Section 1 Filling Limits for Cargo Tanks
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 15 Filling Limits for Cargo Tanks - Section 1 Filling Limits for Cargo Tanks

Section 1 Filling Limits for Cargo Tanks

Parent topic: Chapter 15 Filling Limits for Cargo Tanks

1.1 Definitions

1.1.1  Filling limit (FL) means the maximum liquid volume in a cargo tank relative to the total tank volume when the liquid cargo has reached the reference temperature.

1.1.2  Loading limit (LL) means the maximum allowable liquid volume relative to the tank volume to which the tank may be loaded.

1.1.3 Reference temperature means (for the purposes of this Chapter only):
  1. When no cargo vapour pressure/temperature control, as referred to in Pt 11, Ch 7 Cargo Pressure/Temperature Control , is provided, the temperature corresponding to the vapour pressure of the cargo at the set pressure of the PRVs.
  2. When a cargo vapour pressure/temperature control, as referred to in Pt 11, Ch 7 Cargo Pressure/Temperature Control , is provided, the temperature of the cargo upon termination of loading, during transport or at unloading, whichever is the greatest.

1.1.4  Ambient design temperatures for air and seawater are at their highest daily mean temperatures for the unit’s proposed area of operation based on the 100 year average return period. The ambient temperatures are to be rounded up to the nearest degree Celsius. For initial design, the ambient temperatures may be taken as 45°C for air and 32°C for sea-water.

1.2 General requirements

1.2.1 The maximum filling limit of cargo tanks shall be so determined that the vapour space has a minimum volume at reference temperature allowing for:
  1. tolerance of instrumentation such as level and temperature gauges
  2. volumetric expansion of the cargo between the PRV set pressure and the maximum allowable rise stated in Pt 11, Ch 8, 1.4 Sizing of pressure relieving system
  3. an operational margin to account for liquid drained back to cargo tanks after completion of loading, operator reaction time and closing time of valves, see Pt 11, Ch 5, 2.2 Cargo system valve requirements and Pt 11, Ch 18, 4.2 ESD valve requirements 4.2.1.(d).

1.3 Default filling limit

1.3.1 The default value for the filling limit (FL) of cargo tanks is 98 per cent at the reference temperature. Exceptions to this value shall meet the requirements of Pt 11, Ch 15, 1.4 Determination of increased filling limit.

1.4 Determination of increased filling limit

1.4.1 A filling limit greater than the limit of 98 per cent specified in Pt 11, Ch 15, 1.3 Default filling limit on condition that, under the trim and list conditions specified in Pt 11, Ch 8, 1.2 Pressure relief systems 1.2.18 may be permitted, providing:
  1. no isolated vapour pockets are created within the cargo tank
  2. the PRV inlet arrangement shall remain in the vapour space
  3. allowances need to be provided for:
    1. volumetric expansion of the liquid cargo due to the pressure increase from the MARVS to full flow relieving pressure in accordance with Pt 11, Ch 8, 1.4 Sizing of pressure relieving system 1.4.1
    2. an operational margin of minimum 0,1 per cent of tank volume
    3. tolerances of instrumentation such as level and temperature gauges.

1.4.2 In no case shall a filling limit exceeding 99,5 per cent at reference temperature be permitted.

1.5 Maximum loading limit

1.5.1 The maximum loading limit (LL) to which a cargo tank may be loaded shall be determined by the following formula:

LL =

where:

LL = loading limit as defined in Pt 11, Ch 15, 1.1 Definitions 1.1.2 expressed in per cent;
ρR = relative density of cargo at the reference temperature; and
ρL = relative density of cargo at the loading temperature.

1.5.2 The Administration may allow Type C tanks to be loaded according to the formula in Pt 11, Ch 15, 1.5 Maximum loading limit 1.5.1 with the relative density ρR as defined below, provided that the tank vent system has been approved in accordance with Pt 11, Ch 8, 1.2 Pressure relief systems 1.2.19.

ρR = relative density of cargo at the highest temperature that the cargo may reach upon termination of loading, during storage, or at unloading, under the ambient design temperature conditions described in Pt 11, Ch 15, 1.1 Definitions 1.1.4.

1.6 Information to be provided to the Operator

1.6.1 A document shall be provided to the unit specifying the maximum allowable loading limits for each cargo tank and product, at each applicable loading temperature and maximum reference temperature. The information in this document shall be approved by LR.

1.6.2 Pressures at which the PRVs have been set shall also be stated in the document.

1.6.3 A copy of the above document shall be permanently kept onboard by the Operator.

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