(See Illustration 3.2.1a)
Supervision of all cargo handling operations is carried out
from the Cargo Control Room (CCR): located on deck B
of the accommodation block with a forward looking view. The CCR houses the Cargo Console and the control
cabinets for the ELSAG Bailey (IMS) (see 3.1) a printer
desk, control cabinets for the Emergency Shut Down
System (ESDS), and the analyser cabinet for the ICARE Gas Detection system.
side of the CCR and contains the following: 1 Two IMS operator stations;
2 One operator station for the Custody Transfer System
(CTS) (see 3.3);
3 A CCTV monitor showing the port and stbd manifold areas and trunk deck area.
4 Master fuel gas and nitrogen injection valve control. 5 Automatic winch control.
6 CETENA Loadmaster station
7 ESDS panel
9 Analogue gauges for the main cargo pumps, stripping/spray pumps and auxiliary cargo pump, a wind speed and direction indicator, and analogue cargo tank level indicators.
Printing facilities for 1 and 2 are located on a desk on the starboard bulkhead.
3.2 Cargo Control Room, Cargo Console and Panels - Page 1 Issue: 1
Custody Transfer System Distributed Control System
Telephone
Ship to Shore Communication (Hot Line)
CCTV Alarm Indication Panel
Distributed Control System
ESDS Panel
Track ball Track ball
Load Master Analogue Gauges
For Cargo Pumps
Automatic
Winch Control Master Fuel Gas and Nitrogen Injection Valve Controls
Keyboard Selection Switch
For Emergency Cargo Pump Manual Trip Buttons
3.3 Custody Transfer System
Introduction(See Illustration 3.3.2a)
LNG is bought and sold on its calorific content, normally expressed in Btu’s rather than on a volume or weight basis. However, at the present time there are no practical instruments available to determine the net calorific content transferred during loading and discharge so that for the moment this value is determined partly by measurement and partly by analysis of cargo calculation by means of the following formula:
Total energy transferred Q = Vd HL - VTsPvHv TvPs where :
V = the apparent volume of cargo loaded or discharged
at an average temperature Tl (m3)
d = density of cargo at temperature Tl (kg/m3)
HL = the gross calorific value of the cargo calculated from the component analysis of the cargo (Btu/kg) from mean value of loading and discharge sample (MLNG uses MJ/kg)
Ts = standard temperature (°K)
Tv = average temperature of gas in the cargo tanks either before loading or after discharge (°K)
TL = average temperature of liquid in cargo tanks either before loading or after discharge
Pv = the absolute pressure of the gas in the cargo tanks, that is, gauge pressure of gas + barometric pressure (mbar)
Pv = standard atmospheric pressure (1013mbar)
Hv = the gross heating value of gas vapour at 15.6°C and
1013mbar (Btu/m3). This value is assumed to be a
constant 36,000 Btu/m3 based on pure methane (MLNG uses Btu/scf)
In establishing the value of the cargo transferred to and from the ship, the vessels responsibility is limited to measurement/calculation of the following values: V, Tv, Pv. These measurements/calculations are made by ship and shore representatives and are normally verified by an independent surveyor. The values HL and d are determined ashore at the loading and discharge ports and the calculations are completed by the buyers and sellers. The quantity of cargo delivery is expressed in MMBtu or tonnes.
3.3.1 Custody Transfer Measurements
The Foxboro Custody Transfer CT-IV System provides the high accuracy measurements and data logging of levels, temperatures and pressures required for the calculation of total LNG Cargo loaded or discharged. All Custody Transfer Measurements can be displayed on a video monitor and in addition liquid level measurements of each tank can be individually displayed on the remote 4-Digit. LED-Type digital indicators.
The system automatically scans and prints the values of the selected measurement. In addition, the data is converted to volumetric and mass measure, corrected for the ship’s list and trim based on manual and automatic inputs from a software configured to handle various functions.
The software configuration includes :
1 manually input density data representative of the total cargo from a cargo composition library stored in memory (up to 10 composition and density values is available).
2 2 analog inputs for list and trim. 3 volume expression in cubic metres 4 mass expression in tonnes.
Custody transfer measurement takes place before and after loading and discharging. During gauging, all cargo systems on the ship should be closed and the shore connections isolated or disconnected. No ballast operations should take place during measurement and the vessel should, if possible, be on even keel and upright. However, the operation can be conducted with a slight trim if the corrections included in the tank calibration tables are applied.
Custody Transfer Documents are produced detailing the volumes of cargo and vapour transferred during both loading and discharge operations.
Level measurement
The level measurement system consists of a long coaxial sensor installed in the tank and extends over the full depth in which the level is to be measured.
The liquid level is determined by measuring the electrical capacitance of the sensor segment intersecting the liquid level. The capacitance of this segment is compared with the capacitance of a reference segment located below the liquid surface. The ratio of these two measurements results in the accurate determination of liquid level that is
independent of liquid properties such as dielectric constant, temperature and density.
This ratiometric method of cargo level measurement includes an innovative calibration assurance feature incorporated to maintain the accuracy of the level measuring sub system; this system is called the ON-LINE Validation System.
The level measurement accomplished provides an accuracy of ± 7.5mm over the entire gauging height. The display resolution is 1mm at the system workstation and printer. In the event of failure of the level sensing devices, the
HSH 806 float gauges may be used for level measurement providing that approval is given by the shore representatives.
The accuracy of the float gauge is the same as the capacitance gauge.
! CAUTION
To avoid failure the float of the float gauge must be maintained blocked at the top position except when during the actual measurement.
The total gross number of cubic metres of cargo in the tanks before and after loading or discharging is calculated using the average level reading determined. This volume is corrected for heel, trim, volume, vapour pressure and cargo and vapour temperatures.
The difference in these volumes at the start and end of the
operation will be taken as the apparent volume (m3) of
cargo delivered.
Temperature Measurement
Temperature measurement is accomplished by means of Platinum Resistance transducers (500ohms) providing an accuracy of ± 0.2°C from -165°C to 145°C, ± 0.3°C up to - 120°C and ± 1 - 5°C up to + 80°C.
Data is displayed at the system workstation and printer with a resolution of 0.01°C.
There are five active and five spare temperature sensors in each tank and each reading is recorded.
The determination of whether the temperature point is in vapour or liquid will be made from the liquid level indication. It is safe to assume that any point that indicates a temperature of more than 3° above the LNG temperature must be in the vapour space.
Although the temperature variation over the depth of liquid should be no more than a fraction of a degree, the variation of vapour temperature, particularly at the end of discharge, will be more pronounced.
Pressure measurement
Absolute pressure measurement in each of the cargo tank is determined by intelligent pressure transmitters. The accuracy of the measurement is ± 0.1% of span to a resolution of 1mb at system workstation and printer. Range of measurement is 800 to 1400mb. Ambient temperature effect on the transmitter is ± 0.2% per 55°C change between limits of -30°C and +60°C.
3.3.2 Independent Very High Level Alarm System
Two very high level alarms per tank are provided by independent point sensing elements. Fixed sensors inside the cargo tanks detect the cargo at predetermined levels and system accuracy is ± 6mm.
The very high alarm adjusted at 98.6% of the tank height, when activated will close the corresponding tank filling valve.
The very very high alarm adjusted at 98.5% of the tank
height, when activated will initiate an Emergency Shut Down Alarm (ESD) (refer to Section 5.2). This involves the shutting of manifold and tank loading valve of the tank in question. DCS has facilities to inhibit at 98.5 and 99 to allow opening of tank valve during the level alarm testing. In addition, a blocking function is provided to allow all cargo tank level alarms to be overridden when at sea.
3.3 Custody Transfer System - Page 1 Issue: 1
3.3.2a CTS Measurement Issue: 1 Liquid Dome High Sensing Range 99% 98.5% 100% 75% 50% 25% 0% Mid Sensing Range Low Sensing Range Tank Bottom Feed Through Coaxial Cables to Control Unit via Zener Barriers