for fill heights representative of the permitted filling ranges. In this case, the filling restrictions are to be stated in the approved Loading Manual.

3.4.5  Snow and ice loads

Snow and icing shall be considered, if relevant.

3.4.6  Loads due to operation in ice conditions

Loads due to operation in ice conditions shall be considered for units intended for such service. The effects on the containment system due to additional topside weight as a result of ice accretion, and ice collisions against the hull should be considered, see also Pt 3, Ch 6 Units for Transit and Operation in Ice.

3.5 Accidental loads

3.5.1 Accidental loads are defined as loads that are imposed on a cargo containment system and its supporting arrangements under abnormal and unplanned conditions.

3.5.2  Loads due to flooding

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.

3.5.3  Collision loads

Where collision is defined by the Owner as a credible accidental load case, the requirements in this section are to be followed in order to assess the chocks and supports of the tanks.

Assessment against collision is to be in accordance with Pt 4, Ch 3, 4.16 Accidental loads.

All static loads are to be applied. Environmental loads need not be applied. Acceleration resulting from the collision is to be applied to all of the mass of the model including the cargo in the tanks.

3.5.4  Loss of heading control

Where stern thrusters or other means of heading control are fitted to weathervaning units then the effect of any single failure of the heading control system on the cargo containment system and its supporting arrangements is to be considered (see Pt 3, Ch 10 Positional Mooring Systemsfor thruster assisted mooring systems).

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