Gauging Systems Inc of the US is in the process of re-engineering its latest land-based tank gauging technology and certifying it for marine use. The system is called the Multi-function Tank Gauge (MTG) and the company is in discussions with three shipowners regarding US Coast Guard testing. Gauging Systems Inc claims to produce the world’s most accurate volume tank gauges. They provide readings for volume, mass, level, multi-point spot temperature, average product temperature, multi-strata density, average product density, free water, suspended water, vapour pressure and vapour temperature.
The only differences between the land-based technology and the marine system are the transmitter head
material and sealing flange. The MTG 3012 Multi-function Tank Gauge instrument is composed of multiple (4-12) sensor sections. Each sensor section includes a high-accuracy pressure transducer and provides multiple measured or calculated data variables that can be used for alarm purposes. That means the MTG 3012 can provide primary level and multiple alarm data variables from multiple sensors to provide redundancy. The multiprocessor transmitter card supports 1,000 strapping points and provides volumetric calculations to API, ISO or GOST standards. The system has only one tank entry and one electrical connection. It is bottom referenced, self-calibrating, self-diagnostic, and has no moving parts.
The MTG 3012 calculates all necessary data for volume to custody transfer accuracy within the transmitter head. MODBUS remote terminal unit registers are accessible for calculated data, measured data, diagnostics and sensor data in millivolts. The system’s communications allow the polling and data collection of the raw millivolt readings directly from the sensors, making possible historical monitoring of the sensors for drift or other problems.
If a sensor does fail, the transmitter will turn off that sensor, provide a diagnostic indication of the failed sensor, and formulate all measurements or calculations from the multiple working sensors. A longer span between sensors in calculating level accuracy is the worst-case scenario for a middle sensor failure. The two most critical sensors are the bottom sensor and vapour sensor.
Mass measurement provides the greatest sensitivity and resolution for detecting authorised or unauthorised product movement and leak detection. The MTG 3012 measures both atmospheric pressure and vapour pressure, so is capable of monitoring the tank for overpressure or the formation of a vacuum should pressure relief valves be improperly set or stick. What is more, from the measurement of vapour pressure, vapour temperature, and the number of tank cycles, it is possible to apply real data to hydrocarbon emissions reporting.
Selma of Greece provides tank level gauging and cargo monitoring technologies that the company says meet the latest requirements of ExxonMobil. Requirement H27 specifies that tankers should be fitted with a manifold pressure cargo oil load/discharge monitoring and recording system located in the cargo control room (CCR). Requirement H29/H30 stipulates that existing cargo oil tank (COT) vapour pressure monitoring systems should consist of an independent monitor in the wheelhouse (apart from the existing monitoring system in the CCR) and that four vapour pressure COT alarm set points should be energised (instead of the traditional two set points). Finally, according to Requirement E19, the CCR should have an anemometer recorder (recording wind speed and direction), which is critical during cargo load/discharge operations.
Developed this year, Selma’s Tank Protector and integrated CCR monitoring and alarm system meet all the conditions set by ExxonMobil. The system combines cargo oil tank measurement, ballast tanks/draft level gauging and cargo oil pump temperature monitoring in bearings. The system can also be integrated with an atmospheric oil mist detection system.
The Selma Tank Protector system records the vapour pressure in cargo oil tanks and slop tanks (the typical measuring range is -500mbar to +500 mbar). It also records manifold pressure (measuring 0-16 bar) and wind speed and direction to perform true or relative calculations depending on whether or not it is connected to a GPS system. Recorded data can be reviewed from colour touch monitor graphs and in Excel format via USB memory sticks.
Selma undertakes design, development and installation of cargo oil tank integrated monitoring systems. The company provides measurement of a cargo oil tank’s level, temperature, vapour pressure and high-level alarm systems using Supervising Computing and Data Acquisition Systems and the Programmable Logic Control Platform. Selma’s full product range includes main switchboards, engineroom monitoring and alarm consoles, boiler automation systems, dewatering control systems and navigation consoles.
Krohne Skarpenord of Norway has recently delivered its Cargomaster Tank Monitoring System with Optiwave level radars to Brave Maritime’s newbuilding at Daewoo Shipbuilding & Marine Engineering in South Korea. The system will be designed to fulfil Chevron’s regulations for oil transportation, which include redundant monitoring of cargo tanks.
By using the radar’s high accuracy for ullage measurements in the upper tank levels, operators will have accurate readings for topping off tanks. Further down in the tank where the radar beam has problems recognising a level, the pressure sensor will provide a correct tank level all the way down to the tank bottom. A switch point between the two different sensors has been programmed in the Cargomaster software. The system will use the radar as the primary level instrument above the switch point, and the pressure/level sensor below the switch point. Together, the radar and pressure sensor will also calculate the cargo density.
A 6 + 6 order for tank monitoring systems has also been received for VLCCs on order to NITC in China due for delivery this year and next. Six VLCCs will be built at Dalian Shipbuilding and six at Shanghai Waigaoqiao Shipbuilding. The systems are extensive, and include full redundancy on the monitoring side. Optiwave Cargo Level Radars will be installed on all 15 cargo tanks. The two slop tanks will be equipped with a hybrid level gauging solution combining level radar with a pressure sensor close to the tank bottom to secure level readings in the bottom area as well as density calculations of the slops. The deliveries will also include subsystems from Krohne’s partners Scanjet Macron and Vimex.
Krohne reports good market uptake for its hybrid solution. References include a delivery for a small deck tank on a chemical tanker necessary because a tank radar will pick up interference in the radar wave propagation caused by the cylindrical tank shape. The company says installing a stillwell with all its cleaning issues is not a good option on chemical tankers. Rather, a pure cargo level pressure sensor installation or an Optiflex 4300 C guided level radar installation will provide good monitoring.
Hanla of Korea provides cargo tank measuring systems, remote sounding systems and valve remote control systems. Its cargo monitoring systems are suitable for all types of tankers as well as for FPSOs. Hanla’s remote sounding systems are configured with a level gauging system that is integrated online through the communications unit of the cargo monitoring system or displayed on the level indicator for ballast or engineroom tanks. The company’s range of level gauges includes electro-pneumatic, electric pressure, air purge and microprocessor-controlled models.
Jowa USA provides level gauging solutions based on Metritape, a non-mechanical, continuous level sensor which the company claims offers clear advantages over other level technologies, especially in thick, sticky liquids that cause mechanical level gauges to seize. The Metritape resistance-tape level sensor features full height, independent high alarm, three point temperature and inert gas pressure monitoring in a single deck penetration with a single cable run per tank, which reduces installation costs. The Metritape sensor extends into the cargo well, providing gauging below tank bottom and speeding up discharge time. The flexible sensors follow the contour of J-shaped ballast tanks for high-performance level detection at an affordable price, says Jowa. No tank entry is required for installation or service and the equipment is virtually maintenance-free.
The Metribeam tank radar system can be used for cargo tanks and combined with resistance-tape for ballast, service tanks and draft. The Metribeam tank radar is used to gauge petroleum products and chemicals with temperatures of up to 200°C. Its high signal-to-noise ratio combined with the 24 GHz high frequency electromagnetic wave makes it one of the most accurate and reliable tank gauges in the marine market, according to Jowa. The Metribeam has a measuring range of 0.35 to 50m, making it suitable for any vessel.
The Metrimeter is a single tank instrument that directly interfaces with Metritape level and level/temperature sensors. It provides a 6-digit alphanumeric display, a 40-point bar graph display and six alarm and fault indicators. In addition, it may be used as a slave meter when connected to other systems. Through an extensive menu system, the meter may be set up and configured to suit individual requirements. The Metrimeter can do full height, overfill alarm and two temperature readings. As one is used for each tank, Jowa says it can be very cost effective for shipowners to retrofit if they want an accurate level gauging system for fuel tanks.
NDT International of the US supplies the PLI-1 portable Ultrasonic Liquid Level Indicator made by Class Instrumentation of the UK. The unit’s features include automatic calibration, no tuning controls, dry sensor application and a special power booster for poor condition tanks and cylinders. The unit is accurate to 1.5mm. TST