ISO 10976:2015 pdf download – Refrigerated light hydrocarbon fluids — Measurement of cargoes on board LNG carriers.
See Tables C.4 and C for examples of sections of thermal correction tables for a radar-type level gauge and float-type level gauge, respectively. DensIty correction tables
ensity correction tables may be provided for float-type level gauges to compensate for the float buoyancy as it varies with LNG density. See TabkCJ for an example.
5.6.3 Trim and list measurement General
Tank capacity tables are based on the ship being on an even keel. Trim and list shall be determined by
— taking the draft lore and aft (either manually or by measurement), and/or
— measuring the list of the LNG carrier.
The impact of trim and list varies with the tank type. On an LNG carrier with spherical tanks, due to the centralized location of the level gauge on the tank, trim and list have a minor impact on the uncertainty of the measured quantities. however, fur a membrane tank LNG carrier, the tnrn correction is affected by the large distance from the tank centre to the typical position of the level gauge near the alt tank bulkhead. Trim and list by Inclinometer
Where Inclinometers are used In LNG carrier service, they are predominantly two-axis type and are used to measure trim and list, although they may also be used to measure either individually.
Inclinometers measure trim and/or list based on gravitational principles. The most common methods are capacitance based: otherwise, they make use of electrolytic technology, where a liquid in a precisely designed and closed chamber Is moving. Other types exist, hut only those with servo-assisted technology and an inertial mass/optical sensor within a servo feedback loop give sufficiently accurate and stable measurements. These are electronic instruments which can communicate with the CTMS, preferably using digital signals.
Verification tolerances for inclinometers are provided in Table I. but it should be noted that this tolerance represents the combined influence of inclinometer uncertainty and the possible contributions from structural bending differences between the inclinometer location and the Individual tank locations for the state of load of the LNG carrier, Trim and list by draft nieasurement
An alternative to inclinoineters is draft (alternative spelling: draught) measurement. The draft may be measured manually or automatically, with an electro-pneumatic draft measurement system (with digital communication) being common.
U4 outlines the process for taking draft readings of the vessel to determine trim and list.
5.6.4 Tank gassing-up tables or means of determination
After lay-up or dry dock, the LNG carrier cargo tanks are filled with nitrogen or other inert gas. If the cargo tanks contain nitrogen, the cool-down process may begin without purging. In order to be in a condition to receive cargo, Inert gas may need to be purged with LNG vapour prior to cool down to eliminate high boiling temperature gases, such as carbon dioxide.
LNG carriers usually have gassing-up tables or formulae which are used for determining the quantity of LNG required to gas up the cargo tank(s). These tables give an estimation of the LNG quantity used to gas up the cargo tanks by applying a displacement ratio depending of the type of the cargo tanks (usually between 1,4 and 1.8 for prismatic tanks, and between 1.1 and 1.4 for Moss tanks). Gassing-up tables are usually provided by the tank manufacturer or shipbuilder and should be certified by the class society oran independent company. Some shore terminals rely on meters as means to measure such quantities.
5.6.5 Tank cool-down tables or means of determination General
LNG carriers have cool-down tables or formulae, which are used for determining the quantity of LNGrequired to cool a tank down to a specified temperature.Cool-down tables are usually provided bythe tank manufacturer or shipbuilder and should be certified by the class society or an independentcompany.Other methods, such as those employing spray nozzle flow rate and duration or quantitiesmeasured by meters, may be used.